Kinetic Study in Water−Ethylene Glycol Cationic, Zwitterionic, Nonionic

Langmuir 2004, 20, 9945-9952

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Kinetic Study in Water-Ethylene Glycol Cationic, Zwitterionic, Nonionic, and Anionic Micellar Solutions Amalia Rodrı´guez,† Marı´a Mun˜oz,† Marı´a del Mar Graciani,† Soledad Ferna´ndez Chaco´n,‡ 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, and Departamento de Bioquı´mica, Bromatologı´a, Toxicologı´a y Medicina Legal, Universidad de Sevilla, C/Profesor Garcı´a Gonza´ lez s/n, 41012 Sevilla, Spain Received June 11, 2004. In Final Form: July 21, 2004 The spontaneous hydrolysis of phenyl chloroformate was studied in water-ethylene glycol, EG, cationic, zwitterionic, nonionic, and anionic micellar solutions, the surfactants being tetradecyltrimethylammonium bromide, tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, tricosaoxyethylene glycol ether, and sodium dodecyl sulfate. The dependence of the observed rate constant on surfactant concentration as well as on the percentage by weight of EG, varying from 0 to 50 wt %, was investigated. Information about changes in the critical micelle concentrations, in the micellar ionization degrees (for ionic surfactants), in the aggregation numbers, and in the polarity of the interfacial region of the micelles upon changing the weight percent of EG was obtained through conductivity, surface tension, spectroscopic, and fluorescence measurements. A simple pseudophase model was adequate to rationalize the kinetic data. Micellar medium effects were explained by considering charge-charge interactions and polarity, ionic strength, and water content in the micellar interfacial region. The acceleration of the reaction produced by an increase in the amount of EG present in the mixture was explained on the basis of the substantial decrease in the equilibrium binding constant of phenyl chloroformate molecules to the micelles, resulting in the contribution of the reaction taking place in the bulk water-EG phase being more important. The weight percent of EG did not substantially influence the rate constant in the micellar pseudophase.

Introduction Currently, there is a lot of interest in the nature of micellization and surfactant behavior in polar organic solvents and solvent mixtures.1-13 Changing the solvent quality provides the opportunity to study the role of the so-called solvophobic effect as opposed to the hydrophobic effect in micellization.14 Among the organic solvents studied, ethylene glycol, EG, is of particular interest in that it has many characteristics similar to those of water, and therefore, the study of the aggregation behavior of surfactants in water-EG mixtures provides a better * To whom correspondence should be addressed. E-mail: [email protected]. † Departamento de Quı´mica Fı´sica, Universidad de Sevilla. ‡ Departamento de Bioquı´mica, Bromatologı´a, Toxicologı´a y Medicina Legal, Universidad de Sevilla. (1) Ionescu, L. G.; Fung, D. S. J. Chem. Soc., Faraday Trans. 1 1981, 77, 2907. (2) Cantu, L.; Cotri, M.; Degiorgio, V.; Hoffmann, H.; Ulbricht, W J. Colloid Interface Sci. 1987, 116, 384. (3) Backlund, S.; Bergensta¨ll, B.; Molander, O.; Wa¨rnheim, T. J. Colloid Interface Sci. 1989, 131, 393. (4) Sjo¨ber, M.; Henriksonn, U.; Wa¨rnheim, T. Langmuir 1990, 6, 1205. (5) Gharibi, H.; Palepu, R.; Bloor, D. M.; Hall, D. G.; Wyn-Jones, G. Langmuir 1992, 8, 872. (6) Callaghan, A.; Doyle, R.; Alexander, E.; Palepu, R. Langmuir 1993, 9, 3422. (7) Bakshi, M. S. J. Chem. Soc., Faraday Trans. 1993, 89, 4323. (8) Wa¨rnheim, T. Curr. Opin. Colloid Interface Sci. 1997, 2, 472. (9) Zana, R. Colloids Surf., A 1997, 123-124, 27. (10) Penfold, J.; Staples, E.; Tucker, I.; Cummins, P. J. Colloid Interface Sci. 1997, 185, 424. (11) (a) Carnero Ruiz, C. Colloid Polym. Sci. 1999, 277, 1999. (b) Carnero Ruiz, C. J. Colloid Interface Sci. 2000, 221, 262. (c) Carnero Ruiz, C. Langmuir 2001, 17, 6831. (12) Nagarajan, R.; Wang, Ch.-Ch. Langmuir 2000, 16, 5242. (13) Hazra, P.; Chakrabarty, D.; Sarkar, N. Langmuir 2002, 18, 7872 and references therein. (14) Palepu, R.; Gharibi, H.; Bloor, D. M.; Wyn-Jones, E. Langmuir 1993, 9, 110.

