The swelling of HDTMA smectites as influenced by their preparation and layer charges

Both aqueous and ethanolic solutions of hexadecyltrimethylammonium (HDTMA) bromide have been reacted with layer silicates of different charge. Layer spacings of the resulting intercalates vary with charge and are consistent with the intercalated HDTMA forming: monolayers of flat-lying molecules when layer charges, on a per unit cell basis, are < 0.5 e - ; bilayers of flat-lying molecules for charges between 0.5 and 1.0 e - and tilted paraffin-like arrangements for charges greater than 1.0 e - . Intercalation reactions carried out using aqueous solutions allow salt molecules to be coadsorbed with HDTMA cations. Coadsorption of salt molecules appears not to occur when intercalation reactions are carried out with ethanol. Leaching with ethanol removes salt molecules from intercalates produced in an aqueous medium. For smectites with charges > 0.5 e - , and for vermiculites, the intercalation of salt molecules from an aqueous medium leads to greater layer spacings than for intercalates formed in ethanol or leached with ethanol.Toluene sorption isotherms show that toluene uptake from water by an intercalate is increased when it is formed from a highly charged smectite. Toluene uptake from water also increases when the intercalate contains HDTMA salt molecules. Layer spacings show that HDTMA intercalated layer silicates swell in the presence of neat toluene, and thus sorption from solution is also accompanied by interlayer expansion. Reactive barriers to contain BTEX pollutants ideally should be produced using a clay of the highest charge available, and should have been reacted with aqueous HDTMA. Washing is not necessary.