Adsorption Characterization of Oligo(dimethylsiloxane)-Modified Silicas: An Example of Highly Hydrophobic Surfaces with Non-Aliphatic Architecture

Abstract

This paper describes adsorption characterization of oligo(dimethylsiloxane)-modified silicas, nonpolar, lyophobic adsorbents with non-aliphatic architecture of the surfaces. Synthesis and spectral and thermal characterization of these surfaces have been recently reported in: Fadeev, A. Y.; Kazakevich, Y. V. Langmuir 2002, 18, 2665. Adsorbents were characterized by low-temperature nitrogen adsorption and by liquid-phase adsorption from methanol−water mixtures. Modification of silica with short oligo(dimethylsiloxane)s resulted in a drop of the specific pore volumes and in a decrease of C constants of the Brunauer−Emmett−Teller equation. The observed C constants were in the range of 12−17, indicating low energy of the adsorption interactions for dimethylsiloxane−silicas. The effective thickness of the grafted layers and the molecular volumes of grafted dimethylsiloxy groups were calculated from the change of the pore volumes due to surface modification and from the chemical analysis data. The thickness of the bonded layers increases from ∼3 up to ∼12 Å as grafting density of the [Si(CH3)2O] groups increases from ∼5 to 11 group/nm2, respectively. Molecular volumes of the [Si(CH3)2O] groups undergo substantial increase as grafting density increases, indicating more loose packing of the oligo(dimethylsiloxane) chains on the surface. Excess adsorption isotherms from methanol−water mixtures were measured using the dynamic minor disturbance method. Only preferential adsorption of methanol was observed on the adsorbents of high bonding density of dimethylsiloxane, indicating very low polarity and high uniformity of these surfaces. The absence of the excess water adsorption argues for the absence of accessible silanol groups (or any other polar adsorption centers) on oligo(dimethylsiloxane) surfaces.