Using broadband dielectric spectroscopy, we investigated the effect of hydrostatic pressure on the conductivity relaxation time τ{σ} of the supercooled protic ionic liquid, procainamide hydrochloride, a common pharmaceutical. The pressure dependence of τ{σ} exhibited anomalous behavior in the vicinity of the glass transition T{g}, manifested by abrupt changes in activation volume. This peculiar behavior, paralleling the change in temperature dependence of τ{σ} near T{g}, is a manifestation of the decoupling between electrical conductivity and structural relaxation. Although the latter effectively ceases in the glassy state, free ions retain their mobility but with a reduced sensitivity to thermodynamic changes. This is the first observation of decoupling of ion migration from structural relaxation in a glassy conductor by isothermal densification.

In this paper the molecular dynamics of a common local-anesthetic drug, lidocaine hydrochloride (LD-HCl), and its water mixtures were investigated. By means of broadband dielectric spectroscopy and calorimetric measurements it was shown that even a small addition of water causes a significant effect on the relaxation dynamics of analyzed protic ionic liquid. Apart from the two well-resolved relaxations (σ- and γ-processes) and the β-mode, identified as the JG-process, observed for anhydrous LD-HCl, a new relaxation peak (υ) is visible in the dielectric spectra of aqueous mixtures of this drug. Additionally, the significant effect of the water on the glass transition temperature of LD-HCl was found. The sample characterized with mole fraction of water X(w) = 0.44 reveals the glass transition temperature T(g), 42 K lower than that of anhydrous material (307 K). Finally, it was shown that by amorphization of the hydrochloride salt of lidocaine it is possible to obtain its room temperature ionic liquid form.

Structural dynamics in the glassy state of two protic ionic liquids, carvedilol phosphate and procaine hydrochloride, were characterized from analysis of changes in the conductivity relaxation times during physical aging. The obtained relaxation times, having a magnitude exceeding feasible experimental time scales and thus not directly measurable, are consistent with published data from a method that relies on the presence of a secondary relaxation. We also observe a narrowing of the relaxation dispersion, specific to higher frequencies, that is a consequence of the heterogeneous dynamics of deeply supercooled materials. SECTION: Glasses, Colloids, Polymers, and Soft Matter T he irony of the ∼100 years of research on the glass transition is that so little of it is concerned with glasses per se. This is the case even though the vitreous state is associated with distinct properties that may help solve the glass transition problem, properties such as nonergodicity, an Arrhenius temperature-dependence for the primary relaxation time, and invariance to thermodynamic conditions of both the shape of the relaxation function and the dynamic correlation volume. The limited experimental data on glassy dynamics obviously results from the inability of typical dynamic spectroscopy to measure structural relaxation occurring over prolonged time periods. Approaches to circumvent this difficulty include addition to the sample of chromophores, whose motion can be monitored without the need for motion of the host molecules, 1,2 and measurement of secondary relaxations, from which inferences are drawn concerning the structural dynamics. 3−6The principle material studied herein is carvedilol phosphate (CP, structure in Figure 1), which is comprised of a racemic mixture of two enantiomers and usually exists as the hemihydrate. The substance finds application as an oral medicine, used in the treatment of hypertension and certain heart abnormalities. Our interest is the behavior of glassy CP because the glassy state of pharmaceuticals is an attractive means to control the solubility and bioavailability of drugs. However, glasses are metastable, hence the possibility of crystallization or other physical changes that may degrade biological function. The tendency for such changes is related to the molecular mobility of the glass, which makes pharmaceuticals an important class of materials for studies of glassy dynamics. 7−16 In this work, we describe a new approach, in which the change in the diffusivity of ions due to physical aging is used to define a time constant that characterizes structural relaxation of the glass. The idea underlying this method is that physical aging is governed by the time constant measured in suitable dielectric experiments (e.g., nonlinear spectroscopy 17−20 and hole-burning experiments 21 ); that is, subensembles