Micellar Solutions of Sulfobetaine Surfactants in Water−Ethylene

Langmuir 2005, 21, 7161-7169

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Micellar Solutions of Sulfobetaine Surfactants in Water-Ethylene Glycol Mixtures: Surface Tension, Fluorescence, Spectroscopic, Conductometric, and Kinetic Studies Marı´a del Mar Graciani, Amalia Rodrı´guez, Marı´a Mun˜oz, and Marı´a Luisa Moya´* Departamento de Quı´mica Fı´sica, Universidad de Sevilla, C/Profesor Garcı´a Gonza´ lez s/n, 41012 Sevilla, Spain Received April 1, 2005. In Final Form: May 27, 2005

Micellization in water-ethylene glycol (EG) N-dodecyl, N-tetradecyl, and N-hexadecyl-N,N-dimethyl3-ammonio-1-propanesulfonate (SB3-12, SB3-14, and SB3-16, respectively) micellar solutions, with the weight percent of EG changing within the range 0-40, was studied by means of surface tension measurements. Information about the influence of the added EG on the aggregation number of the sulfobetaine micelles and on the polarity of the interfacial region of micelles was obtained through fluorescence and spectroscopic measurements. Surface tension measurements also provide information about the dependence of the surface excess concentration, the minimum area per surfactant molecule, the surface pressure at the cmc, and the standard Gibbs energy of adsorption on the added weight percent of the organic solvent. The Gordon parameter of the water-EG mixtures was also estimated by means of surface tension measurements. The thermodynamic and structural changes originated by the presence of EG control the micellar kinetic effects observed in the reaction methyl 4-nitrobenzenesulfonate + Br- occurring in the water-EG sulfobetaine micellar solutions. Information about the distribution of bromide ions between the bulk and micellar pseudophases was obtained through conductivity measurements. The kinetic micellar effects were quantitatively explained by using the pseudophase kinetic model.

Introduction Among water-soluble surfactants, zwitterionic compounds have been recognized as especially stable against external variations, particularly ionic strength, and temperature.1 Since they are electrically neutral, they differ from anionic surfactants but also from poly(oxyethylene alkyl ether) nonionic surfactants, especially in their thermal behavior. The properties of solutions of zwitterionic surfactants depend mainly on structural parameters of the amphiphile itself, that is, on its chemical structure.2 Interactions between headgroups, which set the optimal curvature of a surfactant film at an interface,3 are mainly due to dipole-dipole repulsions between zwitterionic groups and the bulkiness of the headgroups.4 These surfactants have recently attracted increasing attention owing to their wide applicability and their increased commercial uses.1,5 Besides, they are compatible with a wide variety of ionic and nonionic surfactants, and these mixtures show interesting synergistic effects.6-10 Studies * To whom all correspondence should be directed. E-mail: [email protected]. Homepage: www.us.es/coloides. (1) Bluestein, B. R.; Hilton, C. L.; Eds. Amphoteric Surfactants, Surface Science Series 12; Marcel Dekker: New York, 1982. (2) (a) Soederman, O.; Carlstroem, G.; Monduzzi, M.; Olsson, U. Langmuir 1988, 4, 1039-1044. (b) Zhao, F.; Rosen, M. J. Phys. Chem. 1988, 88, 6041-6044. (c) Faucompre´, B.; Lindman, B. Langmuir 1987, 3, 383. (3) Israelachvili, J. N.; Mitchell, D. J.; Niham, B. W. J. Chem. Soc., Faraday Trans. 1 1976, 1525. (4) Chevalier, Y.; Storet, Y.; Pourchet, S.; Le Perchec, P. Langmuir 1991, 7, 848. (5) See the series of papers Langmuir 1991, 7, 842. (6) Iwasaki, I.; Ogawa, M.; Esumi, K.; Meguro, K. Langmuir 1991, 7, 30. (7) Shiloah, A.; Blankschtein, D. Langmuir 1997, 13, 3968. (8) Li, F.; Li, G.-Z.; Chen, J.-B. Colloid Surf. A 1998, 145, 167. (9) Mulqueen, M.; Blankschtein, D. Langmuir 2000, 16, 7640.

on zwitterionic surfactants are mainly focused on

CH3-(CH2)mN+(CH3)2(CH2)nSO3sulfobetaine surfactants and

CH3-(CH2)mN+(CH3)2(CH2)nCOOcarboxybetaine surfactants although phosphobetaine surfactants, amine oxide surfactants, and lysolecythin, among others, have also been investigated. These studies show that monodisperse micelles, the size of which does not vary as the surfactant concentration increases, are the current picture of micellar solutions made of zwitterionic surfactants. We are currently interested in the study of micellization processes of different surfactants in water-organic solvent mixtures.11-14 An adequate organic solvent for these studies is ethylene glycol, since it has many characteristics similar to those of water. The molecule is small and can form hydrogen bond networks. It also possesses a high cohesive energy and a fairly high dielectric constant. The ability of ethylene glycol, EG, to bring about selfassociation of conventional amphiphiles can be character(10) Mysselyn-Bauduin, A.; Thibault, A.; Grandjean, J.; Broze, G.; Jerome, R. J. Colloid Interface Sci. 2001, 238, 1. (11) Rodrı´guez, A.; Graciani, M. M.; Mun˜oz, M.; Moya´, M. L. Langmuir 2003, 19, 7206. (12) Graciani, M. M.; Rodrı´guez, A.; Mun˜oz, M.; Moya´, M. L. Langmuir 2003, 19, 8685. (13) Rodrı´guez, A.; Mun˜oz, M.; Graciani, M. M.; Ferna´ndez Pacho´n, S.; Moya´, M. L. Langmuir 2004, 20, 9945. (14) Graciani, M. M.; Mun˜oz, M.; Rodrı´guez, A.; Moya´, M. L. Langmuir in press.

