Charging of water at inert and hydrophobic surfaces. Effect on interfacial properties of silver halides

Isoelectric point of silver halides depends on pH due to charging of interfacial water layer. The isoelectric point of water at inert and hydrophobic surfaces lies at pHa parts per thousand 3 so that water at surfaces is negatively charged in the pH region above 4. Consequently the electroneutrality point of silver halides pAg(eln) in aqueous environment, with respect to adsorption of silver and halide ions, does not correspond to the isoelectric point pAg(iep) measured at pHa parts per thousand 6, as previously assumed. The effect of assumed value of pAg(iep) was examined. The equilibrium constants for adsorption of silver and halide ions on silver chloride and silver bromide were calculated for the range of assumed pAg(iep) values ranging from isoelectric points measured at pH=6 to pH=3. The values of pAg(iep) obtained at pH=3 was taken as pAg(eln) and the corresponding equilibrium constants of interfacial reactions were obtained.