Enhanced acidity and pH-dependent surface charge characterization of successively oxidized graphite oxides

This pH-potentiometric study explores the factors influencing the surface charge developed in aqueous dispersions of graphite oxide (GO) after a series of oxidation treatments. Surface charging curves demonstrate that lamellar surfaces of GO are negatively charged in the entire pH range studied. While these curves were found to be independent from the concentration of the GO suspensions, they were greatly affected by the solution conditions: increasing the pH and the ionic strength promotes the dissociation of acidic surface sites as weaker functional groups progressively participate in the ion exchange process, and the electrolyte provides an effective shielding for the surface charge. Surface densities of dissociated functional groups were determined by the proton binding isotherms and the specific surface areas of GO samples. BET surface areas provided irrationally high values for site densities, while surface areas calculated by geometrical considerations gave acceptable site densities. Formation of more and more oxygen-containing groups upon the subsequent oxidation steps, detected by IR spectroscopy and elemental analysis, resulted in the enhanced acidity of graphite oxide.