10.1021/la050862j CCC: $30.25 © 2005 American Chemical Society Published on Web 07/01/2005

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Table 1. Critical Micelle Concentrations of Various Sulfobetaine Surfactants in Water-EG, Mixtures at 298.2 K SB3-12 103 × cmc/ mol dm-3

SB3-14 104 × cmc/ mol dm-3

SB3-16 105 × cmc/ mol dm-3

wt % EG

a

b

a

b

a

b

0.0 15.0 30.0 40.0

2.7 4.0 6.1 7.1

1.6 2.4 4.1 6.1

2.7 4.0 6.8 10.2

1.9 3.1 5.6 9.4

2.8 4.2 9.1 15.0

2.2 3.7 6.4 12.6

a

In the absence of NaBr. b In the presence of NaBr 0.25 M.

ized by its Gordon parameter,15 G ) γ/V h 1/3, where γ is the solvent surface tension and V h its molar volume. For EG the Gordon parameter is equal to 1.25 J m-3. Since G values above 1.0-1.2 J m-3 seem to be required for selfassociation of the micellar type, micellization is expected to occur in pure EG. In the present work, the micellization process of N-dodecyl, N-tetradecyl, and N-hexadecyl-N,Ndimethyl-3-ammonio-1-propanesulfonate, SB3-12, SB314, and SB3-16, respectively, has been studied in waterEG mixtures with the percentage by weight of EG up to 40% at 298.2 K. Thermodynamic and structural information about the water-EG sulfobetaine micellar solutions was obtained through surface tension, spectroscopic, and fluorescence measurements. Studies on zwitterionic micelles are more scarce than on ionic and nonionic micelles, and to our knowledge, no investigations about the influence of the addition of an organic solvent on the micellization process (as well as on the structure of the micelles formed) of sulfobetaine surfactants have been carried out. The changes provoked by the presence of EG can be shown through the variations observed in the reaction rates of adequate micelle-modified processes taking place in the water-EG sulfobetaine micellar solutions. With this in mind, the reaction between methyl 4-nitrobenzenesulfonate, MBS, and Br- was studied in the water-EG sulfobetaine micellar solutions. This reaction has a wellknown mechanism16 and the contribution of the process occurring in the bulk phase (water-EG mixtures) can be obtained experimentally. Besides, the binding of the bromide anions to the sulfobetaine micelles can be studied through conductivity measurements17 and this will allow us to get information about the effects of the EG presence on the binding affinity of bromide ions to the surface of sulfobetaine micelles. All the measurements were carried out at 298.2 K. Experimental Section Materials. Methyl 4-nitrobenzenesulfonate was from Fluka. N-Dodecyl, N-tetradecyl, and N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonates were from Aldrich, as were NaBr and ethylene glycol. The surfactants were used as received, their cmc’s obtained through surface tension measurements being in agreement with literature data (see Table 1). Reichardt’s dye and hexadecylpyridinium chloride were form Fluka. Pyrene was obtained from Aldrich and was purified before use. Conductivity Measurements. Conductivity was measured with a crison microCM 2201 conductimeter connected to a water (15) Evans, D. F.; Miller, D. D. Organised Solutions. Surfactants in Science and Technology; Friberg, S. E., Lindman, B., Eds.; Dekker: New York, 1992; p 33. (16) Zachariasse, K. A.; Phuc, N. V.; Kozankiewicz, B. J. Phys. Chem. 1981, 85, 2676 and references therein. (17) (a) Ingold, C. K. Structure and Mechanisms in Organic Chemistry, 2nd ed.; Cornell University Press: Ithaca, NY, 1969. (b) Lowry, T. H.; Richardon, K. S. Mechanism and Theory in Organic Chemistry, 3rd ed.; Harper and Row: New York, 1987. (c) Reichardt, C. Solvents and Solvent Effects in Organic Chemistry, 2nd ed.; VCH: Weinheim, Germany, 1988.

flow thermostat maintained at 298.2 ( 0.1 K. The conductivity cell was calibrated with KCl solutions in the appropriate concentration range. Surface Tension Measurements. The air/solution interfacial tensions were measured with a Du Nou¨y tensioameter (KSV 703, Finland). The tensiometer was connected to a water flow criostat kept at 298.2 ( 0.1 K. Prior to each measurement the ring was heated briefly by holding it above a Bunsen burner until glowing. The vessel was cleaned by using chromic sulfuric acid, boiled in distilled water, and then flamed with a Bunsen burner before use. The precision in the measurements was (1 mN m-1. ET Values. Spectra of the assay solutions containing the ET(30) dye were recorded in a Unicam Helios-R spectrophotometer at 298.2 K. These spectra were recorded against a blank consisting of an aqueous or water-EG micellar solution of identical concentration to the assay solution. Five spectra were recorded for each assay solution. The ET value for tetradecyltrimethylammonium bromide, TTAB, aqueous micellar solutions was obtained, this being 53.4 in good agreement with literature.18 Fluorescence Measurements. Fluorescence measurements were made by using a Hitachi F-2500 fluorescence spectrophotometer. The temperature was kept at 298.2 ( 0.1 K by a water flow thermostat connected to the cell compartment. The fluorescence quenching of pyrene by hexadecylpyridinium chloride was studied as in ref 19. The solution was prepared as in ref 20. The probe concentration was kept low enough (2 × 10-6 M) to avoid excimer formation, and the quencher concentration was varied from 5 × 10-5 to 25 × 10-5 M. These values give [pyrene]/[micelles] and [quencher]/[micelles] ratios low enough to ensure Poisson distribution.21 Kinetics. The reaction was followed spectrophotometrically at 298.2 K in a Unicam UV-2 at 280 nm as described in ref 12, the substrate and the NaBr concentrations being 1 × 10-4 and 0.25 M, respectively. Observed first-order rate constants were obtained from the slopes of ln(A∞ - At) against time plots, where At and A∞ are the absorbances at time t and at the end of the reaction, respectively. Under these working conditions the firstorder kinetic plots were linear for more than five half-lives. Each experiment was repeated at least twice, and the rate constants were found to be reproducible within a precision of about 5%.

