Probing of structural relaxation times in the glassy state of sucrose and trehalose based on dynamical properties of two secondary relaxation processes

Influence of High Pressure on the Local Order and Dynamical Properties of the Selected Azole Antifungals

Dielectric studies under various temperature (T) and pressure (p) conditions on five active pharmaceutical ingredients (APIs) with antifungal properties-itraconazole (ITZ), posaconazole (POS), terconazole (TER), ketoconazole (KET), and fluconazole (FLU)-were carried out. We have thoroughly studied the connection between the pressure coefficient of the glass transition temperature (dT_g/dp) and the activation volume of both relaxation modes (ΔV_α, ΔV_δ/α') with respect to the molecular weight (M_w) or molar volume (V_m) in these systems. Besides, high pressure data revealed that the time scale separation between α- and δ- or α'-processes increases with pressure in ITZ and TER. What is more, the activation entropy, which is a measure of cooperativity, calculated from the Eyring model for the secondary (β)-relaxation in ITZ and POS, increased and decreased, respectively, in the compressed samples. To understand these peculiar results, we have carried out X-ray diffraction (XRD) measurements on the pressure-densified glasses and found that pressure may induce frustration in molecular organization and destroy the medium-range order while enhancing the short-range correlations between molecules. This finding allowed us to conclude that varying molecular spatial arrangement is responsible for the extraordinary dynamical behavior of ITZ, POS, and TER at high pressure.

Influence of the Internal Structure and Intermolecular Interactions on the Correlation between Structural (α) and Secondary (β-JG) Relaxation below the Glass Transition Temperature in Neat Probucol and Its Binary Mixtures with Modified Saccharides

Broadband dielectric spectroscopy (BDS) has been used to study the molecular dynamics and aging process in neat probucol (PRO) as well as its binary mixtures with selected acetylated saccharides. In particular, we applied the Casalini and Roland approach to determine structural relaxation times in the glassy state of the examined systems (so-called isostructural times, τ_iso). Next, using the calculated τ_iso, primitive relaxation times of the coupling model were obtained and compared to the experimental secondary β (Johari-Goldstein (JG) type) relaxation times. Interestingly, it turned out that there is a correlation between the β-JG and the structural (α)-relaxation processes below the glass transition temperature (T < T_g) in each investigated sample. This is a new observation compared to previous studies demonstrating that such a relationship exists only in the supercooled liquid state of neat PRO. Moreover, it was revealed that the stretching parameters obtained from the aging procedure are very close to the ones determined by fitting the dielectric data above the T_g with the use of the Kohlrausch-Williams-Watts function, indicating that the aging process is governed by the α-relaxation. Complementary Fourier transform infrared and X-ray diffraction measurements allowed us to find a possible reason for these findings. It was demonstrated that although there are very weak intermolecular interactions between PRO and modified saccharides, the intra- and intermolecular structure of PRO is practically unaffected by the presence of modified saccharides.

Distinguishing different classes of secondary relaxations from vapour deposited ultrastable glasses

Secondary relaxations persistent in the glassy state after structural arrest are especially relevant for the properties of the glass. A major thrust in research in dynamics of glass-forming liquids is to identify what secondary relaxations exhibit a connection to the structural relaxation and are hence more relevant. Via the Coupling Model, secondary relaxations having such connection have been identified by properties similar to the primitive relaxation of the Coupling Model and are called the Johari-Goldstein (JG) β-relaxations. They involve the motion of the entire molecule and act as the precursor of the structural α-relaxation. The change in dynamics of the secondary relaxation by aging an ordinary glass is one way to understand the connection between the two relaxations, but the results are often equivocal. Ultrastable glasses, formed by physical vapour deposition, exhibit density and enthalpy levels comparable to ordinary glasses aged for thousands of years, as well as some particular molecular arrangement. Thus, ultrastable glasses enable the monitoring of the evolution of secondary processes in case aging does not provide any definitive information. Here, we study the secondary relaxation of several ultrastable glasses to identify different types of secondary relaxations from their different relationship with the structural relaxation. We show the existence of two clearly differentiated groups of relaxations: those becoming slower in the ultrastable state and those becoming faster, with respect to the ordinary unaged glass. We propose ultrastability as a way to distinguish between secondary processes arising from the particular microstructure of the system and those connected in properties to and acting as the precursor of the structural relaxation in the sense of the Coupling Model.

