Area Occupied by a Water Molecule Adsorbed on Silica at 298 K: Zeta Adsorption Isotherm Approach

The zeta adsorption isotherm, ZAI, is used with heterogeneity-free adsorption measurements, reported by Baker and Sing, to determine the area of a water molecule adsorbed on TK 800 silica at 298 K. We examine this value by using measured water vapor adsorption isotherms of three non-TK 800 silicas, convert them to thermodynamic adsorption isotherms, and show that they coincide. Then, all four water adsorption measurements are pooled to form a single data set and used to formulate a thermodynamic isotherm for water vapor on silica. We then examine the stability of the adsorbate as a function of the vapor-phase pressure, Pⱽ. This analysis indicates that a phase change occurs for silica when Pⱽ reaches the value of a ZAI constant and this is interpreted as the wetting transition, which is assumed to transform the solid–vapor surface energy to that of the liquid–vapor interface. These assumptions are examined using the ZAI thermodynamic isotherm and the Gibbs adsorption equation to predict the surface energy of any fully hydroxylated silica in the absence of adsorption. Further examination showed that the number of adsorption sites determined by the ZAI corresponds to the surface concentration of silanol groups, which act as centers of water vapor adsorption.