A critical review of the multilayer sorption models and comparison with the sorption site occupancy (SSO) model for wood moisture sorption isotherm analysis

The wood moisture sorption (WMS) isotherm is generally considered to contain information on the water-cell wall interaction and the abundance of water sorption sites (SSs) in wood. The Hailwood-Horrobin (HH) model - as an example of the multilayer surface sorption models - is discussed for its suitability to analyze experimental WMS isotherms, to elaborate the fundamental sorption parameters. Based on multiple independent experimental and theoretical arguments, it was concluded that the basics of the surface multilayer-sorption models do not apply to wood. This is clearly illustrated by applying the analysis to the temperature-dependence of WMS isotherms, to the comparison of adsorption vs. desorption isotherms and to the quantification of SSs in wood. A sorption site occupancy (SSO) model is presented as an alternative for the HH model. It provides a comprehensive, thermodynamically consistent and quantitative basis for the analysis of WMS isotherms. The predicted SS densities are realistic and can be used to quantify sorption hysteresis and cell wall relaxation.