Sorption Hysteresis: A Statistical Thermodynamic Fluctuation Theory

Area of Science:

• Physical Chemistry
• Materials Science
• Thermodynamics

Background:

• Sorption hysteresis is common in porous materials but lacks a unified modeling and energetics assessment approach.
• Existing methods like capillary condensation and interfacial equations of state are insufficient for comprehensive hysteresis analysis.

Purpose of the Study:

• To introduce a statistical thermodynamic framework for modeling sorption hysteresis.
• To simultaneously address hysteresis modeling, energetics, and provide mechanistic insights into isotherm types.

Main Methods:

• Development of a statistical thermodynamic approach.
• Derivation of analytic isotherm equations for hysteresis branches.
• Identification of key parameters: free energy per molecule in pore clusters and cluster size.

Main Results:

• The approach explains sharp yet continuous adsorption-desorption transitions.
• It provides a simple analytic equation for hysteresis branches.
• Mechanistic insights are offered for IUPAC hysteresis types H1, H2(a), H2(b), and isotherm types IV, V.

Conclusions:

• A unified statistical thermodynamic approach effectively models sorption hysteresis and its energetics.
• The method provides both macroscopic and nanoscopic perspectives for understanding hysteresis phenomena.
• This framework offers a powerful tool for analyzing porous materials and their sorption behaviors.