Dielectric spectroscopy of bidisperse colloidal suspensions

Dielectric spectroscopy is used to measure the complex permittivity of bidisperse colloidal suspensions over the frequency range 2.5 kHz ⩽ ω/2π ⩽ 10 MHz using the spectrometer design of Hollingsworth and Saville (A.D. Hollingsworth, D.A. Saville, J. Colloid Interface Sci., 2003). Dielectric spectra of monodisperse polystyrene spheres of two diameters (530 nm and 1 μm) are fit to electrokinetic theory using the surface charge density as an adjustable parameter. Quantitative agreement is found in the dielectric increment and also for the conductivity increment, after considering the effect of added counterions and nonspecific adsorption. Bidisperse suspension spectra are a linear superposition of each particle’s dielectric response. The results provide a simple method to extend standard electrokinetic theory based on a single particle size to dilute suspensions with many particle sizes and verify the sensitivity of the spectrometer.

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► Dielectric spectroscopy measurements of bidisperse polystyrene suspensions.
► Monodisperse suspension spectra agree with standard electrokinetic theory.
► Bidisperse spectra is a linear superposition of monodisperse suspension spectra.
► Validates and extends electrokinetic model calculations to polydisperse suspensions.