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Reactions of Surfactants with Montmorillonite: Adsorption Mechanisms¹

ABSTRACT

Four clays were treated with each of eight surfactants at three treatment rates. Volclay and the clay fraction from two montmorillonitic soils were treated with 100, 50, and 10 mmol /100 g.; and kaolinite was treated with 10, 5, and 1 mmol /100g.

Adsorption was measured quantitatively by the thermogravimetric method. Adsorption mechanisms were investigated by X-ray diffraction and differential thermal analysis.

The surfactants included three anionic, three cationic, and two nonionic compounds. The anionics were not adsorbed in appreciable quantities and anion exchange of kaolinite appeared to be equal to or somewhat greater than that of montmorillonite. Anionics had no effect on the d-spacing of montmorillonite and did not interfere with hydration of clay surfaces.

Cationics were strongly adsorbed by the clay surfaces through ionic bonding in amounts equal to or even greater than the cation-exchange capacities of the clays. The excess material was probably held by van der Waals forces between the alkyl radicals which increased with chain length. Their presence on clays significantly reduced hydration and water content to the extent of making the clay surfaces completely hydrophobic at the high treatment rates.

Nonionics were adsorbed by hydrogen bonding of polar-active groups to oxygen-rich clay surfaces. Their sorption energies were somewhat greater than that of water, and they displaced water when adsorbed. Both cationic and nonionic compounds were held in the interlayer spaces of montmorillonite and tended to form double layers if sufficient material was present.

¹ Contribution of Soil and Crop Sciences Department, Texas Agr. Exp. Sta., Texas A&M University, College Station. Presented before Div. S-2 Soil Sci. Soc. Amer., Columbus, Ohio Nov., 1965. The research was supported in part by the Procter and Gamble Co., Cincinnati, Ohio.

² Research Assistant in Soil Physics, and Professor of Soil Mineralogy, respectively. Present address of senior author: Robert S. Kerr Water Research Center, Ada, Okla.

³ The assistance of Dr. W.F. Bradley, Dep. of Chemical Engineering, University of Texas, Austin, Texas, is gratefully acknowledged.

Received for publication September 27, 1965. Accepted for publication December 13, 1965.