Molecular Simulation Study of Vapor−Liquid Critical Properties of a Simple Fluid in Attractive Slit Pores: Crossover from 3D to 2D

Abstract

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.

Effect of surface-screening parameter of the Yukawa potential model on vapour–liquid phase coexistence and critical-point properties of confined Yukawa fluid

“…The parameter b is also known as the order parameter critical exponent. In the current investigation, we have not used Binder's fourthorder cumulant approach [50] or much better mixed-field finite-size scaling approach to evaluate the pore critical temperature, but a recent study using finite-size scaling approach [51] for the estimation of critical point of LJ fluid in hard slit -pore confinement shows similar behaviour as reported in some other recent work [27,48,49] using the current technique. The critical temperature,T c , estimated from Equation (3) is used to calculate the critical density, r c r c , from the least square fit of the following equation [52]:…”

“…However, the other effective method that uses finite-size scaling approach has shown similar trend of critical-point properties, T c and r c , under slit-pore confinement [51] as depicted by the use of simple rectilinear law approach and current simulation technique in some other work. [27,48] Therefore, the critical-point data obtained using current simulation technique and with simple rectilinear law are reasonably good for qualitative comparisons.…”

“…The simulation cell size in unconfined directions, L x and L y taken as 8 £ 8 away from critical point and 18 £ 18 near the critical point to minimise any significant error because of increased correlation length near the critical-point range. Moreover, similar approach is followed in earlier investigations [27] to minimise any significant finite-size effects on vapour-liquid phase equilibria data near the critical point.…”

“…The above empirical formula has been used to obtain the critical pressure for confined square-well fluids [27] and alkanes. [56] The saturated density profiles (i.e.…”

“…Moreover, molecular simulation methods can provide a microscopic picture of a fluid in the interaction field of the confined space and enable one to examine the underlying physics. [27] Detailed knowledge of the phase coexistence properties of confined fluids is of crucial importance for the interpretation of experimental data on fluids in nanopores. This further can be helpful to optimise the various industrial processes.…”