Ordering processes in porous media

Phase behavior and dynamics of fluids in mesoporous glasses

Equilibrium and dynamical relaxation behavior of fluids confined in disordered mesoporous glasses such as Vycor are studied based on a lattice model using mean field theory and Monte Carlo simulations. Preferential attractive interactions between the solid surfaces and the fluid suppresses macroscopic phase separation, while making the relaxation rate increasingly slow. The free energy landscape characterized by the presence of the many metastable minima separated by finite barriers dominates both the static and dynamic behavior of fluids at low temperature. Our results provide additional insight into the nature of hysteresis in adsorption measurements of gases in porous glasses.

Phase transitions and quantum effects in pore condensates: a path integral Monte Carlo study

Lennard-Jones condensates in cylindrical pores are studied by path integral Monte Carlo simulations with particular emphasis on phase transitions and quantum effects. The pore diameter effect and the influence of the interaction strength between the cylinder wall and the adsorbate particles on the structures and the location of the phase boundaries is studied and the quantum effect on the phase diagram is quantified by path integral Monte Carlo simulations. In case of strong wall-particle interactions good qualitative agreement with recent experimental results for the freezing of Ar-pore condensates is found. Meniscus structures in the solid phase are obtained as well as unexpected condensate structures for the system with the lighter Ne-particles due to quantum delocalization effects. The quantum effect on the freezing temperature can be as large as 10% in these systems.

Phase behavior of confined symmetric binary mixtures

We employ mean-field lattice density functional theory to investigate the phase behavior of a binary (A-B) mixture confined to nanoscopic slit pores with chemically homogeneous walls. We consider only nearest-neighbor interactions in symmetric mixtures, where epsilon(AA)=epsilon(BB) not equal epsilon(AB) and epsilon is a measure of attraction between molecules of like (subscripts AA and BB) and unlike species (subscript AB), respectively. In addition, molecules are exposed to short-range attraction by the substrates separated by z lattice planes where epsilon(W) is the relevant coupling parameter. Moreover, the chemical potentials of both components are the same, that is, mu(A)=mu(B)=mu. In thermodynamic equilibrium (for fixed temperature T and chemical potential mu) the grand-potential density omega[rho,m] (rho identical with [rho(1),...,rho(z)], m identical with [m(1),...,m(z)]) assumes a global minimum which we find by minimizing omega numerically with respect to the order parameters rho(l) identical with rho(A)(l)+rho(B)(l) (total local density) and m(l) identical with (rho(A)(l)-rho(B)(l))/rho(l) (local "miscibility") at lattice plane l parallel to the pore walls. By varying epsilon(AB) three generic types of bulk phase diagrams are observed. On account of confinement (i.e., by varying epsilon(W) as well as z) one may switch between these different types of phase diagrams. This may have profound practical repercussions for experimental nanophase separation since depending on pore width and chemical nature of its walls a bulk gas mixture may undergo capillary condensation and form either a stable mixed or demixed liquid phase.

Simple views on critical binary liquid mixtures in porous glass

A simple scenario, different from previous attempts, is proposed to resolve the problem of the slow phase separation dynamics of binary liquid mixtures confined in porous Vycor glass. We demonstrate that simply mutual diffusion, renormalized by critical composition fluctuations and geometrical hindrance of the porous glass, accounts for the slow phase separation kinetics. Capillary invasion studies of porous Vycor glass by the critical isobutyric acid-water mixture, close to the consolute solution temperature, corroborate our analysis.