ABSTRACT
Abstract-The experimental and theoretical approaches for the determination of the point of zero charge (pzc) on solid inorganic substrates by contact-angle measurements are first reviewed in this paper. The experimental method consists of measuring the water contact angle in air, or in presence of another immiscible liquid, as a function of the pH ofthe water drop. At the pzc, the solid surface is uncharged and the water contact angle has a maximum. The classical thermodynamic analysis to interpret the variation of the water contact angle as a function of pH is presented. We have determined the pzc of bare and functionalized glass. The glass considered has a pzc around 3, whereas glass coated with 3-aminopropyltriethoxysilane (APTS) has a pzc around 7. This value is well corroborated by the data obtained from electrophoretic mobility and pH-potentiometric titration performed on aminated silica particles. Although amine groups of APTS are positively charged below pH ::::: 11, the APTS functionalized glass substrate is positively charged below pH 7 and negatively charged above pH 7. This particular behavior is attributed to the contribution of free SiOH groups on the functionalized glass. For the APTS-functionalized glass substrate, it was demonstrated that the electrical double layer at the solid/water interface behaved as a constant capacitor in wetting. This model was extended to uncoated metal oxide substrates such as chromium and silver oxides. An example of uncharged substrate is also presented with glass functionalized with 3glycidoxypropyltrimethoxysilane, an epoxy silane.
