Abstract
Equilibria of water vapor adsorption on hydrated aluminum oxide were examined by an electrometric method using a hygrometer as sensor, where the admittance of the water-adsorbable dielectric capacitor was measured. A modified Dubinin-Astakhov equation was applied to the description of experimental isotherms. In this expression the water adsorption is characterized by the partial admittance, evaluated by subtracting the background from the sensor admittance. The curve-fitting analyses show temperature dependence of the characteristic admittances and the adsorption characteristic energy. Also, the characteristic energy shows a linear correlation with the admittances. As the admittance is proportional to the permittivity, the influence of adsorption temperature on the admittances is explained by the temperature dependency of the permittivities of adsorbent and adsorbate, from a knowledge of the thermal effect on spontaneous polarization of dielectrics. The linear correlation proves that the polar adsorption is promoted by electric polarization of adsorptive substances. Since the permittivities depend on temperature, it is predictable that the characteristic energy can vary with temperature in polar adsorption systems.
