Hydrophobic Mesoporous Silica Particles Modified With Nonfluorinated Alkyl Silanes

This work reports the preparation of hydrophobic mesoporous silica particles (MSPs) modified with nonfluorinated alkyl silanes. Alkyl silanes were grafted onto the surface of the MSPs as a function of the length of nonfluorinated alkyl chains such as propyltriethoxysilane (C₃), octyltriethoxysilane (C₈), dodecyltriethoxysilane (C₁₂), and octadecyltriethoxysilane (C₁₈). Moreover, the grafting of the different alkyl silanes onto the surface of MSPs to make them hydrophobic was demonstrated using different conditions such as by changing the pH (0, 4, 6, 8, and 13), solvent type (protic and aprotic), concentration of silanes (0, 0.12, 0.24, 0.36, 0.48, and 0.60 M), reaction time (1, 2, 3, and 4 days), and reaction temperature (25 and 40 °C). The contact angles of the alkyl silane-modified MSPs were increased as a function of the alkyl chain lengths in the order of C₁₈ > C₁₂ > C₈ > C₃, and the contact angle of C₁₈-modified MSPs was 4 times wider than that of unmodified MSPs. The unmodified MSPs had a contact angle of 25.3°, but C₁₈-modified MSPs had a contact angle of 102.1°. Furthermore, the hydrophobicity of the nonfluorinated alkyl silane-modified MSPs was also demonstrated by the adsorption of a hydrophobic lecithin compound, which showed the increase of lecithin adsorption as a function of the alkyl chain lengths. The cross-linking ratios of the modified silanes on the MSPs were confirmed by solid-state ²⁹Si-MAS nuclear magnetic resonance (NMR) measurement. Consequently, the hydrophobic modification on MSPs using nonfluorinated alkyl silanes was best preferred in a protic solvent, with a reaction time of ∼24 h at 25 °C and at a high concentration of silanes.