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

ABSTRACT

Electrostatic 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).

GRAPHICAL ABSTRACT

KEYWORDS:

• Electrostatic DPD
• oil/water partitioning
• interfacial kinetics

Acknowledgments

This work was carried out as a part of SUBPRO, a Research-based Innovation Centre within Subsea Production and Processing. The authors gratefully acknowledge the support from SUBPRO, which is financed by the Research Council of Norway, major industry partners and NTNU. The electrostatic part of the DPD model were developed earlier with financial support from IFE via SIS base funding. Dr. T.L. Palmer at UiS/IFE/IOR center of Norway, is acknowledged for suggesting calibration of the DPD simulations to ion concentration ratios rather than absolute pH.

Notes

¹The pH is given by the amount of [OH⁻] via the self protolysis equilibrium of water which dictates [H⁺][OH⁻] = 10⁻¹⁴.

²For example, the recalibrated value is , where is the number of hydronium ions counted in the simulation volume V, and N_A is Avogadros number, and pH* is the pH-value measured directly in the simulations.

Additional information

Funding

This work was supported by the Reserch Council of Norway [Grant Number 237893].