Molecular dynamics in amorphous ergocalciferol

While developing new pharmaceutical products based on drug substances in their amorphous form, the molecular mobility of amorphous active ingredients have to be characterized in detail. The molecular mobility in the supercooled liquid and glassy states of ergocalciferol is studied using broadband dielectric spectroscopy over wide frequency and temperature ranges. Dielectric studies revealed a number of relaxation process of different molecular origi

Relaxation dynamics of amorphous dibucaine using dielectric studies

Using broadband dielectric spectroscopy the molecular mobility of dibucaine is investigated in the supercooled liquid and gassy states, over a wide temperature range for some test frequencies. Above the glass transition temperature T-g, the presence of structural alpha- relaxation peak was observed due to the cooperative motions of the molecule and upon cooling frozen kinetically to form the glass. The secondary relaxation process was perceivable below T-g due to localized motions. The peak loss frequency of alpha-relaxation process shows non-Arrhenius behavior and obeys Vogel-Fulcher-Tammann equation over the measured temperature range whereas the beta- process shows Arrhenius behavior

Revealing the rich dynamics of glass-forming systems by modification of composition and change of thermodynamic conditions

Secondary relaxations have been classified into two types, depending on whether they are related to the structural alpha-relaxation in properties or not. Those secondary relaxations that are related to the a-relaxation may have fundamental importance, and are called the Johari–Goldstein (JG) ß-relaxations. Two polar molecular glass-formers, one flexible and another rigid, dissolved in apolar host with higher glass transition temperature are studied by broadband dielectric spectroscopy at ambient and elevated pressure. The neat flexible glassformer diethylphthalate (DEP) has a resolved secondary relaxation which, unlike the a-relaxation, is insensitive to pressure and hence is not the JG ß-relaxation. In the solution, the JG ß-relaxation of DEP shows up in experiment and its relaxation time tß is pressure and temperature dependent like ta. The result supports the universal presence of the JG ß-relaxation in all glass-formers, and the separation between ta and tß is determined by intermolecular interaction. The rigid glass-former is cyano-benzene (CNBz) and its secondary relaxation involves the entire molecule is necessarily the JG ß-relaxation. The dielectric relaxation spectra obtained at a number of combinations of pressure and temperature at constant ta show not only unchanged is the frequency dispersion of the a-relaxation but also tß. The remarkable results indicate that the JG ß-relaxation bears a strong connection to the alpha-relaxation, and the two relaxations are inseparablewhen considering the dynamics of glass-forming systems. Experimentally, tau_alpha has been found to be a function of the product variables, T/rho^gamma, where rho is the density and gamma is a material constant. From the invariance of the ratio, tau_alphaa/tau_ß, to change of thermodynamic conditions seen in our experiment as well in other systems, it follows that tß is also a function of T/rho^gamma, with the same gamma at least approximately. Since the JG ß-relaxation is the precursor of the a-relaxation, causality implies that the T/rho^gamma-dependence originates from the JG ß-relaxation and is passed on to the alpha-relaxation

Molecular relaxations in amorphous phenylbutazone

Molecular dynamics of phenylbutazone in the supercooled liquid and glassy state is studied using broadband dielectric spectroscopy for test frequencies 1 kHz, 10 kHz and 100 kHz over a wide temperature range. Above the glass transition temperature Tg, the presence of the structural α-relaxation peak was observed which shifts towards lower frequencies as the temperature decreases and kinetically freezes at Tg. Besides the structural α-relaxation peak, a β-process which arises due to the localized molecular fluctuations is observed at lower temperature

Critical Issues of Current Research on the Dynamics Leading to Glass Transition

Glass transition is still an unsolved problem in condensed matter physics and chemistry. In this paper, we critically reexamine experimental data and theoretical interpretations of dynamic properties of various processes seen over a wide time range from picoseconds to laboratory time scales. In order of increasing time, the ubiquitous processes considered include (i) the dynamics of caged molecular units with motion confined within the anharmonic intermolecular potential and where no genuine relaxation has yet taken place; (ii) the onset of the Johari-Goldstein secondary relaxation involving rotation or translation of the entire molecular unit and causing the decay of the cages, to be followed by the cooperative and dynamically heterogeneous motions participated by increasing number of molecules or length scale; and (iii) the terminal primary α-relaxation with the maximum cooperative length-scale allowed by the intermolecular interaction and constraints of the glass former. Some general and important properties found in each of these processes are shown to be interrelated, indicating that the processes are connected, with one being the precursor of the other following it. Thus, a theory of glass transition is neither complete nor fundamental unless all of these processes and their inter-relations have been accounted. In addition to published data, new experimental data are reported here to provide a limited collection of critical experimental facts having an impact on current issues of glass transition research and serving as a guide for the construction of a complete and successful theory in the futur

