Pbc

FD

a

ARR1AA

KGSMSS1

V

1

wfonabsqwloi

iwtb

0d

Colloids and Surfaces A: Physicochem. Eng. Aspects 394 (2012) 46– 56

Contents lists available at SciVerse ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

journa l h omepa g e: www.elsev ier .com/ locate /co lsur fa

hysicochemical study of cationic gemini surfactantutanediyl-1,4-bis(dimethyldodecylammonium bromide) with variousounterions in aqueous solution

arah Khan, Umme Salma Siddiqui, Iqrar Ahmad Khan, Kabir-ud-Din ∗

epartment of Chemistry, Aligarh Muslim University, Aligarh 202 002, U.P., India

r t i c l e i n f o

rticle history:eceived 1 October 2011eceived in revised form6 November 2011ccepted 18 November 2011vailable online 26 November 2011

eywords:

a b s t r a c t

The effect of salts (inorganic and organic) on the characteristic solution properties of bis(quaternaryammonium) gemini surfactant butanediyl-1,4-bis(dimethyldodecylammonium bromide) (referred to as12-4-12) was explored. The results showed that salt counterions induce synergistic effects and greatlyenhance the efficiency of gemini in surface tension reduction as well as the ability of micellization.Furthermore, combinations of salt anions and gemini exhibited thickening of their aqueous solutions.The aggregate morphology is strongly dependent on the nature and size of the counterions. These werealso attributed to the unique molecular structure of gemini surfactant, where the spacer (polymethylene

emini surfactantalticellization

ynergismurface tensionH NMR

chain) links the two quaternary ammonium head groups. The interaction and micellar growth of cationicgemini-salt systems with the inorganic salts have been found to obey Hofmeister series. Also, the anionsof organic salts promote the hydrophobic interaction between the alkyl tails of gemini surfactant. Inaddition, the orientation of the substituents in the aromatic fragment is important too.

© 2011 Elsevier B.V. All rights reserved.

iscosity

. Introduction

The micelle forming surfactants have the ability to catalyze aide variety of reactions, which serve as valuable model processes

or the study of microenvironmental factors affecting the efficiencyf chemical transformations in the realm of science world [1]. Aseutral salts have been found to tune the conformations of proteinsnd other macromolecules by modifying the prevalent hydropho-ic or ionic interactions, exploring the effect of various kinds ofalts in different fields of biology and chemistry is continuing sinceuite long. The processes are usually explained on the basis ofater structure disruption around the amphiphilic compounds by

yophilic salts [2]. The amphiphilic compounds thus become des-lvated and are bound to aggregate or form micelles which resultsn a decrease of CMC and an increase in aggregation number.

There are two opposing tendencies in the micelle formation ofonic surfactants. The removal of hydrocarbon chains from water,

hich favors aggregation, and the electrostatic repulsions amonghe ionic head groups, which disfavor aggregation. A subtle balanceetween the two tendencies makes the system stable. Counterions

∗ Corresponding author. Tel.: +91 571 2700920x3353.E-mail address: kabir7@rediffmail.com ( Kabir-ud-Din).

927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2011.11.024

have the ability to stabilize the ionic surfactant micelles by bindingto the micelles and screening the electrostatic repulsion. Hence, thebinding affinity of a particular counterion influences the process ofmicellization and aggregation.

On the whole, the formation of spherical micelles by theaggregation of monomers in aqueous solutions takes place at con-centrations above a critical micelle concentration (CMC I). In manysystems, at higher concentrations, a transition from spherical torod-like micelles occurs, which is referred to as CMC II. The micellargrowth is mainly favored by the screening of electrostatic repulsionamong the polar head groups and movement of the hydrophobicalkyl chains away from the aqueous environment. This is evidencedby an increase of the micelle aggregation number [3].

Generally, the size and nature of the counterions decides theextent of their influence on the shape and size of micelles. Manyattempts have been made to examine the effect of salts on micelleformation in the light of Hofmeister (lyotropic) series [4]. Ions canbe classified according to their effectiveness as either salting-inor salting-out agents. The Hofmeister series plays vital role in awide range of biological and physicochemical phenomenon, viz. theaction of ion channels in biological membranes, the surface tension

of electrolyte solution, amphiphile micellization, etc.

Organic salts with aromatic carboxylate counterions such asbenzoate and salicylate, habitually called as hydrotropes, aresurface-active and highly water soluble, which can increase the

Physic

sshgssoi

‘hhimpeaCbss

ttcssaDcns

absoc[owbavarImthmftp

sisemo1po(t

viscometer, suspended vertically in a thermostat at 303 ± 0.1 K, asdescribed earlier [27]. The viscometer was cleaned and dried every

F. Khan et al. / Colloids and Surfaces A:

olubility of sparingly soluble solutes in water. Hydrotropes havetructures somewhat similar to surfactants in that they haveydrophilic and hydrophobic groups but, as the hydrophobicroup is generally short, cyclic, and/or branched, they differ fromurfactants. These counterions interact with the micelle-formingurfactants electrostatically as well as hydrophobically, and therientation of hydrophobic moeity at the micellar surface is alsomportant.

Interest herein is focused to third generation surfactants, theGeminis’. These consist of two hydrocarbon tails and two polaread groups connected through a spacer. The gemini surfactantsave been proven better as compared to their conventional analogs

n having higher efficiency in lowering surface tension, possessinguch lower critical micelle concentration (CMC), better wetting

roperties, showing specific rheological and aggregation prop-rties, etc. [5–10]. They are also referred as ‘green surfactants’,s less amount of geminis are used owing to their very lowMC values. Thus, their usage as effective emulsifiers, dispersants,actericidal agents, antifoaming agents, detergents, etc., is a con-equence of their advance features in comparison to conventionalurfactants.

Almost invariably, surfactants are used in presence of addi-ives because of the enhanced performance of the mixtures dueo synergism. Thus, it would be of great relevance to find newombinations where synergism exists. The vitality of the effect ofalts on the aggregation behaviors of ionic surfactants in aqueousolutions is due to a wide range of applications for detergencynd emulsification in industry. A number of studies by Kabir-ud-in et al. [11–20] on the effect of additives (organic/inorganicompounds, nonelectrolytes, surfactants, etc.) using different tech-iques yielded significant results in the field of physicochemicaltudy in gemini solutions.

