Transport of Heptane Molecules across Water–Vapor Interfaces Laden with Surfactants

Abstract

Molecular transport across liquid–vapor interfaces covered by surfactant monolayers plays a key role in applications such as fire suppression by foams. The molecular understanding of such transport, however, remains incomplete. This work uses molecular dynamics simulations to investigate the heptane transport across water–vapor interfaces populated with sodium dodecyl sulfate (SDS) surfactants. Heptane molecules’ potential of mean force (PMF) and local diffusivity profiles across SDS monolayers with different SDS densities are calculated to obtain heptane’s transport resistance. We show that a heptane molecule experiences a finite resistance as it crosses water–vapor interfaces covered by SDS. Such interfacial transport resistance is contributed significantly by heptane molecules’ high PMF in the SDS headgroup region and their slow diffusion there. This resistance increases linearly as the SDS density rises from zero but jumps as the density approaches saturation when its value is equivalent to that afforded by a 5 nm thick layer of bulk water. These results are understood by analyzing the micro-environment experienced by a heptane molecule crossing SDS monolayers and the local perturbation it brings to the monolayers. The implications of these findings for the design of surfactants to suppress heptane transport through water–vapor interfaces are discussed.