Dissipative particle dynamics study on the temperature dependent interfacial tension in surfactant-oil-water mixtures

Effects of cationic (CATB) and anionic (SDBS) surfactants on the interfacial tension (IFT) of surfactant/oil/water system at different temperatures are predicted using coarse-grained dissipative particle dynamics (DPD) at the mesoscopic scale. The DPD repulsive interaction parameters, aij, as a function of temperature (T) are calculated from the T dependent Flory-Huggins parameter χ for different components using all-atom molecular dynamics (MD). This paper provides a method based on DPD dynamics to obtain linear correspondences between the Flory-Huggins parameter χ and the excess repulsion parameter Δa at different temperatures, which is a bridge between DPD and MD. The IFTs are measured in surfactant/oil/water systems at different temperatures to validate simulated results, they are in good agreement. Both experiments and simulations indicate that the interfacial tension decreases with an increase of temperature. Meanwhile, the interfacial information including equilibrated interface morphology, mean interfacial density and RMS end-to-end distance is obtained, by which the capacities of different surfactants to regulate oil-water interfacial properties at different temperatures are evaluated. Compared with CTAB and SDBS, SDBS has a good resistance to a higher temperature, and can maintain a lower interfacial tension in the range of 343-358 K. This indicates that the interfacial tension decreases with the increase of temperature in a certain range and the sensitivity of different surfactants to temperature is different. Furthermore, the impact of inorganic salt (NaCl) on interfacial properties of ionic surfactant (CTAB) is analyzed. It demonstrates that the modified DPD method is a good alternative to study the interfacial characteristics of surfactant/oil/water system at different temperatures.