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Colloids and Surfaces A: Physicochem. Eng. Aspects 414 (2012) 333– 338

Contents lists available at SciVerse ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

jo ur nal homep a ge: www.elsev ier .com/ locate /co lsur fa

ynthesis, characterization and nano-particles synthesis using a simple twoomponent supramolecular gelator: A step towards plausible mechanism ofydrogelation

riyanka Yadav, Amar Ballabh ∗

epartment of Chemistry, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara-390002 (Gujarat), India

i g h l i g h t s

Design, synthesis and characteriza-tion of new class of hydrogelators.Structure–property relationsbetween gelator and non-gelatormolecules.Single crystal X-ray structure of gela-tor molecule without gelling solvent.An insight into a probable mecha-nism of small molecule hydrogela-tion.Synthesis of silver nano-particlesusing newly synthesized hydrogela-tor as template.

g r a p h i c a l a b s t r a c t

r t i c l e i n f o

rticle history:eceived 2 July 2012eceived in revised form 5 August 2012ccepted 8 August 2012vailable online 3 September 2012

a b s t r a c t

Melamine-based mono carboxylate salts of aliphatic dicarboxylic acids are synthesized and character-ized for gelation behaviour towards various solvents including water. Salts 1a (melaminium hydrogenmaleate) and 1e (melaminium hydrogen adipate) were found to be outstanding hydrogelators. The pack-ing of gelator molecule 1a in gel and dried state was probed by single crystal and powder X-ray diffractionstudies. Single crystal X-ray study of gelator molecule with and without its gelling solvent suggested thatsupramolecular assemblies leading to a porous network may be one of the prerequisites for molecules

eywords:ydrogelationupramolecular assemblies-ray diffractionwo-component gelators

to show immobilization of water. An application of two-component gelator1a as a template for silvernano-particle synthesis was demonstrated.

© 2012 Elsevier B.V. All rights reserved.

ilver nano-particlesemplate-directed synthesis

. Introduction

Gels, a type of colloidal system, where liquid phase is immo-

ilized by a solid phase, is always an interesting system for studynd research, due to its intriguing properties, and numerous appli-ations such as templating agent for inorganic/organic materials,

∗ Corresponding author. Tel.: +91 265 2789747; fax: +91 265 2795569.E-mail addresses: bamar.chem@gmail.com, bamar-chem@msubaroda.ac.in

A. Ballabh).

927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.colsurfa.2012.08.032

opto-electronic, smart materials sensitive towards various exter-nal stimuli such as pH, light, sound and temperature [1–3]. Last twodecades have shown an exponential growth of thermo reversiblegels materials based on small molecules commonly known as lowmolecular mass organic gelators (LMOGs). Numerous gelling sys-tems (hydrogelator or organogelator) with a plethora of structuraldiversity are reported in the literature; however, most of them

are discovered by chance than design. Therefore, numerous scien-tists working in the area of research are dedicating their efforts tounderstand the probable mechanism of metastable gel formation inorganic solvent and/or water. Even though, considerable progress

3 : Phys

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34 P. Yadav, A. Ballabh / Colloids and Surfaces A

as been made in finding more and more fascinating applications ofMOGs, a working principle for designing new gelators with tailorade properties and applications, is still desired. In recent years,

working rule for designing and synthesis of a new organogelators established that one-dimensional (1D) hydrogen bond favourselation, as 1D hydrogen bonding may lead to the formation ofnisotropic growth of molecular assemblies, namely, ribbon, fibre,ods and tube, which further entangle among themselves througheak non-covalent interaction to give three-dimensional (3D)etwork capable of immobilizing organic solvents [3]. However,esigning a non-polymeric hydrogelator is still a formidable task.any questions need to be answered before a tailor made hydro-

elator is realized in future. Few of the fundamental questions arei) how hydrogen bonded supramolecular assembly get affected inhe presence of highly polar solvents like water? (ii) role of �–�nd van der Waals interactions in supporting hydrogelation, (iii)ow packing of a molecules in the crystalline state is related withacking in metastable gel state? To answer the last question, Weisst al. [4] had developed an indirect method to probe the packingf gelator molecule in gel, xerogel (dried gel) and bulk state byomparing powder X-ray diffractogram (XRD) of gelator moleculeith simulated single crystal of gelling agent, if any. This method

uffers from inherent drawbacks namely growth of single crystalf gelator molecule is mostly unsuccessful especially in its gellingolvent. Secondly, packing of gelator molecules in bulk crystallinetate is mostly different from xerogel or gel state [3], thirdly, PXRDf gel is masked by strong scattering from solvents making theiffractogram unreliable. Even though, the method developed byeiss et al. suffers from some shortcomings, it is frequently being

