Porous xerogels with a bifunctional surface layer of the composition ≡Si(CH2)3SH/≡Si(CH2)2H3

The sol-gel method with ethanol as a solvent and fluoride ion as a catalyst was used to prepare polysiloxane xerogels containing both 3-mercaptopropyl and n-propyl groups in the surface layer. An increase in the relative amount of n-propyltriethoxysilane in the initial reaction solution was found to result in the formation of xerogels with developed porous structures, which was accompanied by an increase in the specific surface area from 370 to 550 m2/g; simultaneously, other porous structure parameters such as sorption volume and pore size exhibited a tendency to increase. Atomic-force microscopy was used to show that the xerogels synthesized comprised aggregates of mean size 30 nm. An analysis of the IR and 13C cross-polarization magic angle spinning NMR data led us to conclude that the surface layer of bifunctional xerogels contained not only 3-mercaptopropyl and n-propyl groups but also silanol groups, part of nonhydrolyzed alkoxy groups, and H-bonded water molecules. The 29Si cross-polarization magic angle spinning NMR spectra revealed the presence of structural units of the compositions T1 [(≡SiO)Si(OR’)2(CH2CH2CH3) and/or (≡SiO)Si(OR’)2(CH2)3SH, R’ = H, OCH3, or OC2H5], T2 [(≡SiO)2Si(OR’)(CH2CH2CH3) and (≡SiO)2Si(OR’)(CH2)3SH], and T3 [(≡SiO)3SiCH2CH2CH3 and (≡SiO)3Si(CH2)3SH] in the xerogels synthesized.