understanding of the structure of liquids in the micellization process. Despite the interest shown in the effects of EG and other organic solvents on the micellization process of several surfactants, no kinetic studies in these micellar media had been carried out (with the exception of aqueouslinear alcohol micellar solutions), until recently.15-17 To extend our knowledge on the properties of water-ethylene glycol micellar solutions as reaction media, the spontaneous hydrolysis of phenyl chloroformate was studied in water-EG cationic, zwitterionic, nonionic, and anionic micellar solutions. The reaction is a true first-order process with a well-known mechanism (Scheme 1).18,19 Recent literature18 suggests that the C-OAr and C-Cl bonds are not significantly broken in the transition state, in agreement with the formation of a tetrahedral intermediate formed by water-catalyzed addition of OH-. This intermediate is too short-lived to be a true intermediate, but the products are not formed directly from the transition state. Because of this, two arrows separate the transition state and the products in Scheme 1. The kinetic micellar effects observed will only be due to the micellar medium (15) Ionescu, L. G.; Trindade, V. L.; de Souza, E. F. Langmuir 2000, 16, 988. (16) Rodrı´guez, A.; Graciani, M. M.; Mun˜oz, M.; Moya´, M. L. Langmuir 2003, 19, 7206. (17) Graciani, M. M.; Rodrı´guez, A.; Mun˜oz, M.; Moya´, M. L. Langmuir 2003, 19, 8685. (18) (a) Kevill, D. N.; de Souza, M. M. J. Chem. Soc., Perkin Trans. 2 1997, 1721. (b) Queen, A. Can. J. Chem. 1967, 45, 1619. (c) Butler, A. R.; Roberston, I. H.; Bacaloglu, R. J. J. Chem. Soc., Perkin Trans. 2 1974, 1733. (d) Al Lohedan, H.; Bunton, C. A.; Mhala, M. M. J. Am. Chem. Soc. 1982, 104, 6654. (e) Brinchi, L.; Di Profio, P.; Micheli, F.; Savelli, G.; Bunton, C. A. Eur. J. Org. Chem. 2001, 1115. (f) Mun˜oz, M.; Rodrı´guez, A.; Graciani, M. M.; Moya´, M. L. Int. J. Chem. Kinet. 2002, 34, 445. (19) (a) Bunton, C. A. Nucleophilicity; Harris, J. M., McManus, S. P., Eds.; Advances in Chemistry Series, Vol. 215; American Chemical Society; Washington, DC, 1987; Chapter 12. (b) Possidonio, S.; Siviero, F.; El Seoud, E. O. J. Phys. Org. Chem. 1999, 12, 325.

10.1021/la048555l CCC: $27.50 © 2004 American Chemical Society Published on Web 10/15/2004

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Langmuir, Vol. 20, No. 23, 2004 Scheme 1

effect (no micellar concentration effect will be operative). Micellar medium effects on the reaction will depend on the transfer of phenyl chloroformate from the bulk to the micelle, on the reaction mechanism, and on the properties of the interfacial region, such as charge-charge interactions, polarity of the Stern layer, and water content. Comparison between observed rate constants in water and in the micellar pseudophases in water-EG micellar solutions can be done directly and will provide information about the characteristics of the micellar pseudophases where the reaction occurs. The surfactants studied were tetradecytrimethylammonium bromide, TTAB, N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, SB314, dodecyl tricosaoxyethylene glycol ether, Brij35, and sodium dodecyl sulfate, SDS. Extensive information exists on the aqueous micellar solutions of these surfactants; consequently, they were chosen in order to get general information about water-EG cationic, zwitterionic, nonionic, and anionic micellar solutions behaving as reaction media. Thermodynamic and structural information about the water-EG micellar solutions was obtained by using various experimental techniques. Critical micelle concentrations, cmc’s, and micellar ionization degrees of the water-EG ionic surfactant solutions, R, were estimated through conductivity measurements. The cmc’s of the water-EG nonionic and zwitterionic surfactant solutions were obtained by means of surface tension measurements. Reichardt’s ET(30) parameter20 was used to get information on the polarity of the interfacial region. Fluorescence quenching of pyrene by N-hexadecylpyridinium chloride permitted the estimation of the aggregation numbers of the micelles present in the water-EG micellar solutions studied.21 All experiments were done at 298.2 K. Experimental Section Materials. Phenyl chloroformate was obtained from Aldrich, as was EG. TTAB and SB3-14 were from Fluka. Brij35 and SDS were obtained from Aldrich. All the surfactants were used without further purification, their cmc’s being in agreement with literature data. HBr was from Fluka. Reichardt’s dye, ET(30), and pyrene were obtained from Aldrich. ET(30) dye was not purified; pyrene was purified before use.21 Tetraethylammonium bromide and sodium monoethyl sulfate were from Aldrich, as was pyrene-3-carboxaldehyde. Kinetics. The spontaneous hydrolysis of phenyl chloroformate was recorded spectrophotometrically at 270 nm (appearance of phenol), as described in ref 16, in the presence of [HBr] ) 1.5 × 10-3 M. HBr was added in order to suppress the reaction of the organic substrate with basic impurities. The possibility of the substrate reacting with EG (ethylene glycol was not purified before use) was checked. The reaction medium was the commercially pure EG in the absence of HBr. Since the absorbance within the range 220-400 nm did not change, this possibility was excluded. (20) Zachariasse, K. A.; Phuc, N. V.; Kozankiewicz, B. J. Phys. Chem. 1981, 85, 2676 and references therein. (21) Kalyanasundaram, K. Photochemistry in Microheterogeneus Systems; Academic Press: New York, 1987.

