Nonlinear Dielectric Response of Polar Liquids

Abstract

The linear dielectric constant of a polar molecular material is mostly the function of the molecular dipole moment and of the binary correlations between the dipoles. The dielectric response becomes nonlinear for a sufficiently strong electric field gaining a dielectric decrement proportional, in the lowest order, to the squared field magnitude. The alteration of the dielectric response with the electric field is governed by a combination of binary and three- and four-particle dipolar correlations and thus provides new structural information absent in the linear response. Similar higher order correlations between the molecular dipoles enter the temperature derivative of the linear dielectric constant. Mean-field models, often applied to construct theories of linear dielectric response, fail to account for these multi-particle correlations and do not provide an adequate description of the nonlinear dielectric effect. Perturbation theories of polar liquids offer a potential resolution. They have shown promise in describing the elevation of the glass transition temperature by an external electric field. The application of such models reveals a fundamental distinction in polarization of low-temperature glass formers close to the glass transition and high-temperature, low-viscous liquids. The dielectric response of the former is close to the prescription of Maxwell’s electrostatics where surface charge is created at any dielectric interface. On the contrary, rotations of interfacial dipoles are allowed in high-temperature liquids, and they effectively average the surface charge out to zero. Models capturing this essential physics will be required for the theoretical description of the nonlinear dielectric effect in these two types of polar materials.