Broadband-dielectric-spectroscopy study of molecular dynamics in a mixture of itraconazole and glycerol in glassy, smectic-$A$, and isotropic phases

Itraconazole (ITZ) is a thermotropic liquid crystal that exhibits isotropic, nematic, and smectic phases on cooling towards the glass transition upon melting. Over the years, new aspects regarding the liquid-crystalline ordering of this antifungal drug were systematically revealed. It has been shown recently that the temperature range of individual mesophases in ITZ can be modified by adding a small amount of glycerol (GLY). Moreover, above the critical concentration of 5% w/w, a smectic to nematic transition can be avoided. Here we go one step further, and we used broadband dielectric spectroscopy to investigate the new phase behavior of the ITZ-GLY mixture (5% w/w). To confirm the phase transformations of the ITZ-GLY mixture, differential scanning calorimetry was also employed. The analysis of molecular dynamics of the ITZ-GLY mixture in the glassy and isotropic phases revealed features similar to those observed for neat ITZ. Two relaxation processes were identified in the smectic-A phase, with similar temperature dependence, most likely related to the fast rotations around the long axis of a molecule. Additionally, the derivative analysis revealed another low-frequency process hidden under DC conductivity ascribed to the slow rotations about a short axis. We will show that the differences in the molecular organization in the smectic-A and isotropic phases leave a clear fingerprint on the temperature behavior of relaxation times and other dielectric parameters, such as DC conductivity and dielectric strength, for which a pretransition effect has been detected.

Figure 1

(a) The comparison of DSC heating scans for neat ITZ and ITZ-GLY 5% w/w. (b) Phase diagram of ITZ-GLY mixture constructed from DSC data digitalized from [16] (depicted as black symbols). Our results for the ITZ-GLY sample with 5% w/w addition of GLY are shown as red symbols.

Figure 2

(a) Isotropic-to-smectic-$A$ transition observed under POM for ITZ-GLY mixture 5% w/w at $T=350\mathrm{K}$. (b) Smectic-A phase with a characteristic fan-shaped (focal-conic) texture viewed by POM for ITZ-GLY mixture 5% w/w at $T=348\mathrm{K}$ and neat ITZ (c) at $T=340\mathrm{K}$.

Figure 3

Infrared spectra of GLY (red), ITZ (black), and ITZ-GLY mixture with 5% of GLY (blue) being measured at (upper) 293 K and (bottom) 358 K. Data are presented in two spectral regions: (left) $3750–2400\mathrm{c}{\mathrm{m}}^{–1}$ and (right) $1800–400\mathrm{c}{\mathrm{m}}^{-1}$.

Figure 4

Temperature-dependent FTIR spectra of (a) ITZ and (b) ITZ-GLY mixture with 5% of GLY in the frequency range of $600–430\mathrm{c}{\mathrm{m}}^{–1}$. The spectra were shown on a common scale, offset for clarity.

Figure 5

Dielectric loss (a) and dispersion (b) spectra for the ITR-GLY mixture (5% GLY w/w) in the temperature ranges from 173 to 323 K (with $\mathrm{\Delta }T=10\mathrm{K}$) and from 325 to 413 K $\left(\mathrm{\Delta }T=2\mathrm{K}\right)$. The bottom panel shows the variation of the real (d) and imaginary (c) parts of dielectric permittivity with a frequency in the selected temperatures near the smectic-A to isotropic phase transition.

Figure 6

Comparison of different fitting strategies to dielectric loss spectra for ITZ-GLY sample (5% GLY w/w) at $T=345\mathrm{K}$ (smectic phase) and $T=353\mathrm{K}$ (isotropic phase). The upper panels show the analysis results with a single HN function, and the bottom panels with a combination of the D and HN functions. In both cases, the fitting function included the DC-conductivity term. Symbols denote experimental data, solid lines denote resultant fit, and dashed lines denote individual components of the fit.

Figure 7

Temperature dependence of the relaxation times determined for relaxation processes in glassy, smectic-A (SmA), and isotropic phases of ITZ-GL mixture with 5% of GLY. The solid and dashed lines show the fit of the VFT and Arrhenius fit functions, respectively. The temperature regions corresponding to the particular phases are distinguished according to DSC measurements.

Figure 8

The comparison of dielectric loss spectra for ITZ and ITZ-GLY mixture with 5% of GLY at the isochronic condition (for the same ${\tau }_{\alpha }$). The spectra were normalized to the maximum of the α peak. The DC conductivity has been subtracted.

Figure 9

The comparison of dielectric loss spectra measured for ITZ and ITZ-GLY mixture at $T=343\mathrm{K}$ corresponding to the smectic-A phase. The spectra were normalized to the maximum of the α peak. The DC conductivity has been subtracted. The position of low-frequency modes for ITZ and ITZ-GLY is indicated as δ and ${\alpha }^{\prime }$, respectively. The inset shows (a) the comparison of dielectric loss spectra before DC-conductivity subtraction and (b) calculated loss spectra for temperatures close to smectic-A to isotropic transition; instead measuring ε′′ ($f$) the ${\varepsilon }_{\mathrm{der}}^{\prime \prime }\left(f\right)$ data are presented where conductivity contribution is eliminated.

Figure 10

Relaxation map for ITZ and ITZ-GLY mixtures with different GLY content at temperatures corresponding to smectic-A, nematic, and isotropic phases. (a) Results of the linearized derivative-based analysis of the temperature dependence of structural relaxation time for mixture with 1% of GLY. Arrows denote temperatures at which endothermic LC transitions were observed in DSC measurements.

Figure 11

The temperature evolution of (a) DC conductivity, ${\sigma }_{\mathrm{DC}}$, (b) dielectric strength, Δε, characterizing α relaxation, and (c) shape parameters of the HN function for α relaxation observed for ITZ-GLY mixtures with different GLY content.