A study on the progress of mutarotation above and below the Tg and the relationship between constant rates and structural relaxation times

Comprehensive FTIR studies on the progress of mutarotation in d-fructose mixed with maltitol have been carried out over a wide range of temperatures, both above and below the glass transition temperature T_g. In addition to the analysis of single bands, we have developed a completely new approach considering the full spectral range to follow the overall progress of the reaction. We have found that at the calorimetric T_g, there is a clear change in the temperature dependence of constant rates. The activation barrier for mutarotation changes from around 59 kJ mol^-1 (the supercooled state) to around 249 kJ mol^-1 (the glassy state). This dramatic variation in the activation barrier is consistent with the change in the mechanism of this specific chemical conversion, as theoretically considered by Wlodarczyk et al. [Phys. Chem. Chem. Phys., 2014, 16, 4694-4698]. Alternatively, it can also be connected to the change in the viscosity of the sample. Additionally, we investigated the relationship between constant rates (k) of mutarotation, structural relaxation times (τ_α), and dc conductivity (σ_dc) above and below the glass transition temperature. It was found that there was a linear correlation between all these quantities; they scale with various exponents changing at T_g. Our results also indicate that a single activation barrier might not be sufficient to describe the mutarotation process.

Recent developments in the experimental investigations of relaxations in pharmaceuticals by dielectric techniques at ambient and elevated pressure

In recent years, there is a growing interest in improving the physicochemical stability of amorphous pharmaceutical solids due to their very promising applications to manufacture medicines characterized by a better water solubility, and consequently by a higher dissolution rate than those of their crystalline counterparts. In this review article, we show that the molecular mobility investigated both in the supercooled liquid and glassy states is the crucial factor required to understand molecular mechanisms that govern the physical stability of amorphous drugs. We demonstrate that pharmaceuticals can be thoroughly examined by means of the broadband dielectric spectroscopy, which is a very useful experimental technique to explore different relaxation processes and crystallization kinetics as well. Such studies conducted in the wide temperature and pressure ranges provide data needed in searching correlations between properties of molecular dynamics and crystallization process, which are aimed at developing effective and efficient methods for stabilizing amorphous drugs.

Impact of water on molecular dynamics of amorphous α-, β-, and γ-cyclodextrins studied by dielectric spectroscopy

Dielectric, calorimetric, and x-ray diffraction measurements were carried out on α-, β-, and γ-cyclodextrins, which are cyclic saccharides built by, respectively, six, seven, and eight glucose units connected via glycosidic linkage. Differential scanning calorimetry measurements indicated that each carbohydrate has a melting temperature located much above the temperature at which thermal decomposition begins. Moreover, calorimetric data revealed that it is possible to completely dehydrate each cyclodextrin by annealing them above 413 K. Unfortunately, it is impossible to obtain amorphous forms of cyclodextrin by simple cooling of the melt. Thus, a solid state amorphization method has been applied. X-ray diffraction studies demonstrated that by ball milling at room temperature we are able to obtain completely amorphous cyclodextrins. Finally, dielectric measurements were carried out to probe molecular dynamics in the amorphous state of cyclodextrins. It was found that there is only one relaxation process in amorphous hydrated cyclodextrins, while in dried samples two secondary relaxations are present. Moreover, we have shown that water has an enormous effect on the dynamics of both relaxation modes, i.e., with increasing content of water, the activation energy of the slow mode decreases, while that evaluated for the fast mode increases. We were not able to follow the dynamics of the structural relaxation process, because glass transition temperatures of amorphous cyclodextrins were found to lie above thermal degradation points.

Quantifying the Structural Dynamics of Pharmaceuticals in the Glassy State

Structural dynamics in the glassy state of two protic ionic liquids, carvedilol phosphate and procaine hydrochloride, were characterized from analysis of changes in the conductivity relaxation times during physical aging. The obtained relaxation times, having a magnitude exceeding feasible experimental time scales and thus not directly measurable, are consistent with published data from a method that relies on the presence of a secondary relaxation. We also observe a narrowing of the relaxation dispersion, specific to higher frequencies, that is a consequence of the heterogeneous dynamics of deeply supercooled materials.