ANION-EXCHANGE REACTIONS OF LAYERED DOUBLE HYDROXIDES

Layered double hydroxides are antitypes of 2/1 clay minerals. In spite of a high surface charge density, as in micas, they react easily with various types of organic anions. Short- and long-chain anions are rapidly exchanged for the inorganic interlayer anions, and high degrees of reaction are attained. The pronounced preference of primary long-chain compounds as typical of the more highly charged clay minerals is not observed. The small equivalent area forces the alkyl chain compounds (fatty acid anions, dicarboxylates, alkyl sulfates) to point away from the interlamellar surfaces and to form monomolecular films of high regularity. However, the linear carboxylate derivatives are greatly disordered along the stacking direction by mild drying. Secondary alkanesulfonates and technical anionic surfactants, which are mixture of isomers and of compounds with different chain lengths, aggregate in the interlayer spaces forming bimolecular films of constant thickness, and well-ordered organo-double hydroxides are formed, which produce integral series of basal reflections in the X-ray diagrams.