Vapours Adsorption on Non-porous or Porous Solids: Zeta Adsorption Isotherm Approach

The Zeta adsorption isotherm, ZAI, has been extended so it may now be applied to describe the adsorption of molecularly adsorbed gases, of dissociatively adsorbed gases, and of vapours. This isotherm assumes, as a hypothesis to be tested, that the adsorbates consist of differently sized clusters. The ZAI theory is used to derive an expression for predicting the cluster distributions in the adsorbed phase. Cluster distribution predictions are a unique feature of ZAI and they are not available from other adsorption isotherms. For vapours, this isotherm supposes that an adsorbate at pressures less than the saturation-vapour pressure, P_s (T), consists of molecular clusters with 1, 2, 3••• up to a limited maximum value of ζ_m molecules. Since ζ_m is bounded, we show that ZAI has no singularities for any pressure. This allows the adsorption of vapours at pressures greater than P_s to be investigated. The cluster distribution predicts the formation of an adsorbed film at a vapour phase pressure greater than P_s. Further, the ZAI is combined with Gibbsian thermodynamics to obtain an expression for the surface tension at the solid-vapour interface, and a method has been proposed for determining the conditions at which an adsorbed vapour becomes an adsorbed liquid. For molecularly gas adsorption, the same procedure is applied; it is found that ζ_m is unity, meaning there is no clustering. When the same procedure is applied for dissociative gas adsorption, namely for hydrogen adsorbing on metals, ζ_m is found to be two, meaning the adsorbates consist of both adsorbed atoms and molecules and the mole fraction of the adsorbed molecules and atoms are predicted. For porous materials, we present a method by which the values of the pore structure may be determined from the ZAI combined with the necessary conditions for thermodynamic equilibrium. The consistency of the proposed method is evaluated by comparing the values of the material properties obtained by different vapours. These results also agree closely with the TEM images. A total of 27 different fluid-solid systems have been examined successfully with the Zeta adsorption isotherm. ZAI is also used to determine the surface properties of two mesoporous silica templates (MCM-41 and SBA-15) and a highly porous shale.Ph.D