Although the first-order liquid-liquid phase transition (LLT) has been reported to exist in various systems (i.e., phosphorus, silicon, water, triphenyl phosphite, etc.), it is still one of the most ...challenging problems in the field of physical science. Recently, we found that this phenomenon occurs in the family of trihexyl(tetradecyl)phosphonium P
based ionic liquids (ILs) with different anions (Wojnarowska et al in Nat Commun 13:1342, 2022). To understand the molecular structure-property relationships governing LLT, herein, we examine ion dynamics of two other quaternary phosphonium ILs containing long alkyl chains in cation and anion. We found that IL with the anion containing branched -O-(CH
)
-CH
side chains does not reveal any signs of LLT, while IL with shorter alkyl chains in the anion brings a hidden LLT, i.e., it overlaps with the liquid-glass transition. Ambient pressure dielectric and viscosity measurements revealed a peculiar behavior of ion dynamics near T
for IL with hidden LLT. Moreover, high-pressure studies have shown that IL with hidden LLT has relatively strong pressure sensitivity compared to the one without first-order phase transition. At the same time, the former exposes the inflection point indicating the concave-convex character of logτ
(P) dependences.
Abstract
The studies of molecular dynamics in the vicinity of liquid–glass transition are an essential part of condensed matter physics. Various experimental techniques are usually applied to ...understand different aspects of molecular motions, i.e., nuclear magnetic resonance (NMR), photon correlation spectroscopy (PCS), mechanical shear relaxation (MR), and dielectric spectroscopy (DS). Universal behavior of molecular dynamics, reflected in the invariant distribution of relaxation times for different polar and weekly polar glass-formers, has been recently found when probed by NMR, PCS, and MR techniques. On the other hand, the narrow dielectric permittivity function ε*(f) of polar materials has been rationalized by postulating that it is a superposition of a Debye-like peak and a broader structural relaxation found in NMR, PCS, and MR. Herein, we show that dielectric permittivity representation ε*(f) reveals details of molecular motions being undetectable in the other experimental methods. Herein we propose a way to resolve this problem. First, we point out an unresolved Johari–Goldstein (JG) β-relaxation is present nearby the α-relaxation in these polar glass-formers. The dielectric relaxation strength of the JG β-relaxation is sufficiently weak compared to the α-relaxation so that the narrow dielectric frequency dispersion faithfully represents the dynamic heterogeneity and cooperativity of the α-relaxation. However, when the other techniques are used to probe the same polar glass-former, there is reduction of relaxation strength of α-relaxation relative to that of the JG β relaxation as well as their separation. Consequently the α relaxation appears broader in frequency dispersion when observed by PCS, NMR and MR instead of DS. The explanation is supported by showing that the quasi-universal broadened α relaxation in PCS, NMR and MR is captured by the electric modulus
M
*(
f
) = 1/ε*(
f
) representation of the dielectric measurements of polar and weakly polar glass-formers, and also
M
*(
f
) compares favorably with the mechanical shear modulus data
G
*(
f
).
In this paper, we investigate the molecular dynamics and ions transport properties of polymerized imidazolium-based protic ionic liquid HSO3–BVImOTfa new material with potential applications in ...energy storage and electrochemical devices. The results of dielectric measurements, analyzed in modulus M*(f) and conductivity σ*(f) formalisms, combined with temperature-modulated differential scanning calorimetry experiments, have revealed a fundamental difference between the conducting properties of the examined polymer membrane and its low-molecular weight counterpart. Our findings indicated a strong decoupling between conductivity relaxation times τσ (related to the ions migration through the polymer matrix) and segmental dynamics when the ionic transport is controlled by fast proton hopping through the dense hydrogen-bond network. Finally, we also discuss, for the first time, the effect of water content on the glass transition temperature value, relation between the charge and mass diffusion, reflected in the decoupling phenomenon, and the conductivity mechanism of examined polyHSO3–BVImOTf.
Low physical stability is the main reason limiting the widespread use of amorphous pharmaceuticals. One approach to overcome this problem is to mix these drugs with various excipients. In this study ...coamorphous drug–drug compositions of different molar ratios of ezetimib and indapamid (i.e., EZB 10:1 IDP, EZB 5:1 IDP, EZB 2:1 IDP, EZB 1:1 IDP and EZB 1:2 IDP) were prepared and investigated using differential scanning calorimetry (DSC), broadband dielectric spectroscopy (BDS), and X-ray diffraction (XRD). Our studies have shown that the easily recrystallizing ezetimib drug can be significantly stabilized in its amorphous form by using even a small amount of indapamid (8.8 wt %). DSC experiments indicate that the glass transition temperature (T g) of the tested mixtures changes with the drug concentration in accordance with the Gordon–Taylor equation. We also investigated the effect of indapamid on the molecular dynamics of the ezetimib. As a result it was found that, with increasing indapamid content, the molecular mobility of the binary drug–drug system is slowed down. Finally, using the XRD technique we examined the long-term physical stability of the investigated binary systems stored at room temperature. These measurements prove that low-molecular-weight compounds are able to significantly improve the physical stability of amorphous APIs.
Polymerized ionic liquids (PolyILs) are promising candidates for a wide range of technological applications due to their single ion conductivity and good mechanical properties. Tuning the glass ...transition temperature (T g) in these materials constitutes a major strategy to improve room temperature conductivity while controlling their mechanical properties. In this work, we show experimental and simulation results demonstrating that in these materials T g does not follow a universal scaling behavior with the volume of the structural units V m (including monomer and counterion). Instead, T g is significantly influenced by the chain flexibility and polymer dielectric constant. We propose a simplified empirical model that includes the electrostatic interactions and chain flexibility to describe T g in PolyILs. Our model enables design of new functional PolyILs with the desired T g.
