Study of molecular dynamics of pharmaceutically important protic ionic liquid-verapamil hydrochloride. I. Test of thermodynamic scaling

Relaxation dynamics of verapamil hydrochloride (VH), which is a representative of ionic liquids, was studied under isobaric and isothermal conditions by using dielectric spectroscopy. In addition we also carried out pressure-temperature-volume (PVT) measurements. The obtained data enable us to examine the structural α-relaxation time τα as a function of temperature, pressure, and volume. Since the examined sample is a typical ionically conducting material, we employed the dielectric modulus formalism to gain information about α-relaxation process. It was found that application of pressure changes the shape of the modulus spectrum. The α-peak becomes narrower with compression. Consequently, it was also shown that the stretching parameter βKWW increases with pressure. Based on experimental data both the isobaric fragility (mp) at various pressures and isothermal fragility (mT) at various temperatures were calculated. Analyzing the effect of pressure on the dependences τα(T) as well as on the shape parameter of the α-peak it was found that a phenomenological correlation between mp and βKWW established for glass forming liquids is also valid for VH under condition of high compression. The pressure dependences of glass-transition temperature determined from dielectric and volumetric measurements have been compared. Moreover, PVT data allow us to assess the relative contribution of thermal energy and free volume fluctuation to the dramatic slowing down of the molecular dynamics in the vicinity of Tg. It is established from the ratio of the isochronic and isobaric expansivities that the thermal energy has a stronger effect on the relaxation times than the free volume, although the latter contribution is significant. Finally, we also discuss the validity of thermodynamic scaling in the case of VH and examine the scaling exponent γ at various thermodynamic conditions.