Preparation of silicas from silicon complexes: role of cellulose in polymerisation and aggregation control

Abstract

In this paper we present results obtained on the preparation of amorphous silicas from aqueous solutions of the octahedral silicon complex K₂[Si(C₆H₄O₂)₃]· 2H₂O at approximately neutral pH conditions (initial pHs of 6.67 and 7.1) in the presence and absence of cellulose. ¹H solution NMR was used to monitor complex concentrations and a molybdenum blue colorimetric method in conjunction with the NMR data was used to assess concentrations of orthosilicic acid and polymerised silicon species. Transmission electron microscopy was used to investigate the nature of any precipitates formed. At an initial pH of 6.67, the complex decomposes rapidly (> 80% in 2 min) and an equilibrium is set up between the silicon complex, orthosilicic acid and polymerised silicon species which changes little with time after the first hour of the polymerisation process. Raising the initial starting pH to 7.1 leads to fast initial complex breakdown (60% in 2 min) and an equilibrium between the species involved as above. For both systems, polymerisation produces primary particles ca. 1 nm in diameter within 60 min and although the equilibrium between polymerised and monomeric silicon species changes little with time, particles continue to grow in size and disordered necked aggregates built up from particles 2–14 nm in diameter result. For both systems, the levels of orthosilicic acid present in solution are higher than might be expected and it is suggested that transient complexes or hydrogen-bonded species are formed with the catechol molecules which prevents polymerisation from taking place. The addition of insoluble cellulose to the reaction medium under identical pH conditions had no effect on the rate of complex breakdown, the concentration of orthosilicic acid or on the initial rate of polymerisation, but produced ordered sheet-like aggregates of ca. 4 nm diameter silica particles. The role of polysaccharides in the control of polymerisation and aggregation is discussed with reference to biologically produced silicas.