Electrophoretic mobility of model colloids and overcharging: theory and experiment

Abstract

Several theories claim that ion±ion correlations play an important role in the electric double layer of colloids. One of the most outstanding predictions is overcharging, which would take place at high electrolyte concentrations and surface charge densities. The counterion concentration next to the surface can become so large that the particle charge is overcompensated and the overcharging occurs. Sometimes this would also involve a reversal of the ± potential, but this phenomenon has been observed rarely through mobility measurements. This study explores the matter further. The electrophoretic mobility is measured for latex particles with moderate and extremely large surface charge densities at high ionic strengths (up to 2 M) in solutions of symmetric electrolytes. The results are analysed within the so-called hypernetted chain/mean-spherical approximation (HNC/MSA) and a Poisson±Boltzmann approach. In this way, the relevance of ion±ion correlations in practice and the occurrence of overcharging are probed experimentally.

Looking into overcharging in model colloids through electrophoresis: Asymmetric electrolytes

“…The reader interested in additional details about this discussion is referred to our previous work. 22 Here we only highlight the following fact. As can be seen, the experimental mobilities obtained for two electrolytes of different counterions are practically identical.…”

“…This idea is also supported by the previous study for high electrolyte concentrations, in which a radius smaller than 0.43 nm was used. 22 In relation to this, it should be taken into account that the ionic radii used are approximated ͑and averaged͒ values. Israelachvili has pointed out that different methods yield radii within an uncertainty of 0.1 nm.…”

“…22 Measurements were performed with an appropriate setup since the magnitude of mobility at high electrolyte concentrations was expected to be extremely small. The results were analyzed within the so-called hyper-netted-chain/mean-spherical approximation ͑HNC/MSA͒ and a PB approach, which allowed us to probe the relevance of ion size correlations in practice for 2:2 electrolytes.…”

Effective electrostatic interactions arising in core-shell charged microgel suspensions with added salt

“…This appears to be in contrast with the results usually found in hard charged colloids, where the counterion condensation at the particle surface is strongly dependent on the finite size of the ions, leading in some situations to charge inversion of the colloids for multivalent ions. 54,55 Although ion correlations seem to be unimportant, the OZ integral equation method is still a consistent way to determine the ionic density profiles and the effective interactions at once and can be easily extended to the study of more complex situations, where the ion penetration inside the microgel network is restricted by excluded volume effects, leading to superficial ionic condensation.…”

Electrical Forces Determine Glomerular Permeability

“…9,10 However, in Necturus, the filtration-dependent potential exhibited a polarity opposite to that observed in simple in vitro channels with a negatively charged surface. The physical phenomenon called "charged reversal" or "overcharging" [11][12][13][14][15][16][17][18] could be one of several possible physical mechanisms to explain why the negatively charged glomerular filter shows the electrokinetic characteristics of a positively charged filter. "Charge reversal" may occur if additional soluble counter ions (e.g., cations) are bound to the negatively charged filter walls by forces other than electrical interactions (e.g., chemical interactions) and thus change the apparent (i.e., the elec- D).…”