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.
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