Oscillatory shear and high-pressure dielectric study of 5-methyl-3-heptanol

What is the origin of slow relaxation modes in highly viscous ionic liquids?

Room temperature ionic liquids (RTILs) are molten salts consisting entirely of ions and have over the past decades gained increased interest due to their high potential in applications. These structurally complex systems often display multiple relaxation modes in the response functions at lower frequencies, hinting to complex underlying mechanisms. While the existence of these multimodal spectra in the shear mechanical, dielectric, and light scattering response of RTILs has been confirmed multiple times, controversy still surrounds the origin. This paper, therefore, aims to provide additional insights into the multimodal spectra seen in RTILs by presenting new shear mechanical results on seven different RTILs: Pyr1n-TFSI with n = 4, 6, and 8; Pyr18-TFSI mixed with Li-TFSI in two high concentrations; and Cn-mim-BF4 with n = 3 and 8. Dynamic depolarized light scattering was also measured on one of the Pyr18-TFSI Li-salt mixtures. These specific cases were analyzed in detail and put into a bigger perspective together with an overview of the literature. Recent literature offers two specific explanations for the origin of the multimodal shear mechanical spectra: (1) cation-anion time scale separation or (2) combined cation-anion relaxation in addition to a dynamic signal from mesoscale aggregates at lower frequencies. However, neither of these two pictures can consistently explain all the results on different ionic liquids. Instead, we conclude that the origin of the multimodal spectrum is system specific. This underlines the complexity of this class of liquids and shows that great care must be taken when making general conclusions based on specific cases.

High-Pressure and Temperature Effects on the Clustering Ability of Monohydroxy Alcohols

This study examined the clustering behavior of monohydroxy alcohols, where hydrogen-bonded clusters of up to a hundred molecules on the nanoscale can form. By performing X-ray diffraction experiments at different temperatures and under high pressure, we investigated how these conditions affect the ability of alcohols to form clusters. The pioneering high-pressure experiment performed on liquid alcohols contributes to the emerging knowledge in this field. Implementation of molecular dynamics simulations yielded excellent agreement with the experimental results, enabling the analysis of theoretical models. Here we show that at the same global density achieved either by alteration of pressure or temperature, the local aggregation of molecules at the nanoscale may significantly differ. Surprisingly, high pressure not only promotes the formation of hydrogen-bonded clusters but also induces the serious reorganization of molecules. This research represents a milestone in understanding association under extreme thermodynamic conditions in other hydrogen bonding systems such as water.

Experimental and Computational Approach to Studying Supramolecular Structures in Propanol and Its Halogen Derivatives

A series of four alcohols, n-propanol and its halogen (Cl, Br, and I) derivatives, were selected to study the effects of variation in polarity and halogen-driven interactions on the hydrogen bonding pattern and supramolecular structure by means of experimental and theoretical methods. It was demonstrated on both grounds that the average strength of H-bonds remains the same but dissociation enthalpy, the size of molecular nanoassemblies, as well as long-range correlations between dipoles vary with the molecular weight of halogen atom. Further molecular dynamics simulations indicated that it is connected to the variation in the molecular order introduced by specific halogen-based hydrogen bonds and halogen-halogen interactions. Our results also provided important experimental evidence supporting the assumption of the transient chain model on the molecular origin of the structural process in self-assembling alcohols.

On the relationship between the Debye process in dielectric response and a dissociation-association phenomenon in phenyl alcohols

In this paper, we have examined a series of phenyl-substituted primary monohydroxy alcohols (phenyl alcohols, PhAs), from ethanol to hexanol by means of dielectric and Fourier transform infrared (FTIR) spectroscopies supported by the mechanical investigations. The combination of both dielectric and mechanical data allows calculation of the energy barrier, E_a, for dissociation by the Rubinstein approach developed to describe the dynamical properties of self-assembling macromolecules. It was observed that the determined activation energy remains constant, |E_a,RM| ∼ 12.9-14.2 kJ mol^-1, regardless of the molecular weight of the examined material. Surprisingly, the obtained values agree very well with E_a of the dissociation process determined from the FTIR data analysed within the van't Hoff relationship, where E_a,vH ∼ 9.13-13.64 kJ mol^-1. Thus, the observed agreement between E_a determined by both applied approaches clearly implies that in the case of the examined series of PhAs, the dielectric Debye-like process is governed by the association-dissociation phenomenon as proposed by the transient chain model.

