DIELECTRIC-RELAXATION SPECTROSCOPY AND SOME APPLICATIONS IN THE PHARMACEUTICAL SCIENCES

With a few exceptions, dielectric relaxation spectroscopy (DRS) has be en largely neglected by pharmaceutical scientists, despite the potenti al for this technique as a noninvasive and rapid method for the struct ural characterization and quality control of pharmaceutical materials. DRS determines both the magnitude and time dependency of electrical p olarization (i.e. the separation of localized charge distributions) by either measuring the ability of the material to pass alternating curr ent (frequency domain DRS) or by investigating the current that flows on application of a step voltage (time domain DRS). DRS is thus (i) se nsitive to molecular mobility and structure, (ii) non-invasive, and (i ii) employs only mild stresses (a weak electromagnetic field) in order to measure the sample properties. The technique covers a broad-band f requency window (from 10(-5) to 10(11) Hz) and therefore enables the i nvestigation of a diverse range of processes, from slow and hindered m acromolecular vibrations and restricted charge transfer processes (suc h as proton conductivity in nearly dry systems) to the relatively fast reorientations of small molecules or side chain groups. The dielectri c response provides information on (i) structural characteristics of p olymers, gels, proteins, and emulsions, (ii) the interfacial propertie s of molecular films, (iii) membrane properties, (iv) water content an d states of water (and the effects of water as a plasticizer), and (v) lyophilization of biomolecules. This review article details the basis of dielectric theory and the principles of measuring dielectric prope rties (including a comprehensive account of measurement artifacts), an d gives some applications of DRS to the pharmaceutical sciences.