Water adsorption and desorption isotherms and thermodynamic properties of Eucalyptus obliqua woods at different temperatures
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 Qst,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.
Eucalyptus oblique woodswater adsorption and desorption isothermsGAB and H-H modelsthermodynamic properties
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