One of the most powerful methods for investigating dynamicsnd structure at the molecular level is NMR spectroscopy which haseen used to characterize organized assemblies in aqueous micellarystems. Thus, it is being used to probe the location and orientationf molecules in and around the micelles by means of chemi-al shift changes for surfactant and additive proton resonances21,22]. Changes in the chemical shift of the observed resonancesf the –N+CH3 protons clearly depict the electrostatic shieldinghich accompanies the formation of large and organized assem-

lies. The NMR investigations on the micellization and variousggregates on cationic gemini surfactants have been reported pre-iously [12,14,20,23,24]. However, only swollen spherical micellesre produced by the solubilization of additives in the micellar inte-ior [25] and does not contribute much towards micellar growth.n this regard, viscometry has been found very sensitive to the

icellar morphology of macroscopic objects in a colloidal solu-ion. The presence of rod-shaped micelles gives solutions a veryigh viscosity which might be of importance in industrial for-ulations of detergent solutions. These methods may be helpful

or interpreting the relationship between the structure of addi-ive and morphological transition of a surfactant resulting in theirresence.

In our previous study [20], the influence of inorganic/organicalts on the overall micellar structural changes and possible viscos-ty changes in three cationic gemini surfactant solutions (14-s-14,

= 4–6) were studied by 1H NMR and viscosity techniques. How-ver, for a proper understanding of the knowledge of fundamentalicellar solution properties, in the present paper, we report a study

f the micellization and morphological changes of butanediyl-,4-bis(dimethyldodecylammonium bromide) (12-4-12) gemini in

resence of inorganic and organic salts, having different counteri-ns (Scheme 1). These counterions can be divided into two groups:1) inorganic counterions (Br−, NO3

−, SCN−), which are principallyaken from the Hofmeister series, and (2) aromatic carboxylate

ochem. Eng. Aspects 394 (2012) 46– 56 47

counterions (Benz−, Sal−). Surface tension measurements were alsomade to investigate the effect of the above counterions on theadsorption and micellization of 12-4-12 in aqueous solutions. Theresults indeed show a high sensitivity of the self-assemblies of 12-4-12 to a variety of given salts.

2. Materials and methods

2.1. Materials

1,4-dibromobutane (≥98%, Fluka, USA), N,N-dimethyldodecylamine (≥97%, Fluka, Switzerland), ethylacetate(HPLC and Spectroscopy grade, ≥99.7%, Merck, Mumbai), ethanolabsolute (99.8%, E. Merck, Germany), KBr (99%, Merck, India), KNO3(≥99%, Merck, India), KSCN (≥98%, Merck, India), NaBenz (99.5%,Merck, India), NaSal (99.5%, CDH, India) were used as received.

2.2. Synthesis of gemini

The compound 12-4-12 was synthesized by the reflux reactionof a mixture of N,N-dimethyldodecylamine and 1,4-dibromobutane(molar ratio 2.1:1) in dry ethanol at 353 K for 48 h. After removalof the solvent under vacuum, the solid obtained was recrystal-lized four to five times from hexane/ethyl acetate mixture forpurification of the surfactant. It was further characterized by1H NMR analysis. The observed CMC value (1.04 mM) as wellas the NMR data is in good agreement to the earlier literature[8,9,26].

2.3. Surface tension measurements

The surface tension (�) measurements were performed by thering detachment method using a Du Nouy type tensiometer (Hard-son and Co., Kolkata) at 303 K. For each set of experiments, the ringwas cleaned by heating it in alcohol flame. The CMC values wereestimated as intersections of two linear segments, above and belowthe CMC of surface tension vs. log [surfactant] plots. The valuesof surface tension decrease continuously and then become almostconstant along a wide concentration range. The uncertainties on theCMC were estimated to be less than (±0.1–0.3) × 10−5 mol dm−3.The instrument was calibrated against double distilled water at thetime of measurement.

2.4. 1H NMR measurements

The 1H NMR spectra were obtained with a Bruker Avance 400Spectrometer at a proton resonance frequency of 400 MHz at 298 K.D2O (Aldrich, 99.9%) was used to prepare the stock solutions ofgeminis in the absence and presence of salts. About 1 ml of eachsolution was transferred to a 5 mm NMR tube for measurements.Chemical shifts were recorded on the ı (ppm) scale. Tetramethyl-silane (TMS) was used as internal standard. The reproducibility ofchemical shifts was within 0.01 ppm. The line widths at half heightswere measured from spectra and are accurate to ±0.1 Hz.

2.5. Viscosity measurements

Viscosity measurements were carried out using an Ubbelohde

time before use. In order to check the reproducibility, the time of fallfor every viscosity measurement was noted at least two times witha calibrated stopwatch. By doing so, it was found that the viscosityvalues were reproducible within ±0.1%.

48 F. Khan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 394 (2012) 46– 56

S hyldoc

3

3

issfiipN3aoiwsttobsptpiim

cheme 1. Molecular structure of (a) gemini surfactant butanediyl-1,4-bis(dimetarboxylate counterions.

. Results and discussion

.1. Salt effect on micellization and surface activity of gemini

Salt effect is a resultant of various factors acting together includ-ng mainly the screening of electrostatic repulsion, commonlytabilizing the micellar aggregates, and the reduction in alkyl chainolubility [28,29]. The formation and the growth of micelles areavored by the salt addition [30,31], which shows the expectedncrease in surface activity accompanying the lower CMC with thencrease of the salt concentration. The surface tension values of theure gemini as well as of 12-4-12/salt (KBr, KNO3, KSCN, NaBenz,aSal) solutions of different concentrations were measured at03 K (Fig. 1). It is well known that the micellization behaviornd surface activity of an ionic surfactant is sensitively dependentn the chosen counterion [32,33]. The CMC values of pure gem-ni surfactant, obtained from Fig. 1 plots, are in good agreement

ith the available literature [8,9,26]. It is clearly observed that theurface tension decreases with an increase in surfactant concen-ration. At low concentrations the surfactant molecules adsorb athe liquid/air interface until the surface of the solution is totallyccupied. Then the excess molecules tend to self-associate in theulk solution to form micelles, resulting in the constancy of theurface tension. The CMC values obtained from the intersectionoints of �–log (concentration) plots for each additive concentra-ion are given in Table 1. Fig. 2(a and b) shows these CMC values

lotted against the concentration of the added salts. The decrease

n CMC values of the gemini surfactant (Fig. 2 and Table 1) with thencrease in salt concentration is due to the ‘synergistic effect’ for

ixtures of cationic gemini surfactant and the salt counterions. The

decylammonium bromide), 12-4-12, (b) inorganic counterions, and (c) aromatic

CMC values decrease upon increasing salt concentration due to thereduction in the electrostatic repulsion between the intermolecularheadgroups, thus favoring micellization. The electrostatic repulsionmay become almost invariable at high salt concentration, then theCMC values become almost constant [34]. The difference in CMCvalues is found as a function of the nature of counterions. The inor-ganic ions have been found to obey the Hofmeister series, whichis a measure of the ability of the ions to denature protein, thestronger ions have been placed higher on the list. The observed CMCforming efficiency order of the anions in relation to 12-4-12 wasSal− > Benz− > SCN− > NO3

