Determination of the acid-base properties of metal oxide films and of polymers by contact angle measurements

Abstract

The surface isoelectric point for native air-formed oxide films on various metals has been determined by measurement of contact angles at the hexadecane/ aqueous solution interface as a function of the pH of the aqueous phase. Application of Young's equation, the Gibbs equation, and surface equilibria conditions for hydroxylated oxide films leads to a mathematical expression which shows that the contact angle goes though a maximum at the isoelectric point of the oxide. The experimentally determined values for the oxide films on aluminum, chromium, and tantalum are within one to three pH units of the reported isoelectric points for the corresponding bulk oxide powders. The surface isoelectric point of an oxide-covered Ta-implanted A1 surface lies between that of Al2O3 and Ta2O5. The acid-base properties of various polymers, including a commercially available pressure-sensitive adhesive, were determined by measuring the contributions γS + and γS - to the solid surface free energy using the contact angle approach of van Oss and Good. Adhesion measurements for a pressure-sensitive adhesive having a positive surface charge show that the peel strength is greatest when the metal substrate has a surface oxide film of basic character.