Study on Mixed Micelles of Cationic Gemini Surfactants Having

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Study on Mixed Micelles of Cationic Gemini Surfactants Having Hydroxyl Groups in the Spacers with Conventional Cationic Surfactants: Effects of Spacer Group and Hydrocarbon Tail Length Sonu, Amit K. Tiwari, and Subit K. Saha* Department of Chemistry, Birla Institute of Technology and Science, Pilani 333 031, Rajasthan, India S Supporting Information *

ABSTRACT: Gemini surfactants, being more surface-active than their conventional counterparts, have potential applications in various industries. The properties of mixed surfactant systems are far better than those of neat surfactants in many cases, and as a result, mixed surfactants are used in many industrial applications. In the present work, the micellar properties of binary mixtures of the monomeric cationic surfactants hexadecyltrimethylammonium bromide (CTAB), tetradecyltrimethylammonium bromide (TTAB), and dodecyltrimethylammonium bromide (DTAB) with the cationic gemini surfactants 1,3-bis(dodecyl-N,Ndimethylammonium bromide)-2-propanol and 1,4-bis(dodecyl-N,N-dimethylammonium bromide)-2,3-butanediol were studied in aqueous solution at 303.15 K by means of conductivity, steady-state fluorescence, and fluorescence anisotropy techniques. The presence of a small amount of gemini surfactant was found to improve the physicochemical properties of the conventional surfactant. For example, the cmc value of DTAB was reduced to one-sixth of its original value in the presence of 0.1 mole fraction of a gemini surfactant. The spacer group of the gemini surfactant and the hydrocarbon chain of the monomeric surfactant play a significant role in the interactions between the surfactants in mixed micelles. These interactions are greatest when there are similarities in the structures of their hydrocarbon chains; however, the micellization process is favored by increasing hydrophobicity of the monomeric surfactant. The microenvironments of mixed micelles were studied using fluorescence techniques.

1. INTRODUCTION Surfactants are a basic requisite for many technological applications, such as enhanced oil recovery, pharmaceuticals, mineral flotation, detergency, food and cosmetics industries, and many more.1 For application purposes, mixtures of different surfactants are often employed. Surfactant mixtures are known to have superior physicochemical properties compared to the individual surfactants, thereby requiring smaller amounts,2−5 and this is why surfactant mixtures are extensively used in many branches of industry6−8 and biological fields.9,10 The properties of mixed micellar solutions can be optimized conveniently, because one can easily tune the desired feature to the range needed by changing the solution composition. High-performance surface-active compounds are demanded with progress in industrial technology. The synergistic effect (discussed later) of surfactant mixtures is used in the detergent and health-care industries.1,11,12 For example, the critical micelle concentration (cmc) of a surfactant mixture is often lower than that of one of the components, so that the mixture exhibits lower skin irritation than the neat surfactants.13 Some surfactants have very high surface activities but are also costly. The large-scale application of a single surfactant is not economical from the industry point of view because of the high cost of surfactant mass production and purification. Therefore, to achieve almost equivalent activity, it is often possible to mix a more surface-active and expensive surfactant with a comparatively less surface-active and cheaper surfactant in different proportions. To meet these requirements, gemini and conventional mixed surfactant systems would be the best choice. © 2013 American Chemical Society

Gemini or dimeric surfactants are a special class of surfactants that contain two hydrophilic headgroups and two hydrophobic tails covalently linked through a spacer at their headgroups. These surfactants are gaining much more attention because they have unique properties that are superior to those of their monomeric surfactant counterparts. As these surfactants have low cmc values, they are more surface-active,14,15 leading to potential applications such as fabric softeners and detergents, better solubilizing ability, stronger biological activity, and better wetting and foaming16−21 than their conventional monomeric counterparts. Thus, gemini surfactants have more versatile industrial applications.22 The spacer group is a unique part in a gemini surfactant molecule. It affects the aggregation properties of the gemini surfactant.23 Different types of interactions between different surfactant molecules in a mixture are possible. Synergism is the attractive interaction between surfactant molecules, whereas antagonism is repulsive. The synergistic effect of surfactants greatly improves many technological and environmental effects.1,24−26 To calculate the interaction parameters and the composition of the components in the micellar phase, binary mixtures of surfactants have been studied extensively. The first theoretical model given by Lange27 and implemented by Clint28 was based on ideal mixing of the surfactants in the micellar phase. A model based on regular solution theory (RST) was described Received: Revised: Accepted: Published: 5895

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by Rubingh29 for nonideal mixed systems and has been used extensively because of its simplicity. The interaction parameter (β) used in Rubingh’s model indicates the type of interactions between the surfactant molecules. A negative β value indicates attractive interactions, whereas a positive β indicates repulsive interaction.1 The shortcoming of Rubingh’s model is the nonconstancy of the interaction parameter values for a particular surfactant system at different compositions.30 Motomura et al.31 developed a model that is an attempt to overcome the limitations of Rubingh’s model. The mixed micellization behavior of gemini surfactants with conventional surfactants depends strongly on the nature of the spacer group of the gemini surfactants and the chain lengths of both the gemini and conventional surfactants.32,33 Changes in the mixed micellization behavior with variations of the chain length of the simple alkyl chain of the spacer groups have been reported in the literature. However, to our knowledge, studies with a substituted spacer and with variations in the substitution of the spacer of a gemini surfactant as one of the components of a binary mixture are rare.34,35 Mixed micelles formed by nonionic surfactants generally show ideal behavior, whereas those of ionic surfactants show deviations from ideality as a result of attractive or repulsive interactions.36 Mixed micelles consisting of ionic surfactants of the same charge can exhibit ideal and nonideal behaviors depending on the structures of the surfactants.37,38 The interplay between hydrophobic and repulsive interactions among the surfactant molecules has an effect on the formation of micelles.39 Obviously, repulsive interactions among the surfactant molecules become stronger when the surfactant molecules have the same charge. Therefore, attempts have been made to carry out research with these types of surfactant systems with the variation of synergism by making structural changes in the molecules.36−39 In the present work, the properties of mixed micelles of the monomeric cationic surfactants hexadecyltrimethylammonium bromide (CTAB), tetradecyltrimethylammonium bromide (TTAB), and dodecyltrimethylammonium bromide (DTAB) with the two cationic gemini surfactants 1,3-bis(dodecyl-N,Ndimethylammonium bromide)-2-propanol (Gemini-1) and 1,4bis(dodecyl-N,N-dimethylammonium bromide)-2,3-butanediol (Gemini-2) in pure water have been investigated. The structures of the gemini surfactants are presented in Scheme 1. These two gemini surfactants are different because of the

