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Mar 5, 2010 - Department of Chemistry, Aligarh Muslim University, Aligarh-202 002, India. Received December 21, 2009. Revised Manuscript Received Apri...
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Effects of Solvent Media and Temperature on the Self-Aggregation of Cationic Dimeric Surfactant 14-6-14, 2Br- Studied by Conductometric and Fluorescence Techniques Kabir-ud-Din* and P. Ajmal Koya Department of Chemistry, Aligarh Muslim University, Aligarh-202 002, India Received December 21, 2009. Revised Manuscript Received April 13, 2010 To explore how the solvent characteristics influence the self-aggregation of the cationic dimeric surfactant hexamethylene-1,6-bis(tetradecyldimethylammonium bromide) (14-6-14, 2Br-) and to obtain various energetic parameters, conductometric experiments were performed on the binary mixtures of three organic solvents;viz., 2-methoxyethanol (ME)-, acetonitrile (AN)-, and formamide (FA)- water (WR) at different temperatures ranging from 298.15 to 323.15 K. The steady state fluorescence measurements allowed us to calculate the average aggregation number (Nagg) and the air-bulk phase surface tension helped us to correlate the solvent cohesive energy density with Gibbs energy of micellization (ΔG0m). The results showed that, although micellization process becomes less favorable with the increase in volume % of the ME, AN and FA in the binary mixed media, the increment in the cmc below 20% (v/v) of the organic solvents is comparatively less showing the predominance of WR character in the bulk phase. Because of the decrease in Gibbs interfacial energy contribution (ΔG0interf) to ΔG0m, the average aggregation number (Nagg) decreased with the increase in the volume % of the organic solvent in the mixed media. As the enthalpy of micellization became more negative, the corresponding entropy change became less positive and enthalpy-entropy compensation phenomenon was observed for the micellization of 14-6-14, 2Br- in all the mixed media in the studied temperature range.

Introduction In a given medium (usually, water or binary mixtures of either two nonaqueous solvents or non-aqueous solvent with water), amphiphilic molecules self-aggregate together to minimize the unfavorable interaction with the solvent (medium) and form different types of aggregates (depending on their properties and the other physicochemical conditions which are experienced to them). Hydrophobic or, more generally, solvophobic interactions play an important role in raising the above situation and, therefore, several studies have been made by altering the medium (water) properties either by the incorporation of additives1-6 or *Corresponding author. Telephone: þ91 571 2703515. E-mail: kabir7@ rediffmail.com. (1) Myres, D. Surfactant Science and Technology; VCH Publishers: Weinheim, Germany, 1988. (2) Mishra, P. K.; Mishra, B. K.; Behera, G. Colloids Surf. 1991, 57, 1. (3) Bakshi, M. S.; Kaur, G.; Kaur, G. J. Macromolecular Sci. 1999, A36, 697. (4) Kumar, S.; Sharma, D.; Kabir-ud-Din Langmuir 2000, 16, 6821. (5) Kabir-ud-Din; Naqvi, A. Z.; Khan, A. B.; Al-Ahmadi, M. D. A.; Akram, M. J. Chem. Eng. Data 2009, 54, 387. (6) Desai, P. R.; Jain, N. J.; Sharma, R. K.; Bahadur, P. Colloids Surf. A: Physicochem. Eng. Aspects 2001, 178, 57. (7) 7. Jha, R.; Ahluwalia, J. C. J. Phys. Chem. 1991, 95, 7782. (8) Mukherjee, K.; Mukherjee, D. C.; Moulik, S. P. J. Phys. Chem. 1994, 98, 4713. (9) Gracie, K.; Turner, D.; Palepu, R. Can. J. Chem. 1996, 74, 1616. (10) Ruiz, C. C. Colloid Polym. Sci. 1999, 277, 701. (11) Ruiz, C. C.; M.-Bolivar, J. A.; Aguiar, J.; MacIssac, G.; Moroze, S.; Palepu, R. Langmuir 2001, 17, 6831. (12) Ruiz, C. C.; M.-Bolivar, J. A.; Aguiar, J.; MacIssac, G.; Moroze, S.; Palepu, R. Colloid Polym. Sci. 2003, 281, 531. (13) Graciani, M. M.; Munoz, M.; Rodriguez, A.; Moya, M. L. Langmuir 2005, 21, 3303. (14) Akbas, H.; Kartal, C. Colloid J. 2006, 68, 125–130. (15) Rodriguez, A.; Graciani, M. M.; Moya, M. L. Langmuir 2008, 24, 12785. (16) Shrivastava, A.; Ghosh, K. K. J. Surf. Deterg. 2008, 11, 287. (17) Jalali, F.; Rad, A. S. J. Iran Chem. Soc. 2008, 5, 309. (18) Aizawa, H. J. Appl. Crystallogr. 2009, 42, 592. (19) Ramesh, R.; Cassel, S.; Rico-Lattes, I. Langmuir 2009, 25, 6733.

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by mixing with polar/nonpolar nonaqueous solvents.7-21 The importance of studying aggregation behavior of surfactants in water-organic mixed solvent systems is driven by both fundamental and practical considerations.22 Various properties of surfactant solutions, such as their rheology and capability to solubilize water-insoluble compounds, are substantially influenced by the characteristic features of the micelles (like size, shape, etc.) in the given medium.23,24 Nowadays, dimeric surfactants which are composed of two hydrophilic head groups and two hydrocarbon tails attached through a hydrocarbon spacer are gaining wide attention by virtue of their unusual solution and interfacial properties (such as much lower cmc values, high efficiency in reducing the surface tension of water, better wetting properties, high solubilization capacity, etc.) compared to the analogues single head-single tail surfactants.25-34 In addition, some of these novel class of amphiphiles have shown their efficiency against microorganism.35,36 In many instances, for (20) Rodriguez, A.; Graciani, M. M.; Fernandez, G.; Moya, M. L. J. Colloid Interface Sci. 2009, 338, 207. (21) Das, C.; Das, B. J. Chem. Eng. Data 2009, 54, 559. (22) Lee, Y. S.; Woo, K. W. J. Colloid Interface Sci. 1995, 169, 34. (23) Hoffmann, H.; U Ibricht, W. J. Colloid Interface Sci. 1989, 129, 388. (24) Lu, T.; Huang, J.; Li, Z.; Jia, S.; Fu, H. J. Phys. Chem. B 2008, 112, 2909. (25) Devinsky, F.; Masarova, L.; Lacko, I. J. Colloid Interface Sci. 1985, 105, 235. (26) Zana, R.; Benrroau, M.; Rueff, R. Langmuir 1991, 7, 1072. (27) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 115, 10083. (28) Rosen, M. J. CHEMTECH 1993, 23, 30. (29) Song, Li. D.; Rosen, M. J. Langmuir 1996, 12, 1149. (30) Menger, F. M.; Keiper, J. S.; Azov, V. Langmuir 2000, 16, 2062. (31) Menger, F. M.; Keiper, J. S. Angew. Chem., Int. Ed. 2000, 39, 1906. (32) Zana, R. Adv. Colloid Interface Sci. 2002, 97, 203. (33) Zana, R.; Xia, J. Gemini Surfactants: Synthesis, Interfacial and SolutionPhase Behavior and Applications; Marcel Dekker: New York, 2004. (34) Yoshimura, T.; Ishihara, K.; Esumi, K. Langmuir 2005, 21, 10409. (35) Devinsky, F.; Lacko, I.; Bittererova, F.; Tomeckova, L. J. Colloid Interface Sci. 1986, 114, 314. (36) Laatiris, A.; El Achouri, M.; Infante, M. R.; Bensouda, Y. Microbiol. Res. 2008, 163, 645.

