Filling and emptying transitions in cylindrical channels: a density functional approach

The authors use density functional theory in a square gradient approximation to investigate capillary condensation and evaporation in cylindrical channels of finite lengths. The model allows them to systematically explore the effect of the pore's length, width, and surface fields on the location of the transition between "empty" (vapor-filled) and "full" (liquid-filled) states. In general, their results indicate that decreasing the length of the channel drastically reduces the range of pore widths where a transition between liquidlike and vaporlike configurations may be observed. For the wide pores, the transition occurs at very low pressures where the liquid is no longer stable, while for the narrow pores, the transition is hindered by the solid-fluid interactions that favor the vapor phase in lyophobic pores. For the limited range of sizes where the transition can occur, the authors' results confirm the existence of two competing minima that may explain the density oscillations observed in a computer simulation of nanochannels. Comparisons between these results with those generated using a phenomenological model based on the capillary approximation indicate that this simplified approach yields fairly good predictions for medium size pores. However, the capillary approach fails to properly describe the properties of the very small and very large pores.