Micellar solution–solid phase equilibrium for ternary systems containing two N-alkanoyl-N-methylglucamine homologues

The equilibrium of mixed micellar solutions of N-alkanoyl-N-methylglucamines (MEGA-n; n=8,10,12) with solid phase was studied experimentally at various concentrations of the surfactants and for their overall content from 5 to 45–50 wt%. The pure crystalline surfactant precipitates in binary aqueous systems containing MEGA-12 or MEGA-10, the temperature of precipitation is nearly constant in the studied range of concentrations (ca. 321.7 and ca. 304.2 K respectively) The solid phase crystallizing from micellar solutions of MEGA-8 is ice (temperature is close to 273.2 K). The temperature of the micellar solution–solid phase equilibrium in systems containing two surfactants depends on relative content of the surfactants but not on their overall concentration. Two branches are revealed in the solubility diagrams `temperature vs. surfactant-based molar fractionʹ, one branch corresponds to crystallization of the longer-tail surfactant, the other relates to crystallization of the second surfactant or ice (the latter is the case for systems with MEGA-8). A thermodynamic method is formulated for predicting the solubility diagram in the micellar system `two homologous surfactants+waterʹ on the basis of data for the aqueous binaries. The results of prediction are in good agreement with experimental observations. The characteristics of critical micellization and the compositions of the micellar and monomer populations for concentrated surfactant solutions (above the CMC) have been estimated in accord with Nagarajanʹs molecular-based model of mixed micelles. The results of the simulation support the central assumptions of the phenomenological treatment: The entity of micelles can be considered as the macro-phase obeying the ideal behavior.