Published on Web 05/03/2010

DOI: 10.1021/la904812a

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Kabir-ud-Din and Ajmal Koya

Scheme 1. Structure of Hexamethylene-1,6-bis(tetradecyldimethylammonium bromide), 14-6-14, 2Br- (A) and Tetradecyltrimethylammonium Bromide, TTAB (B)

a given pair of conventional and dimeric surfactants, e.g., tetradecyltrimethylammonium bromide (TTAB) and hexamethylene1,6-bis(tetradecyldimethylammonium bromide) (14-6-14, 2Br-), although the chemical moieties are same in both except an additional hydrocarbon chain (see Scheme 1), the dimeric surfactants are favored. However, it can be seen from the literature that the studies on the behavior of amphiphiles in mixed media were mainly focused on the conventional surfactants7-21 rather than the dimeric surfactants, even though the later ones are superior in many counts, as mentioned above. Recently, along with other workers,37-41 we have reported42,43 the micellization of some dimeric surfactants having 14 and 16 tail carbon chain lengths in various mixed media containing water as one component. In the present study, we report the effects of addition of the three organic solvents and the variation in the temperature on the micellization of hexamethylene-1,6-bis(tetradecyldimethylammonium bromide) (14-6-14, 2Br-) cationic dimeric surfactant. The organic solvents chosen for the study, 2-methoxyethanol (ME), acetonitrile (AN), and formamide (FA), represent solvents which have lower and higher polarity than the universal solvent. FA is one of the organic solvents which has closest resemblance in physical parameters (e.g., chohesive energy density, high dielectric constant, etc.) to water and this is the reason why a number of examples of surfactant aggregation in this medium are found in the literature,19 while in the case of ME and AN, the studies are scanty. Besides, aggregation of dimeric surfactants above a 10% composition (12-3-12, 2Br- in 10 wt % WR-AN,38 16-s-16, 2Br- in 10 vol % WR-ME and WR-AN,42 where s = 4, 5, 6) of these mixed media (WR-ME and WR-AN) has not been reported yet. The effect of the temperature on the micellization process, in selected compositions of the mixed media (in addition to pure water), was also studied at four temperatures in the range 298.15-323.15 K to understand the effect of the bulk phase on the thermodynamic parameters related to micellization. Such a study on the micellization process of the dimeric surfactants in the mixed media is rare in the literature.41-43 It is crucial to have knowledge about the effect of water-organic solvent mixed systems for their proper application in diverse areas of chemistry. The present study will give insight about the selection of mixed solvent systems for their use in enhanced oil recovery, (37) Rodriguez, A.; Graciani, M. M.; Munoz, M.; Robina, I.; Moya, M. L. Langmuir 2006, 22, 9519. (38) Rodriguez, A.; Graciani, M. M.; Angulo, M.; Moya, M. L. Langmuir 2007, 23, 11496. (39) Rodriguez, A.; Graciani, M. M.; Munoz, M.; Cordobes, F.; Moya, M. L. J. Phys. Chem. B 2009, 113, 7767. (40) Kolay, S.; Ghosh, K. K.; Quagliotto, P. Colloid Surf. A: Physicochem. Eng. Aspects 2009, 348, 234. (41) Deepti; Ghosh, K. K.; Quagliotto, P. Indian J. Chem. 2009, 48A, 1522. (42) Kabir-ud-Din; Siddiqui, U. S.; Kumar, S.; Dar, A. A. Colloid Polym. Sci. 2006, 28, 807. (43) Kabir-ud-Din; Koya, P. A.; Khan., Z. A. J. Colloid Interface Sci. 2010, 342, 340.

7906 DOI: 10.1021/la904812a

pharmaceutical and cosmetic applications, washings, chemical reactions, etc. Also, from biological point of view, enzymes are being extensively investigated in mixed solvent systems because of their polarity resembles the natural cellular microenvironment than does pure water. In the context of the above, the study proposed herein deserves attention. Since the majority of the work reported in the mixed solvent systems was done only at a few selected compositions of the organic solvents, a study in which the compositions of the mixed systems are varied gradually is necessary for a clear understanding of how a variation in the solvent properties affects the micellization and related properties. So, at a particular temperature (303.15 K), we varied the compositions of the mixed media gradually and made ten different compositions for WR-ME (5-70 vol %) and seven for WR-AN, WR-FA (5-40 vol %). Aggregation number of the studied dimeric surfactant was calculated through steady state fluorescence quenching (SSFQ). In addition, an attempt has also been made to discuss the effect of the studied solvents on the self-association of 14-6-14, 2Brthrough the Gordon parameter.44

Experimental Section Materials. The dimeric surfactant hexamethylene-1,6-bis(tetradecyldimethylammonium bromide) (14-6-14, 2Br-) was synthesized by refluxing N,N-dimethyltetradecylamine (g95%, Fluka) with 1,6-dibromohexane (g97%, Fluka) in dry ethanol for 48 h at 353.15 K and recrystallized in ethanol/ethyl acetate mixture for several times. The obtained product was characterized by 1H, 13C NMR and elemental analysis. 2-Methoxyethanol (ME) (g99%, s.d. Fine-Chem Ltd.), acetonitrile (AN) (g99%, Qualigens Fine Chemicals) and formamide (FA) (g98.5%, s.d. Fine-Chem Ltd.) were mixed thoroughly with appropriate volumes of doubly distilled water at 298.15 K (in all cases) to obtain the various mixed solvent media. Pyrene (probe) and cetylpyridinium chloride (quencher) used for the steady state fluorescence measurements were of highest purity available. Conductivity Measurements. The conductance measurements were performed on an ELICO CM 82 T conductivity bridge equipped with a dip cell having cell constant 1.02 cm-1. All the experiments were done in a thermostated water bath holding the solution under study. The solvent (water or waterorganic solvent mixture) was equilibrated at the desired temperature for at least 30 min before the addition of the suitably prepared concentrated stock solution of 14-6-14, 2Br-. After each addition, the solution was mixed carefully without the formation of foam. The experimental error in the temperature was minimized to (0.2 K. Surface Tension Measurements. The solvent (water or water-organic solvent medium) surface tensions were measured by the ring detachment method using an S. D. Hardson tensiometer (44) Gordon, J. E. The Organic Chemistry of Electrolyte Solutions; Wiley: New York, 1975.

