The Effects of Thermal Treatment on Associations between Fatty Acids and Montmorillonite

The sorption from CCl₄ solutions of acetic, lauric and stearic acids by montmorillonites saturated with H, Li, Na, K, Cs, Ca, Mg, Cu, Al and Fe, was studied by IR and X‐ray methods combined with various thermal treatments. Two distinct species were detected in the clay phase, the carboxylic acid (RCOOH) and the carboxylate anion (RCOO⁻). The ratios of the two sorbed species were found to be dependent both on the nature of the exchangeable cation and on the chain length of the acid. In Al⁻, Fe⁻ and Cu—montmorillonites, the ionic species predominates whereas in Cs—montmorillonite the acidic species predominates. Stearic acid shows the highest tendency to dissociate. The following associations were identified as occurring in the interlayer space: (I) and (II) — linkages between COOH group and oxygen sheet of silicate layer or with hydrophobic structured water; (III) — linkage between a COOH group and an exchangeable cation through a “water bridge”; (IV) — direct linkage between a COOH group and an exchangeable cation; (V) — linkage between a COO⁻ group and an exchangeable cation through a “water bridge” and (VI) — direct linkage between a COO⁻ group and an exchangeable cation. Although several types of associations are associated with every homoionic montmorillonite, some are more predominant. Thus, with Cs, association (I) predominates. With Li, Ca and Mg, association (III) predominates and is transformed to (IV) after thermal dehydration; whereas with Na and K association (IV) appears in freshly prepared sample. Al, Fe and Cu—montmorillonites mainly produce association (V). This structure is transformed into (VI) in the case of Al—montmorillonite by stearic and lauric acids, only. In the case of Cu—montmorillonite, all the three acids are transformed into (VI), but at 200°C decarboxylation of lauric and stearic acids occur.