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

Area of Science:

• Physical Chemistry
• Materials Science
• Nanotechnology

Background:

• Capillary condensation and evaporation are fundamental phenomena in porous materials.
• Understanding these processes at the nanoscale is critical for applications in adsorption, separation, and catalysis.
• Previous models often simplify pore geometry, limiting their applicability to finite-sized systems.

Purpose of the Study:

• To investigate capillary condensation and evaporation in finite-length cylindrical channels.
• To systematically explore the influence of channel length, width, and surface fields on phase transitions.
• To compare theoretical findings with a simplified phenomenological capillary model.

Main Methods:

• Utilized density functional theory (DFT) with a square gradient approximation.
• Simulated capillary condensation and evaporation in cylindrical channels of varying finite lengths and widths.
• Analyzed the transition between vapor-filled (empty) and liquid-filled (full) states.

Main Results:

• Decreasing channel length drastically reduces the range of pore widths exhibiting liquid-vapor transitions.
• In wide pores, transitions occur at pressures where the liquid phase is unstable; in narrow, lyophobic pores, solid-fluid interactions hinder transitions.
• Confirmed the existence of two competing minima, potentially explaining density oscillations observed in nanochannel simulations.

Conclusions:

• Finite channel length significantly alters capillary condensation behavior compared to infinite models.
• The DFT model provides insights into the limitations of simplified capillary approximations, especially for very small and very large pores.
• Results offer a more accurate understanding of fluid behavior in realistic nanoscale confined geometries.