Development of the General Methodology for Determining the Point of Zero Charge of Oxide Films by Contact Angle Titration

The point of zero charge (pH(pzc)) is one of the most important properties in determining the nature of the surface charge on oxide films in contact with aqueous solutions. Contact angle titration is a simple, rapid and direct surface titration approach to determine the pH(pzc) of oxide films, compared with mass titration, potentiometric titration and batch equilibration method. Although contact angle titration has been an effective method to estimate the pH(pzc) in the past, there are some limitations in previous methodologies for the correlation between the contact angle and the surface charge density, due to the consideration of only the negative charged OH- at surface or the absence of a vital part of solution potential for the electrochemical potential of H+ in electrolyte. Based on the work of many previous contributors, two general thermodynamic methods for determining the pHpzc of oxide films by contact angle titration are developed here. The methods provide precise, fundamental understandings of the variation of the contact angle with surface charge density, which is of great significance for interfacial science.