Moisture Content-Water Potential Characteristic Curves for Red Oak and Loblolly Pine


  • Jian Zhang
  • Perry N. Peralta


Sorption, water potential, characteristic curve, isotherm, free energy, enthalpy, entropy, red oak, 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 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.


American Society for Testing and Materials (ASTM). 1998a. Standard test method for capillary-moisture relationships for coarse- and medium-textured soils by porous-plate apparatus. ASTM D2325-68 (1994). Annual Book of ASTM Standards, Philadelphia, PA.nAmerican Society for Testing and Materials (ASTM). 1998b. Standard test method for capillary-moisture relationships for fine-textured soils by pressure-membrane apparatus. ASTM D3152-72 (1994). Annual Book of ASTM Standards, Philadelphia, PA.nBuckingham, E. 1907. Studies on the movement of soil moisture. USDA Soils Bull. 38 pp.nChahal, R. S. 1965. Effect of temperature and trapped air on matric suction. Soil Sci. 100(4):262-266.nCloutier, A., and Y. Fortin. 1991. Moisture content—Water potential relationship of wood from saturated to dry conditions. Wood Sci. Technol. 25:263-280.nCloutier, A., and Y. Fortin. 1993. A model of moisture movement in wood based on water potential and the determination of the effective water conductivity. Wood Sci. Technol. 27:95-114.nCloutier, A., and Y. Fortin. 1994. Wood drying modelling based on the water potential concept: Hysteresis effect. Drying Technol. 12(8):1793-1814.nCloutier, A., Y. Fortin. and G. Dhatt. 1992. A wood drying finite element model based on the water potential concept. Drying Technol. 10(5): 1151-1181.nCloutier, A., C. Tremblay, and Y. Fortin. 1995. Effect of specimen structural orientation on the moisture content—Water potential relationship of wood. Wood Sci. Technol. 29:235-242.nDay, P. R. 1947. The moisture potentials of soils. Soil Sci. 38:391-400.nFortin, Y. 1979. Moisture content—Matric potential relationship and water flow properties of wood at high moisture contents. Ph.D. thesis. University of British Columbia, Vancouver, BC. 187 pp.nGriffin, D. M. 1977. Water potential and wood-decay fungi. Ann. Rev. Phytopathol. 15:319-329.nIwata, S., T. Tabuchi, and B. P. Warkentin. 1995. Soil-water interactions: mechanisms and applications, 2nd ed. Marcel Dekker, New York, N.Y. 440 pp.nKelsey, K. E., and L. N. Clarke. 1956. The heat of sorption of water by wood. Aust. J. Appl. Sci. 7(2): 160-175.nNelson, R. M. 1983. A model for sorption of water vapor by cellulosic materials. Wood Fiber Sci. 15(1):8-88.nPeralta, P. N. 1995. Sorption of moisture by wood within a limited range of relative humidities. Wood Fiber Sci. 27(1):13-21.nPenner, E. 1963. Suction and its use as a measure of moisture contents and potentials in porous materials. Pages 245-252 in A. Wexler, ed. Humidity and moisture: Measurement and control in science and industry, vol. 4. Reinhold Publishing Co. New York, NY.nSaha, R. S., and R. P. Tripathi. 1981. Effect of temperature on the soil-water content-suction relationship. Indian Soc. Soil Sci. 29(2): 143-147.nSchofield, R. K. 1935. The pF of the water in soil. Trans. 3rd Int. Congr. Soil Sci. 2:37-48.nSiau, J. F. 1984. Transport processes in wood. Springer-Verlag, Berlin, Germany. 245 pp.nSiau, J. F. 1995. Wood: Influence of moisture on physical properties. Virginia Polytechnic Institute and State University, Blacksburg, VA. 227 pp.nStamm, A. J. 1964. Wood and cellulose science. Ronald Press, New York, NY. 549 pp.nStamm, A. J., and W. K. Loughborough. 1935. Thermodynamics of the swelling of wood. J. Phys. Chem. 39:121-132.nStone, J. E., and A. M. Scallan. 1967. The effect of component removal upon the porous structure of the cell wall of wood. II. Swelling in water and the fiber saturation point. Tappi 50(10):496-501.nTremblay, C., A. Cloutier, and Y. Fortin. 1996. Moisture content—Water potential relationship of red pine sapwood above the fiber saturation point and determination of the effective pore size distribution. Wood Sci. Technol. 30:361-371.nVeihmeyer, F. J., and N. E. Edlefsen. 1937. Interpretation of soil moisture problems by means of energy changes. Trans. Am. Geophys. Union, 18th Ann. Meet. Hydrol.: 302-318.nWadleigh, C. H., and A. D. Ayers. 1945. Growth and biochemical composition of bean plants as conditioned by soil moisture tension and salt concentration. Plant Physiol. 20:106-132.nZhang, J. 1997. Water potential and its relationship to water flow in loblolly pine and red oak. M.S. thesis. Clemson University, Clemson, SC. 87 pp.n






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