Thermodynamic properties of the water sorption isotherms of wood of limba (Terminalia superba Engl. & Diels), obeche (Triplochiton scleroxylon K. Schum.), radiata pine (Pinus radiata D. Don) and chestnut (Castanea sativa Mill.)

The sorption isotherms of limba (Terminalia superba Engl. & Diels), obeche (Triplochiton scleroxylon K. Schum.), radiata pine (Pinus radiata D. Don) and chestnut (Castanea sativa Mill.), were plotted at 15 °C, 35 °C and 50 °C. The curves were fitted using the Guggenheim-Anderson-de Boer (GAB) model. The thermodynamic properties (net isosteric heat of sorption, total heat of wetting, differential entropy and spreading pressure) were determined from the isotherms to define the energy associated with the sorption processes. Net isosteric heat of sorption was obtained using the Clausius-Clapeyron equation and showed a negative relation with moisture content. Differential entropy showed the same behaviour. These two parameters were higher in desorption than in adsorption. Total heat of wetting was also higher in desorption than in adsorption and higher in chestnut than in the other species. The enthalpy-entropy theory was validated as there was a good linear fit between net isosteric heat of sorption and differential entropy, and the isokinetic temperatures were different from the harmonic mean temperature. Using the enthalpy-entropy theory, it was possible to conclude that the adsorption and desorption processes were enthalpy driven and spontaneous. Spreading pressure increased as water activity increased. The variation of spreading pressure with temperature depends on the species type.