Physicochemical study of cationic gemini surfactant butanediyl-1,4-bis(dimethyldodecylammonium bromide) with various counterions in aqueous solution

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

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Physicochemical study of cationic gemini surfactant butanediyl-1,4-bis(dimethyldodecylammonium bromide) with various counterions in aqueous solution Farah Khan, Umme Salma Siddiqui, Iqrar Ahmad Khan, Kabir-ud-Din ∗ Department of Chemistry, Aligarh Muslim University, Aligarh 202 002, U.P., India

a r t i c l e

i n f o

Article history: Received 1 October 2011 Received in revised form 16 November 2011 Accepted 18 November 2011 Available online 26 November 2011 Keywords: Gemini surfactant Salt Micellization Synergism Surface tension 1 H NMR Viscosity

a b s t r a c t The effect of salts (inorganic and organic) on the characteristic solution properties of bis(quaternary ammonium) gemini surfactant butanediyl-1,4-bis(dimethyldodecylammonium bromide) (referred to as 12-4-12) was explored. The results showed that salt counterions induce synergistic effects and greatly enhance 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 were also attributed to the unique molecular structure of gemini surfactant, where the spacer (polymethylene chain) links the two quaternary ammonium head groups. The interaction and micellar growth of cationic gemini-salt systems with the inorganic salts have been found to obey Hofmeister series. Also, the anions of organic salts promote the hydrophobic interaction between the alkyl tails of gemini surfactant. In addition, the orientation of the substituents in the aromatic fragment is important too. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The micelle forming surfactants have the ability to catalyze a wide variety of reactions, which serve as valuable model processes for the study of microenvironmental factors affecting the efficiency of chemical transformations in the realm of science world [1]. As neutral salts have been found to tune the conformations of proteins and other macromolecules by modifying the prevalent hydrophobic or ionic interactions, exploring the effect of various kinds of salts in different fields of biology and chemistry is continuing since quite long. The processes are usually explained on the basis of water structure disruption around the amphiphilic compounds by lyophilic salts [2]. The amphiphilic compounds thus become desolvated and are bound to aggregate or form micelles which results in a decrease of CMC and an increase in aggregation number. There are two opposing tendencies in the micelle formation of ionic surfactants. The removal of hydrocarbon chains from water, which favors aggregation, and the electrostatic repulsions among the ionic head groups, which disfavor aggregation. A subtle balance between the two tendencies makes the system stable. Counterions

∗ Corresponding author. Tel.: +91 571 2700920x3353. E-mail address: [email protected] ( Kabir-ud-Din). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.11.024

have the ability to stabilize the ionic surfactant micelles by binding to the micelles and screening the electrostatic repulsion. Hence, the binding affinity of a particular counterion influences the process of micellization and aggregation. On the whole, the formation of spherical micelles by the aggregation of monomers in aqueous solutions takes place at concentrations above a critical micelle concentration (CMC I). In many systems, at higher concentrations, a transition from spherical to rod-like micelles occurs, which is referred to as CMC II. The micellar growth is mainly favored by the screening of electrostatic repulsion among the polar head groups and movement of the hydrophobic alkyl chains away from the aqueous environment. This is evidenced by an increase of the micelle aggregation number [3]. Generally, the size and nature of the counterions decides the extent of their influence on the shape and size of micelles. Many attempts have been made to examine the effect of salts on micelle formation in the light of Hofmeister (lyotropic) series [4]. Ions can be classified according to their effectiveness as either salting-in or salting-out agents. The Hofmeister series plays vital role in a wide range of biological and physicochemical phenomenon, viz. the action of ion channels in biological membranes, the surface tension of electrolyte solution, amphiphile micellization, etc. Organic salts with aromatic carboxylate counterions such as benzoate and salicylate, habitually called as hydrotropes, are surface-active and highly water soluble, which can increase the

