Molecular Dynamics Study of the Bulk and Interface Properties of Frother and Oil with Saltwater and Air

Abstract

For water treatment purposes, the separation processes involving surfactants and crude oil at seawater-air interfaces are of importance for the chemical and energy industries. Little progress has been made in understanding the nanoscale phenomena of surfactants on oily saltwater-air interfaces. This work focuses on using molecular dynamics with a united-atom force field to simulate the interface of linear alkane oil, saltwater, and air with three surfactant frothers: methyl isobutyl carbinol (MIBC), terpineol, and ethyl glycol butyl ether. For each frother, although the calculated diffusivities and viscosities are lower than the expected experimental values, our results show that diffusivity trends between each frother agree with experiments but the method cannot be applied for viscosity. Binary combinations of liquid (frother or saltwater)-air and liquid-liquid interfaces are equilibrated to study the density profiles and interfacial tensions. The calculated surface tensions of the frother-air interfaces are like that of oil-air, but lower than that of saltwater-air. Only the MIBC-air and terpineol-air interfaces agreed with our experimental measurements. For the frother-saltwater interfaces, the calculated results showed that terpineol has interfacial tensions higher than those of MIBC-saltwater. The simulated results indicate that the frother-oil systems underwent mixing such that the density profiles depicted large interfacial thicknesses.

Effects of Frothers and Oil at Saltwater–Air Interfaces for Oil Separation: Molecular Dynamics Simulations and Experimental Measurements

“…The complete description of our computational methodology can be found in our previous paper . Here, we limit ourselves to provide only the main aspects relevant for the current study.…”

“…We used a united-atom force field, TraPPE (transferable potentials for phase equilibria), developed by Siepmann et al − They parametrized the force field by fitting bonds, angles, dihedrals, and nonbonded potentials with respect to critical temperatures, saturated liquid densities, and other vapor–liquid data from experiments. The TraPPE force field is applicable for studying oil molecules at interfaces , and has been implemented in our previous work involving binary interfaces with MIBC, terpineol, and EGBE. Figure depicts the frothers’ molecular structures coarse-grained as united atoms with charge distributions describing the amphiphilic nature of surfactants.…”

“…Using solvation free energy, solvation entropy, and radial distribution functions, Horinek et al optimized Lennard-Jones (LJ) parameters for Na + and Cl – ions in SPC/E water . A complete compilation of the parameters can found in our previous work’s methodology and Supporting Information . In addition, LJ parameters of dissimilar atoms were determined by Lorentz–Berthelot combining rules as practiced during TraPPE force field development. − …”

“…Analysis of each interface was primarily focused on surface tensions, density profiles, and pair correlation functions (PCF). Surface tension calculations followed the same method as our previous work and other interface modeling studies. , …”

“…In our previous work, we utilized MD to model binary interfaces of MIBC, terpineol, and ethyl glycol butyl ether (EGBE) with saltwater, oil, and air at different temperatures. The obtained results provided insights into interfacial tension and density profile trends for each frother-solvent pair.…”