COMPARATIVE-STUDY OF THE SURFACE HYDROXYL-GROUPS OF FUMED AND PRECIPITATED SILICAS .3. DRIFT CHARACTERIZATION OF GRAFTED N-HEXADECYL CHAINS

Grafted n-hexadecyl chains have been used to probe the surfaces of a fumed silica and of two silicas prepared in aqueous solutions. The infrared spectra of the thermally treated samples directly evidence that these chains are lying flat on the surfaces in vacuo, as far as the roughness allows. Whereas the n-hexa-decyl chains are comparatively disordered at room temperature before treatment, they can arrange themselves according to a crystal-like structure by heating, mainly around 75-100°C. The ordering was only observed for two of the three silicas studied, the fumed sample and a gel. It is displayed by dramatic spectral changes. Intensities of the CH2 absorptions are strongly increased, and the CH2 and CH3 stretching bands shift to lower wavenumbers. The CH2 bending and rocking modes split into doublets due to correlations between neighboring parallel chains, as in the low-temperature phase of long n-alkane crystals. The formation of the ordered structure by heating the sample is explained by a strong interaction between n-alkyl grafts and the silica surface, larger than between chains: the corresponding interaction energy exceeds the free energy entropic term up to about 250°C. As soon as the first layer is completed, the yield of esterification by 1-alcohols at 200°C is governed both by the multilayer structure and by the available silanols. The vOH stretching band of residual outer silanols is shifted from 3747 cm-1 down to 3695 cm-1 by contact with an alkyl chain. For the precipitated silica, both the vOH absorption still observed around 3730 cm-1 after grafting and the absence of graft structural transition suggest a larger surface roughness than for the other two samples. © 1990, American Chemical Society. All rights reserved.