  Using super-heterodyne Doppler velocimetry with multiple scattering
correction, we extend the opti-cally accessible range of concentrations in
experiments on colloidal electro-kinetics. We here meas-ured the
electro-phoretic mobility and the DC conductivity of aqueous charged sphere
suspensions covering about three orders of magnitude in particle concentrations
and transmissions as low as 40%. The extended concentration range for the first
time allows the demonstration of a non-monotonic con-centration dependence of
the mobility for a single particle species. Our observations reconcile
previ-ous experimental observations made on other species over restricted
concentration ranges. We com-pare our results to state of the art theoretical
calculations using a constant particle charge and the carefully determined
experimental boundary conditions as input. In particular, we consider so-called
realistic salt free conditions, i.e. we respect the release of counter-ions by
the particles, the solvent hydrolysis and the formation of carbonic acid from
dissolved neutral CO2. We also compare to previ-ous results obtained under
similarly well-defined conditions. This allows identification of three
dis-tinct regions of differing density dependence. An ascent during the built
up of double layer overlap which is not expected by theory, an extended plateau
region in quantitative agreement with theoretical expectation based on a
constant effective charge and a sudden decrease which occurs way before the
expected gradual decrease. Our observations suggest a relation of the
non-monotonic behavior to a decrease of particle charge, and we tentatively
discuss possibly underlying mechanisms.
