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Moisture Content-Water Potential Characteristic

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Moisture Content-Water Potential Characteristic MOISTURE CONTENT-WATER POTENTIAL CHARACTERISTIC CURVES FOR RED OAK AND LOBLOLLY PINE Jian Zhangf Former Graduate Research Assistant Department of Forest Resources Clemson University Clemson, SC 29634-1003 and Perry N. Peraltat Assistant Professor Department of Wood and Paper Science North Carolina State University Raleigh, NC 27695-8005 (Received September 1998) ABSTRACT 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 con- tinuity 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 char- acterization of watcr in wood at high moisturc contents and thus avoid the need to extrapolate sorption isothcrm beyond the 98% relativc humidity level as was done in prcvious sorption studics. The results also show that, at a given watcr potential, the moisturc contents of both species decrease with a rise in temperature. This may be due partly to the temperature dcpendence 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 lincarly with moisture contcnt in the hygroscopic range but are nonlinear in the capillary range. Kevwords: Sorption, water potential, characteristic curve, isotherm, free energy, enthalpy, cntropy, red oak, loblolly pine. INTRODUCTION water contained in a piece of wood, it fails to The water present in wood affects many of describe the energy state of water in the ma- terial. A more fundamental property that de- its properties. In most instances, the interac- scribes such state is obtained from basic ther- tion between wood and water gives rise to the modynamic principles. This property is called numerous practical problems associated with the water potential. the processing and utilization of wood. For The concept of water energy in porous me- these reasons, the characterization of moisture dia finds its origin in Buckingham's work on in wood is of fundamental importance. While soils (Buckingham 1907). He introduced the moisture content proves to be a very conve- idea that Aow of water in soil results from a nient parameter for expressing the amount of difference in cavillary- Apotential between two points in the soil. This mechanical approach, t Member of SWST. however, failed to relate capillary potential to Wood und Fiber Science, 31(4), 1999, pp. 360-369 0 1999 hy the Socxety of Wood Sclence and Technology Zhang and Peralra-WATER POTENTIAL OF RED OAK AND LOBLOLLY PINE 361 the energy state of water and thus prevented Since the chemical potential or partial specific the approach from effectively analyzing the free energy of water p, (=dG/dm,) represents various transport phenomena associated with one of the equations of state that correspond soil solutions. Buckingham realized that a rig- to the Gibbs free energy, then it is also de- orous treatment of the subject, with no restric- pendent on the variables T, P, and {mi}: tions imposed on either the water content or the soluble salt content of the soil, would have to use thermodynamic reasoning. Various pro- posals were forwarded in an attempt to satisfy this demand (Schofield 1935; Veihmeyer and Edlefsen 1937; Wadleigh and Ayers 1945; Day 1947). Although they came under differ- This total differential equation could then be ent names, these proposals are all expressions integrated from selected standard conditions of the chemical potential of water in soil rel- (designated by superscript o) resulting in the ative to that of free water at the same temper- following equation: ature. This concept proved to be of immea- surable value in the scientific study of water in soils and in other porous media. Before ,L, - ,Lwo = - S, dT + V, dP Buckingham's work, the only way to describe water in materials was simply as the 'water dm, content.' With the introduction of the water + imw" am, potential concept and using the formalisms of thermodynamics, water in a medium can now be characterized in terms of its energy state (Iwata et al. 1995). To fully describe the state of a system, clas- where S, = dk,/dT = a(aG/dm,)/dT = d(aG/ sical thermodynamics provides a mathematical dT)/dm, = dS/dm, is the partial specific entro- expression relating a thermodynamic potential py of water, J kg-' K-I; and V, = dpJdP = with the independent variables of state. If tem- a(dG/am,)/aP = d(dG/dP)/dm, = dV/dm, is the perature T, pressure P, and the set of compo- partial specific volume of water, J kg-' Pa-' nent masses { m,} are the chosen independent or m" kg-'. The quantity (p, - kWo)is called variables, the thermodynamic potential is the water potential (denoted by $) and has the called the Gibbs free energy G. In differential units of J kg-'; while the terms on the right- form, the Gibbs free energy is given by the hand side of Eq. (4) are the temperature $,, expression pneumatic +,, matric I),, and osmotic $, com- ponents, respectively. Introducing these nota- tions into the integrated form of the chemical potential Eq. (4) yields where S is the entropy, V the volume, and ki the chemical potential of the ith component. If water is one of the i components, the above In soil physics, the standard reference state equation becomes generally used is a hypothetical pool of pure dG = -SdT + VdP + ~,drn, free water at atmospheric pressure, at a given elevation, and at the same temperature as that of the water in the soil; hence the temperature 362 WOOD AND FIBER SCIENCE, OCTOBER 1999, V. 31(4) scribes the combined effect of the capillary this parameter: how it varies relative to such and sorptive forces arising from the presence variables as species, temperature, and moisture of the solid matrix; and the osmotic potential content; how it relates to the transport char- takes into account the presence of solutes in acteristics of wood; and how it applies to the the water (Fortin 1979). While the above various fields of wood technology. This paper equation for water potential was derived using covers the more fundamental scope of the for- only temperature, pressure, and the set of com- mer and thus reports the results of a study ponent masses as the chosen independent var- (Zhang 1997) that investigated the effects of iables of state, other parameters may be in- moisture content and temperature on the water cluded to arrive at a more general equation potential of red oak (Quercus spp.) and lob- with component terms dealing with gravita- lolly pine (Pinus taeda). tional potential, surface interaction potential, electrical potential, etc. (Iwata et al. 1995). MATERIALS AND METHODS However, under normal service conditions of wood, the use of T, P, and {m,} is sufficient. This study focused on the determination of Though originally developed in soil science, the water potential of red oak and loblolly pine the water potential concept has found wide ac- during moisture desorption using the tension- ceptance for the analysis of the interaction be- plate, pressure-plate, and pressure-membrane tween water and other porous materials and, methods. These methods were supplemented therefore, should also find broad applications by equilibration over saturated salt solutions in wood research and utilization. It is expected (isopiestic method) so that a characteristic to shed light on many issues in wood-moisture curve covering the entire moisture content relationship which, to date, have not been re- range was obtained. The measurements were solved by more traditional approaches. Spe- performed at 30° and 55°C. All equilibrations cifically, it should provide insight into the fun- were carried out inside a chamber whose tem- damental driving force for the transport of wa- perature was maintained to within +. l.O°C of ter in wood both below and above the fiber the set temperature. saturation point; eliminate ambiguities regard- Green red oak and loblolly pine boards ing the arbitrary assignment of the fiber satu- were purchased from local sawmills and ration point; clarify the arbitrary categoriza- stored, while wrapped in plastic, in a room tions that prevail in wood science and that pur- maintained at O°C until ready for processing. port to classify different forms of wood mois- The average specific gravity (based on oven- ture (free water, capillary water, hygroscopic dry weight and green volume) of the red oak water); and enable wood scientists to compare and loblolly pine samples was 0.51 and 0.45, quantitatively, with the same scale, the state of respectively. The specimens used for the iso- water in various types of wood (i.e., untreated, piestic method were cut to 50 rnm (R) X 50 chemically modified, composites, etc.). mm (T) X 5 mm (L), while the specimens for Due to a limited number of studies, not the tension-plate, pressure-plate, and pressure- much is known about water potential as far as membrane methods were cut to 25 mm (R) X wood is concerned. Though some studies were 25-mm (T) X 10 mm (L). Larger samples performed in the 1960s (Penner 1963; Stone were used in the former so as to increase and Scallan 1967), most of the information on weight measurement sensitivity.
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