Multi-scale simulation studies on interaction between anionic surfactants and cations

Abstract

In this paper, a dissipative particle dynamics (DPD) simulation method was used to investigate the impact of cations on the rheological properties of dodecyl sulfonate surfactant solutions. In order to obtain reasonable interaction between head groups of the surfactant, the geometric structure and interaction between dodecyl sulfonate and cations are optimized using density function theory (DFT) at the B3LYP/6-31G level. The DFT calculated results indicate that α-methylene nearest the head group can be classified as a part of the polar head. After binding, the charge on polar head decreases, thus greatly reduces the repulsion between the head groups. It is found that the presence of counterions is one of induction factors on the formation of wormlike micelles, thus greatly enhances the viscosity of surfactant solution. With the increasing in shear strengthen, the wormlike micelles are gradually oriented in the x direction and then broken up into small spherical micelles. This process is also shown by the decrease of viscosity, which decreases quickly at the low shear rates, then keeps almost a constant at the moderate shear rates and at last decreases again at the shear rates larger than a critical value. Compared with monovalent cations, divalent cations have a stronger effect on the rheological properties of dodecyl sulfonate solutions.