为了考察水分吸着与解吸过程中木材与吸着水之间的相互作用,本.doc

吸着解吸过程中水分与木材之间的相互作用从介电弛豫及吸附热力学（作者：曹金珍 导师：赵广杰 ）摘要 为了考察吸着与解吸过程中水分与木材之间的相互作用机理，本论文分别从介电弛豫和吸附热力学两个领域对木材中的水分进行了研究。其中在介电弛豫研究中，通过对水分吸着过程（绝干状态20，40，60，80，90，100%RH平衡态）及解吸过程（25，100%RH80%RH60%RH20%RH）中西藏云杉（Picea spinulosa Griff.）试材的介电常数和介电损耗因子的测定，得到了吸着及解吸过程中水分介电弛豫的变化信息。应用Cole-Cole圆弧则对试材的介电性质进行分析后，可以进一步得到水分吸着或解吸过程中木材的静介电常数s，光介电常数，弛豫强度（s-）及衡量弛豫时间分布宽窄的系数（或）的变化。以绝干状态20，60%RH平衡态的吸湿过程为例，将基于吸着水分子回转取向运动的介电弛豫与基于木材无定形区中伯醇羟基回转取向运动的介电弛豫进行分离，并在Eyring的绝对速度反应论的基础上，求得了与吸着水分子进行回转取向运动相关连的热力学量，得到了在吸着过程中吸着水分子与木材吸着点之间的氢键结合随着水分吸着进程的变化情况。在水分平衡状态下所构筑的介电弛豫过程中吸着水分子的回转取向模型基础上，本研究中发展了水分吸着过程中水分子进行回转取向运动的分子模型。在吸附热力学研究中，由于到目前为止Clausius-Clapeyron 公式的应用只局限于水分平衡状态，因此本研究中首先考察了Clausius-Clapeyron 公式对于非平衡状态下水分-木材系统的适用性。通过实验测定了25，50，75三个温度下西藏云杉试材在水分吸着过程（绝干状态到某一恒温恒湿平衡态）及水分解吸过程（从纤维饱和点到某一恒温恒湿平衡态）的各个阶段的水分吸着与解吸等温线。应用基于Clausius-Clapeyron公式的热力学公式以及由水分吸着与解吸等温线中得到的数据，得到了木材中吸着水在各个吸着或解吸阶段的微分吸着热QL，自由能变化G和微分吸着熵S（用TS进行比较）等热力学量。QL基本上对应着水分子与木材实质之间的结合能，QL高的值通常表示水分子与木材实质间有很强的氢键结合作用；G与润胀木材构造暴露木材吸着点所做的功有关；TS值则可以提供有关吸着在木材上的水分子的排列规则性方面的信息。因此，本研究通过考察在水分吸着与解吸过程中这些热力学量的变化规律，得到了有关水分吸着和解吸过程中吸着水分子与木材实质之间相互作用变化的信息。本研究结果归纳如下：1. 在本研究测定的温度和频率范围内 (-5020, 31.6Hz1MHz), 木材在水分吸着与解吸过程中出现了三个介电弛豫过程。在绝干状态，观察到基于木材细胞壁无定形区中伯醇羟基回转取向运动的介电弛豫过程。当木材中含有吸着水时，在低频域观察到弛豫过程。其机理包括两个部分，其一是由吸着水在木材内部的分布不均匀而引起的界面极化，其二是由吸着水中杂质离子的存在而引起的直流电导。在不同的吸着与解吸阶段弛豫过程对应着不同的机理。另外，在高频域出现的弛豫过程是由基于吸着水回转取向运动的介电弛豫过程和弛豫过程两者叠加而成的。2. 在低湿度域的吸湿过程中，弛豫过程在吸湿初期有一个很大的增量，随着吸湿过程的进行逐渐降低；与此对应，在低湿度域的解吸过程中，弛豫过程没有出现单调递减，而是在解吸中期出现了增加的变化趋势。这些现象都与吸着水在木材内的分布不均匀有关，因此在低湿度域，界面极化占主导作用。在高湿度域，弛豫过程随着吸湿（或解吸）的进行始终呈单调递增（或递减）的趋势，这时直流电导是引起弛豫过程的主要原因。在水分吸着（或解吸）过程中，弛豫过程随着吸着（或解吸）的进行逐渐增大（或减小）。3. 在所测定的温度和频率范围内木材的介电性质可以用两组Cole-Cole圆弧则来描述。低频侧的实验值可以用Cole-Cole圆弧则(1)描述，而高频侧的实验值可以用Cole-Cole圆弧则(2)描述。Cole-Cole圆弧则(1)对应着弛豫过程，而Cole-Cole圆弧则(2)对应着弛豫过程。在较低温度条件下，Cole-Cole圆弧则(2)非常明显。随着温度的升高，Cole-Cole圆弧则(1)越来越占优势。因此，采用20的Cole-Cole圆弧则(1)以及-50的Cole-Cole圆弧则(2)来描述吸着与解吸过程中介电参数的变化。4. 由Cole-Cole圆弧则(1)得到的弛豫强度(s-) 与进行离子导电性的水分子的数量有关，而由Cole-Cole圆弧则(2)得到的弛豫强度(s-) 则代表木材中可能进行回转取向运动的伯醇羟基和吸着水分子的总数。在水分吸着及解吸过程中，分别由20的Cole-Cole圆弧则(1)和-50的Cole-Cole圆弧则(2)得到的两组弛豫强度表现出相似的变化趋势：在水分吸着过程中，弛豫强度在低湿度域的变化不大，在高湿度域弛豫强度随着吸湿进行而增强；在水分解吸过程中也可以观察到相似的趋势，即，在低湿度域的变化不大，在高湿度域弛豫强度随着解吸进行明显下降。5. 从绝干状态20，60%RH平衡态的水分吸着过程中，吸着水分子在回转取向过程中的活化焓随着吸湿时间呈线性增加，这说明一个吸着水分子与周围木材吸着点之间的氢键结合数的平均值随着吸湿过程的进行逐渐增多，直至达到平衡状态。6. 水分吸着与解吸过程中的木材-水分系统可以划分为两个区域（V=V1+V2），其中一个区域中的水分子与目标相对湿度达到平衡状态（V1），而另一区域中水分子仍保持原来的初始平衡状态（V2）。因此，木材中水分的热力学量也可以表达为：F（m）= F1（m，t）+F2（m）。在局部平衡假设的基础上，V1区域中水分子的热力学性质，即F1（m，t），也可以根据平衡态热力学进行定义。由于本研究中所采用的吸着过程（初始状态：绝干状态）中V2区域中不存在水分子，而解吸过程（初始状态：纤维饱和点）中V2区域的水分子的热力学量都与液态水基本相似，所以微分热力学量可以近似为零。因此，Clausius-Clapeyron公式可以应用于水分吸着与解吸过程中的木材-水分非平衡系统，尤其是本研究所采用的水分吸着与解吸过程。7. 从由Clausius-Clapeyron公式计算得到的热力学量与含水率的关系曲线可以看出，在水分吸着过程中， QL和TS都随着吸湿的进行逐渐增大。在吸湿初期，QL和TS都出现了负值。这说明在吸湿初期，水分与木材之间的结合能很弱，低于液态水分子之间的相互结合能，并且木材中水分子的排列也比液态水分子无规则。而随着吸着过程的进行，水分与木材之间的结合作用逐渐加强，水分子的排列也趋于规则。G在水分吸着过程的变化不大。8. 如果将QL与含水率的关系转化成QL与相对湿度的关系，还可以发现，QL的最小值出现在相对湿度为60%左右（20），这进一步证实了60%的相对湿度可能对应着多分子层吸着水的产生。9. 在水分解吸过程的任意阶段，在V1区域（即与目标湿度达到平衡的那部分水分所占的木材区域）中的水分子的QL，G和TS随着含水率的增大基本呈下降趋势，除了在812%的含水率区域QL和TS值出现了轻微的增大。在某一温湿度条件的解吸过程中，Q和TS都随着解吸时间下降，而G基本保持不变。10. 木材的吸着滞后包括水分吸着滞后和热力学吸着滞后两个方面。在较低温度条件下，水分吸着滞后表现明显，而在较高温度条件下，吸着滞后主要表现为热力学吸着滞后。有效羟基说可以同时解释水分吸着滞后现象和热力学吸着滞后现象。