Molecular dynamics study on the effect of surfactant mixture on their packing states in mixed micelles

Abstract

Mixture of different surfactant molecules possess better performances than single surfactant, which has importantly scientific and practical significance. In this paper, the effects of different mole ratios on the packing states of sodium dodecyl sulfate (SDS) and n-dodecyl betaine (NDB) in mixed micelles are studied through molecular dynamics simulations. The packing state is characterized by calculating and comparing the configuration entropy and entropy decomposition of SDS and NDB. Results show that mixing with NDB molecules can cause the increase of SDS packing order in mixed micelles, while mixing with large proportions of SDS molecules can cause the decrease of NDB packing order in mixed micelles. Moreover, structural properties that are closely related to the packing state are analyzed. The surface hydrophobicity and hydrophilicity of SDS molecules in micelle can be reversed through changing its mole fraction. Both SDS and NDB terminal carbon atoms in mixed micelles possess opposite distribution tendency to those in pure SDS or NDB micelle. As mixed ratio changes, dominant packing patterns of SDS and NDB in mixed micelles are different. Two represent models of mixed micelle with the change of mole ratios are presented.