Non-Monotonic Concentration Dependence of the Electro-Phoretic Mobility of Charged Spheres in Realistic Salt Free Suspensions

TL;DR

This study extends the concentration range in colloidal electro-kinetics experiments, revealing a non-monotonic dependence of electro-phoretic mobility on particle concentration, and compares results with theoretical models under realistic salt-free conditions.

Contribution

First experimental demonstration of non-monotonic mobility dependence over a wide concentration range in salt-free colloidal suspensions, with detailed comparison to theoretical predictions.

Findings

Mobility shows non-monotonic behavior with concentration.

Three distinct density dependence regions identified.

Results align with theoretical models assuming constant particle charge.

Abstract

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.