Interaction Between Water And Wood During Adsorption And Desorption Processes

In order to investigate the interaction between adsorbed water and wood during moisture adsorption and desorption processes, the dielectric approach and thermodynamic approach are respectively applied in this study.In the research by dielectric approach, the dielectric constant and dielectric loss factor of Sikkim spruce (Picea spinulosa Griff.) specimens were measured during various moisture adsorption processes (from oven-dry state to the equilibrium state in 20℃,40,80,90,100%RH environments, respectively) and desorption processes (25℃,100%RH→80%RH→60%RH→20%RH). Thus, the change of dielectric relaxation during moisture adsorption and desorption processes can be clarified. After analyzing the dielectric properties of wood by use of Cole-Cole plots, the static dielectric constantεs, optic dielectric constantε∞, relaxation strength (ε_s-ε_∞), and the coefficientα(orβ) describing the distribution of relaxation times during adsorption and desorption processes could be obtained. Moreover, taking the adsorption process from oven-dry state to the equilibrium state in 20℃,60%RH environment as an example, the dielectric relaxation based on the reorientation of adsorbed water molecules was separated out from that based on the methylol groups in the amorphous region of wood cell wall. Further the thermodynamic quantities of adsorbed water were calculated based on Eyring's absolute rate reaction theory. As a result, the change of hydrogen bonding between adsorbed water molecules and wood adsorption sites during adsorption process was obtained.In the research by thermodynamic approach, the moisture sorption isotherms of Sikkim spruce were determined at different stages of various adsorption processes (initiated from oven-dry state) and desorption processes (initiated from fiber saturation point) for three temperatures of 25, 50, and 75℃. On the basis of these isotherms, the differential thermodynamic properties including differential sorption heat QL, free energy change⊿G and differential entropy T⊿S of adsorbed water in wood can be worked out by using the Clausius-Clapeyron equation. From the change of these thermodynamic properties during adsorption and desorption processes, some information concerning the interaction between wood and adsorbed water could be obtained.The results from both dielectric and thermodynamic approaches were summarized as follows:19. The mechanism of dielectric relaxation processⅡin lower frequency region includes two parts, one of which is the interfacial polarization resulted from the inhomogeneous distribution of adsorbed water in wood and the other is the electric conduction caused by the impurity ions in adsorbed water. During the adsorption process at low humidity level, there is an abrupt increase at the initial stage of adsorption. It decreases with adsorption process. Correspondingly, during the desorption process at low humidity level, dielectric relaxation processⅡdoes not decrease monotonously with desorption time but appears increasing trend at the medium stage. These phenomena are all concerned with the inhomogeneous distribution of adsorbed water in wood. Therefore, it can be concluded that the interfacial polarization is predominant at low humidity level. While at high humidity level, relaxation processⅡincreases (or decreases) monotonously with adsorption (or desorption) process. In this case, the electric conduction can account for the dielectric relaxation processⅡ.20. The dielectric relaxation processⅢin higher frequency region, which is composed by the relaxation process based on the reorientation of adsorbed water molecules and relaxation processⅠbased on methylol groups, increases (or decreases) with the developing adsorption (or desorption) during moisture adsorption (or desorption) process.21. The dielectric properties in the measured temperature and frequency region can be described by two groups of Cole-Cole plots.22. During the moisture adsorption process from oven-dry state to the equilibrium state in 20℃,60%RH environment, the activation enthalpy of adsorbed water during reorientation increases linearly with adsorption time. It suggests that the average number of hydrogen bonds formed between each water molecule and its surrounding adsorption sites increases with adsorption process until the equilibrium state is reached.23. If the water molecules accessible to phase transition during adsorption or desorption process were taken as the research subject, the Clausius-Clapeyron equation was proved to be valid at non-equilibrium state.24. The analysis for the thermodynamic properties during adsorption process produces following results. At the initial stage of adsorption, some water molecules at lower temperatures (for example, 25℃) can not swell into wood cell wall directly. For higher temperatures (for example, 75℃), most of water molecules entered into wood cell wall directly, but at this moment the hydrogen bonding effect between water molecules and wood adsorption sites is very weak and also these molecules are in an inferior order even to that of liquid water molecules. Therefore, the water in wood during this period, if strictly speaking, can not be called as adsorbed water. With the development of adsorption, the hydrogen bonding effect strengthens gradually and the regularity of water molecules also increases. When the equilibrium state is reached, the average hydrogen bonds between adsorbed water molecules and wood adsorption sites and the regularity of adsorbed water molecules both achieved maximum.25. The analysis for the thermodynamic properties during desorption process produces following results. During desorption process, the region accessible to phase transition extends from the wood surface to the center. Therefore, at the initial stage of desorption, the activated water molecules are less and they require much energy to escape from the bonds from other molecules. At this stage, the water molecules are in a good order since most of them have not been activated. With the development of desorption, the activated water molecules become more and more, which results in a lower bonding effect and a worse regularity. The bonding effect from the surrounding molecules and the regularity of water molecules both decline to minimum while reaching equilibrium state, because all the water molecules at this moment are subjected to activated condition.26. The sorption hysteresis of wood includes two respects, that is, moisture sorption hysteresis and thermodynamic sorption hysteresis. At lower temperatures, moisture sorption hysteresis is very obvious; at higher temperatures, the sorption hysteresis is mainly represented by thermodynamic sorption hysteresis. The effective hydrogen bonding theory seems reasonable in explaining the mechanism of both moisture sorption hysteresis and thermodynamic sorption hysteresis.