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

solubility of sparingly soluble solutes in water. Hydrotropes have structures somewhat similar to surfactants in that they have hydrophilic and hydrophobic groups but, as the hydrophobic group is generally short, cyclic, and/or branched, they differ from surfactants. These counterions interact with the micelle-forming surfactants electrostatically as well as hydrophobically, and the orientation of hydrophobic moeity at the micellar surface is also important. Interest herein is focused to third generation surfactants, the ‘Geminis’. These consist of two hydrocarbon tails and two polar head groups connected through a spacer. The gemini surfactants have been proven better as compared to their conventional analogs in having higher efficiency in lowering surface tension, possessing much lower critical micelle concentration (CMC), better wetting properties, showing specific rheological and aggregation properties, etc. [5–10]. They are also referred as ‘green surfactants’, as less amount of geminis are used owing to their very low CMC values. Thus, their usage as effective emulsifiers, dispersants, bactericidal agents, antifoaming agents, detergents, etc., is a consequence of their advance features in comparison to conventional surfactants. Almost invariably, surfactants are used in presence of additives because of the enhanced performance of the mixtures due to synergism. Thus, it would be of great relevance to find new combinations where synergism exists. The vitality of the effect of salts on the aggregation behaviors of ionic surfactants in aqueous solutions is due to a wide range of applications for detergency and emulsification in industry. A number of studies by Kabir-udDin et al. [11–20] on the effect of additives (organic/inorganic compounds, nonelectrolytes, surfactants, etc.) using different techniques yielded significant results in the field of physicochemical study in gemini solutions. One of the most powerful methods for investigating dynamics and structure at the molecular level is NMR spectroscopy which has been used to characterize organized assemblies in aqueous micellar systems. Thus, it is being used to probe the location and orientation of molecules in and around the micelles by means of chemical shift changes for surfactant and additive proton resonances [21,22]. Changes in the chemical shift of the observed resonances of the –N+ CH3 protons clearly depict the electrostatic shielding which accompanies the formation of large and organized assemblies. The NMR investigations on the micellization and various aggregates on cationic gemini surfactants have been reported previously [12,14,20,23,24]. However, only swollen spherical micelles are produced by the solubilization of additives in the micellar interior [25] and does not contribute much towards micellar growth. In this regard, viscometry has been found very sensitive to the micellar morphology of macroscopic objects in a colloidal solution. The presence of rod-shaped micelles gives solutions a very high viscosity which might be of importance in industrial formulations of detergent solutions. These methods may be helpful for interpreting the relationship between the structure of additive and morphological transition of a surfactant resulting in their presence. In our previous study [20], the influence of inorganic/organic salts on the overall micellar structural changes and possible viscosity changes in three cationic gemini surfactant solutions (14-s-14, s = 4–6) were studied by 1 H NMR and viscosity techniques. However, for a proper understanding of the knowledge of fundamental micellar solution properties, in the present paper, we report a study of the micellization and morphological changes of butanediyl1,4-bis(dimethyldodecylammonium bromide) (12-4-12) gemini in presence of inorganic and organic salts, having different counterions (Scheme 1). These counterions can be divided into two groups: (1) inorganic counterions (Br− , NO3 − , SCN− ), which are principally taken from the Hofmeister series, and (2) aromatic carboxylate

47

counterions (Benz− , Sal− ). Surface tension measurements were also made to investigate the effect of the above counterions on the adsorption and micellization of 12-4-12 in aqueous solutions. The results indeed show a high sensitivity of the self-assemblies of 124-12 to a variety of given salts. 2. Materials and methods 2.1. Materials (≥98%, Fluka, USA), N,N1,4-dibromobutane dimethyldodecylamine (≥97%, Fluka, Switzerland), ethylacetate (HPLC and Spectroscopy grade, ≥99.7%, Merck, Mumbai), ethanol absolute (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 reaction of 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 removal of the solvent under vacuum, the solid obtained was recrystallized four to five times from hexane/ethyl acetate mixture for purification of the surfactant. It was further characterized by 1 H NMR analysis. The observed CMC value (1.04 mM) as well as 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 the ring detachment method using a Du Nouy type tensiometer (Hardson and Co., Kolkata) at 303 K. For each set of experiments, the ring was cleaned by heating it in alcohol flame. The CMC values were estimated as intersections of two linear segments, above and below the CMC of surface tension vs. log [surfactant] plots. The values of surface tension decrease continuously and then become almost constant along a wide concentration range. The uncertainties on the CMC were estimated to be less than (±0.1–0.3) × 10−5 mol dm−3 . The instrument was calibrated against double distilled water at the time of measurement. 2.4.