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Article

Table 1. Various Micellization Parameters for Micellization of 14-6-14, 2Br- (A, B, C) in Water-Organic Solvent (O. S.) Mixed Media at 303.15 K (A) Water-2-Methoxyethanol (WR-ME) System: volume % (o.s.)/v/v

cmca/mM

cmcb/mM

R

ΔG0m/kJ mol-1

ΔG0m,tail/kJ mol-1

ΔG0trans/kJ mol-1

0

0.182 (0.170)c 0.221 0.359 0.419 0.542 0.837 1.100 2.187 2.505 2.935 4.556

0.175

0.380 (0.370)c 0.314 0.347 0.348 0.355 0.379 0.389 0.372 0.404 0.469 0.446

-71.3

-35.6

0

-74.1 -69.0 -67.8 -65.6 -61.6 -59.2 -55.6 -52.6 -47.9 -45.9

-37.0 -34.5 -33.9 -32.8 -30.8 -29.6 -27.8 -26.3 -24.0 -22.9

-2.8 2.3 3.5 5.6 9.7 12.1 15.7 18.7 23.3 25.4

5 10 15 20 25 30 40 50 60 70

0.327 0.507 1.042 2.339 4.549

(B) Water-Acetonitrile (WR-AN) System: volume % (o.s.)/v/v

cmc /mM

cmc /mM

R

ΔG0m/kJ mol-1

ΔG0m,tail/kJ mol-1

ΔG0trans/kJ mol-1

0

0.182 (0.170)c 0.335 0.475 0.925 1.028 1.516 1.715 2.286

0.175

0.380 (0.370)c 0.348 0.343 0.363 0.415 0.443 0.410 0.436

-71.3

-35.6

0

-69.6 -67.6 -62.5 -58.8 -55.0 -55.9 -52.5

-34.8 -33.8 -31.2 -29.4 -27.5 -27.9 -26.3

1.7 3.6 8.8 12.5 16.2 15.4 18.7

ΔG0m,tail/kJ mol-1

ΔG0trans/kJ mol-1

5 10 15 20 25 30 40

a

b

0.428 0.998 1.667 2.238

(C) Water-Formamide (WR-FA) System: volume % (o.s.)/v/v

cmca/mM

cmcb/mM

R

ΔG0m/kJ mol-1

0.182 0.175 0.380 -71.3 -35.6 0 (0.370)c (0.170)c 5 0.284 0.344 -70.8 -35.4 0.4 10 0.446 0.443 0.388 -65.4 -32.7 5.8 15 0.673 0.393 -62.7 -31.4 8.6 20 0.823 0.813 0.435 -59.1 -29.5 12.2 25 1.266 0.466 -55.0 -27.5 16.3 30 1.644 1.640 0.423 -55.7 -27.8 15.6 40 2.315 2.244 0.433 -52.9 -26.5 18.4 a Determined from conductivity measurements. b Determined from surface tension measurements (using Kruss K 11 tensiometer). c Reference 73. 0

(India) at 298.15 K. For each system, the platinum ring was cleaned well and heated briefly in alcoholic flame until it glowed. The precision in the measurements was (0.1 mN m-1. Fluorescence Measurements. A Hitachi F-2500 fluorescence spectrophotometer was used to record the fluorescence spectra of micellar solutions of 14-6-14, 2Br- containing pyrene in different solvent media at 298.15 K. Following instrumental setting was done before the measurements: excitation wavelength, 333 nm; excitation slit width, 5.0 nm; emission slit width, 2.5 nm; scan speed, 60 nm min-1; and the emission spectra were recorded in the range of 350 to 450 nm. A 3 μM pyrene concentration was made in the different solvent (water or water-organic solvent mixtures)14-6-14, 2Br- micellar solutions. A surfactant concentration (3 mM) above the cmc was used in all the measurements. The first and third vibronic peaks appeared at 373 and 384 nm, respectively. Fluorescence quenching was done by the addition of the cetylpyridinium chloride (CPC). [Quencher] was varied slightly to ensure the Poisson distribution45 for the quencher.

Results and Discussion Effect of Organic Solvent Addition on the Micellization Parameters. Table 1 lists the critical micelle concentration (cmc, minimum concentration above which the self-association of (45) Hunter, T. F. Chem. Phys. Lett. 1980, 75, 152.

Langmuir 2010, 26(11), 7905–7914

surfactant monomers takes place), degree of counterion dissociation (R, fraction of the counterion dissociated out of one mole) and values of different free energies of micellization (ΔG0m, ΔG0m,tail, ΔG0trans - explained below) of 14-6-14, 2Br- in water and different mixed media at 303.15 K. Errors in the cmc and R are less than (0.005 mM and (0.01, respectively. The cmc and R values obtained for tetradecyltrimethylammonium bromide (TTAB), which is the monomer of the studied dimeric surfactant, are given in Table S1 (Supporting Information). Usually cmc is determined from the break point in the conductivity (κ) vs [surfactant] profile assuming the conductivity to be linearly related to the surfactant concentration. It is known that the values of cmc often influence the other micellization parameters such as average aggregation number (Nagg, number of monomers that come together to form micelles), ΔG0m, etc., and, therefore, it is important to have an appropriate method to obtain reliable cmc values, especially for studies in the mixed solvent media as the micellization of ionic surfactants in presence of organic solvents probably shows a weak curvature.10 In such cases, the determination of exact break becomes difficult. One of the most efficient methods which are being used recently15,20,39 is that proposed by Carpena et al.46 It is based on the fitting of the conductivity data (46) Carpena, P.; Aguiar, J.; B.-Galvan, P.; Ruiz, C. C. Langmuir 2002, 18, 6054.

DOI: 10.1021/la904812a

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Kabir-ud-Din and Ajmal Koya

Figure 1. Representative plot of specific conductance (κ) versus [14-6-14, 2Br-] dimeric surfactant in 20% WR-ME mixed medium at 303.15 K.

(κ) as a function of surfactant concentration (c) to the integral of Boltzmann-type sigmoidal equation KðcÞ

1 þ eðc - c0 =ΔcÞ ¼ Kð0Þ þ A1 c þ ΔcðA2 - A1 Þln 1 þ e - c0 =Δc

! ð1Þ

Here κ0, A1, A2 and Δc are the conductivity of the solution at zero concentration of the surfactant, premicellar slope, postmicellar slope and width of the transition, respectively. The procedure of the fitting has been explained in our earlier paper.42 The central point on the width of the transition (c0) corresponds to the cmc, and the degree of counterion dissociation (R) was determined from the ratio of postmicellar slope to premicellar slope as R = A2/A1. A representative plot (κ vs [14-6-14, 2Br-]) of fitting using Carpena’s method for the system 14-6-14, 2Br- in 20% WR-ME is shown in Figure 1. The volume % of the three organic solvents (ME, AN, FA) in the binary mixed media were varied (between 0 and 70 for ME, 0 and 40 for both AN and FA) to see as how the compositions of the mixed media affect the micellization parameters at 303.15 K. As observed for other ionic surfactants, an increase in the cmc was observed with the increase in the volume % of the organic solvent in the mixed medium (see Table 1). Generally, the hydrophobic/ solvophobic interaction is considered to be the main driving force behind the self-aggregation of the surfactant monomers. The increase in the cmc clearly shows a decrease in this driving force and, as a result, the transfer of dimeric surfactant monomers from the bulk phase to the micellar region becomes progressively less favorable with the increase in the amount of ME, AN, or FA in the binary mixed media. Moreover, the increase in the cmc was found to be comparatively lesser in WR-ME than WR-AN, or WR-FA mixed media showing a dependence of the nature of the medium on the micellization process. This implies that various properties of the solvents could be responsible for reduction in the tendency of the dimeric surfactant monomers to self-aggregate and they do prefer dual polar-nonpolar surroundings of the mixed systems than pure water. Obviously, a higher concentration of the surfactant would be required for the aggregation to commence with. It can be seen from Table 1 that the increase in the cmc values of 14-6-14, 2Br- is comparatively less below 20 vol % of the organic solvent in the three mixed media showing the 7908 DOI: 10.1021/la904812a

