Effects of additives (methanol and NaCl) from aqueous surfactant solutions on the micellisation of sodium deoxycholate and sodium cholate binary mixture in the temperature interval T = (278.15–318.15) K: The molar excess Gibbs energy and the molar Gibbs energy of the micelle formation

Abstract

Micellisation of a binary mixture of sodium deoxycholate (SD) and sodium cholate (SC) is examined for the aqueous solution without additives (referent system for self- association process) and for the aqueous solution containing methanol, NaCl and methanol + NaCl in the temperature range T = (278.15–318.15) K. Critical micelle concentrations of mono-component surfactants and binary mixtures of surfactants at different molar ratios of mixing were determined experimentally. These values were used to calculate the mole fractions of building units in the binary mixed micelle, the interaction parameter, the molar excess Gibbs energy and the molar Gibbs energy of micellisation. Effects of additives in aqueous surfactants solution on the micellisation of the binary mixture can be observed in two ways, by considering the excess molar Gibbs energy (gije) or by analyzing the molar Gibbs energy of micellisation (ΔgmM0). The impact of methanol in aqueous solution is expressed differently over gije and ΔgmM0, while the effect of Na+ ion is similar for both thermodynamic functions: gije and ΔgmM0. In this paper is performed a detailed analysis of the molar Gibbs energies of micellisation: ΔgmM0(1)=RTxilncmci+xjlncmcj+xilnxi+xjlnxj+βxi-xi2 and ΔgmM0(2)=RTlncmcij and their different loading with absolute errors.