adsorptioncapacitiesofhygroscopicmaterialsbasedon nacl-tio

Research ArticleAdsorption Capacities of Hygroscopic Materials Based onNaCl-TiO2 and NaCl-SiO2 CoreShell Particles

Marie Bermeo1 Nabil El Hadri1 Florent Ravaux1 Abdelali Zaki2 Linda Zou2

and Mustapha Jouiad 13

1Department ofMechanical ampMaterials Science and Engineering Khalifa University of Science and Technology Abu Dhabi UAE2Department of Civil Infrastructure and Environmental Engineering Khalifa University of Science and TechnologyAbu Dhabi UAE3Laboratory of Physics of Condensed Mater (LPMC) University of Picardie Jules Verne Amiens 80039 France

Correspondence should be addressed to Mustapha Jouiad mustaphajouiadu-picardiefr

Received 23 September 2019 Accepted 11 December 2019 Published 13 February 2020

Guest Editor Giorgio Vilardi

Copyright copy 2020Marie Bermeo et alis is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Hygroscopic materials which possess high moisture adsorption capacity were successfully upgraded by the functionalization ofsodium chloride (NaCl) using two nuances of oxides A procedure was developed to first prepare submicron-sized NaCl crystalsthereafter these crystals were coated by choice of either titanium dioxide (TiO2) or silica (SiO2) to enhance the hygroscopicproperties of NaCl and prevent its premature deliquescence After coating several analytical techniques were employed to evaluatethe obtained composite materials Our findings revealed that both composites NaCl-TiO2 and NaCl-SiO2 gave excellent per-formances by exhibiting interesting hydrophilic properties compared to the sole NaCl is was demonstrated by both envi-ronmental scanning electron microscope (ESEM) and water vapor adsorption experiments In particular NaCl-TiO2 compositeshowed the highest water adsorption capacity at low relative humidity and at a faster adsorption rate induced by the high surfaceenergy owing to the presence of TiO2 is result was also confirmed by the kinetics of adsorption which revealed that not onlydoes NaCl-TiO2 adsorb more water vapor than NaCl-SiO2 or sole NaCl but also the adsorption occurred at a much higher rateWhile at room temperature and high relative humidity the NaCl-SiO2 composite showed the best adsorption properties making itideal to be used as a hygroscopic material showing maximum adsorption performance compared to NaCl-TiO2 or sole NaClerefore NaCl-TiO2 and NaCl-SiO2 composites could be considered as promising hygroscopic materials and potential can-didates to replace the existing salt seeding agents

1 Introduction

Hygroscopic materials are characterized for their ability toattract and hold water molecules from their surroundingenvironment is property has led to numerous applica-tions such as drying agents for moisture-sensitive products(eg electronic devices food and pharmaceutical products)dehumidifiers humectants for cosmetic products adsor-bents for industrial adsorption processes [1ndash4] and aerosolsfor rain enhancement and water harvesting [5ndash9] Sodiumchloride (NaCl) is a well-known hygroscopic material whichis commonly used as an adsorbent It is also used as a cloudseeding agent for warm clouds ie hygroscopic cloudseeding to enhance precipitation due to its affinity towards

water [10ndash13] Although NaCl and other natural materialsare widely used for their good adsorption properties itremains a challenge to advance the fabrication of newmaterials with better performances than conventional onesin terms of two main criteria namely higher water uptakeand optimum adsorption kinetics at variable conditions oftemperature and relative pressure

In this regard several complex hygroscopic materialshave been developed based on porous materials such asmetal-organic framework For instance MOF-801 andMOF-841 achieved water adsorption as high as 380 cm3 gminus 1

and 550 cm3 gminus 1 respectively at low relative pressure of 03and this performance was evaluated in the temperaturerange of 15degC to 55degC [14 15] Polyacrylamide (PAM) and

HindawiJournal of NanotechnologyVolume 2020 Article ID 3683629 16 pageshttpsdoiorg10115520203683629

sodium polyacrylate (PAAS) are known as superhygroscopicporousmaterialsey were found to act as a catalyst powderto enhance precipitations with a lower efficiency for PAAS[16] While other composite adsorbents such as crystallineMCM-41 and calcium chloride (CaCl2) have revealed a highwater adsorption capacity of 175 kg kgminus 1 in the temperaturerange of 10ndash15degC and at 78ndash92 relative humidity (RH)[17 18] Although these materials have very good hygro-scopic properties several factors such as their complexprocessing routes and their water stability at differentconditions of pressure and temperature limit their use es-pecially in cloud seeding

For cloud seeding applications the hygroscopic mate-rials need to meet the particle size requirement criterionAccording to earlier studies [19ndash22] based on numericalcorrelations modelling and simulations the optimum sizeof seeding materials is in the range of 05 to 10 μm in di-ameter is particle size range ensures an efficient collision-coalescence process of the cloud condensation nuclei(CCN) is process initiates the water droplet nucleationand enables their growth until they reach the raindropcritical size of about 24 μm in diameter [23] At this stage theraindrops may overcome the updraft velocity inside theclouds and be dragged downwards by their own weight tofall as rain [24 25]

is study concerns the characterization of novelcomposite materials which were developed through rela-tively simple mechanisms and using affordable andabundant materials in nature Here commercial salt (NaCl)as a raw material was optimized in terms of size as percloud seeding requirement to yield to NaCl crystals withsub-micrometric size ereafter they were coated withthin layers of oxides either TiO2 or SiO2 which have goodaffinity towards water ie hydrophilicity [26ndash29] wateradsorption [30 31] and solubility [32ndash36] A character-ization of the benchmark NaCl particles was carried out tohave a baseline reference to be compared to the NaCl-TiO2and NaCl-SiO2 composites obtained after the coatingprocess

11 Hygroscopicity A hygroscopic solid material can beidentified through its hygroscopic point and water uptakecapacity e hygroscopic point (hgp) represents thethreshold value of the relative humidity in the air abovewhich the solid substance starts adsorbing water vapor [37]It describes the relationship at equilibrium between thewater vapor pressure (Psol) and its surrounding environmentwith respect to the partial pressure of the water vapor in theair (PH2O) at a specific temperature hgp is generated as apercentage given by

hgp Psol

PH2O1113888 1113889 times 100 (1)

If the solid is a water-soluble material such as solublesalts then Psol represents the water vapor pressure in thesolid saturated solution since it already contains a certainamount of moisture at equilibrium prior to saturation Inthis case at the hygroscopic point the solid material starts

experiencing the deliquescence process while adsorbingwater vapor from the atmosphere until it starts dissolvingrapidly at quasi-constant RH generally represented by thequasi-vertical line in the isotherm at high RH is deli-quescence property is a characteristic of certain hygroscopicsolid compounds eg water-soluble salts which experiencea phase transition from a solid particle to a liquid drop ieto form a solution [38 39] Hence the hygroscopic point ofthis type of materials at which the deliquescence processoccurs is also called deliquescence point or deliquescencerelative humidity (DRH) [40ndash42]

2 Materials and Methods

21 Material Synthesis e chemical compounds werepurchased from Sigma-Aldrich with analytical grade reetypes of samples were prepared (i) optimized neat NaClwith submicron-range particles (ii) NaCl coated either withtitanium dioxide (NaCl-TiO2) or (iii) silicon dioxide (NaCl-SiO2) Here the optimization of NaCl consists of achieving aparticle size reduction of commercial NaCl to match theaforementioned size requirements (05 to 10 μm) for seedingmaterials Herein an aqueous solution was prepared usingcommercial salt and then 1ml of this solution was added to50ml of 2-propanol while stirring Once a white precipitateis observed the NaCl crystals were extracted by eithercentrifugation or filtration followed by drying in the oven at80degC For the synthesis of NaCl-TiO2 core-shell compositesimilar to what was reported by our previous work [43] firstTiO2 solution was prepared by hydrolysis of titanium (IV)butoxide solution as the first step Solution A was preparedby dispersing 5ml titanium (IV) butoxide in 40ml of 2-propanol en solution B was prepared by mixing 50ml ofdeionized water and 1ml of nitric acid Subsequently so-lution B was added dropwise into solution A under vigorousstirring until the formation of a semitransparent TiO2 so-lution A pH control below 2 is crucial to ensure the overallNaCl-TiO2 particle size in the submicron range To prepareNaCl-TiO2 composite a solution of 50mg of optimizedNaCl and 10ml of 2-propanol was needed en 1ml of theabove-prepared TiO2 solution was poured dropwise into thisNaCl solution e new NaCl-TiO2 solution was stirredvigorously for 60 minutes to be ready for the separationprocess by centrifugation drying at 80degC overnight andcalcination at 250degC during 3 hours

For the synthesis of NaCl-SiO2 core-shell composite firstSiO2 solution was prepared by the hydrolysis of the tet-raethyl orthosilicate (TEOS) solution An acid solution wasprepared by mixing 16ml of hydrochloric acid with 20ml ofethanol and 20ml of deionized water en 46ml of TEOSsolution was added dropwise into the acidic solution undervigorous stirring pH was kept at 2 while stirring at 250 rpmfor 2 hours To prepare NaCl-SiO2 composite a solution of50mg of optimized NaCl and 2ml of 2-propanol wasneeded en 1ml of the above-prepared SiO2 solution waspoured dropwise into this NaCl solution e new NaCl-SiO2 solution was stirred vigorously for 60 minutes to beready for the separation process by centrifugation drying at80degC overnight and calcination at 550degC during 3 hours

2 Journal of Nanotechnology

22 Characterization Morphology and the size of thesynthesized samples were characterized by scanning electronmicroscopy (SEM) FEItrade Quanta 250 operating at 5 KVand their elemental composition analysis was determinedusing energy dispersive X-ray spectroscopy (EDS) fromEDAXtrade Additional composition analysis was performedusing Fourier-transform infrared spectroscopy (FTIR) usinga Brukertrade Optics VERTEX system recorded at a wave-number range of 4000ndash400 cmminus 1 and Raman spectroscopyfrom WiTectrade with a laser source of 532 nm e thin layersof TiO2 and SiO2 coating NaCl crystals were revealed byenergy-filtered transmission electron microscopy (EFTEM)using GATANtrade and electron energy loss spectroscopy(EELS) energy filter mounted on Titan FEItrade transmissionelectron microscope (TEM) operating at 300KV TEMsamples were prepared by dispersion in isopropanol solventof a powder made of particles and applied to 3mm Cu gridscoated with carbon film

In situ water vapor condensation was performed byenvironmental scanning electron microscopy (ESEM)consisting of SEM operating at variable pressure and agaseous secondary electron detector (GSED) designed forgeneral wet imaging e in situ water condensation setup isshown in Figure 1(a) e condensation experiments wereconducted for each sample separately at 10KV at variablepressure and at constant temperature of 1degC To ensurethermal stability on the surface of the sample cooling at 1degCfor a dwell time of 30 minutes was required [44 45]thereafter vapor pressure inside the chamber was increasedgradually until the dew point was reached ie formation ofwater droplets as per the path shown in the water phasediagram in Figure 1(b) e experimental setup allowedobservation of the water droplet nucleation formation andgrowth e entire water droplet process was then recordedand analyzed frame by frame e evaluation of the surfacewettability and deliquescence of the samples was carried outby the comparison between the diameter of the dried par-ticles under high vacuum and the wet particles duringcondensation along with the examination of the alteredsolid particles after condensation

A Quantachrometrade Instrumentsrsquo VSTAR vapor sorptionvolumetric analyzer was used to record the water adsorp-tion-desorption isotherms water adsorption capacity andkinetics of adsorption e relative pressure (PP0) wasvaried within a range 0ndash1 equivalent to 0ndash99 RH eisosteric heat of adsorption was deduced for each samplefrom the adsorption isotherms obtained at temperatures of25degC 35degC and 45degC TA Instrumentstrade DMA Q800 Dy-namic Mechanical Analysis (DMA) in conjunction with aDMA-RH accessory was used to compare qualitatively themechanical integrity which represents the water adsorptioncapacity and the deliquescence of the materials e sampleswere prepared by compacting the powder samples into acircular disc shape Water vapor at controlled RH and atequilibrium state was introduced gradually while the DMAclamp was maintained at a constant compression load of01N at a room temperature e result is given as dis-placements of the clamp in contact with the sample versusRH e DMA-RH operates from 5 to 90 RH using Peltier

elements allowing a precise temperature and RH control ofplusmn01degC and plusmn3 RH respectively

3 Results and Discussion

31 Morphology and Chemical Composition CommercialNaCl exhibits random particle sizes mostly greater than100 μm (Figure 2(a)) To optimize the particle size and toobtain cubic-shaped NaCl particles a systematic analysis ofthe agglomerated particles was performed As a result aremarkable particle size reduction and individual cubic-shaped NaCl particles arose as an effect of 2-propanol(Figure 2(b)) e optimized NaCl particles obtained bycentrifugation have a size ranging from 1 to 6 μm with anaverage size of 23plusmn 09 μm in length (Figure 2(c)) whereasthe ones obtained by filtration have a significantly reducedsize ranging from 05 and 13 μm with an average size of08plusmn 02 μm in length (Figure 2(d)) e resultant particlesize distribution of the optimized NaCl then appeared to fallwithin the aforementioned optimum size range for hygro-scopic cloud seeding agents as described in the literature[19ndash22] Hence these optimized NaCl particles were usedfor the coating process

After the coating process the morphology and compo-sition of the NaCl-TiO2 and NaCl-SiO2 composites wereanalyzed SEM micrographs showed uniform particles iemostly cubic shape and homogeneous size NaCl-TiO2 particlesizes ranging from 03 to 11μm with an average size of0566plusmn 0152μm in length are the smallest particles achieved(Figure 3(a)) Similarly an optimum size for NaCl-SiO2 wasobtained with a particle size range between 09ndash69μm and anaverage of 28plusmn 12μm in length (Figure 3(b) Furthermoreboth composites NaCl-TiO2 and NaCl-SiO2 showed thatduring the coating process the cubic-shaped particles were notdestroyed and they fulfilled the criterion related to the particlesize distribution for seeding agents