predominance of WR character in the bulk phase. A steep increase was observed, however, on increasing the organic solvent beyond 20% in the binary mixed media (Figure S1, Supporting Information). The ability of the ME to break down the three-dimensional structure of water42,47,48 decreases the solvophobicity of the surfactant monomers in this mixed medium. The dipolar aprotic solvent AN can disrupt the micelle formation through a better solvation of the surfactant monomers than pure water.16,42 Since the ε of ME and AN are lower than WR, upon mixing, the polarity of the bulk phase of WR-ME and WR-AN binary systems would decrease with the increase in the amount of the organic solvents. Consequently, repulsion between the ionic head groups increases and this opposes micellization.49 Reversal of the above is expected in the case of WR-FA mixed media as it is more polar than WR. But here too, the results show an increase in the cmc with the increasing volume % of the organic solvent as obtained for TTAB. This could be due to the solvation of the dimeric surfactant molecules by the ionic character of the FA. Moreover, although FA is more polar than WR, the increase in the polarity of the bulk phase with the amount of FA in WR-FA medium is accompanied by an increase in the Gibb’s energy of micellization (ΔG0m) (see Table 1, where the value becomes less negative). A similar explanation has been offered in the literature for the micellization of 12-3-12, 2Br- dimeric surfactant in WR-FA medium.38 The R values were found to be roughly increased with the increase in the volume % of the organic solvents and this could be due to the reduction in the charge density on the micellar surface caused by the decrease in the aggregation number. The values increased from 0.314 to 0.446, 0.348 to 0.436, and 0.344 to 0.433 as the volume % of the organic solvent was varied between 0 and 70, 0 and 40, and 0 and 40 for WR-ME, WR-AN, and WR-FA, respectively. However, the value of R obtained in pure water is little higher than the values in lower volume % of the organic solvents (ME and AN) whereas for both 14-6-14, 2Br- (with an exception to 5 vol % composition) and TTAB, a clear increase was obtained. SSFQ is found to be adequate for estimating Nagg of various amphiphiles in different mixed media as evidenced from the widely reported studies.11-13,20,38,50-56 If [I0], [IQ], [Q], [S], and [M], are, respectively, the fluorescence intensity in the absence of quencher, that in the presence of the quencher, concentration of the quencher, concentration of surfactant, and the unknown micelle concentration in the micellar solution, and if the probe molecule is luminescent only when it occupies an empty micelle, then the ratio of intensities in the presence and absence of the quencher is related as57   IQ ½Q ¼ ln ½M I0

ð2Þ

(47) Guha, P. K.; Kundu, K. K. Can. J. Chem. 1985, 63, 798. (48) Zana, R. Adv. Colloid Interface Sci. 1995, 57, 1. (49) Rosen, M. J. Surfactants and Interfacial Phenomena; Wiley: New York, 2004. (50) Ray, A.; Nemethy, G. J. Phys. Chem. 1971, 75, 809. (51) Glenn, K. M.; Moroze, S.; Palepu, R. M.; Bhattacharya, S. C. J. Dispersion Sci. Technol. 2005, 26, 79. (52) Aguiar, J.; M.-Bolivar, J. A.; Peula, J. M.; Ruiz, C. C. J. Colloid Interface Sci. 2005, 255, 382. (53) D’Errico, G.; Ciccarelli, D.; Ortona, O. J. Colloid Surface Sci. 2005, 286, 747. (54) Rodriguez, A.; Munoz, M.; Graciani, M. M.; Moya, M. L. J. Colloid Interface Sci. 2006, 298, 942. (55) Ruiz, C. C.; D.-Lopez, L.; Aguiar, J. J. Colloid Interface Sci. 2007, 305, 293. (56) Das, D.; Ismail, K. J. Colloid Interface Sci. 2008, 327, 198. (57) Turro, N. J.; Yekta, A. J. Am. Chem. Soc. 1978, 100, 5951.

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Kabir-ud-Din and Ajmal Koya

Article

Table 2. Various Parameters Calculated from SSFQ Technique for 14-6-14, 2Br- Micelles in Different Mixed Media at 298.15 K

Table 3. Surface Tension, Average Molar Volume, and Gordon Parameter Data of Different Mixed Media at 298.15 K

medium

Nagg

Ksv  104

I1/I3

D

medium

γ/mN m-1

Vm/cm3 mol-1

G/J m-3

pure WR 10% WR-ME 20% WR-ME 30% WR-ME 50% WR-ME 10% WR-AN 20% WR-AN 30% WR-AN 10% WR-FA 20% WR-FA 30% WR-FA

28 45 26 14 6 35 16 7 58 27 15

1.121 2.099 1.181 0.662 0.599 1.621 0.803 0.556 3.025 1.354 1.038

1.55 1.50 1.49 1.53 1.75 1.55 1.59 1.80 1.53 1.50 1.57

44 40 39 42 60 44 47 66 42 40 45

pure WR 10% WR-ME 20% WR-ME 30% WR-ME 50% WR-ME 10% WR-AN 20% WR-AN 30% WR-AN 10% WR-FA 20% WR-FA 30% WR-FA

71.60 58.75 52.40 48.90 43.00 56.35 49.00 42.10 68.40 65.10 59.65

18.07 19.58 21.37 23.52 29.43 19.35 20.82 22.53 19.12 20.29 21.62

2.73 2.18 1.89 1.71 1.39 2.10 1.78 1.49 2.56 2.39 2.14

Figure 2. Representative fluorescence (emission) spectra of 3 μM pyrene in 10% WR-ME micellar solution of 14-6-14, 2Br- at different quencher concentrations. The maximum intensity curve is for no quencher concentration and the successive curves are for [Q] = (0.50, 0.99, 1.48, 1.96, 2.44, 2.91, 3.38)  10-5 M. Plot of ln (I0/IQ) vs [CPC] for the same is shown in inset, T = 298.15 K.

which transforms to I0 ln IQ

! ¼

Nagg ½Q ðas ½M ¼ f½S - cmcg=Nagg Þ ½S - cmc

ð3Þ

The values of Nagg in different mixed media (given in Table 2) were calculated from the slopes (= Nagg/{[S] - cmc}) of the linear plots between ln (I0/IQ) vs [Q]. Error in the Nagg was not more than (2. Representative fluorescence (emission) spectra of 3 μM pyrene in 10% WR-ME micellar solution of 14-6-14, 2Br- at different quencher concentrations and its linear plot from which the slope was calculated are shown in Figure 2. The Nagg was found to be decreased in all the three studied mixed media with the increase in the volume % of the ME, AN, and FA. This behavior can be explained on the basis of solventhydrocarbon interfacial tension. Upon the addition of ME, AN, or FA, the surface tension (γ) of the pure water decreases (Table 3). As the variation in the γ is related to interfacial tension, organic solvent (mixed medium)-hydrocarbon interfacial tension becomes smaller than the water-hydrocarbon interfacial tension and Nagg values are expected to be decreased (as obtained), i.e., the contribution of interfacial Gibb’s energy (ΔG0interf)-(an interface is created between micellar core and the bulk phase during the formation of the micelles)-to the Gibb’s energy of micellization (ΔG0m) is minimized with the increase in the volume % of the organic solvent in the mixed medium as it (ΔG0interf) is (58) Camesano, T. A.; Nagarajan, R. Colloids Surf. A 2000, 167, 165.

