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Quantitative Evaluation of the Strong-Force Model for Expansion and Contraction of Vermiculite¹

ABSTRACT

The model used considers expanding forces arising from hydration of positive interlayer ions, negative charge sites, and vermiculite surface. Opposing the hydration forces is the electrostatic contracting force between positive interlayer ions and negative charge sites.

The contribution of each force to net thermodynamic values is considered independently of other forces. Hydration of interlayer cations may then be estimated from values for bulk solution. Hydration of the negative charge sites is estimated from the entropy change during interlayer water sorption by Na vermiculite. Hydration of the vermiculite surface is estimated from the enthalpy change for wetting of an oxygen-silicate surface. The contribution of the electrostatic contracting force is estimated from the lattice enthalpy of an ionic structure containing the interlayer cation and an anion equivalent to the negative charge site.

The thermodynamic estimates based upon the model are in qualitative agreement with water sorption by Li, Na, K, Rb, and Cs vermiculites and are in quantitative agreement with Na vermiculite. These are the only comparisons with the experiment possible at present. Agreement was dependent upon strong hydration of negative charge sites.

The model and the thermodynamic estimates suggest a mechanism for fixation of ions by vermiculite. The hydrating and electrostatic forces appear to be rather closely balanced, resulting in a relatively small enthalpy change favoring water sorption. This permits the entropy contribution from interlayer water molecules, which favors desorption of water, to drive the vermiculite system to spontaneous contraction, fixing interlayer ions such as K⁺.

¹ This investigation was supported, in part, by contract AT(45-1)-1756 with the U.S. Atomic Energy Commission, and is published as Scientific Paper no. 3123, College of Agriculture, Washington State University, Pullman. Project 1717. Presented before Div. S-2, Soil Science Society of America. Nov. 6, 1967, at Washington, D.C.

² Professor of Soils.

Received for publication April 17, 1968. Accepted for publication September 18, 1968.

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