Monitoring solvent dynamics and ion associations in the formation of cubic octamer polyanion in tetramethylammonium silicate solutions

Abstract

NMR methods were utilized to monitor the in situ structural and dynamic changes of various species in highly alkaline tetramethylammonium (TMA) silicate solutions. Quantitative ²⁹Si NMR, ¹H, ²H, and ¹⁷O relaxation NMR, and ¹H and ²⁹Si diffusion NMR of silicates, TMA, H₂O and D₂O demonstrate that the growth of the cubic octamer Q³₈ is accompanied by reduced water mobility and increasing TMA coordination number per Q³₈, which reaches an equilibrium value of 4.5 at 15 °C. Temperature-dependent measurements further reveal that the increased control over speciation by TMA at lower temperatures results from the more stable ion associations via slower solvent motions.