sed for elucidating the mechanism and probable packing of gelatorolecules in the gel state. A gelator molecule with lesser struc-

ural complexity and predictable hydrogen bonding aggregationattern may acts as a model system for understanding the probableechanism of hydrogelation using X-ray diffraction studies.One of most simple gelling systems are two-component gela-

or such as an organic acid/amine salts, charge transfer complexnd hydrogen bonded complex, which provide an opportu-ity to systematically vary one of the component and studytructure–property correlation. In recent years, 1,3,5-triazine moi-ties have appeared as one of the most robust supramolecularcaffold for designing new gelators [5–17]. Nandi et al. had discov-red many two component hydrogelators system using melamines one of the component [11–16]. This new class of supramolecu-ar hydrogelators [11–16] have many hydrogen donor and acceptorites which make the overall supramolecular assembly very intri-ate. As a result, it becomes very difficult to understand and predict

he role of various hydrogen bonding sites and probable van der

aals interaction in immobilization of solvents. So, hydrogelatorsased on a simple system with less number of hydrogen bondingites and functionality is advantageous, which may act as a model

N

N

N

NH2

NH 2H2N

H

HO

1

Scheme 1. List of organic sal

icochem. Eng. Aspects 414 (2012) 333– 338

system to decipher the probable mechanism of hydrogelation andultimately, leads to design of new hydrogelators with tuneableproperties. We decided to synthesize new organic salts of melaminewith few simple aliphatic dicarboxylic acids (Scheme 1), primar-ily due to following reasons, namely, (a) to understand the role of�–� interaction in supramolecular assemblies such as gel, (b) tounderstand the role of hydrophobic interaction (by varying chainlength of dicarboxylic acid) in highly polar environment, (c) role ofspatial arrangement of molecule (fumaric acid and maleic acid) onprobable packing and its effect on gelation behaviour, (d) ease ofsynthesis of organic salts as compared with other multistep syn-thesis of other class of gelators, (e) supramolecular synthons withless number of functional groups which can be predictably ascer-tained due to well known supramolecular synthons of melamineand related compounds [18], (f) to understand the probable rea-sons for the formation of hydrogel in melamine based system usingstructure–property correlation, and (g) application of gelator fibresas a template for nano-material synthesis.

In this paper, we report the synthesis and characterizationof various melamine based salts (1a–1g) with simple aliphaticdicarboxylic acids. Two salts (1a and 1e) turned out to be goodhydrogelators. Fortunately, we were able to grow single crystal of1a with and without gelling solvent i.e. water. Later on, literaturesearch revealed that the single crystals X-ray structure solved byus i.e. melaminium maleate monohydrate, matches exactly with aknown single crystal structure [19]. A detailed study was under-taken to establish the packing of gelator molecules in the gel state,xerogel state, bulk solid and compare it with simulated powder XRDpattern generated using single crystal of 1a (with or without water).To the best of our knowledge, this is a first report which directly cor-relate the structure of gelator molecule interacting with its gellingsolvent in the gel state and packing of the gelator molecules afterevaporation of solvent i.e. xerogel. Moreover, the gelator molecule1a was used for the synthesis of silver nano-particles by simple UVtreatment method of silver salt.

2. Materials and methods

2.1. Synthesis

2.1.1. Synthesis of salts (1a–1g)Various aliphatic dicarboxylic acids (a–g) (maleic acid, fumaric

acid, malonic acid, succinic acid, adipic acid, suberic acid andsebacic acid), and melamine were purchased from Aldrich. Theother chemicals were of the highest commercial grade availableand were used without further purification. The solvents used for

the preparation of gels were of reagent grade. All solvents used inthe synthesis were purified, dried and distilled as required.

Organic salts (1a–1g) were prepared by mixing a hot methanolsolution of dicarboxylic acids (a–g) with a hot aqueous solution of

COOHOOC

OC

COO

H2C

HOOC COOn

a

b

c-g

Where n=1,2,4.6,8

ts synthesized (1a–1g).

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P. Yadav, A. Ballabh / Colloids and Surfaces A

elamine in a 1:1 molar ratio. The mixtures were kept at ambi-nt temperature to cool and dried at rt. The crystalline powderbtained after drying was subjected to characterization by vari-us physicochemical analysis. The salt formation was confirmed byourier transform infrared spectroscopy (FT-IR). All the synthesizedalts showed the characteristic bands for C O symmetrical stretch-ng band for carboxylic group (1697–1720 cm−1) and carboxylate1650–1680 cm−1) indicating the formation of mono-carboxylatealts of dicarboxylic acids. The 1:1 stoichiometry of the synthesizedalts was further confirmed by elemental analyses (see Supportingnformation).

