The dielectric properties of charged nanoparticle colloids at radio and microwave frequencies

Abstract

Charged particle colloids typically consist of particles with negative surface charge suspended in an aqueous electrolyte solution. A layer of ions forms around each particle, and previous research has shown that the dynamics of this charge distribution, induced by an external field, lead to effective and potentially useful dielectric properties (Lyklema J and Van Leeuwen H P 1999 Adv. Colloid and Interface Sci. 83 33, O'Konski C T 1960 J. Phys. Chem. 64 605). Charged nanoparticle colloids are potentially of great benefit because they are extremely stable and their small dimensions make them highly versatile. Possible uses include electromagnetic shielding devices and medical applications. This research focuses on the dielectric properties of charged polystyrene nanoparticle colloids, which are investigated across a range of radio and microwave frequencies. The results show two distinct regions in the permittivity behaviour. The first is the dipolar relaxation of water at microwave frequencies. The second is a high amplitude, low frequency relaxation due to the diffusive motion of ions in the bulk electrolyte beyond the double layer. Effective control of the resulting dielectric response is demonstrated with particle size ranging from 20 to 220 nm. It is shown that the small nanoparticle sizes lead to faster relaxation times than those expected from conventional micron particle sizes, such that the loss extends to microwave frequencies. The results are fitted to a model where the zeta potential, ξ, and double layer thickness, κ−1, are used as fitting parameters.