古代与现代柏木的水分吸附热力学比较研究

A comparative study on moisture sorption thermodynamics of ancient and recent cypress

【目的】探究考古木材与现代木材在水分吸附热力学方面的不同及原因,旨在提高出土饱水木质文物的尺寸稳定性,可以为出土饱水考古木材的保护研究提供理论依据。【方法】以古代与现代柏木为研究对象,分别采用扫描电子显微镜和傅里叶红外光谱表征试材的微观形貌和化学基团;利用动态水分吸附分析分别测定25和50℃下的试材吸湿和解吸等温吸附曲线,并基于Hailwood-Horrobin水分吸着理论进行拟合,结合Clausius-Clapeyron公式分别计算试材的微分吸着热Q_(S)、自由能变化ΔG及微分吸着熵ΔS,分析考古试材与现代试材在吸附热力学量的差异。【结果】与现代木材相比,考古木材的细胞壁腐朽明显,产生大量细胞壁孔洞,并有菌丝体的存在。考古木材纤维素、半纤维素降解严重,而木质素相对含量升高,且极性基团−OH、−COOH含量减少。在一定温度下,考古木材的平衡含水率大于现代木材,而Q_(S)、ΔS值均低于现代木材,并在含水率5%处产生拐点,该拐点对应单分子层吸着水达到饱和;另一方面,考古木材和现代木材的ΔG值差异不大;考古木材在吸湿平衡态下Q_(S)、ΔG、ΔS值小于解吸平衡态的值。【结论】考古木材在经历长时间腐蚀后,细胞壁结构产生腐朽;与现代木材相比,考古木材纤维素、半纤维素降解程度最大,考古木材对水分的吸着减少;考古木材的吸湿性大于现代木材,而热力学值偏低;考古木材存在热力学吸湿滞后现象。

[Objective]This research was carried out to explore the differences and reasons of moisture sorption thermodynamics between archaeological wood and recent wood,and provide a theoretical basis for the protection and research of excavated archaeological wood,and be conducive to improving the dimensional stability of the unearthed moisture-saturated wooden relics.[Method]Ancient and recent cypress(Cupressus sp.)were used as research objects,and the microscopic morphology and chemical groups of the wood samples were characterized by scanning electron microscope and Fourier transform infrared spectroscopy,respectively.The adsorption and desorption isotherms for both wood at 25 and 50℃were measured by dynamic vapor sorption,and fitted based on the Hailwood-Horrobin moisture sorption theory.The differential adsorption heat QS,Gibbs free energy changeΔG and differential adsorption entropyΔS of the wood samples were calculated by Clausius-Clapeyron formula to analyze the discrepancy of sorption thermodynamics between archaeological and recent wood.[Result]Compared with recent wood,the cell wall of archaeological wood was decayed obviously.A large number of cell wall pores and mycelia were found.The cellulose and hemicellulose were degraded seriously with increased relative content for lignin.The content of polar group−OH and−COOH decreased.Under certain temperature conditions,the equilibrium moisture content of archaeological wood was higher than that of recent wood,while the values of QS andΔS were lower than that of recent wood,and an inflection point occurred at the moisture content of 5%,corresponding to a saturation of the monolayer water.On the other hand,there was little difference inΔG between archaeological wood and recent wood.The values of QS,ΔG andΔS of archaeological wood in hygroscopic equilibrium state were lower than those in desorption equilibrium state.[Conclusion]In this study,it was found that the cell wall structure of archaeological wood decayed after a long time of corrosion.Compared with recent wood,the degradation degree of cellulose and hemicellulose in archaeological wood was the greatest.And the moisture sorption of archaeological wood reduced.Under certain temperature conditions,the hygroscopicity of archaeological wood was higher than that of recent wood,but its thermodynamic values were lower.There were thermodynamic sorption hysteresis of archaeological wood.