Fluids in Model Pores or Cavities: The Influence of Confinement on Structure and Phase Behaviour

Abstract

When fluids are confined in narrow pores or capillaries, or in cavities their properties are altered drastically from those in bulk. The effects of wall-fluid forces and of finite-size modify the static structure, the dynamical properties and the phase behaviour of the fluid. These lecture notes summarise some recent investigations of confined fluids. The first lecture describes the use of density functional theory (DFr) and Monte Carlo simulation for determining the equilibrium structure (density profiles) of fluids confined in a small spherical cavity, where packing constraints lead to extreme inhomogeneities and where the results depend on the choice of the ensemble, i.e. whether the system is open (grand canonical) or closed (canonical). The second lecture focuses on simulation studies of the condensation and freezing transitions for simple (Lennard-Jones like) fluids confined in an ideal slit pore and describes the extent to which the location of the phase boundaries depends on the choice of wall-fluid potentials and on the size of the pore. The possible relevance of such studies of idealised model pores for understanding the phase behaviour of fluids confined in real mesoporous solids such as Vycor glass is discussed briefly.

Mineral replacement reactions: from macroscopic observations to microscopic mechanisms

“…Fluids in nely porous media are known to be able to maintain a much higher supersaturation than free uids (Melia and Mof t, 1964;Henisch, 1989;Putnis et al, 1995;Putnis and Mauthe, 2001). The reasons are not well understood, although there have been a number of attempts to explain the thermodynamics of crystallization in porous materials (Ozawa, 1997;Scherer, 1999;Benavente et al, 1999;Evans, 1999), mainly motivated by the fact that increasing the degree of supersaturation increases the force of crystallization (Correns, 1949;Weyl, 1959;Winkler and Singer, 1972;Dewers and Ortoleva, 1990;Rodriguez-Navarro and Doehne, 1999). This force results in salt crystallization damage in porous stonework and masonry, as well as frost heaving.…”

The effect of pore size on cementation in porous rocks

“…Theoretical models of nucleation in small droplets have been proposed that fit the experimental data (Kubota et al . 1986) and it is recognized that such confinement can have a significant effect on the properties and phase behaviour of fluids (Evans 1999). Another manifestation of the influence of pore size on crystallization conditions has been in the use of gels as transport media in crystallization experiments (Henisch 1989).…”

Modifications of the Ornstein–Zernike relation and the LMBW equations in the canonical ensemble via Hilbert-space methods

“…It was already observed by Mermin [8], that this uniqueness relation cannot hold in the CE (in a finite volume), as the density remains the same under a constant shift of the external potential. The direct consequence of this fact is that V H(r, r ′ )dr ′ = 0 (1) in the CE in a finite volume, V . Put differently, if we regard H, C as the kernels of the corresponding integral operators, H, C on some function spaces, the constant functions are zero eigenfunctions of H. This implies that it cannot be straightforwardly inverted and hence the direct correlation function, C, is (or rather seems to be) ill-defined (see appendix).…”

“…Recently there has been renewed interest in the the application of the density functional method to the canonical ensemble (CE). One of the more practical reasons is the investigation of fluids in cavities or pores that are so small that the results depend on the choice of the ensemble (see, e. g., [1] or, for a short review, [2]). One of the methods that have turned out to be quite useful for the description of finite systems consists of the so-called density functional theory.…”

“…In that case the modifications in the LMBW equations can be expressed as pure, seemingly non-local integrations over the container boundaries.Recently there has been renewed interest in the the application of the density functional method to the canonical ensemble (CE). One of the more practical reasons is the investigation of fluids in cavities or pores that are so small that the results depend on the choice of the ensemble (see, e. g., [1] or, for a short review, [2]). One of the methods that have turned out to be quite useful for the description of finite systems consists of the so-called density functional theory.However, in its customary form this method requires the use of a grand canonical ensemble (GCE), i. e., an open system.…”