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

Abstract

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.

Atomistic-Scale Energetic Heterogeneity on a Membrane Surface

“…A 10 nm by 10 nm by 1 nm polyvinylidene fluoride (PVDF) membrane with a density of 1.75 g/cm 3 , which is the same as that reported in Velioglu et al [ 18 ], was used in this study, with the force field parameters adopted from Lachet et al [ 19 ]. Per that reported earlier [ 20 ], the PVDF was made hydrophilic by adding 98 hydroxyl and 98 carboxyl functional groups randomly onto the PVDF surface to mimic the hydrophilic PVDF used experimentally. Three probe molecules were evaluated, namely argon, carbon dioxide and water, which represent different sizes and natures.…”

Investigation of membrane fouling using molecular dynamics simulation

“…Some interaction energy results have been found to be poorly correlated to membrane fouling. Our earlier study found that none of the interaction energy term is sufficient to explain the relative flux trend of feeds containing anionic, cationic and nonionic surfactants [35]. Tiwari et al [118] observed the affinity of two proteins despite the repulsive protein-protein interaction energy, and found that protein-solvent and protein-ion interactions play critical roles.…”

“…Simulation models have to be validated against experiments to ensure reliability and accuracy. Composition of feed/membrane [92,93] Conductivity [36,94] Cross-linking degree [95,96] Contact angle [97,98] Density of grafting sites on the membrane surface [26,29] DOTM A [55] Density of membrane [30,40,99,100] Flux decline profile [30,33,37,101] Spectroscopy (e.g., XPS B , FTIR C ) [35,102,103] Isotherm of static adsorption (e.g., by QCM-D D ) [104][105][106] Zeta potential [107] Rejection of foulant [35,102] SEM E…”

“…A schematic of umbrella sampling is shown in Fig. 2-9A for a surfactant-membrane system [35]. The sequence of implementing the umbrella sampling procedure is detailed as follows.…”

“…(A) Schematic illustration of umbrella sampling simulation, with hydrophilic PVDF membrane as representative surface, and CTAB as adsorbate. Configurations of CTAB are taken by snapshots from data files of previous study [35]. Reaction coordinate 𝜉 is defined with respect to nitrogen atom in the polar headgroup of CTAB (atom in blue).…”