Dielectric and Shear Mechanical Spectra of Propanols: The Influence of Hydrogen-Bonded Structures

We present a dielectric and shear mechanical study of 1-propanol and three phenylpropanols. Contrary to other monoalcohols, the phenylpropanols do not show a bimodal behavior in their dielectric response, but instead show a single, rather narrow process. Combined dielectric and light scattering spectra (Böhmer, T.; et al. J. Phys. Chem. B 2019, 123, 10959) have shown that this single peak may be separated into a self- and a cross-correlation part, thus indicating that phenylpropanols do display features originating from hydrogen-bonded structures. The shear mechanical spectra support that interpretation, demonstrating a subtle, yet clear, low-frequency polymer-like mode, similar to what is found in other monoalcohols. An analysis of the characteristic time scales found in the spectra shows that shear alpha relaxation is faster than the dielectric alpha and that time scale separation of the dielectric Debye and alpha processes is temperature independent and nearly identical in all the phenylpropanols.

Supramolecular structures of self-assembled oligomers under confinement

We study the molecular origin of a prepeak (PP) observed at low q values in the structure factors of three oligomers in a bulk (poly(mercaptopropyl)methylsiloxane, PMMS, poly(methylmercaptopropyl)-grafted-hexylmethacrylate, PMMS-g-HMA, and poly(methylphenyl)siloxane, PMPS) in order to understand the lowering of the PP intensity detected for oligomers confined in cylindrical pores with low diameter. For this purpose, we use a combination of X-ray diffraction measurements and coarse-grained bead-spring molecular dynamics simulations. Our molecular modelling demonstrated that the planarity of the pendant groups triggers the self-association of oligomers into nanoaggregates. However, the formation of oligomeric nanodomains is not sufficient for building-up the PP. The latter requires spatial disturbance in the arrangement of the side groups of oligomers within clusters. Importantly, our numerical analysis revealed that the increasing degree of the confinement of oligomers limits their aggregation and consequently lowers the amplitude of the PP observed in the experimental data.

Evolution of microscopic heterogeneity and dynamics in choline chloride-based deep eutectic solvents

Deep eutectic solvents (DESs) are an emerging class of non-aqueous solvents that are potentially scalable, easy to prepare and functionalize for many applications ranging from biomass processing to energy storage technologies. Predictive understanding of the fundamental correlations between local structure and macroscopic properties is needed to exploit the large design space and tunability of DESs for specific applications. Here, we employ a range of computational and experimental techniques that span length-scales from molecular to macroscopic and timescales from picoseconds to seconds to study the evolution of structure and dynamics in model DESs, namely Glyceline and Ethaline, starting from the parent compounds. We show that systematic addition of choline chloride leads to microscopic heterogeneities that alter the primary structural relaxation in glycerol and ethylene glycol and result in new dynamic modes that are strongly correlated to the macroscopic properties of the DES formed.

Tailoring the macroscopic properties of deep eutectic solvents requires knowing how these depend on the local structure and microscopic dynamics. The authors, with computational and experimental tools spanning a wide range of space- and timescales, shed light into the relationship between micro and macroscopic properties in glyceline and ethaline.

Effect of confinement on the dynamics of 1-propanol and other monohydroxy alcohols

