Study of interfacial properties of water + methane + oil three-phase systems by a simple molecular simulation protocol

A straightforward molecular simulation protocol has been proposed to study the interfacial properties of the three-phase fluid systems. With this protocol, the interfctcial tension (IFT) can be calculated based on the pressure tensor of the entire simulation box of the two-phase systems split from the three-phase system without calculating the debatable pressure tensor profile. Water + methane + oil (decane, hexadecane, and toluene) three-phase systems was studied at different temperatures (323–423 K) and pressures (up to around 20 MPa). Reasonable agreement was found among the results obtained from molecular simulation, density gradient theory with the cubic-plus-association equation of state, and available experimental data in the literature. The IFT of the aqueous phase + vapor phase in the three-phase systems is smaller than the IFT in water + methane two-phase systems. Importantly, the reduction of IFT of aqueous phase + vapor phase in the three-phase systems containing decane or hexadecane is moderate while that in the three-phase systems containing toluene is significant, which can be explained by the stronger enrichment of toluene in the interfacial region in contrast to that of decane or hexadecane. Meanwhile, methane accumulates in the interfacial region of the aqueous phase + decane/hexadecane-rich phase in the three-phase systems, which causes the reduction of IFT with pressure while the opposite pressure effect was reported in the water + decane/hexadecane two-phase systems. The interfacial properties of the oil-rich phase + vapor phase in the three-phase systems are hardly affected by water due to the small amount of dissolved water. Furthermore, the calculated spreading coefficient of different types of oil in contact with water + methane under three-phase conditions follows this order: toluene > decane > hexadecane.