The hygroscopic properties of the Siberian conifers bark: thermal analysis and sorption

This article deals with the hygroscopic properties of the coniferous bark species in Siberia. Isotherms of moisture sorption in the bark of the Siberian larch Larix sibirica Ledeb., the Scotch pine Pinus sylvestris L. and the Siberian fir Abies sibirica Ledeb., when the relative vapor pressure changes from 0.1 to 0.8 at a temperature of (20.0 ± 1.5) °C practically coincided. Therefore, to analyze the sorption properties of the system bark–water vapor within the framework of the Brunauer–Emmet–Teller (BET), Guggenheim–Andersen–de Boer (GAB), Frenkel–Holsey–Hill (FHH), Zimm–Lundberg (ZL), Flory–Huggins (FH), and the theory of volume filling of micropores (MVF) used for «generalized isotherm» that approximates experimental data. The isotherms of moisture sorption by bark were used to quantify the most important hygroscopic characteristics. We used BET and GAB to calculate monolayer capacity (um), specific inner surface area (Ssp) and ZL equation to find water vapor content and relative pressure when water clusters start to form in the bark samples; FHH, FH model and MVF theory to obtain fractal dimension of «sorption surface», sorbent-sorbate interaction parameter and characteristic sorption energy (Ес), respectively. Based on the precision methods of thermal analysis – thermogravimetry (TG), differential thermogravimetry (DTG) and differential scanning calorimetry (DSC), a fractional picture of thermal desorption of bound water and energy spent on its removal when heating samples at a constant rate. The values of um, Ssp indicate differences between these values obtained as a result BET models and the average value of the characteristic sorption energy Eс (MVF) indicates the absence or very small internal mechanical stresses in the cortex. From the analysis of the rate of nonisothermal drying according to the fourth derivative of the DTG circuit with respect to temperature, differences in the temperature ranges and thethermal desorption intensity of bound moisture by samples of bark of the Siberian larch, Scotch pine, and the Siberian fir were established; the heat of thermal desorption of bound water was 38.1, 38.0 and 45.6 kJ/mol Н2О accordingly.