Volume study on the exclusion of lithium naphthylsulfonate from lithium decylsulfonate micelles

Densities of aqueous solutions of lithium 1-naphthylsulfonate (1-LiNSO)–lithium decylsulfonate (LiDeSO) and 2-LiNSO–LiDeSO mixtures were measured as a function of total molality and composition of the mixtures. The partial derivative of the solution with respect to the total molality was calculated for the monomer and micellar regions. It was found that the values of the partial derivative are larger for the 2-LiNSO–LiDeSO system than for the 1-LiNSO–LiDeSO system. This fact is attributable to the larger value of the partial molar volume of monomer of 2-LiNSO than that of 1-LiNSO. For the two systems, the micellar molar volume of the mixtures varied linearly with the composition from the partial molar volumes of 1- and 2-LiNSO in their single systems to the micellar molar volume in the single LiDeSO system. Miscibility of the solutes in the mixed micelles was examined by drawing the critical micelle concentration (CMC) vs composition diagrams. The diagrams for the 1- and 2-isomers coincided with each other and showed that molecules of 1- and 2-LiNSO are excluded from the micelles. The contribution of the micelle-unforming component to the volume of micelle formation is positive and large because of the exclusion from the micelles. On the other hand, the contribution of the micelle-forming component to the volume of micelle formation is unchanged. The dependence of the monomer molalities of LiNSO and LiDeSO on the total molality, evaluated by means of the CMC vs composition diagrams, substantiated the validity of the approximations used in the derivation of the equations in this study.