Interactions between hydroxy-aluminium species and homoionic Na- and Ca-montmorillonite particles, as manifested by ζ potential, suspension stability and X-ray diffraction

The interactions between AllaO4(OH)24(H20)~ ~ and higher molecular weight hydroxy-Al species with homoionic Na- and Ca-montmorillonite have been studied by measuring adsorption of the hydroxy-Al on to the clay, turbidity of the resulting suspensions, electrokinetic potential, and d(001) basal spacing. The isolated "Alia" ions are adsorbed according to a cation-exchange process which causes flocculation of the tactoids at low concentrations. At higher concentrations, the adsorption of either isolated "All3" or/and higher molecular weight species is mainly responsible for the dispersion of clay particles with a net positive surface charge (~ ~ + 50 mV). Consequently, the tactoids are destroyed. In aqueous solution, hydroxy-A1 species have a strong affinity for smectites and formation of Al-interlayers modifies the swelling properties. The specific surface area of Al-clay can be 10 times that of pure Na- or Ca-montmorillonite and then the solid becomes a molecular sieve which has been studied for its catalytic properties (Lahav et aL, 1978; Lahav & Shani, 1978; Occelli & Tindwa, 1983; Pinnavaia et al., 1984; Plee et al., 1985), and agricultural applications (Rengasamy & Oades, 1978; Oades, 1984). Often the interaction mechanisms at the molecular scale have not been described well because the nature of the AI species was not correctly known. Recently, Plee et al. (1985), using 27A1 solid-state high-resolution nuclear magnetic resonance (NMR), showed that the AllaO4(OH) 7+ polycation has a high affinity for the interlamellar space of smectites producing a basal spacing d(001) ~ 1.7-1.8 nm. The polycation is thus protected from the aqueous environment.