Prediction of surface charge properties on the basis of contact angle titration models

Contact angle titration has recently been introduced as a feasible alternative to the streaming potential technique for the determination of the surface isoelectric point, as well as the surface zeta potential. In this context, a few theoretical models have been developed. However, each model was discretely developed for a specific surface for the evaluation of either the isoelectric point or the surface zeta potential. The surface potential measured using these models also differ considerably with that measured by widely applied streaming potential analysis method. In this study, all three different contact angle based theoretical frameworks, namely Holmes-Farley and Hurwitz unified, Gibbs-Young, and Lippmann-Young equations were examined for the first time for the evaluation of both the isoelectric point and the surface electrical potential on a variety of surfaces with different physicochemical heterogeneity (e.g., glass, polyamide membrane, and polypropylene films). The results obtained from the contact angle titration curves were compared with the streaming potentials. After critical evaluation of the limitation of the existing models, new theoretical frameworks were developed considering the necessary modifications of the former models. The comparison of the contact angle-based data with the results obtained by a streaming potential analyzer indicated that the Lippmann-Young equation exhibited the highest deviations (δ>100%) for all the samples. The modification of Holmes-Farley and Hurwitz unified model reduced the deviation from 95.4% to 66.3% for glass, and from 82.5% to 38.8% for polyamide (PA) membrane, while the modification of Gibbs-Young model reduced the deviation from 47.2% to 41.8% for glass and from 423.6% to 270% for polyamide membrane. However, no obvious pH dependence of the contact angle titration was observed for the hydrophobic polypropylene film with an exception at the surface isoelectric point where the lowest contact angle was observed. The findings of this study will help to identify the appropriate model for the determination of both the isoelectric point and surface electrical potential on a particular surface of physicochemical heterogeneity as well to understand the surface phenomena that should be considered while developing the theoretical models for a particular surface of interest.