C12E6 and SDS surfactants simulated at the vacuum-water interface.

The effect of surface coverage on the aggregate structure for the nonionic hexaethylene glycol monododecyl ether (C(12)E(6)) and anionic sodium dodecyl sulfate (SDS) surfactants at vacuum-water interface has been studied using molecular dynamics simulations. We report the aggregate morphologies and various structural details of both surfactants as a function of surface coverage. Our results indicate that C(12)E(6) tail groups orient less perpendicularly to the vacuum-water interface compared to SDS ones. Interfacial C(12)E(6) shows a transition from gaslike to liquidlike phases as the surface density increases. However, even at the largest coverage considered, interfacial C(12)E(6) aggregates show more disordered structures compared to SDS ones. Both surfactants exhibit a non-monotonic change in planar mobility as the available surface area per molecule varies. The results are interpreted on the basis of the molecular features of both surfactants, with particular emphasis on the properties of the surfactant heads, which are nonionic, long, and flexible for C(12)E(6), as opposed to ionic, compact, and rigid for SDS.