Acid-base properties of carbon and graphite fiber surfaces

Carbon and graphite fiber surfaces representing a graded series in graphitic character were analyzed by inverse gas chromatography, programmed thermal desorption, and monofilament wetting. Lewis acids and bases were used as probes to determine the effect of electrooxidation and radio frequency glow discharge plasma on surface acid-base properties. Physical adsorption isotherms computed from chromatograms were analyzed with the CAEDMON (Computed Adsorption Energy Distribution in the MONolayer) algorithm to obtain histograms of surface area fraction versus adsorptive energy. Histograms from isotherms using the Lewis base show fiber surface energetics to be bimodal, with most of the surface presenting dispersion force or weak acid-base attraction to the adsorbates. A small surface area fraction of acid sites shows strong chemisorptive properties, and this area fraction increases with the degree of oxidation. Plasma treatment is effective at enhancing surface acidity, especially on high modulus graphite fibers that resist conventional electrooxidative processes. Analysis of chemisorption by programmed thermal desorption suggests that oxidative processes damage fiber surfaces, and that plasma treatment enhances surface acidity while causing less surface damage than excessive electrooxidation. Monofilament wetting with a basic liquid shows trends that are qualitatively in accord with those evinced by gas adsorption.