Results Table 1 shows the cmc values of the sulfobetaine surfactants solutions in water-EG mixtures in the presence and in the absence of NaBr 0.25 M. These values were obtained by means of surface tension measurements, as the concentrations at the point of intersection of the two linear portions of the γ-log([surfactant]) plots (Figure 1 is representative). The cmcs corresponding to the aqueous surfactant solutions are in good agreement with those in the literature.22 Figure 1 shows the dependence of the surface tension on the logarithm of surfactant concentration for the water-EG SB3-14 surfactant solutions in the presence of NaBr 0.25 M. Values of the aggregation numbers of the micelles present in the water-EG sulfobetaine micellar solutions obtained by studying the fluorescence quenching of pyrene by hexadecylpyridinium chloride are listed in Table 2. The influence of the quencher concentration on the (18) Rodrı´guez, A.; Graciani, M. M.; Guinda, M. A.; Mun˜oz, M.; Moya´, M. L. Langmuir 2000, 16, 3182. (19) Velazquez, M. M.; Costa, M. B. J. Chem. Soc., Faraday Trans. 1990, 86, 4043. (20) Rodrı´guez Prieto, M. F.; Rı´os Rodrı´guez, M. C.; Mosquera Gonza´lez, M.; Rı´os Rodrı´guez, A. M.; Mejuto Ferna´ndez, J. C. J. Chem. Educ. 1995, 72, 662. (21) (a) Tachiya, M. Chem. Phys. Lett. 1975, 33, 179. (b) Infelta, P.; Gratzel, M. J. Phys. Chem. 1979, 70, 179. (c) Tachiya, M. In Kinetics of Nonhomogeneous Processes; Freeman, G. R., Ed.; John Wiley and Sons: London, 1987; p 575. (d) Infelta, P. P. Chem. Phys. Lett. 1979, 61, 88. (e) Hunter, T. F. Chem. Phys. Lett. 1980, 75, 152. (22) Frescura, V. A.; Marconi, D: M: D.; Zanette, D.; Nome, F.; Blasko, A.; Bunton, C. A. J. Phys. Chem. 1995, 99, 11494.

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Figure 1. Dependence of surface tension on log[surfactant] for SB3-14 micellar solutions in the presence of NaBr 0.25 M. T ) 298.2 K. Table 2. Aggregation Numbers of Various Sulfobetaine Micelles in Water-EG, Mixtures at 298.2 K wt % EG SB3-12 SB3-14 SB3-16

0.0

15.0

30.0

40.0

56 67 71

50 52 55

44 45 53

39 41 48

intensity of the pyrene fluorescence in water-EG SB312, with 15 wt % EG micellar solutions, is shown in Figure 2. The aggregation numbers found in aqueous solution are in agreement with literature data.23 The surface excess concentration, Γmax, and the minimum area per surfactant molecule, Amin, at the air/solvent interface were obtained using the surface tension measurements and the following equations:

∂γ 1 Γmax ) RT ∂ ln C

[

Amin )

]

Γmax NA

T,P

(1) (2)

here R is the gas constant, NA is Avogadro’s number, γ is the surface tension value, and C is the concentration of surfactant in solution. The value of the surface pressure at the cmc, Πcmc, were obtained by using eq 3, where γo is the surface tension of the solvent and γcmc is the surface tension at the cmc. Γmax, Amin, and Πcmc values are listed in Table 3.

Πcmc ) γo - γcmc

(3)

The ability of a solvent to bring about self-association of conventional amphiphiles can be characterized by its h 1/3, where γ is the solvent Gordon parameter,24 G ) γ/V surface tension and V h its molar volume. Table 4 shows γ, V h , and the Gordon parameter for the water-EG mixtures used as bulk phase in the sulfobetaine micellar solutions investigated. (23) (a) Li, F.; Li, G.-Z.; Chen, J.-B. Colloid Surf. A 1998, 145, 167. (b) Di Profio, P.; Germani, R.; Savelli, G.; Cerichelli, G.; Chiarini, M.; Mancini, G.; Bunton, C. A.; Gillitt, N. D. Langmuir 1998, 14, 2662. (c) Malliaris, A.; Boens, N.; Van der Auweraer, M.; de Schryver, F. C.; Reekmans, S. Chem. Phys. Lett. 1989, 155, 587. (24) Brinchi, L.; Di Profio, P.; Germani, R.; Savelli, G.; Spretti, N.; Bunton, C. A. Eur. J. Org. Chem. 2000, 3849.

Figure 2. Influence of N-hexadecylpyridinium concentration on the intensity of the pyrene fluorescence in water-EG SB312 solutions with 15 wt % EG at 298.2 K. Table 3. Surface Excess Concentration, Γmax, Minimum Area Per Surfactant Molecule, Amin, Surface Pressure at the cmc, Πcmc, and the Standard Gibbs Energy of Adsorption, ∆G°ad, for SB3-12, SB3-14 and SB3-16 in Water-EG, Mixtures at 298.2 K wt % EG

106 × Γmax/ mol m-2

00.0 15.0 30.0 40.0

2.7 2.5 2.2 2.0

00.0 15.0 30.0 40.0 00.0 15.0 30.0 40.0

1020 × Amin/ m2

Πcmc/ mN m-1

-∆G°ad/ kJ mol-1

SB3-12 59.8 67.1 75.0 81.3

31.7 24.7 21.6 19.8

26.0 23.6 22.4 22.1

3.2 2.9 2.7 2.6

SB3-14 52.4 56.2 61.4 62.7

34.4 28.2 24.2 22.2

31.1 28.8 27.1 25.6

3.6 3.4 2.9 2.7

SB3-16 45.5 49.0 56.2 60.7

36.2 29.1 25.4 23.7

35.8 33.4 31.7 30.6

Table 4. Solvent Surface Tension, γ, Solvent Molar Volume, V h , and Gordon Parameter, G, for Some Water-EG, Mixturesa wt % EG

γ (mN m-1)

V h (dm3 mol-1)b

G ) γ/V h 1/3 (J m-3)

0 15.0 30.0 40.0 100

71.8 64.3 60.6 58.5 48.0

18.07 23.7 29.4 33.2 55.9

2.74 2.24 1.96 1.82 1.25

a T ) 298.2 K. Data corresponding to the pure solvents were taken from ref 25. b Estimated considering V h )V h EGXEG + V h water (1 - XEG).

Table 5. Reichardt’s Parameter, ET(30), for Various Sulfobetaine Water-EG, Micellar Solutions at 298.2 K ET(30) wt % EG

0.0

15.0

30.0

40.0

SB3-13 SB3-14 SB3-16

52.7 52.0 51.6

52.8 52.3 51.8

53.2 52.6 52.8

53.8 53.4 52.4

Table 5 shows the ET Reichardt’s parameter values for the sulfobetaine micellar solutions in water-EG mixtures. Figure 3 shows the changes in the observed rate constant for the reaction methyl 4-nitrobenzenesulfonate, MBS, with bromide ions (see Scheme 1) upon changing the

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del Mar Graciani et al. Table 6. Dependence of the Second Order Rate Constant, k2,bulk/dm3 mol-1 s-1, for the Reaction Methyl 4-Nitrobenzenesulfonate + Br- on the Amount of Ethylene Glycol, EG, Present in Water-EG Mixturesa wt % EG

0

15

30

40

104 × k2,bulk (dm3 mol-1 s-1)

