Dielectric and conductometric properties of highly heterogeneous colloidal systems

Abstract

This review focuses on the behavior of highly heterogeneous colloidal systems under the influence of an external, frequency-dependent, electric field. Heterogeneous systems, which lie between bulk and molecularly dispersed systems, consist, in a simple meaning of the word, of a dispersed phase distributed uniformly in a finely divided state in a dispersion medium and represent a class of materials characterized by an extremely large surface-to-volume ratio. In these systems, the nanoscopic or mesoscopic size of the components leads to a situation where their original properties can be only partially important, or may be even completely lost, and, on the contrary, where the effects associated to the interface dominate. Moreover, at least in the most relevant systems both from a theoretical and practical point of view, the interfaces are often charged. This makes dielectric spectroscopy methods, which probe the charge distribution in the different regions of the system, particularly suitable to investigate the electrical properties of this class of materials. We shall limit ourselves to a discussion of relatively simple heterogeneous systems, such as polymeric colloidal particles dispersed in an electrolyte solution, biological cell suspensions and, finally, microemulsions. Each of these systems represents a family of colloids used as an example to outline some of their most important characteristics and to evidentiate a number of phenomena that are directly linked to the presence of a huge surfaces. For each of the above-stated systems, we will review the complex phenomenology they present, giving particular emphasis to the effects totally or partially induced by the presence of a large interface.