Rodrı´guez et al. The kinetics was followed for more than 5 half-lives in all the water-EG micellar media. The observed rate constants were obtained from the slopes of the ln(A∞ - At) against time plots, At and A∞ being the absorbances at time t and at the end of the reaction, respectively. Each experiment was repeated at least twice, and the observed rate constants were reproducible within a precision of better than 5%. The temperature was maintained at 298.2 ( 0.1 K using a water-jacketed cell compartment connected to a water flow thermostat. Conductivity Measurements. Conductivity was measured with a Crison microCM 2201 conductimeter connected to a water 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 tensiometer (KSV 703, Finland). The tensiometer was connected to a water flow cryostat maintained 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 values for aqueous micellar solutions of TTAB, SB314, Brij35, and SDS obtained were 53.4, 52, 52.8, and 57.5, in good agreement with those in the literature.20,22 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 N-hexadecyl pyridinium was studied as in ref 23. The solutions were prepared as in ref 24. The probe concentration was kept low enough (2 × 10-6 mol dm-3) to avoid excimer formation, and the quencher concentration was varied from 5 × 10-5 to 25 × 10-5 mol dm-3. These values give [pyrene]/[micelles] and [quencher]/[micelles] ratios low enough to ensure a Poisson distribution.25

Results Figures 1-4 show the dependence of the rate constant of the spontaneous hydrolysis of phenyl chloroformate on surfactant concentration in water-EG TTAB, SB3-14, Brij35, and SDS micellar solutions, with the EG concentration varying within the range of 0-50 wt %. The kinetic data shown in Figure 1 were taken from ref 16. The values of the rate constant for the reaction studied in water-EG binary mixtures, in the absence of surfactant, were also taken from ref 16. The cmc’s and micellar ionization degrees, R, corresponding to the water-EG TTAB and SDS micellar solutions are listed in Table 1. These values were obtained through conductivity measurements, in the absence and in the presence of HBr 1.5 × 10-3 M, by using the Williams method.26 Data corresponding to water-EG TTAB solu(22) Rodrı´guez, A.; Mun˜oz, M.; Graciani, M. M.; Ferna´ndez, G.; Moya´, M. L. New J. Chem. 2001, 25, 1084. (23) Velazquez, M. M.; Costa, M. B. J. Chem. Soc., Faraday Trans. 1990, 86, 4043. (24) 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. (25) (a) Tachiya, M. Chem. Phys. Lett. 1975, 33, 289. (b) Infelta, P.; Gratzel, M. J. Chem. Phys. 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. (26) Williams, R.; Phillips, J. N.; Mysels, K. J. Trans. Faraday Soc. 1955, 51, 728.

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Figure 1. Dependence of the observed rate constant, kobs/s-1, for the spontaneous hydrolysis of phenyl chloroformate on surfactant concentration in water-EG TTAB micellar solutions at 298.2 K.

Figure 3. Dependence of the observed rate constant, kobs/s-1, for the spontaneous hydrolysis of phenyl chloroformate on surfactant concentration in water-EG Brij35 micellar solutions at 298.2 K.

Figure 2. Dependence of the observed rate constant, kobs/s-1, for the spontaneous hydrolysis of phenyl chloroformate on surfactant concentration in water-EG SB3-14 micellar solutions at 298.2 K.

Figure 4. Dependence of the observed rate constant, kobs/s-1, for the spontaneous hydrolysis of phenyl chloroformate on surfactant concentration in water-EG SDS micellar solutions at 298.2 K.

tions were taken from ref 16. The values obtained for the aqueous TTAB and SDS surfactant solutions were in agreement with those in the literature.27 The cmc’s corresponding to water-EG SDS solutions listed in Table 1 are in agreement with previous data.7,28 Figure 5 shows the dependence of the specific conductivity, κ, of waterEG SDS solutions on surfactant concentration in the absence of HBr. One can see in Figure 5 that an increase in the percentage by weight of EG results in a less abrupt change in conductivity in going from the premicellar surfactant concentration range to the postmicellar surfactant concentration range, as compared to that in pure water. This was also true for the conductivity measurements done in the presence of HBr, and it introduces some uncertainties in the evaluation of the cmc values corresponding to the high weight percent of EG by using the Williams method.26 To investigate this point, the Phillips method29 was used to estimate the cmc’s, the latter applied

Table 1. Critical Micelle Concentrations and Micellar Ionization Degrees, r, of SDS and TTAB in Water-EG Mixtures at 298.2 K (Obtained through Conductivity Measurements)

(27) Van Os, N. M.; Haak, J. R.; Rupert, L. A. Physicochemical Properties of Selected Anionic, Cationic and Nonionic Surfactants; Elsevier: New York, 1993. (28) (a) Lee, D. J.; Huang, W. H. Colloid Polym. Sci. 1996, 274, 160. (b) Gracie, K.; Turner, D.; Palepu, R. Can. J. Chem. 1996, 74, 1616. (29) Phillips, J. N. Trans. Faraday Soc. 1955, 51, 561.

103 × cmc (mol dm-3) wt % EG

no HBr

0.00 20.0 35.0 50.0

3.62 4.59 6.30 12.9

0.00 20.0 35.0 50.0

8.12 9.24 13.0 22.0

a

R no HBr

[HBr] ) 1.5 × 10-3 M

TTABa 3.00 4.23 6.39 11.9

0.23 0.25 0.26 0.30

0.21 0.24 0.26 0.30

SDS 6.88 7.90 10.7 19.5

0.26 0.31 0.46 0.52

0.24 0.31 0.43 0.50

[HBr] ) 1.5 × 10-3 M

Data taken from ref 16.

through an integration by the Runge-Kutta method and a least-squares Levenberg-Marquardt fitting, as described in ref 30. The cmc’s obtained by using the two methods coincide for all the surfactant solutions studied. This means that the cmc’s and R values obtained through

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Figure 5. Specific conductivity, κ/µS cm-1, of water-EG SDS solutions as a function of surfactant concentration at 298.2 K.

Figure 6. Surface tension, τ/mN m-1, of water-EG SB3-14 solutions as a function of surfactant concentration at 298.2 K.

