Hygroscopic wood moisture: single and dimerized water molecules at hydroxyl-pair sites?

A new molecular model is developed for wood moisture sorption in the hygroscopic range of relative humidity (RH < 95%). This model incorporates realistic physical characteristics of wood moisture sorption that cannot be explained by commonly used moisture sorption theories. The model considers a fixed number of active sorption sites over the full hygroscopic RH range. Each site is constituted by a pair of wood hydroxyl groups, which may be occupied by single or dimerized water species. The equilibrium occupation statistics of the sorption sites are controlled by the temperature and RH conditions and appear generally applicable to wood as well as modified wood. The relative amounts of monomer and dimer water can be calculated from the model for any equilibrium moisture content at any temperature and RH, which can be used for future spectroscopic identification and verification. The sigmoid wood moisture adsorption isotherm is calculated to be exclusively composed of monomers at low RH < 50%, while at RH = 100%, moisture is exclusively bound as water dimers, which is supported by a statistical cluster calculation. This state change in adsorbed water species from strongly bound monomers at low RH to relatively loosely bound dimers at high RH fundamentally differs from multilayer or hydration sorption models where a monolayer of strongly bound water remains present at high RH as a substrate layer for loosely bound secondary water.