Moisture content-water potential characteristic curves for red oak and loblolly pine

This report describes the results of a study performed to measure the water potential of loblolly pine and red oak over the full range of moisture con tent during desorption. The matric potential as measured by the tension pla te, pressure plate, and pressure membrane methods exhibited good continuity with the total water potential as measured by the isopiestic method. This not only proves the validity of the water potential measurements but also s hows that the osmotic potential component of the total water potential is n egligible at low moisture content. The characteristic curves allow characte rization of water in wood at high moisture contents and thus avoid the need to extrapolate sorption isotherm beyond the 98% relative humidity level as was done in previous sorption studies. The results also show that, at a gi ven water potential, the moisture contents of both species decrease with a rise in temperature. This may be due partly to the temperature dependence o f the surface tension of water and to the fact that entrapped air expands w hen heated, thus displacing water out of the capillaries. The temperature d ependence of water potential was used to calculate the enthalpy change, the free energy change, and the product of absolute temperature and entropy ch ange associated with moisture sorption. The data show that the logarithms o f these thermodynamic properties vary linearly with moisture content in the hygroscopic range but are nonlinear in the capillary range.