Gellability zone for aluminium alkoxides

Ethyl acetoacetate modi®ed aluminium sec-butoxide can be hydrolysed and gelled with alkaline, neutral or acidic media within a pH range of 3.0±8.5. Alumina precipitation occurs if pH values at any point in the sol±gel transition are outside this range. Nine di€erent gelling agents have been evaluated for pH dependence of gel point. Based on stoichiometric considerations, only those systems with a pH of 5.7±7.7 at their gel points can be fully hydrolysed before gelling and the extent of hydrolysis is reduced with an increase in the acidity/basicity of the system. In NaOH/HNO3 hydrolysed precursors, the extent of hydrolysis at the gel point is about 55%. An unhydrolysed precursor has a pH of 6.4 which increases rapidly as aluminium hydroxide formation begins and a pH of 5.7±7.7 for complete hydrolysis prior to gelling is only possible with weak acids, weak alkalis, surfactant solutions and standard bu€ers. pH data in the solto gel-transition is measured for NaOH, NH4OH, soya-PC, Milli-Q water, a bu€er of pH 7.0, standard solution of Tris-HCl, phenol, acetic and nitric acid. Based on observations a pH dependent ``gellability zone'' is proposed with the sol±gel-precipitate states existing at the inside±periphery±outside of this zone.