Moisture Content-Water Potential Characteristic Curves for Red Oak and Loblolly Pine
Keywords:Sorption, water potential, characteristic curve, isotherm, free energy, enthalpy, entropy, red oak, loblolly pine
AbstractThis 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 content during desorption. The matric potential as measured by the tension plate, 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 shows that the osmotic potential component of the total water potential is negligible at low moisture content. The characteristic curves allow characterization 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 given water potential, the moisture contents of both species decrease with a rise in temperature. This may be due partly to the temperature dependence of the surface tension of water and to the fact that entrapped air expands when heated, thus displacing water out of the capillaries. The temperature dependence of water potential was used to calculate the enthalpy change, the free energy change, and the product of absolute temperature and entropy change associated with moisture sorption. The data show that the logarithms of these thermodynamic properties vary linearly with moisture content in the hygroscopic range but are nonlinear in the capillary range.
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