High pressure dielectric studies on the structural and orientational glass

High-pressure studies in the supercooled and glassy state of the strongly associated active pharmaceutical ingredient-ticagrelor

In this paper, the molecular dynamics at different thermodynamic conditions of hydrogen-bonded (H-bonded) active pharmaceutical ingredient-ticagrelor (TICA) have been investigated. Extensive high-pressure (HP) dielectric studies revealed surprising high sensitivity of the structural (α)-relaxation to compression. They also showed that unexpectedly the shape of the α-peak remains invariable at various temperature (T) and pressure (p) conditions at constant α-relaxation time. Further infrared measurements on the ordinary and pressure densified glasses of the examined compound indicated that the hydrogen-bonding pattern in TICA is unchanged by the applied experimental conditions. Such behavior was in contrast to that observed recently for ritonavir (where the organization of hydrogen bonds varied at high p) and explained the lack of changes in the width of α-dispersion with compression. Moreover, HP dielectric measurements performed in the glassy state of TICA revealed the high sensitivity of the slow secondary (β)-relaxation (Johari-Goldstein type) to pressure and fulfillment of the isochronal superpositioning of α- and JG-β-relaxation times. Additionally, it was found that the activation entropy for the β-process, estimated from the Eyring equation (a high positive value at 0.1 MPa) slightly increases with compression. We suggested that the reason for that are probably small conformational variations of TICA molecules at elevated p.

Studies on the nature and pressure evolution of phase transitions in 1-adamantylamine and 1-adamantanol

In this paper, several experimental techniques, i.e., differential scanning calorimetry, X-ray diffraction, Fourier transform infrared, Raman, and broadband dielectric spectroscopy were applied to study the nature of the phase transitions in 1-adamantylamine (1-NH₂-ADM, C₁₀H₁₇N) and 1-adamantanol (1-OH-ADM, C₁₀H₁₆O). Calorimetric measurements showed one and three endothermic peaks in thermograms for the latter and the former substance, respectively. Indeed, results of spectroscopic investigations indicated that the observed thermal events in 1-NH₂-ADM correspond to transitions between various plastic crystal (PC) phases (I, II, III, IV), while the endothermic process in 1-OH-ADM can be assigned to a phase transition between the PC and the ordinary crystal (OC). Especially interesting were the outcomes of dielectric studies carried out both at ambient and high-pressure conditions, during heating and cooling cycles. They showed: i) noticeable changes in the frequency dependencies of the imaginary (ε^'') and real (ε^') parts of the complex dielectric permittivity that occurred around temperatures of the characteristic endothermic events detected by the calorimetry, and ii) significant fluctuations of ε^'' and ε^' at pressures attributed to the respective phase transitions. Moreover, the pressure coefficients of the phase transition temperatures were estimated to be approximately equal to 0.2 K/MPa for both compounds. In turn, volume variation (ΔV) at the PC (II)-PC (III) and PC (III)-PC (IV) transition temperatures for 1-NH₂-ADM was essentially different than ΔV for the PC-OC transition in 1-OH-ADM.

How does pressure affect the molecular dynamics, intramolecular interactions, and the relationship between structural (α) and secondary (JG-β) relaxation above and below the glass transition temperature in binary mixtures of H-bonded API - probucol and acetylated saccharides?

In this paper, the molecular dynamics as well as inter- and intramolecular interactions in the homogenous solid dispersions (SDs) of active pharmaceutical ingredient - probucol (PRO) with acetylated glucose (acGLU), acetylated sucrose (acSUC), and sucrose acetoisobutyrate (aibSUC), prepared in 5:1 molar ratio, have been investigated using broadband dielectric (BD) and Fourier transform infrared (FTIR) spectroscopy. Importantly, high pressure dielectric measurements revealed that as for neat PRO, a breakdown of the isochronal structural (α) and JG-β exact superpositioning, due to increasing separation between both processes under compression, can also be detected in its mixtures with acetylated saccharides (acSACCHs). Furthermore, the analysis of temperature dependences of JG-β-relaxation times for PRO and PRO-acSACCH SDs at selected isobaric conditions indicated the increase in the cooperativity of the secondary process (reflected in the value of the activation entropy, ΔS_β) at elevated pressure in all systems. The mere addition of the small amount of excipient to neat PRO (p = 0.1 MPa) resulted in a greater value of ΔS_β (it was the most noticeable in the case of aibSUC). Further FTIR studies carried out on the pressure densified glasses of PRO, and binary mixtures suggested that the observed changes in the cooperativity of the JG-β-process, as well as the failure of the exact isochronal superpositioning of α- and JG-β relaxation times, are due to varying H-bond pattern in the examined single- and two-component systems at high compression/in the presence of saccharide.

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.

