Influence of interfacial water layer on surface properties of silver halides: effect of pH on the isoelectric point.

The hypothesis that pH dependent charge of interfacial water affects electrokinetic charge and electrokinetic potential of hydrophobic colloids, but not the (inner) surface potential was tested. It was found that isoelectric points of silver chloride, bromide and iodide shift to the higher pAg values in the acidic solutions, but that surface potential did not depend on pH. Isoelectric points of water at inert surfaces lie in the range 2<pH<4. In the neutral environment around pH≈7, the interfacial water is negatively charged due to preferential accumulation of OH(-) ions, with respect to H(+) ions, thus increasing the negative electrokinetic charge of silver halide particles. In the acidic region, the isoelectric points of silver halides were shifted to the higher pAg values by 0.5-1.0 pAg units depending on the type of silver halide, as well on the ionic strength of the solution. At pH≈3, the interfacial water is almost uncharged so that electrokinetic charge of silver halide particles is due to adsorption of silver and halide ions only. The conclusion was supported by mass titration experiments. Consequently, the electroneutrality points of silver halide surfaces correspond to values obtained in the acidic media. For silver chloride, pAg(eln)=5.3; for silver bromide, pAg(eln)=5.5; and for silver iodide, pAg(eln)=4.8.