Search

The low frequency dielectric response of aqueous solutions containing 0, 1, 5, and 10% w/v polyvinylpyrrolidone (PVP) was studied to characterize the low temperature relaxation behavior of these systems. Complementary modulated temperature differential scanning calorimetry (MTDSC) studies allowed measurement of the glass transition temperature for these materials, corresponding to the behavior of the nonfrozen phase. Dielectric investigations in the frequency range of 10 6 to 10 −2 Hz were performed on the systems in the liquid state, with a Maxwell–Wagner response noted for both the PVP solutions and water. The solid-phase responses were studied over a range of temperatures down to − 70°C, with a relaxation peak observed for the PVP systems in the kilohertz region. The spectra were modeled using the Havriliak–Negami equation and the corresponding relaxation times were calculated, with a satisfactory fit to the Arrhenius equation noted. The calculated activation energies were similar to literature values for the dielectric relaxation of water. It is suggested that the dielectric response is primarily a reflection of the relaxation behavior of the water molecules in the nonfrozen fraction, thereby indicating that the dielectric technique may yield insights into specific components of frozen aqueous systems. © 2001 Wiley-Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 90:157–164, 2001

Thermotropic phase transitions of glyceryl monoolein–water systems have been investigated using dielectric spectroscopy over a frequency range of 10−2–106 Hz and a temperature range of 15–100°C, and also using differential interference contrast (DIC) microscopy fitted with a hot stage as a support technique. The frequency and temperature dependencies of the dielectric permittivity of systems undergoing transitions between lamellar, cubic, reverse hexagonal and L2 phases are reported. It was demonstrated that the frequency dependent response is highly sensitive to changes in phase and a model is described which allows the dielectric behaviour to be interpreted in terms of an equivalent circuit diagram, thereby allowing the response to be related to specific structural features within the sample. Similarly, it was demonstrated that the phase transition temperatures and ranges could be identified using the dielectric technique, yielding results which were in good agreement with those obtained using DIC microscopy for those systems whereby the phase changes are optically identifiable. The study therefore suggests that low frequency dielectric analysis is a potentially highly useful tool for the identification of thermotropic phase changes.