Effect of Double Bonds in the Formation of Sodium Dodecanoate and Sodium 10-Undecenoate Mixed Micelles in Water Marı ´a B. Sierra, ² Marcela A. Morini, ² Pablo C. Schulz,* ,² Elena Junquera, ‡ and Emilio Aicart ‡ Departamento de Quı ´mica, UniVersidad Nacional del Sur, AVenida Alem 1253, 8000 Bahı ´a Blanca, Argentina, and Departamento de Quı ´mica Fı ´sica I, Facultad de Ciencias Quı ´micas, UniVersidad Complutense de Madrid, 28040-Madrid, Spain ReceiVed: April 9, 2007; In Final Form: July 24, 2007 The micellization of an aqueous mixture of sodium dodecanoate (SDD) and sodium 10-undecenoate (SUD) was studied with the theory of mixed micellization. A strong nonideality was found, with a preferential composition of mixed micelles. This phenomenon was interpreted on the basis of the interaction between the vinyl group and water by hydrogen bonding. The importance of the aliphatic π electrons and water was stated. Introduction In previous papers on mixed micelles we have found that the presence of double bonds in the hydrocarbon chain of one of the surfactants leads to an interaction which does not follow the presumptions of the theory of mixed micelles. Cationic- anionic mixtures do not precipitate, even at a 1:1 proportion, and micelles have a preferential proportion which is not that of 1:1, but that which produces the complete substitution of the micelle saturated hydrocarbon/water interface by an unsaturated hydrocarbon/water one. 1-3 The presence of the double bond also gives an unusual nonideal behavior of the partial molar volume of mixed micelles, which indicates that the effect is originated in the hydrocarbon micelle core. 4 Computer simulation supports the explanation that the interaction between the π electrons of the double bond and water produces a reduction in energy which is responsible for this behavior. 4,5 In an anionic-anionic mixture (sodium oleate-sodium dehydrocholate) we also have found a behavior which may be caused by this interaction. 6 However, in this case the unusual structure of one of the components (sodium dehydrocholate) may also play a role in the nonideal interaction. To unequivocally elucidate the effect of the double bond, it is necessary to eliminate all the other possible causes of nonideality. So we have studied an anionic-anionic mixture of soaps having almost the same chain length: sodium dode- canoate (SDD) and sodium 10-undecenoate (SUD). This last surfactant was chosen because it was extensively studied in this laboratory. 2-4,7,8 Theory The regular solution theory has been widely used to model the thermodynamic nonidealities of mixed micelles; it has been shown to accurately model critical micelle concentration (cmc) values 9 and monomer-micelle equilibrium compositions 10 in surfactant systems exhibiting negative deviations from ideality. However, it must be pointed out that the theoretical validity of using regular solution theory to describe nonideal mixing in mixed surfactant micelles has been questioned. 11 Although this theory assumes that the excess entropy of mixing is zero, it has been demonstrated that in some surfactant mixtures this as- sumption is not true. 12,13 However, the pseudophase separation model and regular solution theory combination remains as an extensively used and convenient method for analyzing experi- mental data. A mixture of two different surfactants 1 and 2 form micelles with composition X 1 and X 2 , in equilibrium with solution monomers of composition R 1 and R 2 . These mole fractions are on a surfactant-only basis, so that At the cmc 14 * To whom correspondence should be addressed. E-mail: pschulz@ criba.edu.ar. ² Universidad Nacional del Sur. ‡ Universidad Compultense de Madrid. Figure 1. Critical micelle concentration of SUD-SDD mixed micelles as a function of the total composition of the surfactants mixture, determined by conductivity, dye solubilization, and rhodamine 6G. The full line represents the ideal composition computed with eq 13. The broken line is an eye guide. X 1 + X 2 ) 1 (1) R 1 +R 2 ) 1 (2) R 1 γ 1,m cmc 1 ) X 1 γ 1,M cmc M (3) 11692 J. Phys. Chem. B 2007, 111, 11692-11699 10.1021/jp072766s CCC: $37.00 © 2007 American Chemical Society Published on Web 09/20/2007