关键词： 木材，吸着水，吸着过程，解吸过程，介电弛豫，吸附热力学Interaction between Water and Woodduring Adsorption and Desorption Processes from Dielectric and Thermodynamic Approaches(Cao Jinzhen Directed by prof. Zhao Guangjie)Abstract 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%RH80%RH60%RH20%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 constant s, optic dielectric constant , relaxation strength (s-), and the coefficient (or ) 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 Eyrings 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. On the basis of the constructed reorientation model of water molecules during dielectric relaxation in previous research, the authors also developed a molecular model to illustrate the reorientation behavior of water molecules during dielectric relaxation in the adsorption process from oven-dry state to 20, 60%RH equilibrium state. In the research by thermodynamic approach, the application of the Clausius-Clapeyron equation to non-equilibrium wood-water system was discussed first because until now its application was still limited in equilibrium region. Then 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. By use of the Clausius-Clapeyron equation and the data from the sorption isotherms, the differential thermodynamic properties including differential sorption heat QL, free energy change G and differential entropy TS of adsorbed water in wood can be worked out by using the Clausius-Clapeyron equation. QL is essentially corresponding to the binding energy between the water molecules and wood substances. A high QL value suggests that there is strong hydrogen bonding effect between water molecules and wood substances. G is related to the work involved in making sorption sites available by swelling the wood structure. TS value provides some information on the regularity of water molecules adsorbed on wood. Therefore, from the change of these thermodynamic properties during adsorption and desorption processes, some information concerning the interaction between wood and adsorbed water was obtained.The results from both dielectric and thermodynamic approaches were summarized as follows:1 Within the measured temperature and frequency range (-5020, 31.6Hz1MHz), three dielectric relaxation processes could be observed. At oven-dry state, relaxation process appeared, which was based on the reorientation of methylol groups in the amorphous region of wood cell wall. After wood adsorbed water, relaxation process can be observed in lower frequency region. The mechanism of this relaxation process 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. Different mechanisms work at different stages of adsorption and desorption. In addition, there is dielectric relaxation process in higher frequency region, which is composed by the relaxation process based on the reorientation of adsorbed water molecules and relaxation process .2. 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 process does 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 process increases (or decreases) monotonously with adsorption (or desorption) process. In this case, the electric conduction is the main cause for dielectric process . The dielectric relaxation process in higher frequency region, increases (or decreases with the developing adsorption (or desorption) during moisture adsorption (or desorption) process.3. The dielectric properties in the measured