In this study, we employed dielectric spectroscopy to investigate the effect of temperature and pressure on the ion dynamics of phosphonium ionic liquids (ILs) differing by the length of an alkyl ...chain, P666,n TFSI (n = 2, 6, 8, 12). We found that both temperature and pressure dependence of dc-conductivity (σdc) determined for all examined ILs herein exhibit unique characteristics, unusual for aprotic ILs. Two regions differing by ion self-organization have been identified from the derivative analysis of σdc(T –1) data. On the other hand, isothermal measurements performed at elevated pressure revealed a unique concave–convex character of σdc(P) dependences, resulting in a clear minimum in the pressure behavior of activation volume. Such an inflection point characterizing the pressure dependence of σdc in P666,n TFSI ILs can be considered an inherent feature of ion dynamics governed by structural self-assembly. Our results offer a unique perspective to link the ion mobility at various T–P conditions to the nanostructural organization of ionic systems.
From an exhaustive examination of the molecular dynamics in practically all van der Waals molecular glass formers ever probed by dielectric spectroscopy, we found that the width of the α-loss peak at ...or near the glass transition temperature T_{g} is strongly anticorrelated with the polarity of the molecule. The larger the dielectric relaxation strength Δε(T_{g}) of the system, the narrower is the α-loss peak. This remarkable property is explained by the contribution from the dipole-dipole interaction potential V_{dd}(r)=-Dr^{-6} to the attractive part of the intermolecular potential, making the resultant potential more harmonic, and the effect increases rapidly with the dipole moment μ and Δε(T_{g}) in view of the relation, D∝(μ^{4}/kT_{g})∝kT_{g}Δε(T_{g})^{2}. Since the novel correlation discovered encompasses practically all van der Waals molecular glass formers studied by dielectric spectroscopy, it impacts the large dielectric research community as well as those engaged in solving the glass transition problem.
Display omitted
To improve solubility of tadalafil (Td), a poorly soluble drug substance (3μg/ml) belonging to the II class of the Biopharmaceutical Classification System, its six different solid ...dispersions (1:1, w/w) in the following polymers: HPMC, MC, PVP, PVP-VA, Kollicoat IR and Soluplus were successfully produced by freeze-drying. Scanning electron microscopy showed a morphological structure of solid dispersions typical of lyophilisates. Apparent solubility and intrinsic dissolution rate studies revealed the greatest, a 16-fold, increase in drug solubility (50μg/ml) and a significant, 20-fold, dissolution rate enhancement for the Td/PVP-VA solid dispersion in comparison with crystalline Td. However, the longest duration of the supersaturation state in water (27μg/ml) over 24h was observed for the Td solid dispersion in HPMC. The improved dissolution of Td from Td/PVP-VA was confirmed in the standard dissolution test of capsules filled with solid dispersions. Powder X-ray diffraction and thermal analysis showed the amorphous nature of these binary systems and indicated the existence of dispersion at the molecular level and its supersaturated character, respectively. Nevertheless, as evidenced by film casting, the greatest ability to dissolve Td in polymer was determined for PVP-VA. The crystallization tendency of Td dispersed in Kollicoat IR could be explained by the low Tg (113°C) of the solid dispersion and the highest difference in Hansen solubility parameters (6.8MPa0.5) between Td and the polymer, although this relationship was not satisfied for the partially crystalline dispersion in PVP. Similarly, no correlation was found between the strength of hydrogen bonds investigated using infrared spectroscopy and the physical stability of solid dispersions or the level of supersaturation in aqueous solution.
In this Letter we report the relation between ionic conductivity and structural relaxation in supercooled protic ionic liquids (PILs) under high pressure. The results of high-pressure dielectric and ...volumetric measurements, combined with rheological and temperature-modulated differential scanning calorimetry experiments, have revealed a fundamental difference between the conducting properties under isothermal and isobaric conditions for three PILs with different charge transport mechanisms (Grotthuss vs vehicle). Our findings indicate a breakdown of the fractional Stokes-Einstein relation and Walden rule when the ionic transport is controlled by fast proton hopping. Consequently, we demonstrate that the studied PILs exhibit significantly higher conductivity than one would expect taking into account that they are in fact a mixture of ionic and neutral species. Thus, the examined herein samples represent a new class of "superionic" materials desired for many advanced applications.
We present investigations of the charge transport in an ionic glass-former carvedilol dihydrogen phosphate (CP) at various T–P–V thermodynamic conditions in terms of density scaling concept. The ...studied material was found to reveal superprotonic properties both at ambient and elevated pressure, as proved by the Walden rule. Surprisingly, from the isobaric conductivity data, the relaxation times τσ presented in volume formalism showed no visual evidence of a liquid–glass transition. The different behavior of relaxation dynamics above and below T g was only revealed from the analysis of log τσ(V sp) data at isochronal conditions. The τσ experimental data of CP plotted as a function of (TV γ)−1 satisfy the thermodynamic scaling criterion in the supercooled liquid as well as in the amorphous regime, however with a different γ coefficient (γSL = 1.12; γG = 0.48). Nevertheless, by introducing the idea of fictive temperature T f, the transport properties of glassy and supercooled Grotthuss-type conductors measured at various T–P points obey the universal scaling with the use of a single γ parameter.
PDF
