Thermodynamics of silica nanoparticle self-assembly in basic solutions of monovalent cations

Silica nanoparticle formation is studied using microcalorimetry to measure the heats of reaction evolved from the addition of tetraethylorthosilicate (TEOS) to basic aqueous solutions of monovalent cations. This approach tracks a series of reactions, beginning with the formation of soluble species (i.e., monomers/oligomers), followed by the self-assembly of nanoparticles at a critical silica concentration (CAC). Contributions to the net enthalpy arise from three primary reactions: TEOS hydrolysis, silanol dissociation, and silica condensation. Analyses are performed in the presence of tetrapropylammonium (TPA) ion, which serves as a structure-directing agent (SDA) in hydrothermal syntheses of the all-silica zeolite, silicalite-1. Plots of enthalpy versus TEOS concentration exhibit sigmoidal behavior with a 18 ± 3 kJ/mol reduction in the magnitude of enthalpy, which is related to the CAC and is independent of solution alkalinity, but dependent on the identity of the cation. Solutions of Na⁺ and various tetraalkylammonium (TAA) cations display similar sigmoidal curves with much lower changes in the magnitude of enthalpy at the CAC (∼5 kJ/mol SiO₂). Nanoparticle formation may involve endothermic reactions, derived from protonation of negatively charged silica to form neutral species, which participate in condensation, thereby providing an explanation for the previously observed exothermic-endothermic crossover as a function of time during silicalite-1 syntheses.