Electrseapilllary Phenomena at the Stress-Annealed Pyrolytic Graphite Electrode
.@grdro-&uebec institute of Research, Varennes, Quebec, Canada
(Received March 17, 1.973)
P~blicationcosts assisted by the Hydro-Quebec Institute of Research
Mefliscus rise vs. potential relationships were measured on a partially immersed cleavage orientation of stressannealed pyrolytic graphite in both aqueous and DMF-HzO electrolytic solutions. From this experimental inforrcation as well as the experimentally determined value of the graphite's surface tension, the interfacial tension as well as charge us. potential plots were calculated using the equations YBL = 7 8
- ~ ~ ( 2 Kh 2K2h4)'/z
where ~ B L YS, , and YL are the solid-electrolyte, solid-gas, and liquid-gas interfacial tensions, p is the surface charge, E is the potential, h is the meniscus rise, and K is a constant for each solvent. The electroca,pillary information obtained on cleavage graphite is compared to that obtained on a mercury electrode. The lower charge density on graphite as compared to mercury is believed to be due to the low surface tension value of cleavage graphite surface which cannot adsorb the solvent and therefore prevent the adsorption of solvated ions. The higher charge density on cleavage graphite in DMF-H20 solutions as compared to aqueous solutions is due to the numerically lower solvation energy in DMF-H20 compared to water. The chemical inertness of cleavage graphite is believed to limit anion adsorption. Differential capacity values calculated from the charge us. potential plots a t the potential of zero charge of cleavage graphite in aqueous solutions (-4 pF'/cm2) agree with differential capacity minimum values obtained by Randin and Yeager using direct ac impedance methods. The low surface tension of cleavage graphite seems to be responsible for its low capacity aad slight capacity dependence on potential.
entroduetion Recent studies of meniscus rise os. potential at partially immersed electrodes1 -5 have provided a new and powerful tool for studying electrocapillary phenomena at the solid electrode-electrolyte interphase. Akasurements at a partially immersed mercury-plated gold electrode have shown that the interfacial tension as well as surface charge which are obtained from meniscus rise v8. potential plots5 are in agreement with those obtained from direct interfacial tension measurements on liquid mercury. An understanding of the electrocapillary phenomena at solid electrodes on the basis of meniscus rise vs. po1 entia1 measurements requires the most rigid and seleclive choice of the electrode material. Surface roughness and inhomogeneity as well as chemical and elec1,rochemical activity associated with solid surfaces complicate the nature of the interphase and likewise {,heinterpretation of experimental data. One solid suriace which satisfies most of the requirements of an ideal polarized electrode is that of cleavage orientat,ion of stress-annealed pyrolytic graphite. This surfacc is chemically inert with satisfied valencies and high hydrogen and oxygen overvoltage.6a The surface is srnoot,h, homogeneous, and can be easily peeled off, which ailoa s its reproducible renewal before each experiment. Very recently the absolute surface tension of this surface \ias determined.6b Consequently, the T h e Journal of Physical Chemistrv, Vol. 76,No. 19,1978
estimation of the interfacial tension (in absolute values) as well as the surface charge at the cleavage graphiteelectrolyte interphase has become possible. Bwause of the low surface tension magnitude of cleavage graphite (about 35 dyn/cm) and the criteria of the meniscus rise technique, one is limited to solvents of high surfacc tension. In addition to water, a 50% DlIF-50% HzO mixture (51 dyn/cm) was chosen for the study. (A pure DMF solvent cannot be applied in the study because it wets the surface with a small contact angle (about S o ) and leaves little room for variation with the potential. The maximum meniscus rise that can be rcalieed is that corresponding t o zero contact angle as shown by eq 1.) It is the purpose of the present work to study the electrocapillary phenomena at the clravage graphiteelectrolyte interphase using the meniscus rise method, with the particular object of determining the interfacial tension, the charge, and capacity magnitudes. The results will be compared to that of a mercury electrode. (1) I. Morcos and H. Fischer, J . Electround Chem. Interfacial Electrochem., 17, 7 (1968). (2) 1. Morcos, ibid., LO, 479 (1969). (3) I . Morcos, Collect. Czech. Chem. Commun., 36, 659 (1971). (4) 1. Morcos, J . Colloid Interface Sei., 37, 410 (1971).
(5) I. Morcos, J . Chem. Phys., 56, 3996 (1972). (6) (a) I. Morcos and E. Yeager, Electrochim. Acta, 15, 953 (1970) (b) I. Morcos, J . Chem. Phys., in press.
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~ ~ e c eh ~ ~ and qr u emethod ~ of ~ study ~ has ~ ~ ~ ~ ~ ~ ~ in a number of previous publicag equilibrium meniscus rise was ~ ~ as a kunci,aoru e ~of the potential ~ ~at partially ~ ~ ~ e ~ ~ ~ ~ ~ s ~ ~ e e sr c ~pyrolytic ~ ~e ~ ~~ graphite ~~ ~ ~ plates ~ ~ e d of 4 Y 2.5 crn f i r ~ e
