Sodium silicates and silicic acids
This thesis describes a study of silicic acids and silica-rich sodium silicates. It involves investigations into solid materials and the species present in solutions. The relative rates of aqueous dissolution of III Na2Si2O5 crystals and Na2O2SiO2 glass at room temperature were determined (Chapter 3) The results show that glass aged in the air dissolves more slowly than fresh glass and that the fresh glass initially dissolves more slowly than the crystals. The former phenomenon is caused by the formation of a leached layer in the glass surface during storage whereas the latter is explained by the differences between the structures of the two materials. The Na+ ions can freely diffuse along the planes between the silicate sheets in the crystals but their movement is inhibited by the random silicate network in the glass. A new sodium silicate (phase d) is formed during the dissolution of the crystals. The conditions for the hydrothermal synthesis of the sodium polysilicate hydrates were established (Chapter k). All the natural ones can be precipitated from solutions of Na2O:SiO2 ratio less than 1:2 between 70 and 135° C. Some of these compounds were found to be metastable and certain of those reported previously were shown to be identical to others. They appear to have sheet structures but their small crystal size precluded single-crystal X-ray structure determinations. Electron diffraction studies showed that the octosilicate (Na2O.8SiO2.9H2O) under vacuum has four-fold symmetry in the hk reflections with a = b = 7 37. From X-ray powder diffraction, d001=9.51. Magadiite (Na2O.l4SiO2.9H2O) and kenyaite (Na2O.22SiO2.10H2O) have off-orthogonal hk patterns under vacuum ; a = b = 7.32, = 93.2 . Their d001 values are 14.3 and 18.2 respectively. These three materials lose water reversibly on gentle heating and under vacuum. This is accompanied by a shrinkage in d001 It would appear that all of the sodium polysilicate hydrates rapidly form crystalline silicic acids when treated with dilute mineral acids. Each converts into a different acid. The compounds are not members of a solid solution series although some might show structural similarity. The polymerisation of silicic acids in solution, between pH 1.8 and 3.5 and at a variety of temperatures, was followed by trimethylsilylation, 29Si n.m.r., Raman spectroscopy and classic light-scattering (Chapter 5). The monosilicic acid was produced by hydrolysis of Si(OGH3)4 in dilute HC1. Maximum gel time occurs around pH 1.8 whereas the minimum rate of loss of monosilicic and other low molecular weight acids was found around pH 3.5. The explanation for this apparent paradox is that the polymerisation is a multi-step process. The low molecular weight species grow to oligomers/polymers which combine to form the gel. These oligomers/ polymers are more persistent at pH 1.8 than at pH 3.5, hence gelation is more rapid at the latter. Methanol appears to have little effect on the rate of polymerisation of monosilicic acid but it does stabilise the larger polymers. No evidence was found for the existence of cyclic trimeric silicic acid under these conditions. The results from all the techniques used axe in good agreement which confirms the validity of their use when studying silicate solutions of low pH.