Surface characterization of clay particles via dielectric spectroscopy

Abstract

This work deals with the high frequency dielectric relaxation of clay (sodium montmorillonite, or NaMt) suspensions. By high frequency it is meant that the permittivity will be determined in the region where the Maxwell-Wagner-O’Konski relaxation takes place, roughly, the MHz frequency range. The applicability of dielectric determinations for the characterization of the electrical properties of these complex systems is demonstrated. In fact, standard electrophoresis measurements only allow to detect that the charge of the particles becomes slightly more negative upon increasing pH. Much more information is obtained from the high frequency electric permittivity for different concentrations of solids and pHs. From the characteristic frequencies of the relaxation it is possible to detect separate processes for parallel and perpendicular orientations of the clay platelets, modelled as oblate spheroids with a high aspect ratio. In addition, using a suitable model the surface conductivity of the clay particles can be estimated. Our data indicate that this quantity is minimum around pH 7, which is admitted as representative of the isoelectric point of the edges of the clay platelets. Data are also obtained on the amplitude (value of the relative permittivity at low frequency minus that at high frequency) of the relaxation, and it is found that it depends linearly on the volume fraction of solids, and that it is minimum at pH 5. These results are considered to be a manifestation of the fact that particle interactions do not affect the electric conduction inside the electric double layer, while the special behaviour at pH 5 is related to the existence of aggregates at pH 5, which increase the effective size of the particles and provoke a reduction of their effective conductivity.