Use of p-Toluenesulfonic Acid for the Controlled Grafting of Alkoxysilanes onto Silanol Containing Surfaces:  Preparation of Tunable Hydrophilic, Hydrophobic, and Super-Hydrophobic Silica

The modification of Aerosil 200 has been carried out using methoxysilanes in toluene reflux, with p-toluenesulfonic acid as the catalyst. Both trimethoxyalkyl silanes (methyl, ethyl, propyl, butyl, hexyl, octyl, and octadecyl) and trialkylmethoxy silanes (trimethyl and dimethyloctyl) have been used. The surface has been studied by 29Si NMR, 13C NMR, elemental analysis, thermogravimetry, water contact angle, and BET analysis. When incorporating trimethoxysilanes, a plateau of modification was achieved after 1 h of reflux, while when using trialkylmethoxy silanes, a longer time of about 7 h was required. The average number of molecules incorporated in both cases has been well above those reported by other authors in similar reactions and in much shorter times. Depending on the modification agent and on the experimental conditions, the resulting organosilicas are in seven cases superhydrophobic, in three cases hydrophobic, and in two cases hydrophilic. Two structural origins for superhydrophobicity have been identified in these samples: almost complete disappearance of water accessible surface silanols (smallest methoxysilanes) and shielding of would-be water accessible surface silanols by long aliphatic tails. These features can be very precisely controlled.