Dielectric properties of water adsorbed by kaolinite clays

Abstract

The dielectric loss observed for adsorbed water on kaolinite clays is interpreted in terms of a Debye dipolar mechanism in preference to a Maxwell–Wagner mechanism. The variation of relaxation times, τ, with coverage shows a decrease from low coverage values of ≈10–4 s to limiting values at monolayer coverage of ≈10–6 s. These times imply a very tightly bound adsorbate at low coverages. The more rigidly bound a water molecule is the longer it takes to orientate in the direction of the applied field. At monolayer coverages τ has values similar to the extrapolated value for ice at 298 K, indicating a monolayer that has a co-operative structure similar to that of ice.

The effect of Na+ and Cs+ counter-ions on the structure of the adsorbate is discussed in terms of the enthalpy of hydration of the cations, and the strength to which they are bound to the surface; the weaker they are bound, the easier for the cations to form complete hydration shells.

A low frequency loss, very marked for H2O + Cs kaolinite and H2O + Na kaolinite, is attributed to d.c. conductance in which mobile cations act as charge carriers with adsorbed water as the conducting path.