Investigation of Surfactant-Membrane Interaction Using Molecular Dynamics Simulation with Umbrella Sampling

Membranes are effective for removing oil emulsions in oily wastewater treatments, which is important for environmental remediation as well as recovery of oil for economic benefits. Surfactants play a critical role in stabilizing the oil emulsions, but their effects on the inevitable membrane fouling phenomena remain poorly understood. The focus here is the interesting flux enhancement relative to water conferred by the cationic cetyltrimethylammonium bromide (CTAB) surfactant. Molecular dynamics simulations were performed to understand the interactions between three different surfactants and a hydrophilic polyvinylidene fluoride (PVDF) surface. Unbiased MD simulations quantify the surfactant-water, surfactant-membrane, and water-membrane interactions, but none appears well-correlated to the relative flux trends due to the interplay of all three interactions. To account for all interactions concurrently, umbrella sampling was performed to obtain the potential of mean force (PMF) curves. The adsorption of all three surfactants is driven by enthalpy (rather than entropy), and CTAB was found to have the most attractive binding free energy, smallest equilibrium distance, and looser water network near the PVDF surface, which are tied to the experimental observation of flux enhancement and highest retention. The Angstrom-scale results here reveal the need to consider all the interactions simultaneously rather than separately to fully account for mechanisms underlying membrane fouling by surfactants.