Langmuir 2004, 20, 10419-10426
10419
Stability, Interaction, Size, and Microenvironmental Properties of Mixed Micelles of Decanoyl-N-methylglucamide and Sodium Dodecyl Sulfate J. M. Hierrezuelo, J. Aguiar, and C. Carnero Ruiz* Grupo de Fluidos Estructurados y Sistemas Anfifı´licos, Departamento de Fı´sica Aplicada II, Escuela Universitaria Polite´ cnica, Universidad de Ma´ laga, Campus de El Ejido, 29013 Ma´ laga, Spain Received July 9, 2004. In Final Form: September 14, 2004 The mixed micellization between the nonionic surfactant decanoyl-N-methylglucamide (MEGA-10) and the common sodium dodecyl sulfate (SDS) in aqueous solutions of 0.1 M NaCl was investigated by the fluorescence probe method. The critical micelle concentrations were determined by the pyrene 1:3 ratio method. The experimental data are discussed in light of two mixing thermodynamic models within the framework of the pseudophase separation model, including the conventional regular solution theory and a recent treatment proposed by Maeda (J. Phys. Chem. B 2004, 108, 6043). This last approach provides a more appropriate description of the mixed system, particularly in two aspects: the nature of the interactions responsible for the stability of the mixed micelle and the behavior of the excess free energy per monomer of the system. By using the static quenching method, the mean micellar aggregation numbers of mixed micelles in the whole range of compositions were obtained. It was found that the micellar aggregation number initially increases with the content of the ionic component, then remains roughly constant, and, finally, decreases slightly for high content of this component. This behavior was analyzed taking into account the effects produced by the presence of the charged headgroups of sodium dodecyl sulfate, as this component increases its participation in the mixed micelle. The micropolarity of the mixed micelles was studied by the pyrene 1:3 ratio index. It was observed that the increasing participation of the ionic component induces the formation of micelles with a more dehydrated structure. Data of micellar microviscosity were obtained by using different methods, including fluorescence intensity measurements of Auramine O and steady-state fluorescence anisotropy of rhodamine B and diphenylbutadiene. The results obtained from these experiments are in good agreement and suggest the formation of mixed micelles with a less ordered structure as the content of SDS increases.
Introduction The physicochemical properties of the aqueous solutions constituted by two or more surfactants often present a very different character in comparison to those formed by the single surfactants. The change in these properties is due to the occurrence of interactions of a different nature between the component surfactants. The interest in the study of these systems is motivated by aspects of both applied and theoretical character. On one hand, these systems often present superior properties to those with a single surfactant component, showing numerous advantages in many technical applications.1-4 For example, mixed micelles of ionic and nonionic surfactants show an expanded colloidal stability when compared with the pure nonionic system. In addition, the size of an ionic micelle, which usually forms small globular aggregates at a low surfactant concentration, may be increased upon the addition of a nonionic surfactant. The improvement of these two properties, stability and size, would enhance the capability of incorporating different solutes in the micellar phase, and this is an important topic in many applications of micellar solutions.5 Furthermore, there are other important properties of great interest in several applications of surfactants which are determined by the micellar composition. A representative case refers to the * Corresponding author. E-mail:
[email protected]. (1) Hill, R. M. In Mixed Surfactant Systems; Ogino, K., Abe, M., Eds.; Marcel-Dekker: New York, 1993; p 317. (2) Hoffmann, H.; Po¨ssnecker, G. Langmuir 1994, 10, 381. (3) Desai, T. R.; Dixit, S. G. J. Colloid Interface Sci. 1996, 177, 471. (4) Kronberg, B. Curr. Opin. Colloid Interface Sci. 1997, 2, 456. (5) de Oliveira, H. P. M.; Gehlen, M. H. Langmuir 2002, 18, 3792.