− > Br−. Both NaBenz and NaSal decreasethe CMC values, indicating that C6H5COO− and C6H4(OH)COO−

anions can greatly enhance the close packing of the cationic geminisurfactant molecules at air–water interface. Moreover, the effi-ciency in reducing the surface tension without salt was much lower,indicating that the packing of the surfactant molecules in the micel-lar structure at zero salt concentration was not as tight as that inmicelles formed in the presence of salts. Without effective shieldingby salt, the two charges on the gemini head groups keep the surfac-tant molecules away from each other due to electrostatic repulsion.When salt was added to screen the effective charges in the headgroup, the electrostatic repulsion was weakened and its workingrange was also shortened, making the hydrophobic interactionsrelatively stronger.

The primary driving force in micellization is the hydrophobiceffect associated with the chain association [35] by promoting the

release of water molecules which solvate the apolar chain. Theamphiphilic monomers are gathered by hydrophobic effect whichis a result of a net entropy increase of the whole solution. The bal-ancing force is the electrostatic force between the surfactant head

F. Khan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 394 (2012) 46– 56 49

0.50.0-0.5-1.0-1.5

40

50

60

70a

γ ( m

Nm

-1)

log [surfactant]

[KB r] (mM) 0. 0 1. 0 2. 0 5. 0 8. 0

0.50.0-0.5-1.0-1.535

40

45

50

55

60

65

70b

γ (m

Nm

-1)

log [surfactant]

[KNO3] ( mM) 0.0 1.0 2.0 5.0 8.0

0.50.0-0.5-1.0-1.5

40

50

60

70

c

γ ( m

Nm

-1)

log [surfactant]

[K SCN] (mM) 0.0 1.0 2.0 5.0 8.0

0.50.0-0.5-1.0-1.5-2.030

40

50

60

70

d

γ ( m

Nm

-1)

log [surfactant]

[NaBen z] (m M) 0.0 1.0 2.0 5.0 8.0

0.50.0-0.5-1.0-1.5-2.0-2.5

32

40

48

56

64

72e

γ ( m

Nm

-1)

log [surfactant]

[Na Sal] ( mM) 0.0 1.0 2.0 5.0 8.0

F tion oK

gPfricmpp

ig. 1. Representative plots for the variation of surface tension (�) with concentraNO3, (c) KSCN, (d) NaBenz, and (e) NaSal.

roups with their counterions and water at the micellar surface.olar and neutral ion pairs, that are less hydrated than free ions, areormed between the hydrated and charged headgroups and counte-ions. As a result, water is released into bulk solution with increasen entropy. These interactions depend on the hydrophobicity of the

ounterions. Hydrophobic counterions have tendency to interactore strongly with micellar interface, resulting in stronger ion-

air formation, which favors micellization, and lowers CMC. In theresent case also it is reflected by the results shown in Table 1.

f gemini surfactant 12-4-12 at different fixed concentrations of salts: (a) KBr, (b)

In case of ionic gemini surfactants, the added counterions, inaddition to the normal effect of shortening the distance betweenthe heads (due to reduction of electrostatic repulsion) and pro-moting the hydrophobic interaction, exhibit their due share on thespacer as well which further promotes the hydrophobic interac-

tion between the spacer and the alkyl tails and thus spacer chainstrongly folds [36]. As a result, the gemini molecules are more andmore tightly packed at the interface and the �CMC considerablydecreases with increasing concentration of salt. The decrease in

50 F. Khan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 394 (2012) 46– 56

Table 1CMC, C20, CMC/C20, ˘CMC, � max, Amin, �G0

m, �G0ads

for cationic gemini surfactant 12-4-12 in presence of salts at 303 K.

Additive (mM) CMC (mM) C20 (mM) CMC/C20 ПCMC (mN m−1) 107 � max (mol m−2) Amin ( ´̊A2) −�G0m (kJ mol−1) −�G0

ads(kJ mol−1)

KBr0 1.04 0.870 1.20 20.37 9.88 167.90 27.42 29.481 0.78 0.370 2.09 25.21 10.07 164.75 28.14 30.652 0.69 0.280 2.44 26.48 11.02 150.66 28.46 30.865 0.61 0.220 2.74 27.23 11.25 147.58 28.75 31.178 0.54 0.100 5.16 30.52 11.62 142.77 29.06 31.68KNO3

1 0.64 0.290 2.20 25.54 10.29 161.34 28.64 31.142 0.61 0.280 2.21 26.75 11.28 147.13 28.74 31.115 0.59 0.240 2.44 27.87 11.69 142.00 28.86 31.248 0.51 0.110 4.52 32.33 11.97 138.66 29.18 31.88KSCN1 0.56 0.180 3.11 29.82 10.38 159.91 28.99 31.862 0.51 0.110 4.46 31.44 12.20 135.98 29.19 31.775 0.49 0.080 5.89 33.56 13.23 125.42 29.32 31.858 0.38 0.050 7.09 34.95 14.79 112.22 29.93 32.29NaBenz1 0.50 0.150 3.25 32.45 12.13 136.82 29.26 31.932 0.46 0.100 4.40 34.57 14.40 115.21 29.48 31.885 0.40 0.040 8.76 35.87 14.63 113.45 29.80 32.268 0.36 0.030 11.06 37.62 14.84 111.87 30.05 32.59NaSal1 0.33 0.100 3.20 33.27 12.65 131.17 30.28 32.91

�n

f(Gngap

(

2 0.26 0.050 5.63 35.46

5 0.09 0.020 4.55 37.08

8 0.07 0.010 5.75 39.65

CMC value indicates that addition of salts enhances the effective-ess of 12-4-12 in surface tension reduction.∏

CMC (the surface pressure at CMC), � max (the maximum sur-ace excess), Amin (the minimum surface area per molecule), �G0

mthe standard Gibbs energy of micellization), �G0

ads (the standardibbs energy of adsorption) (see Supporting Information for defi-ition of terms and the equations used to evaluate the parametersiven in Table 1) values obtained at different concentrations ofdded salts in 12-4-12 solutions are given in Table 1. The followingoints emerge:

(i) With the increasing salt concentration, the∏

CMC increases

(see Table 1).