EDS elemental composition analysis for the studiedsamples is represented in Figure 4 A reference EDS spec-trum obtained from the optimized NaCl showed Na and Clpeaks (Figure 4(a)) which indicates no contaminationduring the optimization process NaCl-TiO2 compositeshowed EDS spectra of high peaks of Na and Cl and verysmall peaks of Ti and O2 (Figure 4(b)) Likewise NaCl-SiO2composite showed EDS spectra of high peaks of Na and Cland small peaks of Si and O2 (Figure 4(c)) In both cases theEDS composition quantification results indicate that theapproximate weight percentage of the oxides in bothcomposites is below 5

FTIR analysis revealed the composition of both com-posites NaCl-TiO2 and NaCl-SiO2 (Figure 5) Sole TiO2showed FTIR spectra of high absorbance band in the region500ndash1000 cmminus 1 which is certainly due to the bonds betweenthe titanium and the oxygen Ti-O is is confirmed by theFTIR spectra of the NaCl-TiO2 composite where the bondTi-O is present However due to the amount of TiO2 coatingin the composite sample (le5) the absorbance band ob-served is significantly low as expected (Figure 5(a)) Simi-larly the FTIR spectra of the NaCl-SiO2 composite showedthe presence of three distinctive peaks of low intensity at

Journal of Nanotechnology 3

Electron gunElectron beamMagnetic condenser lenses

X + Y scanning coils

Stigmator

GSED

Condensed droplets

Peltier cooling stageSample holder

(a)

SolidLiquid

Vapor

Melting Vaporization

CondensationFreezing

Con

dens

atio

n

P (Pa)

700

500

10 T (degC)

(b)

Figure 1 (a) Experimental setup for in situ water condensation (ESEM) [42] (b) water condensation was achieved at a temperature of 1degCand increasing the pressure above 500 Pa

200 microm

(a)

10 microm

(b)

10 microm

(c)

10 microm

(d)

Figure 2 SEM micrographs showing different obtained sizes of NaCl Sequence 1 (a) commercial NaCl (b) NaCl after using 2-propanoland (c) NaCl obtained by centrifugation Sequence 2 (a) and (b) followed by (d) NaCl obtained after filtration


1050 cmminus 1 790 cmminus 1 and 430 cmminus 1 compared to the highabsorbance peaks observed for the sole SiO2 at the samewavenumber (Figure 5(b))

Additionally Raman spectroscopy was also used toconfirm the characteristic structural fingerprint of thesamples NaCl NaCl-TiO2 and TiO2 (Figure 6) As expecteddue to the symmetry of NaCl crystals no Raman signal wasdetected for NaCl however TiO2 revealed Raman spectrawith several peaks in 0ndash1000 cmminus 1 range and a prominentpeak at 152 cmminus 1 is is a characteristic peak of the anatasephase of TiO2 [46 47] A zoomed-in (inset) figure shows thatNaCl-TiO2 possesses a peak at a Raman shift of 152 cmminus 1approximately which confirms the presence of TiO2 in thecomposite material

Undoubtedly the oxides TiO2 and SiO2 are present inthe NaCl-TiO2 and NaCl-SiO2 composites respectivelyHence in order to verify the quality of TiO2 and SiO2coatings in terms of dispersion and homogeneity EFTEMand EELS observations were conducted As a result anevenly distributed thin layer of TiO2 on the surface of theNaCl-TiO2 composite was observed e TiO2 coatingthickness is approximately 8 nm which varies slightly fromone particle to another (Figure 7)

Likewise an evenly distributed thin layer of SiO2 was ob-served on the surface of the NaCl-SiO2 composite (Figure 8)e thickness of the coating of SiO2 surrounding the NaClparticle seems to vary approximately between 8nm and 12nm

For both EFTEM-EELS images the interaction of theelectron beam and the sample creates a shadow effect ob-served in both images where the intensity of the thin layereither Ti or Si seems to be concentrated in some edges of theparticle

32Water SorptionAnalysis A solid substance is consideredas an efficient hygroscopic material when it shows high wateradsorption capacity and high rate of adsorption is isgenerally the case for porous material possessing highspecific surface area or micropore volume as well as a largepore network [48] ese characteristics can be estimated

through the isotherms obtained during the sorption analysisBesides the hygroscopic materials being considered as cloudseeding agents are required to exhibit a high-water ad-sorption capacity especially at low RH and low temperatureHence both water adsorption capacity and kinetics of ad-sorption along with deliquescence are essential parametersto determine the best seeding agent

In this sense the hygroscopic properties of the suc-cessfully synthesized NaCl-TiO2 and NaCl-SiO2 compositescompared to neat NaCl were studied to determine theirperformances under specific conditions Adsorption iso-therms of these materials were obtained at three differenttemperatures 25degC 35degC and 45degC (Figure 9) e ad-sorption isotherms exhibited particular performances for allsamples in two different stages at low relative pressure (stage1) from 0 to 06 of relative pressure (ie 0 to 60 RH) andat high relative pressure (stage 2) from 06 to 1 of relativepressure (ie 60 to 99 RH)

At stage 2 multilayer adsorption mechanism andsubsequently full deliquescence of the solid adsorbents arereflected on the very steep lines against a very slight in-crease in relative pressure e optimized NaCl has com-parable water uptakes at all temperatures and a hygroscopicpoint at approximately 75 RH which are in accordancewith the Extended Aerosol Inorganics Model IV (E-AIM)[49ndash51] and previous studies [52ndash56] e water uptake ofneat NaCl above the hygroscopic point at 25degC (3 datapoints) was correlated to E-AIM as it showed an experi-mental deviation (Figure 9(a)) Several studies have ex-amined that anatase TiO2 [27] and amorphous SiO2[57ndash59] exhibit a moderate water uptake as the relativepressure increases us the steep isotherms of the NaCl-TiO2 and NaCl-SiO2 composites at high relative pressureimply that they were boosted by the deliquescence of NaCl(Figures 9(b) and 9(c))

At stage 1 new graphs were obtained from the full rangeof relative pressure ese graphs were plotted separately inorder to comprehend the adsorption process behavior(Figures 9(d)ndash9(f )) Here the temperature seems to be anactive kinetics parameter influencing the water uptake at low

5 microm

(a)

5 microm

(b)

Figure 3 SEM micrographs of (a) NaCl-TiO2 and (b) NaCl-SiO2


relatively pressure even though this stage is related to amonolayer adsorption mechanism Nevertheless a similartrend among the samples was observed ie the higher thetemperature the higher the water uptake across the ad-sorption isotherms As expected a slight change in wateruptake for NaCl was observed as the relative pressure in-creases [60ndash62]us a nonsignificant influence of NaCl canbe inferred on the NaCl-TiO2 and NaCl-SiO2 composites atlow relative pressure

e best hygroscopic materials for cloud seeding ap-plications are those capable of adsorbing high amounts ofmoisture especially at very low relative pressure ie low RHis is in consideration of the variable pressure profile in theatmosphere and low relative RH especially in low cloudcover conditions [63 64] Accordingly special attention wasgiven to the analysis of the isotherms at stage 1 (low relativepressure) to evaluate the performances of the sampleserefore a comparison of the adsorption isotherms of the

ClNa

ClCl

306K

272K

232K

240K

170K

136K

102K

068K

034K

000K000 100 200 300 400 500 600 700 800 900

Energy (eV)

Inte

nsity

AU

(a)

Presence of titaniumand oxygen

Ti TiClO

TiCl

ClNa

000 067 134 201 268 335 402 469 536 603

729

648

567

486

405

324

243

162

81

0

(b)

Presence of siliconand oxygen

Cl

Na

Cl

SiSiCl

O000 13 26 39 52 65 78 91 104 117 130

810K

720K

630K

540K

450K

360K

270K

180K

090K

000K

(c)

Figure 4 EDS spectra of (a) reduced size NaCl (b) NaCl-TiO2 and (c) NaCl-SiO2


samples were plotted at stage 1 at temperatures of 25degC 35degCand 45degC (Figure 10)

According to the IUPAC classification for the adsorptionisotherms for gas-solid equilibria [61] at low relativepressure NaCl-TiO2 composite followed a sigmoid-shapedisotherm curve thus reflecting a type IV isotherm Mono-layer-multilayer adsorption attributed to high energy ofadsorption is the characteristic of this isotherm [65 66]Undoubtedly NaCl-TiO2 exhibited the highest water uptakedue to the contribution of TiO2 e surface energy of thisoxide facilitates the interaction between the water vapormolecules and the surface of the NaCl-TiO2 Similarly NaCl-SiO2 and NaCl followed the type IV isotherm but with lowerwater uptake which indicates less interaction of watermolecules with these samples attributed to a minor surface

energy of SiO2 and NaCl respectively Overall and for alltemperatures NaCl-TiO2 shows the highest performance incomparison with NaCl and NaCl-SiO2 At 25degC NaCl-TiO2showed an increase of water uptake up to 360 with respectto NaCl or NaCl-SiO2 At 35degC and 45degC the increase ofwater uptake reached more than 1000 Moreover by in-creasing the temperature the water uptake for NaCl-TiO2increased from 347mg gminus 1 at 25degC to 1401mg gminus 1at 45degCese results indicate that TiO2 significantly enhances thehygroscopicity of the NaCl-TiO2 composite promoting it aspotential candidate for cloud seeding

At stage 2 (high relative pressure) the effect of thecoating on hygroscopicity was not conclusive as it gavemixed results (Figure 11)

It is important to point out that the highest relativepressure at which the maximum water uptake was measuredis not accurately 1 (100 RH) for all the samplese highestrelative pressures oscillate in the range of (09768ndash09832)As a result the difference between water uptake at bothextreme values of relative pressure may be significant Forexample as per the E-AIM model NaCl at 25degC (29815K)has a water uptake of 24349mg gminus 1 at 9768 RH and33555mg gminus 1 at 9832 RH erefore for close values ofwater uptake among the samples the difference may benegligible At 25degC and 35degC NaCl-SiO2 and NaCl exhibitedhigher water uptake than NaCl-TiO2 At 45degC an interestingphenomenon occurs At relative pressure between 075 and09 all isotherms overlap however above 09 relativepressure NaCl-TiO2 registered the highest water uptakewhich was above 45000mg gminus 1 with a clear tendency ofincreased water uptake with temperature ese resultssuggest that NaCl-TiO2 may also be attractive for applica-tions requiring high temperature and high relative pressureMoreover although the water uptake of NaCl-SiO2 hasdecreased at high temperature it still reached 30000mgmiddot gminus 1this indicates that NaCl-SiO2 could be a more suitablehygroscopic material at moderate temperature and highrelative pressure as it reaches its best performance at roomtemperature

0 1000 2000 3000 4000

Abso

rban

ce (a

u)

Wavenumber (cmndash1)

NaCl-TiO2TiO2NaCl

(a)

NaCl-SiO2SiO2NaCl

0 1000 2000 3000 4000

Abso

rban

ce (a

u)


(b)

Figure 5 FTIR spectra of (a) NaCl-TiO2 and (b) NaCl-SiO2

(a)

(b)

NaCl-TiO2TiO2NaCl

500 1000 1500 2000 2500 30000Raman Shi (cmndash1)

800

1000

1200

1400

Ram

an in

tens

ity (a

u)

0 100 200 300 400 500750

800

850

900

Ram

an in

tens

ity (a

u)

Raman shi (cmndash1)

Figure 6 Raman spectra of NaCl TiO2 and NaCl-TiO2 e inert(0 to 500 cmminus 1) highlights the peak at 152 cmminus 1 approximatelywhich is the characteristic of TiO2


According to the hygroscopicity classification de-scribed by Callahan et al [67] using the concept ofequilibriummoisture content (EMC) introduced earlier byScot et al [68] the isotherms showed that NaCl-TiO2 is avery good hygroscopic material as the water uptake ieequilibrium moisture content or EMC occurs from verylow relative humidity even below 40 RH as mentionedin the classification Although NaCl and NaCl-SiO2showed lower water uptake at relative humidity below 70RH they are classified as very hygroscopic materials as wellbecause the increase in the EMC is higher than 5 oc-curring below 60 RH Another recent classification de-scribed by Murikipudi et al [69] using a different methodbased on the sorption analysis lead to the same conclusionsabout these materials classifying them as very good hy-groscopic materials

Additionally a per the adsorption isotherms at very lowrelative pressure the specific surface area for each samplewas determined (Table 1) NaCl-TiO2 exhibited the highestspecific surface area which is coherent with its adsorptioncapacity compared to the other samples NaCl and NaCl-SiO2 exhibited similar specific surface areas which is alsoconsistent with their respective adsorption capacity

To study the kinetics of water adsorption the wateruptake as a function of time was recorded for each RH(Figure 12) Each point of the plots represents the time that asample took to reach a maximum water uptake ie wateradsorption at equilibrium at a fixed relative pressure

e early stages of the water adsorption process are of amajor interest because they allow identifying not only theability of the material to adsorb high quantity of watermolecules but also indicate at which rate the adsorption hasoccurred At all temperatures NaCl-TiO2 registered fasterrate of adsorption and higher water uptake especially for thefirst 60 minutes which is critical for cloud seeding appli-cation It is certain that once the clouds are seeded rain isexpected to fall within the first hour [70] is interestingcharacteristic exhibited by NaCl-TiO2 is attributed to thehigh energy interaction between the solid adsorbent andwater vapor in the presence of TiO2 For NaCl and NaCl-SiO2 the kinetics of water adsorption is quite similar andthe adsorption rate is slower in comparison with NaCl-TiO2Moreover the kinetics of adsorption indicates that it de-pends on temperature In fact the water uptake recorded foran equivalent time at equilibrium is higher at 45degC comparedto the respective ones at 35degC and 25degC

