JOURNAL OF COLLOID AND INTERFACE SCIENCE 192, 363–367 (1997) ARTICLE NO. CS975007 Zeta Potential as a Tool to Characterize Plasma Oxidation of Carbon Fibers Ana B. Garcı B a, Ana Cuesta, Miguel A. Montes-Mora ´n, Amelia Martı B nez-Alonso, and Juan M. D. Tasco ´n 1 Instituto Nacional del Carbo ´n, CSIC, Apartado 73, 33080 Oviedo, Spain Received March 3, 1997; accepted May 23, 1997 been restricted to date to mesocarbon microbeads (3–5). Two different types of carbon fibers (ultrahigh modulus and The application of electrokinetic measurements to carbon high strength carbon fibers) were surface-treated in an oxygen fiber surfaces is even more scarce. Grundke et al. (25) plasma under equivalent conditions. Changes in the fiber surface showed differences between the zeta potential – pH curves chemistry were followed by electrokinetic measurements (zeta po- of untreated and oxidized carbon fibers, but the oxidation tential). The oxygen plasma treatment resulted in a displacement method employed was not specified. of the isoelectric point of carbon fibers toward lowerpHs, evidence In this work electrokinetic mobility measurements were of an increase in the surface acidity. The possible simultaneous carried out to compare the acid–base characteristics of car- formation of basic functional groups was also inferred and sup- bon fibers oxidized to various extents in an oxygen plasma. ported by results obtained by inverse gas chromatography and wettability measurements. Differences were more evident with ul- The carbon fiber samples used were representative for the trahigh modulus carbon fibers than with high strength carbon main types of this material (ultrahigh modulus and high fibers. It is concluded that the measurement of the electrokinetic strength ) , and they were plasma-treated under equivalent properties constitutes a useful technique to follow the evolution conditions to attain various burnoff (BO) degrees. of the surface chemical characteristics of different types of carbon fibers.  1997 Academic Press EXPERIMENTAL Key Words: zeta potential; oxygen plasma; carbon fibers; surface acidity and basicity. Materials Ultrahigh modulus Thornel P120 from Amoco (HM sam- INTRODUCTION ple) and high strength Sigrafil C320 from Sigri Great Lakes Carbon (HT sample) carbon fibers, both untreated and un- Oxygen plasma constitutes a novel and promising reactive sized, were selected as starting materials. medium for the surface chemical modification of different Samples, ground to õ63 mm, were treated to various BO types of carbon materials, including graphites (1, 2), meso- degrees by varying the time of exposure to a microwave carbon microbeads (3–5), glassy carbons (6–8), carbon (2450 MHz) generated oxygen plasma. Treatments were blacks (9, 10), and carbon fibers (11–17). The oxygen carried out in a Technics Plasma 200-G reactor at 150 W plasma treatment of the latter type of material deserves par- and with an oxygen flow rate to sustain 1.0 mbar pressure ticular interest as it can be used to improve the carbon fiber – inside the reaction chamber. matrix adhesion in composites by introducing oxygenated Electrokinetic Measurements functionalities at the carbon fiber surfaces. This would lead, in turn, to composite materials with an improved interlami- Electrokinetic potentials ( zeta potentials ) of parent and nar shear strength. oxidized samples were determined using a Laser Zee Meter Measurements of the electrokinetic or zeta potential have 501. The electrophoretic mobilities were converted into zeta long been used to study the surface chemistry of different potentials according to Smoluchowski’s equation. For each types of solids (18). This technique has been successfully determination, 0.05 g of sample was dispersed in 100 ml of applied to a range of carbon materials ( 19 – 24 ) , with special 0.01 M KCl solution and the slurry magnetically stirred for emphasis placed on activated carbons. However, its applica- at least 18–24 h before the measurements were carried out. tion to characterize the surface chemical modifications ac- To generate zeta potentials versus pH curves, and from them companying the plasma treatment of carbon materials has the isoelectric point ( IEP ) , the pH of the slurry was adjusted using dilute HCl and KOH solutions followed by magnetic 1 To whom correspondence should be addressed. stirring to equilibrium (usually 18–24 h). 363 0021-9797/97 $25.00 Copyright  1997 by Academic Press All rights of reproduction in any form reserved.