MOISTURE ADSORPTION ISOTHERMS AND THERMODYNAMIC CHARACTERISTICS OF TANNIC ACID

Moisture adsorption isotherms of tannic acid were determined at 5, 15, and 35o
C with the use of the static gravimetric method in the range 0.113–0.980 aw (aw is the water activity). It was shown that tannic acid adsorbed more water
at 5o C. The experimental data fi tted well to the Guggenheim–Anderson–de Boer and Yanniotis–Blahovec equations, giving the corresponding parameters by nonlinear regression. The monolayer moisture content, number of monolayers, and the surface area of sorption were demonstrated to decrease with increasing temperature. Mesopores dominated below the monolayer moisture content followed by the formation of macropores. The variation of the differential enthalpy and entropy with the moisture content showed that water was strongly bound to the surface of tannic acid below the moisture content 5.0 g water/100 g dry basis. The adsorption process was found to be enthalpy-driven; however, it was not spontaneous at a low moisture content, as follows from the enthalpy–entropy compensation theory. The variation of the net integral enthalpy and entropy (at a constant spreading pressure) with the moisture content exhibited maximum and minimum values, respectively. This behavior indicated that water molecules were strongly bound to the tannic acid surface at the moisture content up to its monolayer values.
Moisture adsorption isotherms of tannic acid were determined at 5, 15, and 35o
C with the use of the static gravimetric method in the range 0.113–0.980 aw (aw is the water activity). It was shown that tannic acid adsorbed more water
at 5o C. The experimental data fi tted well to the Guggenheim–Anderson–de Boer and Yanniotis–Blahovec equations, giving the corresponding parameters by nonlinear regression. The monolayer moisture content, number of monolayers, and the surface area of sorption were demonstrated to decrease with increasing temperature. Mesopores dominated below the monolayer moisture content followed by the formation of macropores. The variation of the differential enthalpy and entropy with the moisture content showed that water was strongly bound to the surface of tannic acid below the moisture content 5.0 g water/100 g dry basis. The adsorption process was found to be enthalpy-driven; however, it was not spontaneous at a low moisture content, as follows from the enthalpy–entropy compensation theory. The variation of the net integral enthalpy and entropy (at a constant spreading pressure) with the moisture content exhibited maximum and minimum values, respectively. This behavior indicated that water molecules were strongly bound to the tannic acid surface at the moisture content up to its monolayer values.
Author:  L. Červenka and L. Cacková
Keywords:  tannic acid, moisture sorption, thermodynamic properties
Page:  1168