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_eff in regulated systems could, in general, decrease with increasing η. We test this hypothesis for nonpolar dispersions by determining Z_eff(η) over a wide range of packing fractions (10⁻⁵ ≤ 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_eff 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.

中文翻译：

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非极性胶体的电荷调节† ‡

由于弱电离的表面基团的解离（或吸附），单个胶体通常带有电荷。大小取决于颗粒周围的精确化学环境，在浓缩分散液中，该化学环境是胶体堆积分数η的函数。理论研究表明，调节系统中的有效电荷Z _eff通常可随η的增加而降低。我们通过确定测试该假设对非极性分散体ž _EFF（η在宽范围的包装部分（10）^-5 ≤ η≤0.3）结合使用小角度X射线散射和电泳迁移率测量。所有分散体完全保留在液相状态中。我们发现颗粒电荷与填充分数的关系复杂，Z _eff最初在低浓度时降低，最后在高η时增加。我们将非单调密度依赖性归因于从低η的浓度独立筛选到高填充分数的交换，其中反离子数超过盐离子，静电筛选变为η-依赖。高浓度下电荷稳定的效率可能解释了已报道的浓缩纳米颗粒分散体异常高的稳定性。