Sorption and thermodynamic properties of Terminalia superba Engl. & Diels and Triplochiton scleroxylon K. Schum. through the 15, 35 and 50 °C sorption isotherms

Abstract

The hygroscopicity and thermodynamic properties of Terminalia superba Engl. & Diels (limba) and Triplochiton scleroxylon K. Schum. (obeche) wood were studied and the species were compared. The 15, 35 and 50 °C isotherms were plotted by applying the saturated salts method and fitted using the GAB model. The thermodynamic parameters were obtained from the isotherms through the integration method of the Clausius–Clapeyron equation. Infrared spectra were used to study possible chemical modifications in the cell wall, and X-ray diffractograms were used to analyse the crystal structure of the cell wall. Obeche had shorter crystallite length and a lower crystallinity index, giving rise to a higher number of amorphous zones and thus a higher number of sorption sites in the monolayer. As a result, the saturation moisture content in the monolayer (X_m) is greater in obeche than in limba. In terms of the thermodynamic properties, in both species the net isosteric heat of sorption decreases as the equilibrium moisture content increases, and more energy is required in desorption than in adsorption.