The unsupported hydrophobic silica membranes were gener- ally produced by using co-precursor method. In the present work, the hydrophobic properties of TEOS based silica membrane materials were obtained by using MTES as the hydrophobic addi- tive. In the sol–gel process, initially TEOS was acid-hydrolyzed and condensed as described by the following reactions:Hydrolysis:
SiðOC2H5Þ4 þ 4H2O ! SiðOHÞ4 þ 4C2H5OH Condensation:Water condensation: nSiðOHÞ4 ! nSiO2 þ 2nH2 O
Alcohol condensation:
SiðOHÞ4 þ SiðOC2H5Þ4 ! 2SiO2 þ 4C2H5OH
However, using MTES as a co-precursor in the sol–gel process, the hydrogen from –OH groups on the silica surface are replaced by „Si–CH3 through –O–SiCH3 bonds as shown in the following reaction: