Fourier transform infrared (FT-IR) and Raman spectroscopy were employed to study the hydrothermal
stability and the influence of surface functional groups on the surface wettability of methyl-modified
silica films. The surface free energy parameters of the silica films were determined using the Lifshitz-van
der Waals/acid–base approach. The thermal decomposition mechanisms of the CH3 groups in the methylmodified
silica material are proposed. The results show that with the increase of methyltriethoxysilane
(MTES)/tetraethylorthosilicate (TEOS) ratio, the surface free energy and surface wettability of the silica
films decrease greatly. This is mainly because of the contribution of the acid–base term; the intensity of
Si–CH3 groups increases at the expense of the intensity of O–H groups in the samples. The surfaces of the
methyl-modified silica films exhibited predominantly monopolar electron-donicity. The contact angle on
the silica film surface reaches its maximum value when calcination is performed at 350 8C.
Thermogravimetric analysis implies that some low molecular weight species, such as H2, CH4, and C,
are eliminated upon thermal decomposition of the –CH3 groups. The Si–CH3 and –CH3 vibrational bands
diminish in intensity as the calcination temperature is increased, disappearing completely when the
calcination temperature is increased to 600 8C. When the calcination temperature is increased to 750 8C,
the free carbon and CSi4 species will be formed.