Structural properties of hydroxyl-substituted alkyl benzenesulfonates at the water/vapor and water/decane interfaces

Molecular dynamics simulations have been performed to investigate the structural properties of hydroxyl-substituted alkyl benzenesulfonate monolayers formed at the water/vapor and water/decane interfaces. We report a detailed study of the interfacial properties—liquid density profile, hydrogen bond structure, surfactant aggregate structure and order parameter—of the novel anionic surfactant, sodium 2-hydroxy-3-decyl-5-octylbenzenesulfonate (C10C8OHphSO3Na). Simulation results show that: with increasing number of surfactant molecules, the average number of intramolecular hydrogen bonds per surfactant molecule in the monolayer decreases, but the structures forming the intramolecular hydrogen bonds still play a dominant role; the hydrophobic part of the alkyl tail chain, especially the decyl substituent on the third carbon atom in the benzene ring, becomes straighter, and more ordered towards the external interface at higher surfactant coverage; two-dimensional radial distribution functions can describe the characteristic of surfactant aggregate structures and highlight the decane phase effect on the orientation of the hydrophobic part of the surfactant; the surfactant molecules readily form long-range hydrogen bonded structures. Our results are an important complement to experimental studies. We used the all-atom model by employing the GROMACS and ffAMBER programs in the simulations, which provides a new way to simulate the interfacial behavior of alkyl benzenesulfonate surfactants.