Molecular dynamics in the supercooled liquid and glassy states of bezafibrate and binary mixture of fenofibrate

Two solubility limited pharmaceuticals bezafibrate and fenofibrate, belonging to BCS class II drugs, were compared, by investigating molecular dynamics of the former and compared with the neat and as a probe in an apolar host polystyrene of mole. wt. 800 (PS800) of the latter by broadband dielectric spectroscopy (BDS), other supporting thermal and spectroscopic experimental means and density functional theory (DFT) calculations. The BDS experiments from frozen glassy state to vibrant molten state of the title drugs were done to explore the role of parameters describing their complex molecular dynamics and the stabiliy of amorphous phase, such as glass transition temperature (Tg), temperature dependence of α-relaxation times (τα), width of structural dispersion and its role on the occurrence of Johari-Goldstein (JG) β-relaxation, fragility index (m) and apparent activation energy of secondary processes. Realizing that most of the pharmaceuticals have narrow α-dispersion which limits the visibility of JG β-relaxation as a separate process in the dielectric window, to resolve the conundrum, we have further explored the amorphous dynamics of fenofibrate as a probein an apolar host polystyrene of molecular weight 800 (PS800) having higher Tg than the polar probe. The Tg of bezafibrate and PS800-fenofibrate mixture were determined to be 298 K and 275 K respectively, whereas the fractional exponent of Kohlrausch–Williams–Watts function βKWW of bezafibrate was determined to be 0.68 which is closer to that of fenofibrate, whereas that of PS800-fenofibrate mixture was found to be much broader with a value of 0.39, all in the vicinity of Tg. Even though βKWW decreased in binary mixture of fenofibrate when compared to its pure form due to concentration fluctuations and other aspects, but cold crystallization tendency was absent and Tg is elevated than that of fenofibrate. Due to higher Tg than that of fenofibrate, amorphous bezafibrate is more stable than fenofibrate. Finally, using quantum computational calculations using DFT, the relevant flexible parts of the two drug molecules, whose motions manifests the secondary γ-relaxation was identified by calculating potential energy barriers and dipole moments on relaxed dihedral angle scans, as no experimental technique is available for this purpose.