Moisture Adsorption Isotherms and Thermodynamic Characteristics of Tannic Acid

Abstract

Moisture adsorption isotherms of tannic acid were determinate at 5 °C, 15°C, and 35 °C using static gravimetric method in the range of 0.113–0.980 aw. Tannic acid adsorbed more water at 5 °C in comparison with 15 °C and 35 °C. The experimental data fitted well to the Guggenheim-Anderson-de Boer’s and Yanniotis-Blahovec equations giving their parameters by non-linear regression. Monolayer moisture content, number of monolayers and surface area of sorption decreased with increasing the temperature. Mezopores dominated below the monolayer moisture content followed by the formation of macropores. The variation of differential enthalpy and entropy with the moisture content showed the water is strongly bound to the surface of tannic acid below 5.0 g water/100 g dry basis. The adsorption process was found to be an enthalpy-driven; however, it was not spontaneous in low moisture content as resulting from enthalpy-entropy compensation theory. The variation of the net integral enthalpy and entropy (at the constant spreading pressure) with the moisture content exhibited maximum and minimum values, respectively. This behaviour indicates that water molecules are strongly bound the tannic acid surface up to its monolayer values.