Sodium hydroxide catalysed silica sol-gel synthesis: Physicochemical properties of silica nanoparticles and their post-grafting using C8 and C18 alkyl-organosilanes

Sodium hydroxide (NaOH) has been shown to result in fast nucleation of highly monodispersed silica nano-particles (SNPs). However, limited work has been reported on the physicochemical properties of the resulting SNPs and their surface modification. Herein, we demonstrate that NaOH results in poor hydrolysis of silica precursor, SNPs with adsorbed sodium ions and thermally unstable siloxane cross-linkage. The sodium ions are removed by refluxing SNPs in a 4 vol% hydrochloric acid solution. This step was also found to significantly improve the thermal stability of SNPs. The surface chemistry of the SNPs was easily modified with 0.74 mmol/g of octyltrimethoxysilane and 0.42 mmol/g of octadecyltrimethoxysilane. Functionalised SNPs exhibited super-hydrophobicity (water contact angle >= 150 degrees) and displayed a decrease between 2.46% to 3.03% in moisture sorption at 95% relative humidity. The results reveal that the acid treatment will be crucial in the design of hydroxylated and thermally stable SNPs from NaOH-catalysed synthesis.

Automated summary

Sodium hydroxide (NaOH) plays an important role in the nucleation of silica nano-particles (SNPs), but little is known about the physicochemical properties and surface modification of the resulting SNPs. By treating SNPs with hydrochloric acid, the sodium ions can be removed and the thermal stability of SNPs can be significantly improved. The surface chemistry of SNPs can be easily modified with octyltrimethoxysilane and octadecyltrimethoxysilane, resulting in super-hydrophobicity and reduced moisture sorption.