(ii) Compared to pure gemini surfactant solutions, the solutionswith salts have a greater preference to be adsorbed at air/waterinterface. In the presence of salts, the repulsion among the head

864200.30

0.45

0.60

0.75

0.90

1.05a

CM

C (m

M)

[salt] (mM)

KBr KNO3 KSCN

Fig. 2. Values of CMC of the gemini surfactant 12-4-12 at differen

14.88 111.57 30.86 33.2417.31 95.90 33.55 35.6920.44 81.21 34.25 36.19

groups decreases and causes the adsorption of more geminimolecules at the interface.

iii) With the addition of salts, as the values of � max increase, thevalues of Amin decrease and the trend is followed in all thecases. The progressive charge shielding and closer packing ofthe gemini surfactant ions in the surface cause a decrease inthe Amin. This suggests that the orientation of the gemini sur-factant molecule at the interface is almost perpendicular to theinterface [37].

(iv) The �G0m and �G0

ads decrease with increasing the salt concen-trations. The standard state for the surfactant is a hypotheticalmonolayer at its minimum surface area per molecule, but atzero surface pressure. Hydrophobicity is the main cause for

adsorption, which leads an amphiphile towards the air/waterinterface. All the �G0

m and �G0ads values are negative, which

imply that the adsorption of the surfactants at the air/mixtureinterface takes place spontaneously.

86420

0.0

0.2

0.4

0.6

0.8

1.0

b

CM

C (m

M)

[salt] (mM)

NaBenz NaSal

t concentrations of (a) inorganic salts and (b) organic salts.

F. Khan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 394 (2012) 46– 56 51

Table 2Micellar compositions (X1

m, X�1 ), interaction parameters (ˇm, ˇ� ), and activity coefficients (f1m, f2m, f1� , f2� ) for mixed gemini surfactant 12-4-12–salt systems.

˛salt X1m ˇm f1m f2m X1

� ˇ� f1� f2�

NaBenz0.06 0.213 −12.39 0.000462 0.569763 0.275 −17.49 0.000101 0.2662300.12 0.236 −12.41 0.000718 0.500443 0.305 −18.82 0.000113 0.1732410.25 0.271 −12.73 0.001153 0.392622 0.344 −21.14 0.000111 0.0818670.35 0.294 −13.27 0.001339 0.317511 0.367 −23.28 0.000088 0.043445NaSal0.06 0.246 −15.49 0.000150 0.391590 0.293 −20.87 0.000029 0.1665500.12 0.273 −16.12 0.000199 0.300652 0.327 −23.42 0.000024 0.081653

6666

6880

3

tarstetIiagfbpaEmo5

stˇ(psfait

0.25 0.337 −20.57 0.000118 0.090.35 0.354 −21.58 0.000122 0.06

.2. Surfactant–organic salt counterion interactions

In comparison to inorganic salts, besides electrostatic interac-ion, organic salts have additional hydrophobic interaction [38]. Theromatic counterions have aptitude to penetrate the head groupegion leading to micellar growth at lower loading of the micellarurface as compared to less weakly penetrating inorganic coun-erions. They induce strong hydrophobic interaction and reducelectrostatic repulsion between the cationic headgroups, leading toightly packed and reduced curvature surfactant aggregates [39].n general, the CMC values found to decrease with the increasen mole fraction of the hydrotropes. The results indicate that thedded hydrotropes are assisting in the micelle formation of theemini surfactants, as they form mixed micelles with gemini sur-actants. Therefore, in order to investigate the nature of interactionetween the constituents in the mixed micelles, the interactionarameters (ˇm and ˇ�) have been calculated for mixed micellesnd mixed monolayer, which are given in Table 2 (calculated usingqs. (S6)–(S9)). The interaction parameter for mixed micelle for-ation is calculated using Rubingh’s theory [40]. The CMC values

f NaBenz and NaSal used in the calculation are 320 mM and60.1 mM, respectively [41].

A positive ̌ value means repulsive interaction among mixedpecies, whereas a negative ̌ value means an attractive interac-ion; the more negative its value, the greater the interaction. Them values are negative at all mole fractions of the mixed systems

Table 2), suggesting that the interaction between the two com-onents is more attractive in mixed micelles as compared to theelf-interaction of the two components before mixing. As the mole

raction of organic salts increases, the ˇm values become more neg-tive. There is an increase in the attractive interaction with increasen salt concentration which is due to the intercalation of the salts inhe micelles of the gemini surfactants [40], resulting in an increase

Fig. 3. 400 MHz 1H NMR spectrum of 50 mM 12

0.359 −25.99 0.000022 0.0350550.378 −28.35 0.000017 0.017398

in the hydrophobic interactions. It is also evident by a decrease inCMC values with increasing salt concentration (Table 1).

Rosen’s approach reveals increased synergism in mixed mono-layer as compared to that in mixed micelles which gets enhancedwith the addition of salt. The ˇ� also shows the similar trend(Table 2), i.e., the mixtures of organic salt/gemini surfactant showstronger attractive interaction at the solution/air interface. This isdue to the steric factor which is more important in micelle forma-tion than in monolayer formation at a planar interface. Increasedbulkiness in the hydrophobic group causes greater difficulty forincorporation into the curved mixed micelle compared to that ofaccommodating at planar interface.

3.3. Salt effect on the morphology of gemini surfactant

In order to study the morphological changes caused by theaddition of salts, 1H NMR and viscometric measurements wereperformed well above the CMC values, as both of these are theconvenient methods for the simultaneous monitoring of morpho-logical transition in aggregates. A signature effect is observed onthe interaction of the salt anions with the cationic gemini 12-4-12,as revealed by 1H NMR studies.

The 1H NMR spectrum of pure 50 mM 12-4-12 in D2O is rep-resented in Fig. 3. The various protons attached to carbon atomsare labeled. Fig. 4(a) shows the variation of line width at half height(lw) of the signal relative to –N+CH3 group versus [12–4–12] and theresults are also confirmed by the viscosity measurements (Fig. 4b).The concentration of gemini surfactant is much higher (≥43 times)than its CMC, which ensures that the observed chemical shifts are

of aggregates of gemini surfactants. Tables S1 and S2 represent thechemical shifts in the 1H NMR spectra of 12-4-12 in presence ofinorganic and organic salt counterions. The nature of the added saltsalters the chemical environment of gemini leading to the variations

-4-12 gemini surfactant in D2O at 298 K.

