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.

Relaxation dynamics of verapamil hydrochloride (VH), which is a representative of ionic liquids, was studied under isobaric and isothermal conditions by using dielectric spectroscopy. In addition we also carried out pressure-temperature-volume $\left(PVT\right)$ measurements. The obtained data enable us to examine the structural $\alpha$ -relaxation time ${\tau }_{\alpha }$ as a function of temperature, pressure, and volume. Since the examined sample is a typical ionically conducting material, we employed the dielectric modulus formalism to gain information about $\alpha$ -relaxation process. It was found that application of pressure changes the shape of the modulus spectrum. The $\alpha$ -peak becomes narrower with compression. Consequently, it was also shown that the stretching parameter ${\beta }_{\text{KWW}}$ increases with pressure. Based on experimental data both the isobaric fragility $\left(m_p\right)$ at various pressures and isothermal fragility $\left(m_T\right)$ at various temperatures were calculated. Analyzing the effect of pressure on the dependences ${\tau }_{\alpha }\left(T\right)$ as well as on the shape parameter of the $\alpha$ -peak it was found that a phenomenological correlation between $m_p$ and ${\beta }_{\text{KWW}}$ established for glass forming liquids is also valid for VH under condition of high compression. The pressure dependences of glass-transition temperature determined from dielectric and volumetric measurements have been compared. Moreover, $PVT$ data allow us to assess the relative contribution of thermal energy and free volume fluctuation to the dramatic slowing down of the molecular dynamics in the vicinity of $T_g$ . It is established from the ratio of the isochronic and isobaric expansivities that the thermal energy has a stronger effect on the relaxation times than the free volume, although the latter contribution is significant. Finally, we also discuss the validity of thermodynamic scaling in the case of VH and examine the scaling exponent $\gamma$ at various thermodynamic conditions.