Langmuir 2010, 26(11), 7905–7914

proportional to bulk phase/micelle core interfacial tension.58,59 However, although Nagg of 14-6-14, 2Br- decreases with the increase in the volume % of the organic solvents in all the three mixed media, the values obtained at 10% composition of the mixed media is little higher than that in the water. This may be the reason for the higher value of R in the pure water at least in 10 vol % composition of the ME where a more difference in that value is observed, due the decrease in the charge on the micellar surface by the decrease in the Nagg. The Nagg values of 12-6-12, 2Br-,39 14-6-14, 2Br- (this study), and 16-6-16, 2Br- 60 at 20, 3, and 2 mM in pure water are 27, 28, and 28, respectively. Regardless of their high concentration in the solution than cmc, Nagg values are smaller for the three dimeric surfactants in pure water and this is supposed to be due to the presence of large spacer between the head groups. An explanation for smaller Nagg values of wateralkyltrimethylammonium bromide micellar solutions is that the bulky quencher, like CPC, is not very effective in quenching process, because of very slow diffusion of that toward an excited probe molecule, especially in large micelles, and, as a result, the complete quenching in all the micelles containing both the probe and the quencher is susceptible.61 It is reported that the Nagg determined through SSFQ is smaller than that determined by time-resolved fluorescence quenching (TRFQ) for bis(dodecyldimethylammonium bromide) surfactants.38 Moreover, it should be noted that, as the water-organic solvent systems are more complicated than pure water (to favor the aggregation) and a fraction of the pyrene may get transferred to the bulk phase from the micellar phase, especially in the mixed medium containing a higher volume % of the organic solvent [we are highly thankful to one of the reviewers for this valuable suggestion] (see below), the values reported herein should be viewed as approximate and care should be given to the effect of solvent on Nagg. Other results which are obtained from SSFQ are the micropolarity (I1/I3), Stern-Volmer constant (KSV) and apparent dielectric constant (D) of the medium around the pyrene, and they are also given in the Table 2. The ratio of intensity of first and third vibronic peaks, i.e., I1/I3, of pyrene fluorescence emission spectrum in presence of surfactants is taken as the index of micropolarity of the medium (around pyrene). The variation in this ratio was different with the increase in the composition of organic solvent in different mixed media and higher values obtained in 50% WR-ME and 30% WR-AN indicate weakening of micellar hydrophobic environment.62 This is expected, at least for the mixed media containing a higher volume % of organic solvent (where a lower Nagg were obtained), due to (59) Nagarajan, R.; Wang, C.-C. Langmuir 2000, 16, 5242. (60) Azum, N.; Naqvi, A. Z.; Akram, M.; Kabir-ud-Din J. Colloid Interface Sci. 2008, 328, 429. (61) Stam, J.; Depaemelaere, S.; De Schryver, F. C. J. Chem. Educ. 1998, 75, 93. (62) Bakshi, M. S.; Bhandari, P. J. Photochem. Photobiol. A: Chem. 2007, 186, 166.

DOI: 10.1021/la904812a

7909

Article

Kabir-ud-Din and Ajmal Koya

Figure 3. Stern-Volmer plots for the fluorescence quenching of 3 μM pyrene in various water-organic solvent micellar solution of 14-6-14, 2Br- at 298.15 K, composition of organic solvent = 20 vol %.

penetration of the solvent molecules into the palisade layer of the micelles caused by the formation of less densely packed micelles. The result can further be evaluated by determining the first order quenching rate constant, called Stern-Volmer constant, using the relation: I0/IQ = 1 þ KSV[Q], which gives an idea about the bimolecular quenching and unimolecular decay as it being the product of rate constant of quenching process and lifetime of the probe in the absence of bimolecular quenching.63 The KSV values were calculated from the slope of the plot drawn between I0/IQ against [Q] (see Figure 3 for representative plots in various mixed media at 20 vol % compositions). The greater the solubility of the probe and quencher, the higher would be the KSV value. From Figure 3, it can be inferred that at a particular composition of the mixed media, the pyrene and CPC experience a different hydrophobic environment in the three mixed media. It seems that the presence of FA induces a stronger environment, while AN gives the weakest. The apparent dielectric constant of the medium (D) was then calculated from the relation: I1/I3 = 1.00461 þ 0.01253D. Their values (Table 2) also support the above discussion. The values of the Gordon parameter,44 which can be used to characterize an organic solvent to bring about the self-association of the amphiphiles along with average molar volumes (Vm) of various mixed media, are given in the Table 3. The relation G ¼ γ=Vm

1=3

was used to evaluate G (where γ is the surface tension of the pure or mixed solvent and Vm their molar volume). Here, the molar volumes of the various media were calculated by the summation of the Vm of the pure solvents (water and ME, AN or FA) corresponding to their mole fractions, as 2

Vm ¼ Σ x i Vi i ¼1

where xi and Vi are the mole fraction and molar volume of the ith component, respectively. This parameter is considered to be useful in understanding the cohesive energy density which is (63) R.-Mukherjee, K. K. Fundamentals of Photochemistry: Wiley Eastern: New Delhi, India, 1992.

7910 DOI: 10.1021/la904812a

Figure 4. Plots of Gibbs energy of micellization, ΔG0m, versus Gordon parameter, G, of the bulk phase: (a) in WR-ME and WR-AN; (b) in all the mixed media, T = 298.15 K.

related to the solubility behavior as the same intermolecular forces have to be overcome during solvation. Figure 4 shows the plots between ΔG0m and G for the micellization of 14-6-14, 2Br- in all the studied mixed media at 298.15 K. It can be seen that a reduction in the Gordon parameter decreases the spontaneity of the aggregation in all the cases and this trend fits well to yield a straight line with a correlation coefficient, r ≈ 0.96 (with an exception to WR-FA mixed media) showing a linear dependence of the bulk phase with ΔG0m for the studied dimeric surfactant in different mixed media at 298.15 K. On inclusion of the values in WR-FA mixed media and pure water, the r value reduced to 0.67. On the basis of the above facts, one can understand that, at least for water-organic solvent binary mixed media, which are less polar than water, the cohesive energy density of the bulk phase seems to be playing a notable role in determining the contribution of the solvophobic effect on the ΔG0m of the 14-6-14, 2Br- in the studied mixed media. The same type of result has been obtained for the micellization of 12-3-12, 2Br- in various mixed media, where also a deviation, as obtained in the present study, in the WR-FA mixed medium was observed.38 Effect of Temperature on Cmc and r. Conductivity measurements were performed, containing some selected volume percentages of the organic solvents (10, 20, 30, and 50 v/v for ME; 10, 20, and 30 v/v for AN and FA), in the binary mixed media in addition to the pure water at four temperatures ranging from 298.15 to 323.15 K. The values of cmc and R along with other energetic parameters of 14-6-14, 2Br- in different media are given in Table 4. One can see an increase in their values (cmc and R) with the rise in temperature at all the volume % of the different organic solvents; the increase in the cmc values becomes intensified at least for higher volume % in the binary mixed media. This can be understood clearly from the plot of cmc difference (cmcdiff, the difference in the cmc at higher and lower temperatures for a particular system: in this study, cmcdiff = cmc323.15K - cmc298.15K) vs volume % of the organic solvent shown in Figure S2 (Supporting Information). The cmcdiff value is less in pure water (0.094) compared to that in mixed media. It (Figure S2, Supporting Information) also makes clear that the magnitude of the cmcdiff is less in the case of WR-ME mixed medium than the WR-AN and WR-FA for the micellization of Langmuir 2010, 26(11), 7905–7914

