Isosteric Heat of Adsorption of Water Vapor on Bismuth Iron Molybdate Measured by the Method of Constant Surface Conductance

The sensitivity of the four-probe electrical conductance of compressed Bi3FeO4(MoO4)2 powder to humid air was measured under isothermal conditions from 24 to 60 °C at 4 deg intervals. The data were analyzed as the logarithm of the conductance change due to humid air vs the logarithm of the humidity. This was done under quasistatic conditions, so that in the oven chamber the adsorbed surface water was always in equilibrium with the vapor in the air. From this, a series of isoconductance curves were calculated at a number of distinct values. Plotted as the logarithm of the absolute humidity vs 1000 K/T these curves were good fits to a straight line, and via the Clausius−Clapeyron equation gave the isosteric heats of adsorption. The results showed hydrophobic behavior with latent heats ranging from about 0.2 eV (19 kJ/mol, most likely due to single isolated hydrogen bonds) for a very dry surface up to the bulk-water value of 0.45 eV (44 kJ/mol, due to water molecule clustering) for surfaces exposed to high humidity. Previous analysis has shown that the number of layers of adsorbed water varies from one to four for this material, with a conductance change increase measured over 3−4 orders of magnitude.