We report the effect of confinement on the dynamics of three monohydroxy alcohols (1-propanol, 2-ethyl-1-hexanol, and 4-methyl-3-heptanol) differing in their chemical structure and, consequently, in the dielectric strength of the "Debye" process. Density functional theory calculations in bulk 1-propanol identified both linear and ring-like associations composed of up to five repeat units. The simulation results revealed that the ring structures, with a low dipole moment (∼2 D), are energetically preferred over the linear assemblies with a dipole moment of 2.18 D per repeat unit. Under confinement in nanoporous alumina (in templates with pore diameters ranging from 400 to 20 nm), all dynamic processes were found to speed up irrespective of the molecular architecture. The characteristic freezing temperatures of the α and the Debye-like processes followed the pore size dependence: T_a,D=T_a,D ^bulk-A/d^1/2, where d is the pore diameter. The characteristic "freezing" temperatures for the Debye-like (the slow process for confined 1-propanol is non-Debye) and the α-processes decrease, respectively, by 6.5 and 13 K in confined 1-propanol, by 9.5 and 19 K in confined 2-ethyl-1-hexanol, and by 9 and 23 K in confined 4-methyl-3-heptanol within the same 25 nm pores. In 2-ethyl-1-hexanol, confinement reduced the number of linearly associated repeats from approximately heptamers in the bulk to dimers within 25 pores. In addition, the slower process in bulk 2-ethyl-1-hexanol and 4-methyl-3-heptanol, where the signal is dominated by ring-like supramolecular assemblies, is clearly non-Debye. The results suggest that the effect of confinement is dominant in the latter assemblies.

Isomeric effects in structure formation and dielectric dynamics of different octanols

The understanding of the microstructure of associated liquids promoted by hydrogen-bonding and constrained by steric hindrance is highly relevant in chemistry, physics, biology and for many aspects of daily life. In this study we use a combination of X-ray diffraction, dielectric spectroscopy and molecular dynamics simulations to reveal temperature induced changes in the microstructure of different octanol isomers, i.e., linear 1-octanol and branched 2-, 3- and 4-octanol. In all octanols, the hydroxyl groups form the basis of chain-, cyclic- or loop-like bonded structures that are separated by outwardly directed alkyl chains. This clustering is analyzed through the scattering pre-peaks observed from X-ray scattering and simulations. The charge ordering which pilots OH aggregation can be linked to the strength of the Debye process observed in dielectric spectroscopy. Interestingly, all methods used here converge to the same interpretation: as one moves from 1-octanol to the branched octanols, the cluster structure evolves from loose large aggregates to a larger number of smaller, tighter aggregates. All alcohols exhibit a peculiar temperature dependence of both the pre-peak and Debye process, which can be understood as a change in microstructure promoted by chain association with increased chain length possibly assisted by ring-opening effects. All these results tend to support the intuitive picture of the entropic constraint provided by branching through the alkyl tails and highlight its capital entropic role in supramolecular assembly.

Unusual Debye relaxation in 4-methyl-2-pentanol evidenced by high-pressure dielectric studies

The Debye relaxation is the main signal in the dielectric measurements of monoalcohols arising from the hydrogen-bonded superstructures, but its physics remains to be cleared. In this work, a monoalcohol of 4-methyl-2-pentanol is studied using dielectric spectroscopies recorded at high pressures. The dynamic parameters of the Debye and structural relaxations are extracted. The calculation of the Kirkwood factor of the Debye relaxation indicates chain-like H-bond molecular configurations. Remarkably, we found that both ratios of the relaxation strength and relaxation time between the Debye and structural dynamics, Δε _D/Δε _α and τ _D/τ _α , decreases upon compression, indicating a positive correlation. This is different from the results reported in primary 2-ethyl-1-hexanol and secondary 4-methyl-3-heptanol, where the two ratios are inversely correlated. The discussion and interpretation of these different results are provided.

The impact of chemical structure on the formation of the medium-range order and dynamical properties of selected antifungal APIs

In this paper, we have analyzed structural, thermal, and dynamical properties of four azole antifungals: itraconazole (ITZ), posaconazole (POS), terconazole (TER) and ketoconazole (KET), differing mainly in the length of the rod-like backbone and slightly in side groups. Our investigations clearly demonstrated that the changes in the chemical structure result in a different ability to form the medium-range order (MRO) and variation in thermal and dynamical properties of these pharmaceuticals. Direct comparison of the diffractograms collected for glassy and crystalline materials indicated that the MRO observed in the former phases is related to maintaining the local molecular arrangement of the crystal structure. Moreover, it was shown that once the MRO-related diffraction peaks appear, additional mobility (δ- or α' relaxation), slower than the structural (α)-process, is also detected in dielectric spectra. This new mode is connected to the motions within supramolecular nanoaggregates. Detailed analysis of dielectric and calorimetric data also revealed that the variation in the internal structure and MRO of the examined pharmaceuticals have an impact on the glass transition temperature (Tg) shape of the α-process, isobaric fragility, molecular dynamics in the glassy state and number of dynamically correlated molecules. These findings could be helpful in an understanding the influence of different types of intermolecular MRO on the properties of substances having a similar chemical backbone.

