Molecular Dynamics Simulations of Interfacial Tensions and Contact Angles of the Nitrogen plus Oil plus Brine plus Rock System

XY Yao and AKN Nair and MFAC Ruslan and SY Sun and BC Yan, INDUSTRIAL & ENGINEERING CHEMISTRY RESEARCH, 64, 3831-3840 (2025).

DOI: 10.1021/acs.iecr.4c04630

Molecular dynamics (MD) simulations of the N2+hexane+H2O system (two- phase) were conducted at 343-443 K and 60-150 MPa. The MD results agreed reasonably well with the density gradient theory (DGT) results based on the CPA EoS. The interfacial tensions (IFTs) were found to increase with pressure and decrease with temperature. An important finding is that the IFTs only slightly decreased with increasing N2 mole fraction in the N2/hexane-rich phase (x N2 ). In general, N2 shows a positive surface excess, and hexane shows a negative surface excess. The increase in the IFT with pressure indicates that the IFT behavior is dominated by the negative surface excess of hexane. MD simulations of the corresponding N2+hexane+H2O+silica (hydrophilic) system showed that the water contact angles (CAs) are not greatly affected by pressure or temperature. Importantly, the water CAs slightly decreased with increasing x N2 , and the adhesion tensions increased with increasing x N2 . MD simulations of the N2+hexane+brine system were also conducted (salt concentration (c s) up to 5.4 mol/kg NaCl). The MD results agreed reasonably well with the DGT results based on the CPA EoS with the Debye-Huckel contribution. Here, Na+ and Cl- were excluded from the interfacial regions. The solubility of N2 in the H2O-rich phase decreased with increasing c s, because of the salting-out effect. The IFTs increased linearly with increasing c s. MD simulations of the corresponding N2+hexane+brine+silica (hydrophilic) system showed that the water CAs increase with increasing c s. Our previous studies showed that the CO2+hexane and CO2+hexane+silica (hydrophilic) systems in the presence of water or brine gave generally similar results. However, for example, the adhesion tensions of the CO2+hexane+H2O+silica (hydrophilic) system decreased with increasing x CO2 .

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