50 nm

(a)

Chlorine L mapTitanium L map

50 nm

(b)

10 nm

(c)

10 nm


(d)

Figure 7 EFTEM-EELS high-resolution images of NaCl-TiO2 (a b) one particle of NaCl-TiO2 (c d) inset shows the detail of the thicknessof TiO2 surrounding the NaCl particle


In addition using the adsorption isotherms at varioustemperatures the isosteric heat of adsorption can be de-termined e isosteric heat of adsorption also called en-thalpy of adsorption (ΔHads) indicates the strength of theinteraction (bond) between the adsorbate (water molecules)and the solid adsorbent [71 72] is parameter representsthe energy necessary for the heat of vaporization of moisturein a material during the adsorption process to exceed thelatent heat of vaporization of pure water at a certain tem-perature [73] It is derived from the ClausiusndashClapeyronequation [48]

δ lnP

δ(1T)

(ΔH)

R (2)

where P is the absolute pressure T is the temperature ∆H isthe enthalpy of adsorption and R is the ideal gas constant

e values of ΔHads were deduced from the data ofpressure and temperature in equilibrium conditions rep-resented by the three isotherms e results are given as aslope ΔHadsR of the plot ln (P) versus (T minus 1) (Figure 13)ΔHads extracted from the plots of Figure 13 is shown in

Figure 14 A particular variation of the NaCl-TiO2 composite

was observed At the beginning of water adsorption ΔHads ishigh around 452 kJmiddotmolminus 1 and then it drops to414 kJmiddotmolminus 1 approximately

is indicates that the high activity of polar sites on thesurface of the solid adsorbent (ie high energy interaction)are covered with water molecules to form a monomolecularlayer [74 75] which is consistent with the previous results ofthe adsorption kinetics obtained for the first 60 minutes andthe significant water uptake at stage 1 (low relative pressure)Moreover the low values of ΔHads are due to the progressiveand slower filling of the less available sites known to havelower bonding activation energies [76 77] Additionallythose initial values of ΔHads are higher than the latent heat ofvaporization of pure water (431 kJmiddotmolminus 1 at 45degC) At theseconditions the energy of binding between the condensedwater molecules and the adsorption sites of the solid ad-sorbent is higher than the energy of binding between themolecules of pure water that hold them together in the liquidphase which is favorable for the adsorption process to occur[78 79]

In the contrast NaCl-SiO2 showed high ΔHads relativelyconstant throughout all the water adsorption stagesis can

50 nm

(a)

50 nm

Sodium L mapChlorine L mapSilicon L map

(b)

10 nm

(c)

10 nm


(d)

Figure 8 EFTEM-EELS high resolution images of NaCl-SiO2 (a b) one particle of NaCl-SiO2 (c d) inset shows the detail of the thicknessof SiO2 surrounding the NaCl particle


NaCl25degC (experimental)25degC (E-AIM model)25degC (deviation)35degC45degC

0

10

20

30

40W

ater

upt

ake (

103 m

g gndash1

)

06 0800 02 1004PP0

(a)

PP0

NaCl-TiO225degC35degC45degC

00 02 04 06 08 10

Wat

er u

ptak

e (10

3 mg

gndash1)

0

10

20

30

40

(b)

PP0

25degC35degC45degC

NaCl-SiO2

Wat

er u

ptak

e (10

3 mg

gndash1)

0

10

20

30

40

02 04 06 08 1000

(c)

Wat

er u

ptak

e (m

g gndash1

)

NaCl

2

0

4

6

8

10

00 01 02 03 04 05 06PP0

25degC35degC45degC

(d)

Wat

er u

ptak

e (m

g gndash1

)

NaCl-TiO2

120

100

80

60

40

20

000 01 02 03 04 05 06

PP0

25degC35degC45degC

(e)

Wat

er u

ptak

e (m

g gndash1

)NaCl-SiO2

0

2

4

6

8

10

00 01 02 03 04 05 06PP0

25degC35degC45degC

(f )

Figure 9Water sorption analysis with isotherms at 25degC 35degC and 45degC (a) and (d) for NaCl (b) and (e) for NaCl-TiO2 and (c) and (f) forNaCl-SiO2 at relative pressure (0 to 1) top and an inset for low values of relative pressure (0 to 06) bottom e isotherm of NaCl at 25degCshows a deviation obtained experimentally after the hygroscopic point and the correction as per the E-AIM model

000

30

60

90

120

25degC

Wat

er u

ptak

e (m

g gndash1

)

NaCl-TiO2NaCl-SiO2NaCl

01 02 03 04 05 06PP0

(a)

0

30

60

90

120

Wat

er u

ptak

e (m

g gndash1

)

35degCNaCl-TiO2NaCl-SiO2NaCl

00 01 02 03 04 05 06PP0

(b)

0

30

60

90

120

Wat

er u

ptak

e (m

g gndash1

)


00 01 02 03 04 05 06PP0

(c)

Figure 10 Comparison of isotherms ofNaCl NaCl-TiO2 andNaCl-SiO2 at low relative pressure from0 to 07 at (a) 25degC (b) 35degC and (c) 45degC


be considered as a valuable behavior in terms of adsorptionprocess However NaCl has a slightly mixed trend of theisosteric heat of adsorption at low water uptake ΔHads isslightly above the latent heat of vaporization of pure waterbut for high water uptake up to 6000mgmiddotgminus 1 approximatelyΔHads decreases to lower values and then again ΔHadssuddenly increases to 42 kJmiddotmolminus 1 during the last wateruptake range In addition ΔHads for NaCl remains lower in

all cases than the ones measured or NaCl-SiO2is suggeststhat NaCl-SiO2 can be considered as a better adsorbentcompared to NaCl

33 Wettability Analysis e water affinity of the sampleswas analyzed through in situ water condensation usingESEM (Figure 15)

06 07 08 09 100

10

20

30

40

Wat

er u

ptak

e (10

3 mg

gndash1)

PP0

25degCNaCl-TiO2NaCl-SiO2NaCl (Exp amp E-AIM)

(a)


Wat

er u

ptak

e (10

3 mg

gndash1)

PP0

06 07 08 09 100

10

20

30

40

(b)

45 degCNaCl-TiO2NaCl-SiO2NaCl

Wat

er u

ptak

e (10

3 mg

gndash1)

PP0

06 07 08 09 100

10

20

30

40

(c)

Figure 11 Comparison of isotherms of NaCl NaCl-TiO2 and NaCl-SiO2 at high relative pressure from 07 to 1 at (a) 25degC (b) 35degC and(c) 45degCe isotherm of NaCl at 25degC includes the experimental data and the correction as per the E-AIMmodel after the hygroscopic pointas shown in Figure 9(a)

Table 1 Measured specific surface area of NaCl NaCl-TiO2 and NaCl-SiO2 at 25degC 35degC and 45degC

MaterialSpecific surface areaa (m2middotgminus 1)

25degC 35degC 45degCNaCl 992 1205 1532NaCl-TiO2 4914 20186 17530NaCl-SiO2 983 1367 1520aAdsorbate water vapor PP0 005ndash035

0 20 40 60 800

10

20

30

40

Wat

er u

ptak

e (m

g gndash1

)

Time (min)

25degCNaCl-TiO2

NaCl-SiO2

NaCl

(a)

35degCNaCl-TiO2

NaCl-SiO2

NaCl

Wat

er u

ptak

e (m

g gndash1

)

0 20 40 60 800

10

20

30

40

Time (min)

(b)

0 20 40 60 800

10

20

30

40

Time (min)

Wat

er u

ptak

e (m

g gndash1

)

45degCNaCl-TiO2

NaCl-SiO2

NaCl

(c)

Figure 12 Kinetics of adsorption for NaCl NaCl-TiO2 and NaCl-SiO2 at (a) 25degC (b) 35degC and (c) 45degC


Since the surface wettability of these particles is themajor interest to determine the extent of hydrophilicity anddeliquescence individual particles were analyzed epressure was increased gradually until the nucleation of thefirst water droplets was observed At equilibrium the waterdroplets stopped growing while the pressure was still in-creasing ie the water droplets reached their maximumdiameter which represents the maximum water adsorptioncapacity e diameter of the droplets was then measuredand compared for the three types of samples e final stepconsisted of reducing the pressure gradually until driedsamples were observed ie no more physical changes in theremaining particles is last step clarified the under-standing of the effect of the deliquescence process BothNaCl-TiO2 andNaCl-SiO2 composites have developed largerwater droplets compared to neat NaCl DD0 which definesthe ratio between the maximum diameter reached by thewater droplet at equilibrium to the length of the solid

particle was determined to be asymp344 asymp376 and asymp226 forNaCl-TiO2 NaCl-SiO2 and NaCl respectively is indi-cates that both composite samples are more hydrophilic innature thanNaCl It is noteworthy to emphasizing that the insitu condensation experiments at low temperature andpressure emulate the water vapor condensation process inthe clouds with a close approximation specifically in warmclouds which are the main target for hygroscopic cloudseeding Furthermore once the condensation is achievedthe moisture was gradually removed until the particles aredried e results showed that the size of neat NaClremained relatively unchanged after the condensation ex-periments which suggests that both slow adsorption and alimited deliquescence process have occurred during thecourse of the experiment for neat NaCl In contrast after thecondensation experiments both composites NaCl-TiO2 andNaCl-SiO2 turned to a dispersed fine powder or they wereentirely disintegrated which indicates a good deliquescence

310 315 320 325 330 335283032343638404244

ln P

T ndash1 (103 Kndash1)

NaCl

(a)

310 315 320 325 330 335T ndash1 (103 Kndash1)

283032343638404244

ln P

NaCl-TiO2

(b)

310 315 320 325 330 335T ndash1 (103 Kndash1)

283032343638404244

ln P

NaCl-SiO2

(c)

Figure 13 Variation of ln (P) vs (Tminus 1) of (a) NaCl (b) NaCl-TiO2 and (c) NaCl-SiO2

0 4000 8000 12000 16000 20000

34

36

38

40

42

44

46

Isos

teric

hea

t of a

dsor

ptio

n (k

Jmol

)

Water uptake (mgg)

NaCl-TiO2

NaCl-SiO2

NaCl

(a)

1000 2000 3000 4000 5000

34

36

38

40

42

44

46

Isos

teric

hea

t of a

dsor

ptio

n (k

Jmol

)

Water uptake (mgg)

NaCl-TiO2

NaCl-SiO2

NaCl

(b)

Figure 14 Isosteric heat of adsorption (ΔHads) of NaCl NaCl-TiO2 and NaCl-SiO2 (a) full scale (b) the inset shows the range500ndash5000mgmiddotgminus 1


erefore these results allowed to infer that both compositesadsorb more water and deliquesce more effectively than neatNaCl

34 Humidity-Controlled Dynamic Mechanical Analysis(DMA-RH) DMA-RH results are illustrated in Figure 16Two stages can be identified in stage 1 from 0 to 50ndash70RH the materials showed a distinctive water uptake relatedto an initial swelling and a subsequent softening of the areain contact with the DMA clamp us a negative dis-placement due to the penetration of the clamp inside thedisc-shaped samples is recorded while the moisture is in-creased in the chamber In stage 2 from 50ndash70 RH to 90RH approximately the materials experienced a steepernegative displacement since the clamp load has overcome

the resilience of the materials which became softer due to thehigh amount of water adsorbed At this stage samples aredeformed or dispersed is behavior can be linked to thedeliquescence process of the solid particles in water as seenin ESEM experiments and the isotherms

NaCl presents no measurable water uptake at low relativehumidity (stage 1) is is interpreted by a minor displace-ment as observed on the DMA-RH plot At high RH (stage 2)a large displacement is recorded and the deliquescenceseemed to occur at 50ndash65 RH where a sudden drop of thecurve was observed In contrast the NaCl-TiO2 compositestarted adsorbing water at the very beginning of RH (stage 1)and a significant displacement is recorded which stabilizedand formed a plateau up to 60 RH and then a rapid dropwas observed in the range of 60ndash75 RH at which the NaCl-TiO2 composite deliquesces (stage 2) is indicates that

Aercondensation

Beforecondensation

Duringcondensation

NaCl NaCl

D0

NaCl

D

5 microm 5 microm 5 microm

(a)

Aercondensation

Beforecondensation

Duringcondensation

NaClTiO2NaClTiO2

D0

NaClTiO2

D


(b)

Aercondensation

Beforecondensation

Duringcondensation

NaClSiO2


NaClSiO2

D0

NaClSiO2

D

(c)

Figure 15 In situ condensation experiments inside ESEM at three key stages before condensation (left dry samples) during con-densation (center samples at equilibrium) and after condensation (right dried sample after water is removed) performed respectivelyon (a) NaCl (b) NaCl-TiO2 and (c) NaCl-SiO2


NaCl-TiO2 retained higher amount of moisture compared toNaCl Moreover NaCl-SiO2 exhibited 3 stepwise water up-takes ranging from 0 to 20 RH as step 1 ranging from 20 to60 RH as step 2 and starting above 60 RH as step 3 whichsuggests a multilayer adsorption is indicates that NaCl-SiO2 is behaving like NaCl-TiO2 in terms of water uptake butat lower kinetics as NaCl-SiO2 needs two stages of adsorptionat low relative pressure to achieve similar water uptake thanNaCl-TiO2 Both water uptake curves showed that NaCl-SiO2deliquesces approximately at the same relative humidity asNaCl-TiO2 (60 to 75 RH) erefore the two compositesNaCl-TiO2 and NaCl-SiO2 have demonstrated better wateradsorption and deliquescence compared to NaCl but at higherrate for NaCl-TiO2