Kabir-ud-Din and Ajmal Koya

Article

Table 4. Values of Cmc, r, and Thermodynamic Parameters for Micellization of 14-6-14, 2Br- in Various Mixed Media at Different Temperatures (A) Water-2-Methoxyethanol (WR-ME) System: volume% (o.s.)/v/v

T/K

cmc/mM

R

ΔG0m/kJ mol-1

ΔG0m,tail/kJ mol-1

ΔG0trans/kJ mol-1

ΔH0m/kJ mol-1

ΔS0m/J K-1 mol-1

0 0 0 0

298.15 303.15 313.15 323.15

0.158 0.182 0.215 0.252

0.369 0.380 0.391 0.397

-71.6 -71.3 -72.0 -72.9

-35.8 -35.6 -36.0 -36.4

0 0 0 0

-26.7 -27.4 -28.9 -30.6

150.5 144.8 137.4 130.8

10 10 10 10

298.15 303.15 313.15 323.15

0.302 0.359 0.495 0.668

0.294 0.347 0.331 0.390

-72.0 -69.0 -70.3 -67.1

-36.0 -34.5 -35.1 -33.5

-0.4 2.3 1.7 5.8

-55.3 -54.6 -59.1 -59.8

56.1 47.4 35.8 22.7

20 20 20 20

298.15 303.15 313.15 323.15

0.456 0.542 0.664 1.148

0.303 0.355 0.356 0.378

-68.5 -65.6 -66.5 -64.0

-34.2 -32.8 -33.2 -32.0

3.1 5.6 5.5 8.9

-65.4 -64.8 -69.0 -72.1

10.2 2.9 -8.0 -24.9

30 30 30 30

298.15 303.15 313.15 323.15

0.672 1.100 1.477 1.813

0.368 0.389 0.394 0.396

-62.1 -59.2 -59.2 -59.7

-31.0 -29.6 -29.6 -29.9

9.5 12.1 12.8 13.2

-41.8 -42.5 -45.1 -47.9

67.9 55.2 44.9 36.5

50 50 50 50

298.15 303.15 313.15 323.15

2.216 2.505 2.923 3.225

0.392 0.404 0.425 0.440

-53.0 -52.6 -52.4 -52.8

-26.5 -26.3 -26.2 -26.4

18.6 18.7 19.6 20.1

-29.5 -30.2 -31.5 -33.1

78.8 74.1 66.6 60.8

(B) Water-Acetonitrile (WR-AN) System: volume% (o.s.)/v/v

T/K

cmc/mM

R

ΔG0m/kJ mol-1

ΔG0m,tail/kJ mol-1

ΔG0trans/kJ mol-1

ΔH0m/kJ mol-1

ΔS0m/J K-1 mol-1

0 0 0 0

298.15 303.15 313.15 323.15

0.158 0.182 0.215 0.252

0.369 0.380 0.391 0.397

-71.6 -71.3 -72.0 -72.9

-35.8 -35.6 -36.0 -36.4

0.0 0.0 0.0 0.0

-26.7 -27.4 -28.9 -30.6

150.5 144.8 137.4 130.8

10 10 10 10

298.15 303.15 313.15 323.15

0.386 0.475 0.574 0.625

0.346 0.343 0.371 0.387

-67.6 -67.6 -67.1 -67.7

-33.8 -33.8 -33.5 -33.9

4.1 3.6 4.9 5.1

-43.3 -44.9 -46.7 -49.1

81.3 75.0 64.9 57.7

20 20 20 20

298.15 303.15 313.15 323.15

0.823 1.028 1.321 1.629

0.400 0.415 0.425 0.454

-59.9 -58.8 -58.8 -57.9

-29.9 -29.4 -29.4 -28.9

11.7 12.5 13.2 15.0

-37.4 -38.1 -40.3 -41.8

75.4 68.2 59.1 49.8

30 30 30 30

298.15 303.15 313.15 323.15

1.587 1.715 2.036 2.391

0.435 0.410 0.482 0.454

-54.1 -54.9 -51.3 -53.6

-27.0 -27.9 -26.5 -27.6

17.5 16.7 20.0 18.4

-25.2 -25.8 -24.9 -27.3

96.9 97.8 87.1 83.9

(C) Water-Formamide (WR-FA) System: volume% (o.s.)/v/v

T/K

cmc/mM

R

ΔG0m/kJ mol-1

ΔG0m,tail/kJ mol-1

ΔG0trans/kJ mol-1

ΔH0m/kJ mol-1

ΔS0m /J K-1 mol-1

0 0 0 0

298.15 303.15 313.15 323.15

0.158 0.182 0.215 0.252

0.369 0.380 0.391 0.397

-71.6 -71.3 -72.0 -72.9

-35.8 -35.6 -36.0 -36.4

0.0 0.0 0.0 0.0

-26.7 -27.4 -28.9 -30.6

150.5 144.8 137.4 130.8

10 10 10 10

298.15 303.15 313.15 323.15

0.332 0.446 0.565 0.831

0.371 0.388 0.393 0.415

-67.0 -65.4 -65.9 -64.4

-33.5 -32.7 -33.0 -32.2

4.6 5.8 6.1 8.5

-61.2 -62.4 -66.2 -69.1

19.3 10.2 -1.0 -14.5

20 20 20 20

298.15 303.15 313.15 323.15

0.698 0.823 1.182 1.937

0.402 0.435 0.419 0.419

-60.8 -59.1 -59.9 -58.9

-30.4 -29.5 -30.0 -29.5

10.8 12.2 12.1 14.0

-57.3 -57.4 -62.2 -66.2

11.8 5.4 -7.4 -22.6

30 30 30 30

298.15 303.15 313.15 323.15

1.398 1.644 2.177 2.979

0.420 0.423 0.450 0.470

-55.7 -55.7 -54.5 -53.5

-27.9 -27.8 -27.3 -26.7

15.9 15.6 17.5 19.4

-47.9 -49.4 -51.4 -53.7

26.3 20.7 10.0 -0.7

Langmuir 2010, 26(11), 7905–7914

DOI: 10.1021/la904812a

7911

Article

Kabir-ud-Din and Ajmal Koya

14-6-14, 2Br- dimeric surfactant surfactant. A similar type of behavior has been obtained previously for the aggregation of 14-5-14, 2Br- in another three binary mixed media.43 Generally, for an ionic surfactant, a variation in the temperature can affect the aggregation of the surfactant solution in two ways: (1) by inhibiting the micellization due to disruption of the water structure around the hydrophobic group and this increases the cmc, and (2) by favoring the micellization due to decrease in the degree of hydration of the hydrophilic groups and this decreases the cmc. In the present study, predominance of the first effect over the second seems to be happening as evidenced from the cmc values. The gradual rise in the R with respect to temperature (T) could be due to the decrease in the surface charge density on the micelle and, hence, more fraction of the dimeric surfactant and counterions are preferred to stay in dissociated form, as the increase in the T reduces the aggregation number of the ionic surfactants.64,65 Thermodynamics of Micellization. The two main approaches which have received wide acceptance among the colloid research groups for the interpretation of the energetics of the micellization are phase-separation and mass-action ones. For the ionic surfactants, the mass-action approach is usually preferred65 and various thermodynamic parameters can be deduced from the temperature dependence of the cmc values as explained below: According to this model, the micellization of the dimeric surfactant 14-6-14, 2Br- can be written by (since one dimeric surfactant monomer can give three ions in solution, one cation and two counterions) nG2þ þ 2ðn - pÞBr - SM2pþ