difference in the chemical structures of their spacer groups. It is known that the hydroxyl groups of the spacer have a significant effect on the aggregation properties of gemini surfactants.40−44 Compared to Gemini-1, Gemini-2 has one additional methylene group and one extra substituted hydroxyl group in the spacer. The present study investigated how the hydroxylgroup-substituted spacers of Gemini-1 and Gemini-2 affect their micellization process as compared to that of gemini surfactants without any hydroxyl group(s) in the spacer. This study focused on the effect of the hydroxyl-group-substituted spacer on the deviation from ideality of mixed micelles of gemini and monomeric surfactants. To our knowledge, this type of effect on mixed micellization behavior has not previously been studied. In addition, the effects of the hydrocarbon chain lengths of a series of monomeric surfactants on the interactions between the two components of the mixed micelles and the cmc values of mixed micelles were investigated. The present study also demonstrates quantitatively how the surface activity of a monomeric surfactant is enhanced in the presence of a gemini surfactant and how a gemini surfactant can be made cost-effective in the presence of a monomeric surfactant. To study the microenvironment of mixed micelles, we determined the micropolarity and microviscosity using the probe para-N,N-dimethylaminocinnamaldehyde (DMACA, Scheme 1). Instead of using two different probes, namely, pyrene and diphenylhexatriene (DPH)45 for the determination of micropolarity and microviscosity, respectively, in the present work, a single probe molecule was employed for both purposes. The determination of micropolarity and microviscosity is important from the biological point of view, because alteration of the viscosity of the cell membrane can lead to many diseases, such as cell malignancy, hypercholesterolemia, atherosclerosis, and diabetes.46,47 Moreover, there are some biological processes for which a change in microviscosity is accompanied by a simultaneous change in micropolarity.46

2. MATERIALS AND METHODS 2.1. Materials. The monomeric cationic surfactants DTAB and TTAB were obtained from Alfa Aesar (Ward Hill, MA), and CTAB and pyrene were obtained from Aldrich Chemical Co. (Milwaukee, WI); all were used as received. DMACA was also obtained from Aldrich Chemical Co. and recrystallized from aqueous ethanol. A single spot was noticed for the recrystallized compound on a thin layer chromatography plate. The procedures for the synthesis, purification, and characterization of the gemini surfactants used in this study were reported in our earlier publication.48 2.2. Conductivity Measurements. The cmc values of all of the mixed surfactant systems were determined by conductivity measurements. These measurements were performed using a direct-reading Eutech Instruments conductivity meter (model PC 510) equipped with a dip cell (cell constant = 1.0 cm−1). The conductivity cell was calibrated with a standard KCl solution with a specific conductivity of 1413 μS cm−1 obtained from Merck (Mumbai, India). All solutions were prepared in triply distilled water with a specific conductivity of 2−4 μS cm−1. Molar fraction (Xi) was used to expresses the composition of the solutions of the respective surfactant, defined as

Scheme 1. Chemical Structures of Gemini Surfactants and Fluorescent Probe Molecules

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dx.doi.org/10.1021/ie303616j | Ind. Eng. Chem. Res. 2013, 52, 5895−5905

Industrial & Engineering Chemistry Research Xi =

[Si ] [Si ] + [Sj ]

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Table 1. Values of cmc (10−4 M) of Monomeric Surfactant + Gemini-1/Gemini-2 Systems from Conductivity and Fluorescence Measurements at 303.15 K

(1)

where [Si] and [Sj] are the molar concentrations of surfactants i and j, respectively, in the mixed surfactant solutions. The conductivity measurements were carried out in the form of titrations by adding concentrated stock solutions of aqueous surfactants in pure water over the whole mole fraction range, keeping the total surfactant concentration at least 10 times higher than the cmc of each component. A thermostat (Julabo F 25) with a temperature accuracy of ±0.01 °C was used to maintain the temperature constant. For the determination of a cmc value, experimental values of specific conductivity, κ, were plotted as a function of total surfactant concentration, [S]total ([S]total = [monomeric surfactant] + [Gemini-1] or [Gemini2]) at several constant values of the bulk mole fraction of a given monomeric surfactant, α1. The determination of the cmc using the method of Williams et al.49 is sometimes difficult, because choosing the exact break point in the plot of specific conductance (κ) versus total surfactant concentration can become tedious.50 Therefore, to overcome this problem, the method of Carpena et al.51 was applied in the present study to calculate the cmc and degree of counterion dissociation (g) from the conductivity data for all mixed micellar systems. The details of Carpena et al.’s method are described elsewhere.51,52 2.3. Steady-State Fluorescence Measurements. The steady-state fluorescence intensity and anisotropy measurements were made with a Horiba Jobin Yvon Fluoromax-4 scanning spectrofluorimeter. The final concentrations of DMACA and pyrene in all solutions were 5 and 2 μM, respectively, and the excitation wavelengths for DMACA and pyrene were 378 and 339 nm, respectively. Steady-state fluorescence anisotropy values (r) were calculated by the equation48 r=

IVV − GIVH IVV + 2GIVH

α 0.0 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 α 0.0 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

CTAB + Gemini-1a,b

TTAB + Gemini-1

DTAB + Gemini-1

c, d

6.90 (6.90) 7.10 (7.32) 7.20 7.43 (7.36) 7.66 7.78 (7.74) 7.99 8.24 (8.13) 8.40 9.20 (9.12) CTAB + Gemini-2 7.87 8.03 8.20 8.25 8.33 8.50 8.74 8.82 9.10

(8.10) (8.05) (8.30) (8.66) (8.71)

7.99 (7.94) 8.80 9.30 (9.36) 9.94 11.70 (11.42) 12.80 15.20 (15.10) 19.40 36.60 (36.14) TTAB + Gemini-2

8.22 (8.28) 8.96 9.96 (10.06) 10.60 12.00 (12.30) 14.40 18.00 (18.03) 23.60 146.00 (151.01) DTAB + Gemini-2