52 F. Khan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 394 (2012) 46– 56

20015010050020

25

30

35

40

45a

line

wid

th (H

z)

[surfactant] (mM)

250200150100500

0

10

20

30

40

50

60b

ηr

[surfactant] (mM)

F ht of ta

os[trptwwiagiFnv

t(i

F2

ig. 4. Influence of gemini concentration on variations in (a) line width at half heignd (b) relative viscosities at 303 K.

f chemical shifts (ı), line width at half height (lw), and linehapes, thus signifying the micellar growth of gemini surfactant21,22,42–44]. A variation in the spin lattice/spin–spin relaxationimes changes line width, thereby, providing information regardingelative mobility of particular groupings within different micellarhases. Due to the slow mobility of the rod-like micelles (segmen-al motions or rotation) relaxation time decreases, as a result, lineidth increases (Fig. 5a). Thus, a peak broadening or increase in lineidth signifies the structural transition from spherical to nonspher-

cal micelles [45]. In the present case, the chemical shift variationsre more pronounced for the protons located near cationic headroups of gemini surfactant as revealed in Fig. 6 (the values of chem-cal shifts for other peaks are given in Table S1 and also shown inigs. S1–S3). This was the reason for considering –N+CH3 group sig-als for the observed chemical shift changes on surfactant or saltariation.

As observed from our results, upon addition of inorganic salts,

he chemical shift values of all the protons of gemini shift downfieldFigs. S1–S3). For the present surfactant, the variations of chem-cal shifts are different for different salts. It clearly indicates that

50403020100

32

34

36

38

40

42

44

a

line

wid

th (H

z)

[salt] (mM)

KB r KNO3 KS CN

b

η

ig. 5. Influence of salt concentration on variations in (a) line width at half height of th98 K, and (b) relative viscosities at 303 K.

he 1H NMR signal corresponding to the –N+CH3 group of 12-4-12 gemini at 298 K,

the saturation of Br− and NO3− ions at the double layer of gemini

surfactant aggregates occurs and beyond this concentration theanions would remain free in the bulk solution. Thus, due to thereduction of the surface electrostatic potential between the sur-factant head groups, surfactant aggregation is promoted and adownfield shift of proton is observed. However, on the addition ofKSCN, a prominent downfield shift is observed as compared to thatof other two salts. Signals are broadened with an increase in eachsalt concentration as confirmed by the line width values of –N+CH3protons as shown in Fig. 5(a) and the results are also reflected bythe viscometric measurements (Fig. 5b). In case of inorganic salts,the lw values of KSCN show a prominent change as compared to theother two salts (KBr/KNO3). It has been reported earlier [46] thatthe broadening of the proton resonances is due to the end-over-endtumbling motion of rod-shaped micelles. With increasing concen-tration of salt, this tumbling motion slows down due to growthof micelle as well as increase in steric intermicellar interactions

resulting in the increase in lw values (Fig. 5a). At high concen-tration of salts, the lw values almost level off due to the fact thatthe end-over tumbling motion are not a dominating factor in this

50403020100

0

15

30

45

60

r

[salt] (mM)

KBr KNO3 KSCN

e 1H NMR signal corresponding to the –N+CH3 group of 50 mM 12-4-12 gemini at

F. Khan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 394 (2012) 46– 56 53

504030201003.12

3.14

3.16

3.18

3.20

3.22

chem

ical

shi

ft (δ

, ppm

)

[salt] (mM)

KB r KN O3 KSCN

Fs

rtTmihbmamrlptp

bsiptimellvsilchKw

tcate

ii

13.513.012.512.011.511.038.5

39.0

39.5

40.0

40.5

41.0SCN

-

NO3-

Br-

line

wid

th (H

z)

LN

NaBenz has same skeleton as NaSal, the only difference is the lack of–OH group on benzene ring. Because of the presence of –OH groupon benzene ring the salicylate ion is expected to show complicated

50403020100

3.06

3.08

3.10

3.12

chem

ical

shi

ft (δ

, ppm

)

NaBenz NaSal

ig. 6. Influence of salt concentration on variations in chemical shift of the 1H NMRignal corresponding to the –N+CH3 group of 50 mM 12-4-12 gemini at 298 K.

egion, instead, the combination of lateral diffusion and reorien-ation of the local axes in terms of reorientation rate dominate.hese motions mainly depend on the persistence length of the rodicelles rather than on the whole length of micellar aggregates. The

norganic ions follow Hofmeister series. Br− and NO3− ions are fully

ydrated and have low polarizability. As a result, they are weaklyound to the cationic head groups and are located in the uppericellar sheath. Irrespective of its almost same micellar size [47]

nd polarizability, NO3− ion is responsible for more pronounced

icellar growth as observed by high lw values, which is due to itsespective position in Hofmeister series. The ability of a particu-ar counterion to promote aggregation appears to be related to itsosition in lyotropic series of anions. This series is a measure ofhe ability of the ions to denature proteins, the stronger ions beinglaced higher on the list.

In general, it is proposed that in aqueous medium, hydropho-ic/chaotropic counterions are bound more strongly to the micellarurface than hydrophilic/kosmotropic counterions. As a result, theons of the former category have been found more effective inromoting the micellar growth of ionic surfactants than those ofhe latter category [48]. Large chaotropic anions penetrate deeplynto the interfacial region of the monolayer. Larger anions are

ore hydrophobic and hence prefer to stay in the bilayer interior,xplained by a less structured hydration shell. KSCN (containingarge SCN− ion having a weakly distributed charge) induces micel-ar growth more efficiently as reflected by increase in lw and �r

alues (Fig. 5a and b); also evident by more prominent downfieldhifts of protons of –N+CH3 head group as compared to other twoons (Br− and NO3

−). In Fig. 7, we have attempted to correlate theine width with lyotropic number (LN) [49], which provides a directorrelation between the lyotropic number (LN) and line width atalf height (lw) values of 12-4-12 in presence of 10 mM salts (KBr,NO3, KSCN). It is clearly shown that our data correlate quite wellith the lyotropic number (LN) of the ions.

The present results support the fact that the nature and struc-ure of salts are the key factors responsible for the aggregation ofationic gemini surfactant. Inorganic salts mainly affect surfactantggregation by reduction of the electrostatic repulsion betweenhe cationic head groups. Also, SCN− promotes aggregation morefficiently than Br− and NO −.

3

The refined structures of organic salts play very significant rolen their adsorption on the surface of aggregates [50], as hydrophobicnteraction between organic counterions and surfactant aggregates

Fig. 7. Correlation between the lyotropic number (LN) and the line width of inor-ganic salts (KBr, KNO3, and KSCN).

is determined by the position of substituent group on the benzenering of the counterion [51], which obviously influences the mor-phology of surfactant aggregates directly.

