Thoughts on the ideal behavior of mixed micelles and the appropriate application of regular solution theory (RST)

Solutions of mixed surfactants are often considered as solvent mixtures. Usually, mixed micellar aggregates are considered as a homogeneous mixture of solvents dispersed in a solution. But the transposition of the usual thermodynamic models of solvent mixtures to mixed micelles is not always so obvious. We discussed this point in this paper by considering several cases of surfactant mixtures. A major problem is to define the molar fraction of each surfactant in the aggregate especially when a charged surfactant is employed in the mixture, because possible dissociation of the components of the mixture must be considered in the bulk as well in the micelle. This definition is crucial especially for the characterization of the ideal behavior which is usually described by the Clint relation, as well as for the application of regular solution theory (RST) which is the most frequently applied model for interpreting the behavior of surfactant mixtures. We show in this paper how the definition of the molar fraction can change the equations and the interpretations.

DTABr–Brij35 mixtures in 0.3 M NaBr can be well described by an ideal behavior supposing that the DTABr generated 1.4 particles.Figure optionsDownload high-quality image (74 K)Download as PowerPoint slideResearch highlights
► Transposition of the usual thermodynamic behavior models of solvent mixtures to mixed micelles is not always obvious.
► Ionic surfactants in mixed micelles can be either totally associated to their counterions or partly or completely dissociated.
► Clint’s equation must be applied carefully for mixed micelles of charged and uncharged surfactants.
► Application of RST to mixed micelles depends on the presence or not of ionic surfactants.