1H

NMR measurements

The 1 H NMR spectra were obtained with a Bruker Avance 400 Spectrometer at a proton resonance frequency of 400 MHz at 298 K. D2 O (Aldrich, 99.9%) was used to prepare the stock solutions of geminis in the absence and presence of salts. About 1 ml of each solution was transferred to a 5 mm NMR tube for measurements. Chemical shifts were recorded on the ı (ppm) scale. Tetramethylsilane (TMS) was used as internal standard. The reproducibility of chemical shifts was within 0.01 ppm. The line widths at half heights were measured from spectra and are accurate to ±0.1 Hz. 2.5. Viscosity measurements Viscosity measurements were carried out using an Ubbelohde viscometer, suspended vertically in a thermostat at 303 ± 0.1 K, as described earlier [27]. The viscometer was cleaned and dried every time before use. In order to check the reproducibility, the time of fall for every viscosity measurement was noted at least two times with a calibrated stopwatch. By doing so, it was found that the viscosity values were reproducible within ±0.1%.

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

Scheme 1. Molecular structure of (a) gemini surfactant butanediyl-1,4-bis(dimethyldodecylammonium bromide), 12-4-12, (b) inorganic counterions, and (c) aromatic carboxylate counterions.

3. Results and discussion 3.1. Salt effect on micellization and surface activity of gemini Salt effect is a resultant of various factors acting together including mainly the screening of electrostatic repulsion, commonly stabilizing the micellar aggregates, and the reduction in alkyl chain solubility [28,29]. The formation and the growth of micelles are favored by the salt addition [30,31], which shows the expected increase in surface activity accompanying the lower CMC with the increase of the salt concentration. The surface tension values of the pure gemini as well as of 12-4-12/salt (KBr, KNO3 , KSCN, NaBenz, NaSal) solutions of different concentrations were measured at 303 K (Fig. 1). It is well known that the micellization behavior and surface activity of an ionic surfactant is sensitively dependent on the chosen counterion [32,33]. The CMC values of pure gemini surfactant, obtained from Fig. 1 plots, are in good agreement with the available literature [8,9,26]. It is clearly observed that the surface tension decreases with an increase in surfactant concentration. At low concentrations the surfactant molecules adsorb at the liquid/air interface until the surface of the solution is totally occupied. Then the excess molecules tend to self-associate in the bulk solution to form micelles, resulting in the constancy of the surface tension. The CMC values obtained from the intersection points of ␥–log (concentration) plots for each additive concentration are given in Table 1. Fig. 2(a and b) shows these CMC values plotted against the concentration of the added salts. The decrease in CMC values of the gemini surfactant (Fig. 2 and Table 1) with the increase in salt concentration is due to the ‘synergistic effect’ for mixtures of cationic gemini surfactant and the salt counterions. The

CMC values decrease upon increasing salt concentration due to the reduction in the electrostatic repulsion between the intermolecular headgroups, thus favoring micellization. The electrostatic repulsion may become almost invariable at high salt concentration, then the CMC values become almost constant [34]. The difference in CMC values is found as a function of the nature of counterions. The inorganic ions have been found to obey the Hofmeister series, which is a measure of the ability of the ions to denature protein, the stronger ions have been placed higher on the list. The observed CMC forming efficiency order of the anions in relation to 12-4-12 was Sal− > Benz− > SCN− > NO3 − > Br− . Both NaBenz and NaSal decrease the CMC values, indicating that C6 H5 COO− and C6 H4 (OH)COO− anions can greatly enhance the close packing of the cationic gemini surfactant molecules at air–water interface. Moreover, the efficiency in reducing the surface tension without salt was much lower, indicating that the packing of the surfactant molecules in the micellar structure at zero salt concentration was not as tight as that in micelles formed in the presence of salts. Without effective shielding by salt, the two charges on the gemini head groups keep the surfactant molecules away from each other due to electrostatic repulsion. When salt was added to screen the effective charges in the head group, the electrostatic repulsion was weakened and its working range was also shortened, making the hydrophobic interactions relatively stronger. The primary driving force in micellization is the hydrophobic effect associated with the chain association [35] by promoting the release of water molecules which solvate the apolar chain. The amphiphilic monomers are gathered by hydrophobic effect which is a result of a net entropy increase of the whole solution. The balancing force is the electrostatic force between the surfactant head

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

70

b [KBr] (mM) 0.0 1.0 2.0 5.0 8.0

[KNO3] (mM) 0.0 1.0 2.0 5.0 8.0

70 65 60

-1

-1

γ ( mNm )

60

γ (mNm )

a

49

50

55 50 45

40

40 35

-1.5

-1.0

-0.5

0.0

-1.5

0.5

-1.0

-0.5

c

[NaBenz] (mM) 0.0 1.0 2.0 5.0 8.0

70

γ ( mNm )