microenvironmental properties, which play a decisive role in areas such as micellar catalysis. Note, for instance, that the local polarity or micropolarity can modify not only the velocity but also the mechanism of the reaction.6 On the other hand, it is also important to get appropriate mixing thermodynamic models able not only to interpret the experimental results but also to predict the behavior of a determined mixed system. Hines7 has reviewed the most relevant advances in theoretical studies of mixed surfactant systems up to 2001. However, new approaches arise continually in this area. Recently, Maeda8 has reported a phenomenological procedure for analyzing the stability and the synergism of ionic/nonionic mixed micelles in salt solutions. Among the more frequently studied mixed systems, those constituted by a polyoxyethylene nonionic surfactant and an ionic one have attracted considerable attention due to their wide use in many industrial applications.5,8-18 (6) Abe, M.; Ogino, K. In Mixed Surfactant Systems; Ogino, K., Abe, M., Eds.; Marcel-Dekker: New York, 1993; p 1. (7) Hines, J. D. Curr. Opin. Colloid Interface Sci. 2001, 6, 350. (8) Maeda, H. J. Phys. Chem. B 2004, 108, 6043. (9) Zhang, H.; Dubin, P. L. J. Colloid Interface Sci. 1997, 186, 264. (10) Feitosa, E.; Brown, W. Langmuir 1998, 14, 4460. (11) Shiloach, A.; Blankschtein, D. Langmuir 1998, 14, 7166. (12) Penfold, J.; Staples, E.; Thompson, L.; Hines, J.; Thomas, R. K.; Lu, J. R.; Warren, N. J. Phys. Chem. B 1999, 103, 5204. (13) Rodenas, E.; Valiente, M.; del Sol Villafruela, M. J. Phys. Chem. B 1999, 103, 4549. (14) Carnero Ruiz, C.; Aguiar, J. Mol. Phys. 1999, 97, 1095; 2000, 98, 699. (15) Carnero Ruiz, C.; Aguiar, J. Langmuir 2000, 16, 7946. (16) Matsubara, H.; Muroi, S.; Kameda, M.; Ikeda, N.; Ohta, A.; Aratono, M. Langmuir 2001, 17, 7752.
10.1021/la048278i CCC: $27.50 © 2004 American Chemical Society Published on Web 10/28/2004
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Langmuir, Vol. 20, No. 24, 2004
Recently, alkylpolyglucoside (APG) surfactants have been attracting increasing interest due to their peculiar solution properties. APG surfactants are nonionic and are characterized by having a hydroxyl sugar group as the hydrophilic moiety, and they show a solution behavior substantially different from the ethoxylated nonionic ones. For instance, APG surfactants have both stronger lipophobicity and hydrophilicity, and their temperature dependence of the solution properties is much less pronounced, not showing the clouding phenomenon.19,20 This kind of surfactant has a number of properties which make it very interesting in numerous applications in the biomembrane field.21 In addition, APG surfactants are biodegradable and considered dermatologically safe and, therefore, are very suitable materials in applications related to cosmetic preparations, cleaning products, or food technology.22 However, the number of studies concerned with mixtures of APG with other surfactants is rather scarce.23-32 This paper deals with a fluorescence probe study on the characterization of the mixed system formed by the APG surfactant decanoyl-N-methylglucamide (MEGA-10) and the conventional ionic one sodium dodecyl sulfate (SDS) in 0.1 M NaCl. The interactions in the system were determined by analyzing the variation of the critical micelle concentration (cmc) through the whole composition range, as determined by the pyrene 1:3 ratio method. The experimental data were analyzed by the regular solution theory, and the results were compared with those obtained by a new approach recently proposed by Maeda.8 In the second part, by using the static quenching method, we study the variation of the micellar mean aggregation number as a function of the system composition. Finally, by selecting different fluorescent probes, we have obtained information on the microenvironmental properties of the mixed system. Experimental Section Materials. The nonionic surfactant MEGA-10 and the quencher cetylpyridinium chloride (CPyC) were obtained from Sigma, whereas SDS was acquired from Fluka. These products were used as received. The fluorescence probes pyrene, auramine O (AuO), and