9.02 (8.97) 9.85 10.40 (10.60) 11.20 13.10 (13.33) 15.10 18.40 (18.49) 20.10

9.35 (10.96) 10.10 11.00 (11.12) 12.10 13.50 (13.52) 15.80 20.40 (20.40) 32.60

cmc measured by the conductivity method. bSD = ±0.02. ccmc measured by the fluorescence method given in parentheses. dSD = ±0.04. a

ratio of intensities of first and third major vibrational peaks (I1/ I3) in its fluorescence spectrum. However, an exact and direct determination of the cmc is not possible with I1/I3 for pyrene because of various ambiguities reported in the literature.53−56 To avoid all of these ambiguities, we measured the cmc values of pure and mixed surfactants by monitoring the change in fluorescence intensity with increasing concentration of surfactant of the twisted-intramolecular-charge-transfer (TICT) fluorescence probe DMACA. This probe is highly sensitive to changes in the micropolarity and microviscosity of the environment.57 Figure S2 (Supporting Information) presents the fluorescence spectra of DMACA with varying concentrations of TTAB + Gemini-1 in aqueous medium for αTTAB = 0.8 at 303.15 K. At concentrations below the cmc, the fluorescence intensity is very low, and a minimal enhancement in intensity with increasing surfactant concentration in the solution is observed. However, just above the cmc, there is a sharp increase in the intensity with a concomitant blue shift of the peak maximum (Figure S2, Supporting Information). This result suggests the transfer of DMACA from the polar bulk aqueous medium to the hydrophobic environment of the micelles.58 Figure 1 presents plots of the fluorescence intensity of DMACA as a function of the total concentration of TTAB + Gemini-1 at αTTAB = 0.8 and as a function of the concentration of pure TTAB at 303.15 K. The cmc values of pure surfactants estimated by the fluorescence method are in good agreement with both the conductivity and literature data (Table S1, Supporting Information). For mixed systems, the cmc values were determined using the fluorescence technique at bulk mole fractions of monomeric surfactants of only 0.2, 0.4, 0.6, and 0.8 to avoid recording a large number of emission spectra. These cmc values are consistent with those obtained using conductivity measurements and are also included in Table 1.

(2)

where IVV and IVH represent the vertically and horizontally polarized emission intensities, respectively, obtained by excitation with vertically polarized light. G is a correction factor, defined as G = IHV/IHH, where IHV and IHH represent the vertically and horizontally polarized emission intensities, respectively, obtained by excitation with horizontally polarized light.

3. RESULTS AND DISCUSSION 3.1. Determination of cmc. A representative plot of κ versus total surfactant concentration ([surfactants]total) with fitted data obtained after applying Carpena et al.’s method for the DTAB + Gemini-1 system, for αDTAB = 0.8 at 303.15 K, is shown by Figure S1 (Supporting Information). The cmc values obtained by this method for the pure surfactants were in good agreement with the literature values and are summarized in Table S1 (Supporting Information). The values of cmc for the mixed micellar systems obtained by using Carpena et al.’s method51 are reported in Table 1, and these values were used for the calculation of mixed micellar properties. To avoid criticism30 for determining the cmc by conductivity measurements, we also used the fluorescence method to estimate cmc values and compared the results with those determined by conductivity measurements. Pyrene is a wellknown probe for the determination of the cmc based on the 5897



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that the process of micellization was progressively delayed with increasing α1. Because of the lower surface activity of a monomeric surfactant as compared to that of either Gemini-1 or Gemini-2, the former has a lower tendency to form micelles than the latter, which results in an increase in the cmc with increasing α1. The presence of two hydrocarbon chains in a gemini surfactant molecule results in excess surface hydrophobicity in mixed systems, which leads to significant synergistic interactions in its binary mixtures with monomeric surfactants.60,61 A micelle is formed only when the hydrophobic interactions predominate over the Coulombic repulsive interactions among the surfactant molecules.39 Gemini surfactant molecules might contribute to the effective hydrophobicity of mixed micelles, leading to a decrease in the cmc.62 One can see that the cmc values of the mixed micelles were between the individual cmc values of the corresponding monomeric and gemini surfactants and, more precisely, closer to the cmc of the pure gemini surfactant than to that of the pure monomeric surfactant. It is pertinent to note from the data in Table 1 that the cmc values of monomeric + Gemini-2 mixed surfactant systems are higher than those of monomeric + Gemini-1 mixed surfactant systems. This could be because of the less surface-active nature of Gemini-2 as compared to Gemini-1, as discussed in section 3.2. The data in Table 1 also show that, in the presence of a very small amount of gemini surfactant, the cmc values of conventional surfactants are markedly reduced. For example, the cmc of pure DTAB is 146.0 × 10−4 M (Table 1), but at 0.9 mole fraction of DTAB (i.e., at 0.1 mole fraction of Gemini-1), the cmc is reduced to 23.60 × 10−4 M (Table 1), which is onesixth of the original value. This indicates that, in the presence of a very small amount of Gemini-1, the surface-active properties of DTAB increase significantly. However, in the presence of Gemini-2, the reduction in the cmc value is smaller than that for Gemini-1, because Gemini-2 is less surface-active than Gemini-1. On the other hand, the cmc of pure Gemini-1 is 6.9 × 10−4 M (Table 1). In the presence of a monomeric surfactant, the cmc is increased, but this increase in cmc is not very high. For example, even at 0.5 bulk mole fraction of DTAB, the cmc of the mixed system is 10.6 × 10−4 M (only 1.5 times the cmc of pure Gemini-1). These observations led us to conclude that the use of gemini surfactants can be made costeffective by mixing them with conventional surfactants at the expense of very little surface activity of the gemini surfactants. Although the cmc value of CTAB is close to those of gemini surfactants, the reason for choosing CTAB as one of the monomeric surfactants is to show the effect of variations in the hydrocarbon chain length. 3.3.2. Interactions and Thermodynamic Properties. To determine the ideality in mixed micelle formation, one can use Clint’s equation,28 which relates the theoretical cmc (cmc*) of ideal binary mixture to the experimental cmc values (cmc1 and cmc2) of the pure components

Figure 1. Fluorescence intensity of DMACA as a function of the total concentration of TTAB + Gemini-1 at αTTAB = 0.8 and (inset) as a function of the concentration of pure TTAB at 303.15 K. λex = 378 nm.