ð4Þ

-

2þ

where G represents the dimeric cations, Br the counterions of the 14-6-14, and M2pþ the aggregate of n monomers with an effective charge of 2p.The Gibbs energy of micelle formation per mole of the dimeric surfactant, ΔG0m, is given by "

ΔG

0

m

#   -1 p ln aM 2pþ þ ln aG2þ þ 2 1 ln aBr - ð5Þ ¼ RT n n

where R, T have their usual meanings and a’s are the respective activities. In general, for a micelle formed with an adequate number of monomer units the first term in the parentheses would be small and can be neglected. Under this situation, the activities of the corresponding ions can be replaced by their activities at the cmc and this may be equal to the critical micelle concentration expressed in mole fraction scale, Xcmc, of the dimeric surfactant. By the above approximation, for the studied dimeric surfactant, ΔG0m value depends on both cmc and R and can be written as66 ΔG0 m ¼ 2RTð1:5 - RÞln Xcmc

ð6Þ

Here, the ratio of cmc of 14-6-14, 2Br- to the total concentration of all components in the system was calculated to get the Xcmc (i.e., the values were calculated as Xcmc = cmc/(cmc þ number of moles of the solvent), either water or water-organic solvent mixed medium). For the micellization in pure water, the number of moles of solvent is taken as 55.556 mol dm-3. It should (64) del Rio, J. M.; Prieto, G.; Sarmiento, F.; Mosquera, V. Langmuir 1995, 11, 1511. (65) Attwood, D.; Florence, A. T. Surfactant Systems: Their Chemistry, Pharmacy and Biology; Chapman and Hall: London and New York, 1983. (66) Zana, R. Langmuir 1996, 12, 1208.

7912 DOI: 10.1021/la904812a

be noted that the total number of moles of solvent decreases with the increase in the amount of the organic solvent in the mixed medium. The other Gibbs energies, such as Gibbs energy of transfer (ΔG0trans) and Gibbs energy of micellization per alkyl tail of the dimeric surfactant (ΔG0m,tail), were obtained according to ΔG0trans =ΔG0m(water-organic solvent mixed medium) - ΔG0m(pure water) and ΔG0m.tail = ΔG0m/2.The values of various Gibbs free energies of the 14-6-14, 2Br-, both in pure water and in the three water-organic mixed solvent systems at 303.15 K, are also listed in Table 1. The ΔG0m values were found to be negative in all the cases and the positive values of ΔG0trans increased with the increase in the volume percentages of the organic solvent. It can be seen from the Table 1 that, though variations are there on the increment in the values of ΔG0m in the three different waterorganic solvent mixed systems, the values increase (become less negative) as the volume % of the organic solvent increases in the system showing that the micellization process is less spontaneous in the studied water-organic solvent binary mixed media than that in pure. If we compare the various compositions of the mixed media up to 30 vol % of the organic solvents, at a particular temperature, it is seen that the micellization process of 14-6-14, 2Br- is relatively more spontaneous in WR-ME mixed solvent system (decrease in the value of ΔG0m is less) than in the WR-AN and WR-FA mixed solvent systems. Obviously, the changes in the values of ΔG0trans also support the above discussion. The ΔG0trans values are more in WR-AN and WR-FA than in WR-ME. The magnitude of the ΔG0m is mainly controlled by the bulk phase properties. According to the theory of surfactant aggregation proposed by Nagarajan et al.,59 various Gibbs energies, such as (i) ΔG0trans, that accounts for the tail transfer free energy when it is moved from the bulk phase to the hydrophobic core of the micelle, (ii) ΔG0def, that accounts for the tail deformation Gibbs energy as the surfactant tail inside the micelle has a different conformation due to the molecular packing requirements, (iii) ΔG0interf, that accounts for the interfacial Gibbs energy between the hydrocarbon aggregate core-solvent interface (which has been discussed briefly earlier and it mainly controls the Nagg of the micelle and the dependence of this on the cmc is usually small), (iv) ΔG0ste, and (v) ΔG0ionic, that account for the steric and ionic Gibbs energy contributions for the ionic head groups at the micelle, contribute to ΔG0m. It is known that the ΔG0trans is mainly responsible for the delay in the micellization of surfactants in the mixed media13,15 and their value depends on the transfer Gibbs free energies from pure water and the organic solvents in addition to their mutual interaction. As the addition of organic solvent modifies the bulk phase making it more preferable than pure water for surfactant molecules,67 the transfer of the hydrophobic tail from the bulk phase to the micellar region becomes less favorable, and hence ΔG0m value increases (becomes less negative). This is in agreement with the micellization results of 14-6-14, 2Br- in WR-ME, WR-AN, and WR-FA mixed media. Since ΔG0m,tail is half of the ΔG0m, its values are expected to be roughly equal to ΔG0m of TTAB, which is the monomer of the studied dimeric surfactant. For this purpose, although TTAB was studied well in different mixed media,10,21,68-70 we were not able to locate the literature data except in WR-FA mixed medium15 (these values are given in Table S1, Supporting Information). At a (67) Marcus, Y. Ion Solvation: Wiley: London, 1986. (68) Palepu, R.; Gharibi, H.; Bloor, D. M.; W.-Jones, E. Langmuir 1993, 9, 110. (69) Rodriguez, A.; Graciani, M. M.; Munoz, M.; Moya, M. L. Langmuir 2003, 19, 7206. (70) Naorem, H.; Devi, S. D. J. Surf. Sci. Technol. 2006, 22, 89.

Langmuir 2010, 26(11), 7905–7914

Kabir-ud-Din and Ajmal Koya

Article

Figure 5. Plots of enthalpy change values (ΔHom) against the

volume % of different organic solvents for 14-6-14, 2Br- dimeric surfactant in different mixed media at 298.15 K. Solid lines are for visual purposes.

particular volume % (we converted weight % data from the literature into volume %, as it is less than the corresponding studied volume %, the actual value might be slightly more), ΔG0m,tail values are approximately similar for the conventional and dimeric surfactants in WR-FA mixed medium. Regarding the values of ΔG0trans, it has been pointed out previously37 that, at a particular composition, their values remain practically same for the dimeric surfactant 12-s-12, 2Br- (where s = 3-5) with its monomeric counterpart dodecyltrimethylammonium bromide (DTAB) in WR-EG mixed medium. In the case of 14-6-14, 2Br-, the ΔG0trans values were found to be higher than that of its counterpart TTAB in the WR-FA mixed medium (see Table 1 and Table S1, Supporting Information). A similar type of increase in their values can be seen for 12-6-12, 2Br- in WR-EG mixed medium,39 and this is supposed to be due to the effect of the large spacer in between the heads of the dimeric surfactant molecule. The corresponding enthalpy change, ΔH0m, accompanied by the micellization of 14-6-14, 2Br- in different mixed media, can be calculated by the expression     dðln Xcmc Þ dR - ln Xcmc ΔH 0 m ¼ -2RT 2 ð1:5 - RÞ dT dT P P ð7Þ At a particular composition of the water-organic solvent mixed media, the variation of R with temperature is not so much and, therefore, the second term in the above expression can be neglected and it becomes ΔH

0

m

  dðln Xcmc Þ ¼ -2RT ð1:5 - RÞ dT P 2

ð8Þ

The values of ln Xcmc at a particular composition in all the water-organic solvent mixed media were plotted against the temperature, T. A linear plot was obtained for each of the systems studied and the slope of these plots was taken as the values of d(ln Xcmc)/dT. Accordingly, the entropy change, ΔS0m, was calculated by the expression ΔS0 m ¼

ΔH 0 m - ΔG0 m T

Langmuir 2010, 26(11), 7905–7914

ð9Þ

Figure 6. Enthalpy-entropy compensation plots for the micellization of 14-6-14, 2Br- in WR-FA mixed media.