4.5

4.7

5.5

6.6

a

T ) 298.2 K.

of the weight percentage of EG, are listed in Table 6. The value in water is in agreement with literature data.24 Discussion Table 1 shows that an increase in the weight percent of EG present in the mixture results in an increase in the critical micelle concentration in all of the surfactant solutions studied, in the absence as well as in the presence of NaBr 0.25 M. One can also see in this table that the presence of this moderately high concentration of NaBr decreases the cmc, although the decrease is not large. Zwitterionic micelles, despite being formally neutral, bind ions.25 An accepted description of the driving force for binding in the case of sulfobetaines, and analogous monomer-forming aggregates, is the large surface positive charge density. This distinct radial charge separation, although electroneutral, generates a dipole moment, which in turn, can attract charged species.26 Radiactive, tracer self-diffusion, and fluorescence quenching data have shown that anions bind more strongly than cations,27 the binding appearing to be essentially of an electrostatic nature, although ion specificity cannot be neglected. However, the charge taken by zwitterionic micelles remains low and this explains why the micellar aggregation number and the phase diagrams of zwitterionic surfactants do not substantially change with the addition of salts.25 This would also explain the small change observed in the cmc in the presence of NaBr 0.25 M with respect to its absence in the sulfobetaine surfactants studied. To quantify the effect of the presence of ethylene glycol in the mixture on the micellization process, the Gibbs energy of micellization, ∆G°M, can be calculated by using the following equation:28

∆G°M ) RT ln cmc

Figure 3. Dependence of the observed rate constant, kobs in s-1, for the reaction methyl 4-nitrobenzenesulfonate + Br- on surfactant concentration at 298.2 K. (a) SB3-12; (b) SB3-14; (c) SB3-16. Scheme 1

surfactant concentration in the SB3-12, SB3-14, and SB316 water-EG micellar solutions studied. The second-order rate constant values, kbulk/M-1 s-1, for the reaction MBS + Br- in homogeneous water-EG mixtures, as a function

(4)

Table 7 summarizes the Gibbs energy values obtained by using eq 4 for the different surfactant solutions studied. In regard to these values, it is convenient to indicate that eq 4 is applicable when the aggregation number is large. Table 2 shows that the aggregation number of the sulfobetaine micelles diminish with increasing the weight percent of EG. Therefore, the ∆G°M values listed in Table 7 for the higher wt % EG levels have to be considered as approximated. To explain the influence of the amount of EG present in the mixture on the micellization process as well as on the size of the aggregates, the main solvent dependent factors responsible for the changes experienced by a surfactant molecule when it is transferred from a solvent (25) Bluestein, B. R.; Hilton, C. L.; Eds. Amphoteric Surfactants, Surfactant Sci. Ser. 12; Dekker: New York, 1982. (26) Chevalier, Y.; Kamenka, N.; Chorro, N.; Zana, R. Langmuir 1996, 12, 3225 and references therein. (27) (a) Kamenka, N.; Chevalier, Y.; Zana, R. Langmuir 1995, 11, 3351. (b) Kamenka, N.; Chevalier, Y.; Zana, R. Langmuir 1995, 11, 4234. (c) Bunton, C. A.; Mhala, M. M.; Moffat, J. R. J. Phys. Chem. 1987, 93, 854. (d) Baptista, M. S.; Politi, M. J. J. Phys. Chem. 1990, 95, 5936. (28) (a) Evans, D. F.; Wennestro¨m, H. The Colloidal Domain: Whre Physics, Chemistry and Biology Meet; VCH: New York, 1994. (b) Desnoyers, J. E.; Perron, G. Langmuir 1996, 12, 4044. (c) Zana, R. Langmuir 1996, 12, 1208.

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Table 7. Gibbs Energy of Micelle Formation, ∆G°M, and Gibbs Energy of Transfer, ∆G°trans, for SB3-12, SB3-14, and SB3-16 Water-EG, Surfactant Solutionsa wt % EG

30

40

-∆G°M/kJ mol-1 ∆G°trans/kJ mol-1

SB3-12 14.3 13.7 0.69

12.6 1.7

12.2 2.1

-∆G°M/kJ mol-1 ∆G°trans/kJ mol-1

SB3-14 20.4 19.1 0.98

18.1 2.3

17.1 3.3

-∆G°M/kJ mol-1 ∆G°trans/kJ mol-1

SB3-16 25.8 24.9 1.1

23.0 2.7

21.8 3.9

a

0

15

T ) 298.2 K.

numbers) would cause the cmc to increase. The competition between these two factors can cause an increase or a decrease in the cmc as the weight percent of EG augments. As a result, the dependence of the cmc on this energy contribution is always much weaker when compared to the dependence of the cmc on the tail transfer Gibbs energy, and this explains the increase in the cmc as the amount of EG present in the mixture increases. If this is true, the presence of EG is expected to affect the micellization process more the longer the hydrocarbon chain of the surfactant is. To investigate this point, the effect of EG on the micellization process was estimated through the Gibbs energy of transfer, ∆G°trans, which can be written as28b

∆G°trans ) (∆G°M)water-EG - (∆G°M)water to a micellar aggregate will be taken into account. (i) The surfactant tail transfer Gibbs energy considers the fact that the surfactant tail is removed from contact with the solvent mixture and transferred to the hydrophobic core of the micelles. This contribution will depend on the transfer Gibbs energies from pure water and pure ethylene glycol and on the nature of the interactions between the two solvents and it accounts for the solvophobic effect. ii) The aggregate-core solvent interfacial Gibbs energy takes into account that the formation of a micelle creates an interface allowing for contact between the hydrophobic core and the solvent mixture. (iii) The headgroup interaction Gibbs energy accounts for the electrostatic interactions among the dipoles of the headgroups of the zwitterionic surfactant molecules located at the micellar surface. Besides these factors, the headgroups steric interactions and the deformation of the surfactant tails inside the micelles also have an influence on the Gibbs energy of micellization. The magnitude of the surfactant tail transfer Gibbs energy is considerably smaller in pure ethylene glycol than in water and this is the dominant contribution responsible for the observed increase in the cmc when the weight percent of EG in the mixture increases. To quantify this effect the Gibbs energy for the transfer of one methylene CH2 group from the water-EG bulk phase to the micellar pseudophase, ∆G°CH2, was estimated by using eq 5:29

