Moisture desorption isotherms and thermodynamic properties of ecalyptus globulus wood

Abstract

This paper aimed to analyze the moisture desorption isotherms and specific thermodynamic properties of Algerian Eucalyptus globulus wood using the static gravimetric method. The study was conducted at height temperatures and relative humidity levels ranging from 5% to 90%. The desorption isotherms exhibited a sigmoidal shape, categorized as "type II." A suitable "thermodynamic" equation was chosen to describe the desorption isotherms under controlled conditions. The Clausius-Clapeyron relationship was employed to determine the isosteric heat of desorption. The results demonstrated that, for equilibrium moisture content below 4%, the differential enthalpy and entropy decreased exponentially with increasing moisture content. Specifically, the differential enthalpy decreased from 36 kJ/mol to 5 kJ/mol, while the entropy decreased from 76 J/ (mol K) to 13 J/ (mol K). The enthalpy-entropy compensation theory in the desorption reaction was validated by the observed difference between the isokinetic temperature and the harmonic temperature. Moreover, the wood-water desorption process was spontaneous, as confirmed by the negative value of Gibbs free energy (-1690 J/mol). With increasing temperature, the spreading pressure dropped; at 40°C, it was 2.47978 J/m², and at 80°C, it was 0.82328 J/m². Conversely, the rate of increase was 0.11% J/m² per relative humidity as the water activity increased.