3.2. Pure Gemini Surfactants: Aggregation Behavior. The spacer group of a gemini surfactant plays a significant role in its aggregation properties. A gemini surfactant with a flexible hydrophilic spacer has a higher affinity toward micelle formation, which leads to a lower cmc, smaller ionization degree, and larger aggregation number than for a gemini surfactant with a rigid hydrophobic spacer.43,59 The hydroxyl groups in the spacers of Gemini-1 and Gemini-2 enhance their micellization processes. The cmc values of Gemini-1 and Gemini-2 are less than those of their corresponding gemini surfactants (12−3−12 and 12−4−12, respectively) without any hydroxyl group(s) in the spacer,30,40 which is most likely due to the hydrophilic nature43 of the spacer groups of Gemini-1 and Gemini-2. Although the spacer group of Gemini-2 is expected to be more hydrophilic than that of Gemini-1, the cmc of the former was found to be higher than that of the latter (Table 1). Because of the presence of intramolecular hydrogen bonding between the hydroxyl groups of the spacer group of Gemini-2, this spacer group is more rigid than that of Gemini-1. Moreover, there are four −CH2− groups in the spacer of Gemini-2 as compared to three −CH2− groups in the spacer of Gemini-1. Therefore, the increase in hydrophobicity of the spacer group of Gemini-2 with one additional −CH2− group might predominate over the increase in the hydrophilicity with one additional −OH group. Thus, the Gemini-2 surfactant molecule, with a rigid and less hydrophilic spacer than the Gemini-1 surfactant molecule, faces difficulty in forming aggregates because of the difficulty of locating a less hydrophilic spacer at the micelle−water interface and the steric inhibition of the rigid spacer group. As a result, the cmc of Gemini-2 was found to be higher than that of Gemini-1. Moreover, in the case of Gemini-1, because of the greater extent of intermolecular hydrogen bonding, the number of interfacial water molecules released will be greater, and hence, the increase in entropy during the formation of aggregates will also be greater. The extent of intermolecular hydrogen bonding is lower in the case of Gemini-2, as two hydroxyl groups already participate in intramolecular hydrogen bonding. As a result, aggregates of Gemini-1 surfactant molecules form at a lower surfactant concentration than for Gemini-2. Therefore, Gemini-1 is more active toward micelle formation than Gemini-2. 3.3. Mixed Surfactant Systems. 3.3.1. Aggregation Behavior. The data in Table 1 show that the cmc values increased with increasing bulk mole fraction (α1) of each monomeric surfactant with a given gemini surfactant (Gemini-1 or Gemini-2) in the mixed surfactant systems. It is noteworthy

α1 (1 − α1) 1 = + cmc* cmc1 cmc 2

(3)

where α1 is the mole fraction of surfactant 1 (the monomeric surfactant) in the mixed solution and cmc1 and cmc2 are the cmc values of pure surfactant 1 (monomeric surfactant) and surfactant 2 (Gemini-1 or Gemini-2), respectively. To identify the type of interactions between the components as well as the deviation from ideality, the theoretical and experimental cmc 5898



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previously mentioned negative deviations from ideality support the conclusion that the hydrophobic interactions in the system predominate over the Coulombic repulsive interactions. The β values were negative but not constant for all binary combinations throughout the entire range of mole fractions.30 The β parameter should be independent of micellar composition, so an average value is commonly used in the literature.30,65 The average values of the interaction parameter (βav) for the mixtures of CTAB, TTAB, and DTAB with Gemini-1 were −0.231, −0.838, and −2.242, respectively, and those for the mixtures of CTAB, TTAB, and DTAB with Gemini-2 were −0.064, −0.702, and −2.070, respectively. These values indicate that the attractive interaction increased with decreasing chain length of the monomeric surfactants. A similar trend was reported by Rodriguez et al.30 The maximum interaction was observed for the DTAB + Gemini-1 mixed system. This can be explained in terms of the interactions between the hydrocarbon chains. As the chain lengths of the monomeric and gemini surfactants become equal, a comfortable association of the components in the mixture is observed. Bakshi et al.66 reported that monomeric and gemini surfactants of equal chain lengths give compatibility at the level of the headgroup region. The results also show that the magnitudes of the βav values were lower for Gemini-2 than for Gemini-1 with any monomeric surfactant. On the basis of the discussion in section 3.2, it can be stated that, because Gemini-1 is more surface-active than Gemini-2, the association of monomeric surfactant molecules with Gemini-1 in mixed systems is expected to be friendlier than that with Gemini-2. The cmc data for mixed micelles summarized in Table 1 show some interesting trends. Although the interactions between the gemini and monomeric surfactants are a maximum when the surfactants have equal hydrocarbon chain lengths, the process of micellization becomes more favorable with increasing hydrophobicity of the individual surfactants. This is attributed to the decrease in cmc values with increasing chain length of the monomeric surfactants (Table 1). Friendlier interactions depend on the similarity in the structures of the components of mixed micelles; however, the hydrophobicity of the surfactant molecules is the driving force for the formation of micelles. The micelle mole fraction (X1) values for monomeric surfactant + Gemini-1/Gemini-2 mixed systems calculated using Rubingh’s model are reported in Table 2 and Table S2 (Supporting Information), respectively. The data on mixed micellar properties presented in various tables are provided at 0.2, 0.4, 0.6, and 0.8 bulk mole fractions of each mixed system only to save space. For the calculation of X1 values, the models of Motomura et al.31 and Rodenas et al.67 were also used. The micelle mole fractions at various bulk mole fractions calculated using these models were close to those calculated by Rubingh’s method (Table 2). The micelle mole fraction in the ideal state (Xideal 1 ) was evaluated by applying the equation proposed by Clint28,66 (Table 2)

values were plotted as a function of the mole fraction of monomeric surfactant in the solution at 303.15 K, and Figure 2

Figure 2. Critical micelle concentrations (cmc and cmc*) versus bulk mole fraction, α1, for monomeric surfactant + Gemini-1 systems.