4 Conclusions

e successful optimization in terms of size and morphologyof the commercial NaCl as per cloud seeding requirementsallowed the development of the NaCl-TiO2 and NaCl-SiO2composites e effective coating process of NaCl preservedthe cubic-shaped particles and size (05 to 69 μm) as percloud seeding agent requirements

Water adsorption analysis revealed that the NaCl-TiO2composite adsorbed high amount of water vapor at a fasterrate in the cloud seeding conditions (low relative pressure)compared to NaCl and NaCl-SiO2 In addition NaCl-TiO2exhibited very promising adsorption properties for high-temperature applications as high as 45degC Moreover NaCl-SiO2 was found to be potentially suitable hygroscopic ma-terial for high relative pressure applications while reachingits highest performance at room temperature

In situ water condensation inside the ESEM and theDMA-RH tests confirmed that NaCl-TiO2 composite pos-sesses the best performances in terms of water adsorptioncapacity and deliquescence compared to NaCl LikewiseNaCl-SiO2 showed a comparable behavior as NaCl-TiO2 butat lower kinetics

erefore both composites NaCl-TiO2 and NaCl-SiO2could be considered as promising hygroscopic materials andas potential candidates to replace the existing salt seedingagents for rain enhancement

Data Availability

e (SEM and TEM images the movies of the in situ ESEMand the graphs) data used to support the findings of thisstudy are available from the corresponding author uponrequest

Conflicts of Interest

e authors declare that there are no conflicts of interestregarding the publication of this paper

Acknowledgments

e financial support was provided by the National Centerof Meteorology and Seismology Abu Dhabi UAE under theUAE Research Program for Rain Enhancement

Supplementary Materials

Water vapor adsorption kinetics of coated and uncoatedNaCl inset is an illustration of the adsorption amongsamples (Supplementary Materials)

References

[1] J U Keller and R Staudt Gas Adsorption Equilibria Ex-perimental Methods and Adsorptive Isotherms Springer Sci-ence amp Business Media Berlin Germany 2005

[2] M N Golubovic H D M Hettiarachchi and W M WorekldquoSorption properties for different types of molecular sieve andtheir influence on optimum dehumidification performance ofdesiccant wheelsrdquo International Journal of Heat and MassTransfer vol 49 no 17-18 pp 2802ndash2809 2006

[3] X Zheng T S Ge R Z Wang and L M Hu ldquoPerformancestudy of composite silica gels with different pore sizes anddifferent impregnating hygroscopic saltsrdquo Chemical Engi-neering Science vol 120 pp 1ndash9 2014

[4] S Yildirim B Rocker M K Pettersen et al ldquoActive pack-aging applications for foodrdquo Comprehensive Reviews in FoodScience and Food Safety vol 17 no 1 pp 165ndash199 2018

[5] R T Bruintjes ldquoA review of cloud seeding experiments to en-hance precipitation and some new prospectsrdquo Bulletin of theAmericanMeteorological Society vol 80 no 5 pp 805ndash820 1999

[6] G K Mather D E Terblanche F E Steffens and L FletcherldquoResults of the South African cloud-seeding experimentsusing hygroscopic flaresrdquo Journal of Applied Meteorologyvol 36 no 11 pp 1433ndash1447 1997

[7] Y Tu R Wang Y Zhang and J Wang ldquoProgress and ex-pectation of atmospheric water harvestingrdquo Joule vol 2 no 8pp 1452ndash1475 2018

[8] K R Spurny ldquoAtmospheric condensation nuclei P J Coulier1875 and J Aitken 1880 (historical review)rdquo Aerosol Scienceand Technology vol 32 no 3 pp 243ndash248 2000

[9] D M Chate P S P Rao M S Naik G A Momin P D Safaiand K Ali ldquoScavenging of aerosols and their chemical speciesby rainrdquo Atmospheric Environment vol 37 no 18pp 2477ndash2484 2003

0 10 20 30 40 50 60 70 80 90ndash10

ndash8

ndash6

ndash4

ndash2

0D

ispla

cem

ent (μm

)

Relative humidity ( RH)

NaCl-TiO2

NaCl-SiO2

NaCl

Figure 16 Humidity-controlled dynamic mechanical analysis ofNaCl NaCl-TiO2 and NaCl-SiO2


[10] M Curic and D Janc ldquoWet deposition of the seeding agentafter weather modification activitiesrdquo Environmental Scienceand Pollution Research vol 20 pp 6344ndash6350 2013

[11] R R Czys and R Bruintjes ldquoA review of hygroscopic seedingexperiments to enhance rainfallrdquo =e Journal of WeatherModification vol 26 pp 41ndash52 1994

[12] B A Silverman ldquoA critical assessment of hygroscopic seedingof convective clouds for rainfall enhancementrdquo Bulletin of theAmericanMeteorological Society vol 84 no 9 pp 1219ndash12302003

[13] R T Bruintjes V Salazar T A Semeniuk P BuseckD W Breed and J Gunkelman ldquoEvaluation of hygroscopiccloud seeding flaresrdquo =e Journal of Weather Modificationvol 44 pp 69ndash94 2012

[14] H Furukawa F Gandara Y-B Zhang et al ldquoWater ad-sorption in porous metal-organic frameworks and relatedmaterialsrdquo Journal of the American Chemical Society vol 136no 11 pp 4369ndash4381 2014

[15] H Kim S Yang S R Rao et al ldquoWater harvesting from airwith metal-organic frameworks powered by natural sunlightrdquoScience vol 356 no 6336 pp 430ndash434 2017

[16] H X Zhu X D Li and R J Yang ldquoA novel catalyst of warm-cloud seeding to enhance precipitationrdquo IOP ConferenceSeries Materials Science and Engineering vol 137 Article ID012002 2016

[17] J G Ji R Z Wang and L X Li ldquoNew composite adsorbentfor solar-driven fresh water production from the atmo-sphererdquo Desalination vol 212 no 1-3 pp 176ndash182 2007

[18] D Kumar K Schumacher C du Fresne von HohenescheM Grun and K K Unger ldquoMCM-41 MCM-48 and relatedmesoporous adsorbents their synthesis and characterisationrdquoColloids and Surfaces A Physicochemical and EngineeringAspects vol 187-188 pp 109ndash116 2001

[19] Y Segal A Khain M Pinsky and D Rosenfeld ldquoEffects ofhygroscopic seeding on raindrop formation as seen fromsimulations using a 2000-bin spectral cloud parcel modelrdquoAtmospheric Research vol 71 no 1-2 pp 3ndash34 2004

[20] D Caro W Wobrock and A I Flossmann ldquoA numericalstudy on the impact of hygroscopic seeding on the devel-opment of cloud particle spectrardquo Journal of Applied Mete-orology vol 41 no 3 pp 333ndash350 2002

[21] W A Cooper R T Bruintjes and G K Mather ldquoCalcula-tions pertaining to hygroscopic seeding with flaresrdquo Journal ofApplied Meteorology vol 36 no 11 pp 1449ndash1469 1997

[22] N Kuba and M Murakami ldquoEffect of hygroscopic seeding onwarm rain clouds - numerical study using a hybrid cloudmicrophysical modelrdquo Atmospheric Chemistry and Physicsvol 10 no 7 pp 3335ndash3351 2010

[23] D Rosenfeld and G Gutman ldquoRetrieving microphysicalproperties near the tops of potential rain clouds by multi-spectral analysis of AVHRR datardquo Atmospheric Researchvol 34 no 1ndash4 pp 259ndash283 1994

[24] C G Keyes Jr G W Bomar T P DeFelice D A Griffithand DW LangerudGuidelines for Cloud Seeding to AugmentPrecipitation American Society of Civil Engineers RestonVA USA 2016

[25] D K Farmer C D Cappa and S M Kreidenweis ldquoAt-mospheric processes and their controlling influence on cloudcondensation nuclei activityrdquo Chemical Reviews vol 115no 10 pp 4199ndash4217 2015

[26] J Rouquerol F Rouquerol P Llewellyn G Maurin andK S Sing Adsorption by Powders and Porous Solids Prin-ciples Methodology and Applications Academic PressCambridge MA USA 2nd edition 2013

[27] V Bolis C Busco M Ciarletta et al ldquoHydrophilichydro-phobic features of TiO2 nanoparticles as a function of crystalphase surface area and coating in relation to their potentialtoxicity in peripheral nervous systemrdquo Journal of Colloid andInterface Science vol 369 no 1 pp 28ndash39 2012

[28] H Cheng and A Selloni ldquoHydroxide ions at the wateranataseTiO2(101) interface structure and electronic states from firstprinciples molecular dynamicsrdquo Langmuir vol 26 no 13pp 11518ndash11525 2010

[29] U Aschauer Y He H Cheng S-C Li U Diebold andA Selloni ldquoInfluence of subsurface defects on the surfacereactivity of TiO2 water on anatase (101)rdquo =e Journal ofPhysical Chemistry C vol 114 no 2 pp 1278ndash1284 2010

[30] J Goniakowski and M J Gillan ldquoe adsorption of H2O onTiO2 and SnO2(110) studied by first-principles calculationsrdquoSurface Science vol 350 no 1-3 pp 145ndash158 1996

[31] Z Knez and Z Novak ldquoAdsorption of water vapor on silicaalumina and their mixed oxide aerogelsrdquo Journal of Chemicalamp Engineering Data vol 46 no 4 pp 858ndash860 2001

[32] G B Alexader ldquoe effect of particle size on the solubility ofamorphous silica in waterrdquo=e Journal of Physical Chemistryvol 61 no 11 pp 1563-1564 1957

[33] G B Alexander W M Heston and R K Iler ldquoe solubilityof amorphous silica in waterrdquo =e Journal of PhysicalChemistry vol 58 no 6 pp 453ndash455 1954

[34] V Lenher andH BMerrill ldquoe solubility of silicardquo Journal of theAmerican Chemical Society vol 39 no 12 pp 2630ndash2638 1917

[35] G Okamoto T Okura and K Goto ldquoProperties of silica inwaterrdquo Geochimica et Cosmochimica Acta vol 12 no 1-2pp 123ndash132 1957

[36] J D Rimstidt and H L Barnes ldquoe kinetics of silica-waterreactionsrdquo Geochimica et Cosmochimica Acta vol 44 no 11pp 1683ndash1699 1980

[37] A G Tereshchenko ldquoDeliquescence hygroscopicity of water-soluble crystalline solidsrdquo Journal of Pharmaceutical Sciencesvol 104 no 11 pp 3639ndash3652 2015

[38] K Hameri M Vakeva H-C Hansson and A LaaksonenldquoHygroscopic growth of ultrafine ammonium sulphateaerosol measured using an ultrafine tandem differentialmobility analyzerrdquo Journal of Geophysical Research Atmo-spheres vol 105 no D17 pp 22231ndash22242 2000

[39] D J Cziczo J B Nowak J H Hu and J P D AbbattldquoInfrared spectroscopy of model tropospheric aerosols as afunction of relative humidity observation of deliquescenceand crystallizationrdquo Journal of Geophysical Research Atmo-spheres vol 102 no D15 pp 18843ndash18850 1997

[40] C N Cruz and S N Pandis ldquoDeliquescence and hygroscopicgrowth of mixed Inorganicminus Organic atmospheric aerosolrdquoEnvironmental Science amp Technology vol 34 no 20pp 4313ndash4319 2000

[41] I N Tang ldquoPhase transformation and growth of aerosolparticles composed of mixed saltsrdquo Journal of Aerosol Sciencevol 7 no 5 pp 361ndash371 1976

[42] D S Covert R J Charlson and N C Ahlquist ldquoA study ofthe relationship of chemical composition and humidity tolight scattering by aerosolsrdquo Journal of Applied Meteorologyvol 11 no 6 pp 968ndash976 1972

[43] Y Tai H Liang A Zaki et al ldquoCoreshell microstructureinduced synergistic effect for efficient water-droplet forma-tion and cloud-seeding applicationrdquo ACS Nano vol 11no 12 pp 12318ndash12325 2017

[44] K Marbou A Al Ghaferi and M Jouiad ldquoIn-situ charac-terization of Wettability alteration in Hopgrdquo SOP Transac-tions on Nanotechnology vol 2374 pp 1ndash10 2015


[45] N Miljkovic R Enright and E N Wang ldquoEffect of dropletmorphology on growth dynamics and heat transfer duringcondensation on superhydrophobic nanostructured surfacesrdquoACS Nano vol 6 no 2 pp 1776ndash1785 2012

[46] C Aprile L Maretti M Alvaro J C Scaiano and H GarcialdquoLong-lived (minutes) photoinduced charge separation in astructured periodic mesoporous titania containing 246-tri-phenylpyrylium as guestrdquo Dalton Transactions no 40pp 5465ndash5470 2008

[47] Y L Du Y Deng and M S Zhang ldquoVariable-temperatureRaman scattering study on anatase titanium dioxide nano-crystalsrdquo Journal of Physics and Chemistry of Solids vol 67no 11 pp 2405ndash2408 2006

[48] D D Do Adsorption Analysis Equilibria and Kinetics Im-perial College Press London UK 1st edition 1998

[49] S L Clegg K S Pitzer and P Brimblecombe ldquoermody-namics of multicomponent miscible ionic solutions ldquoex-tended AIM aerosol thermodynamics modelrdquordquo=e Journal ofPhysical Chemistry A vol 96 no pp 9470ndash9479 1992

[50] A S Wexler and S L Clegg ldquoAtmospheric aerosol models forsystems including the ions H+ NH4

+ Na+ SO42minus NO3

minus Clminus Brminus and H2Ordquo Journal of Geophysical Research vol 107no D14 2002

[51] E Friese and A Ebel ldquoTemperature dependent thermodynamicmodel of the system H+minus NH4

