Description of an n-Alkane/Water Phase Boundary in Presence of Polyethylene Glycol Ethers of Fatty Alcohols with the Aid of Coarse-Grained Multilayer Quasi-Chemical Model and Molecular Dynamics

Abstract

Liquid interfaces are modeled for n-alkane + water systems with an without added non-ionic surfactants: polyethylene glycol ethers of fatty alcohols (C_nE_m). Coarse-grained molecular dynamic (MD) simulation with the MARTINI force field is combined with a coarse-grained version of a multilayer quasi-chemical model (MQM) of a non-uniform fluid mixture. The effect the choice of the monomer’s unit size has on predicting the interfacial tension and mutual solubility of n-alkanes and water is demonstrated using the MQM. The interfacial tension’s dependence on the length of the n-alkane chain and the structure of the added surfactant molecule are predicted satisfactorily. A predicted drop in the interfacial tension upon adsorption of the surfactant is consistent with the MD data. Liquid-liquid phase diagrams are calculated, and the non-uniform of the surfactant between the hydrocarbon and aqueous phases is described, depending on the ratio of the hydrophobic and hydrophilic parts of the C_nE_m molecule. The coarse-grained MQM is used to obtain normal and tangental pressure profiles and data on the local structure for flat and spherical phase boundaries. A conclusion is reached on the limited applicability of the coarse-grained approach within the MQM.