Density fluctuations near the liquid-gas critical point of a confined fluid

Abstract

We report the results of an experimental study of the effect of a dilute silica network on liquid-gas critical phenomena in carbon dioxide (CO2). Using small-angle neutron scattering, we measured the correlation length of the density fluctuations in bulk (xi(bulk)) and confined CO2 (xi(conf)) as a function of temperature and average fluid density. We find that quenched disorder induced by an aerogel suppresses density fluctuations: xi(conf) loses the Ising model divergence characteristic of xi(bulk) and does not exceed the size of pores in the homogeneous region.

Diffusion of interacting particles in discrete geometries: Equilibrium and dynamical properties

“…This can be seen as the result of the confinement, which prevents the formation of stable macroscopic phases [41][42][43]. We discuss how such behavior is reproduced in our model.…”

Supercritical Fluids in Confined Geometries

“…9.4) as well as individual fluids near the liquid-gas critical point as it can probe the local structure and thermodynamic properties of fluids in small pores with sizes of the order on 10-1000 Å. SANS was applied in [18,19] to study the temperature variation of the correlation length (ξ) of the density fluctuations as well as susceptibility (χ) of CO 2 confined in a random porous matrix of a silica aerogel with $96 % porosity, a surface area of 400 m 2 /g, a density of 0.1 g/cm 3 , and pore size distribution peaked around the nominal pore diameters 2R P $ 60-70 Å. Aerogel was obtained from Oscellus Technologies, Livermore, CA and shaped into a cylinder (17 mm OD, 10 mm length), which fit tightly into the SANS high-pressure cell (Sect. 4.2, Fig.…”

“…(10.6), whereas dash line corresponds to the Porod law I~Q À4. Reprinted figure with permission from [18]. Copyright (2004) by the American Physical Society…”

Helium adsorption in silica aerogel near the liquid-vapor critical point

“…On the other hand, aerogels could be the ideal system in which to study liquid-vapor critical behavior in the dilute impurity limit. However, a recent direct measurement of density fluctuations in carbon dioxide confined in aerogel 15 showed little evidence for a diverging correlation length at the LVCP and thermal fluctuations may be less important than those introduced by the disorder in the aerogel structure 16 .…”