Gellability zone for aluminium alkoxides

Abstract

Ethyl acetoacetate modified 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 different 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/HNO 3 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 buffers. pH data in the sol- to gel-transition is measured for NaOH, NH 4OH, soya-PC, Milli-Q water, a buffer 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. ©