Table 2. Critical Micelle Concentrations of SB3-14 and Brij35 in Water-EG Mixtures at 298.2 K (Obtained through Surface Tension Measurements)

Table 3. Reichardt’s Parameter, ET(30), for TTAB, SB3-14, Brij35, and SDS Water-EG Micellar Solutions at 298.2 K

wt % EG SB3-14, 104 × cmc (mol dm-3) Brij35, 105 × cmc (mol dm-3)

wt % EG

0.0

20.0

35.0

50.0

2.70 6.00

4.76 8.45

8.16 11.7

20.1 34.6

TTABa ET(30) (kcal mol-1) SB3-14 ET(30) (kcal mol-1) Brij35 ET(30) (kcal mol-1) SDS ET(30) (kcal mol-1)

0.0

20.0

35.0

50.0

53.4 52.0 52.8 57.1

53.5 52.4 53.0 57.1

53.5 52.6 53.3 57.0

54.2 53.4 53.6 57.0

the Williams method, shown in Table 1, are reliable. It also gives reliability to the R values obtained from the ratios of the slopes of the plots of the specific conductivity, κ, against [surfactant] above and below the cmc. The cmc’s of the Brij35 and SB3-14 water-EG surfactant solutions were estimated by means of surface tension measurements and are listed in Table 2. The influence of the [HBr] ) 1.5 × 10-3 M on the cmc’s of the zwitterionic and nonionic surfactant solutions was investigated, the experimental results showing that the critical micelle concentrations are practically the same in the presence and in the absence of HBr, at the acid concentration used. The values corresponding to the surfactant aqueous solutions were in agreement with literature data.31,32 Figure 6 shows the dependence of the surface tension on the logarithm of surfactant concentration for the waterEG SB3-14 surfactant solutions. One can see that no minimum was found, this indicating that surface-active impurities were not present in the solutions. This was also the case for the water-EG Brij35 surfactant solutions. Table 3 summarizes the values of the Reichardt parameter, ET(30), obtained through spectroscopic measurements for the several water-EG micellar solutions used as reaction media. Table 4 shows the aggregation numbers, Nagg, of the micelles present in the water-EG SDS, Brij35, and SB314 micellar solutions obtained by studying the fluorescence quenching of pyrene by N-hexadecylpyridinium chloride. The influence of the quencher concentration on the intensity of pyrene fluorescence in water-EG SDS, with 20 wt % EG, micellar solutions is shown in Figure 7. In a previous work,16 this method was used to estimate the aggregation number corresponding to the micellar ag-

Figure 7. The influence of quencher (N-hexadecylpyridinium chloride) concentration on the intensity of pyrene fluorescence in water-EG SDS solutions, with 20 wt % EG, at 298.2 K. [SDS] ) 0.02 M.

(30) Mosquera, V.; Garcı´a, M.; Varela, L. M. In Handbook of Surfaces and Interfaces of Materials; Nalwa, H. S., Ed.; Academic Press: New York, 2001; Vol. 3, p 405. (31) Mun˜oz, M.; Rodrı´guez, A.; Graciani, M. M.; Ortega, F.; Vazquez, M.; Moya´, M. L. Langmuir 1999, 15, 7876. (32) Frescura, V. L. A.; Marconi, D. M. O.; Zanette, D.; Nome, F.; Blasco, A.; Bunton, C. A. J. Phys. Chem. 1995, 99, 1194.

gregates present in the water-EG TTAB micellar solutions. The method renders aggregation numbers that are too low, but the results indicated that an increase in the weight percent of EG results in a decrease in the aggregation number of the cationic micelles. This is also

a Data taken from ref 16. E (35% weight EG) ) 60.6 kcal mol-1; T ET(50% weight EG) ) 59.8 kcal mol-1; ET(100% weight EG) ) 56.3 kcal mol-1.

Table 4. Aggregation Numbers of SDS, Brij35, and SB3-14 Micelles in Water-EG Mixtures at 298.2 Ka wt % EG SDS Brij35 SB3-14 wt % EG SDS Brij35 SB3-14 0.00 20.0 a

55 43

43 37

67 48

35.0 50.0

41 27

34 31

45 39

Nagg numbers given in this table are approximate (see the text).

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Table 5. Standard Free Energy of Micelle Formation, ∆G°M, and the Effect of EG on the Free Energy of Micellization, ∆G(EG)°M, for TTAB, SB3-14, Brij35, and SDS Micellar Solutions at 298.2 K wt % EG

-∆G°M

∆G(EG)°M

wt % EG

∆G°M

∆G(EG)°M

21.84 (21.78) 18.32 (18.66)

2.81 6.33

TTABa 0.00 24.65 (25.76) 20.0 23.43 (23.83)

1.31

35.0 50.0

0.00 20.36 20.0 18.95

SB3-14 35.0 1.41 50.0

17.62 15.38

2.74 4.98

0.00 24.08 20.0 23.23

Brij35 35.0 0.85 50.0

22.43 19.74

1.65 4.34

0.00 20.75 (21.71) 20.0 19.61 (20.32)

SDS 35.0 1.14 50.0

16.45 (17.76) 14.00 (14.63)

4.30 6.75

a The values in parentheses correspond to the surfactant solutions in the presence of [HBr] ) 1.5 × 10-3 M.

