Adsorption of a fluid in an aerogel: Integral equation approach

Abstract

We present a theoretical study of the phase diagram and the structure of a fluid adsorbed in high-porosity aerogels by means of an integral-equation approach combined with the replica formalism. To simulate a realistic gel environment, we use an aerogel structure factor obtained from an off-lattice diffusion-limited cluster-cluster aggregation process. The predictions of the theory are in qualitative agreement with the experimental results, showing a substantial narrowing of the gas-liquid coexistence curve (compared to that of the bulk fluid), associated with weak changes in the critical density and temperature. The influence of the aerogel structure (nontrivial short-range correlations due to connectedness, long-range fractal behavior of the silica strands) is shown to be important at low fluid densities.

Effects of porous confinement on the structural properties of the Gaussian core model

“…The observations we made in discussing Figure 3 and 4 agree with the consensus on the strong influence of the matrix on the properties of the adsorbed fluid [3,40].…”

“…In contrast, in the model aerogel of Ref. [40] fluid-matrix correlations were generally enhanced at low k.…”

“…On the other hand, Krakoviack et al [40] observed a similar -yet much more pronounced -correlation peak in S f f (k) at low wave-vectors in a model of fluid adsorbed in highly porous aerogel.…”

Computer simulation of the phase diagram for a fluid confined in a fractal and disordered porous material

“…A model of "quenchedannealed" binary mixture was first studied by Madden and Glandt 15 and then widely used in dealing with the * Author to whom correspondence should be addressed; e-mail: rovere@fis.uniroma3.it problem of fluids phase separation in porous materials. Following this model, several integral equation theories and computer simulation studies have been successively proposed, in which the porous material is described by a random matrix of spheres 16,17,18,19,20 .…”

“…It has been shown that the structural properties and fractal dimension of DLCA gels well agree with experimental data on silica aerogels. A theoretical study of thermodynamical and structural properties of a Lennard-Jones fluid adsorbed in a highly porous DLCA aerogel by means of an integral equation approach has been recently proposed 17 . The phase diagram as well as the structural correlations of the confined fluid are found to be influenced by the specific properties of the gel, such as its connectivity and fractal behavior, particularly at low fluid densities.…”