Molecular simulation study of vapor-liquid critical properties of a simple fluid in attractive slit pores: crossover from 3D to 2D.

We present the effect of surface attraction on the vapor-liquid equilibria of square well (SW) fluids in slit pores of varying slit width from quasi 3D to 2D regime using molecular simulation methodologies. Four to five distinct linear regimes are found for shift in the critical temperature with inverse slit width, which is more prominent at higher surface fluid interaction strength. On the other hand, shift in the critical density and the critical pressure does not show any specific trend. Nevertheless, critical density and pressure show the sign of approaching toward the 3D bulk value with increase in the slit pore width, H, beyond 40 molecular diameters. The crossover from 3D to 2D behavior for attractive pores is observed around 14-16 molecular diameters, which is significantly different from the crossover behavior in the hydrophobic slit pore. Critical properties for H <or= 2 molecular diameters are indifferent to the surface characteristics. Corresponding state plot displays fluctuating positive deviation of spreading pressure for large pores and negative deviation for small pores from the bulk saturation value. Such behavior is more accentuated at stronger surface-fluid interaction strength. We also present vapor-liquid surface tensions of the SW fluid for different attractive planar slit-pores of variable slit-widths. Vapor-liquid surface tension or interfacial width values are insensitive to the surface-fluid interaction strength for slit width, H <or= 2 molecular diameters. At a given slit width and temperature, vapor-liquid interfacial width is found to decrease with increasing wall-fluid interaction for H > 2. However, interfacial properties approaches to the bulk value with increasing slit width. On the other hand, surface tension at a reduced temperature displays a nonmonotonic behavior with the change in H, which is in good agreement with the nature of the corresponding scaled interfacial width.