Characterization of fluorinated PAN-based carbon fibers by zeta-potential measurements

An alternative technique in order to increase the fiber surface polarity is the mild direct fluorination of carbon fibers. We studied the influence of various fluorination agents at different temperatures and pressures on the chemical surface composition, the bulk structure and the wetting behavior of two high tensile strength (HT) carbon fibers. Zeta (zeta)-potential measurements using the streaming potential method (applied in the EKA) were performed to characterize the electrokinetic properties of fluorinated carbon fibers. Since fluorine is able to form different kinds of chemical bonds with graphite like materials (ionic, semi-ionic or covalent C-F bonds). The time dependence of the zeta-potential reflects the surface composition of the obtained fiber material, due to the dissociation of such ionic C-F groups. The pH dependence of the zeta-potential of the fluorinated carbon fibers were determined in 10(-3) mol l(-1) KCI electrolyte solutions indicating an increased acidic character of the fluorinated fiber surfaces compared to the unmodified carbon fibers. From the KCl-concentration dependence of the zeta-potential it is possible to acquire information about the degree of interaction between the solid surface and the ions of the electrolyte solution, based on specific or electrostatic interactions. The molar adsorption-free energies Phi(K+) for potassium and Phi(cl-) for chloride ions by the fiber surfaces were calculated from the concentration c(max) at the maximum zeta-potential zeta(max). (C) 1999 Elsevier Science B.V. All rights reserved.