temperature and frequency region can be described by two groups of Cole-Cole plots. Within the measured temperature and frequency range, the data in lower frequency side are described by Cole-Cole plots (1) and the other group is Cole-Cole (2). At lower temperatures, Cole-Cole plots (2) are obvious. But with increasing temperature, Cole-Cole plots (1) become more and more predominant. Thus, the Cole-Cole plots (1) at 20 and the Cole-Cole plots (2) at 50 are presented to illustrate the change of dielectric parameters during adsorption and desorption processes.4. The relaxation strength (s-) obtained from Cole-Cole plots (1) is associated with the amount of water molecules subjected to electric conduction, while the (s-) obtained from Cole-Cole plots (2) mainly represents the total amount of methylol groups and adsorbed water molecules possible to reorient. The common characteristic of the two groups of (s-) values are as follows. During adsorption process, they have not change much at lower humidity level but increase with adsorption time at higher humidity level. Similar trends can be found during desorption process. Namely, they change little at low humidity level but decrease obviously during the desorption process from high humidity level to low humidity level. 5. 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.6. The wood-water system during water sorption process can be divided into two regions (V=V1+V2), in one of which the water molecules reach equilibrium with the target relative humidity (V1), and in the other they remain their original equilibrium state (V2). Thus, the thermodynamic properties of water in wood were expressed as F（m）= F1（m，t）+F2（m）. On the basis of the Assumption of Local Equilibrium, the thermodynamic properties in V1 region, that is, F1（m，t）, can be defined according to the equilibrium thermodynamics. In this study, the adsorption process initiated from oven-dry state, so there are no water molecules in V2 region; the desorption process initiated from fiber saturation point, at which state the water molecules have similar thermodynamic properties with liquid water and the differential thermodynamic properties are regarded as zero. Therefore, the Clausius-Clapeyron equation was verified to be applicable to the wood-water system at non-equilibrium states during adsorption and desorption processes, especially for those applied in this study.7. It could be found from the curves of the thermodynamic properties against moisture content that, during the adsorption process, the differential heat of sorption QL and the excess energy TS associated with water sorption by wood all increase gradually with the adsorption time. At the early stages of adsorption, QL and TS both appear negative values. It suggests that, during this period, the binding energy between water and wood is weaker than the interaction between liquid water molecules and also the water in wood is less ordered than is liquid water. With the development of adsorption, the binding energy between water and wood becomes more and more strong, and also the regularity of water molecules becomes better. The free energy change G has little changed during adsorption.8. When we change the relationship between QL and moisture content into