On the Applicability of the Regular Solution Theory to Multicomponent Systems

Abstract

AbstractThe critical micelle concentration (CMC) of aqueous mixtures of decyl‐ (C10TAB), dodecyl‐ (C12TAB), and tetradecyl trimethyl ammonium (C14TAB) bromides has been studied in the complete triangular diagram. The results have been analyzed on the basis of the multicomponent regular solution theory for mixed micelles (MRST). It has been found that the mixtures of both, two or three components, contrary to the MRST theory assumptions, have a nonideal behavior. Both the experimental CMC, (CMCijk)exp, and that computed with the MRST, (CMCijk)calc), were lower than the ideal one, (CMCijk)id, indicating an attractive interaction. Moreover, in the majority of the triangular diagram (CMCijk)exp differs significantly from (CMCijk)calc. This suggests that the assumption in the MRST that the interaction among the three different homologue molecules in the mixed micelle computed using the binary regular solution theory interaction parameters obtained from binary mixtures may be an oversimplification. This has been reinforced by analysis of the intramicellar activity coefficients of components, indicating differences in the surrounding of the component molecules when they are in bi‐ or tricomponent micelles. The excess free energy of micellization in three‐component systems indicates that the more stable mixed micelles are those having an excess of the shorter components (C10TAB–C12TAB) and <50 % of the longer one (C14TAB). Furthermore, the common statement that mixtures of homologue surfactants are ideal is not supported by this work.