Computing contact angles for oil-water-rock systems via thermodynamic integration

Wettability of rock surfaces with respect to oil and water, which is characterized by the contact angle, is an important factor that determines the efficacy of enhanced oil recovery operations. Experimental determination of contact angles for oil-water-rock systems is expensive and time-consuming due to the extremely long times needed for the establishment of adsorption equilibrium at the liquid-solid interface. Hence, molecular simulations form an attractive tool for computing contact angles. In this work, we use the cleaving wall technique that was developed previously in our group [R. K. R. Addula and S. N. Punnathanam, J. Chem. Phys. 153, 154504 (2020)] to compute solid-liquid interfacial free energy, which is then combined with Young's equation to compute the oil-water contact angle on silica surfaces. The silica surface is modeled with the INTERFACE force field that has been developed to accurately reproduce experimental data. We have considered three different surface chemistries of silica, namely, Q₂, Q₃, and Q₄, in this study. Our calculations reveal that while the Q₂ and Q₃ surfaces are completely wetted by water, the Q₄ surface is partially non-wetted by water. All the simulations needed for this calculation can be performed using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) molecular package. This should facilitate wider adoption of the Young's equation route to compute contact angles for systems comprised of complex molecules.