[ ]

ln cmc ) const + n

∆G°CH2 RT

(5)

where n is the number of carbon atoms in the surfactant chain. From the slope of the plots of ln cmc against n for a homologous series of surfactants ∆G°CH2 values can be estimated. Despite having only three points, these plots were found to be good straight lines for water and the three water-EG mixtures studied and the ∆G°CH2 values obtained were -2.9, -2.8, -2.6, and -2.4 kJ mol-1 for pure water, 15wt %, 30wt %, and 40wt % EG, respectively. The aggregate core solvent interfacial Gibbs energy is smaller in ethylene glycol than in water because the ethylene glycol-hydrocarbon interfacial tension is smaller than the water-hydrocarbon interfacial tension. This is principally responsible for the decrease observed in the micellar aggregation numbers when the weight percent of EG increases. There is also a dependence of the cmc on this energy contribution. On one hand, the lower hydrocarbon-EG interfacial tension would cause the cmc to decrease, but on the other hand, the larger areas per molecule (resulting from the decrease in the aggregation (29) Gharibi, H.; Palepu, R.; Bloor, D. M.; May, D. G.; Wyn-Jones, E. Langmuir 1992, 8, 782.

(6)

Table 7 shows the ∆G°trans values obtained by using eq 6. One can see from this table that the trend was ∆G°trans(SB3-12) < ∆G°trans(SB3-14) < ∆G°trans(SB3-16) as expected. The dependence of the ability of water-EG mixtures to bring about the self-association of conventional amphiphiles on the weight percent of EG can be related to its cohesive energy density,30 which can be characterized by the Gordon parameter, G ) γ/V h 1/3, where γ is the solvent surface tension and V h its molar volume. Table 4 shows the Gordon parameter values for the different mixtures used as bulk phases in the micellar solutions studied. The G parameter points out that an increase in the weight percent of EG results in a decrease in the solvent cohesiviness, thereby improving the solvation of the hydrocarbon tails in the bulk phase and decreasing the solvophobic effect. The increase in the solubility of the hydrocarbon tails in the water-EG mixtures provoked an increase in the cmc. The investigation of interfacial properties of surfactants in solution can provide information about solute-solute and solvent-solute interactions. The surface excess concentration, Γmax, and the minimum area per surfactant molecule, Amin, obtained by using eqs 1 and 2, are listed in Table 3. One can see that the value of Γmax increases, and that of Amin decreases, when the length of the hydrocarbon chain increases. This shows that the more hydrophobic the surfactant molecules are, the stronger the tendency is of those molecules to escape from the solvent to the air/solvent interface (there is less affinity between solvent and surfactant molecules), resulting in a more packed surface. An increase in the weight percent of EG in the mixture results in a decrease in Γmax, and in an increase in Amin. Changes in the water structure due to the presence of EG, interactions between EG and surfactant molecules, and the presence of EG at the interface can be responsible for these variations. In relation to solvent-solvent interactions, binary aqueous mixtures can be classified into three groups according to their excess molar thermodynamic functions of mixing.28 The waterEG mixture belongs to the typically nonaqueous negative group.31 This group is characterized by a negative excess Gibbs energy, ∆Gexc (with |∆Hexc| > |T∆Sexc|), and ethylene glycol is considered a water structure breaker since its presence disrupts the hydrogen bond network of water due to the strong interactions between water and ethylene glycol molecules. In regard to the solvent-surfactant interactions, an increase in the weight percent of EG results in a decrease in the dielectric constant, in the (30) Ramadan, M.; Evans, D. F.; Lumry, R.; Philson, S. J. Phys. Chem. 1985, 89, 3405. (31) Franks, F. Hydrogen Bonded Solvents Systems; Covington, A., Jones, P., Eds.; Taylor and Francis: London, 1968.

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Reichardt parameter, ET, in the Gutman donor number, DN, or in the π* polarity index32 of the mixture, this meaning that the bulk phase will be a better solvent for the surfactant molecules and their tendency to be adsorbed at the interface will decrease. As a consequence, the surface excess concentration decreases and the minimum area per surfactant molecule at the air/mixture interface increases upon increasing the weight percent of EG in the mixture. The effectiveness of a surface-active molecule is measured by the surface pressure at the cmc, Πcmc. Table 3 shows that an increase in the hydrocarbon chain length of the surfactant results in a stronger adsorption of the sulfobetaine molecules at the air/solvent interface, reducing more effectively the surface tension. Besides, an increase in the amount of EG present in the mixture results in a diminution of Πcmc, that is, in a decrease in the surface activity of the sulfobetaine molecules studied. This could be explained by the improved solubility of the sulfobetaine molecules in the mixture when the weight percent of EG increases. Although other factors involving changes in the water-EG structure and the presence of EG molecules at the air/solvent interface can also contribute. The Gibbs energy of adsorption, ∆G°ad, can be calculated by using the equation33

∆G°ad ) ∆G°M - Πcmc/Γmax

(7)

The standard state for the adsorbed surfactant here is a hypothetical monolayer at its minimum surface area/ molecule, but at zero surface pressure. The last term in eq 7 expresses work involved in transferring the surfactant molecule from a monolayer at zero surface pressure to the micelle. All of the ∆G°ad values listed in Table 3 are negative, indicating that the adsorption of the surfactant at the air/mixture interface takes place spontaneously. This process is more spontaneous the longer the hydrocarbon chain of the surfactant is and the lower the weight percent of EG in the mixture is. Besides, ∆G°ad values are more negative than their corresponding ∆G°M values. This points out that, when a micelle is formed, work has to be done to transfer the surfactant molecules in the monomeric form at the surface to the micellar stage through the aqueous medium.34 Table 5 shows the Reichardt parameter, ET(30), for the sulfobetaine water-EG micellar solutions studied. The value of this parameter is equal to the lowest energy transition of the indicator N-phenol betaine, ET(30), dissolved in a given solvent, in kcal mol-1 32 and gives information about the polarity of the region surrounding the dye molecules. Since NMR measurements have shown that the ET(30) molecules are predominantly solubilized in the micellar surface region,15 from this parameter one can get information about the polarity of the interfacial region. One can see from Table 5 that the polarity of the interfacial region follows the trend SB3-12 > SB3-14 > SB3-16. Besides, an increase in the amount of EG present in the mixture brings about an increase in the polarity of the interfacial region. The same trend was found for the alkyltrimethylammonium bromides with 12, 14, and 16 methylene units in the hydrocarbon chain.10 These results could be related to the aggregation numbers. A decrease (32) Marcus, Y. Ion Solvation; Wiley: London, 1985. (b) Sindreu, R. J.; Moya´, M. L.; Sa´nchez Burgos, F.; Gonza´lez, G. J. Solution Chem. 1996, 25, 289. (33) Rosen, M. J.; Cohen, A. W.; Dahanayake, M.; Hua, X. J. Phys. Chem. 1982, 86, 541. (34) Anad, K.; Yadav, O. P.; Singh, P. P. Colloid Surf. 1991, 55, 345.

