The roles of bound water chemical potential and gas phase diffusion in moisture transport through wood

Abstract

A mechanistic description and quantitative mathematical expressions are developed for the net migration rate through wood of water in the bound and vapor phases. Diffusion of bound water is driven by the gradient in the chemical potential of the bound water molecules while water vapor diffusion is driven, by the gradient in the mole fraction of water in the gas phase. Vapor and bound water are assumed always to be in local thermodynamic equilibrium. An original mathematical derivation grounded on fundamental thermodynamic relationships is applied to the bound water chemical potential in order to express the rate of bound water diffusion in terms of only the local temperature and water vapor pressure. Published experiments on nonisothermal moisture migration rates in wood are compared to the solutions of this equation and also others which have been recently proposed in the literature. Results from the equation developed in this paper are in closest agreement with the reported experimental data. This success may be attributed both to the thermodynamically correct expression derived for bound water chemical potential and to recognition of the important contribution of gas phase diffusion to total moisture migration rates.