Studying the Intermolecular Interactions, Structural Dynamics, and Non-Equilibrium Kinetics of Cilnidipine Infiltrated into Alumina and Silica Pores

In the present study, the behavior of the calcium channel blocker cilnidipine (CLN) infiltrated into silica (SiO₂) and anodic aluminum oxide (AAO) porous membranes characterized by a similar pore size (d = 8 nm and d = 10 nm, respectively) as well as the bulk sample has been investigated using differential scanning calorimetry, broadband dielectric spectroscopy (BDS), and Fourier-transform infrared spectroscopy (FTIR) techniques. The obtained data suggested the existence of two sets of CLN molecules in both confined systems (core and interfacial). They also revealed the lack of substantial differences in inter- and intramolecular dynamics of nanospatially restricted samples independently of the applied porous membranes. Moreover, the annealing experiments (isothermal time-dependent measurements) performed on the confined CLN clearly indicated that the whole equilibration process under confinement is governed by structural relaxation. It was also found that the β_anneal parameters obtained from BDS and FTIR data upon equilibration of both confined samples are comparable (within 10%) to each other, while the equilibration constants are significantly different. This finding strongly emphasizes that there is a close connection between the inter- and intramolecular dynamics under nanospatial restriction.

Dramatic slowing down of the conformational equilibrium in the silyl derivative of glucose in the vicinity of the glass transition temperature

The vitrification process is usually preceded by a significant change (around 6-8 decades) in the viscosity, structural relaxation times, or diffusion that occurs in a relatively small range of temperatures in fragile liquids. Along with this phenomenon, conformations of the molecules vary as well. In fact, this process is studied in bulk polymers and high molecular weight materials deposited in the form of thin films. On the other hand, spatial rearrangement of small glass formers in the supercooled liquid state has not been intensively investigated, so far. Herein, data obtained from measurements carried out using various experimental techniques on supercooled 1,2,3,4,6-penta-O-(trimethylsilyl)-d-glucopyranose (S-GLU) have revealed that rotations of silyl moieties along with the deformation in the saccharide ring are significantly slowed down in the vicinity of the glass transition temperature (Tg). These intramolecular reorganizations affect the structural relaxation time, atomic pair distribution function, integrated intensity, as well as a number of bands and signals observed, respectively, in the Raman and NMR spectra. Data reported herein offer a better understanding of the conformational variation and time scale of this process in the complex and flexible molecules around the Tg.

Unexpected Crossover in the kinetics of mutarotation in the supercooled region: the role of H-bonds

Intra- and intermolecular studies on the molten L-sorbose have been carried out at variable temperature conditions to determine the crosover temperature (T_c). In addition, isothermal time-dependent FTIR and Raman measurements were performed to probe the pace of mutarotation and activation energy of this reaction in the studied saccharide, which varied from 53–62 kJ/mol up to 177–192 kJ/mol below and above T_c, respectively. To explain the change in activation barrier for the mutarotation a complementary analysis using difference FTIR spectra collected around T_c = 365 K in the hydroxyl region has been done. It was found that the alteration of kinetic parameters and molecular dynamics around T_c are strictly related to the variation in the strength of H-bonds which above T_c are significantly weaken, increasing the freedom of rotation of functional groups and movement of individual molecules. That phenomenon most likely affects the proton transfer, underlying molecular mechanism of mutarotation, which may lead to the significant increase in activation barrier. The new insight into a molecular aspect of the mutarotation around T_c has created an opportunity to better understanding the relationship between physics of condensed matter and the potential role of H-bonds dynamics on the progress of the chemical reaction in highly viscous systems.

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.