.1.2. Synthesis of silver nano-particlesSilver nano-particles were synthesized by the simple method of

eduction of silver salts using UV radiation [20,21]. Silver nitrate,gNO3(5 mg) and compound 1a (40 mg) were taken in a quartz test

ube containing water (2 mL) and heated till the solid dissolved.fter the formation of stable colourless gel at room temperature,

t was irradiated with a mercury vapour UV lamp. The silver saltoaded gel showed remarkable colour change from colourless toale pink within 15 min of irradiation. Prolong exposure of UV lightor an hour or so, on gel containing silver nano-particles displayedolour change from pale pink to dark brown.

.2. Gelation test

Salts 1a–1g were tested for their gelation behaviour in variousolvents of different polarity by test tube method. 10 mg of theample was taken in a test tube, and 1 mL of solvent was addedo it. The suspension of salt in solvent was heated in an oil-bath forew minutes till the solute dissolved completely. The hot solutionas kept at room temperature for observation. The inverted test-

ube method was used to verify the gel formation. Tgel (gel-to-solonversion temperature) was measured using a small glass beadsweighing about 63 mg) placed on the upper surface of gel formedn a glass tube. The test tube was heated in oil-bath till the glassall fell on the bottom of the test tube. The temperature at whichemi-solid mass lost its weight bearing capacity and gel convertnto a sol was noted as Tgel.

.3. SEM measurement

Hot solution of a gelator (50 �L) is placed on a SEM sampleolder and allowed to form gel, which was then dried under vac-um. The dried gel was then subjected to gold sputtering usingolaron SC 7620 sputter coater. The gold-coated sample of 1a wassed directly for viewing using LEO 1430VP SEM instrument. TheEM image of xerogel of 1e was recorded on JEOL JSM5610 LV SEMnstrument after carbon coating.

.4. FT-IR measurement

FT-IR spectra of salts were recorded on Perkin-Elmer RX-FTIR.ew milligrams of solid sample was grinded together with KBranhydrous) using mortar pestle and a pallet was made for FT-IRtudy. Liquids or gel sample were directly poured on KRs-5 windownd subjected for FT-IR measurement.

.5. Powder X-ray diffraction study

Powder diffraction patterns of neat gelator, xerogel (water,

lowly evaporated) and gel (water) of 1a was recorded on XPERThilips (CuK� radiation). The silver nano-particles loaded on gela-or fibres was subjected to powder diffraction studies using Bruker8 (CuK� radiation) diffractometer.

icochem. Eng. Aspects 414 (2012) 333– 338 335

2.6. Single crystal X-ray study of 1a

Crystal of 1a was obtained from ethanol in a slow evapora-tive condition at room temperature. Diffraction data for 1a wascollected using MoK� (� = 0.7107 A) radiation on a SMART APEXdiffractometer equipped with CCD area detector. Data collection,data reduction, structure solution/refinement was carried out usingthe software package of SMART APEX. Graphics was generatedusing MERCURY 3.0.

The structure of 1a was solved by direct methods and refinedin a routine manner. In all cases, non-hydrogen atoms aretreated anisotropically. Whenever possible, the hydrogen atomsare located on a difference Fourier map and refined. In other cases,the hydrogen atoms are geometrically fixed (for crystallographicparameters-see Supporting information) (CCDC No. 877950).

2.7. DRS (diffuse reflectance spectroscopy) study of nano-particles

The sample for DRS study was prepared by drying the gel con-taining silver nano-particles at room temperature. The powder wasmixed with BaSO4 and the spectrum was recorded using PerkinElmer UV-Visible Spectrometer.

2.8. TEM studies

TEM images of nano-particles loaded on gelator fibres wererecorded on Philips CM200 operated between working voltage of20–200 kV. Sample was prepared by drying the gel-phase networkcontaining the silver nano-particles under vacuum. The powderedsample was dispersed in water with the help of vigorous sonica-tion. Samples were deposited on carbon-coated copper grids anddirectly imaged after drying under IR lamp.

3. Results and discussion

3.1. Gelation studies

Salts synthesized were subjected to gelation test in various sol-vents with varied polarity ranging from highly polar solvents suchas water, ethanol, methanol to very less polar solvents such astoluene and dodecane (see supporting information Table No. 1).Out of seven salts synthesized two turned out to be good hydrogela-tors. Salts 1a and 1e showed good thermal stability (Tgel values lyingbetween 75 and 95 ◦C) and minimum gelation concentration (MGC)values greater than 1 wt%. A graph of Tgel verses concentration ofgelator molecules in wt% (w/v) showed a gradual increase in Tgelup to a certain concentration of gelator then plateau was observed.Understandably, the increase in gelator concentration improvesthe self-aggregation and stability of supramolecular assembly notbeyond certain critical concentration. The nature of graph for Tgelversus concentration of gelator is very frequent for supramoleculargelators (Fig. 1).

