Specific Surface Area, Wetting, and Surface Tension of Materials from N2 Vapor Adsorption Isotherms

We propose a method for determining the specific surface area, A(s), of a nanopowder from its measured (mass-specific) N-2 vapor adsorption isotherm. The reported cross-sectional area of an adsorbed N-2 molecule and the (mass-specific) zeta adsorption isotherm are used to calculate A(s). The (area-specific) zeta adsorption isotherm is then determined using the calculated A(s). The method is demonstrated by applying it to three nanopowders of each of three materials (alpha-alumina, carbon, and silica). The values of A(s) and mass-specific adsorption isotherms vary widely. However, when A(s) is used to convert the reported adsorption measurements from mass-specific to area-specific, it is found that they very nearly coincide with the calculated (area-specific) zeta adsorption isotherm. The zeta adsorption isotherm is then used to determine the entropy of the adsorbate. It indicates the wetting phase transition when the substrate is cooled sufficiently. We assume that the surface tension of the solid-vapor interface is transformed to that of the liquid-vapor at wetting. We then combine the zeta adsorption isotherm with the Gibbs adsorption equation. After integrating the result and applying the wetting condition to evaluate the integration constant, we obtain expressions for the surface tension of the three solids in the absence of adsorption.