+minus Na+minus SO42minus minus NO3

minus minus Clminus minus H2Ordquo=e Journal of Physical Chemistry A vol 114 no 43pp 11595ndash11631 2010

[52] S Ghorai B Wang A Tivanski and A Laskin ldquoHygroscopicproperties of internally mixed particles composed of NaCl andwater-soluble organic acidsrdquo Environmental Science ampTechnology vol 48 no 4 pp 2234ndash2241 2014

[53] Y Liu Z Yang Y Desyaterik P L Gassman H Wang andA Laskin ldquoHygroscopic behavior of substrate-depositedparticles studied by micro-FT-IR spectroscopy and comple-mentary methods of particle analysisrdquo Analytical Chemistryvol 80 no 3 pp 633ndash642 2008

[54] C Peng B Jing Y-C Guo Y-H Zhang and M-F GeldquoHygroscopic behavior of multicomponent aerosols involvingNaCl and dicarboxylic acidsrdquo =e Journal of PhysicalChemistry A vol 120 no 7 pp 1029ndash1038 2016

[55] F D Pope B J Dennis-Smither P T Griffiths S L Cleggand R A Cox ldquoStudies of single aerosol particles containingmalonic acid glutaric acid and their mixtures with sodiumchloride I Hygroscopic growthrdquo =e Journal of PhysicalChemistry A vol 114 no 16 pp 5335ndash5341 2010

[56] C B Richardson and T D Snyder ldquoA study of heterogeneousnucleation in aqueous solutionsrdquo Langmuir vol 10 no 7pp 2462ndash2465 1994

[57] C-P Lin H Chen A Nakaruk P Koshy and C C SorrellldquoEffect of annealing temperature on the photocatalytic activity ofTiO2 thin filmsrdquo Energy Procedia vol 34 pp 627ndash636 2013

[58] Q Ma H He and Y Liu ldquoIn situ DRIFTS study of hygro-scopic behavior of mineral aerosolrdquo Journal of EnvironmentalSciences vol 22 no 4 pp 555ndash560 2010

[59] Z Mouline K Asai Y Daiko S Honda S Bernard andY Iwamoto ldquoAmine-functionalized polycarbosilane hybridsfor CO2-selective membranesrdquo Journal of the European Ce-ramic Society vol 37 no 16 pp 5213ndash5221 2017

[60] G E Ewing and S J Peters ldquoAdsorption of water on NaClrdquoSurface Review and Letters vol 04 no 04 pp 757ndash770 1997

[61] B Wassermann S Mirbt J Reif J C Zink and E MatthiasldquoClustered water adsorption on the NaCl (100) surfacerdquo =eJournal of Chemical Physics vol 98 no12 pp10049ndash10060 1993

[62] D D Weis and G E Ewing ldquoWater content and morphology ofsodium chloride aerosol particlesrdquo Journal of Geophysical Re-search Atmospheres vol 104 no D17 pp 21275ndash21285 1999

[63] C J Walcek ldquoCloud cover and its relationship to relativehumidity during a springtime midlatitude cyclonerdquo MonthlyWeather Review vol 122 no 6 pp 1021ndash1035 1994

[64] R T Wetherald and S Manabe ldquoAn investigation of cloudcover change in response to thermal forcingrdquo ClimaticChange vol 8 no 1 pp 5ndash23 1986

[65] J B Condon Surface Area and Porosity Determinations byPhysisorption Measurements and =eory Elsevier Amster-dam Netherlands 2006

[66] K S W Sing ldquoReporting physisorption data for gassolidsystems with special reference to the determination of surfacearea and porosity (Provisional)rdquo Pure and Applied Chemistryvol 54 no 11 pp 2201ndash2218 1982

[67] J C Callahan GW Cleary M Elefant G Kaplan T Kenslerand R A Nash ldquoEquilibrium moisture content of pharma-ceutical excipientsrdquo Drug Development and Industrial Phar-macy vol 8 no 3 pp 355ndash369 1982

[68] M W Scott H A Lieberman and F S Chow ldquoPharma-ceutical applications of the concept of equilibrium moisturecontentsrdquo Journal of Pharmaceutical Sciences vol 52 no 10pp 994ndash998 1963

[69] V Murikipudi P Gupta and V Sihorkar ldquoEfficientthroughput method for hygroscopicity classification of activeand inactive pharmaceutical ingredients by water vaporsorption analysisrdquo Pharmaceutical Development and Tech-nology vol 18 no 2 pp 348ndash358 2013

[70] S Patil ldquoA study of artificial rainfallrdquo International Journal ofMultifaceted and Multilingual Studies vol 3 2016

[71] E Shotton and N Harb ldquoe effect of humidity and tem-perature on the equilibrium moisture content of powdersrdquoJournal of Pharmacy and Pharmacology vol 17 no 8pp 504ndash508 1965

[72] S Builes S I Sandler and R Xiong ldquoIsosteric heats of gas andliquid adsorptionrdquo Langmuir vol 29 no 33 pp 10416ndash10422 2013

[73] N A Aviara O O Ajibola and U O Dairo ldquoPH-postharvesttechnologyrdquo Biosystems Engineering vol 83 no 4 pp 423ndash431 2002

[74] A H Al-Muhtaseb W A M McMinn and T R A MageeldquoWater sorption isotherms of starch powdersrdquo Journal ofFood Engineering vol 61 no 3 pp 297ndash307 2004

[75] A H Al-Muhtaseb W A M McMinn and T R A MageeldquoWater sorption isotherms of starch powders Part 2 ther-modynamic characteristicsrdquo Journal of Food Engineeringvol 62 no 2 pp 135ndash142 2004

[76] S Sircar R Mohr C Ristic and M B Rao ldquoIsosteric heat ofadsorption theory and experimentrdquo =e Journal of PhysicalChemistry B vol 103 no 31 pp 6539ndash6546 1999

[77] M Yazdani P Sazandehchi M Azizi and P GhobadildquoMoisture sorption isotherms and isosteric heat for pista-chiordquo European Food Research and Technology vol 223 no 5pp 577ndash584 2006

[78] M Masuzawa and C Sterling ldquoGel-water relationships inhydrophilic polymers thermodynamics of sorption of watervaporrdquo Journal of Applied Polymer Science vol 12 no 9pp 2023ndash2032 1968

[79] E Tsami ldquoNet isosteric heat of sorption in dried fruitsrdquoJournal of Food Engineering vol 14 no 4 pp 327ndash335 1991


sodium polyacrylate (PAAS) are known as superhygroscopicporousmaterialsey were found to act as a catalyst powderto enhance precipitations with a lower efficiency for PAAS[16] While other composite adsorbents such as crystallineMCM-41 and calcium chloride (CaCl2) have revealed a highwater adsorption capacity of 175 kg kgminus 1 in the temperaturerange of 10ndash15degC and at 78ndash92 relative humidity (RH)[17 18] Although these materials have very good hygro-scopic properties several factors such as their complexprocessing routes and their water stability at differentconditions of pressure and temperature limit their use es-pecially in cloud seeding

For cloud seeding applications the hygroscopic mate-rials need to meet the particle size requirement criterionAccording to earlier studies [19ndash22] based on numericalcorrelations modelling and simulations the optimum sizeof seeding materials is in the range of 05 to 10 μm in di-ameter is particle size range ensures an efficient collision-coalescence process of the cloud condensation nuclei(CCN) is process initiates the water droplet nucleationand enables their growth until they reach the raindropcritical size of about 24 μm in diameter [23] At this stage theraindrops may overcome the updraft velocity inside theclouds and be dragged downwards by their own weight tofall as rain [24 25]

is study concerns the characterization of novelcomposite materials which were developed through rela-tively simple mechanisms and using affordable andabundant materials in nature Here commercial salt (NaCl)as a raw material was optimized in terms of size as percloud seeding requirement to yield to NaCl crystals withsub-micrometric size ereafter they were coated withthin layers of oxides either TiO2 or SiO2 which have goodaffinity towards water ie hydrophilicity [26ndash29] wateradsorption [30 31] and solubility [32ndash36] A character-ization of the benchmark NaCl particles was carried out tohave a baseline reference to be compared to the NaCl-TiO2and NaCl-SiO2 composites obtained after the coatingprocess

11 Hygroscopicity A hygroscopic solid material can beidentified through its hygroscopic point and water uptakecapacity e hygroscopic point (hgp) represents thethreshold value of the relative humidity in the air abovewhich the solid substance starts adsorbing water vapor [37]It describes the relationship at equilibrium between thewater vapor pressure (Psol) and its surrounding environmentwith respect to the partial pressure of the water vapor in theair (PH2O) at a specific temperature hgp is generated as apercentage given by

hgp Psol

PH2O1113888 1113889 times 100 (1)

If the solid is a water-soluble material such as solublesalts then Psol represents the water vapor pressure in thesolid saturated solution since it already contains a certainamount of moisture at equilibrium prior to saturation Inthis case at the hygroscopic point the solid material starts

experiencing the deliquescence process while adsorbingwater vapor from the atmosphere until it starts dissolvingrapidly at quasi-constant RH generally represented by thequasi-vertical line in the isotherm at high RH is deli-quescence property is a characteristic of certain hygroscopicsolid compounds eg water-soluble salts which experiencea phase transition from a solid particle to a liquid drop ieto form a solution [38 39] Hence the hygroscopic point ofthis type of materials at which the deliquescence processoccurs is also called deliquescence point or deliquescencerelative humidity (DRH) [40ndash42]

2 Materials and Methods

21 Material Synthesis e chemical compounds werepurchased from Sigma-Aldrich with analytical grade reetypes of samples were prepared (i) optimized neat NaClwith submicron-range particles (ii) NaCl coated either withtitanium dioxide (NaCl-TiO2) or (iii) silicon dioxide (NaCl-SiO2) Here the optimization of NaCl consists of achieving aparticle size reduction of commercial NaCl to match theaforementioned size requirements (05 to 10 μm) for seedingmaterials Herein an aqueous solution was prepared usingcommercial salt and then 1ml of this solution was added to50ml of 2-propanol while stirring Once a white precipitateis observed the NaCl crystals were extracted by eithercentrifugation or filtration followed by drying in the oven at80degC For the synthesis of NaCl-TiO2 core-shell compositesimilar to what was reported by our previous work [43] firstTiO2 solution was prepared by hydrolysis of titanium (IV)butoxide solution as the first step Solution A was preparedby dispersing 5ml titanium (IV) butoxide in 40ml of 2-propanol en solution B was prepared by mixing 50ml ofdeionized water and 1ml of nitric acid Subsequently so-lution B was added dropwise into solution A under vigorousstirring until the formation of a semitransparent TiO2 so-lution A pH control below 2 is crucial to ensure the overallNaCl-TiO2 particle size in the submicron range To prepareNaCl-TiO2 composite a solution of 50mg of optimizedNaCl and 10ml of 2-propanol was needed en 1ml of theabove-prepared TiO2 solution was poured dropwise into thisNaCl solution e new NaCl-TiO2 solution was stirredvigorously for 60 minutes to be ready for the separationprocess by centrifugation drying at 80degC overnight andcalcination at 250degC during 3 hours

For the synthesis of NaCl-SiO2 core-shell composite firstSiO2 solution was prepared by the hydrolysis of the tet-raethyl orthosilicate (TEOS) solution An acid solution wasprepared by mixing 16ml of hydrochloric acid with 20ml ofethanol and 20ml of deionized water en 46ml of TEOSsolution was added dropwise into the acidic solution undervigorous stirring pH was kept at 2 while stirring at 250 rpmfor 2 hours To prepare NaCl-SiO2 composite a solution of50mg of optimized NaCl and 2ml of 2-propanol wasneeded en 1ml of the above-prepared SiO2 solution waspoured dropwise into this NaCl solution e new NaCl-SiO2 solution was stirred vigorously for 60 minutes to beready for the separation process by centrifugation drying at80degC overnight and calcination at 550degC during 3 hours














Stigmator

GSED

Condensed droplets


(a)

SolidLiquid

Vapor



Con

dens

atio

n

P (Pa)

700

500

10 T (degC)

(b)


200 microm

(a)

10 microm

(b)

10 microm

(c)

10 microm

(d)













5 microm

(a)

5 microm

(b)





ClNa

ClCl

306K

272K

232K

240K

170K

136K

102K

068K

034K

000K000 100 200 300 400 500 600 700 800 900

Energy (eV)

Inte

nsity

AU

(a)


Ti TiClO

TiCl

ClNa

000 067 134 201 268 335 402 469 536 603

729

648

567

486

405

324

243

162

81

0

(b)


Cl

Na

Cl

SiSiCl

O000 13 26 39 52 65 78 91 104 117 130

810K

720K

630K

540K

450K

360K

270K

180K

090K

000K

(c)








0 1000 2000 3000 4000

Abso

rban

ce (a

u)


NaCl-TiO2TiO2NaCl

(a)

NaCl-SiO2SiO2NaCl

0 1000 2000 3000 4000

Abso

rban

ce (a

u)


(b)


(a)

(b)

NaCl-TiO2TiO2NaCl


800

1000

1200

1400

Ram

an in

tens

ity (a

u)

0 100 200 300 400 500750

800

850

900

Ram

an in

tens

ity (a

u)








50 nm

(a)


50 nm

(b)

10 nm

(c)

10 nm


(d)




δ lnP

δ(1T)

(ΔH)

R (2)







50 nm

(a)

50 nm


(b)

10 nm

(c)

10 nm


(d)




0

10

20

30

40W

ater

upt

ake (

103 m

g gndash1

)

06 0800 02 1004PP0

(a)

PP0


00 02 04 06 08 10

Wat

er u

ptak

e (10

3 mg

gndash1)

0

10

20

30

40

(b)

