Surface conductivity and conduction mechanisms on adsorption of vapours on silica

Abstract

Surface conductivity of a high surface-area porous silica adsorbent has been studied upon adsorption of CH₃NO₂, C₆H₆, dioxane, H₂O and D₂O. For low surface coverages an exponential dependence between conductance and amount of gas adsorbed was found, whereas in the capillary condensation region the exponential dependence changed gradually to a linear one. The temperature dependence of the conductivity, appearance of polarization, and, with D₂O, the detection of electrolytic products proved that the conductivity is of an ionic type. Energies of activation for conduction depended on coverage and decreased with the amount adsorbed. Measurements were performed on surfaces modified by heat treatment, deuteration and enrichment in Na⁺ ions. Possible mechanisms for surface conductivity are suggested.