Analysis of thermodynamic properties of moisture-desorption isotherms of Eucalyptus gomphocephala wood

Abstract

ABSTRACT The aim of this paper was to determine the moisture desorption isotherms and certain thermodynamic properties of Eucalyptus gomphocephala wood using the static gravimetric method at five temperatures: 40°C, 50°C, 60°C, 70°C and 80°C, and relative humidity ranging from 5% to 90%. A “thermodynamic” equation was selected to describe the desorption isotherms under controlled conditions. Desorption isotherms were a “type II” sigmoid. The Clausius–Clapeyron relationship was applied to determine the isosteric heat of desorption. Results showed that for equilibrium moisture content less than 6%, the differential enthalpy and entropy decreased with increasing moisture content according to an exponential law from 26 to 0.5 kJ/mol and from 44 to 0.16 J/(mol K), respectively. The difference in isokinetic and harmonic temperature confirmed the enthalpy-entropy compensation theory of desorption reaction. The negative value of Gibbs free energy (−208.2 kJ/mol) indicated the spontaneous wood-water desorption reaction. Spreading pressure decreased with increasing temperature from 1 J/m² at 40°C to 0.16 J/m² at 80°C and increased with increasing water activity with the rate of 0.014% J/m² per relative humidity.