in Nag supposes a less packed micelle, where the penetration of solvent molecules (water and ethylene glycol) in the palisade layer would be easier, this resulting in a more polar environment for the polarity probe. Figure 3 shows the dependence of the observed rate constant for the reaction methyl 4-nitrobenzenesulfonate, MBS, + Br- on the surfactant concentration in the waterEG sulfobetaine micellar solutions studied. Before discussing the kinetic data, it is necessary to point out that the reaction of the organic substrate with water can make a contribution to the MBS + Br- process under study.35 In this regard, the observed rate constants were corrected, when necessary, using the procedure described previously.12,14,36 kobs increases upon increasing surfactant concentration, reaching a plateau at high enough concentration of sulfobetaine. This dependence can be explained by considering that an increase in the surfactant concentration results in further incorporation of the organic substrate into the sulfobetaine micelles, where the bromide ions concentration is higher than that in the bulk phase since sulfobetaine micelles bind anions. Therefore, this provokes an increase in the contribution of the reaction taking place in the micellar pseudophase, this ending in an increase in kobs. When the MBS molecules are fully bound to the micelles, the process ocurrs wholly in the micellar pseudophase, and the observed rate constant reaches its maximum value. Figure 3 also shows that, for a given surfactant concentration, the observed rate constant decreases as the weight percent of EG present in the mixture increases. These variations in kobs upon changing wt % EG should be a reflection of the thermodynamic and structural changes produced in the micellar solutions by the presence of different amounts of the organic solvent. If pseudophase kinetic models are considered, the observed rate of the reaction is assumed to be equal to the sum of the reaction occurring in the bulk phase (the water-EG mixture), and in the micellar pseudophase and the observed rate constant can be written as37

kobs )

k2,bulk[Brbulk-] + (km 2 /Vm)[Brm ]Km

1 + Km[surfactantm]

(8)

Here [Brbulk-] and [Brm-] are the bromide ion concentrations in the bulk and micellar pseudophases referred to -1 the total solution volume. (km 2 /Vm) ) k2m (s ) is the second-order rate constant in the micellar pseudophase written with concentrations as a molar ratio, [Brm-]/ [surfactant], and k2,bulk is the second-order rate constant of the reaction in the bulk phase. Vm is the molar reaction volume expressed in liters per mole, and Km is the equilibrium binding constant which describes the distribution of the organic substrate molecules between the bulk and the micellar pseudophases. [surfactant] is the micellized surfactant concentration, equal to the total surfactant concentration minus the cmc. The bromide ion concentrations [Brbulk-] and [Brm-] can be estimated by (35) Bonan, C.; Germani, R.; Ponti, P. P.; Savelli, G.; Bunton, C. A. J. Phys. Chem. 1990, 94, 5333. (36) Graciani, M. M.; Rodrı´guez, A.; Mun˜oz, M.; Moya´, M. L. Langmuir 2002, 18, 3476. (37) (a) Berezin, I. V.; Martinek, K.; Yatsimirskii, A. K. Russ. Chem. Rev. 1973, 42, 787. (b) Romsted, L. S. In Surfactants in Solution; Mital, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; Vol. 2, p 1015. (c) Bunton, C. A.; Nome, F.; Quina, F.; Romsted, L. S. Acc. Chem. Res. 1991, 24, 357. (d) Bunton, C. A.; Yao, J.; Romsted, L. S. Curr. Opin. Colloid Interface Sci. 1996, 65, 125. (38) (a) Bacaloglu, R.; Bunton, C. A.; Cerichelli, G.; Ortega, F. J. Phys. Chem. 1990, 94, 5068. (b) Bunton, C. A.; Moffatt, J. R. J. Phys. Chem. 1988, 92, 2896.

Micellar Solutions of Sulfobetaine Surfactants

Langmuir, Vol. 21, No. 16, 2005 7167 Table 9. Fitting Parameters for the Reaction Methyl 4-nitrobenzenesulfonate + Br- in Water-EG, Sulfobetaine Micellar Solutionsa wt % EG

15

30

40

Km/dm3 mol-1 103 × (k2m ) k2m/Vm)/s-1

SB3-12 43 2.0

35 1.8

24 1.7

18 1.5

Km/dm3 mol-1 103 × (k2m ) k2m/Vm)/s-1

SB3-14 74 2.2

54 2.0

38 1.7

25 1.6

Km/dm3 mol-1 103 × (k2m ) k2m/Vm)/s-1

SB3-16 95 2.5

62 2.3

47 2.0

31 1.9

a

Figure 4. Plots of the variation of the specific conductivity, κ in µS cm-1, vs the SB3-16 concentration added to water-EG sodium bromide solutions with [NaBr] ) 5 × 10-3 M and weight percent of EG varying from 0 to 40 at 298.2 K. Table 8. Equilibrium Binding Constants of Bromide Ions to Various Sulfobetaine Micelles, KBr-, in Water-EG, Mixtures at 298.2 K KBr-/dm3 mol-1 wt % EG

0

15

30

40

SB3-12 SB3-14 SB3-16

4.9 4.4 4.5

4.3 3.6 3.9

3.7 3.2 3.6

3.8 3.3 3.7

considering that the incorporation of the bromide anions into the sulfobetaine micelles can be described by eq 939

KBr- )

[Brm-] [Brbulk-]([surfactantm] - [Brm-])

(9)

