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.