the trend found for the anionic, nonionic, and zwitterionic micellar solutions studied, as shown in Table 4. The Nagg values obtained in aqueous micellar solutions of SDS and Brij35 were in good agreement with literature data.24,33,34 We found little about the aggregation numbers of sulfobetaine micelles in aqueous solutions. Reference 35 gives an aggregation number within the range of 83-130 for SB3-14. The value listed in Table 4, 67, seems too low. Therefore, Nagg values shown in Table 4 for SB3-14 waterEG micellar solutions have to be considered as approximated. Discussion Before discussing kinetic data, the information obtained on the water-EG micellar solutions used as reaction media will be considered. Tables 1 and 2 show that for all the surfactants studied, an increase in the amount of EG present in the solution results in an increase in the critical micelle concentration and a decrease in the micellar ionization degree in the case of ionic surfactants. To quantify this effect, the Gibbs free energy of micellization, ∆G°M, can be calculated by using eqs 1 and 2 for ionic and neutral micelles, respectively:36,37

∆G°M ) (2 - R)RT ln cmc

(1)

∆G°M ) RT ln cmc

(2)

where R is the micellar ionization degree. Table 5 summarizes the ∆G°M values obtained for the different surfactant solutions. In the case of the ionic surfactants, Table 1 shows that in the presence of HBr the cmc’s were lower than in its absence, although the micellar ionization degree was practically the same in both cases. The decrease in the cmc can be explained by considering that the presence of an electrolyte (the HBr) brought about a screening of the electrostatic repulsion of dissociated headgroups, this screening resulting in a decrease of the electrostatic contribution to the free enthalpy of micel(33) Bales, B. L. J. Phys. Chem. B 2001, 105, 6798. (34) Phillies, G. D. J.; Hunt, R. H.; Strang, K.; Sushkin, N. Langmuir 1995, 11, 3408. (35) Le Maire, M.; Champeil, P.; Moller, J. V. Biochim. Biophys. Acta I 2000, 508, 86. (36) Evans, D. F.; Wennestro¨m, H. The Colloidal Domain: Where Physics, Chemistry and Biology Meet; VCH: New York, 1994. (37) Desnoyers, J. E.; Perron, G. Langmuir 1996, 12, 4044.

lization. The ∆G°M values in parentheses listed in Table 5 correspond to the ionic surfactant solutions in the presence of HBr. One can see that the trend found in ∆G°M upon varying the weight percent of EG is the same in the presence as in the absence of HBr, so no further comments will be made about the influence of hydrobromic acid on micellization. It is also worth noting that eqs 1 and 2 are applicable when the aggregation numbers are large. Table 4 and previous data8,12 show that for all the surfactants studied an increase in the amount of EG present in the solution produces a decrease in the aggregation number. Therefore, the ∆G°M values listed in Table 5 for the higher wt % EG levels have to be considered as approximated. The influence of the amount of EG present in the mixture on cmc, R, and Nagg can be rationalized by considering the following solvent-dependent contributions to the free energy of micellization: (i) the surfactant tail transfer free energy, which accounts for the solvophobic effect; (ii) the aggregate-core solvent interfacial free energy; (iii) the headgroup interaction free energy. The magnitude of the surfactant tail transfer free energy is considerably smaller in ethylene glycol than in water, and it is the dominant contribution responsible for the increase in the cmc of ionic, nonionic, and zwitterionic surfactants as the amount of EG in the mixture increases. The aggregatecore solvent interfacial free energy is smaller in ethylene glycol solutions than in water because of the considerably smaller ethylene glycol-hydrocarbon interfacial tension compared to the water-hydrocarbon interfacial tension. This is the dominant contribution responsible for the decrease in the micellar aggregation number of ionic, nonionic, and zwitterionic micelles upon increasing the weight percent of EG in the mixture. There is a dependence of the cmc on this free energy contribution, but it is much weaker than that on the surfactant tail transfer free energy. In regard to the headgroup interaction free energy, the importance of this contribution follows the trend ionic > zwitterionic > nonionic. For ionic surfactants, one has to consider the electrostatic interactions between the charged headgroups located at the aggregate surface. The dielectric constant of EG is lower than that of water (37.7 as compared to 78.39 at 298.2 K38). An increase in the weight percent of EG from 0 to 50% results in a decrease in the dielectric constant from 78.39 to 65.75, at 298.2 K.38 This decrease should lead to an increase in the magnitude of ionic interaction energy in water-EG solutions compared with aqueous solutions. However, the large increase in the cmc provoked by the increase in the amount of EG present in the mixture results in an increase in the ionic strength present in the solution. The decrease in ionic headgroup repulsion due to the higher ionic strength more than compensates for the increase in ionic headgroup repulsion due to the decrease in the dielectric constant. The weaker ionic headgroup repulsions due to the higher ionic strengths are responsible for the increase in the micellar ionization degrees. Concerning the micellar size, one would expect that the decrease in the ionic repulsions in water-EG mixtures compared to water will result in larger Nagg values. However, this effect is largely compensated for by the contribution of the decrease in the interfacial energy, responsible for the decrease in Nagg. In the case of zwitterionic and nonionic surfactants, dipole interactions are operative in the headgroup interaction free energy, these interactions being more important for zwitterionic than for nonionic surfactants. The effect of EG on the micellization process can be estimated by using eq 3:11c (38) Marcus, Y. Ion Solvation; Wiley: London, 1985.

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∆G(EG)°M ) ∆G°M(EG-water) - ∆G°M(water)

Rodrı´guez et al.

