Silica gel dissolution in aqueous alkali metal hydroxides studied by 29SiNMR

Abstract

The rate of silica gel dissolution in aqueous alkaline media was investigated using 29Si nuclear magnetic resonance spectroscopy. A profound difference of alkali metal hydroxides on the dissolution rate of amorphous silica gel was observed. The dissolution rate increases in order LiOH ≈ CsaOH) < (RbOH ≈ NaOH) < KOH, as was confirmed by the β = silicomolybdate complexation method. Silica gel dissolution involved formation of monomeric silicic acid, Q 0. The monomeric anions oligomerise into dimer species, which in turn form cyclic and lineartrimer species. The structure of highly polymerised silicate species depends on the alkali metal cation, i.e. low pH silicate solutions have structurally different silicate species as a function of alkali metal hydroxide, as is shown by 29Si-NMR spectroscopy. Potassium cations gave rise to more polymerised silica species compared with other alkali metal cations studied. On the contrary, when lithium hydroxide and silica gel are mixed in a molar SiO 2/Li 2O = 1:1, a microcrystalline phase is formed which consists of lithium silicate crystals. Crystallisation of the lithium silicate proceeds via monomeric silica being in solution.