Deep Eutectic Solvents: A Review of Fundamentals and Applications

Deep eutectic solvents (DESs) are an emerging class of mixtures characterized by significant depressions in melting points compared to those of the neat constituent components. These materials are promising for applications as inexpensive "designer" solvents exhibiting a host of tunable physicochemical properties. A detailed review of the current literature reveals the lack of predictive understanding of the microscopic mechanisms that govern the structure-property relationships in this class of solvents. Complex hydrogen bonding is postulated as the root cause of their melting point depressions and physicochemical properties; to understand these hydrogen bonded networks, it is imperative to study these systems as dynamic entities using both simulations and experiments. This review emphasizes recent research efforts in order to elucidate the next steps needed to develop a fundamental framework needed for a deeper understanding of DESs. It covers recent developments in DES research, frames outstanding scientific questions, and identifies promising research thrusts aligned with the advancement of the field toward predictive models and fundamental understanding of these solvents.

Are hydrogen supramolecular structures being suppressed upon nanoscale confinement? The case of monohydroxy alcohols

In this paper, the molecular dynamics, H-bonding pattern and wettability of the primary and secondary monohydroxyalcohols, 2-ethyl-1-hexanol (2E1H), 2-ethyl-1-butanol (2E1B) and 5-methyl-3-heptanol (5M3H) infiltrated into native and functionalized silica and alumina pores having pore diameters, d = 4 nm and d = 10 nm, have been studied with the use of Broadband Dielectric (BDS) and Fourier Transform InfraRed (FTIR) spectroscopies, as well as contact angle measurements. We found significant differences in the behavior of alcohols forming chain- (2E1H, 2E1B) or micelle-like (5M3H) supramolecular structures despite of their similarities in the wettability and interfacial energy. It turned out that nanoassociates as well as H-bonds are more or less affected by the confinement dependently on the chemical structure and alcohol order. Moreover, a peculiar behavior of the self-assemblies at the interface was noted in the latter material (5M3H). Finally, it was found that irrespectively to the sample, type of pores, functionalization, the temperature evolution of Debye relaxation times, τ_D, of the confined systems deviates from the bulk behavior always at similar τ_D due to vitrification of the interfacial layer. This finding is a clear indication that unexpectedly dynamics (mobility) of the supramolecular structures close to the hydrophilic and hydrophobic surfaces is similar in each system.

Density Scaling Based Detection of Thermodynamic Regions of Complex Intermolecular Interactions Characterizing Supramolecular Structures

In this paper, applying the density scaling idea to an associated liquid 4-methyl-2-pentanol used as an example, we identify different pressure-volume-temperature ranges within which molecular dynamics is dominated by either complex H-bonded networks most probably leading to supramolecular structures or non-specific intermolecular interactions like van der Waals forces. In this way, we show that the density scaling law for molecular dynamics near the glass transition provides a sensitive tool to detect thermodynamic regions characterized by intermolecular interactions of different type and complexity for a given material in the wide pressure-volume-temperature domain even if its typical form with constant scaling exponent is not obeyed. Moreover, we quantify the observed decoupling between dielectric and mechanical relaxations of the material in the density scaling regime. The suggested methods of analyses and their interpretations open new prospects for formulating models based on proper effective intermolecular potentials describing physicochemical phenomena near the glass transition.

Studies on the internal medium-range ordering and high pressure dynamics in modified ibuprofens

