Adsorption study of alkyl-silicas and methylsiloxy-silicas

We report the synthesis and adsorption study of the lyophobic porous silica s. Four adsorbents were prepared and tested: (1) octyl-silica, (2) hexadecy l-silica, (3) bis(trimethylsiloxy) -silica, and (4) oligo(dimethylsiloxane) -silica. Octyl- and hexadecyl-silicas were prepared via the reaction of sil ica with (CH3)(2)NSi(CH3)(2)CnH2n divided by1 (n=8 and 16), the reactions w ere 2 carried under the optimized conditions yielding high bonding densitie s of alkyl groups similar to2.9-3.0 groups/nm(2) and highly uniform non-pol ar adsorbents. Bis(trimethylsiloxy)-silica was prepared via the reaction of silica with ClSi(CH3)(2)(CH2)(10)Si(CH3)[OSi(CH3)(3)](2). Oligo(dimethylsi loxane)-silica was prepared via the reaction of silica with ClSi(CH3)(2)-[O Si(CH3)(2)](2)-Cl. Adsorption of small organic compounds (n-alkanes, alkylb enzene, benzene, diethyl ether) was investigated using two methods, classic al static adsorption and gas chromatography. Thermodynamic parameters (heat , Gibbs energy, and entropy) of the adsorption of organic compounds were st udied as a function of the nature of adsorbate and of the nature of the bon ded layer as well. The results obtained suggest penetration of the adsorbat e molecules into the bonded layer and the importance of this process in the retention mechanism in gas chromatography. Energy of the dispersion intera ctions with the surface decreases in the following order: n-C16H33(CH3)(2)S i -->n-C8H17(CH3)(2)Si --> [(CH3)(3)SiO](2)Si(CH3)-(CH2)(10)(CH3)(2)Si -->[ {(CH3)(2)SiO}(2)](x)-(CH3)(2)Si-. Energy of the electrostatic and hydrogen bonding interactions with the surface, as assessed from the adsorption of b enzene and diethyl ether molecules, decreases in the opposite direction, in dicating that alkyl-silicas are less polar adsorbents than methylsiloxy-sil icas, (C) 2001 Published by Elsevier Science B.V.