presents such a plot as a representative figure for monomeric surfactant + Gemini-1 systems. It can be seen that, for all of the systems, the experimental cmc values are less than the theoretically calculated cmc values (cmc*) at each mole fraction of the conventional surfactant.56 These results indicate nonideal behavior. Because the experimental cmc lies below cmc*, this indicates a negative deviation from ideality. These results are quite consistent with the findings of Rodriguez et al.30 The negative deviation indicates that there are attractive interactions between the components of the investigated mixed surfactant systems.30 The deviation from ideality varies with the chemical nature of the spacer group of the gemini surfactants and also with the chain length of the monomeric surfactants.30 For binary mixtures of a monomeric surfactant and Gemini-1, the deviation from ideality was found to increase with decreasing chain length of monomeric surfactant. For Gemini-1, the deviation from ideality for the mixed micellization of CTAB with Gemini-1 is low. The maximum deviation was observed for the DTAB + Gemini-1 system. The same trends were also observed for binary mixtures of monomeric surfactant + Gemini-2, as shown in Figure S3 (Supporting Information); however, the deviations from ideality were less than those for the monomeric surfactant + Gemini-1 systems. These results clearly show the effect of the spacer group of the gemini surfactants. An explanation for possible reasons for all of these phenomena is given later. The experimental results were interpreted quantitatively using Rubingh’s model.29 Using this theory, it is possible to calculate the micelle mole fraction (X1) and the interaction parameter (β) from the equations [X12 ln(cmcα1/cmc1X1)] (1 − X1)2 ln[cmc(1 − α1)/cmc 2(1 − X1)]

β=

=1 (4)

ln(cmcα1/cmc1X1) (1 − X1)2

(5)

β indicates the magnitude of the interactions between the components in the mixed micelle and the extent of deviation from ideality.30,63,64 The larger the negative value of β, the stronger the attractive interaction between the two types of surfactant molecules. The interaction parameter β was found to have a negative value for all mixed surfactant systems investigated (not shown). These results show that the formation of mixed micelles is due to the attractive interactions between the components. The negative values of β and the

X1ideal = {(α1cmc 2)/[α1cmc 2 + (1 − α1)cmc1]}

(6)

For the mixture CTAB + Gemini-1, the X1 values calculated using various methods were very close to Xideal 1 . These results are shown in Figure 3 as a representative example, where only the X1 values calculated using Rubingh’s model are plotted. This figure shows that this system deviates slightly from ideality. The greater the difference between X1 and Xideal 1 , the 5899



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headgroup region.66 For the mixed systems of monomeric and Gemini-2 surfactants, similar types of interactions were observed, as shown by Figure S4 (Supporting Information). However, the magnitudes of deviations from ideality were less than those for the mixed monomeric surfactant + Gemini-1 systems. This is due to the fact that the participation of Gemini2 molecules in the mixed micelles is less than that of the Gemini-1 molecules, because the former are less surface-active than the latter. This result is supported by the higher micelle mole fraction (X2) of Gemini-1 than of Gemini-2 at any bulk composition, as reported in Table 2 and Table S2 (Supporting Information), respectively. Singh et al.33 observed the effect of the spacer group of the gemini surfactant on zwitterionic− gemini mixed surfactant system. They reported that the synergism in the mixed micellar system decreases with increasing length of the spacer. However, they worked with gemini surfactants with unsubstituted spacers. As discussed in section 3.2, the spacer of Gemini-2 is rigid, whereas the spacer of Gemini-1 is comparatively flexible. As a result of this rigidity, the hydrocarbon chains of Gemini-2 would not get proper room in the core of the micelles. Thus, the hydrophobic interactions between Gemini-2 and monomeric surfactants are less than those between Gemini-1 and monomeric surfactants. Moreover, as discussed previously, the spacer group of Gemini2 is expected to be less hydrophilic than that of Gemini-1. These results are also supported by the calculation of activity coefficients (Table 2). If β were equal to 0, the activity coefficients would be unity, and the mixture of surfactants would be ideal.68 The activity coefficients for the monomeric ( f1) and gemini (f 2) surfactants were calculated by the equations

Table 2. Various Mixed Micellar Parameters for Monomeric Surfactant + Gemini-1 Mixed Systems Based on Conductivity Measurement α1

Xideal 1

XRubingh 1

0.2 0.4 0.6 0.8

0.156 0.331 0.527 0.748

0.172 0.341 0.524 0.724

0.2 0.4 0.6 0.8

0.045 0.112 0.220 0.429

0.071 0.172 0.281 0.454

0.2 0.4 0.6 0.8

0.012 0.031 0.066 0.159

0.044 0.113 0.220 0.321

XMotomura 1

XRodenas 1

CTAB + Gemini-1 0.151 0.171 0.317 0.340 0.521 0.533 0.742 0.733 TTAB + Gemini-1 0.072 0.075 0.178 0.167 0.299 0.286 0.464 0.447 DTAB + Gemini-1 0.042 0.053 0.136 0.116 0.199 0.223 0.355 0.302

f1

f2

0.887 0.937 0.958 0.980

0.997 0.987 0.949 0.864

0.615 0.591 0.683 0.729

0.997 0.978 0.943 0.804

0.256 0.241 0.224 0.307

0.997 0.977 0.888 0.768

Xideal 1 ,

Figure 3. Micellar mole fractions, X1 and versus bulk mole fraction, α, for monomeric surfactant + Gemini-1 systems.

f1 = exp[β(1 − X1)2 ]

(7)

f2 = exp[β(X1)2 ]

(8)

It can be seen that the activity coefficient values in all mixed systems were less than unity, showing the nonideal behavior with attractive interactions.64 However, in the case of the CTAB + Gemini-1/Gemini-2 systems, the deviations from ideality are very low and are supported by activity coefficient values close to unity. The maximum differences between the f1 and f 2 values were found for the DTAB + Gemini-1/Gemini-2 systems. For both the TTAB + Gemini-1 and DTAB + Gemini1 systems, the activity coefficients of the gemini surfactants are higher than those of the monomeric surfactants. It is also noteworthy that the activity coefficient values of the gemini surfactants are higher than that of CTAB below α1 = 0.5, whereas this trend is reversed above α1 = 0.5. All of these results are in accordance with the cmc and X1 values already discussed. Quantitatively, any deviation from ideality can be demonstrated by the excess Gibbs energy of the micelles (GE). A positive value of GE indicates that the interactions in the mixed micelles are less attractive or more repulsive than those in the one-component micelles, and the reverse is true for a negative GE value.69 GE was calculated by the equation

