Molecular Dynamics Simulation of "Quasi-Gemini" Anionic Surfactant at the Decane/Water Interface

Anionic surfactants play a key role in many industrial fields such as drug delivery, detergent, oil displacement and food processing because of their unique amphiphilic properties. The structure of surfactant in oil-water system has a great influence on the interfacial behavior. It is of great significance to study the structure and interfacial properties of surfactants. In this paper, the all-atomic molecular dynamics method was used to study the aggregation behavior of nonylphenol-substituted series of alkyl sulfonate surfactants (C-n-NPAS) at the decane/water interface. The effects of different sulfoalkyl chain lengths on the interfacial properties of nonylphenol-substituted alkyl sulfonate surfactants were investigated by analyzing the interface thickness, interface formation energy, interfacial tension, the radial distribution function and coordination number. Simulation results have shown that the interfacial thickness increases at first and then decreases as the length of sulfoalkyl chain increases. The same trend was found in the results of the interface formation energy (IFE). The absolute value of IFE follows the order of C-12-NPAS>C-14-NPAS>C-10-NPAS>C-16-NPAS>C-8-NPAS, indicating that the C 12 -NPAS is the most stable system in terms of energy which should be attribute to the stronger aggregation ability. Moreover, it is observed that the trend of interfacial tension is in agreement with that of interface formation energy and the interface thickness. Surfactant C-12-NPAS induces the minimum interfacial tension. The calculation results are consistent with the experimental data. Furthermore, the radial distribution function and the coordination number of water around the surfactant headgroup were obtained for evaluating the interaction strength between the hydrophilic headgroup and water molecules. The results are well in accordance with the trend of the interface formation energy and interfacial tension. This indicates that the length of alkyl tail affect the interaction between hydrophilic headgroup and water indirectly. Simulation results suggest that the length of alkyl tail plays a dominant role in the interfacial behaviors. We expect that the results of this study could be valuable for the understanding of mechanism and the design of high performance surfactants.