60 -1

-1

γ ( mNm )

60

0.5

d

[KSCN] (mM) 0.0 1.0 2.0 5.0 8.0

70

0.0

log [surfactant]

log [surfactant]

50

50

40

40

-1.5

-1.0

-0.5

0.0

0.5

30 -2.0

-1.5

log [surfactant]

e

-1

-0.5

0.0

0.5

log [surfactant]

[NaSal] (mM) 0.0 1.0 2.0 5.0 8.0

72

64

γ ( mNm )

-1.0

56

48

40

32 -2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

log [surfactant] Fig. 1. Representative plots for the variation of surface tension () with concentration of gemini surfactant 12-4-12 at different fixed concentrations of salts: (a) KBr, (b) KNO3 , (c) KSCN, (d) NaBenz, and (e) NaSal.

groups with their counterions and water at the micellar surface. Polar and neutral ion pairs, that are less hydrated than free ions, are formed between the hydrated and charged headgroups and counterions. As a result, water is released into bulk solution with increase in entropy. These interactions depend on the hydrophobicity of the counterions. Hydrophobic counterions have tendency to interact more strongly with micellar interface, resulting in stronger ionpair formation, which favors micellization, and lowers CMC. In the present case also it is reflected by the results shown in Table 1.

In case of ionic gemini surfactants, the added counterions, in addition to the normal effect of shortening the distance between the heads (due to reduction of electrostatic repulsion) and promoting the hydrophobic interaction, exhibit their due share on the spacer as well which further promotes the hydrophobic interaction between the spacer and the alkyl tails and thus spacer chain strongly folds [36]. As a result, the gemini molecules are more and more tightly packed at the interface and the  CMC considerably decreases with increasing concentration of salt. The decrease in

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

Table 1 0 0 , Gads for cationic gemini surfactant 12-4-12 in presence of salts at 303 K. CMC, C20 , CMC/C20 , ˘ CMC ,  max , Amin , Gm Additive (mM) KBr 0 1 2 5 8 KNO3 1 2 5 8 KSCN 1 2 5 8 NaBenz 1 2 5 8 NaSal 1 2 5 8

ПCMC (mN m−1 )

107  max (mol m−2 )

Amin (A´˚ 2 )

0 −Gm (kJ mol−1 )

0 −Gads (kJ mol−1 )

1.20 2.09 2.44 2.74 5.16

20.37 25.21 26.48 27.23 30.52

9.88 10.07 11.02 11.25 11.62

167.90 164.75 150.66 147.58 142.77

27.42 28.14 28.46 28.75 29.06

29.48 30.65 30.86 31.17 31.68

0.290 0.280 0.240 0.110

2.20 2.21 2.44 4.52

25.54 26.75 27.87 32.33

10.29 11.28 11.69 11.97

161.34 147.13 142.00 138.66

28.64 28.74 28.86 29.18

31.14 31.11 31.24 31.88

0.56 0.51 0.49 0.38

0.180 0.110 0.080 0.050

3.11 4.46 5.89 7.09

29.82 31.44 33.56 34.95

10.38 12.20 13.23 14.79

159.91 135.98 125.42 112.22

28.99 29.19 29.32 29.93

31.86 31.77 31.85 32.29

0.50 0.46 0.40 0.36

0.150 0.100 0.040 0.030

3.25 4.40 8.76 11.06

32.45 34.57 35.87 37.62

12.13 14.40 14.63 14.84

136.82 115.21 113.45 111.87

29.26 29.48 29.80 30.05

31.93 31.88 32.26 32.59

0.33 0.26 0.09 0.07

0.100 0.050 0.020 0.010

3.20 5.63 4.55 5.75

33.27 35.46 37.08 39.65

12.65 14.88 17.31 20.44

131.17 111.57 95.90 81.21

30.28 30.86 33.55 34.25

32.91 33.24 35.69 36.19

CMC (mM)

C20 (mM)

1.04 0.78 0.69 0.61 0.54

0.870 0.370 0.280 0.220 0.100

0.64 0.61 0.59 0.51

CMC/C20

 CMC value indicates that addition of salts enhances the effectiveness of 12-4-12 in surface tension reduction. CMC (the surface pressure at CMC),  max (the maximum sur0 face excess), Amin (the minimum surface area per molecule), Gm 0 (the standard (the standard Gibbs energy of micellization), Gads Gibbs energy of adsorption) (see Supporting Information for definition of terms and the equations used to evaluate the parameters given in Table 1) values obtained at different concentrations of added salts in 12-4-12 solutions are given in Table 1. The following points emerge:



(i) With the increasing salt concentration, the CMC increases (see Table 1). (ii) Compared to pure gemini surfactant solutions, the solutions with salts have a greater preference to be adsorbed at air/water interface. In the presence of salts, the repulsion among the head

a

1.05

KBr KNO3 KSCN

groups decreases and causes the adsorption of more gemini molecules at the interface. (iii) With the addition of salts, as the values of  max increase, the values of Amin decrease and the trend is followed in all the cases. The progressive charge shielding and closer packing of the gemini surfactant ions in the surface cause a decrease in the Amin . This suggests that the orientation of the gemini surfactant molecule at the interface is almost perpendicular to the interface [37]. 0 and G0 decrease with increasing the salt concen(iv) The Gm ads trations. The standard state for the surfactant is a hypothetical monolayer at its minimum surface area per molecule, but at zero surface pressure. Hydrophobicity is the main cause for adsorption, which leads an amphiphile towards the air/water 0 and G0 interface. All the Gm values are negative, which ads imply that the adsorption of the surfactants at the air/mixture interface takes place spontaneously.

b

NaBenz NaSal

1.0

0.90

0.75

CMC (mM)

CMC (mM)

0.8

0.60

0.6

0.4

0.2

0.45

0.0

0.30 0

2

4

[salt] (mM)

6

8

0

2

4

6

[salt] (mM)

Fig. 2. Values of CMC of the gemini surfactant 12-4-12 at different concentrations of (a) inorganic salts and (b) organic salts.

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

51

Table 2 Micellar compositions (X1 m , X1 ), interaction parameters (ˇm , ˇ ), and activity coefficients (f1 m , f2 m , f1  , f2  ) for mixed gemini surfactant 12-4-12–salt systems. ˛salt NaBenz 0.06 0.12 0.25 0.35 NaSal 0.06 0.12 0.25 0.35

X1 m

ˇm

f1 m

f2 m

X1 

ˇ

f1 

f2 

0.213 0.236 0.271 0.294

−12.39 −12.41 −12.73 −13.27

0.000462 0.000718 0.001153 0.001339

0.569763 0.500443 0.392622 0.317511

0.275 0.305 0.344 0.367

−17.49 −18.82 −21.14 −23.28

0.000101 0.000113 0.000111 0.000088

0.266230 0.173241 0.081867 0.043445

0.246 0.273 0.337 0.354

−15.49 −16.12 −20.57 −21.58

0.000150 0.000199 0.000118 0.000122

0.391590 0.300652 0.096666 0.066880

0.293 0.327 0.359 0.378

−20.87 −23.42 −25.99 −28.35

0.000029 0.000024 0.000022 0.000017

0.166550 0.081653 0.035055 0.017398

3.2. Surfactant–organic salt counterion interactions In comparison to inorganic salts, besides electrostatic interaction, organic salts have additional hydrophobic interaction [38]. The aromatic counterions have aptitude to penetrate the head group region leading to micellar growth at lower loading of the micellar surface as compared to less weakly penetrating inorganic counterions. They induce strong hydrophobic interaction and reduce electrostatic repulsion between the cationic headgroups, leading to tightly packed and reduced curvature surfactant aggregates [39]. In general, the CMC values found to decrease with the increase in mole fraction of the hydrotropes. The results indicate that the added hydrotropes are assisting in the micelle formation of the gemini surfactants, as they form mixed micelles with gemini surfactants. Therefore, in order to investigate the nature of interaction between the constituents in the mixed micelles, the interaction parameters (ˇm and ˇ ) have been calculated for mixed micelles and mixed monolayer, which are given in Table 2 (calculated using Eqs. (S6)–(S9)). The interaction parameter for mixed micelle formation is calculated using Rubingh’s theory [40]. The CMC values of NaBenz and NaSal used in the calculation are 320 mM and 560.1 mM, respectively [41]. A positive ˇ value means repulsive interaction among mixed species, whereas a negative ˇ value means an attractive interaction; the more negative its value, the greater the interaction. The ˇm values are negative at all mole fractions of the mixed systems (Table 2), suggesting that the interaction between the two components is more attractive in mixed micelles as compared to the self-interaction of the two components before mixing. As the mole fraction of organic salts increases, the ˇm values become more negative. There is an increase in the attractive interaction with increase in salt concentration which is due to the intercalation of the salts in the micelles of the gemini surfactants [40], resulting in an increase