The various thermodynamic parameters obtained using above procedures for 14-6-14, 2Br- in pure water and WR-ME, WR-AN and WR-FA mixed media at the selected volume % of the organic solvents are listed in the Table 4. At a particular composition of the water-organic solvent mixed system, the ΔG0m values were found to become slightly more or less negative with the rise in temperature suggesting that the micellization of 14-6-14, 2Br- in these media is only slightly dependent on the studied temperature range. The enthalpy of micellization, ΔH0m, was negative and also varied accordingly as ΔG0m with the rise in temperature for a particular composition of the mixed media. However, at all the studied T, on plotting the variation of ΔH0m against the volume % of the organic solvents, a maximum in the value of ΔH0m with respect to composition of the medium was observed. The plots obtained at 298.15 K are shown in Figure 5. One can see from Figure 5 and Table 4 that the micellization of 14-6-14, 2Br- is exothermic in nature and its magnitude varies with both the composition of the mixed media and the variation in the temperature. At all the four temperatures, the magnitude of ΔH0m was found to be more in the mixed media corresponding to 10% WR-AN, 10% WR-FA, and 20% WR-ME. This behavior could be due to various interactions between the solute-solvent and solvent-solvent. During the micellization of 14-6-14, 2Br-, one can expect a disruption of some of the H-bonds or any other type of interaction between WR-WR and organic solvent- organic solvent (if any interaction exists there - endothermic process) and they may be partially reformed at the end (exothermic process). The overall magnitude of enthalpy would be depending on the overall energies of the two processes and which one exceeds over the other. The values of ΔS0m obtained are positive, which decrease with the increase in temperature (in WR-FA mixed medium, at higher T, even a negative entropy was observed) for a particular composition and show a rough decrease with the volume % of the organic solvent at a particular temperature. This decrease in the entropy indicates that the tendency of micellization reduces at higher T. As the enthalpy of micellization is negative and becomes more negative with the rise in temperature, accordingly, the positive entropy change becomes less positive and enthalpyentropy compensation (EEC) effect was observed for the micellization of 14-6-14, 2Br- in all the studied systems. The EEC plots obtained for the micellization of the studied dimeric surfactant in WR-FA mixed media are shown in Figure 6. DOI: 10.1021/la904812a

7913

Article

Kabir-ud-Din and Ajmal Koya

Table 5. Compensation Temperature (Tc), Enthalpy of Compensation (ΔH*m), and the Corresponding r Values of 14-6-14, 2Br- in Different Mixed Media in the Temperature Range of 298.15 K-323.15 K medium

Tc/K

ΔH*m/kJ mol-1

r

pure WR 10% WR-ME 20% WR-ME 30% WR-ME 50% WR-ME 10% WR-AN 20% WR-AN 30% WR-AN 10% WR-FA 20% WR-FA 30% WR-FA

199.4 163.2 213.8 194.1 197.0 235.7 180.3 73.2 245.3 277.6 220.6

-56.49 -63.81 -66.77 -54.25 -44.88 -62.43 -50.78 -32.49 -65.58 -59.89 -53.78

0.991 0.899 0.960 0.948 0.991 0.992 0.991 0.478 0.991 0.986 0.998

Similar types of plots with other systems also resulted in satisfactory r values. It is considered that the micellization process involves a chemical part and a solvation part71 and the observed linear relationship can be interpreted by72 ΔH0m = ΔH*m þ TcΔS0m. The intercept of the line (ΔH*m) and the slope (Tc, compensation temperature) provide the information about solute-solute interaction (chemical) and solute-solvent (solvation) interaction. Their values obtained in different mixed media along with r are given in the Table 5 and the variation in their values also supports the influence of the organic solvent on the modification of the bulk phase.

Conclusions The various micellization and energetic parameters of 14-6-14, 2Br- dimeric surfactant in WR, WR- ME, WR-AN and WR-FA mixed media have been determined with the help of conductometric and fluorescence techniques by varying both the amount of organic content (% v/v) in the bulk phase and the temperature. The following conclusions were drawn: As the binary mixed media are preferred by 14-6-14, 2Br- dimeric surfactant molecules due to solvophobic effect, the addition of (71) Lumry, R.; Rajender, S. Biopolymers 1970, 9, 1125. (72) Frank, H. S.; Evans, M. W. J. Chem. Phys. 1954, 13, 507. (73) Azum, N.; Naqvi, A. Z.; Akram, M.; Kabir-ud-Din J. Chem. Eng. Data 2009, 54, 1518.

7914 DOI: 10.1021/la904812a

ME, AN, or FA makes self-aggregation less favorable than that in pure water. The bulk phase energy density described through its Gordon parameter showed a linear relation with the Gibbs energy of micellization (ΔG0m) in WR-ME and WR-AN mixed media (a small deviation in this behavior was observed in WR-FA mixed medium). At a particular composition of the mixed media, although the micellization is not favored as in pure water, it seems that it is comparatively more favored in WR-ME than others (WR-AN, WR-FA). The gradual variation in the amount of organic solvent in the bulk phase reveals that the increment in the cmc is less below 20% (v/v) of ME, AN and FA and the cmcdiff varied with the compositions of the binary mixed media. The decrease in the average aggregation number of micelles in the mixed media with the increase in the volume % of the organic solvent is due to the decrease in interfacial Gibbs energy. Various thermodynamic parameters of micellization manifest that the process is exothermic, and with the increase in the volume % of the ME, AN, or FA in the binary mixed medium it becomes less favorable. With the rise in temperature, the predominance of enthalpic contribution over entropic contribution toward Gibbs energy of micellization (ΔG0m) was observed and a linear correlation was obtained in the enthalpy-entropy compensation plot for the micellization of 14-6-14, 2Br- in all the binary mixed media in the studied temperature range. Acknowledgment. UGC is gratefully acknowledged for providing a fellowship to P.A.K. Note Added after ASAP Publication. This article was published ASAP on May 3, 2010. Due to a production error, equation 5 was incorrect. The correct version was published on May 10, 2010. Supporting Information Available: Table of micellization parameters of TTAB in WR-FA mixed medium at 303.15 K and figures showing the plots of cmc and cmcdiff vs volume % of the organic solvents in the mixed media are given. This material is available free of charge via the Internet at http:// pubs.acs.org.

Langmuir 2010, 26(11), 7905–7914