A theoretical study of the effect of long-chain aliphatic alcohols on the stability of surfactant micelles

Abstract

Employing the theoretical approach and computational methods described previously [9], calculations have been made on the two-dimensional size/composition distribution of spherical micelles composed of sodium dodecylsulphate (SDS) and dodecylalcohol (DOH) monomers. These calculations show fair agreement with experimental CMC and solubilization capacity results. For rod-shaped micelles our calculations indicate that the critical condition for the formation of rods, i.e. β=0, where βkT is the difference in molecular free energy per SDS/DOH unit between the middle part of the micelle and the solution, is approached at significantly lower salt concentrations upon raising the DOH/SDS ratio.

The incorporation of DOH in the SDS micelles is driven by the free energy of mixing the monomers within the micelles and by the reduction obtained of their electrostatic free energy. Since a slight swelling is noted, there is a counteracting increase in chain conformational free energy. Moreover, the hydrophobic driving force of aggregation is considerably weaker for DOH than for SDS because the monomer concentration of DOH is comparatively low.