Ionic structure in caustic aluminate solutions and the precipitation of gibbsite

The structure of caustic aluminate solutions in relation to the precipitati on of gibbsite was investigated using vibrational spectroscopy and molecula r dynamics simulations. Results from the molecular dynamics simulations ind icate that aluminate ions form clusters as a function of time and that thes e clusters are stabilised by sodium ions. While the method used has the lim itation that bond formation is forbidden, the predicted clustering would ce rtainly facilitate polyaluminate anion formation. It, is proposed that obse rved additional bands in vibrational spectra of concentrated aluminate solu tions, as compared with those of dilute solutions, result from vibrations o f these clusters (and any polyaluminate ions which arise from them). The ab sence of spectral features characteristic of a distinct interfacial alumina te layer at the growing crystal surface is explained by clustering througho ut the bulk solution, and the participation of such clusters (and polyanion s) in the growth process.