Effects of Silanol Density, Distribution, and Hydration State of Fumed Silica on the Formation of Self-Assembled Monolayers of n-Octadecyltrichlorosilane

The saturation adsorption of octadecyltrichlorosilane (OTS) on two fumed silicas [Aerosil 380 (A380), Aerosil OX50 (OX50)] was investigated as a function of their hydration states. The amount of water adsorbed onto the fumed silica was in the order superhydrated A380 > as is A380 ∼ superhydrated OX50 > as is OX50. The amount of OTS adsorbed onto the Aerosil samples was in the order superhydrated OX50 > superhydrated A380 ≫ as is A380 ∼ as is OX50. This ordering was explained by the effects of the underlying silica substrate: (i) the effective surface area available for adsorption was greater for the less aggregated OX50 than for A380; (ii) the increased number of non-nearest-neighbor isolated silanols on OX50 provided a greater number of sites for direct hydrogen bonding of OTS; (iii) increased water adsorption resulted in increased OTS adsorption as reported in the literature; 1 (iv) however, some “mobile” water not tightly bound to the silica surface of A380 could be rinsed off, along with attached OTS molecules. The alkyl chains were ordered conformationally and laterally for the superhydrated fumed silica, where the amount of adsorbed OTS was greater than about 60%. In the case of the as is samples, where the amount of adsorbed OTS was approximately 34%, the OTS on OX50 was liquidlike, but the OTS on A380 was conformationally ordered but with no lateral order. This was attributed to the effects of the underlying substrate: (i) OX50 was more hydrophobic, with more siloxane bridges on its surface, and had a greater proportion of isolated silanols; (ii) A380 was more hydrophilic owing to the greater amount of adsorbed water. Therefore, at the same surface coverage, there were (i) fewer large islands of OTS on A380, in which the hydrophobic alkyl chains pointed away from the hydrophilic surface, and (ii) a larger number of small islands of OTS on OX50 that could nonspecifically adsorb on the hydrophobic sites on the surface.