Fig. 12 Molecular distribution and velocity profiles of water in PH pores of different widths at a water concentration of 71.43%. The acceleration is 0.002 nm/ps2. Hydrocarbon is not shown for clarity. The red dots represent adsorbed water and the yellow dots are those in the water bridge. In a 5 nm PH nanopore, acceleration causes the adsorbed layer to exchange both mass and velocity with water at the pore center. Increasing the pore size to 10 nm or 15 nm, as revealed by the flatter velocity profile, no mass or velocity exchange occurs between the adsorbed layer and water in the bridge or pore center.
Effect of Water Concentration
Fig.13 shows the water (Fig.13a) and hydrocarbon (Fig.13b) velocity profiles at 0.002 nm/ps2 in a 10 nm PH nanopore for different values of water concentration. The velocity of water decreases with increasing water concentration (Fig. 13a) because of the increased thickness of the water bridge. The thickness of the water bridge at water concentrations of 18.87%, 58.82%, 71.43% and 80.00% is 0.94 nm, 1.24 nm, 1.60 nm and 1.96 nm respectively. This is shown in Fig. S9 in Supporting Information.
Fig. 13b shows an increased hydrocarbon velocity for an initial increase in the water concentration which has been attributed to the creation of smoother surfaces for hydrocarbon flow69. However, when the water concentration is increased, the width of the water bridge progressively increases (shown in Fig S10), thereby hampering hydrocarbon flow.
Fig. 13 also indicates flat velocity profiles exist for both the oil and water phases. Because of the hydrophilic surface, hydrocarbon molecules are responding to the acceleration and are not strongly influenced by the pore surfaces, leading to a flatter velocity profile.