Determination of the Surface Isoelectric Point of Oxide Films on Metals by Contact Angle Titration

✍ Scribed by E. McCafferty; J.P. Wightman

• Publisher: Elsevier Science
• Year: 1997
• Tongue: English
• Weight: 208 KB
• Volume: 194
• Category: Article
• ISSN: 0021-9797
• DOI: 10.1006/jcis.1997.5138

✦ Synopsis

If the pH is greater than the isoelectric point, the surface The surface isoelectric point for the native air-formed oxide will acquire a negative charge: films on aluminum, chromium, and tantalum has been determined by measurement of contact angles at the hexadecane/aqueous solution interface as a function of pH of the aqueous phase. Applica--MOH sur f / OH 0 `-MO 0 sur f / H 2 O tion of Young's equation, the Gibbs equation, and surface equilibria conditions for hydroxylated oxide films leads to a mathematical or expression which shows that the contact angle goes though a maximum at the isoelectric point of the oxide. The experimentally determined isoelectric point of oxide-covered chromium is 5.2 to

-MOH sur f `-MO 0 sur f / H / (aq).

[2]

5.3, of oxide-covered aluminum is 9.5, and of oxide-covered tantalum is approximately 00.7. These values for the oxide films are within one to three pH units of the reported isoelectric points for The nature of the surface charge on an oxide film is important the corresponding bulk oxide powders. The oxide-covered metal with regard to a number of applications involving metals, surfaces were cleaned by argon plasma treatment prior to measuresuch as adhesion (2), corrosion (3-5), and catalysis (6). ment of contact angles, in that XPS measurements showed this In the area of corrosion, for example, previous work by one treatment to be effective in reducing the thickness of the carbon of the present authors (3, 4) has shown that certain implanted contamination layer. In addition, interfacial tensions were meaions cause an increase in the pitting potential of aluminum sured at the hexadecane/aqueous solution interface and were obwhereas other implanted ions result in a decrease in pitting served to have only a slight dependence on the pH of the aqueous potential, and an explanation has been given in terms of the phase.