New insight into the mixed micelles thermodynamics modelling

ERICA PATRICA SCHULZ; GUILLERMO A. DURAND; PABLO CARLOS SCHULZ

Guimaraes/Braga

Congreso; 6th Iberian Meeting on Colloids and Interfaces; 2015

Centre of Physics of University of Minho (CFUM), BragaER

Surfactant mixtures have wide applicability in fields such as cosmetics, biology, and pharmacology. The Regular Solution Theory (RST) [1,2] is the most frequently applied model for interpreting the behavior of surfactant mixtures. One of the major problems is to define the molar fraction of each surfactant in the aggregate since its value is fixed by the partition equilibria of the species between the aggregate and the surrounding medium, and only the total composition of the micellar solution is accessible to the experimenter. When the micelle is considered as a pseudo-phase and the activities of the micelle components are known for several mixtures, the Gibbs?Duhem relation allows the corresponding compositions of the aggregate to be determined. The regular solution approximation assumes that the excess entropy of mixing is zero. The classically employed RST applies the symmetric Margules-type formulations [3] in order to model excess molar thermochemica1 properties. Moreover, the extended version employed for multicomponent systems assummes that the ternary o higher order interactions can be described only by binary interactions [4,5]. This modelling approachs fails in representing many systems [6,7].In the present work we present a novel approach based on the global minimization of the free energy constrained by the thermodinamics of mixed micellization, which simultaneously solves the whole model for all the compositions in the phase diagram. This procedures assures the application of the Gibbs-Duhem relation. Besides, we consider asymmetric Margules-type formulations which is a more general case than the symmetric ones. Within this frame, multicomponent systems are much better respresented considering higher order interactions. We used a well-known, state of the art, tool/language to solve the algebraic model, GAMS (General Algebraic Modelling System) [8], which provides the interface for the solving packages BARON and DICOPT [9].