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On the Interaction of Water Molecules and Montmorillonite Surfaces¹

ABSTRACT

The adsorption of water was studied on monoionic montmorillonite saturated with Cs, Rb, Li, Na, NH₄, and Ca ions. These ions vary in their influence on the structure of water in solution. Thus the effect on the adsorbed water by exchange ions could be compared with the effect of these ions in solution. Heats of wetting of the montmorillonite samples were determined as a function of the equilibrium relative vapor pressure. Entropy values were calculated from the heats of wetting combined with values of the surface potential from adsorption isotherms using the Gibbs surface excess equation. The decrease in heats of wetting with increasing relative vapor pressure was discontinuous near p/p_o = 0.1. This was interpreted as indicating a change at the surface of the clay mineral probably related with ion hydration effects. Consequently the obtained entropy changes could not be assigned entirely to the adsorbed water molecules. At low relative vapor pressures the entropy values strongly reflected the influence of the exchange ion which was found to be similar to that exerted by these ions in solution. At higher relative vapor pressures the entropy of the system was about 4 e.u. below that of liquid water which was nearly the same as the likely value for the silicate sheet itself obtained in earlier studies.

¹ Department of Agricultural Biochemistry and Soil Science, The Waite Agr. Res. Inst., University of Adelaide, South Australia.

² Lecturer in Soil Physics.

Received for publication November 19, 1968. Accepted for publication February 28, 1969.