Studies on the internal medium-range ordering and high pressure dynamics in modified ibuprofens

Broadband dielectric spectroscopy (BDS), combined with the X-ray diffraction (XRD) and Fourier transform infrared (FTIR) techniques, was used to study the dynamics of the primary (α) relaxation process and slow mode (SM), as well as structural properties and intermolecular interactions, in the methyl-, isopropyl-, hexyl-, and benzyl derivative of a well-known pharmaceutical, ibuprofen (IBU). Unexpectedly, the XRD and FTIR methods revealed the formation of medium-range ordering together with some molecular organization, which probably leads to the creation of small aggregates at the scale of several microns at lower temperatures. Moreover, high pressure dielectric experiments revealed that the SM (observed in the ambient pressure data) is not detected in the loss spectra of compressed IBU esters, which is consistent with the results reported previously for propylene carbonate and dioxolane derivatives. This finding can be interpreted as connected to either the comparable time scale of the structural dynamics and slow mode or suppression of the motions responsible for the latter process at elevated pressure. Additionally, it was found that the pressure coefficient of the glass transition temperature (dTg/dp) and activation volume (ΔV) change with molecular weight (Mw) in a non-monotonic way. It might be related to various chemical structures, conformations, and intermolecular interactions, as well as different architecture of supramolecular aggregates in the investigated compounds.

Activation volume in superpressed glass-formers

In pressurized glass-forming systems, the apparent (changeable) activation volume Va(P) is the key property governing the previtreous behavior of the structural relaxation time (τ) or viscosity (η), following the Super-Barus behavior: [Formula: see text], T = const. It is usually assumed that Va(P) = V#(P), where [Formula: see text] or [Formula: see text]. This report shows that Va(P) ≪ V#(P) for P → Pg, where Pg denotes the glass pressure, and the magnitude V#(P) is coupled to the pressure steepness index (the apparent fragility). V#(P) and Va(P) coincides only for the basic Barus dynamics, where Va(P) = Va = const in the given pressure domain, or for P → 0. The simple and non-biased way of determining Va(P) and the relation for its parameterization are proposed. The derived relation resembles Murnaghan - O'Connel equation, applied in deep Earth studies. It also offers a possibility of estimating the pressure and volume at the absolute stability limit. The application of the methodology is shown for diisobutyl phthalate (DIIP, low-molecular-weight liquid), isooctyloxycyanobiphenyl (8*OCB, liquid crystal) and bisphenol A/epichlorohydrin (EPON 828, epoxy resin), respectively.

Studying structural and local dynamics in model H-bonded active ingredient - Curcumin in the supercooled and glassy states at various thermodynamic conditions

Different experimental techniques were applied to study thermal and structural properties, strength of H-bonds, possible keto-enol tautomerism and molecular dynamics at various thermodynamic conditions in the H-bonded active substance, curcumin (CRM). Dielectric measurements revealed dynamical features of examined compound that are uncharacteristic for the associated systems. This includes enormously large pressure coefficient of the glass transition temperature and prominent drop of the fragility with compression. Simultaneously, the shape of α-process slightly broadened at elevated pressures. Infrared investigations demonstrated that this effect is related to the variation in the population of H-bonds. Moreover, we studied the changes in the structural and dynamical properties of the glasses prepared upon cooling of the melt (ordinary glass, OG) and the one obtained after compression of CRM in the liquid phase and decompression at T = 293 K (dense glass, DG). Interestingly, during the aging of the latter sample, a clear shift of the β-relaxation towards higher frequencies was noted. This unexpected result indicated that the density of DG is probably getting smaller with time. Complementary X-ray diffraction experiments confirmed this supposition. Additionally, they showed that in DG there are traces of polymorph II of CRM that has a higher density than initial crystals (polymorph I). Finally, infrared studies demonstrated that H-bond pattern in DG is slightly different with respect to OG. Furthermore, Raman investigations suggested that probably keto-enol tautomerism might be shifted towards diketo form in the glass obtained at high compression. Our investigations are very interesting in the context of better understanding of the behavior of associated systems at high compression as well as provide a better insight into dynamics of higher density glasses produced at elevated pressures.

Anhydrosaccharides-A new class of the fragile plastic crystals

In this paper, 1,6-anhydro-β-D-glucopyranose (anhGLU), 1,6-anhydro-β-D-mannopyranose (anhMAN), and 1,6-anhydro-β-D-galactopyranose (anhGAL), three new materials that form the Orientationally Disordered Crystal (ODIC) phase, have been thoroughly investigated using various experimental techniques. All measurements clearly indicated that these compounds possess a series of very interesting physical properties that are considerably different than those reported for ordinary plastic crystals. X-Ray diffraction investigations have revealed enormously long-range static correlations between molecules, reaching even 120 Å. Moreover, dielectric studies showed that besides Freon 113, the investigated anhydrosaccharides are the most fragile systems that form the ODIC phase. Further analysis of Fourier transform infrared spectra indicated that such peculiar behavior of anhydrosaccharides might be closely related to multidirectional H-bonds of various strengths that most likely affect the number of available conformations, density states, and the potential barriers in the energy landscape of these compounds. This is consistent with the results from previous reports [L. C. Pardo, J. Chem. Phys. 124, 124911 (2006) and Th. Bauer et al., J Chem. Phys. 133, 144509 (2010)] showing that the higher fragility of Freon 112 as well as a mixture of 60% succinonitrile and 40% glutaronitrile (60SN-40GN) can be closely related to the enhanced conformational ability and additional disorder introduced by various substituents, which further make energy landscape more complex. Finally, by studying the properties of 2,3,4-tri-O-acetyl-1,6-anhydro-β-D-glucopyranose (ac-anhGLU) it was found that besides the shape of the molecules, H-bonds or generally strong intermolecular interactions are extremely important parameters contributing to the ability to form the plastic phase. This is in line with current observations that in most cases the ODIC phase is created in highly interacting compounds.