Broadband dielectric spectroscopy (BDS), combined with the X-ray diffraction (XRD) and Fourier transform infrared (FTIR) techniques, was used to study the dynamics of the primary (α) relaxation process and slow mode (SM), as well as structural properties and intermolecular interactions, in the methyl-, isopropyl-, hexyl-, and benzyl derivative of a well-known pharmaceutical, ibuprofen (IBU). Unexpectedly, the XRD and FTIR methods revealed the formation of medium-range ordering together with some molecular organization, which probably leads to the creation of small aggregates at the scale of several microns at lower temperatures. Moreover, high pressure dielectric experiments revealed that the SM (observed in the ambient pressure data) is not detected in the loss spectra of compressed IBU esters, which is consistent with the results reported previously for propylene carbonate and dioxolane derivatives. This finding can be interpreted as connected to either the comparable time scale of the structural dynamics and slow mode or suppression of the motions responsible for the latter process at elevated pressure. Additionally, it was found that the pressure coefficient of the glass transition temperature (dTg/dp) and activation volume (ΔV) change with molecular weight (Mw) in a non-monotonic way. It might be related to various chemical structures, conformations, and intermolecular interactions, as well as different architecture of supramolecular aggregates in the investigated compounds.

Mesoscale Organization and Dynamics in Binary Ionic Liquid Mixtures

The impact of mesoscale organization on dynamics and ion transport in binary ionic liquid mixtures is investigated by broad-band dielectric spectroscopy, dynamic-mechanical spectroscopy, X-ray scattering, and molecular dynamics simulations. The mixtures are found to form distinct liquids with macroscopic properties that significantly deviate from weighted contributions of the neat components. For instance, it is shown that the mesoscale morphologies in ionic liquids can be tuned by mixing to enhance the static dielectric permittivity of the resulting liquid by as high as 100% relative to the neat ionic liquid components. This enhancement is attributed to the intricate role of interfacial dynamics associated with the changes in the mesoscopic aggregate morphologies in these systems. These results demonstrate the potential to design the physicochemical properties of ionic liquids through control of solvophobic aggregation.

Collective Mesoscale Dynamics of Liquid 1-Dodecanol Studied by Neutron Spin-Echo Spectroscopy with Isotopic Substitution and Molecular Dynamics Simulation

The collective dynamics of liquid 1-dodecanol was investigated at a length scale matching the mesoscale structure arising from the segregation of hydrophilic and hydrophobic domains. To this end, neutron spin-echo experiments were performed on a series of partially deuterated samples and the relevant collective dynamics of the hydroxyl groups with respect to the alkyl chains was extracted from the linear combination of the intermediate scattering functions of these samples. The resulting collective dynamics is slower than the single particle dynamics as determined by the measurement on the nondeuterated sample. The experimental results are in excellent agreement with molecular dynamics simulation, which allows further insight into the mechanism of the molecular motions. The results indicate that two factors are responsible for the slower collective dynamics. The first one is the slower dynamics of the hydroxyl group, with respect to the alkyl chains, owing to hydrogen bonding, and the second one is the presence of mesoscale structuring.

Carboxylic acids in aqueous solutions: Hydrogen bonds, hydrophobic effects, concentration fluctuations, ionization, and catalysis

In the frequency range between 100 kHz and 2 GHz, ultrasonic absorption spectra have been measured for a series of carboxylic acids from formic to enanthic acid, including constitutional isomers. Also investigated have been the spectra for mixtures with water of short-chain formic, acetic, propionic, butyric, and isobutyric acid, in each case covering the complete composition range. The neat carboxylic acids feature two Debye-type relaxation terms with relaxation times between 5.6 and 260 ns as well as 0.14 and 1.4 ns, respectively, at room temperature. Depending on the composition, mixtures with water reveal an additional Debye relaxation term in the intermediate frequency range (acetic acid) or a term subject to a relaxation time distribution (propionic, butyric, and isobutyric acid). The relaxations of the neat acids are assigned to the equilibrium between monomers and single-hydrogen-bonded linear dimers and between linear and twofold-hydrogen-bonded cyclic dimers. The latter equilibrium is considerably catalyzed by hydronium and carboxylate ions. Several mixtures with water indicate one of the up to three Debye relaxations to reflect the protolysis of the organic acid. The term with underlying relaxation time distribution is due to noncritical fluctuations in the local concentrations. The Debye relaxations are evaluated to yield the parameters of the relevant elementary chemical reactions, such as the rate and equilibrium constants and the isentropic reaction volumes. A comparison of the correlation length of concentration fluctuations with data for other aqueous systems confirms the idea that the hydrophobic part of the organic constituent promotes the formation of a micro-heterogeneous liquid structure, whereas the hydrophilic moiety is of minor importance in this respect. The high-frequency limiting absorption suggests the equilibrium between conformers of linear dimers to contribute to the spectra well above the frequency range of measurements.