A critical look at the structures of gelator and non-gelatorsmolecules suggested few important observations such as saltsmade up of molecules having cis and trans orientation of dicar-boxylic groups (1a and 1b) is totally dissimilar in gelationbehaviour towards solvents, and some critical aliphatic carbonchain length was required to induce gelation behaviour throughvan der Waals/hydrophobic interaction. The spatial orientation ofmolecules may leads to a different overall packing in the threedimensional network, porous or interpenetrating network, which

seems to be a guiding principle for a molecule to show hydroge-lation or non-gelation. The salts containing less than four carbonatoms (1c and 1d) in aliphatic back bone turned out to be insuf-ficient to immobilized solvent through hydrophobic interaction

336 P. Yadav, A. Ballabh / Colloids and Surfaces A: Physicochem. Eng. Aspects 414 (2012) 333– 338

Ft

abal

3

cgmi

3

gdemhsvdgrmbdomeio

Fig. 3. Single crystal X-ray structures of 1a (A) with gelling solvent water (regen-erated using reference [19]). Red round circle represents water molecules in spacefill style and other molecules are shown in wireframe style and (B) without gelling

ig. 1. Effect of gelators 1a and 1e concentration (in wt%) on sol-to-gel transitionemperature (Tgel).

nd surface tension. It is also noteworthy that a long chain car-on chain (1f and 1g) disturbs the delicate balance of hydrophilicnd hydrophobic interaction of gelator molecules with solvents andeads to non-gelation of solvents.

.2. SEM characterization

To get an insight into the gelator morphology SEM studies werearried out on xerogel (dried gel) of 1a and 1e. Xerogel of bothelators showed entangled fibrous network of molecules whichight have immobilized the solvent through various non-covalent

nteractions (Fig. 2).

.3. X-ray diffraction studies

The structure–property correlation between molecules and itselling behaviour was established by single crystal and powderiffraction studies. Fortunately, 1a gave crystals in water andthanol as melaminium maleate monohydrate and melaminiumaleate. The crystal structure of melaminium maleate mono-

ydrate found to be exactly matching with the known crystaltructure [19]. However, the single crystal of 1a without sol-ent turned out to be a new structure, which prompted us too a comparative study of gelator molecule interacting with itselling solvent in the gel state with xerogel state. Literature searchevealed that very few incidences to probe the packing of gelatorolecules interacting with its gelling solvent were successful either

y single crystal studies [10,22,23] or ab initio crystal structure pre-iction [17]. We decided to compare the single crystal structuref melaminium maleate monohydrate structure and melaminium

aleate as shown in Fig. 3. The structure reported by Janczak

t al. [19] showed a three-dimensional robust network capable ofmmobilizing water in the void whereas as packing of 1a with-ut water seems very different from the hydrous structure. The

Fig. 2. SEM images of xerogel of (A) 1a a

solvent (water). Hydrogen atoms are omitted for clarity in (A). (For interpretationof the references to colour in this figure legend, the reader is referred to the webversion of this article.)

three-dimensional hydrogen bonding pattern of melaminiummaleate salt showed no space for any solvent to occupy.

A detailed study of packing of 1a in gelling solvent (gel state),pure crystalline state, xerogel state was carried out using powderX-ray diffraction method, which was further compared with simu-lated powder X-ray pattern of 1a·H2O and 1a single crystal data asshown in Fig. 4.The XRD pattern of gel of 1a in water showed somepeaks (2� = 9.37, 10.86, 12.68, 15.28, 17.63, 20.73) which matchedwell with the peaks in simulated hydrated single crystal. XRD peaksof 1a in gel state at higher angle were difficult to identify due tostrong scattering from water molecules. An important peak in allthe powder XRD pattern of 1a at 2� equal to 27.7◦ correspondsto “d” spacing of 3.2 A, is representing � � interaction betweenmelamine molecules [17].

3.4. Synthesis and characterization of silver nano-particles

Organogelators or hydrogelators are preferable choice astemplate for nano-materials synthesis due to well-defined

nd (B) 1e in water at 2 wt% (w/v).

