Are the mixtures of homologous surfactants ideal?

The interaction between homologous surfactants in mixed micelles was studied by the Regular Solution Theory of mixed micelles. The interaction is independent of the nature of the polar head groups and attractive and the interaction parameter βMβM depends linearly on the difference in chain length ΔnCΔnC. The interaction becomes ideal at ΔnC=0.75±0.06ΔnC=0.75±0.06. Above ΔnC≈5ΔnC≈5, the dependence remains linear but the slope increased 2.7 times. The phenomenon is explained as the effect of the reduction of the hydrocarbon/water micelle interface and a better packing of the chains in the micelle core, caused by the inclusion of a shorter homologous surfactants. This reduction can be more effective when ΔnC⩾5ΔnC⩾5.

The interaction between homologous surfactants in mixed micelles is not ideal as commonly supposed in the theoretical treatments. The nonideality (represented by the interaction parameter βMβM) is independent of the polar head nature, but depends on the difference in chain length (ΔnC)(ΔnC) between the two components. βMβM is negative for ΔnC>0.75ΔnC>0.75 and becomes more attractive for ΔnC⩾5ΔnC⩾5. This behavior is explained on the basis of a better packing in the micelle core and a reduction in the water/hydrocarbon contact.Figure optionsDownload high-quality image (65 K)Download as PowerPoint slide