Comprehensive structural and dynamical study of alkali silicate solutions: Influence of hydroxide concentration and alkali nature

This study provides an extensive and comprehensive structural and dynamical characterization of alkali silicate solutions (“waterglass”), with a focus on the impact of hydroxide concentration and alkali cation type (Li⁺, Na⁺, K⁺) on their molecular and macromolecular properties. Combining 29 Si NMR spectroscopy, NMR relaxometry, UV-Vis spectroscopy (Hammett acidity function), and simultaneous small and wide-angle X-ray scattering (SWAXS), we systematically explore the silicate species distribution, size, and interactions in solution. Our results demonstrate that hydroxide concentration controls both the degree of silicate polymerization and the supramolecular organization of oligomers. Increasing sodium hydroxide concentration induces decondensation, leading to a marked decrease in the effective radius from 7.36 to 3.44 Å and, consequently, to an increase in specific surface area, as confirmed by NMR relaxometry. The nature of the alkali cation also plays a critical role: attractive or repulsive inter-oligomer interactions, modulated by specific ion effects, significantly influence the structural evolution of the silicate network. These findings highlight the importance of understanding the molecular-scale structure and dynamics of alkali silicate solutions, in order to predict and tune macroscopic properties such as viscosity, dispersibility, adhesion, and impermeabilization, which are essential for numerous industrial applications. • Decondensation of dissolved silicates enhanced by addition of alkali hydroxide • The type of alkali cation modifies the nature of the interaction between silicate species • Silicate species act as a reservoir of hydroxide ions • Paramagnetic impurities provided a powerful probe of accessible specific surface area