Abstract:

Recently, the domestic demand for messmate (Eucalyptus obliqua) woods is rising, and there is no efficient and feasible conventional drying schedule for this particular wood so far. In-depth research on hygroscopicity and thermodynamic properties of messmate woods is highly needed. The water sorption characteristics of messmate woods at different temperature levels (30℃, 45℃, 60℃ and 75℃) were studied using the constant temperature and humidity chamber. The water adsorption and desorption isotherms and associated sorption hysteresis, as well as the thermodynamic properties such as the effective specific surface area S, net isosteric heat of sorption Q_st, total heat of wetting W₀, differential entropy ΔS, Gibbs free energy change ΔG, expansion pressure Φ and enthalpy-entropy compensation, were analyzed. The obtained water sorption isotherms belonged to type Ⅱ isotherms and could be well-fitted by the GAB and H-H models (R²>0.999). At constant temperature levels, the equilibrium moisture content (EMC) increased with water activity. At the constant water activity, the EMC and associated sorption hysteresis reduced with the increased temperature. The effective specific surface area decreased with the temperature. Overall, the isosteric heat of sorption and differential entropy for the water adsorption process were negative but positive for the desorption process. Furthermore, the absolute values of both isosteric heat of sorption and differential entropy increased with EMC first and then decreased gradually approaching zero. The absolute values of total heat of wetting of the desorption process, 83.7kJ/mol, was much higher than that of the adsorption process, 32.2kJ/mol. A good linear fit between the net isosteric heat of sorption and differential entropy and different isokinetic and harmonic mean temperatures indicated the establishment of the enthalpy-entropy compensation theory. Furthermore, both adsorption and desorption processes were enthalpy driven. The adsorption process was spontaneous, but the desorption process was non-spontaneous. The spreading pressure increased with the water activity. It was difficult to assess the influence of temperature on the spreading pressure for the adsorption process, but for the desorption process, the spreading pressure increased with the temperature.

Key words: Eucalyptus oblique woods, water adsorption and desorption isotherms, GAB and H-H models, thermodynamic properties

摘要：

近年来斜叶桉（Eucalyptus obliqua）木材广受国内市场欢迎，需求量大，但尚无高效可行的常规干燥工艺，需对其吸湿性及热力学性质进行深入研究。本文利用恒温恒湿箱法研究了斜叶桉在不同温度下（30℃、45℃、60℃、75℃）的等温吸湿、解吸特性，并通过GAB和H-H模型对吸湿和解吸等温线进行拟合，对吸湿滞后现象，以及有效比表面积S、净等量吸湿热Q_st、总润湿热W₀、微分熵ΔS、吉布斯自由能变ΔG、扩张压力Φ、焓熵补偿等热力学性质进行了分析。结果表明，斜叶桉木材的吸湿和解吸等温线为Ⅱ型，且GAB和H-H模型均适用于木材-水分体系（R²大于0.999）。恒定温度下，试样平衡含水率（EMC）随水分活度的增加而增加。恒定水分活度下，EMC和吸湿滞后程度均随温度的升高而降低。有效比表面积随着温度升高而降低。总体上，吸湿过程净等量吸湿热和微分熵为负值，而解吸过程为正值，净等量吸湿热和微分熵的大小均随EMC先增大后减小，直至趋近于0。解吸过程总润湿热绝对值为83.7kJ/mol，远大于吸湿过程的32.2kJ/mol。等速温度和平均调和温度不一致，焓熵补偿理论成立。吸湿、解吸过程均为焓驱动，不同的是吸湿为自发过程，解吸为非自发过程。扩张压力会随着水分活度的升高而升高，吸湿过程中，温度对于扩张压力的影响无明显规律，而在解吸过程中，扩张压力随温度的升高而升高。

关键词: 斜叶桉木材, 吸湿、解吸等温线, GAB和H-H模型, 热力学性质