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The Adsorption and Reactions of Enzymes and Proteins on Clay Minerals: IV. Kaolinite and Montmorillonite¹

ABSTRACT

With kaolinite and montmorillonite the initial sorption of proteins is rapid; about three-fourths of the maximum uptake occurs in the first few minutes in suspension. Equilibrium is only reached in a matter of hours. Sorption isotherms are characterized by a rapid initial rise at low equilibrium protein concentrations followed by saturation at equilibrium values of about 0.5 mg. per ml. Sorption of proteins by the clays is low above the isoelectric points of the proteins.

The sorption of lysozyme by sodium montmorillonite in suspension results in the release of essentially all the bound sodium into the aqueous phase. The interlayer crystal lattice expansion of montmorillonite-protein complexes corresponds to a monolayer of adsorbed protein in some cases. Montmorillonite was used as a "caliper" to show that sorbed molecules of heat-denatured and native lysozyme are of the same thickness.

The maximum adsorption of lysozyme by kaolinite is closely related to surface area and only incidentally related to surface charge of kaolinite.

d(001) spacings are not proportional to the amount of alanine sorbed on montmorillonite. Adsorption as a function of pH reveals abrupt changes near the pH regions corresponding to ionization of the amino and carboxyl groups.

¹ Contribution from the Department of Soils and Plant Nutrition, University of California, Berkeley. Presented before Div. III, Soil Science Society of America, Davis, Calif., Aug. 17, 1955. Data from the Ph.D. thesis of G. H. Peterson, 1957.

² Professor of Soil Chemistry, Research Assistant, and Associate Soil Chemist, respectively.

Received for publication December 19, 1957. Accepted for publication February 11, 1958.