Synthesis and structure-related adsorption characteristics of bifunctional polysiloxane xerogels with methyl and 3-mercaptopropyl groups

Polysiloxane xerogels containing 3-mercaptopropyl and methyl groups in the surface layer were synthesized by the sol-gel method with ethanol used as a solvent and fluoride ion used as a catalyst. It is established that an increase in the relative content of methyltriethoxysilane in the initial reaction mixture results in formation of xerogels with a developed porous structure. The tendency for an increase in other characteristics of porous structure, the sorption volume and pore size, is also observed. The analogous effect is found upon increasing relative content of tetraethoxysilane with a constant ratio between two trifunctional silanes. By means of atomic force microscopy, it is shown that the xerogels are composed of aggregated particles with mean sizes of 35–45 nm. These results correlate with the data of scanning electron microscopy. On the basis of the data of IR spectroscopy and ¹³C CP/MAS NMR spectroscopy, it is concluded that the surface layers contain not only 3-mercaptopropyl and methyl groups but also silanol groups, a part of the unhydrolyzed alkoxy groups, and water molecules involved in the formation of hydrogen bonds. The results obtained by ²⁹Si CP/MAS NMR spectroscopy testify that, in synthesized xerogels, the structural units of T¹ type (≡SiO)Si(OR′)₂CH₃ and/or (≡SiO)Si(OR′)₂(CH₂)₃SH, R′ = H, OCH₃ or OC₂H₅] are absent and the structural units of T³ type (≡SiO)₃SiCH₃ and (≡SiO)₃Si(CH₂)₃SH] dominate compared to the units of T² type (≡SiO)₂Si(OR′)CH₃ and (≡SiO)₂Si(OR′)(CH₂)₃SH]. These results are an indirect indication of enhanced hydrolytic stability of surface layers in such xerogels.