Development of electrochemical DPD molecular simulations for oil/water partitioning of organic acids at varying pH

Abstract

ABSTRACTElectrostatic DPD simulations were developed to model the partitioning of organic acid from the lipid phase to the aqueous phase with increasing pH. The ionization-kinetic mechanisms were accounted for by implementing proton transfer between acid and base via double well Morse potentials, as reported by Lee et al. The DPD model was extended with hydration of the ionized acid A− at the oil/water interface. Thermodynamics predict a rapid increase of acid concentration in the water above a certain value in pH, corresponding to high ionization fraction of acid in water. This transition was regenerated by a careful tuning of the DPD interaction parameters. However, relatively high acid and ion concentrations are needed to provide a sufficient number of molecules in a limited simulation volume. Consequently, the simulations operate at very high and very low pH. High pH was obtained by adding a base providing OH− that accept protons from the acid at the interface. Likewise, low pH was obtained by adding H+, either forming hydronium or recombining with the ionized acid A− at the interface.In conclusion, the current simulation model contains the essential kinetic ionization mechanisms in partitioning. It seems reasonable to use the simulation model to study molecular transport over the interface in the presence of surfactant layers (surfactant indigenous to the oil, or added to the water).