rhodamine B (RB) were also from Sigma, and diphenylbutadiene (DPB) was from Aldrich. All these substances were also used without further purification. Stock solutions of surfactants and quencher were prepared in water and those of (17) Carnero Ruiz, C.; Aguiar, J. Colloids Surf., A 2003, 224, 221. (18) Sharma, K. S.; Rodgers, C.; Palepu, R. M.; Rakshit, A. K. J. Colloid Interface Sci. 2003, 268, 482. (19) Shinoda, K.; Carlsson, A.; Lindman, B. Adv. Colloid Interface Sci. 1996, 64, 253. (20) So¨derman, O.; Johansson, I. Curr. Opin. Colloid Interface Sci. 2000, 4, 391. (21) Bonincontro, A.; Briganti, G.; D’Aprano, A.; La Mesa, C.; Sesta, B. Langmuir 1996, 12, 3206. (22) Garcı´a, M. T.; Ribosa, I.; Campos, E.; Sanchez Leal, J. Chemosphere 1997, 35, 545. (23) Drummond, C. J.; Warr, G. G.; Griesser, F.; Ninham, B. W.; Evans, D. F. J. Phys. Chem. 1985, 89, 2103. (24) Griffiths, P. C.; Stilbs, P.; Paulsen, K.; Howe, A. H.; Pitt, A. R. J. Phys. Chem. B 1997, 101, 915. (25) Hines, J. D.; Thomas, R. K.; Garret, P. R.; Rennie, G. K.; Penfold, J. J. Phys. Chem. B 1997, 101, 9216. (26) Arai, T.; Takasugi, K.; Esumi, K. J. Colloid Interface Sci. 1998, 197, 94. (27) Sierra, M. L.; Svensson, M. Langmuir 1999, 15, 2301. (28) Okano, T.; Tamura, T.; Abe, Y.; Tsuchida, T.; Lee, S.; Sugihara, G. Langmuir 2000, 16, 1508. (29) Liljekvist, P.; Kronberg, B. J. Colloid Interface Sci. 2000, 222, 159. (30) Sulthana, S. B.; Rao, P. V. C.; Bhat, S. G. T.; Nakano, T. Y.; Sugihara, G.; Rakshit, A. K. Langmuir 2000, 16, 980. (31) Rosen, M. J.; Sulthana, S. B. J. Colloid Interface Sci. 2001, 239, 528. (32) Del Burgo, P.; Junquera, E.; Aicart, E. Langmuir 2004, 20, 1587.
Hierrezuelo et al. fluorescence probes in absolute ethanol. Water was doubly distilled, and all the experiments were carried out with freshly prepared solutions. Apparatus. All fluorescence measurements were recorded on a SPEX FluoroMax-2 spectrofluorometer in the “S” mode. This apparatus is equipped with a thermostated cell housing and fitted with a 150-W xenon lamp and 1 cm × 1 cm quartz cells. Fluorescence anisotropy measurements were recorded in the same apparatus provided with a polarization accessory, which uses the L-format instrumental configuration33 and an automatic interchangeable wheel with Glan-Thompson polarizers. The steady-state fluorescence anisotropy values were determined as
r)
IV - GIH IV + 2GIH
(1)
where the subscripts of the fluorescence intensity values (I) refer to vertical (V) and horizontal (H) polarizer orientation. The software supplied by the manufacturer automatically determined the instrumental configuration factor G, required for the L-format configuration. The anisotropy values were averaged over an integration time of 10 s, and a minimum number of three measurements were made for each sample. The anisotropy values of the probes in micellar media presented in this work are the mean value of three individual determinations. All the fluorescence measurements were made at 30.0 ( 0.1 °C. Methods. The pyrene 1:3 ratio method34,35 was used to obtain the cmc values in each binary surfactant mixture. Different aqueous solutions containing MEGA-10 and SDS in several proportions were prepared. The composition of the solutions was expressed in molar fraction (Rj) of the respective surfactant, defined as
Rj )
[Sj] [Si] + [Sj]
(2)
where [Si] and [Sj] refer to the molar concentration of the component surfactants. Working solutions of lower concentration were prepared by adding proper volumes of the pyrene ethanolic solution. The volume of this solution was small enough (0.1% of the total volume) so that the solvent did not have an effect on the micellar system. From these solutions, fluorescence emission spectra were recorded using an excitation wavelength of 335 nm, and the fluorescence intensities at the wavelengths corresponding to the first (I1) and third (I3) vibronic bands, located near 373 and 384 nm, were measured. The ratio I1/I3 is the socalled pyrene 1:3 ratio. The cmc determination in each sample was repeated at least two times. When we did not find a good reproducibility, the experiment was repeated a third time. The errors in cmc values are estimated to be
