TSDC and DSC Investigation on the Molecular Mobility in the Amorphous Solid State and in the Glass Transformation Region of Two Benzodiazepine Derivatives: Diazepam and Nordazepam.

In this work we study the molecular mobility in the amorphous solid state and in the glass transformation region of two compounds, diazepam and nordazepam; these are two benzodiazepines, a family of psychotropic drugs with sedative, anxiolytic and muscle-relaxing properties. The experimental techniques used are thermostimulated currents (TSC) and differential scanning calorimetry (DSC). TSC is a time-dependent technique recognized for its high resolving power; the use of this technique in the depolarization and polarization modes (TSDC and TSPC respectively), provides results that confirm and complement results of dielectric relaxation spectroscopy (DRS) published recently. On the other hand, the variation with the heating rate of the temperature position of the DSC glass transition signal also allowed the estimation of the activation energy at Tg and of the dynamic fragility of the two glass formers.

Thermal behavior and molecular mobility studies in the supercooled liquid and glassy states of carvedilol and loratadine.

The thermal behavior of carvedilol and loratadine was studied by differential scanning calorimetry (DSC). The glass-forming ability, as well as the the tendency for crystallization from the glass (glass stability) and from the metastable and equilibrium melt were also investigated by DSC. In addition this technique was also used to characterize the glass transition of carvedilol and loratadine by determining the activation energy of the structural relaxation, the dynamic fragility, and the heat capacity jump associated with the glass transformation. Different aspects of the molecular mobility in carvedilol and loratadine were analyzed by Thermally Stimulated Depolarization Currents (TSDC), while in carvedilol the Dielectric Relaxation Spectroscopy (DRS) technique was also used. Carvedilol stands out for its high values of specific heat jump and dynamic fragility, which has been attributed to the particular mobility of this glass-former in the glass transformation region, a consequence of specific characteristics of its molecular structure. These molecular features are also at the origin of a relaxation above Tg that has been detected and characterized by TSDC; the DRS investigation allowed to better understand the molecular dynamics in carvedilol in the amorphous solid, in the metastable liquid state and in the glass transformation region. Finally, the secondary relaxations in loratadine were studied by TSDC, while those in carvedilol were studied by the two dielectric techniques and the results were compared and discussed.

Thermal Behavior and Slow Relaxation Dynamics in Amorphous Efavirenz: A Study by DSC, XRPD, TSDC, and DRS.

The analysis of the thermal behavior of efavirenz showed a high glass-forming ability and good glass stability of this glass-forming liquid at room temperature. No polymorphic forms were formed either by cold crystallization or by recrystallization from solvent acetone. The determination of the dynamic fragility by the differential scanning calorimetry, thermally stimulated depolarization currents (TSDC), and dielectric relaxation spectroscopy (DRS) techniques is unanimous in suggesting efavirenz as a moderately fragile liquid. With DRS, secondary relaxations were detected, however, with weak intensities that did not allow the respective kinetic analysis; in contrast, TSDC allows clearly resolving the components of the secondary ß-relaxation below Tg, with activation energies distributed between about 75 and 90 kJ mol-1 and Arrhenius prefactors of the order of 10-13 s. In this regard, the TSDC technique proved to be more effective compared to DRS in characterizing the secondary relaxation. The glass forming ability and glass stability found for efavirenz have been discussed in terms of various thermodynamic and kinetic parameters such as the reduced glass transition temperature, Tgred, the dynamic fragility, m, the stretching exponent, ßKWW, the melting entropy, ΔSfus, and the molecular stiffness. The exceptionally low value of efavirenz fusion entropy was highlighted as a key feature of the thermal behavior of this glass-forming liquid.