PP0

25degC35degC45degC

NaCl-SiO2

Wat

er u

ptak

e (10

3 mg

gndash1)

0

10

20

30

40

02 04 06 08 1000

(c)

Wat

er u

ptak

e (m

g gndash1

)

NaCl

2

0

4

6

8

10

00 01 02 03 04 05 06PP0

25degC35degC45degC

(d)

Wat

er u

ptak

e (m

g gndash1

)

NaCl-TiO2

120

100

80

60

40

20

000 01 02 03 04 05 06

PP0

25degC35degC45degC

(e)

Wat

er u

ptak

e (m

g gndash1

)NaCl-SiO2

0

2

4

6

8

10

00 01 02 03 04 05 06PP0

25degC35degC45degC

(f )


000

30

60

90

120

25degC

Wat

er u

ptak

e (m

g gndash1

)


01 02 03 04 05 06PP0

(a)

0

30

60

90

120

Wat

er u

ptak

e (m

g gndash1

)


00 01 02 03 04 05 06PP0

(b)

0

30

60

90

120

Wat

er u

ptak

e (m

g gndash1

)


00 01 02 03 04 05 06PP0

(c)






06 07 08 09 100

10

20

30

40

Wat

er u

ptak

e (10

3 mg

gndash1)

PP0


(a)


Wat

er u

ptak

e (10

3 mg

gndash1)

PP0

06 07 08 09 100

10

20

30

40

(b)


Wat

er u

ptak

e (10

3 mg

gndash1)

PP0

06 07 08 09 100

10

20

30

40

(c)





0 20 40 60 800

10

20

30

40

Wat

er u

ptak

e (m

g gndash1

)

Time (min)

25degCNaCl-TiO2

NaCl-SiO2

NaCl

(a)

35degCNaCl-TiO2

NaCl-SiO2

NaCl

Wat

er u

ptak

e (m

g gndash1

)

0 20 40 60 800

10

20

30

40

Time (min)

(b)

0 20 40 60 800

10

20

30

40

Time (min)

Wat

er u

ptak

e (m

g gndash1

)

45degCNaCl-TiO2

NaCl-SiO2

NaCl

(c)





310 315 320 325 330 335283032343638404244

ln P


NaCl

(a)

310 315 320 325 330 335T ndash1 (103 Kndash1)

283032343638404244

ln P

NaCl-TiO2

(b)

310 315 320 325 330 335T ndash1 (103 Kndash1)

283032343638404244

ln P

NaCl-SiO2

(c)


0 4000 8000 12000 16000 20000

34

36

38

40

42

44

46

Isos

teric

hea

t of a

dsor

ptio

n (k

Jmol

)

Water uptake (mgg)

NaCl-TiO2

NaCl-SiO2

NaCl

(a)

1000 2000 3000 4000 5000

34

36

38

40

42

44

46

Isos

teric

hea

t of a

dsor

ptio

n (k

Jmol

)

Water uptake (mgg)

NaCl-TiO2

NaCl-SiO2

NaCl

(b)







Aercondensation

Beforecondensation

Duringcondensation

NaCl NaCl

D0

NaCl

D


(a)

Aercondensation

Beforecondensation

Duringcondensation

NaClTiO2NaClTiO2

D0

NaClTiO2

D


(b)

Aercondensation

Beforecondensation

Duringcondensation

NaClSiO2


NaClSiO2

D0

NaClSiO2

D

(c)




4 Conclusions





Data Availability




Acknowledgments




References










0 10 20 30 40 50 60 70 80 90ndash10

ndash8

ndash6

ndash4

ndash2

0D

ispla

cem

ent (μm

)


NaCl-TiO2

NaCl-SiO2

NaCl













































+ Na+ SO42minus NO3














































Stigmator

GSED

Condensed droplets


(a)

SolidLiquid

Vapor



Con

dens

atio

n

P (Pa)

700

500

10 T (degC)

(b)


200 microm

(a)

10 microm

(b)

10 microm

(c)

10 microm

(d)













5 microm

(a)

5 microm

(b)





ClNa

ClCl

306K

272K

232K

240K

170K

136K

102K

068K

034K

000K000 100 200 300 400 500 600 700 800 900

Energy (eV)

Inte

nsity

AU

(a)


Ti TiClO

TiCl

ClNa

000 067 134 201 268 335 402 469 536 603

729

648

567

486

405

324

243

162

81

0

(b)


Cl

Na

Cl

SiSiCl

O000 13 26 39 52 65 78 91 104 117 130

810K

720K

630K

540K

450K

360K

270K

180K

090K

000K

(c)








0 1000 2000 3000 4000

Abso

rban

ce (a

u)


NaCl-TiO2TiO2NaCl

(a)

NaCl-SiO2SiO2NaCl

0 1000 2000 3000 4000

Abso

rban

ce (a

u)


(b)


(a)

(b)

NaCl-TiO2TiO2NaCl


800

1000

1200

1400

Ram

an in

tens

ity (a

u)

0 100 200 300 400 500750

800

850

900

Ram

an in

tens

ity (a

u)








50 nm

(a)


50 nm

(b)

10 nm

(c)

10 nm


(d)




δ lnP

δ(1T)

(ΔH)

R (2)







50 nm

(a)

50 nm


(b)

10 nm

(c)

10 nm


(d)




0

10

20

30

40W

ater

upt

ake (

103 m

g gndash1

)

06 0800 02 1004PP0

(a)

PP0


00 02 04 06 08 10

Wat

er u

ptak

e (10

3 mg

gndash1)

0

10

20

30

40

(b)

PP0

25degC35degC45degC

NaCl-SiO2

Wat

er u

ptak

e (10

3 mg

gndash1)

0

10

20

30

40

02 04 06 08 1000

(c)

Wat

er u

ptak

e (m

g gndash1

)

NaCl

2

0

4

6

8

10

00 01 02 03 04 05 06PP0

25degC35degC45degC

(d)

Wat

er u

ptak

e (m

g gndash1

)

NaCl-TiO2

120

100

80

60

40

20

000 01 02 03 04 05 06

PP0

25degC35degC45degC

(e)

Wat

er u

ptak

e (m

g gndash1

)NaCl-SiO2

0

2

4

6

8

10

00 01 02 03 04 05 06PP0

25degC35degC45degC

(f )


000

30

60

90

120

25degC

Wat

er u

ptak

e (m

g gndash1

)


01 02 03 04 05 06PP0

(a)

0

30

60

90

120

Wat

er u

ptak

e (m

g gndash1

)


00 01 02 03 04 05 06PP0

(b)

0

30

60

90

120

Wat

er u

ptak

e (m

g gndash1

)


00 01 02 03 04 05 06PP0

(c)






06 07 08 09 100

10

20

30

40

Wat

er u

ptak

e (10

3 mg

gndash1)

PP0


(a)


Wat

er u

ptak

e (10

3 mg

gndash1)

PP0

06 07 08 09 100

10

20

30

40

(b)


Wat

er u

ptak

e (10

3 mg

gndash1)

PP0

06 07 08 09 100

10

20

30

40

(c)





0 20 40 60 800

10

20

30

40

Wat

er u

ptak

e (m

g gndash1

)

Time (min)

25degCNaCl-TiO2

NaCl-SiO2

NaCl

(a)

35degCNaCl-TiO2

NaCl-SiO2

NaCl

Wat

er u

ptak

e (m

g gndash1

)

0 20 40 60 800

10

20

30

40

Time (min)

(b)

0 20 40 60 800

10

20

30

40

Time (min)

Wat

er u

ptak

e (m

g gndash1

)

45degCNaCl-TiO2

NaCl-SiO2

NaCl

(c)





310 315 320 325 330 335283032343638404244

ln P


NaCl

(a)

310 315 320 325 330 335T ndash1 (103 Kndash1)

283032343638404244

ln P

NaCl-TiO2

(b)

310 315 320 325 330 335T ndash1 (103 Kndash1)

283032343638404244

ln P

NaCl-SiO2

(c)


0 4000 8000 12000 16000 20000

34

36

38

40

42

44

46

Isos

teric

hea

t of a

dsor

ptio

n (k

Jmol

)

Water uptake (mgg)

NaCl-TiO2

NaCl-SiO2

NaCl

(a)

1000 2000 3000 4000 5000

34

36

38

40

42

44

46

Isos

teric

hea

t of a

dsor

ptio

n (k

Jmol

)

Water uptake (mgg)

NaCl-TiO2

NaCl-SiO2

NaCl

(b)







Aercondensation

Beforecondensation

Duringcondensation

NaCl NaCl

D0

NaCl

D


(a)

Aercondensation

Beforecondensation

Duringcondensation

NaClTiO2NaClTiO2

D0

NaClTiO2

D


(b)

Aercondensation

Beforecondensation

Duringcondensation

NaClSiO2


NaClSiO2

D0

NaClSiO2

D

(c)




4 Conclusions





Data Availability




Acknowledgments




References










0 10 20 30 40 50 60 70 80 90ndash10

ndash8

ndash6

ndash4

ndash2

0D

ispla

cem

ent (μm

)


NaCl-TiO2

NaCl-SiO2

NaCl













































+ Na+ SO42minus NO3












































5 microm

(a)

5 microm

(b)





ClNa

ClCl

306K

272K

232K

240K

170K

136K

102K

068K

034K

000K000 100 200 300 400 500 600 700 800 900

Energy (eV)

Inte

nsity

AU

(a)


Ti TiClO

TiCl

ClNa

000 067 134 201 268 335 402 469 536 603

729

648

567

486

405

324

243

162

81

0

(b)


Cl

Na

Cl

SiSiCl

O000 13 26 39 52 65 78 91 104 117 130

810K

720K

630K

540K

450K

360K

270K

180K

090K

000K

(c)








0 1000 2000 3000 4000

Abso

rban

ce (a

u)


NaCl-TiO2TiO2NaCl

(a)

NaCl-SiO2SiO2NaCl

0 1000 2000 3000 4000

Abso

rban

ce (a

u)


(b)


(a)

(b)

NaCl-TiO2TiO2NaCl


800

1000

1200

1400

Ram

an in

tens

ity (a

u)

0 100 200 300 400 500750

800

850

900

Ram

an in

tens

ity (a

u)








50 nm

(a)


50 nm

(b)

10 nm

(c)

10 nm


(d)




δ lnP

δ(1T)

(ΔH)

R (2)







50 nm

(a)

50 nm


(b)

10 nm

(c)

10 nm


(d)




0

10

20

30

40W

ater

upt

ake (

103 m

g gndash1

)

06 0800 02 1004PP0

(a)

PP0


00 02 04 06 08 10

Wat

er u

ptak

e (10

3 mg

gndash1)

0

10

20

30

40

(b)

PP0

25degC35degC45degC

NaCl-SiO2

Wat

er u

ptak

e (10

3 mg

gndash1)

0

10

20

30

40

02 04 06 08 1000

(c)

Wat

er u

ptak

e (m

g gndash1

)

NaCl

2

0

4

6

8

10

00 01 02 03 04 05 06PP0

25degC35degC45degC

(d)

Wat

er u

ptak

e (m

g gndash1

)

NaCl-TiO2

120

100

80

60

40

20

000 01 02 03 04 05 06

PP0

25degC35degC45degC

(e)

Wat

er u

ptak

e (m

g gndash1

)NaCl-SiO2

0

2

4

6

8

10

00 01 02 03 04 05 06PP0

25degC35degC45degC

(f )


000

30

60

90

120

25degC

Wat

er u

ptak

e (m

g gndash1

)


01 02 03 04 05 06PP0

(a)

0

30

60

90

120

Wat

er u

ptak

e (m

g gndash1

)


00 01 02 03 04 05 06PP0

(b)

0

30

60

90

120

Wat

er u

ptak

e (m

g gndash1

)


00 01 02 03 04 05 06PP0

(c)






06 07 08 09 100

10

20

30

40

Wat

er u

ptak

e (10

3 mg

gndash1)

PP0


(a)


Wat

er u

ptak

e (10

3 mg

gndash1)

PP0

06 07 08 09 100

10

20

30

40

(b)


Wat

er u

ptak

e (10

3 mg

gndash1)

PP0

06 07 08 09 100

10

20

30

40

(c)





0 20 40 60 800

10

20

30

40

Wat

er u

ptak

e (m

g gndash1

)

Time (min)

25degCNaCl-TiO2

NaCl-SiO2

NaCl

(a)

35degCNaCl-TiO2

NaCl-SiO2

NaCl

Wat

er u

ptak

e (m

g gndash1

)

0 20 40 60 800

10

20

30

40

Time (min)

(b)

0 20 40 60 800

10

20

30

40

Time (min)

Wat

er u

ptak

e (m

g gndash1

)

45degCNaCl-TiO2

NaCl-SiO2

NaCl

(c)





310 315 320 325 330 335283032343638404244

ln P


NaCl

(a)

310 315 320 325 330 335T ndash1 (103 Kndash1)

283032343638404244

ln P

NaCl-TiO2

(b)

310 315 320 325 330 335T ndash1 (103 Kndash1)

283032343638404244

ln P

NaCl-SiO2

(c)


0 4000 8000 12000 16000 20000

34

36

38

40

42

44

46

Isos

teric

hea

t of a

dsor

ptio

n (k

Jmol

)

Water uptake (mgg)

NaCl-TiO2

NaCl-SiO2

NaCl

(a)

1000 2000 3000 4000 5000

34

36

38

40

42

44

46

Isos

teric

hea

t of a

dsor

ptio

n (k

Jmol

)

Water uptake (mgg)

NaCl-TiO2

NaCl-SiO2

NaCl

(b)







Aercondensation

Beforecondensation

Duringcondensation

NaCl NaCl

D0

NaCl

D


(a)

Aercondensation

Beforecondensation

Duringcondensation

NaClTiO2NaClTiO2

D0

NaClTiO2

D


(b)

