Surface charge properties of oxides and hydroxides formed on metal substrates determined by contact angle titration

Interactions of naturally oxidized or chemically treated flat metal surfaces with the environment, such as the atmosphere or a polymer coating are extremely important for the practical applications. These interactions are governed by the surface chemistry defined by the number and ionization constants of the surface hydroxyl groups along with the surface charge properties. In the present study, a robust methodology was developed to characterize the chemistry of flat surfaces using the contact angle titration procedure. The point of zero charge on the surface of a series of metal oxides and hydroxides, determined using the proposed methodology, showed consistent correlation with the corresponding values for oxides and hydroxides studied in the powder form. A new model based on the concept of polar and disperse components of the work of adhesion and a simplified version of the triple layer was proposed to explain the evolution of the contact angle with pH. This model covers the wide range of pH and ionic strengths of the test solutions and directly accounts for the adsorption process and speciation on the surface. The adhesion of polymer coatings to the chemically treated metal substrates was also explored. It was shown that there is a linear correlation between the point of zero charge on the surface and the strength of adhesion determined by means of the feathering test. This finding emphasizes the significance of the surface charge properties for predicting the adhesion of coatings.