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

► XPS data show the average formal oxidation state of Si in fumed silica is +1. ► Less hydrophilic character for fumed silica than SiOx with +3 or +4 oxidation states. ► H2O on fumed silica surface facilitates conversion of Si(III) to Si(IV).