Low-Frequency Dielectric Investigations into the Phase Behavior of Glyceryl Monoolein/Water Systems

Abstract

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 -10 6 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. It is proposed that the dielectric responses of the lamellar, cubic, and hexagonal phases exhibited by these systems may be described in terms of equivalent circuit models, which may in turn be related to specific structural features of the sample.

Rheological and Dielectric Characterization of Monoolein/Water Mesophases in the Presence of a Peptide Drug

“…On increasing the temperature, both the frequency and temperature sweeps indicated a dramatic decrease in moduli corresponding to the transition to the reverse hexagonal phase system. The dielectric data for the unloaded systems was in good agreement with previous studies 19,20 in that the charge motility decreased for the cubic phase system in comparison with the lamellar systems. However, there was a discrepancy noted between the rheological and dielectric data in terms of the 22% w/w systems at 208C; the rheological studies indicated an increase in moduli compared to the 10% w/w systems which is consistent with the expected behavior of the cubic as opposed to lamellar phase.…”

“…5c) is characterized by a high frequency bulk process with a greater imaginary component (higher conductance) than that of the lamellar phase and a sub-Hz barrier process due to the presence of an adsorbed layer at electrode. 19 As already seen for the 22% water sample, systems analyzed at 20 and 378C showed similar behavior at both temperatures and characteristic features of the cubic phase. As the frequency decreased to sub-Hz values both loss and real components approached a common permittivity value, corresponding to the low frequency dispersion caused by barrier layers on the electrodes.…”

“…The dielectric behavior of the system containing 22% w/w water is shown in Figure 5b. At 20 and 378C, the spectrum shape is different from that obtained with the system containing the 10% of water, exhibiting behavior more typical of a cubic structure as observed in previous work; 19,20 in this respect there does appear to be a discrepancy between the rheological and dielectric data in that the former indicated the system to be in the lamellar phase at 208C. The response of the cubic phase consisted of a high frequency bulk process, as observed for the lamellar phase, together with an inverse frequencydependent dielectric loss component (e 00 a (o) À1 ) over a frequency range of approximately six decades (1-10 6 Hz).…”

“…Three sets of gel sample containing 10%, 22%, and 30% (w/w) of water were prepared according to a previously presented procedure. 19 A weighed amount of Monomuls 90018 was heated in a water bath at 608C, then water added at the same temperature to give the correct (w/w) gel composition. The system was stirred at 2000 rpm using a mechanical Heidolph mixer.…”

“…12,17,18 Our previous work examined the use of dielectric analysis as a novel means of studying monoolein mesophases. 14,19,20 The aim of the present study is twofold. Firstly, we present a rheological study of the monoolein systems in conjunction with dielectric analysis as a means of further characterizing the properties and behavior of these gel systems, and secondly we examine the structural effect of adding a model peptide, cyclosporin A (Cyc A), to the monoolein gels.…”