In case of both the organic salts, NaBenz and NaSal, a signifi-cant change in chemical shifts of –N+CH3 protons is observed for12-4-12 (as shown in Fig. 8, the values for other peaks are given inTable S2). With the addition of these salts, the protons in the alkylchain (1-H, 2-H) move downfield and they show a significant vari-ation than the inorganic salts (Figs. S4 and S5 and Table S2). Also,the peak of 3-H proton merges to 2-H proton. As the salt concen-tration is increased, all of the peaks are broadened. The line widthat half height (lw) values corresponding to –N+CH3 signal are plot-ted in Fig. 9(a) against the concentration of added organic salt. Theviscosity of the solution also rises upon addition of organic salts, ascan be seen in Fig. 9(b).

The variation of 1H NMR spectra of these two organic salts in thesurfactant solutions reflects their interaction with the surfactant.

[salt] (mM)

Fig. 8. Influence of salt concentration on variations in chemical shift of the 1H NMRsignal corresponding to the –N+CH3 group of 50 mM 12-4-12 gemini at 298 K.

54 F. Khan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 394 (2012) 46– 56

50403020100

32

36

40

44

48

52a

line

wid

th (H

z)

Na Benz Na Sal

6050403020100

0

20

40

60

80b

ηr

Na Benz NaS al

F of th2

abilcitd

FN

[salt] (mM)

ig. 9. Influence of salt concentration on variations in (a) line width at half height98 K, and (b) relative viscosities at 303 K.

dsorption properties. Due to the possible polar hydrogen bondsetween –OH group and neighboring acid group, other types of

nteractions lead to influence the specific orientation at the micel-ar surface. With an increase in salt concentration, an increase in

hemical shift values is observed for the protons lying in the vicin-ty of the core carbon atoms, whereas a decrease is observed forhe protons of carbon atoms in the vicinity of head group. Also, theisappearance of peaks together with peak broadening is indicative

ig. 10. (a) 400 MHz 1H NMR spectra of NaBenz: (i) 50 mM pure NaBenz; (ii) at differentMR spectra of NaSal: (i) 50 mM pure NaSal; (ii) at different concentrations of NaSal in p

[salt] (mM)

e 1H NMR signal corresponding to the –N+CH3 group of 50 mM 12-4-12 gemini at

of the presence of grown micelles in the system. This broadeningis due to restricted mobility of NaBenz and NaSal, as a result of theincrease in micellar size on which the anions are adsorbed.

The 1H NMR spectra of NaBenz and NaSal in D2O, with and with-

out 12-4-12, are shown in Fig. 10(a and b). The spectra are of firstorder and consist of three multiplets in each case (all due to ringprotons). The values of 1H NMR chemical shifts of 50 mM 12-4-12in presence of NaBenz and NaSal salts are given in Table S2.

concentrations of NaBenz in presence of 50 mM 12-4-12 at 298 K. (b) 400 MHz 1Hresence of 50 mM 12-4-12 at 298 K.

Physic

itbHrosob(

mpibceppsfahoiit

4

Kgh

[

[

[

[

[

[

[

[

[

[

[

[

[

[

F. Khan et al. / Colloids and Surfaces A:

Also, the spectra of NaSal (Fig. 10b) show that the –OH protons not observed separately as it is labile and exchanges with deu-erium in D2O, therefore merges with the solvent peak. In case ofoth the organic salts, NaBenz and NaSal, the ring protons 3-H, 4-, 5-H shift to lower chemical shift values, whereas the 6-H proton

emains more or less unshifted. This means that in the presencef 12-4-12 micelles, the meta and para protons of NaSal/NaBenzhift to a more nonpolar environment than water, whereas thertho protons remain in the same polar environment. The peaks areroadened dramatically as the concentration of salts is increasedsee Fig. 9a and b).

As above, we can conclude that the orientation of the NaSalolecule on the micellar surface is such that the –COO− group

rojects away from the positively charged micellar surface, induc-ng some sort of a charge separation. The induced charge separationy the separation of negatively charged –COO− from the positivelyharged micellar surface increases the energy of the system. It isxpected that the total energy of the system is decreased by com-ensating the negative charge on –COO− group by attaching it to theositively charged surface of a second micelle (having an adsorbedolubilizate). The process thus goes on and dimers, trimers, etc., areormed. This surely is in line with the surface active nature of NaSalnd also one of the significant factors responsible for the observedydrotropic action of NaSal [52]. This indeed is reflected by thebserved pronounced micellar growth in case of NaSal. The mostmportant factors responsible for the micellar growth are the pack-ng of the hydrocarbon chains and the extent of repulsion betweenhe cationic head groups [53].

. Conclusions

In this paper we have examined the effect of salts (KBr, KNO3,SCN, NaBenz, NaSal) on the physicochemical properties of theemini surfactant 12-4-12 in aqueous solution. Following are theighlighting points:

The CMC of the gemini surfactants decreases with increasing thesalt concentration; this is mainly the result of a reduction in chargedensity per surface area of the micelle which leads to lowering ofCoulombic repulsions between the head groups.For the above anions, the effect on the CMC parallels the anionradius; in fact, greater the anion radius, greater the polarizibility,and lower the heat of dehydration.These factors will enhance the attraction between the polariz-able gemini cation and the added anion and will determine thelowering of the CMC.Also, the same trend was observed for the effect of salts onthe micellar structural transition at the concentration range wellabove the CMC (CMC II).The ability to promote aggregation decreases in the order:NaSal > NaBenz > KSCN > KNO3 > KBr.The 1H NMR studies revealed that the micellar growth of gem-ini surfactant in the presence of organic salts is mainly due tothe strong binding of hydrophobic counterions with the surfac-tant head group. The structure of the counterion, hydrophobicity,and substituent in the counterion are the main factors responsiblefor the micellar growth in presence of organic counterions.Out of the two organic salts used herein, NaSal is found to be moreeffective than NaBenz.In case of NaSal, the concept of protruding COO− groups has been

invoked in accounting for the role of calcium ions in the biologicalcells [54(a)]. The two cells unite by a calcium ion bridge throughthe projecting carboxylic groups. This is also known in case ofdust particles binding to the cloth in detergency [54(b)]. Thus, the

[

ochem. Eng. Aspects 394 (2012) 46– 56 55

interaction between gemini surfactant and salts is mainly relatedto the nature of counterions.

Acknowledgements

Authors are thankful to UGC, CSIR, New Delhi, for granting fel-lowships, and SAIF, CDRI, Lucknow, for providing 1H NMR facilities.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.colsurfa.2011.11.024.

