Dynamic double layer effects on ac-induced dipoles of dielectric nanocolloids

Normal and tangential surface ionic currents around low-permittivity nanocolloids with surface charges are shown to produce three different conductive mechanisms for ac-induced dipoles, all involving dynamic space charge accumulation at the double layer/bulk interface with a conductivity jump. However, the distinct capacitor dimensions and diffusive contributions produce three disparate crossover frequencies at which the induced dipole reverses direction relative to the bulk field. A highly conducting collapsed diffuse layer, with bulk ion mobility, renders the particle conductive and produces an ionic strength independent crossover frequency for weak electrolytes. A precipitous drop in crossover frequency occurs at high ionic strengths when charging occurs only at the poles through field focusing around the insulated colloid. A peculiar maximum in crossover frequency exists between these two asymptotes for colloids smaller than a critical size when normal charging of the diffuse layer occurs over the entire surface. The crossover frequency data for latex nanocolloids of various sizes in different electrolytes of wide ranging ionic strengths are collapsed by explicit theoretical predictions without empirical parameters.