Determination of the acid-base properties for 3-amino, 3-chloro and 3-mercaptopropyltrimethoxysilane coatings on silica surfaces by XPS

The acid-base properties of three silanes, aminopropyltrimethoxysilane (APS), chloropropyltrimethoxysilane (CPS) and mercaptopropyltrimethoxysilane (MPS), as films deposited onto silica plates are compared by the measurement of elemental binding energy (BE) values using X-ray photoelectron spectroscopy (XPS). It is found that nitrogen from APS, chlorine from CPS and sulphur from MPS all interact with the silica surface as a base. From a comparison of oxygen O 1s and silicon Si 2p BE values for the three silanes with those of pure silica, mechanisms to describe the modes of attachment of the silane heteroatoms to the silica surface have been proposed. The influence of X-ray beam damage on atomic concentrations or BE values is found to be significant in all cases. Tests simulating outdoor weathering to determine changes in the silane film acid-base character from BE shifts are briefly discussed. (C) 1997 Elsevier Science B.V.