greater the deviation from ideality.32 The X1 value is higher than Xideal up to bulk mole fraction of 0.5, after which X1 is 1 lower than Xideal 1 . For TTAB + Gemini-1, the difference between X1 and Xideal is greater than that for CTAB + Gemini-1, showing 1 a larger deviation from ideality. For the formation of mixed aggregates, Gemini-1 participates more even at higher concentrations of TTAB. The mixed micelle content of TTAB is higher than that of Gemini-1 only at αTTAB = 0.9. Further, for the system DTAB + Gemini-1, there is a large difference between X1 and Xideal 1 . All of these results further support the conclusion that the deviation from ideality increases with decreasing difference between the hydrocarbon chain lengths of the monomeric and gemini surfactants. If the hydrocarbon chain of a monomeric surfactant is longer than that of a gemini surfactant, then the former would not allow sufficient room to the latter to adjust its hydrocarbon chains in the core of the micelle.66 As the chain lengths of the monomeric and gemini surfactants become equal, there will be more interactions between them because their chains are adjusted well in the core of the micelle. This is why the maximum interaction was observed in the case of the DTAB + Gemini-1 system. Interestingly, for the DTAB + Gemini-1 system, the micellar content of Gemini-1 is higher than that of DTAB even at a high bulk composition of DTAB because of their good adjustment in the micelle core. A longer hydrocarbon tail of the monomeric surfactant compared to the gemini surfactant might also induce incompatibility in the

GE = RT[X1 ln f1 + (1 − X1)ln f2 ]

(9)

where R and T are the gas constant and the absolute temperature, respectively. f1 and f 2 were calculated using eqs 7 and 8, respectively. The calculated GE values for all mixtures were negative at all bulk mole fractions. For consideration of 5900



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the Motomura et al.31 and Rodenas et al.67 approaches to the activity coefficients, f1 and f 2 were calculated by the equations30,33 X1micellef1 cmc1 = α1cmc exp

(10)

X 2micellef2 cmc 2 = α2cmc exp

(11)

activity of Gemini-2 than Gemini-1. However, similar trends in cmc values for mixed systems of both Gemini-1 and Gemini-2 with monomeric surfactants could be a result of the predominant effect of the hydrophobicity of the surfactant tails over other possible effects. It is also pertinent to note that the micellization process is comparatively more spontaneous with the Gemini-1 surfactant than with the Gemini-2 surfactant. 3.4. Microenvironmental Properties. The micropolarity around a probe molecule reveals important information regarding the probable location of the probe molecule in the micelle.72−74 With the introduction of a suitable environmental polarity- and viscosity-sensitive fluorescent probe, it could be possible to predict the micellization process. Because TICT fluorescence is highly sensitive to the polarity and viscosity of environment, in the present study, the DMACA molecule, which exhibits TICT fluorescence,57 was employed to characterize the microenvironment of the mixed micelles. The fluorescence spectra of DMACA were recorded at 0.2, 0.4, 0.6, and 0.8 bulk mole fractions of monomeric surfactants for all of the mixed systems at λex = 378 nm. The TICT band shows a blue shift in all of the mixtures as compared to that in pure water58 (Figure S2, Supporting Information). For the evaluation of microenvironmental properties, we focused on the TICT fluorescence band. It has been reported in the literature that a TICT band is more sensitive to the environment than a locally excited (LE) band.75−77 The emission spectra of DMACA in dioxane−water mixtures were recorded, and then the emission maxima (in terms of wavenumbers) were plotted against ET(30), a solvent polarity parameter78 (Figure S5, Supporting Information). For the measurement of the micropolarity of mixed micelles, the emission spectra of DMACA in mixed surfactant solutions at a concentration well above the cmc (10 times) were recorded. Micropolarity sensed by DMACA was estimated by correlating the fluorescence peak maxima in mixed micellar systems with those in dioxane−water mixtures of different percentages. Micropolarity values expressed on the equivalent scale of ET(30) are reported in Tables S5 and S6 (Supporting Information) for monomeric surfactant + Gemini-1 and monomeric surfactant + Gemini-2 mixed systems, respectively. The average ET(30) value for all of the systems was found to be 51.6, and we can say that the micropolarity of DMACA binding sites in the mixed micelles is close to that of ethanol [ET(30) = 51.9]. These results show that the probe molecules reside in the micelle−water interface. Panja et al.58 reported the orientations of DMACA in different types of micelles. In cationic micelles, the acceptor part (CO) of DMACA oriented toward the positively charged Stern layer, and the donor part (−NMe2) oriented toward the Gouy−Chapman layer. The micropolarities of the mixed systems were also determined using pyrene as a probe molecule. The I1/I3 value for pyrene in ethanol was found to be 1.05 (intensities measured at 375 and 386 nm for I1 and I3, respectively, at λex = 339 nm). The average value of I1/I3 for all mixed systems was calculated to be 1.05 (Tables S5 and S6, Supporting Information). This indicates that the pyrene molecule also resided at the micelle−water interface. To obtain further information regarding the microenvironment around the probe molecule, the steady-state fluorescence anisotropy of DMACA was estimated in various mixed micellar systems measuring fluorescence intensities at 490 nm.79 Fluorescence anisotropy experiments are often carried out to estimate the microviscosity of an environment.59,80 To determine the microviscosity of each mixture, we first

E

In all cases, the values of G were found to be negative. This indicates that, for the formation of mixed micelles, there are attractive interactions between the surfactant molecules.30 The GE values for monomeric surfactant + Gemini-1 and monomeric surfactant + Gemini-2 systems at some selective mole fractions are reported in Tables S3 and S4 (Supporting Information), respectively. The magnitude of GE increases with decreasing the chain length of the monomeric surfactants. This result is also in accordance with the fact that the nonideality increases with decreasing chain length of the monomeric surfactants. The stability of the micelles can be studied by the calculation of the standard Gibbs free energy of mixed micelle formation, ΔG°m, which was calculated by the equation70,71 ΔGm° = (3 − 2g )RT ln Xcmc

