The electrophoretic mobility of charged colloidal spheres suspended in deionized water was measured as a function of the packing fraction ⌽ increasing from about 10 Ϫ6 to 2ϫ10 Ϫ3 . With increasing packing fraction, the mobility first increases linearly with the logarithm of the packing fraction and then saturates at a high value unaffected by the freezing transition. The electrostatic potential ⌿(r) was calculated numerically by solving the nonlinearized Poisson-Boltzmann equation in a cell model under conditions of charge regulation. The potential first is constant at low packing fraction and then decreases roughly linearly with log(⌽). In both cases the qualitative change in packing fraction dependence occurs once a significantly ͑typically 10-20 %͒ increases above 0 a given by the residual small ion concentration. Qualitatively similar behavior was found for particles of different size and surface chemistry and also under conditions of added salt. None of the theoretical approaches presently available is able to capture this interesting and complex behavior observed under low salt conditions.

We report on comprehensive measurements of the electrophoretic mobility µ of a highly charged spherical colloid in deionized or low salt aqueous suspensions, where fluid and crystalline order develops with increased packing fraction . We propose the existence of a 'universal' shape of the µ( ) showing three distinct regimes: a logarithmic increase, a plateau and a logarithmic decrease. The position and the height of the plateau are found to be influenced by the particle surface properties and the electrolyte concentration. In particular, it starts once the counter-ion concentration becomes equal to the concentration of background electrolyte. This coincides only loosely with the range of where fluid order is developing. Also the better defined first order freezing transition is observed to be uncorrelated to the shape of the µ( ) curve.

We studied the electrophoretic mobility µ of highly charged colloidal spheres in very dilute low salt aqueous suspension. We combined experiments on individual particles and ensemble averaged measurements. In both cases µ was observed to be independent of particle size and surface chemistry. Corresponding effective charges Z * µ , however, scaled with the ratio of particle size to Bjerrum length λ B : Z * µ = Aa/λ B with a coefficient A ≈ 2. Our results are discussed in comparison to other charge determination experiments and charge renormalization theory and with respect to the issue of charge polydispersity.

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