in the hydrophobic interactions. It is also evident by a decrease in CMC values with increasing salt concentration (Table 1). Rosen’s approach reveals increased synergism in mixed monolayer as compared to that in mixed micelles which gets enhanced with the addition of salt. The ˇ also shows the similar trend (Table 2), i.e., the mixtures of organic salt/gemini surfactant show stronger attractive interaction at the solution/air interface. This is due to the steric factor which is more important in micelle formation than in monolayer formation at a planar interface. Increased bulkiness in the hydrophobic group causes greater difficulty for incorporation into the curved mixed micelle compared to that of accommodating at planar interface. 3.3. Salt effect on the morphology of gemini surfactant In order to study the morphological changes caused by the addition of salts, 1 H NMR and viscometric measurements were performed well above the CMC values, as both of these are the convenient methods for the simultaneous monitoring of morphological transition in aggregates. A signature effect is observed on the interaction of the salt anions with the cationic gemini 12-4-12, as revealed by 1 H NMR studies. The 1 H NMR spectrum of pure 50 mM 12-4-12 in D2 O is represented in Fig. 3. The various protons attached to carbon atoms are 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 the results 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 the chemical shifts in the 1 H NMR spectra of 12-4-12 in presence of inorganic and organic salt counterions. The nature of the added salts alters the chemical environment of gemini leading to the variations

Fig. 3. 400 MHz 1 H NMR spectrum of 50 mM 12-4-12 gemini surfactant in D2 O at 298 K.

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

60

b

a 45

50

40

35

30

ηr

line width (Hz)

40

20

30

10 25 0 20 0

50

100

150

0

200

50

100

150

200

250

[surfactant] (mM)

[surfactant] (mM)

Fig. 4. Influence of gemini concentration on variations in (a) line width at half height of the 1 H NMR signal corresponding to the –N+ CH3 group of 12-4-12 gemini at 298 K, and (b) relative viscosities at 303 K.

of chemical shifts (ı), line width at half height (lw), and line shapes, thus signifying the micellar growth of gemini surfactant [21,22,42–44]. A variation in the spin lattice/spin–spin relaxation times changes line width, thereby, providing information regarding relative mobility of particular groupings within different micellar phases. Due to the slow mobility of the rod-like micelles (segmental motions or rotation) relaxation time decreases, as a result, line width increases (Fig. 5a). Thus, a peak broadening or increase in line width signifies the structural transition from spherical to nonspherical micelles [45]. In the present case, the chemical shift variations are more pronounced for the protons located near cationic head groups of gemini surfactant as revealed in Fig. 6 (the values of chemical shifts for other peaks are given in Table S1 and also shown in Figs. S1–S3). This was the reason for considering –N+ CH3 group signals for the observed chemical shift changes on surfactant or salt variation. As observed from our results, upon addition of inorganic salts, the chemical shift values of all the protons of gemini shift downfield (Figs. S1–S3). For the present surfactant, the variations of chemical shifts are different for different salts. It clearly indicates that

a

b

KBr KNO3 KSCN

44

the saturation of Br− and NO3 − ions at the double layer of gemini surfactant aggregates occurs and beyond this concentration the anions would remain free in the bulk solution. Thus, due to the reduction of the surface electrostatic potential between the surfactant head groups, surfactant aggregation is promoted and a downfield shift of proton is observed. However, on the addition of KSCN, a prominent downfield shift is observed as compared to that of other two salts. Signals are broadened with an increase in each salt concentration as confirmed by the line width values of –N+ CH3 protons as shown in Fig. 5(a) and the results are also reflected by the viscometric measurements (Fig. 5b). In case of inorganic salts, the lw values of KSCN show a prominent change as compared to the other two salts (KBr/KNO3 ). It has been reported earlier [46] that the broadening of the proton resonances is due to the end-over-end tumbling motion of rod-shaped micelles. With increasing concentration of salt, this tumbling motion slows down due to growth of micelle as well as increase in steric intermicellar interactions resulting in the increase in lw values (Fig. 5a). At high concentration of salts, the lw values almost level off due to the fact that the end-over tumbling motion are not a dominating factor in this

KBr KNO3 KSCN

60

45

40 38

ηr

line width (Hz)

42

30

36 15 34 32

0 0

10

20

30

[salt] (mM)

40

50

0

10

20

30

40

50

[salt] (mM)

Fig. 5. Influence of salt concentration on variations in (a) line width at half height of the 1 H NMR signal corresponding to the –N+ CH3 group of 50 mM 12-4-12 gemini at 298 K, and (b) relative viscosities at 303 K.