References

[1] K.L. Mittal (Ed.), Micellization, Solubilization and Microemulsions, PlenumPress, New York, 1977.

[2] M.De. Vijlder, Ion-pair formation as a determining factor in the effectivenessof the interaction of electrolytes with amphiphilic azo dyes in water, J. Chem.Soc. Faraday Trans. I 81 (1985) 1369–1373.

[3] S. Hayashi, S. Ikeda, Micelle size and shape of sodium dodecyl sulfate in con-centrated sodium chloride solutions, J. Phys. Chem. 84 (1980) 744–751.

[4] S. Glasstone, Text Book of Physical Chemistry, 2nd ed., Macmillan, London,1960.

[5] M.J. Rosen, Surfactants and Interfacial Phenomena, 3rd ed., Wiley-Interscience,New York, 2004.

[6] M.J. Rosen, D.C. Tracy, Gemini surfactants, J. Surf. Deter. 1 (1998) 547–554.[7] R. Zana, in: K. Holmberg (Ed.), Novel Surfactants, Marcel Dekker, New York,

1998.[8] R. Zana, M. Benrraou, R. Rueff, Alkanediyl-alpha, omega-

bis(dimethylalkylammonium bromide) surfactants. 1. Effect of the spacerchain length on the critical micelle concentration and micelle ionizationdegree, Langmuir 7 (1991) 1072–1075.

[9] Y. You, J. Zhao, R. Jiang, J. Cao, Strong effect of NaBr on self-assembly of qua-ternary ammonium gemini surfactants at air/water interface and in aqueoussolution studied by surface tension and fluorescence techniques, Colloid Polym.Sci. 287 (2009) 839–846.

10] S. De, V.K. Aswal, P.S. Goyal, S. Bhattacharya, Role of spacer chain length indimeric micellar organization. Small angle neutron scattering and fluorescencestudies, J. Phys. Chem. 100 (1996) 11664–11671.

11] U.S. Siddiqui, G. Ghosh, Kabir-ud-Din, Dynamic light scattering studies of addi-tive effects on the microstructure of aqueous gemini micelles, Langmuir 22(2006) 9874–9878.

12] Kabir-ud-Din, W. Fatma, Z.A. Khan, A 1H NMR study of 1,4-bis(N-hexadecyl-N, N-dimethylammonium)butane dibromide/sodium anthranilate system:spherical to rod-shaped transition, Colloid Polym. Sci. 284 (2006) 1339–1344.

13] Kabir-ud-Din, U.S. Siddiqui, S. Kumar, Viscometric studies on aqueous geminimicelles in the presence of additives, Colloid Surf. A 301 (2007) 209–213.

14] Kabir-ud-Din, W. Fatma, Z.A. Khan, A.A. Dar, 1H NMR and viscometric studieson cationic gemini surfactants in presence of aromatic acids and salts, J. Phys.Chem. B 111 (2007) 8860–8867.

15] Kabir-ud-Din, W. Fatma, S. Khatoon, Z.A. Khan, A.Z. Naqvi, Surface andsolution properties of alkanediyl-�,�-bis(dimethylcetylammonium bromide)gemini surfactants in the presence of additives, J. Chem. Eng. Data 53 (2008)2291–2300.

16] U.S. Siddiqui, S. Kumar, Kabir-ud-Din, Structural transition of bifunctional sur-factants, Monatsh. Chem. 140 (2009) 457–462.

17] I.A. Khan, R. Mohammad, Md.S. Alam, Kabir-ud-Din, Effect of alkylamine chainlength on the critical micelle concentration of cationic gemini surfactantbutanediyl-�,�-bis(dimethylcetylammonium bromide) surfactant, J. Disp. Sci.Technol. 30 (2009) 1486–1493.

18] I.A. Khan, R. Mohammad, Md.S. Alam, Kabir-ud-Din, The interaction of cationicgemini surfactant 1,4-butanediyl-�,�-bis(dimethylcetylammonium bromide)with primary linear alkanols, J. Disp. Sci. Technol. 31 (2010) 129–137.

19] U.S. Siddiqui, F. Khan, I.A. Khan, Kabir-ud-Din, Synergism in cationic gemini-additive systems, Phys. Chem. Liq. 49 (2011) 72–80.

20] U.S. Siddiqui, F. Khan, I.A. Khan, Kabir-ud-Din, Role of added counterions in themicellar growth of bisquaternary ammonium halide surfactant (14-s-14): 1HNMR and viscometric studies, J. Colloid Interface Sci. 355 (2011) 131–139.

21] U.R.K. Rao, C. Manohar, B.S. Valaulikar, R.M. Iyer, Micellar chain model for theorigin of the viscoelasticity in dilute surfactant solutions, J. Phys. Chem. 91(1987) 3286–3291.

22] S.J. Bachofer, U. Simonis, Determination of the ion exchange constants of fouraromatic organic anions competing for a cationic micellar interface, Langmuir12 (1996) 1744–1754.

23] L. Wattebled, A. Laschewsky, Effects of organic salt additives on the aggrega-

tion behavior of dimeric (gemini) surfactants in aqueous solution, Langmuir 23(2007) 10044–10052.

24] (a) T. Lu, J. Huang, Z. Li, S. Jia, H. Fu, Effect of hydrotropic salt on the assemblytransitions and rheological responses of cationic gemini surfactant solutions,J. Phys. Chem. B 112 (2008) 2909–2914;

5 Physic

[

[

[

[

[

[

[

[

[

[

[[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[[

6 F. Khan et al. / Colloids and Surfaces A:

(b) R. Jiang, J. Zhao, X. Hu, X. Pei, L. Zhang, Rich aggregate morphologies inducedby organic salts in aqueous solutions of a cationic gemini surfactant with a shortspacer, J. Colloid Interface Sci. 340 (2009) 98–103.

25] J.C. Eriksson, G. Gillberg, NMR-studies of the solubilization of aromatic com-pounds in cetyltrimethylammonium bromide solution, Acta Chem. Scand. 20(1966) 2019–2027.

26] F.M. Menger, J.S. Keiper, Gemini surfactants, Angew. Chem. Int. Ed. 39 (2000)1906–1920.

27] Kabir-ud-Din, S. Kumar, V.K. Aswal, P.S. Goyal, Effect of the addition of n-alkylamines on the growth of sodium decyl sulfate micelles, J. Chem. Soc.Faraday Trans. 92 (1996) 2413–2415.

28] T.R. Carale, Q.T. Pham, D. Blankschtein, Salt effects on intramicellar interactionsand micellization of nonionic surfactants in aqueous solutions, Langmuir 10(1994) 109–121.

