On Condensation and Evaporation Mechanisms in Disordered Porous Materials

Area of Science:

• Materials Science
• Physical Chemistry
• Statistical Mechanics

Background:

• Sorption isotherms are crucial for porous material characterization but exhibit hysteresis.
• Understanding condensation and evaporation mechanisms is vital for accurate pore size distribution.
• Disordered porous materials present complex behaviors due to network effects.

Purpose of the Study:

• To investigate helium sorption hysteresis in Vycor glass.
• To compare experimental data with a model incorporating single-pore and collective effects.
• To determine the validity of the Barrett-Joyner-Halenda method for disordered porous materials.

Main Methods:

• High-precision measurements of helium condensation and evaporation.
• Volumetric measurements for average fluid density.
• Light scattering for spatial density fluctuations.
• Lattice simulations for collective effects.

Main Results:

• The model accurately reproduces temperature-dependent isotherm shapes and optical signals.
• Evaporation mechanisms transition from surface to bulk percolation with increasing temperature.
• Network effects significantly influence condensation, causing pores to fill at equilibrium pressure.
• Experimental results support the applicability of the Barrett-Joyner-Halenda approach for pore size distribution.

Conclusions:

• The developed model successfully explains helium sorption hysteresis in Vycor glass.
• The study validates the extended Kelvin equation within the Barrett-Joyner-Halenda framework for disordered porous materials.
• Network effects are critical for understanding sorption phenomena in porous media.