(12)

where g is the degree of dissociation and Xcmc is the cmc expressed on a mole fraction scale. In all cases, the ΔGm ° values were found to be negative (Tables S3 and S4, Supporting Information). The values of ΔG°m indicate that mixed micelle formation is a spontaneous process for all of the mixtures. The spontaneity of the micellization process increased with increasing bulk mole fraction of gemini surfactant, and this phenomenon was significantly prominent for the TTAB/DTAB + Gemini-1 surfactant systems. It is worth mentioning that the average values of ΔGm ° for DTAB, TTAB, and CTAB with Gemini-1 were found to be −60.45, −64.22, and −69.87 kJ mol−1, respectively, and the average values for counterion dissociation, g, for DTAB, TTAB, and CTAB with Gemini-1 were 0.386, 0.328, and 0.262, respectively (Table S3, Supporting Information). One can see that both ΔGm ° and g decreased with increasing hydrocarbon chain length of the monomeric surfactants. This result is consistent with the fact that the micellization process becomes favorable with increasing hydrophobicity of one of the components of the mixed micelle that leads to a decrease in cmc values (Table 1). For Gemini-2 as one of the components of the mixed micelles, the trend was slightly different. The average values of ΔGm ° for DTAB, TTAB, and CTAB with Gemini-2 were −58.44, −49.90, and −68.79 kJ mol−1, respectively, and the average values of counterion dissociation, g, for DTAB, TTAB, and CTAB with Gemini-2 were 0.416, 0.472, and 0.271, respectively (Table S4, Supporting Information). The different trend for Gemini-2 might be because one or more free energy terms contribute differently to ΔG°m.61 The most probable factor responsible for difference in ΔGm ° values could be different extents of influence on the conformation of linked tails inside the core because of the difference in the chemical structure of spacers.61 The longer spacer of Gemini-2 as compared to Gemini-1 might help in better accommodating the hydrocarbon chains of DTAB in the core of the micelles. The corresponding Gibbs free energy term contributing to ΔG°m could be more negative for DTAB than for TTAB as one of the components of mixed micelle. However, the greater ΔGm ° value for DTAB + Gemini-2 as compared to DTAB + Gemini-1 is because of lower surface 5901



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determined the fluorescence anisotropy of DMACA in glycerol−water mixtures of various compositions using eq 2, as presented by Figure S6 (Supporting Information).59,80 By correlating the anisotropy of DMACA in surfactant mixtures with that in glycerol−water mixtures, we first determined the microenvironment on the equivalent scale of the composition of glycerol−water mixtures. We then determined the microviscosity from the correlation diagram between the viscosity of the glycerol−water mixtures and the compositions of the same mixtures (Figure S7, Supporting Information). The fluorescence anisotropy values of DMACA in various mixed micellar systems of monomeric surfactant + Gemini-1/Gemini-2 surfactants and corresponding microviscosity values are reported in Tables S5 and S6, respectively (Supporting Information). The microviscosity determined in this way was more or less same for each mixed system. The average value of microviscosity was found to be 1.39 cP. It is pertinent to note that, similarly to the micropolarity, the microviscosity of the environment around the probe was also same for each mixed system. 3.5. Binding of DMACA with Mixed Surfactant Systems. To determine the strength of the binding of the probe molecule (DMACA) with the mixed micelles, binding constant (KS) values were calculated. The emission intensity of DMACA increased with increasing concentration of surfactant (Figure 1). The binding constant was calculated using the equation81 F − F0 = KSFm − KSF [S]total − cmc

results also support our aforementioned discussion that the hydrophobicity of the micelles increases with increasing hydrocarbon chain length of the monomeric surfactant in the mixed micelles with gemini surfactants. With the increase in the hydrophobicity of the mixed micelle, the DMACA binding becomes stronger. The data are also consistent with the fact that the mixed micelles of monomeric surfactants with Gemini1 were more hydrophobic than those with Gemini-2.

4. CONCLUSIONS The cationic gemini surfactant Gemini-1, with a flexible and hydrophilic spacer group, is more active in aggregating than the cationic gemini surfactant Gemini-2, with a comparatively rigid and less hydrophilic spacer group. The mixed micellar properties of these gemini surfactants with cationic conventional surfactants with varying chain length were studied. The cmc values of all of the mixed systems were determined conductometrically, as well as using a TICT fluorescence probe molecule. The use of gemini surfactants can be made costeffective by mixing them with conventional surfactants at the expense of very slight decrease in the surface activity of the gemini surfactants. The surface activity of monomeric surfactants can be enhanced significantly by mixing them with gemini surfactants. The cmc values of all of the mixed systems were found to be between the individual cmc values of the gemini and monomeric surfactants. Different theoretical models have been used to analyze mixed surfactant systems. The interactions between the gemini and monomeric surfactant molecules in the mixed micelles were found to be attractive. Interestingly, it was found that, although the extent of interactions is a maximum when there are similarities in the structures of the hydrocarbon chains, the process of micellization is favored with increasing hydrophobicity of the monomeric surfactant. The longer spacer of a gemini surfactant might help in better accommodating a hydrocarbon tail of the same length of a monomeric surfactant in the core of a micelle. The corresponding Gibbs free energy term might contribute to the negative ΔG°m value. The participation of gemini surfactant molecules in mixed micelle formation was found to increase with decreasing spacer chain length and number of substituted hydroxyl groups in it. The cmc value of each mixed micellar system decreased with increasing bulk mole fraction of gemini surfactant. All of these results show that the mixed micellar properties can be tuned by changing the chemical structures of the spacer group and hydrocarbon chain, as well as by changing the solution composition. The formation of mixed micelles was found to be thermodynamically spontaneous for all of the mixed systems investigated herein. Keeping in mind the fact that micelles are used as drug molecule solubilizers and as biomimicking systems, the microviscosity and micropolarity of all of the mixed systems were determined using the viscosityand polarity-sensitive TICT fluorescence probe DMACA. The micropolarity of the environment around the probe molecule was found to be equivalent to that of ethanol. The average value of the microviscosity of the mixed systems was 1.39 cP. The binding constant of the fluorescent probe molecule increased with increasing content of gemini surfactant in the mixed micelle and also with increasing chain length of the monomeric surfactants. This is because of the increase in the hydrophobicity of the mixed micelles for either of the two factors. DMACA can potentially be used to estimate cmc values to avoid ambiguities in the use of pyrene.

