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The critical micelle concentration of sodium oleate-hexadecyltrimethylammonium bromide mixtures was determined as a function of the mixture composition and analyzed with Rubingh's theory, showing a strong nonideality. The hydrophile-lipophile balance (HLB) of the mixtures was also measured and the validity of the additivity rule for the HLB of surfactants mixtures was tested. Results were compared with the aggregation behavior of that mixture. Both the HLB and the critical micelle concentration dependencies on the mixture composition exhibit a negative deviation when compared with ideal behavior, which is explained on the basis of the interactions between components and surfactant partitioning between the bulk and the interface.

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The critical micelle concentration (CMC) of aqueous mixtures of decyl-(C 10 TAB), dodecyl-(C 12 TAB), and tetradecyl trimethyl ammonium (C 14 TAB) 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, (CMC ijk ) exp , and that computed with the MRST, (CMC ijk ) calc ), were lower than the ideal one, (CMC ijk ) id , indicating an attractive interaction. Moreover, in the majority of the triangular diagram (CMC ijk ) exp differs significantly from (CMC ijk ) 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 (C 10 TAB-C 12 TAB) and \50 % of the longer one (C 14 TAB). Furthermore, the common statement that mixtures of homologue surfactants are ideal is not supported by this work.