Octadecylsilane-modified silicas prepared by grafting and sol-gel methods

Silica bearing octadecylsilane groups were synthesized by grafting and the sol-gel method. In the former, commercial silica was chemically modified with octadecyltrimethoxisilane (ODS), while in the latter, it was produced by reacting tetraethoxysilane and ODS in alkaline milieu. Carbon content was much lower in the case of grafting reaction (1.2 and 3.5 wt.%) in comparison to that achieved with the sol-gel method (2.5-68.6 wt.%). Hybrid silicas were characterized by X-ray photoelectronic spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), 13C resonance nuclear magnetic spectroscopy (NMR), laser scattering and scanning electronic microscopy (SEM). Alkyl chain conformation was shown to depend on the preparation route. Grafting yields hybrid silica having a liquid-like conformation, while in silica prepared by the sol-gel method, the C18 chains present a more ordered and crystalline conformation. The presence of alkyl chains increases the particle size distribution for hybrid silicas prepared by the grafting method.