Identification of solvated species present in concentrated and dilute sodium silicate solutions by combined 29Si NMR and SAXS studies

The high siliceous species solubility of a sodium silicate solution is due to the formation of a Si 7O 18H 4N 4 neutral complex as major species. [Display omitted] . ► High siliceous species solubility observed in Na silicate solution in basic medium (pH = 11.56) results from the formation of a Si 7O 18H 4Na 4 neutral complex. ► Si 7O 18H 4Na 4 species are gathered into aggregates in the intial solution. ► Charged species are formed by dilution due to the dissociation of neutral species. ► Dilution leads to increase colloids size by polycondensation reaction and to decrease the size of aggregates. Both concentrated and diluted sodium silicate solutions have been investigated by combining 29Si NMR spectroscopy and SAXS experiments. The chemical nature of the entities responsible for the high siliceous species solubility observed in such alkaline concentrated sodium silicate solutions and their evolution according to dilution have been identified. For the most concentrated solution ([Si] = 7 mol/l; pH = 11.56; Si/Na atomic ratio = 1.71), the results evidence the preponderant presence of neutral Si 7O 18H 4Na 4 complexes, which behave like colloids of about 0.6–0.8 nm able to form very small aggregates with an average size lower than 3 nm. Addition of distilled water to this initial concentrated solution leads, on one hand, to a doubling of the colloid size, i.e. 1.2–1.5 nm, and, on the other hand, to a progressive decrease of the aggregate size until their total disappearance. Such a behavior could be explained by considering, first, the dissociation of the neutral Si 7O 18H 4Na 4 complexes present in the concentrated solution into Na + ions and charged (Si 7O 18H 4Na 4− n ) n − complexes (with 1 ⩽ n ⩽ 4) and, second, the condensation of these siliceous charged species in order to form larger (Si 7 y O 18 y − z H 4 y −2 z Na (4− n )y) n y− colloids. The mean size of these colloids suggests that the condensation occurs between 2 and 8 (Si 7O 18H 4Na 4− n ) n − groups.