Condensed Matter Physics, 2016, vol. 19, No. 1, 13604
Phase transitions of fluids in heterogeneous pores
(Department of Physical Chemistry, University of Chemistry and Technology Prague, 166 28 Praha 6, Czech Republic;
Laboratory of Aerosols Chemistry and Physics, Institute of Chemical Process Fundamentals, Academy of Sciences, 16502 Prague 6, Czech Republic)
We study phase behaviour of a model fluid confined between two unlike parallel walls in the presence of long range (dispersion) forces.
Predictions obtained from macroscopic (geometric) and mesoscopic arguments are compared with numerical solutions of a non-local density
functional theory. Two capillary models are considered. For a capillary comprising two (differently) adsorbing walls we show that simple
geometric arguments lead to the generalized Kelvin equation locating very accurately capillary condensation, provided both walls are only
partially wet. If at least one of the walls is in complete wetting regime, the Kelvin equation should be modified by capturing the effect
of thick wetting films by including Derjaguin's correction. Within the second model, we consider a capillary formed of two competing walls,
so that one tends to be wet and the other dry. In this case, an interface localized-delocalized transition occurs at bulk two-phase coexistence and
a temperature T*(L) depending on the pore width L. A mean-field analysis shows that for walls exhibiting first-order wetting transition
at a temperature Tw, Ts > T*(L) > Tw, where the spinodal temperature Ts can be associated with the
prewetting critical temperature, which also determines a critical pore width below which the interface localized-delocalized transition does not occur.
If the walls exhibit critical wetting, the transition is shifted below Tw and for a model with the binding
potential W(l)=A(T)l-2+B(T)l-3+..., where l is the location of the liquid-gas interface, the transition can
be characterized by a dimensionless parameter κ=B/(AL), so that the fluid configuration with delocalized interface is stable in the
interval between κ=-2/3 and κ ~ -0.23.
capillary condensation, wetting, Kelvin equation, adsorption, density functional theory, fundamental measure theory
68.08.Bc, 05.70.Np, 05.70.Fh