Aercondensation

Beforecondensation

Duringcondensation

NaClSiO2


NaClSiO2

D0

NaClSiO2

D

(c)




4 Conclusions





Data Availability




Acknowledgments




References










0 10 20 30 40 50 60 70 80 90ndash10

ndash8

ndash6

ndash4

ndash2

0D

ispla

cem

ent (μm

)


NaCl-TiO2

NaCl-SiO2

NaCl













































+ Na+ SO42minus NO3




































ClNa

ClCl

306K

272K

232K

240K

170K

136K

102K

068K

034K

000K000 100 200 300 400 500 600 700 800 900

Energy (eV)

Inte

nsity

AU

(a)


Ti TiClO

TiCl

ClNa

000 067 134 201 268 335 402 469 536 603

729

648

567

486

405

324

243

162

81

0

(b)


Cl

Na

Cl

SiSiCl

O000 13 26 39 52 65 78 91 104 117 130

810K

720K

630K

540K

450K

360K

270K

180K

090K

000K

(c)








0 1000 2000 3000 4000

Abso

rban

ce (a

u)


NaCl-TiO2TiO2NaCl

(a)

NaCl-SiO2SiO2NaCl

0 1000 2000 3000 4000

Abso

rban

ce (a

u)


(b)


(a)

(b)

NaCl-TiO2TiO2NaCl


800

1000

1200

1400

Ram

an in

tens

ity (a

u)

0 100 200 300 400 500750

800

850

900

Ram

an in

tens

ity (a

u)








50 nm

(a)


50 nm

(b)

10 nm

(c)

10 nm


(d)




δ lnP

δ(1T)

(ΔH)

R (2)







50 nm

(a)

50 nm


(b)

10 nm

(c)

10 nm


(d)




0

10

20

30

40W

ater

upt

ake (

103 m

g gndash1

)

06 0800 02 1004PP0

(a)

PP0


00 02 04 06 08 10

Wat

er u

ptak

e (10

3 mg

gndash1)

0

10

20

30

40

(b)

PP0

25degC35degC45degC

NaCl-SiO2

Wat

er u

ptak

e (10

3 mg

gndash1)

0

10

20

30

40

02 04 06 08 1000

(c)

Wat

er u

ptak

e (m

g gndash1

)

NaCl

2

0

4

6

8

10

00 01 02 03 04 05 06PP0

25degC35degC45degC

(d)

Wat

er u

ptak

e (m

g gndash1

)

NaCl-TiO2

120

100

80

60

40

20

000 01 02 03 04 05 06

PP0

25degC35degC45degC

(e)

Wat

er u

ptak

e (m

g gndash1

)NaCl-SiO2

0

2

4

6

8

10

00 01 02 03 04 05 06PP0

25degC35degC45degC

(f )


000

30

60

90

120

25degC

Wat

er u

ptak

e (m

g gndash1

)


01 02 03 04 05 06PP0

(a)

0

30

60

90

120

Wat

er u

ptak

e (m

g gndash1

)


00 01 02 03 04 05 06PP0

(b)

0

30

60

90

120

Wat

er u

ptak

e (m

g gndash1

)


00 01 02 03 04 05 06PP0

(c)






06 07 08 09 100

10

20

30

40

Wat

er u

ptak

e (10

3 mg

gndash1)

PP0


(a)


Wat

er u

ptak

e (10

3 mg

gndash1)

PP0

06 07 08 09 100

10

20

30

40

(b)


Wat

er u

ptak

e (10

3 mg

gndash1)

PP0

06 07 08 09 100

10

20

30

40

(c)





0 20 40 60 800

10

20

30

40

Wat

er u

ptak

e (m

g gndash1

)

Time (min)

25degCNaCl-TiO2

NaCl-SiO2

NaCl

(a)

35degCNaCl-TiO2

NaCl-SiO2

NaCl

Wat

er u

ptak

e (m

g gndash1

)

0 20 40 60 800

10

20

30

40

Time (min)

(b)

0 20 40 60 800

10

20

30

40

Time (min)

Wat

er u

ptak

e (m

g gndash1

)

45degCNaCl-TiO2

NaCl-SiO2

NaCl

(c)





310 315 320 325 330 335283032343638404244

ln P


NaCl

(a)

310 315 320 325 330 335T ndash1 (103 Kndash1)

283032343638404244

ln P

NaCl-TiO2

(b)

310 315 320 325 330 335T ndash1 (103 Kndash1)

283032343638404244

ln P

NaCl-SiO2

(c)


0 4000 8000 12000 16000 20000

34

36

38

40

42

44

46

Isos

teric

hea

t of a

dsor

ptio

n (k

Jmol

)

Water uptake (mgg)

NaCl-TiO2

NaCl-SiO2

NaCl

(a)

1000 2000 3000 4000 5000

34

36

38

40

42

44

46

Isos

teric

hea

t of a

dsor

ptio

n (k

Jmol

)

Water uptake (mgg)

NaCl-TiO2

NaCl-SiO2

NaCl

(b)







Aercondensation

Beforecondensation

Duringcondensation

NaCl NaCl

D0

NaCl

D


(a)

Aercondensation

Beforecondensation

Duringcondensation

NaClTiO2NaClTiO2

D0

NaClTiO2

D


(b)

Aercondensation

Beforecondensation

Duringcondensation

NaClSiO2


NaClSiO2

D0

NaClSiO2

D

(c)




4 Conclusions





Data Availability




Acknowledgments




References










0 10 20 30 40 50 60 70 80 90ndash10

ndash8

ndash6

ndash4

ndash2

0D

ispla

cem

ent (μm

)


NaCl-TiO2

NaCl-SiO2

NaCl













































+ Na+ SO42minus NO3







































0 1000 2000 3000 4000

Abso

rban

ce (a

u)


NaCl-TiO2TiO2NaCl

(a)

NaCl-SiO2SiO2NaCl

0 1000 2000 3000 4000

Abso

rban

ce (a

u)


(b)


(a)

(b)

NaCl-TiO2TiO2NaCl


800

1000

1200

1400

Ram

an in

tens

ity (a

u)

0 100 200 300 400 500750

800

850

900

Ram

an in

tens

ity (a

u)








50 nm

(a)


50 nm

(b)

10 nm

(c)

10 nm


(d)




δ lnP

δ(1T)

(ΔH)

R (2)







50 nm

(a)

50 nm


(b)

10 nm

(c)

10 nm


(d)




0

10

20

30

40W

ater

upt

ake (

103 m

g gndash1

)

06 0800 02 1004PP0

(a)

PP0


00 02 04 06 08 10

Wat

er u

ptak

e (10

3 mg

gndash1)

0

10

20

30

40

(b)

PP0

25degC35degC45degC

NaCl-SiO2

Wat

er u

ptak

e (10

3 mg

gndash1)

0

10

20

30

40

02 04 06 08 1000

(c)

Wat

er u

ptak

e (m

g gndash1

)

NaCl

2

0

4

6

8

10

00 01 02 03 04 05 06PP0

25degC35degC45degC

(d)

Wat

er u

ptak

e (m

g gndash1

)

NaCl-TiO2

120

100

80

60

40

20

000 01 02 03 04 05 06

PP0

25degC35degC45degC

(e)

Wat

er u

ptak

e (m

g gndash1

)NaCl-SiO2

0

2

4

6

8

10

00 01 02 03 04 05 06PP0

25degC35degC45degC

(f )


000

30

60

90

120

25degC

Wat

er u

ptak

e (m

g gndash1

)


01 02 03 04 05 06PP0

(a)

0

30

60

90

120

Wat

er u

ptak

e (m

g gndash1

)


00 01 02 03 04 05 06PP0

(b)

0

30

60

90

120

Wat

er u

ptak

e (m

g gndash1

)


00 01 02 03 04 05 06PP0

(c)






06 07 08 09 100

10

20

30

40

Wat

er u

ptak

e (10

3 mg

gndash1)

PP0


(a)


Wat

er u

ptak

e (10

3 mg

gndash1)

PP0

06 07 08 09 100

10

20

30

40

(b)


Wat

er u

ptak

e (10

3 mg

gndash1)

PP0

06 07 08 09 100

10

20

30

40

(c)





0 20 40 60 800

10

20

30

40

Wat

er u

ptak

e (m

g gndash1

)

Time (min)

25degCNaCl-TiO2

NaCl-SiO2

NaCl

(a)

35degCNaCl-TiO2

NaCl-SiO2

NaCl

Wat

er u

ptak

e (m

g gndash1

)

0 20 40 60 800

10

20

30

40

Time (min)

(b)

0 20 40 60 800

10

20

30

40

Time (min)

Wat

er u

ptak

e (m

g gndash1

)

45degCNaCl-TiO2

NaCl-SiO2

NaCl

(c)





310 315 320 325 330 335283032343638404244

ln P


NaCl

(a)

310 315 320 325 330 335T ndash1 (103 Kndash1)

283032343638404244

ln P

NaCl-TiO2

(b)

310 315 320 325 330 335T ndash1 (103 Kndash1)

283032343638404244

ln P

NaCl-SiO2

(c)


0 4000 8000 12000 16000 20000

34

36

38

40

42

44

46

Isos

teric

hea

t of a

dsor

ptio

n (k

Jmol

)

Water uptake (mgg)

NaCl-TiO2

NaCl-SiO2

NaCl

(a)

1000 2000 3000 4000 5000

34

36

38

40

42

44

46

Isos

teric

hea

t of a

dsor

ptio

n (k

Jmol

)

Water uptake (mgg)

NaCl-TiO2

NaCl-SiO2

NaCl

(b)







Aercondensation

Beforecondensation

Duringcondensation

NaCl NaCl

D0

NaCl

D


(a)

Aercondensation

Beforecondensation

Duringcondensation

NaClTiO2NaClTiO2

D0

NaClTiO2

D


(b)

Aercondensation

Beforecondensation

Duringcondensation

NaClSiO2


NaClSiO2

D0

NaClSiO2

D

(c)




4 Conclusions





Data Availability




Acknowledgments




References










0 10 20 30 40 50 60 70 80 90ndash10

ndash8

ndash6

ndash4

ndash2

0D

ispla

cem

ent (μm

)


NaCl-TiO2

NaCl-SiO2

NaCl













































+ Na+ SO42minus NO3






































50 nm

(a)


50 nm

(b)

10 nm

(c)

10 nm


(d)




δ lnP

δ(1T)

(ΔH)

R (2)







50 nm

(a)

50 nm


(b)

10 nm

(c)

10 nm


(d)




0

10

20

30

40W

ater

upt

ake (

103 m

g gndash1

)

06 0800 02 1004PP0

(a)

PP0


00 02 04 06 08 10

Wat

er u

ptak

e (10

3 mg

gndash1)

0

10

20

30

40

(b)

PP0

25degC35degC45degC

NaCl-SiO2

Wat

er u

ptak

e (10

3 mg

gndash1)

0

10

20

30

40

02 04 06 08 1000

(c)

Wat

er u

ptak

e (m

g gndash1

)

NaCl

2

0

4

6

8

10

00 01 02 03 04 05 06PP0

25degC35degC45degC

(d)

Wat

er u

ptak

e (m

g gndash1

)

NaCl-TiO2

120

100

80

60

40

20

000 01 02 03 04 05 06

PP0

25degC35degC45degC

(e)

Wat

er u

ptak

e (m

g gndash1

)NaCl-SiO2

0

2

4

6

8

10

00 01 02 03 04 05 06PP0

25degC35degC45degC

(f )


000

30

60

90

120

25degC

Wat

er u

ptak

e (m

g gndash1

)


01 02 03 04 05 06PP0

(a)

0

30

60

90

120

Wat

er u

ptak

e (m

g gndash1

)


00 01 02 03 04 05 06PP0

(b)

0

30

60

90

120

Wat

er u

ptak

e (m

g gndash1

)


00 01 02 03 04 05 06PP0

(c)






06 07 08 09 100

10

20

30

40

Wat

er u

ptak

e (10

3 mg

gndash1)

PP0


(a)


Wat

er u

ptak

e (10

3 mg

gndash1)

PP0

06 07 08 09 100

10

20

30

40

(b)


Wat

er u

ptak

e (10

3 mg

gndash1)

PP0

06 07 08 09 100

10

20

30

40

(c)





0 20 40 60 800

10

20

30

40

Wat

er u

ptak

e (m

g gndash1

)

Time (min)

25degCNaCl-TiO2

NaCl-SiO2

NaCl

(a)

35degCNaCl-TiO2

NaCl-SiO2

NaCl

Wat

er u

ptak

e (m

g gndash1

)

0 20 40 60 800

10

20

30

40

Time (min)

(b)

0 20 40 60 800

10

20

30

40

Time (min)

Wat

er u

ptak

e (m

g gndash1

)

45degCNaCl-TiO2

NaCl-SiO2

NaCl

(c)





310 315 320 325 330 335283032343638404244

ln P


NaCl

(a)

310 315 320 325 330 335T ndash1 (103 Kndash1)

283032343638404244

ln P

NaCl-TiO2

(b)

310 315 320 325 330 335T ndash1 (103 Kndash1)

283032343638404244

ln P

NaCl-SiO2

(c)


0 4000 8000 12000 16000 20000

34

36

38

40

42

44

46

Isos

teric

hea

t of a

dsor

ptio

n (k

Jmol

)

Water uptake (mgg)

NaCl-TiO2

NaCl-SiO2

NaCl

(a)

1000 2000 3000 4000 5000

34

36

38

40

42

44

46

Isos

teric

hea

t of a

dsor

ptio

n (k

Jmol

)

Water uptake (mgg)

NaCl-TiO2

NaCl-SiO2

NaCl

(b)







Aercondensation

Beforecondensation

Duringcondensation

NaCl NaCl

D0

NaCl

D


(a)