(13)

where F, Fo, and Fm are the fluorescence intensities of DMACA at intermediate concentrations of surfactants, in water, and when completely bound to the mixed micelle, respectively. Figure 4 shows a typical plot of (F − Fo)/([S]total − cmc)

Figure 4. Plot of (F − Fo)/([S]total − cmc) versus F in the DTAB + Gemini-1 mixed system at αDTAB = 0.2.

versus fluorescence intensity, F, of DMACA in the DTAB + Gemini-1 mixed system at αDTAB = 0.2 as a representative example. From the slope of a fitted straight line, KS was calculated. The KS values for monomeric surfactant + Gemini1/Gemini-2 mixed systems are listed in Tables S5 and S6, respectively (Supporting Information). The binding constant increased with increasing content of gemini surfactant in the mixed micelles. This could be due to the more hydrophobic nature of the gemini surfactants as compared to the monomeric surfactants. The average values of KS for DMACA with mixed micelles of DTAB, TTAB, and CTAB with Gemini-1 were 256, 290, and 513 M−1, respectively. The same values with Gemini-2 surfactant were 201, 281, and 429 M−1, respectively. These 5902



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(12) Domingo, X. In Amphoteric Surfactants; Lomax, E. G., Eds.; Marcel Dekker: New York, 1996; pp 75−190. (13) Hall-Manning, T. J.; Holland, G. H.; Rennie, G.; Revell, P.; Hines, J.; Barratt, M. D.; Basketter, D. A. Skin Irritation Potential of Mixed Surfactant Systems. Food Chem. Toxicol. 1998, 36, 233. (14) Devínsky, F.; Masárová, L.; Lacko, I. Surface Activity and Micelle Formation of Some New Bisquaternary Ammonium Salts. J. Colloid Interface Sci. 1985, 105, 235. (15) Devínsky, F.; Lacko, I.; Bittererová, F.; Tomečková, L. Relationship between Structure, Surface Activity, and Micelle Formation of Some New Bisquaternary Isosteres of 1,5-Pentanediammonium Dibromides. J. Colloid Interface Sci. 1996, 114, 314. (16) Menger, F. M.; Keiper, J. S. Gemini Surfactants. Angew. Chem., Int. Ed. 2000, 39, 1906. (17) Zana, R. Dimeric and Oligomeric Surfactants. Behavior at Interfaces and in Aqueous Solution: A Review. Adv. Colloid Interface Sci. 2002, 97, 205. (18) Rosen, M. J. Geminis: A New Generation of Surfactants. CHEMTECH 1993, 23, 30. (19) Devínsky, F.; Lacko, I.; Imam, T. Relationship between Structure and Solubilization Properties of Some Bisquaternaryammonium Amphiphiles. J. Colloid Interface Sci. 1993, 143, 336. (20) Bai, G.; Wang, J.; Yan, H.; Li, Z.; Thomas, R. K. Thermodynamics of Molecular Self-Assembly of Cationic Gemini and Related Double Chain Surfactants in Aqueous Solution. J. Phys. Chem. B 2001, 105, 3105. (21) Ansari, W. H.; Fatma, N.; Panda, M.; Kabir-ud-Din. Solubilization of Polycyclic Aromatic Hydrocarbons by Novel Biodegradable Cationic Gemini Surfactant Ethane-1,2-diyl bis(N,Ndimethyl-N-hexadecylammoniumacetoxy)dichloride and Its Binary Mixtures with Conventional Surfactants. Soft Matter 2013, 9, 1478. (22) Tsubone, K. The Interaction of an Anionic Gemini Surfactant with Conventional Anionic Surfactants. J. Colloid Interface Sci. 2003, 261, 524. (23) Zana, R.; Xia, J. Gemini Surfactants: Synthesis Interfacial and Solution-Phase Behavior and Applications; Marcel Dekker: New York, 2004. (24) Garcia, M. T.; Ribosa, I.; Leal, J. S.; Cornelles, F. Monomer− Micelle Equilibrium in the Diffusion of Surfactants in Binary Systems through Collagen Films. J. Am. Oil Chem. Soc. 1992, 69, 25. (25) Rhein, L. D.; Simion, F. A.; Hill, R. L.; Cagan, R. H.; Mattai, J.; Maibach, H. I. Human Cutaneous Response to a Mixed Surfactant System: Role of Solution Phenomena in Controlling Surfactant Irritation. Dermatologica 1990, 180, 18. (26) Kibbey, T. C. G.; Hayes, K. F. A. Multicomponent Analysis of the Sorption of Polydisperse Ethoxylated Nonionic Surfactants to Aquifer Materials: Equilibrium Sorption Behavior. Environ. Sci. Technol. 1997, 31, 1171. (27) Lange, H. Uber Die Mizellenbildung in Mischlosunggen Homologer Paraffinkettensalze. Kolloid Z. 1953, 131, 96. (28) Clint, J. H. Micellization of Mixed Nonionic Surface Active Agents. J. Chem. Soc., Faraday Trans. 1975, 171, 1327. (29) Rubingh, D. N. Mixed micelle solutions. In Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum Press: New York, 1979; Vol. 2, pp 337−354. (30) Rodríguez, A.; Graciani, M. M.; Moreno-Vargas, A. J.; Moyá, M. L. Mixtures of Monomeric and Dimeric Surfactants: Hydrophobic Chain Length and Spacer Group Length Effects on Non Ideality. J. Phys. Chem. B 2008, 112, 11942. (31) Motomura, K.; Yamanaka, M.; Aratono, M. Thermodynamic Consideration of the Mixed Micelle of Surfactants. Colloid Polym. Sci. 1984, 262, 948. (32) Azum, N.; Naqvi, A. Z.; Akram, M.; Kabir-ud-Din. Studies of Mixed Micelle Formation between Cationic Gemini and Cationic Conventional Surfactants. J. Colloid Interface Sci. 2008, 328, 429. (33) Singh, K.; Marangoni, D. G. Synergistic Interactions in the Mixed Micelles of Cationic Gemini with Zwitterionic Surfactants: The pH and Spacer Effect. J. Colloid Interface Sci. 2007, 315, 620.

ASSOCIATED CONTENT

S Supporting Information *

Figures used for the calculation of cmc, figures and tables of mixed micelle properties of monomeric surfactant + Gemini-1/ Gemini-2 systems, figures for the calculation of microenvironmental properties. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91 1596 515279. Fax: +91 1596 244183. E-mail: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.K.S. acknowledges the Aditya Birla Groups for financial support under a major research project; the Council of Scientific and Industrial Research (CSIR) for financial support under a major research project [01(2213)/08/EMR-II]; the University Grants Commission (UGC) Special Assistance Programme [F.540/14/DRS/2007 (SAP-I)]; and the Department of Science and Technology (DST) FIST program, Government of India. S. acknowledges UGC for financial support under a junior research fellowship, and A.K.T. acknowledges CSIR for financial support under a senior research fellowship.

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Study on Mixed Micelles of Cationic Gemini Surfactants Having

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