This equation was shown to fit conductivity data well for added salts in sulfobetaine micelles and has permitted the estimation of KX for several anions.39,40 Figure 4 shows the variations of the specific conductivity, κ, against the SB3-16 concentration added to water-EG mixtures containing NaBr 5 × 10-3 M. Solid lines in this figure were fitted to the experimental conductivity data following the procedure described in the literature40,41 and considering the KBr- values listed in Table 8 for the different water-EG mixtures. The KBr- values corresponding to the aqueous solutions are in agreement with literature data.40,41 Table 8 shows that KBr- decreases when the amount of EG present in the mixture increases, for the three surfactants studied, although the diminution is not large. An increment in the weight percent of EG in the mixture results in a small increase in the polarity of the interfacial region of micelles and in a decrease in the polarity of the bulk phase (the dielectric constant changes from 78.3 to 66.6 upon varying wt % of EG from 0 to 40, at 298.2 K32a) but also in an increase in the ionic strength present in the bulk phase due to a substantial increase in the critical micelle concentrations. These changes, together with possible variations in the micellar volume (39) Bunton, C. A.; Gan, L. H.; Moffatt, J. R.; Romsted, L. S.; Savelli, G. J. Phys. Chem. 1981, 85, 4118. (40) (a) Chorro, N.; Kamenka, K.; Faucompre´, B.; Partyka, S.; Lindeimer, N.; Zana, R. Colloid Surf. A 1990, 110, 249. (b) Rodrı´guez, A.; Graciani, M. M.; Guinda, A.; Mun˜oz, M.; Moya´, M. L. Langmuir 2000, 16, 3182. (41) Rodrı´guez, A.; Graciani, M. M.; Guinda, A.; Mun˜oz, M.; Moya´, M. L. Langmuir 2000, 16, 3182.

0

T ) 298.2 K.

into which bromide ions are incorporated, could be responsible for the observed variations in KBr- values. Solid lines in Figure 3a-c are the result of fitting the experimental kinetic data by using eq 8. To do this, the bromide ion concentrations in the bulk and micellar pseudophases were calculated by taking into account eq 9 and the mass balance. From them, one can write

KBr-[Brm-]2 - (KBr-[surfactantm] + KBr-[BrT-] + 1) + KBr-[surfactantm][BrT-] ) 0 (10) This equation permits the calculation of the bromide ions concentrations in the bulk and micellar pseudophases as a function of the surfactant concentration (and by taking into account that NaBr 0.25 M are present in the micellar reaction media) using the KBr- values listed in Table 8. For the fittings, the micellized surfactant concentrations were calculated by considering the cmc’s in the presence of NaBr 0.25 M summarized in Table 1 and the experimental second-order rate constants in the bulk phase, k2b, were taken form Table 6. In this way, only two adjustable parameters, Km and k2m, are present in eq 8. Their values obtained from the fittings are listed in Table 9. It is worth noting that only one set of adjustable parameters gives the best fittings. For this to happen, the reaction was studied in a wide surfactant concentration range from just above the cmc to a surfactant concentration high enough to reach saturation. Nonetheless, only substantial changes in the adjustable parameter values are worth discussing due to the simplicity of the model used to rationalize the experimental kinetic data. The equilibrium binding constants, Km, shown in Table 9 are small, in agreement with previous results36,42 and they show the trend Km(SB3-16) > Km(SB3-14) > Km(SB312) for all of the water-EG micellar reaction media. Besides, an increase in the amount of EG present in the bulk phase results in a decrease in Km for the three surfactants investigated. Changes in Km values will depend on the changes in the characteristics of the two pseudophases in which methyl 4-nitrobenzenesulfonate molecules distributes. MBS molecules are expected to be located at the micellar surface.42 Therefore, for a given bulk phase, and considering the ET(30) values listed in Table 5, one would anticipate Km values to follow the trend Km(SB3-16) > Km(SB3-14) > Km(SB3-12) since the polarity of the interfacial region follows the trend SB3-16 < SB314 < SB3-12 and the tendency of the organic substrate molecules to incorporate in the interfacial region of the micelles will increase upon decreasing its polarity. Experimental results agree with the expectations. With (42) Brinchi, L.; Di Profio, P.; Germani, R.; Savelli, G.; Bunton, C. A. Langmuir 1997, 13, 4583.

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regard to the dependence of Km on the weight percent of EG, this dependence can be explained by considering that an increase in the amount of organic solvent in the mixture results in a decrease in the polarity of the bulk phase and in an increase in the polarity of the interfacial region (see Table 5). This means that the bulk phase becomes a better solvent for the methyl 4-nitrobenzenesulfonate molecules, whereas the affinity of the MBS molecules for the interfacial region decreases. Therefore, the tendency of the organic substrate molecules to incorporate into the micelles is expected to decrease, also decreasing the Km values, as in fact is found. Table 9 shows that k2m ) (km 2 /Vm) values are similar for the three sulfobetaines studied, k2m decreasing a little upon increasing the weight percent of EG. To compare the reactivity in the bulk phase and in the sulfobetaine micelles, the second-order rate constants km 2 have to be calculated. Vm is usually considered similar for sulfobetaine micelles and for alkyltrimethylammonium micelles.45 Therefore, taking into account that the molar reaction volumes were estimated to be equal to 0.37, 0.33, and 0.30 dm3 mol-1 for N-hexadecyl, N-tetradecyl, and N-dodecyltrimethylammonium bromides aqueous micellar solutions, respectively,44 the km 2 values calculated for the reaction MBS + Br- in SB3-12, SB3-14, and SB3-16 aqueous micellar solutions are 9.2 × 10-4, 7.3 × 10-4, and 6.0 × 10-4 dm3 mol-1 s-1, respectively, to be compared to 4.5 × 10-4 dm3 mol-1 s-1, the second-order rate constant for the process in water (see Table 6). In regard to the water-EG micellar solutions, Vm values are not known, and the second-order rate constants in the micellar pseudophases could not be calculated. Nonetheless, it is interesting to indicate that, in the case of water-EG Triton X-100 micellar solutions, it was found that an increase in the weight percent of EG in the mixture originates a decrease in the aggregation number and in the hydrodynamic radius of the micelles, thus resulting in a small decrease in Vm.46 Table 2 shows that an increase in the amount of organic solvent present in the micellar solutions provokes a decrease in the aggregation number. No data about the dependence of the hydrodynamic radius of sulfobetaine micelles on EG content was found in the literature, but it seems reasonable to assume that no large changes in Vm are expected. Therefore, taking the k2,bulk and (km 2 /Vm) values in Tables 6 and 9 into account one can conclude that the reaction methyl 4-nitrobenzenesulfonate + Br- has a similar rate in the bulk and in the micellar pseudophases of sulfobetaine water-EG micellar solutions. It seems that a weak catalytic effect is working when the reaction occurs in the micellar pseudophases, but its magnitude is small, and considering the simplicity of the model used, to discuss it would be rather speculative. This result differs from that found for the reaction MBS + Br- in water-EG alkyltrimethylammonium micellar solutions, where a substantial catalytic effect is operative when the reaction occurs in the micellar pseudophase.11 The main factors involved in this catalysis would be the electrophilic interaction of the ammonium headgroups and the forming nitrobenzebesulfonate ions and disruption of the hydration shell of the bromide ions.47 In alkyltrimethylammonium micellar solutions the methyl 4-nitrobenzenesulfonate molecules are expected to be located (43) Bacaloglu, R.; Bunton, C. A.; Cerichelli, G.; Ortega, F. J. Phys. Chem. 1989, 93, 1490. (44) Imae, T.; Ikeda, S. J. Phys. Chem. 1986, 90, 5216. (45) Baptista, M. S.; Cuccovia, I.; Chaimovich, H.; Politi, M. J.; Reed, W. F. J. Phys. Chem. 1992, 96, 6442. (46) Carnero Ruiz, C.; Molina-Bolivar, J. A.; Aguiar, J.; MacIsaac, G.; Moroze, S.; Palepu, R. Langmuir 2001, 17, 6831.