Aercondensation

Beforecondensation

Duringcondensation

NaClTiO2NaClTiO2

D0

NaClTiO2

D


(b)

Aercondensation

Beforecondensation

Duringcondensation

NaClSiO2


NaClSiO2

D0

NaClSiO2

D

(c)




4 Conclusions





Data Availability




Acknowledgments




References










0 10 20 30 40 50 60 70 80 90ndash10

ndash8

ndash6

ndash4

ndash2

0D

ispla

cem

ent (μm

)


NaCl-TiO2

NaCl-SiO2

NaCl













































+ Na+ SO42minus NO3



































δ lnP

δ(1T)

(ΔH)

R (2)







50 nm

(a)

50 nm


(b)

10 nm

(c)

10 nm


(d)




0

10

20

30

40W

ater

upt

ake (

103 m

g gndash1

)

06 0800 02 1004PP0

(a)

PP0


00 02 04 06 08 10

Wat

er u

ptak

e (10

3 mg

gndash1)

0

10

20

30

40

(b)

PP0

25degC35degC45degC

NaCl-SiO2

Wat

er u

ptak

e (10

3 mg

gndash1)

0

10

20

30

40

02 04 06 08 1000

(c)

Wat

er u

ptak

e (m

g gndash1

)

NaCl

2

0

4

6

8

10

00 01 02 03 04 05 06PP0

25degC35degC45degC

(d)

Wat

er u

ptak

e (m

g gndash1

)

NaCl-TiO2

120

100

80

60

40

20

000 01 02 03 04 05 06

PP0

25degC35degC45degC

(e)

Wat

er u

ptak

e (m

g gndash1

)NaCl-SiO2

0

2

4

6

8

10

00 01 02 03 04 05 06PP0

25degC35degC45degC

(f )


000

30

60

90

120

25degC

Wat

er u

ptak

e (m

g gndash1

)


01 02 03 04 05 06PP0

(a)

0

30

60

90

120

Wat

er u

ptak

e (m

g gndash1

)


00 01 02 03 04 05 06PP0

(b)

0

30

60

90

120

Wat

er u

ptak

e (m

g gndash1

)


00 01 02 03 04 05 06PP0

(c)






06 07 08 09 100

10

20

30

40

Wat

er u

ptak

e (10

3 mg

gndash1)

PP0


(a)


Wat

er u

ptak

e (10

3 mg

gndash1)

PP0

06 07 08 09 100

10

20

30

40

(b)


Wat

er u

ptak

e (10

3 mg

gndash1)

PP0

06 07 08 09 100

10

20

30

40

(c)





0 20 40 60 800

10

20

30

40

Wat

er u

ptak

e (m

g gndash1

)

Time (min)

25degCNaCl-TiO2

NaCl-SiO2

NaCl

(a)

35degCNaCl-TiO2

NaCl-SiO2

NaCl

Wat

er u

ptak

e (m

g gndash1

)

0 20 40 60 800

10

20

30

40

Time (min)

(b)

0 20 40 60 800

10

20

30

40

Time (min)

Wat

er u

ptak

e (m

g gndash1

)

45degCNaCl-TiO2

NaCl-SiO2

NaCl

(c)





310 315 320 325 330 335283032343638404244

ln P


NaCl

(a)

310 315 320 325 330 335T ndash1 (103 Kndash1)

283032343638404244

ln P

NaCl-TiO2

(b)

310 315 320 325 330 335T ndash1 (103 Kndash1)

283032343638404244

ln P

NaCl-SiO2

(c)


0 4000 8000 12000 16000 20000

34

36

38

40

42

44

46

Isos

teric

hea

t of a

dsor

ptio

n (k

Jmol

)

Water uptake (mgg)

NaCl-TiO2

NaCl-SiO2

NaCl

(a)

1000 2000 3000 4000 5000

34

36

38

40

42

44

46

Isos

teric

hea

t of a

dsor

ptio

n (k

Jmol

)

Water uptake (mgg)

NaCl-TiO2

NaCl-SiO2

NaCl

(b)







Aercondensation

Beforecondensation

Duringcondensation

NaCl NaCl

D0

NaCl

D


(a)

Aercondensation

Beforecondensation

Duringcondensation

NaClTiO2NaClTiO2

D0

NaClTiO2

D


(b)

Aercondensation

Beforecondensation

Duringcondensation

NaClSiO2


NaClSiO2

D0

NaClSiO2

D

(c)




4 Conclusions





Data Availability




Acknowledgments




References










0 10 20 30 40 50 60 70 80 90ndash10

ndash8

ndash6

ndash4

ndash2

0D

ispla

cem

ent (μm

)


NaCl-TiO2

NaCl-SiO2

NaCl













































+ Na+ SO42minus NO3



































0

10

20

30

40W

ater

upt

ake (

103 m

g gndash1

)

06 0800 02 1004PP0

(a)

PP0


00 02 04 06 08 10

Wat

er u

ptak

e (10

3 mg

gndash1)

0

10

20

30

40

(b)

PP0

25degC35degC45degC

NaCl-SiO2

Wat

er u

ptak

e (10

3 mg

gndash1)

0

10

20

30

40

02 04 06 08 1000

(c)

Wat

er u

ptak

e (m

g gndash1

)

NaCl

2

0

4

6

8

10

00 01 02 03 04 05 06PP0

25degC35degC45degC

(d)

Wat

er u

ptak

e (m

g gndash1

)

NaCl-TiO2

120

100

80

60

40

20

000 01 02 03 04 05 06

PP0

25degC35degC45degC

(e)

Wat

er u

ptak

e (m

g gndash1

)NaCl-SiO2

0

2

4

6

8

10

00 01 02 03 04 05 06PP0

25degC35degC45degC

(f )


000

30

60

90

120

25degC

Wat

er u

ptak

e (m

g gndash1

)


01 02 03 04 05 06PP0

(a)

0

30

60

90

120

Wat

er u

ptak

e (m

g gndash1

)


00 01 02 03 04 05 06PP0

(b)

0

30

60

90

120

Wat

er u

ptak

e (m

g gndash1

)


00 01 02 03 04 05 06PP0

(c)






06 07 08 09 100

10

20

30

40

Wat

er u

ptak

e (10

3 mg

gndash1)

PP0


(a)


Wat

er u

ptak

e (10

3 mg

gndash1)

PP0

06 07 08 09 100

10

20

30

40

(b)


Wat

er u

ptak

e (10

3 mg

gndash1)

PP0

06 07 08 09 100

10

20

30

40

(c)





0 20 40 60 800

10

20

30

40

Wat

er u

ptak

e (m

g gndash1

)

Time (min)

25degCNaCl-TiO2

NaCl-SiO2

NaCl

(a)

35degCNaCl-TiO2

NaCl-SiO2

NaCl

Wat

er u

ptak

e (m

g gndash1

)

0 20 40 60 800

10

20

30

40

Time (min)

(b)

0 20 40 60 800

10

20

30

40

Time (min)

Wat

er u

ptak

e (m

g gndash1

)

45degCNaCl-TiO2

NaCl-SiO2

NaCl

(c)





310 315 320 325 330 335283032343638404244

ln P


NaCl

(a)

310 315 320 325 330 335T ndash1 (103 Kndash1)

283032343638404244

ln P

NaCl-TiO2

(b)

310 315 320 325 330 335T ndash1 (103 Kndash1)

283032343638404244

ln P

NaCl-SiO2

(c)


0 4000 8000 12000 16000 20000

34

36

38

40

42

44

46

Isos

teric

hea

t of a

dsor

ptio

n (k

Jmol

)

Water uptake (mgg)

NaCl-TiO2

NaCl-SiO2

NaCl

(a)

1000 2000 3000 4000 5000

34

36

38

40

42

44

46

Isos

teric

hea

t of a

dsor

ptio

n (k

Jmol

)

Water uptake (mgg)

NaCl-TiO2

NaCl-SiO2

NaCl

(b)







Aercondensation

Beforecondensation

Duringcondensation

NaCl NaCl

D0

NaCl

D


(a)

Aercondensation

Beforecondensation

Duringcondensation

NaClTiO2NaClTiO2

D0

NaClTiO2

D


(b)

Aercondensation

Beforecondensation

Duringcondensation

NaClSiO2


NaClSiO2

D0

NaClSiO2

D

(c)




4 Conclusions





Data Availability




Acknowledgments




References










0 10 20 30 40 50 60 70 80 90ndash10

ndash8

ndash6

ndash4

ndash2

0D

ispla

cem

ent (μm

)


NaCl-TiO2

NaCl-SiO2

NaCl













































+ Na+ SO42minus NO3





































06 07 08 09 100

10

20

30

40

Wat

er u

ptak

e (10

3 mg

gndash1)

PP0


(a)


Wat

er u

ptak

e (10

3 mg

gndash1)

PP0

06 07 08 09 100

10

20

30

40

(b)


Wat

er u

ptak

e (10

3 mg

gndash1)

PP0

06 07 08 09 100

10

20

30

40

(c)





0 20 40 60 800

10

20

30

40

Wat

er u

ptak

e (m

g gndash1

)

Time (min)

25degCNaCl-TiO2

NaCl-SiO2

NaCl

(a)

35degCNaCl-TiO2

NaCl-SiO2

NaCl

Wat

er u

ptak

e (m

g gndash1

)

0 20 40 60 800

10

20

30

40

Time (min)

(b)

0 20 40 60 800

10

20

30

40

Time (min)

Wat

er u

ptak

e (m

g gndash1

)

45degCNaCl-TiO2

NaCl-SiO2

NaCl

(c)





310 315 320 325 330 335283032343638404244

ln P


NaCl

(a)

310 315 320 325 330 335T ndash1 (103 Kndash1)

283032343638404244

ln P

NaCl-TiO2

(b)

310 315 320 325 330 335T ndash1 (103 Kndash1)

283032343638404244

ln P

NaCl-SiO2

(c)


0 4000 8000 12000 16000 20000

34

36

38

40

42

44

46

Isos

teric

hea

t of a

dsor

ptio

n (k

Jmol

)

Water uptake (mgg)

NaCl-TiO2

NaCl-SiO2

NaCl

(a)

1000 2000 3000 4000 5000

34

36

38

40

42

44

46

Isos

teric

hea

t of a

dsor

ptio

n (k

Jmol

)

Water uptake (mgg)

NaCl-TiO2

NaCl-SiO2

NaCl

(b)







Aercondensation

Beforecondensation

Duringcondensation

NaCl NaCl

D0

NaCl

D


(a)

Aercondensation

Beforecondensation

Duringcondensation

NaClTiO2NaClTiO2

D0

NaClTiO2

D


(b)

Aercondensation

Beforecondensation

Duringcondensation

NaClSiO2


NaClSiO2

D0

NaClSiO2

D

(c)




4 Conclusions





Data Availability




Acknowledgments




References










0 10 20 30 40 50 60 70 80 90ndash10

ndash8

ndash6

ndash4

ndash2

0D

ispla

cem

ent (μm

)


NaCl-TiO2

NaCl-SiO2

NaCl













































+ Na+ SO42minus NO3




































310 315 320 325 330 335283032343638404244

ln P


NaCl

(a)

310 315 320 325 330 335T ndash1 (103 Kndash1)

283032343638404244

ln P

NaCl-TiO2

(b)

310 315 320 325 330 335T ndash1 (103 Kndash1)

283032343638404244

ln P

NaCl-SiO2

(c)


0 4000 8000 12000 16000 20000

34

36

38

40

42

44

46

Isos

teric

hea

t of a

dsor

ptio

n (k

Jmol

)

Water uptake (mgg)

NaCl-TiO2

NaCl-SiO2

NaCl

(a)

1000 2000 3000 4000 5000

34

36

38

40

42

44

46

Isos

teric

hea

t of a

dsor

ptio

n (k

Jmol

)

Water uptake (mgg)

NaCl-TiO2

NaCl-SiO2

NaCl

(b)







Aercondensation

Beforecondensation

Duringcondensation

NaCl NaCl

D0

NaCl

D


(a)

Aercondensation

Beforecondensation

Duringcondensation

NaClTiO2NaClTiO2

D0

NaClTiO2

D


(b)

Aercondensation

Beforecondensation

Duringcondensation

NaClSiO2


NaClSiO2

D0

NaClSiO2

D

(c)




4 Conclusions





Data Availability




Acknowledgments




References










0 10 20 30 40 50 60 70 80 90ndash10

ndash8

ndash6

ndash4

ndash2

0D

ispla

cem

ent (μm

)


NaCl-TiO2

NaCl-SiO2

NaCl













































+ Na+ SO42minus NO3






































Aercondensation

Beforecondensation

Duringcondensation

NaCl NaCl

D0

NaCl

D


(a)

Aercondensation

Beforecondensation

Duringcondensation

NaClTiO2NaClTiO2

D0

NaClTiO2

D


(b)

Aercondensation

Beforecondensation

Duringcondensation

NaClSiO2


NaClSiO2

D0

NaClSiO2

D

(c)




4 Conclusions





Data Availability




Acknowledgments




References










0 10 20 30 40 50 60 70 80 90ndash10

ndash8

ndash6

ndash4

ndash2

0D

ispla

cem

ent (μm

)


NaCl-TiO2

NaCl-SiO2

NaCl













































+ Na+ SO42minus NO3



































4 Conclusions





Data Availability




Acknowledgments




References










0 10 20 30 40 50 60 70 80 90ndash10

ndash8

ndash6

ndash4

ndash2

0D

ispla

cem

ent (μm

)


NaCl-TiO2

NaCl-SiO2

NaCl













































+ Na+ SO42minus NO3












































































+ Na+ SO42minus NO3


































adsorptioncapacitiesofhygroscopicmaterialsbasedon nacl-tio

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