(3)

Values of ∆G(EG)°M are listed in Table 5. This table shows that the importance of the influence of EG on the micellization process follows the trend ionic > zwitterionic > nonionic for the surfactants investigated. However, this comparison cannot be done directly since the hydrophobic chain length is not similar in all the surfactants considered and variation in the surfactant tail transfer free energy is the main factor controlling changes in the cmc. The spontaneous hydrolysis of phenyl chloroformate is expected to occur in the micellar surface region (see below).18d To get some information about the polarity of this interfacial region in the different micellar aggregates in water-EG mixtures, the Reichardt and Dimroth ET parameter was estimated. ET is equal to the lowest energy transition of the indicator N-phenol betaine, ET(30), dissolved in a given solvent, in kcal mol-1.39 It is frequently used as a polarity index. NMR measurements have shown that the ET(30) molecules are predominantly solubilized in the micellar surface region,20 and therefore, from this parameter one can obtain information about the polarity of the interfacial region. Table 3 shows, for all the wt % EG levels studied, the polarity of the interfacial regions increases in the order SB3-14 < Brij35 < TTAB < SDS. Besides, an increase in the amount of EG present in the mixture seems to increase ET, meaning that the polarity of the micellar interfacial region increases. Only in the case of SDS does the addition of EG to the micellar solution produce no change in Reichardt’s parameter. That is, the polarity of the interfacial region of SDS micelles would be similar in the absence and in the presence of EG up to 50 wt %. The possible influence of impurities in the commercial EG on the ET(30) values was checked and ruled out.16 The changes in polarity upon changing the weight percent of EG were investigated by using another two methods in the presence of 0 and 50 wt % of EG in the mixture. The pyrene 1:3 ratio method40 and the solventdependent fluorescence of pyrene-3-carboxaldehyde41 were used. Both methods rendered the same result: the polarity of the micellar interfacial region is higher in the presence of 50 wt % EG than in the absence of EG in the mixture for water-EG TTAB, SB3-14, and Brij35 micellar solutions but does not change in the case of water-EG SDS micellar solutions. The changes in the interfacial region polarity upon varying the weight percent of EG found in TTAB, SB3-14, and Brij35 micellar solutions could be related to the decrease in the aggregation number provoked by an increase in the amount of EG present in the mixture. A decrease in Nagg results in a less-packed micelle where the penetration of solvent molecules (water as well as EG) in the palisade layer would be easier, this ending in a more polar environment for the polarity probe. However, this would not explain the results obtained for water-EG SDS micellar solutions. Another possibility is that the average solubilization site of the polarity probe would be further away from the micellar core when the amount of EG in the mixture increases. In this regard, the high ET parameter value obtained for aqueous SDS micellar solutions, as compared to those found in TTAB, SB3-14, and Brij35 micelles, was explained by considering that the solubilization site of the ET(30) molecules in the SDS (39) (a) Reichardt, C. Solvent Effects in Organic Chemistry; Verlag Chemie: Weinheim, 1979. (b) Reichardt, C. Angew. Chem., Int. Ed. Engl. 1979, 18, 98. (40) Kalyanasundaram, K. Photochemistry in Microeheterogeneous Systems; Academic Press: New York, 1987. (41) Kalyanasundaram, K.; Thomas, J. K. J. Phys. Chem. 1977, 81, 2176.

micelles was more distant from the micellar core than in the other micellar solutions.22 As in the case of ET(30) molecules, pyrene and pyrene-3-carboxaldehyde molecules are solubilized predominantly in the micellar surface region.42,43 If their solubilization site is similar to that of the Reichardt dye molecules in SDS micelles, the near independence of the polarity parameters on changes in weight percent of EG found in SDS micelles by using the three methods could be due to the probe molecules’ position being less sensitive to an easier penetration of solvent molecules when the aggregation number of the micelles decreases. Figures 1-4 show the dependence of the reaction rate of the phenyl chloroformate spontaneous hydrolysis on surfactant concentration in several water-EG TTAB, SB314, Brij35, and SDS micellar solutions. This process follows an addition-elimination mechanism, with the addition step being determining, as shown in Scheme 1. One can see that in all cases the process is retarded by the presence of micelles. However, the decrease in the observed rate constant is not large as compared to water, this indicating that the reaction occurs in a relatively wet region of the micelles. Given that the core of the micelles is supposed to be dry, the reaction seems to take place in the Stern region. This idea is in agreement with literature data showing that phenyl chloroformate molecules reside in the surface regions of micelles.18e Deacylations are slowed by a decrease in the solvent polarity and in the water content. Since the interfacial micellar region is less polar and has a smaller water content than the bulk phase, the observed decrease in kobs when [surfactant] increases found in all the micellar reaction media investigated is in agreement with expectations. Figures 1-4 also show that for a given [surfactant], the higher the amount of EG present in the mixture, the larger the kobs. To rationalize the kinetic data, the observed rate constant can be written as44

kobs )

kbulk + kmKm[surfactantm] 1 + Km[surfactantm]

(4)

