From an exhaustive examination of the molecular dynamics in practically all van der Waals molecular glass formers ever probed by dielectric spectroscopy, we found that the width of the $\alpha$-loss peak at or near the glass transition temperature $T_g$ is strongly anticorrelated with the polarity of the molecule. The larger the dielectric relaxation strength $\mathrm{\Delta }\epsilon (T_g)$ of the system, the narrower is the $\alpha$-loss peak. This remarkable property is explained by the contribution from the dipole-dipole interaction potential $V_{dd}(r)=-D{r}^{-6}$ to the attractive part of the intermolecular potential, making the resultant potential more harmonic, and the effect increases rapidly with the dipole moment $\mu$ and $\mathrm{\Delta }\epsilon (T_g)$ in view of the relation, $D\propto ({\mu }^4/k{T}_g)\propto k{T}_g{[\mathrm{\Delta }\epsilon (T_g)]}^2$. Since the novel correlation discovered encompasses practically all van der Waals molecular glass formers studied by dielectric spectroscopy, it impacts the large dielectric research community as well as those engaged in solving the glass transition problem.

Low physical stability is the main reason limiting the widespread use of amorphous pharmaceuticals. One approach to overcome this problem is to mix these drugs with various excipients. In this study coamorphous drug–drug compositions of different molar ratios of ezetimib and indapamid (i.e., EZB 10:1 IDP, EZB 5:1 IDP, EZB 2:1 IDP, EZB 1:1 IDP and EZB 1:2 IDP) were prepared and investigated using differential scanning calorimetry (DSC), broadband dielectric spectroscopy (BDS), and X-ray diffraction (XRD). Our studies have shown that the easily recrystallizing ezetimib drug can be significantly stabilized in its amorphous form by using even a small amount of indapamid (8.8 wt %). DSC experiments indicate that the glass transition temperature (T_g) of the tested mixtures changes with the drug concentration in accordance with the Gordon–Taylor equation. We also investigated the effect of indapamid on the molecular dynamics of the ezetimib. As a result it was found that, with increasing indapamid content, the molecular mobility of the binary drug–drug system is slowed down. Finally, using the XRD technique we examined the long-term physical stability of the investigated binary systems stored at room temperature. These measurements prove that low-molecular-weight compounds are able to significantly improve the physical stability of amorphous APIs.

A detailed investigation on the molecular dynamics of ionic deep eutectic solvents (acetamide + lithium nitrate/sodium thiocyanate) is reported. The study was carried out employing dielectric relaxation spectroscopy covering seven decades in frequency (10⁻¹-10⁶ Hz) and in a wide temperature range from 373 K down to 173 K, accessing the dynamic observables both in liquid and glassy state. The dielectric response of the ionic system has been presented in the dynamic window of modulus formalism to understand the conductivity relaxation and its possible connection to the origin of localized motion. Two secondary relaxation processes appear below glass transition temperature. Our findings provide suitable interpretation on the nature of secondary Johari-Goldstein process describing the ion translation and orientation of dipoles in a combined approach using Ngai’s coupling model. A nearly constant loss feature is witnessed at shorter times/lower temperatures. We also discuss the ac conductivity scaling behavior using Summerfield approach and random free energy barrier model which establish the time-temperature superposition principle. These experimental observations have fundamental importance on theoretical elucidation of the conductivity relaxation and glass transition phenomena in molten ionic conductors.