Comparison of the zeta potential with the diffuse layer potential from charge titration

The hypernetted chain (HNC) theory is used to compare charge titration and zeta potential data for several colloids. It is found that the HNC diffuse layer potential is closer to the zeta potential than that calculated with t he traditional Poisson-Boltzmann (PB) approximation, and hence the amount o f counterion binding required to bring the two into conformity is reduced. Further improvement is obtained by using an effective PB diffuse layer pote ntial, calculated from both the numerical HNC theory and the analytic exten ded PB approximation. The rationale for using the effective PB potential is that the conversion from the measured electrophoretic mobility to the zeta potential invokes the PB approximation. In the case of silver iodide quant itative agreement is obtained without any fitting parameters, which confirm s that no counterion binding occurs. For the metal oxides analyzed (titaniu m dioxide, aluminum oxide, and silicon dioxide) the two measurements can on ly be reconciled by postulating counterion binding or surface charge mobili ty.