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.