The influence of the structure of selected Brij and Tween homologues on the thermodynamic stability of their binary mixed micelles

Abstract In order to construct micelles with desirable characteristics and predictable behaviour, insights in micellization mechanisms and factors leading to micelle stabilization are necessary. In this study, Brij S10, Brij S20, Tween 20, Tween 80 and their binary systems were examined. Spectrofluorimetry was used in order to obtain experimental values of critical micelle concentrations from which thermodynamic parameters were calculated. The aim of the study was to determine the nature of the excess Gibbs energy in examined systems. Analysis of the function β  =  f ( T ), allowed us to define how variations in surfactants’ structure affect the thermodynamic stabilization of binary systems. Results show that the difference in the length of hydrophobic segments in mixed micelles is responsible for the additional stabilization of Brij S10/Tween 20 and Brij S20/Tween 20 binary systems. In comparison to them, mixed micelles containing alkyl chains of the same length have lower excess entropy values. Introduction of olefin bond in surfactant’s alkyl chain (Tween 80) destabilizes the binary system and reduces its excess entropy values. Elongation of hydrophilic segments results in a favourable enthalpic and an unfavourable entropic contribution to the excess Gibbs energy.