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 si licas [Aerosil 380 (A380), Aerosil OX50 (OX50)] was investigated as a funct ion of their hydration states. The amount of water adsorbed onto the fumed silica was in the order superhydrated A380 > as is A380 similar to superhyd rated OX50 > as is OX50. The amount of OTS adsorbed onto the Aerosil sample s was in the order superhydrated OX50 >:superhydrated A380 >> as is A380 si milar to as is OX50. This ordering was explained by the effects of the unde rlying silica substrate: (i) the effective surface area available for adsor ption was greater for the less aggregated OX50 than for A380; (ii) the incr eased number of non-nearest-neighbor isolated silanols on OX50 provided a g reater number of sites for direct hydrogen bonding of OTS; (iii) increased water adsorption resulted in increased OTS adsorption as reported in the li terature;(1) (iv) however, some "mobile" water not tightly bound to the sil ica surface of A380 could be rinsed off, along with attached OTS molecules. The alkyl chains were ordered conformationally and laterally for the super hydrated fumed silica, where the amount of adsorbed OTS was greater than ab out 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 attribute d to the effects of the underlying substrate: (i) OX50 was more hydrophobic , with more siloxane bridges on its surface, and had a greater proportion o f isolated silanols; (ii) A380 was more hydrophilic owing to the greater am ount of adsorbed water. Therefore, at the same surface coverage, there were (i) fewer large islands of OTS on A380, in which the hydrophobic alkyl cha ins pointed away from the hydrophilic surface, and (ii) a larger number of small islands of OTS on OX50 that could nonspecifically adsorb on the hydro phobic sites on the surface.