High-pressure dielectric studies on 1,6-anhydro-β-D-mannopyranose (plastic crystal) and 2,3,4-tri-O-acetyl-1,6-anhydro-β-D-glucopyranose (canonical glass)

Broadband Dielectric Spectroscopy was applied to investigate molecular dynamics of two anhydrosaccharides, i.e., 1,6-anhydro-β-D-mannopyranose, anhMAN (hydrogen-bonded system) and 2,3,4-tri-O-acetyl-1,6-anhydro-β-D-glucopyranose, ac-anhGLU (van der Waals material), at different thermodynamic conditions. Moreover, the reported data were compared with those recently published for two other H-bonded systems, i.e., 1,6-anhydro-β-D-glucopyranose (anhGLU) and D-glucose (D-GLU). A direct comparison of the dynamical behavior of the materials with a similar chemical structure but significantly differing by the degrees of freedom, complexity, and intermolecular interactions made it possible to probe the impact of compression on the fragility, Temperature-Pressure Superpositioning and pressure coefficient of the glassy crystal/glass transition temperatures (dT_gc/dp ; dT_g/dp). Moreover, the correlation between dT_gc/dp determined experimentally from the high-pressure dielectric data and the Ehrenfest equation has been tested for the plastic crystals (anhGLU and anhMAN) for the first time. Interestingly, a satisfactory agreement was found between both approaches. It is a quite intriguing finding which can be rationalized by the fact that the studied materials are characterized by the low complexity (lower degrees of freedom with respect to the molecular mobility) as well as ordered internal structure. Therefore, one can speculate that in contrast to the ordinary glasses the dynamics of the plastic crystals might be described with the use of a single order parameter. However, to confirm this thesis further, pressure-volume-temperature (PVT) experiments enabling calculations of the Prigogine Defay ratio are required.

Liquid-solid directional composites and anisotropic dipolar phases of polar nanoregions in disordered perovskites

Using temperature-resolved dielectric spectroscopy in the range of 75-320 K we have inspected the solid-like and liquid-like arrangements of nanometric dipoles (polar nanoregions) embedded in sodium-enriched potassium-tantalate-niobate (KNTN), a chemically-substituted complex perovskite crystal hosting inherent substitutional disorder. The study of order versus direction is carried out using Fröhlich entropy measurements and indicates the presence of four long-range symmetry phases, two of which are found to display profoundly anisotropic features. Exotic phases are found for which the dipoles at one fixed temperature manifest a liquid reorientational response along one crystal axis and a solid-like behavior along another axis. The macroscopic anisotropy observed in the sequence of different phases is found to match a microscopic order-disorder sequence typical of nominally pure perovskites. Moreover, experimental demonstration of the onset of a frozen state above transitions is provided.

Observation of the nearly constant loss in super rigid saccharides: in search of a hidden crossover in dynamics deep in the glassy state

The molecular dynamics of three saccharides: D-glucose, 1,6-anhydro-D-glucose (levoglucosan) and 1,6:2,3-dianhydro-β-D-mannopyranose of various degrees of freedom, number of hydroxyl groups and internal structures was investigated over a wide range of temperatures and frequencies by means of Broadband Dielectric Spectroscopy (BDS). Despite the pronounced variety in the physicochemical properties of the carbohydrates, no change in the shape of the structural relaxation process was observed in the vicinity of the glass transition temperature (β(KWW) = 0.5). On the other hand further studies of the Debye-Stokes-Einstein relationship between dc conductivity and structural dynamics revealed some significant changes connected with the ability to form strong H-bonded structures. Moreover the presence of nearly constant loss (NCL) at moderate frequencies and just below the T(g) in the glassy state of levoglucosan and 1,6:2,3-dianhydro-β-D-mannopyranose was noticeable. We followed the temperature evolution of ε'' located at frequencies f = 0.1 kHz and f = 1 kHz, where the NCL is detected. Interestingly, a clear change in the dynamics far below the glass transition was observed in both compounds. This crossover (T(c)), found in different materials, and studied by various experimental techniques, is usually interpreted as being caused by the freezing of the Johari-Goldstein (JG) relaxation process. Alternatively it can also be due to the increasing anharmonicity in the density of vibrational states. Interestingly, it was shown that the slope of ε''(T) measured above the T(c) slightly changes while below the T(c) stays constant after physical aging. This is related to the densification of the sample that might result in steric hindrance and suppression of some kind of motion occurring in the glassy state, involving the larger parts of the molecules.

Isochronal superposition and density scaling of the intermolecular dynamics in glass-forming liquids with varying hydrogen bonding propensity

We have tested the idea of thermodynamic scaling T⁻¹ρ^γ and isochronal superposition in glass-forming liquids with varying propensity to form hydrogen bonds.