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

KBr KNO3 KSCN

3.22

SCN

41.0

3.20

40.5

-

-

NO3

line width (Hz)

chemical shift (δ, ppm)

53

3.18

3.16

3.14

40.0

39.5

39.0

3.12 0

10

20

30

40

-

Br

38.5 11.0

50

11.5

12.0

[salt] (mM)

12.5

13.0

13.5

LN

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

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

region, instead, the combination of lateral diffusion and reorientation of the local axes in terms of reorientation rate dominate. These motions mainly depend on the persistence length of the rod micelles rather than on the whole length of micellar aggregates. The inorganic ions follow Hofmeister series. Br− and NO3 − ions are fully hydrated and have low polarizability. As a result, they are weakly bound to the cationic head groups and are located in the upper micellar sheath. Irrespective of its almost same micellar size [47] and polarizability, NO3 − ion is responsible for more pronounced micellar growth as observed by high lw values, which is due to its respective position in Hofmeister series. The ability of a particular counterion to promote aggregation appears to be related to its position in lyotropic series of anions. This series is a measure of the ability of the ions to denature proteins, the stronger ions being placed higher on the list. In general, it is proposed that in aqueous medium, hydrophobic/chaotropic counterions are bound more strongly to the micellar surface than hydrophilic/kosmotropic counterions. As a result, the ions of the former category have been found more effective in promoting the micellar growth of ionic surfactants than those of the latter category [48]. Large chaotropic anions penetrate deeply into the interfacial region of the monolayer. Larger anions are more hydrophobic and hence prefer to stay in the bilayer interior, explained by a less structured hydration shell. KSCN (containing large SCN− ion having a weakly distributed charge) induces micellar growth more efficiently as reflected by increase in lw and r values (Fig. 5a and b); also evident by more prominent downfield shifts of protons of –N+ CH3 head group as compared to other two ions (Br− and NO3 − ). In Fig. 7, we have attempted to correlate the line width with lyotropic number (LN) [49], which provides a direct correlation between the lyotropic number (LN) and line width at half height (lw) values of 12-4-12 in presence of 10 mM salts (KBr, KNO3 , KSCN). It is clearly shown that our data correlate quite well with the lyotropic number (LN) of the ions. The present results support the fact that the nature and structure of salts are the key factors responsible for the aggregation of cationic gemini surfactant. Inorganic salts mainly affect surfactant aggregation by reduction of the electrostatic repulsion between the cationic head groups. Also, SCN− promotes aggregation more efficiently than Br− and NO3 − . The refined structures of organic salts play very significant role in their adsorption on the surface of aggregates [50], as hydrophobic interaction between organic counterions and surfactant aggregates

is determined by the position of substituent group on the benzene ring of the counterion [51], which obviously influences the morphology of surfactant aggregates directly. In case of both the organic salts, NaBenz and NaSal, a significant change in chemical shifts of –N+ CH3 protons is observed for 12-4-12 (as shown in Fig. 8, the values for other peaks are given in Table S2). With the addition of these salts, the protons in the alkyl chain (1-H, 2-H) move downfield and they show a significant variation 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 concentration is increased, all of the peaks are broadened. The line width at half height (lw) values corresponding to –N+ CH3 signal are plotted in Fig. 9(a) against the concentration of added organic salt. The viscosity of the solution also rises upon addition of organic salts, as can be seen in Fig. 9(b). The variation of 1 H NMR spectra of these two organic salts in the surfactant solutions reflects their interaction with the surfactant. NaBenz has same skeleton as NaSal, the only difference is the lack of –OH group on benzene ring. Because of the presence of –OH group on benzene ring the salicylate ion is expected to show complicated

NaBenz NaSal

chemical shift (δ, ppm)

3.12

3.10

3.08

3.06

0

10

20

30

40

50

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

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

a

b

NaBenz NaSal

52

NaBenz NaSal

80

48

44

ηr

line width (Hz)