Here kbulk and km are the rate constants for the reaction in the water-EG bulk phase and in the micellar pseudophase, respectively. Km is the equilibrium binding constant of the phenyl chloroformate molecules to the micellar aggregates present in the reaction media and [surfactantm] is the micellized surfactant concentration, equal to the total surfactant concentration minus the cmc. Kinetic data shown in Figures 1-4 were rationalized by using eq 4. The kbulk values used were the experimental ones, taken from ref 16. The cmc’s considered were those shown in Tables 1 and 2 for the kinetic working conditions, that is, in the presence of [HBr] ) 1.5 × 10-3 M. Solid lines in Figures 1-4 resulted from the fittings of the kinetic data by using eq 4. One can see that the agreement between the theoretical and the experimental data was good. Table 6 shows the values of the adjustable parameters obtained from those fittings. Table 6 shows that the equilibrium binding constant values are small in all the micellar media studied. This (42) (a) Lianos, P.; Zana, R. J. Colloid Interface Sci. 1981, 84, 100. (b) Zana, R.; Strazielle, C.; Lianos, P. J. Colloid Interface Sci. 1981, 80, 208. (43) (a) Gratzel, M.; Kalyanasundram, K.; Thomas, J. H. J. Am. Chem. Soc. 1974, 96, 7869. (b) Eriksson, J. C.; Gillberg, G. Acta Chem. Scand. 1966, 20, 2019. (44) (a) Menguer, F. M.; Portnoy, C. E. J. Am. Chem. Soc. 1967, 89, 4698. (b) Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, L. S. Acc. Chem. Res. 1991, 24, 357. (c) Romsted, L. S.; Bunton, C. A.; Yao, J. Curr. Opin. Colloid Interface Sci. 1997, 2, 622.

Water-Ethylene Glycol Micellar Solutions

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Table 6. Equilibrium Binding Constants, Km/dm3 mol-1, and Rate Constants in the Micellar Pseudophase, km/s-1, for the Spontaneous Hydrolysis of Phenyl Chloroformate in Water-EG TTAB, SB3-14, Brij35, and SDS Micellar Solutions Obtained from the Fittings of the Kinetic Data by Using Equation 4 at 298.2 K wt % EG

K m/ dm3 mol-1

103 × km/ s-1

0.0 20.0

100 80

4.4 4.2

0.0 20.0

207 74

0.0 20.0 0.0 20.0 a

Km/ dm3 mol-1

103 × km/ s-1

TTABa 35.0 50.0

56 30

4.4 4.1

8.1 7.3

SB3-14 35.0 50.0

52 35

7.0 6.8

174 100

2.5 2.2

Brij35 35.0 50.0

67 44

2.1 2.1

69 47

0.2 0.4

SDS 35.0 50.0

30 21

0.2 0.2

wt % EG

Data taken from ref 16.

is in agreement with Km values found for other related organic substrates in aqueous micellar solutions.45 For a given surfactant, when the amount of EG present in the mixtures increases, the equilibrium binding constant decreases. This is so for all the surfactants investigated, and it could be explained by considering two factors. As mentioned before, an increase in the content of EG in the mixture results in a decrease in the polarity of the waterEG bulk phase.38 This means that the water-EG bulk phase will be a better solvent for the organic substrate molecules than the aqueous bulk phase, thus diminishing the Km values. On the other hand, when the weight percent of EG increases, the polarity of the interfacial region, where the phenyl chloroformate molecules are located, seems to increase (particularly for the high wt % EG levels). As a consequence, the affinity of the organic substrate molecules for the micelles will decrease, thus decreasing Km. In regard to the rate constant values in the micellar pseudophase, Table 6 shows that for any of the wt % EG levels studied km(SDS) < km(Brij35) < km(TTAB) < km(SB3-14). In a previous work,18f the fractional charge at the reaction center was estimated using the Eyring equation and considering the micellar kinetic effects observed for the spontaneous hydrolysis of phenyl chloroformate in cationic and anionic micellar solutions, the reaction being substantially faster in the presence of cationic micelles than in the presence of anionic micelles. An apparent fractional charge in the transition state equal to -0.25 was estimated. Taking into account the simplifications involved in the model used, the conclusion is that a negative fractional charge develops at the reaction center, in agreement with the mechanism proposed in Scheme 1. Since the presence of various amounts of EG in the mixtures does not affect the ratio km(TTAB)/ km(SDS), it is reasonable to conclude that the mechanism followed by the reaction is the same in all the water-EG micellar solutions studied, with a negative fractional charge developed in the transition state. Medium effects exerted by micelles on the spontaneous hydrolysis of phenyl chloroformate can be due to various factors. Among them, the transfer of the organic substrate from the water-EG bulk phase to the micelles (through the Km values) and the mechanism followed for the reaction have been already considered. Now the influence of (45) Possidonio, S.; Siviero, F.; El Seoud, E. O. J. Phys. Org. Chem. 1999, 12, 325.