The Johari-Goldstein beta-relaxation of glass-forming binary mixtures

The present paper shows, by means of broadband dielectric measurements, that the primary alpha- and the secondary Johari-Goldstein (JG) beta-processes in binary mixtures are strongly correlated. This occurs for different polar rigid probes dissolved in apolar glass-forming solvents, over a wide temperature and pressure range. reserved. We found that the coupling parameter n = 1 - beta(KWW) and the ratio between alpha- and beta-relaxation time reduce on increasing the size of the solute solved within the same apolar matrix. Moreover, such a ratio is invariant when calculated at different combinations of P and T maintaining either the primary or the JG relaxation times constant. Dielectric spectra measured at different T-P combinations but with an invariant alpha-relaxation time are well superposed in both the alpha- and beta-frequency ranges. Experimental results can be rationalized by Coupling Model equation. (C) 2010 Elsevier B.V. All rights reserved

Resolution of problems in soft matter dynamics by combining calorimetry and other spectroscopies

In several current important problems in different areas of soft matter physics, controversy persists in interpreting the molecular dynamics observed by various spectroscopies including dielectric relaxation, light scattering, nuclear magnetic resonance, and neutron scattering. Outstanding examples include: (1) relaxation of water in aqueous mixtures, in molecular sieves and silica-gel nanopores, and in hydration shell of proteins; and (2) dynamics of each component in binary miscible polymer blends, in mixtures of an amorphous polymer with a small molecular glassformer, and in binary mixtures of two small molecular glassformers. We show the applications of calorimetry to these problems have enhanced our understanding of the dynamics and eliminated the controversies

Molecular dynamics of amorphous pharmaceutical fenofibrate studied by broadband dielectric spectroscopy

Fenofibrate is mainly used to reduce cholesterol level in patients at risk of cardiovascular disease. Thermal transition study with the help of differential scanning calorimetry (DSC) shows that the aforesaid active pharmaceutical ingredient (API) is a good glass former. Based on our DSC study, the molecular dynamics of this API has been carried out by broadband dielectric spectroscopy (BDS) covering wide temperature and frequency ranges. Dielectric measurements of amorphous fenofibrate were performed after its vitrification by fast cooling from a few degrees above the melting point (Tm=354.11 K) to deep glassy state. The sample does not show any crystallization tendency during cooling and reaches the glassy state. The temperature dependence of the structural relaxation has been fitted by single Vogel–Fulcher–Tamman (VFT) equation. From VFT fit, glass transition temperature (Tg) was estimated as 250.56 K and fragility (m) was determined as 94.02. This drug is classified as a fragile glass former. Deviations of experimental data from Kohlrausch–Williams–Watts (KWW) fits on high-frequency flank of α-peak indicate the presence of an excess wing in fenofibrate. Based on Ngai׳s coupling model, we identified the excess wing as true Johari–Goldstein (JG) process. Below the glass transition temperature one can clearly see a secondary relaxation (γ) with an activation energy of 32.67 kJ/mol

Recent progress in understanding relaxation in complex systems

The evolution of dynamics with time in complex interacting systems is shown to be general. The caged dynamics exemplified by the nearly constant loss (NCL), is followed in time and terminated by the primitive relaxation, which is called the Johari-Goldstein beta-relaxation in the case of glass-forming substances. The latter is the precursor of the many-body relaxation process which increases in length-scale with time until the terminal relaxation is reached to determine the transport coefficient. Using experimental data, we show these three ubiquitous features in the dynamics are inter-related as shown by correlations in their properties, and hence all must be considered together. The dynamics of hydrated proteins are chosen here as an example for detailed discussion. These three major processes and their inter-relations are found in hydrated proteins. An interpretation of the data follows along the line of other complex systems exhibiting similar dynamics. (c) 2010 Elsevier B.V. All rights reserved