Fumed Silica Surface Research Articles

The Fumed Silica Surface: A Study by NMR

High-resolution solid-state NMR techniques were used to investigate the surface structure of Cab-O-Sil fumed silica. 1H NMR results obtained from CRAMPS, MAS-only and relaxation studies reveal the existence of both hydrogen-bonded silanols and isolated silanols on the Cab-O-Sil surface. A systematic dehydration study of fumed silica was carried out, with results on the quantity of each type of silanol on the surface at various dehydration stages. 29Si CP-MAS experiments, including CP spin dynamics studies and various other relaxation studies, were employed to probe hydrogen bonding and the local structural environments of various hydroxyl groups of silica surfaces. 29Si CP-MAS experiments on water-treated and deuterium-exchanged Cab-O-Sil indicate the existence of interparticle silanols and internal silanols in fumed silica. 1H and 29Si NMR show that for fumed silica both isolated and hydrogen-bonded silanols are present on the surface of an untreated sample, in contrast to the case of silica gel, where all silanols of an untreated sample are hydrogen bonded.

Preparation of Self-Assembled Fullerene-Diaminododecane Multilayer Films on (MeO)Si(CH3)2(CH2)4NH2-Modified Oxide Surfaces

Abstract Self-assembled fullerene-diaminoaodecane multilayer films were prepared on planar silicon wafer and quartz plate and on nonplanar fumed silica surfaces, and the growth of each layer and its uniformity was charackrized by using FTIR, UV, contact angle measurement and ellipsometric measurement. It is suggested that fullerene-diaminododecane layers are not completely packed on oxide surfaces, but the growrh of each layer was confirmed.

29-Silicon NMR evidence for the improved chromatographic siloxane bond stability of bulky alkylsilane ligands on a silica surface

A stable bond stationary phase for reversed-phase high-performance liquid chromatography, with a diisobutyl- n-octadecylsilane derivatized surface, was studied using 29Si cross-polarization magic angle spinning (CP MAS) NMR. Fumed silica surfaces (Aerosil), trimethylsilylated to different extents, were used to illustrate the effect of ligand surface loading on the hydrogen bonding contribution to the ligand silane CP MAS NMR signal. Spectral comparison of the diisobutyl- n-octadecylsilane derivatized silica with the conventional dimethyl- n-octadecylsilane derivatized silica revealed significantly decreased hydrogen bonding of residual silanols to the ligand siloxane bond in the diisobutyl- n-octadecyl phase. This illustrates the increased steric protection of the ligand siloxane bond by the bulky alkyl substituents, which is assumed to be the reason for the improved hydrolytic stability at low pH of this phase.

Study of modified silicas by inverse gas chromatography. Influence of chain length on the conformation of n-alcohols grafted on a pyrogenic silica

A fumed silica surface was systematically modified by the grafting of n-alkyl chains with increasing carbon numbers. The samples were characterized by the dispersive component of the surface energy, their specific interaction potential and enthalpies of adsorption of polar, in particular, alcohol probes. It is shown that the variation of the surface properties and adsorption capacities depend on the chain length of the graft. For instance, minimum values are recorded when the surface coverage by methylene groups corresponds either to one or two CH2 surface layers. This behaviour is related to the mobility of the grafted alkyl chains, mobility which was examined by solid state NMR.