Interaction of water with clay surfaces

Abstract
In clay suspensions and gels water molecules are distributed between two populations in fast exchange. The physical meaning of this distribution is discussed here. From the molecular dynamics point of view it is shown that the surface force fields perturb the rotational motion on a distance z* extending to approximately 10~9m. The product times the surface area available to water defines the volume of a surface phase b. In the bulk phase a the rotational diffusional correlation time is not affected and the pulsed field gradient nuclear magnetic technique shows that the self-diffusion coefficient of water in that phase is the same as in pure liquid. The effects of salt concentration, pressure and temperature on phase b are studied, the main finding being that in the surface phase the well-developed random hydrogen bond network characteristic of liquid water is, at least, partly destroyed. Freezing the the gel suppresses the rapid exchange between the two phases. From the thermodynamic point of view it is not possible with the present data to decide whether phase b has thermodynamic significance or if it constitutes a heterogeneity within the aqueous phase.

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