Search

Herein, broadband dielectric (BDS) and Fourier transform infrared spectroscopy (FTIR), together with differential scanning calorimetry (DSC) and X-ray diffraction (XRD), were applied to study the molecular dynamics, molecular interactions as well as physical stability of an amorphous Active Pharmaceutical Ingredient (API)-probucol (PRO)-infiltrated into anodic aluminum oxide (AAO) membranes of pore size, d ~ 10-160 nm. Interestingly, the behavior of examined substance strongly depends on the applied thermal protocol. Remarkably, for the first time, we observed that the structural dynamics of the slowly cooled PRO under confinement is significantly enhanced when compared to thatofthe quenched material. This unusual behavior was interpreted as a result of surface-induced effects (including the formation of well-resolved interfacial H-bonded layer and adsorption-desorption processes near the interface) that are magnified by the extremely high sensitivity to density fluctuation of studied PRO, reflected in the enormous pressure coefficient of the glass transition temperature dTg/dp = 427 K/GPa. In fact, FTIR investigations revealed that PRO tends to self-associate under confinement and forms a strongly bonded interfacial layer, which controls the variation in the structural dynamics of core molecules. Finally, we observed that the tendency to crystallize of confined API is reduced with respect to the bulk, even though the critical size of PRO nuclei (rc ~ 3 nm) is significantly lower than the smallest examined pore size. Nevertheless, after few weeks of storage, the investigated substance crystallized in larger pores, while it remained stable in the nanochannels of d = 10 nm. A combination of XRD and DSC measurements indicated that the infiltrated PRO forms two polymorphs, the stable form I (dominating in bulk) and unstable form II (prevailing under confinement). That means that porous matrices might be used to obtain and maintain prolonged stability of unstable polymorphic forms of API.

Infrared and dielectric spectroscopy, X-ray diffraction, and density functional theory computations have been used to study the hydrogen-bonding pattern, molecular dynamics, internal structure, and dipole moment distribution in 2-ethyl-1-hexanol, 2-ethyl-1-hexylamine, 2-ethyl-1-hexanethiol, and 1-phenyl-2-butanol. Dielectric investigations revealed that Kirkwood–Frohlich correlation factor is much larger or lower than the unity in the vicinity of the glass transition, dependent on the compound. It indicates that change in the functionality of the H-bonding moiety influences the architecture of the supramolecular nanoassemblies. Further thorough experimental and theoretical considerations confirmed this hypothesis. Moreover, it was found that not the strength of hydrogen bonds itself, but the diverse population of nanoassociates, their size, and the spatial organization of the molecules in clusters have a strong influence on the appearance/absence as well as the intensity of the Debye relaxation. The resul...

The behavior of the low- and high-molecular weight glass formers confined in nanoporous templates remains an unsolved puzzle despite the intensive long-term studies in this matter. Special effort is taken to understand the enhancement of segmental or structural dynamics and the depression of the glass-transition temperatures, Tgs in materials infiltrated in pores of the nanometric size. In this paper, we have analyzed dielectric, calorimetric, and contact angle data collected for various systems to determine which factors are responsible for these effects. It turned out that with increasing interfacial energy, molecules attached to the pore walls (interfacial layer) vitrify at higher temperatures. Moreover, the dynamics of core molecules starts to deviate from bulk-like behavior. Therefore, a greater depression of the glass-transition temperature, Tg, of this fraction of molecules is noted. Also, it was found that the sensitivity of structural dynamics to the density fluctuations, quantified by the pressu...

Herein, we tested the validity of the thermodynamic scaling of two (α and δ) relaxation processes observed in the isotropic and nematic phases of itraconazole, that is classified as a liquid crysta...

In this paper, we have investigated the molecular dynamics above and below the glass-transition temperature of bisphenol-A diglycidyl ether (known as DGEBA, Mn = 340 g/mol) infiltrated in nanoporous alumina (AAO) templates of various pore sizes by means of dielectric and Raman spectroscopies. It was found that the temperature dependence of the structural relaxation times is different under confinement with respect to the bulk sample even in the high-temperature regime. Interestingly, below the glass-transition temperature, the slow secondary process (β) was not detected in dielectric loss spectra of confined DGEBA, while the relaxation times of the faster secondary process (γ) were unaffected by the pore size. To explain this phenomenon, two different scenarios, considering either suppression of the motions related to this mobility or enhancement of its dynamics, were taken into account. Additional annealing experiments, which lead to density perturbation, enabled us to recover bulk-like temperature depen...

The dynamics and thermodynamics of confined triphenyl phosphite (TPP) were studied using broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC). Geometric confinement in channels having length scales commensurate with the molecular size of TPP causes bifurcation of the dynamics: two populations are observed, distinguished by their reorientational mobilities and glass transition temperatures. Upon cooling, significant changes in the relaxation process and temperature dependence occur due to the slow vitrification of the molecules in close proximity to the interface. Such a kinetic aspect of glass formation is unusual. This surface interaction alleviates constraints on the molecules, allowing their glass transition to shift to lower temperatures. Simultaneously, it was observed that the structural relaxation process shifts to lower frequencies, and the distribution of the relaxation times becomes narrower upon annealing. This effect is especially visible at lower frequencies, in...

Dielectric spectroscopy and differential scanning calorimetry (DSC) were applied to study the molecular dynamics and thermal properties of a low-molecular-weight glass-forming liquid, salol (phenyl...