Role of the functional group in n-octydimethylsilanes in the synthesis of C 8 reversed-phase silica packings for high-performance liquid chromatography

Abstract

C 8 reversed-phase packings have been sysnthesised by reaction of a 25-nm pore-size high-performance liquid chromatographic silica (10 μm, a s(BET) = 297 m 2 g -1) with 2,4-lutidine as base and dichloromethane and N,N-dimethylflormamide as solvents, or without solvents and with the following silanes: n-octyldimenthylchlorosilane (C 8-Cl), n-octyldimethylhydroxysilane (C 8-OH), n-octyldimethylmethoxysilane (C 8-OCH 3), n-octyldimethylethoxysilane (C 8-OC 2H 5), n-octyldimethyl(dimethylamino) silane [C 8-N(CH 3) 2], n-octyldimethyl(trifluoroacetoxy)silane (C 8- OCOCF 3), and bis-( n-octyldimethylsiloxane) (C 8-O-C 8). C 8-Cl, C 8-OH and C 8-OCH 3 each form a reactive intermediate with 2,4-lutidine, favouring the reaction with the hydroxyl groups of the silica. In the C 8-Cl reaction with silica, an amount of water larger than half of the stoichiometric amount of C 8-Cl prresent in the starting reaction leads to a drastic decline in the ligand density of bonded C 8 groups. This is associated with the formation of C 8-O-C 8, the latter exhibiting an extremely low reactivity. The highest reactivity of the silanes studied was observed for C 8-N(CH 3) 2, since this already contains a reactive Si bond and furthermore eliminates the volatile dimethylamine, which is a co-product of silanization. The ligand density of the bonded C 8 groups in the reaction of C 8-N(CH 3) 2 with silica was affected scarcely at all by the thermal pretreatment temperature (up to 800 K) of the silica, and the hydroxyl group concentration of the silica showed an approximately linear decrease at temperatures of 500–900 K. Under comparable conditions, the reactives of the silanes followed the sequence C 8-N(CH 3) 2 > C 8-OCOCF 3 > C 8-Cl ⋍ C 8-OH ≈ C 8-OCH 3 ≈ C 8-OC 2H 5 ⋍ C 8-O-C 8, with a maximum lignad density of 4.10 μmol m -2 for C 8-N(CH 3) 2. In accordance with their reaction mechanisms, distinct patterns in the course of the ligand densities achieved by the silanes as a function of the reaction temperature were obtained.