A solution thermodynamics definition of the fiber saturation point and the derivation of a wood-water phase (state) diagram

The fiber saturation point (FSP) is an important concept in wood-moisture relations that differentiates between the states of water in wood and has been discussed in the literature for over 100 years. Despite its importance and extensive study, the exact theoretical definition of the FSP and the operational definition (the correct way to measure the FSP) are still debated because different methods give a wide range of values. In this paper, a theoretical definition of the FSP is presented based on solution thermodynamics that treats the FSP as a phase boundary. This thermodynamic interpretation allows FSP to be calculated from the chemical potentials of bound and free water as a function of moisture content, assuming that they are both known. Treating FSP as a phase boundary naturally lends itself to the construction of a phase diagram of water in wood. A preliminary phase diagram is constructed with previously published data, and the phase diagram is extended to a state diagram by adding data on the glass transition temperatures of the wood components. The thermodynamic interpretation and resulting state diagram represent a potential framework for understanding how wood modification may affect wood-moisture relations.