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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

A series of ternary gel systems based on cetostearyl alcohol (CSA) and cetomacrogol 1000 or sodium lauryl sulfate have been studied using oscillatory rheology, differential interference contrast (DIC) microscopy, cryoscanning electron microscopy (cryo-SEM), and low-frequency dielectric analysis in order to elucidate the nature of the lamellar structures formed in relation to composition. The effects of altering the concentration of CSA (0.25% to 8% w/w) for 1% and 2% w/v cetomacrogol 1000 and 0.5% and 1% w/v sodium lauryl sulfate systems have been investigated, with marked increases in the storage and loss moduli seen on increasing the concentration of CSA for both surfactants. DIC microscopy indicated that at low CSA concentrations, needlelike structures were seen which, on increasing the concentration, were observed to congregate into nuclei. At concentrations of 4% CSA and above, neospherical structures were also observed. Cryo-SEM revealed that the needlelike objects were sheet structures ascribed to lamellar gel phases, while the nuclei were folded "rosettes" formed by those sheets, with the spherical structures being ascribed to cetostearyl alcohol. It was also noted that the lamellae were more tightly folded at 8% w/w CSA, which may be associated with the higher rheological moduli for these systems. Low-frequency dielectric analysis was performed over a frequency range of 10(4) Hz to 10(-2) Hz. A decrease in both the dielectric loss and capacitance was observed as the concentration of cetostearyl alcohol was increased. The dielectric data were described in terms of an equivalent circuit model based on a modified Maxwell-Wagner response. A good correlation was found between the fitted and experimental data and the effect of altering the gel composition on specific features of the equivalent circuit are discussed.

The influence of propantheline bromide incorporation on the phase structure of glyceryl monoolein/water systems has been investigated using low-frequency dielectric spectroscopy over a frequency range of 10(-2) to 10(6) Hz at 20 degrees C. The responses of glyceryl monoolein systems composed of 10% and 30% w/w were measured and the spectra modeled using an equivalent circuit based on the Maxwell-Wagner theory.1,2 Marked changes in the dielectric responses of the systems were noted on addition of the propantheline bromide at concentrations up to 10% w/w. For the lamellar (10% w/w water) glyceryl monoolein systems, an increase in the imaginary permitivity was seen, corresponding to an increase in conductivity due to the presence of additional ionic species within the system. Evidence was also obtained for the incorporation of the drug directly into the lipid bilayers, particularly at higher concentrations (10% drug) at which dielectric behavior corresponding to bilayer disruption was seen. Incorporation of 3% and 5% w/w drug into the cubic phase systems (30% w/w water) resulted in a change to the lamellar phase. However, circuit modeling indicated that the system formed structures which showed features of both the lamellar and cubic phases at 3% w/w drug loadings. The study has therefore demonstrated that dielectric analysis may provide a novel means of studying the effects of drug incorporation on the phase behavior of complex gel systems.

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.

A series of glyceryl monoolein/water liquid crystal systems containing 15%, 20%, 30%, and 35% w/w water have been investigated using dielectric spectroscopy over a frequency range of 10-3−106 Hz and a temperature range of 20−90 °C in order to facilitate association of the lyotropic and thermotropic phase behavior with the dielectric response. The phase boundaries of the mixes were determined independently using hot stage microscopy. The frequency-dependent dielectric spectra of systems containing lamellar, cubic, and hexagonal phases are reported and have been modeled using circuits corresponding to generalized Maxwell−Wagner responses. More specifically, the systems were found to correspond to a parallel bulk capacitance and conductance in series with a Maxwell−Wagner blocking capacitance, which was associated with an adsorbed layer at the electrode surfaces. An additional dispersion was observed corresponding to a series RC circuit, which was associated with charge transport across the lipid bilayers. I...

The objective of the study was to investigate the use of low frequency dielectric spectroscopy as a means of characterizing the thermal transitions of an amorphous drug substance, indometacin, with particular emphasis on modelling the response using the Dissado-Hill function. The low frequency dielectric behaviour of indometacin was measured over a temperature range of 10–160°C and a frequency range of 10−3-106 Hz. Modulated temperature differential scanning calorimetry (MTDSC) studies were also performed on equivalent samples, showing a glass transition, recrystallization and melting. Isothermal low frequency dielectric spectra of the sample at temperatures below recrystallization showed the dynamic dielectric relaxation associated with the amorphous phase, while changes in the real and imaginary permittivities were observed that were associated with recrystallization and subsequent melting. A small discontinuity was observed immediately above the recrystallization process in the MTDSC and dielectric data, suggested to correspond to a solid state transformation. The use of the Dissado-Hill function as a means of modelling the dielectric behaviour has also been described. The study suggests that low frequency dielectric spectroscopy, used in conjunction with MTDSC and Dissado-Hill modelling, maybe a useful tool for the characterization of amorphous and crystalline drugs.

A range of creams based on Aqueous Cream BP have been analyzed using low-frequency dielectric spectroscopy, with accompanying circuit modeling in combination with rheological and microscopic supportive techniques, to explore the use of the dielectric approach as a novel means of characterizing cream systems. Creams based on the formula for Aqueous Cream BP were produced by hand-mixing and mechanical mixing, with and without the inclusion of the preservative phenoxyethanol. Dielectric analysis was performed over a frequency range of 10(-2)-10(5) Hz. Cream samples were also examined using stress scan rheology and differential interference contrast microscopy. Dielectric analysis indicated that the presence of preservative decreased the capacitance and loss of the creams. The responses were modeled in terms of a dispersive capacitance in series with two RC circuits (series and parallel). Rheological studies indicated higher viscosities for the hand-mixed and unpreserved systems. Differential interference contrast microscopy showed marked differences in the distribution of the oil droplets, depending on the method of mixing. The study has demonstrated that dielectric spectroscopy, with accompanying circuit analysis, may be used as a means of modeling the structure of cream systems. The investigation has also shown that the formulation and preparation method of Aqueous Cream BP may have a profound effect on sample structure.