Molecular simulation of the water-triolein-oleic acid mixture: Local structure and thermodynamic properties

An artificial oil-in-water emulsion analogous to those found in bioresources or food matrices is simulated and studied experimentally. It is composed of one of the major natural free fatty acids (the oleic acid, OA) and the corresponding triacylglyceride (trioleic glyceride, or triolein, GTO). Because of the large time and length scales involved, the molecular simulations are done with the Martini force field, a coarse-grained model. This allowed us to study the water-OA-GTO system at different compositions with more than 20 000 molecules and up to 2 μs. Interfacial tension was measured using the pendant drop method and compared with molecular simulation results. We observe very good agreement at high OA concentrations and deviations up to 15% at low OA concentrations. The water solubility in the lipid phase is in fair agreement with experiments, between 0.03 and 0.32 mol/l, rising with the OA content. The area occupied by OA and GTO at the interface between water and the pure product fitted with experimental data (A_OA = 36.6 Ų and A_GTO = 152.1 Ų). The consistency between simulation and experimental results allowed a structural analysis of the interface. A bilayer structure of the lipids at the water/oil interface is proposed, containing preferentially oleic acid but also triolein. Through all these results, the usefulness of coarse-grained simulation for the description of water-oil interfacial organization is demonstrated. This method will be used later to bring local information on the organization of target compounds, necessary in biomass fractionation processes or food additive formulations, for example.