del Mar Graciani et al.

at the micellar surface, with the aromatic π system interacting with the quaternary ammonium headgroups and the methyl group not buried in the micellar interior.43 This interaction should increase the reactivity of MBS in alkyltrimethylammonium micelles with respect to water because the 4-nitrobenzenesulfonate ion will become a better leaving group. In sulfobetaine micellar solutions, the π system is also expected to be interacting with the ammonium centers. However, the presence of the sulfonate headgroups of the zwitterionic surfactant molecules in the vicinity will result in repulsive interactions between the 4-nitrobenzenesulfonate ion and the anionic headgroups of the sulfobetaine molecules, and as a consequence, a less important acceleration is expected in sulfobetaine micelles than in alkyltrimethylammonium bromide micelles. With regard to bromide ions, they will be close to the positively charged ammonium centers in alkyltrimethylammonium bromide micelles as well as in sulfobetaine micelles. One could expect that the presence of the trimethylene group in the proximity of the ammonium centers in sulfobetaine micelles ended in a more important loss of the anion hydration shell than in the case of alkyltrimethylammonium bromide micelles, where the cationic headgroups are exposed to water. However, this would make the bromide ions more reactive in sulfobetaine micelles than in the cationic micelles, in disagreement with the observations. A comment can be made with respect to the changes in (km 2 /Vm) upon changing the EG weight percent. For the three surfactants investigated, this magnitude seems to decrease slightly by increasing the content of EG in the mixture. This result could be related to the increase in the polarity of the interfacial region (see Table 5) provoked by the increase in the weight percent of EG. An increase in the polarity of the reaction medium favors reactions in which charge is dispersed in the transition state,48 as is the case of the process studied,42 as can be seen in Table 6. This could also explain that km 2 /Vm(SB3-16) > m km 2 /Vm(SB3-14) > k2 /Vm(SB3-12) since the ET(30) values listed in Table 5 show that the polarity of the interfacial regions of the sulfobetaine micelles follows the opposite trend. Nevertheless, the changes are small and this paragraphhas to be taken as a comment only. In conclusion, the addition of ethylene glycol up to a weight percent of 40% to sulfobetaine aqueous micellar solutions results in an increase in the critical micelle concentrations, in a decrease in the micellar aggregation number and in an increase in the polarity of the interfacial micellar region. When the amount of organic solvent in the mixture increases, the ability of the bulk phase to bring about self-association of conventional amphiphiles decreases, as shown through a decrease in the Gordon parameter. The solvent cohesiviness diminishes, thereby increasing the solubility of the hydrocarbon tails and decreasing the solvophobic effect. The decrease in the surface excess concentration and, as a consequence, the increase in the minimum area per surfactant molecule upon increasing wt % of EG are also related to the improving solubility of the surfactant molecules in the bulk phase when the amount of EG in the micellar (47) (a) Bohme, K. D.; Young, L. B. J. Am. Chem. Soc. 1970, 92, 7354. (b) Bohme, K. D.; Mackay, G. I.; Pay, J. D. J. Am. Chem. Soc. 1974, 96, 4027. (c) Tanaka, K.; Mackay, G. I.; Payzant, J. D.; Bohme, D. K. Can. J. Chem. 1976, 74, 1643. (d) Olmsted, W. E.; Braumen, J. I. J. Am. Chem. Soc. 1977, 99, 4219. (e) Henchan, M.; Paulson, J. F.; Hiel, P. M. J. Am. Chem. Soc. 1983, 105, 5509. (f) Dewar, M. J.; Storch, D. M. J. Chem. Soc., Chem. Commun. 1985, 94. (48) Di Profio, P.; Germani, R.; Savelli, G.; Cerichelli, G.; Spretti, N.; Bunton, C. A. J. Chem. Soc., Perkin Trans. 2 1996, 1505 and references therein.

Micellar Solutions of Sulfobetaine Surfactants

solutions increases. The surface pressure at the cmc and the standard Gibbs energy of adsorption become smaller the larger the weight percent of EG in the solution is. These thermodynamic and structural changes control the micellar effects observed in the reaction methyl 4-nitrobenzenesulfonate + Br- occurring in the water-EG SB3-12, SB3-14, and SB3-16 zwitterionic micellar solutions. The observed slow of the reaction upon increasing the weight percent of EG present in the mixture is mainly the result of three factors: (i) the decrease in the equilibrium binding constant of the methyl 4-nitrobenzenesulfonate molecules to the zwitterionic micelles due to the bulk phase being a better solvent for the organic substrate than pure water and to the interfacial region of the sulfobetaine micelles becoming more polar upon increasing wt % of EG, (ii) the decrease in the bromide ion concentration at the micellar surface due to a decrease in

Langmuir, Vol. 21, No. 16, 2005 7169

the equilibrium binding constant of the bromide ions to the micellar surface of the sulfobetaine micelles, and (iii) the decrease in the second-order rate constant k2m ) (km 2 /Vm) by increasing the weight percent oh EG in the mixture due to the interfacial region of the sulfobetine micelles becoming more polar the higher the weight percent of EG is. Acknowledgment. This work was financed by the DGCYT (Grant BQU2002-00691) and Consejerı´a de Educacio´n y Ciencia de la Junta de Andalucı´a (FQM-274). The authors thank Professor Ana Troncoso, from the University of Seville, Spain, for helping us in the fluorescence measurements. LA050862J