Phase equilibria of fluid interfaces and confined fluids

Abstract

Phase transitions at fluid interfaces and in fluids confined in pores have been investigated by means of a density functional approach that treats attractive forces between fluid molecules in mean-field approximation and models repulsive forces by hard-spheres. Two types of approximation were employed for the hard-sphere free energy functional: (a)tile well-known local density approximation (LDA) that omits short-ranged correlations and (b) a non-local smoothed density approximation (SDA) that includes such correlations and therefore accounts for the oscillations of the density profile near walls. Three different kinds of phase transition were considered: (i) wetting transition. The transition from partial to complete wetting at a single adsorbing wall is shifted to lower temperatures and tends to become first-order when the more-realistic SDA is employed. Comparison of the results suggests that the LDA overestimates the contact angle 0 in a partial wetting situation. (ii) capillary evaporation of a fluid confined between two parallel hard walls. This transition, from dense 'liquid' to dilute 'gas', occurs in a supersaturated fluid (p > Psat)" The lines of capillary coexistence calculated in the LDA and SDA are rather close, suggesting that non-local effects are not especially important in this case. (iii) capillary condensation of fluids confined between two adsorbing walls or in a single cylindrical pore. For a partial wetting situation the condensation pressures p(<Psat) obtained from the SDA are in remarkably good agreement with the macroscopic Laplace (or Kelvin) prediction for wall separations H or pore radii Rc ~ 5a; a is a molecular diameter. While, because of different packing, the density profiles of the fluid differ considerably between slits and cylinders this has little effect on the coexistence line until H or R, ~ or. In contrast to the LDA the SDA describes two-dimensional-like liquid-gas coexistence for very narrow pores (H < a) and temperatures below the two-dimensional critical temperature and this has ramifications for the existence of capillary critical points

Zeolite microporosity studied by molecular simulation

“…Molecular simulations (Monte Carlo and Molecular Dynamics) have been used to obtain a better understanding of sorption phenomena in porous materials [20,21,22,23,24,25,26,27,28,29,30,31,17,32,33]. These microscopic methods describe the guest-host system at a molecular level, in contrast to classical methods that are based on macroscopic thermodynamic assumptions.…”

A Monte Carlo test of the Fisher–Nakanishi–Scaling theory for the capillary condensation critical point

“…However, a prerequisite for the clarification of pattern formation 2,3 and dynamics 4,5 is a good understanding of the interplay between bulk and surface effects on thermodynamics and phase behavior in this finite-size geometry [6][7][8] : although theoretical aspects of phase transitions and critical phenomena in confined geometry have been considered since a long time [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] , this is still a topic of active current research [29][30][31][32][33] even for one of the most well-known phenomena, namely "capillary condensation" 34,35 . By "capillary condensation" one means the finding, already discovered in the 19th century 34 , that in a capillary the condensation of a gas occurs already at a lower pressure p than the …”

Probabilistic Approach to the Length-Scale Dependence of the Effect of Water Hydrogen Bonding on Hydrophobic Hydration

“…Among various models for F h [ρ(r)], the weighted density approximation (WDA) 30,36,37 with a weight function independent of weighted density represents an optimal combination of accuracy and simplicity. It is non-local with respect to ρ(r); it takes into account short-ranged correlations and captures the fluid density oscillations near a hard wall.…”