Molecular Dynamics Simulation Studies On The Selfassembly Of Zwitterionic Surfactant Mixed Systems

Surfactants can self-assemble into a wide variety of micellar structures such as spherical, wormlike, vesicle, or lamellae in solution due to the coexistence of both hydrophilic head group and hydrophobic tail in the same molecule. Some surfactants have been widely used in various fields due to the unique rheological properties of their micellar structures. Multi-surfactant systems usually exhibit many fascinating characteristics which are not found in single surfactant systems, such as complex phase behaviors, ultra-low surface tension and enhanced viscosity and so on. Understanding multi-surfactant systems can bring great technical breakthrough across a range of industries and in different aspects of people’s daily life. In this paper, we investigated the self-assembled structures of anionic(sodium-dodecyl-sulfate, SDS) and zwitterionic(tetradecyl-dimethyl-ammonium-propane-sulfonate, TPS) surfactant mixtures and the impact of octanol on the self-assembly structures in the zwitterionic surfactant.First, we performed molecular dynamical simulations of multi-surfactant systems at different molar ratios of SDS and TPS(XTPS = [TPS]/[TPS]+[SDS]) and different Ca2+ concentration in aqueous solution. The results indicated that when Ca2+ concentration is kept constant, the micelle structure evolves from interdigitate to wormlike and eventually to spherical with the increase of XTPS. Wormlike micelle was formed when XTPS equaled 0.6. Furthermore, with XTPS fixed at 0.6, micelle structure also evolves from interdigitate to wormlike to spherical with the increase of Ca2+ concentration. Ca2+ ions form salt bridges between the heads of surfactant, which can effectively screen the electrostatic repulsion between the heads which assisted the formation of wormlike micelle. However, excessive Ca2+ destabilizes the wormlike micelle and induces the formation of spherical micelles due to high local charge density.Secondly, we performed molecular dynamical simulations to study the effect of octanol(OCT) concentration to the self-assembled structure of zwitterionic surfactant TPS in aqueous solutions. We found that in the low octanol concentration systems, the micellar structure remained spherical. With increasing octanol concentration, the micelles became larger and more compact. Wormlike micellar was obtained as the concentration of octanol continued to increase. By analyzing the radical distribution functions of TPS and OCT molecules, we found that octanol tends to embed in micelles, which enlarges the hydrophobic portion of the micelles, and reduces the surface area of hydrophilic heads that is exposed to the solution due to the electrostatic attraction between octanol and TPS. In addition, hydrogen bonds were formed between the octanol molecules which further stabilized micelles and made the micelle more compact. The simulation results are in good agreement with experimental observations.