60

40

40 20 36

0

32 0

10

20

30

40

50

[salt] (mM)

0

10

20

30

40

50

60

[salt] (mM)

Fig. 9. Influence of salt concentration on variations in (a) line width at half height of the 1 H NMR signal corresponding to the –N+ CH3 group of 50 mM 12-4-12 gemini at 298 K, and (b) relative viscosities at 303 K.

adsorption properties. Due to the possible polar hydrogen bonds between –OH group and neighboring acid group, other types of interactions lead to influence the specific orientation at the micellar surface. With an increase in salt concentration, an increase in chemical shift values is observed for the protons lying in the vicinity of the core carbon atoms, whereas a decrease is observed for the protons of carbon atoms in the vicinity of head group. Also, the disappearance of peaks together with peak broadening is indicative

of the presence of grown micelles in the system. This broadening is due to restricted mobility of NaBenz and NaSal, as a result of the increase in micellar size on which the anions are adsorbed. The 1 H NMR spectra of NaBenz and NaSal in D2 O, with and without 12-4-12, are shown in Fig. 10(a and b). The spectra are of first order and consist of three multiplets in each case (all due to ring protons). The values of 1 H NMR chemical shifts of 50 mM 12-4-12 in presence of NaBenz and NaSal salts are given in Table S2.

Fig. 10. (a) 400 MHz 1 H NMR spectra of NaBenz: (i) 50 mM pure NaBenz; (ii) at different concentrations of NaBenz in presence of 50 mM 12-4-12 at 298 K. (b) 400 MHz 1 H NMR spectra of NaSal: (i) 50 mM pure NaSal; (ii) at different concentrations of NaSal in presence of 50 mM 12-4-12 at 298 K.

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

Also, the spectra of NaSal (Fig. 10b) show that the –OH proton is not observed separately as it is labile and exchanges with deuterium in D2 O, therefore merges with the solvent peak. In case of both the organic salts, NaBenz and NaSal, the ring protons 3-H, 4H, 5-H shift to lower chemical shift values, whereas the 6-H proton remains more or less unshifted. This means that in the presence of 12-4-12 micelles, the meta and para protons of NaSal/NaBenz shift to a more nonpolar environment than water, whereas the ortho protons remain in the same polar environment. The peaks are broadened dramatically as the concentration of salts is increased (see Fig. 9a and b). As above, we can conclude that the orientation of the NaSal molecule on the micellar surface is such that the –COO− group projects away from the positively charged micellar surface, inducing some sort of a charge separation. The induced charge separation by the separation of negatively charged –COO− from the positively charged micellar surface increases the energy of the system. It is expected that the total energy of the system is decreased by compensating the negative charge on –COO− group by attaching it to the positively charged surface of a second micelle (having an adsorbed solubilizate). The process thus goes on and dimers, trimers, etc., are formed. This surely is in line with the surface active nature of NaSal and also one of the significant factors responsible for the observed hydrotropic action of NaSal [52]. This indeed is reflected by the observed pronounced micellar growth in case of NaSal. The most important factors responsible for the micellar growth are the packing of the hydrocarbon chains and the extent of repulsion between the cationic head groups [53].

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

The CMC of the gemini surfactants decreases with increasing the salt concentration; this is mainly the result of a reduction in charge density per surface area of the micelle which leads to lowering of Coulombic repulsions between the head groups. For the above anions, the effect on the CMC parallels the anion radius; in fact, greater the anion radius, greater the polarizibility, and lower the heat of dehydration. These factors will enhance the attraction between the polarizable gemini cation and the added anion and will determine the lowering of the CMC. Also, the same trend was observed for the effect of salts on the micellar structural transition at the concentration range well above the CMC (CMC II). The ability to promote aggregation decreases in the order: NaSal > NaBenz > KSCN > KNO3 > KBr. The 1 H NMR studies revealed that the micellar growth of gemini surfactant in the presence of organic salts is mainly due to the strong binding of hydrophobic counterions with the surfactant head group. The structure of the counterion, hydrophobicity, and substituent in the counterion are the main factors responsible for the micellar growth in presence of organic counterions. Out of the two organic salts used herein, NaSal is found to be more effective 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 biological cells [54(a)]. The two cells unite by a calcium ion bridge through the projecting carboxylic groups. This is also known in case of dust particles binding to the cloth in detergency [54(b)]. Thus, the

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