Inflection point in the Debye relaxation time of 2-butyl-1-octanol

We report a striking anomaly in the pressure dependent Debye-relaxation time of the branched monohydroxy alcohol 2-butyl-1-octanol. Evidence of a crossover from slower to faster than exponential pressure dependency was obtained at different temperatures via high pressure broadband dielectric spectroscopy. At the same time, viscosity measurements reveal similar behavior in the viscosity, respectively, the structural relaxation time, indicating a similar origin of the phenomena.

Connecting structurally and dynamically detected signatures of supramolecular Debye liquids

The monohydroxy alcohol 2-ethyl-1-hexanol mixed with the halogen-substituted alkyl halides 2-ethyl-1-hexyl chloride and 2-ethyl-1-hexyl bromide was studied using synchrotron-based x-ray scattering. In the diffraction patterns, an oxygen-related prepeak appears. The concentration dependence of its intensity, shape, and position indicates that the formation of the hydrogen-bonded associates of monohydroxy alcohols is largely hindered by the halogen alkane admixture. Using dielectric spectroscopy and high-resolution rheology on the same liquid mixtures, it is shown that these structural features are correlated with the relaxation mechanisms giving rise to supramolecular low-frequency dynamics.

Associating Imidazoles: Elucidating the Correlation between the Static Dielectric Permittivity and Proton Conductivity

Broadband dielectric spectroscopy is employed to investigate the impact of supramolecular structure on charge transport and dynamics in hydrogen-bonded 2-ethyl-4-methylimidazole and 4-methylimidazole. Detailed analyses reveal (i) an inverse relationship between the average supramolecular chain length and proton conductivity and (ii) no direct correlation between the static dielectric permittivity and proton conductivity in imidazoles. These findings raise fundamental questions regarding the widespread notion that extended supramolecular hydrogen-bonded networks facilitate proton conduction in hydrogen bonding materials.

Structural Relaxation and Viscoelasticity of a Higher Alcohol with Mesoscopic Structure

This work studied the slow dynamics of liquids with mesoscopic structure and its relation to shear viscosity. Quasielastic scattering measurements were made on a liquid higher alcohol, 3,7-dimethyl-1-octanol, using γ-ray time-domain interferometry at a synchrotron radiation facility, SPring-8. The quasielastic scattering spectra were measured to determine the structural relaxation at two wavenumbers of the prepeak and the main peak of the static structure factor. It was found that relaxation at the prepeak is more than 10 times slower than that at the main peak. Compared with the viscoelastic spectrum, which exhibits bimodal relaxation, the relaxations at the prepeak and the main peak were shown to correspond to the slower and faster modes of the viscoelastic relaxation, respectively. This indicates that the dynamics of the mesoscopic structure represented as the prepeak contributes to the shear viscosity through the slowest mode of the viscoelastic relaxation.

Unusual dielectric response of 4-methyl-1,3-dioxolane derivatives

In this paper, we applied broadband dielectric spectroscopy (BDS) to investigate the molecular dynamics of three 4-methyl-1,3-dioxolane derivatives (MD) whose chemical structures differ in the length of non-polar alkyl side chains.

Coupling between the mesoscopic dynamics and shear stress of a room-temperature ionic liquid

Shear viscosity of an ionic liquid is governed by the dynamics of the charge-alternation mode irrespective of the presence of the domain structure.

Dynamic-Mechanical and Dielectric Evidence of Long-Lived Mesoscale Organization in Ionic Liquids

Experimental evidence of the dynamics of mesoscopic structure in room-temperature ionic liquids-a feature expected to correlate with many physicochemical properties of these materials-remains limited. Here, we report the observation of slow, sub-α relaxations corresponding to dynamics of nanoscale hydrophobic aggregates in a systematic series of 1-alkyl-3-methylimidazolium-based ionic liquids from detailed analysis of dynamic-mechanical and broad-band dielectric spectra. The emergence of the sub-α relaxations correlates with increases in the zero-shear viscosity and static dielectric permittivity, constituting direct evidence of the influence of mesoscale aggregation on the physicochemical properties of ionic liquids.