Fig. 4 Configurations and number density profiles of water (in blue) and hydrocarbon (in black) in hydroxyl-hydroxyl (H-H) and potassium-hydroxyl (P-H) pores with a width of 5 nm. Illite is not shown for clarity. The results show that P-H surfaces favor the formation of a water bridge, while adsorption is dominant in H-H pore systems.
The distribution of water in P-H pores is in marked contrast to the H-H nanopores. Fig. 4c shows the formation of a water bridge (corresponding to the peak in the number density of water shown in a blue line) at a water concentration of 18.87%. Increasing the water concentration to 58.82% as shown in Fig.4d leads to the formation of two water bridges indicated by the presence of two peaks in the water density profile. This phenomenon is commonly referred to as ‘capillary condensation’84–86. However, the water bridge in the P-H nanopore persists even after increasing the pore size to 10 nm and 15 nm as shown in Fig.S-3 in Supporting Information. The water bridges observed in this work are not solely due to ‘capillary condensation’ which usually occurs when two water films are adequately close to each other87. Our work in this section tries to shed light on the existence of water bridges in P-H nanopores.
It is important to note that under certain conditions, a water bridge can form in an H-H clay-hosted pore, typically at higher water concentrations and for smaller pore widths. Table 1 shows the conditions under which a water bridge occurs.
Table 1 . The matrix below shows the conditions investigated in this study under which a water bridge will form.