Theory of Sorption Hysteresis in Nanoporous Solids: II. Molecular condensation

TL;DR

This paper develops a theoretical framework for understanding sorption hysteresis in nanoporous solids, attributing it to hindered molecular condensation influenced by pore size, temperature, and wetting properties, with implications for cement-based materials.

Contribution

It introduces a comprehensive mean-field model combining van der Waals theory and hierarchical wetting to explain sorption hysteresis in nanopores, validated against experimental data.

Findings

Hysteresis depends on temperature and pore size.

Larger hysteresis critical temperature at high vapor pressure.

Interfacial energy barriers influence sorption sequences.

Abstract

Motivated by the puzzle of sorption hysteresis in Portland cement concrete or cement paste, we develop in Part II of this study a general theory of vapor sorption and desorption from nanoporous solids, which attributes hysteresis to hindered molecular condensation with attractive lateral interactions. The classical mean-field theory of van der Waals is applied to predict the dependence of hysteresis on temperature and pore size, using the regular solution model and gradient energy of Cahn and Hilliard. A simple "hierarchical wetting" model for thin nanopores is developed to describe the case of strong wetting by the first monolayer, followed by condensation of nanodroplets and nanobubbles in the bulk. The model predicts a larger hysteresis critical temperature and enhanced hysteresis for molecular condensation across nanopores at high vapor pressure than within monolayers at low vapor pressure. For heterogeneous pores, the theory predicts sorption/desorption sequences similar to those seen in molecular dynamics simulations, where the interfacial energy (or gradient penalty) at nanopore junctions acts as a free energy barrier for snap-through instabilities. The model helps to quantitatively understand recent experimental data for concrete or cement paste wetting and drying cycles and suggests new experiments at different temperatures and humidity sweep rates.