properties of the interfacial region, where the reaction occurs, will be taken into account. In regard to chargecharge interactions, electrostatic interactions between the negatively charged carbonyl oxygen of the transition state (see Scheme 1) and the headgroups of the surfactant molecules forming the micelles will attract cationic micelles and repel anionic micelles. No electrostatic interactions are expected for nonionic surfactants. In the case of sulfobetaine surfactants, the charge density due to the cationic ammonium centers is higher than that at the anionic sulfonate centers and the interfacial electrical potential is about 30 mV (as compared to 140 mV for cationic micelles and -130 mV for anionic micelles).46-48 Therefore, if charge-charge interactions were the main factor operating on reactivity the expected trend would be km(TTAB) > km(SB3-14) > km(Brij35) > km(SDS), for all the wt % EG levels investigated. Table 6 shows that this is not the observed trend, which means that other factors have to be considered. Polarity and water content of the interfacial region as well as the high ionic strength present in the Stern layer of ionic micelles could influence reactivity. Rate constants of spontaneous hydrolysis decrease upon addition of organic solvents to water.49 That was also the case for the reaction under study18f and can be explained taking into account that the addition of an organic solvent diminishes the water content and decreases the polarity of the reaction medium. A decrease in polarity results in a larger destabilization of the polar transition state with respect to the initial state, thus retarding the reaction. If the ET parameter was taken as an indicator of the polarity of the interfacial region of micelles, a larger inhibition would be expected for sulfobetaine, nonionic, and cationic micellar solutions (in that order) and a smaller inhibition for the anionic micellar solutions. In regard to the water content in the Stern region of various micelles, estimations of water concentrations between 33 and 45 M for anionic, nonionic, and cationic micellar solutions were done.50 This indicates that large concentrations of water are available in the Stern region, in agreement with the not too large inhibition of the reaction when it takes place in the micellar pseudophase with respect to the process occurring in the aqueous phase. However, it is not possible to quantify the contribution of this factor and, besides, water activity, the reduced mobility of water molecules, and so forth could also influence reactivity. With regard to the influence of the high ionic strength present in the interfacial regions of ionic micellar solutions, the calculated ionic concentrations in the micellar Stern region of SDS and hexadecyltrimethylammonium bromide, CTAB, are 2.7-5.4 and 2.9-5.7 mol dm-3, respectively.51 It seems reasonable to assume that the ionic concentration in CTAB and TTAB micellar Stern regions is similar. To investigate the effect of ionic concentration on the reaction studied, two model salts were used on the basis of their resemblance to the (46) (a) Kamenka, N.; Chevalier, Y.; Zana, R. Langmuir 1995, 11, 3351. (b) Kamenka, N.; Chorro, M.; Chevalier, Y.; Levy, H.; Zana, R. Langmuir 1995, 11, 4234. (47) Ferna´ndez, M. S.; Fromherz, P. J. Phys. Chem. 1977, 81, 1755. (48) (a) Drummond, C. J.; Grieser, F.; Healy, T. W. J. Chem. Soc., Faraday Trans. 1 1989, 85, 537. (b) Almgrem, M.; Rydholm, R. J. Phys. Chem. 1979, 83, 360. (49) (a) Ingold, C. K. Structure and Mechanism in Organic Chemistry, 2nd ed.; Cornell University Press: Ithaca, NY, 1969; Chapter 7. (b) Kevill, D. N.; Anderson, S. W. J. Org. Chem. 1991, 56, 1845. (50) (a) Chandhuri, A.; Loughlin, J. A.; Romsted, L. S.; Yao, J. J. Am. Chem. Soc. 1993, 115, 8351. (b) Chaudhuri, A.; Romsted, L. S. J. Am. Chem. Soc. 1991, 113, 5052. (c) Angeli, A. D.; Cipiani, A.; Germani, R.; Savelli, G.; Cerchelli, G.; Bunton, C. A. J. Colloid Interface Sci. 1988, 121, 42. (d) Romsted, L. S.; Yao, L. Langmuir 1996, 12, 425. (51) Buurma, N. J.; Herransz, A.; Engberts, J. B. F. N. J. Chem. Soc., Perkin Trans. 2 1999, 113.

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micellar headgroups of TTAB and SDS: tetraethylammonium bromide, TMBA, and sodium monomethyl sulfate, NMS. The spontaneous hydrolysis of phenyl chloroformate was studied in homogeneous aqueous solutions in the presence of various TMBA concentrations up to 3.5 M and in the presence of various NMS concentrations up to 4 M. The diminution in the observed rate constant value was no larger than 15% in both cases (kobs > 12 s-1). The km values in Table 6 for TTAB and SDS micellar solutions show that the retardation of the reaction due to the incorporation of the substrate into the micellar interfacial region is much larger. Therefore, the ionic strength effect is not the main contribution to the medium micellar effects, although it could be responsible for the lower km value found in aqueous TTAB micellar solutions with respect to that in aqueous SB3-14 micellar solutions. The presence of different amounts of EG in the mixture does not substantially affect the km values obtained from the fittings. The small decrease in the polarity of the interfacial region would affect the reaction accelerating the process. However, the spontaneous hydrolysis of phenyl chloroformate is not particularly sensitive to changes in the polarity of the reaction medium and considering the simplicity of the model used to discuss the kinetic data only substantial changes in km are worth discussing. The fact that the trend km(SDS) < km(Brij35) < km(TTAB) < km(SB3-14) remained in the presence as well as in the absence of EG points out that the factors considered to explain the micellar medium effects in aqueous micellar solutions are also operative in waterEG micellar solutions.

Rodrı´guez et al.

Summarizing, for water-EG TTAB, SB3-14, Brij35, and SDS micellar solutions an increase in the amount of ethylene glycol present in the mixture results in (i) an increase in the cmc; (ii) an increase in the micellar ionization degree, in the case of ionic surfactants; (iii) a decrease in the aggregation number; (iv) a small increase in the polarity of the micellar interfacial region with the exception of SDS micelles. Kinetic micellar effects on the spontaneous hydrolysis of phenyl chloroformate show that the presence of micelles inhibits the reaction. The acceleration of the process upon increasing the amount of EG present in the mixture, for all the micellar solutions investigated, is mainly due to the decrease in the equilibrium binding constant of the organic substrate molecules to the micelles. This results in the contribution of the reaction occurring in the bulk water-EG phase being more important, and as a consequence, the observed rate constant increases. The trend km(SDS) < km(Brij35) < km(TTAB) < km(SB3-14) holds independently of the weight percent of EG present in the mixture and can be rationalized by considering charge-charge interactions and polarity, ionic strength, and water content effects. Other specific factors operating on reactivity cannot be neglected. 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. LA048555L