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

The surface isoelectric point for native air-formed oxide films on various metals has been determined by measurement of contact angles at the hexadeca ne/aqueous solution interface as a function of the pH of the aqueous phase. Application of Young's equation. the Gibbs equation, and surface equilibri a conditions for hydroxylated oxide films leads to a mathematical expressio n which shows that the contact angle goes though a maximum at the isoelectr ic point of the oxide. The experimentally determined values for the oxide f ilms on aluminum, chromium, and tantalum are within one to three pH units o f the reported isoelectric points for the corresponding bulk oxide powders. The surface isoelectric point of an oxide-covered Ta-implanted Al surface lies between that of Al2O3 and Ta2O5 The acid-base properties of various po lymers, including a commercially available pressure-sensitive adhesive, wer e determined by measuring the contributions gamma(S)(+) and gamma(S)(-) to the solid surface free energy using the contact angle approach of van Oss a nd Good. Adhesion measurements for a pressure-sensitive adhesive having a p ositive surface charge show that the peel strength is greatest when the met al substrate has a surface oxide film of basic character.