Mass transport across densely packed surfactant covered oil-water interfaces in microemulsions plays a key role in numerous applications in a wide variety of areas such as separations, reactions, drug delivery, and detoxification. In this talk we present results of molecular modeling and development of a theoretical model for transport of solute molecules across hexadecane-water interfaces covered by Brij surfactants. For simplicity of the modeling process, we have chosen to investigate transport across the flat interface.

In our earlier study we have shown that the presence of the surfactant monolayer at the interface provides little resistance to transport of small spherical solute. In the current presentation, we discuss the extension of our work to investigations of transport mechanism of solutes of larger sizes, various degrees of hydrophobicity, and non-spherical shapes. In addition, we will investigate the effects of changing the length of surfactant molecules on the transport properties.

The studies are performed using a coarse-grained molecular dynamics model, which represents groups of several atoms as united atoms (beads). This allows one to significantly speed up the calculations while retaining the key system dynamics. We assume that the transport of the solute can be described by a generalized Langevin equation for the solute and obtain the parameters for this equation (potential of mean free force and the autocorrelation function of the stochastic force) from molecular dynamics simulations with the solute center of mass constrained in the direction normal to the interface.

We observe non-trivial behavior of the stochastic force acting on the solute: the decay of this function strongly depends on the distance from the interface and the correlation times differ by two orders of magnitude within a narrow region of the interface. This phenomenon is related to the density fluctuations of the surfactant as well as water and oil molecules around the solute. We further discuss implications of this phenomenon on the transport properties.