P. Yadav, A. Ballabh / Colloids and Surfaces A: Physicochem. Eng. Aspects 414 (2012) 333– 338 337

Fig. 4. Powder XRD pattern of salts 1a in gel state, dried gel(xerogel), bulk solid,s

sadgpstX

pp

imulated powder pattern obtained from single crystal X-ray data.

upramolecular assembly of gelator molecules in a solvent suchs fibres, rods and tubes [24]. We used gelator 1a as structureirecting agent for silver nano-material synthesis. Silver salt loadedels were subjected to UV treatment and formation of silver nano-article was visibly evident as seen in Fig. 5. The formation ofilver nano-particles was confirmed by various physicochemicalechniques such as diffuse reflectance spectroscopy (DRS), powder-ray diffraction and TEM.

DRS spectra (Fig. 6a) showed the plasma absorption with aeak maximum at 414 nm indicating the formation of silver nano-articles [25,26].

Fig. 6. (A) DRS spectra of the silver nano-particles

Fig. 7. TEM image of silver nano-particle

Fig. 5. Photographic image of silver nano-particles within the gel.

The structure of silver nano-particle was further investigatedby XRD analysis. A typical XRD pattern of the silver nano-particleis shown in Fig. 6b.Two distinct diffraction peaks were observed at2� values of 39.5 and 45.6, corresponding to the (1 1 1) and (2 0 0)crystalline planes, respectively [27]. The broad nature of the XRDpeaks could be attributed to the nano-size of the particles.

The formation of nano-particles of silver can be visuallyobserved through a continuous colour change from colourless topale brown within 15 min, and finally to dark brown in 60 min.Morphologies of the gelators 1a and 1e was characterized by SEM.The TEM images of the silver nano-particles within the gel networkare shown in Fig. 7.The size of silver nano-particles attached withgelator fibres was found to be in the range of 10–20 nm.

In the present case, the gelators fibres are made up of chargedspecies such as melaminium and mono-carboxylate, the overall

supramolecular assemblies may have some charge, which mightbe responsible for attracting some silver ion towards itself. The sil-ver ion anchored with gelator fibres should be acting as nucleating

and (B) XRD pattern of silver nano-particles.

s formed within the xerogel fibre.

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38 P. Yadav, A. Ballabh / Colloids and Surfaces A

ite where the growth of silver nano-particles were taking placefter UV irradiation.

. Conclusions

We have demonstrated two new LMOGs salts melaminiumydrogen maleate (1a) and melaminium hydrogen adipate (1e) as

novel class of non-polymeric hydrogelator. The ease of synthe-is of these salts in almost quantitative yield gives an advantagever other class of melamine-based hydrogelator/organogelators6–9,28,29] which may require tedious multi-step synthesis. To theest of our knowledge these salts are the lowest molecular weightelamine based bi-component gelators. Moreover, the formation

f porous hydrogen bonded network in hydrated structure of 1aeems to be one of the prerequisites for hydrogelation and thisbservation matches well with the observation made by from var-ous research groups [17,22,23]. XRD pattern of gelator 1a matchesemarkably well with bulk crystalline, xerogel and hydrated singlerystal (simulated) recognizes a similar morph and porous networkn all these states. However, position and intensity of few peaks inhe XRD of 1a in the gel state suggests that the packing of gelator

olecules may be similar to its xerogel and crystalline state butot necessary be identical. On the other hand, simulated XRD pat-ern of unhydrated single crystal showed a different diffractogramhan simulated XRD of hydrated single crystal of 1a, prompted uso believe that the excessive heating and drying of gel may leado irreversible breakdown of the porous network and formation ofew hydrogen bonded three-dimensional system represented byingle crystal of 1a (without solvent). We propose molecules capa-le of forming three-dimensional supramolecular assembly withoid may act as a potent hydrogelator. A designing strategy ofupramolecular synthesis of porous network may lead to the dis-overy of new aqueous gelling agent with tunable and predictableroperty. The success of salt 1a as a structure directing agent forpherical silver nano-particles in the range of 10–20 nm, may bettributed to the overall charged supramolecular assembly of 1a aslso demonstrated by Weiss et al. [30]. The colloidal suspension ofilver nano-particles stabilized by charged supramolecular assem-ly of 1a in water was found to be stable for few months at roomemperature. Work is underway to understand the role of electro-tatic interaction of gelator fibres on nano-materials synthesis andow to control the shape, size and morphology of nano-materialssing organic salts based gelators.

cknowledgments

P.Y. is thankful to UGC, New Delhi for UGC-RFSMS fellowshipnd A.B. would like to acknowledge UGC, New Delhi for financialupport [Major Research Project-F. No. 37-379/2009(SR)]

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.colsurfa.2012.08.032.

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