29] T. Imae, S.J. Ikeda, Sphere-rod transition of micelles of tetradecyltrimethy-lammonium halides in aqueous sodium halide solutions and flexibility andentanglement of long rodlike micelles, Phys. Chem. 90 (1986) 5216–5223.

30] M.R. Bohmer, L.K. Koopal, J. Lyklema, Micellization of ionic surfactants: calcu-lations based on a self-consistent field lattice model, J. Phys. Chem. 95 (1991)9569.

31] E. Roelants, F.C.D. Schryver, Parameters affecting aqueous micelles of CTAC,TTAC, and DTAC probed by fluorescence quenching, Langmuir 3 (1987)209–214.

32] T.F. Tadros, Applied Surfactants: Principles and Applications, Wiley VCH, Wein-heim, 2005.

33] D. Myers, Surfactants Science and Technology, 3rd ed., Wiley-Interscience,Hoboken, NJ, 2006.

34] X. Wang, Y. Li, J. Li, J. Wang, Z. Guo, H. Yan, Salt effect on the complex for-mation between polyelectrolyte and oppositely charged surfactant in aqueoussolution, J. Phys. Chem. B 109 (2005) 10807–10812.

35] C. Tanford, The Hydrophobic Effect, Wiley, New York, 1980.36] Y. You, R. Jiang, T.T. Ling, J.X. Zhao, Bending of the flexible spacer chain of gem-

ini surfactant induced by hydrophobic interaction, Chin. J. Chem. 27 (2009)469–471.

37] K. Anand, O.P. Yadav, P.P. Singh, Studies on the surface and thermodynamicproperties of some surfactants in aqueous and water + 1,4-dioxane solutions,Colloids Surf. 55 (1991) 345–358.

38] K. Bijma, J.B.F.N. Engberts, Effect of counterions on properties of micellesformed by alkylpyridinium surfactants. 1. Conductometry and 1H-NMR chem-ical shifts, Langmuir 13 (1997) 4843–4849.

39] S. Gravsholt, Viscoelasticity in highly dilute aqueous solutions of pure cationicdetergents, J. Colloid Interface Sci. 57 (1976) 575–577.

40] D.N. Rubingh, in: K.L. Mittal (Ed.), Solution Chemistry of Surfactants, vol. 1,

Plenum, New York, 1979.

41] (a) S. Kumar, N. Parveen, Kabir-ud-Din, Additive-induced association in uncon-ventional systems: a case of the hydrotrope, J. Surf. Deter. 8 (2005) 109–113;(b) I.A. Khan, A.J. Khanam, Z.A. Khan, Kabir-ud-Din, Mixing behavior of anionichydrotropes with cationic surfactants, J. Chem. Eng. Data 55 (2010) 4775–4779.

[

ochem. Eng. Aspects 394 (2012) 46– 56

42] J. Ulmius, H. Wennerstrom, Proton NMR bandshapes for large aggregates;micellar solutions of hexadecyltrimethylammonium bromide, J. Magn. Reson.28 (1977) 309–312.

43] N.D. Gillitt, G. Savelli, C.A. Bunton, Premicellization of dimethyl di-n-dodecylammonium chloride, Langmuir 22 (2006) 5570–5571.

44] X. Cui, S. Mao, M. Liu, H. Yuan, Y. Du, Mechanism of surfactant micelle formation,Langmuir 24 (2008) 10771–10775.

45] F.A.L. Anet, Novel spin–spin splitting and relaxation effects in the proton NMRspectra of sodium salicylate in viscoelastic micelles, J. Am. Chem. Soc. 108(1986) 7102–7103.

46] U. Olsson, O. Soderman, P. Gudringt, Characterization of micellar aggregatesin viscoelastic surfactant solutions. A nuclear magnetic resonance and lightscattering study, J. Phys. Chem. 90 (1986) 5223–5232.

47] S. Berr, R.R.M. Jones, J.S. Johnson Jr., Effect of counterion on the size and chargeof alkyltrimethylammonium halide micelles as a function of chain length andconcentration as determined by small-angle neutron scattering, J. Phys. Chem.96 (1992) 5611–5614.

48] (a) L. Romsted, J. Yao, Arenediazonium salts: new probes of the interfacial com-positions of association colloids. 4.1–3 Estimation of the hydration numbers ofaqueous hexaethylene glycol monododecyl ether, C12E6, micelles by chemicaltrapping, Langmuir 12 (1996) 2425–2432;(b) L. Romsted, J. Yao, Arenediazonium salts: new probes of the interfacialcompositions of association colloids. 5.1–4 Determination of hydration num-bers and radial distributions of terminal hydroxyl groups in mixed nonionicCmEn micelles by chemical trapping, Langmuir 15 (1999) 326–336;(c) L. Romsted, J. Zhang, I. Cuccovia, M. Politi, H. Chaimovich, Concentrationof urea in interfacial regions of aqueous cationic, anionic, and zwitterionicmicelles determined by chemical trapping, Langmuir 19 (2003) 9179–9190;(d) L. Romsted, Do amphiphile aggregate morphologies and interfacialcompositions depend primarily on interfacial hydration and ion-specificinteractions? The evidence from chemical trapping, Langmuir 23 (2007)414–424;(e) Y. Geng, L. Romsted, F.M. Menger, Specific ion pairing and interfacial hydra-tion as controlling factors in gemini micelle morphology. Chemical trappingstudies, J. Am. Chem. Soc. 128 (2006) 492–501.

49] W. Kunz, P. Lo. Nostro, B.W. Ninham, The present state of affairs with Hofmeis-ter effects, Curr. Opin. Colloid Interface Sci. 9 (2004) 1–18.

50] V.K. Aswal, Effect of the hydrophilicity of aromatic counterions on the structureof ionic micelles, J. Phys. Chem. B 107 (2003) 13323–13328.

51] H. Rehage, H. Hoffmann, Shear induced phase transitions in highly dilute aque-ous detergent solutions, Rheol. Acta 21 (1982) 561–563.

52] M.E.L. Mabain, E. Hutchinson, Solubilization, Academic, New York, 1955.53] J.N. Israelachvili, D.J. Mitchell, B.W. Ninham, Theory of self-assembly of hydro-

carbon amphiphiles into micelles and bilayers, J. Chem. Soc. Faraday Trans. 2(72) (1976) 1525–1568.

54] (a) M.N. Jones, Biological Interfaces, Elsevier, Oxford, 1975;(b) J.T. Davies, E.K. Rideal, Interfacial Phenomena, Academic, New York,1963.