The surprising oxidation state of fumed silica and the nature of water binding to silicon oxides and hydroxides

X-ray photoelectron spectroscopy (XPS) data demonstrate that the average formal oxidation state of silicon in fumed silica (CAB-O-SIL®) is +1 as opposed to the usual +4. Electronic structure calculations on model molecular clusters and comparison with XPS data for silicon compounds in known oxidation states suggest less hydrophilic character for CAB-O-SIL® than the oxides of silicon with Si in an average formal +3 or +4 oxidation state. Once the +3 oxidation state is formed, water on the fumed silica surface facilitates the ready conversion of the Si in an average +3 oxidation state to an average +4 oxidation state.