Charge regulation of nonpolar colloids.
basic_science · Level V
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- Record sourced from PubMed, PMID 29164218.
- Also identified by DOI 10.1039/c7sm01825h.
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Abstract
Individual colloids often carry a charge as a result of the dissociation (or adsorption) of weakly-ionized surface groups. The magnitude depends on the precise chemical environment surrounding a particle, which in a concentrated dispersion is a function of the colloid packing fraction η. Theoretical studies have suggested that the effective charge Z<sub>eff</sub> in regulated systems could, in general, decrease with increasing η. We test this hypothesis for nonpolar dispersions by determining Z<sub>eff</sub>(η) over a wide range of packing fractions (10<sup>-5</sup> ≤ η ≤ 0.3) using a combination of small-angle X-ray scattering and electrophoretic mobility measurements. All dispersions remain entirely in the fluid phase regime. We find a complex dependence of the particle charge as a function of the packing fraction, with Z<sub>eff</sub> initially decreasing at low concentrations before finally increasing at high η. We attribute the non-monotonic density dependence to a crossover from concentration-independent screening at low η, to a high packing fraction regime in which counterions outnumber salt ions and electrostatic screening becomes η-dependent. The efficiency of charge stabilization at high concentrations may explain the unusually high stability of concentrated nanoparticle dispersions which has been reported.