Thermodynamic modeling of precipitation behavior in mixed micellar systems

Abstract

Results of thermodynamic modeling of solubility diagrams for various types of mixed micellar systems (nonionic surfactants with added alkanols, mixtures of two nonionics, two anionics, anionic plus semipolar surfactants) are summarized. Phase behavior over a wide range of surfactant concentrations is under study and the model description is aimed at prediction of the solubility diagrams in ternary systems on the basis of data for aqueous solutions of the pure surfactants. In this case the pseudo-phase separation approach is quite efficient. When the total surfactant concentration in mixed solutions far exceeds the CMC it is expedient to describe the dependence of the dissolution (precipitation) temperature on the relative content of two surfactants using the concentration variables of the micellar pseudo phase. It follows from thermodynamic equations that a substantial decrease of the precipitation temperature can be produced by additives forming mixed micelles with the basic surfactant, and the distribution coefficient between micelles and the aqueous surroundings is an important characteristics of the additive. In some cases (mixtures of nonionic surfactants, solutions of ionic surfactants without a salt background at concentrations substantially above the CMC) the composition of mixed micelles is very close to the overall surfactant based composition of the solution. That is why in the both cases the eutectic type curves ‘dissolution temperature versus surfactant-based mole fraction’ can be modeled in a similar manner which is simpler than calculations for ionic systems based on the solubility product approach. For mixtures with added salts the ion exchange between micelles and the aqueous surroundings should be taken into account. The specificity of systems whose phase behavior is affected by chemical reactions (protonation, complex formation) is discussed and a version of the pseudo-phase model is proposed for description of the solubility diagrams in aqueous sodium dodecylsulfate–alkyl amine oxide mixtures. Experimental results obtained by the author and collegues and data from literature serve to illustrate the model performance.