Adsorption study of alkyl-silicas and methylsiloxy-silicas

We report the synthesis and adsorption study of the lyophobic porous silicas. Four adsorbents were prepared and tested: (1) octyl-silica, (2) hexadecyl-silica, (3) bis(trimethylsiloxy)-silica, and (4) oligo(dimethylsiloxane)-silica. Octyl- and hexadecyl-silicas were prepared via the reaction of silica with (CH3)2NSi(CH3)2CnH(2n+1) (n=8 and 16), the reactions were carried under the optimized conditions yielding high bonding densities of alkyl groups approximately 2.9-3.0 groups/nm2 and highly uniform non-polar adsorbents. Bis(trimethylsiloxy)-silica was prepared via the reaction silica with ClSi(CH3)2(CH2)10Si(CH3)[OSi(CH3)3]2. Oligo(dimethylsiloxane)-silica was prepared via the reaction of silica with ClSi(CH3)2-[OSi(CH3)2]2-Cl. Adsorption of small organic compounds (n-alkanes, alkylbenzene, benzene, diethyl ether) was investigated using two methods, classical static adsorption and gas chromatography. Thermodynamic parameters (heat, Gibbs energy, and entropy) of the adsorption of organic compounds were studied as a function of the nature of adsorbate and of the nature of the bonded layer as well. The results obtained suggest penetration of the adsorbate molecules into the bonded layer and the importance of this process in the retention mechanism in gas chromatography. Energy of the dispersion interactions with the surface decreases in the following order: n-C16H33(CH3)2Si- > n-C8H17(CH3)2Si- > [(CH3)3SiO]2Si(CH3)-(CH2)10(CH3)2Si- > -[[(CH3)2SiO]2]x-(CH3)2Si-. Energy of the electrostatic and hydrogen bonding interactions with the surface, as assessed from the adsorption of benzene and diethyl ether molecules, decreases in the opposite direction, indicating that alkyl-silicas are less polar adsorbents than methylsiloxy-silicas.