Phase diagram of a symmetric binary fluid in a porous matrix

Abstract

The phase behavior of a binary symmetric fluid in thermal equilibrium with a porous matrix has been studied with the optimized random phase approximation and grand canonical Monte Carlo simulations. Depending on the matrix properties and the matrix-fluid and fluid-fluid interactions we find three types of phase diagram characterized by a tricritical point, a tricritical point with a triple point, or a critical end point. Small changes in the properties of the matrix or in the interactions are demonstrated to lead to drastic modifications of the phase diagram of the fluid, in qualitative agreement with observations in experimental studies. We show, in particular, that the change between the different types of phase diagram is triggered not only by the fluid-fluid interactions (internal parameters) but also by the properties of the matrix and of the matrix-fluid potentials (external parameters).

Phase Transitions and Critical Behaviour of Binary Liquid Mixtures

“…!-line; from Ref. [22] mentioned above) and exists only in a small range of y-values, extending roughly from 2 to À0.5. The phase diagram is again of type III for the more strongly repulsive matrix±¯uid interaction y À1.…”

“…Apart from the stable phase transitions we have also depicted in these ®gures the metastable phase equilibria; they are of relevance for the dynamic properties of the system (for a more detailed discussion, see Refs. [2,22]). A comparison with computer simulations shows that the MSA gives reasonably good results for the bulk case.…”

“…Weis [22] (for details of the simulation, see references in this paper). The phase diagrams presented are projections of the three dimensional (T, &, x) phase diagram onto the (x 1/2) plane.…”

“…The computer simulations were carried out for four different matrix densities & 0 0, 0.05, 0.15, and 0.3 at 0.7, y 1, z' 2.5, and r c 2.5 ' (for technical details, see Ref. [22] and references quoted therein). The results are shown in Fig.…”

Explicit spatial description of fluid inclusions in porous matrices in terms of an inhomogeneous integral equation

“…From the theoretical perspective, the advent of the Replica Ornstein Zernike (ROZ) approach in the early nineties 1-3 provided a powerful alternative to direct molecular simulation for the description of fluid inclusions in disordered porous systems. Since then, the ROZ approximation has been much exploited to describe templated 4-7 and sponge-like materials 8,9 , and a large variety of inclusions, such as simple binary mixtures 10 illustrating their phase behavior 11 , colloid/polymer mixtures 12 , electrolytes [13][14][15][16] , and associating fluids 17,18 . This approach yields average thermodynamic properties, fluid-fluid, and fluid-matrix correlations, but if one is interested in the explicit spatial distribution of the fluid/adsorbate for a given configuration of the matrix an alternative approach is needed, aside from resorting to molecular simulation.…”