Nanoscopic length scale dependence of hydrogen bonded molecular associates' dynamics in methanol

Contrast dependence of scattering profiles for poly(ethylene glycol) in water: Investigation by small-angle neutron scattering with 3He spin filter and small-angle x-ray scattering

The small-angle neutron scattering (SANS) and small-angle x-ray scattering (SAXS) measurements were performed for deuterated and non-deuterated poly(ethylene glycol) (d-PEG and h-PEG, respectively) in D2O and a D2O/H2O mixed solvent (Mix) to compare the scattering profiles. To determine the coherent scattering intensity of SANS, a 3He spin filter was utilized. The scattering profiles determined by the SANS measurements were analyzed in terms of the wormlike chain model with touched beads along the contour of the chain. However, the SAXS profiles were not explained by the same model with uniform beads but with beads each consisting of a core and a shell having different electron densities. To explore the chain thickness determined from the SANS profile, the scattering intensities for different combinations of d-PEG/D2O, d-PEG/Mix, h-PEG/D2O, and h-PEG/Mix were also examined.

Dynamics of molecular associates in methanol/water mixtures

The dynamics of molecular associates in a methanol/water mixture was investigated using quasielastic neutron scattering. By measuring the signal from four methanol/water samples differing only by their isotopic composition, the relative motion of the water to methanol molecules, i.e. their mutual dynamics, was determined at the nanoscale. The thus obtained nanoscopic mutual diffusion coefficient signals a significantly slower process than the single particle diffusion of either methanol or water in the system as well as their macroscopic mutual diffusion. The data do not provide any indication of microsegregation in this preeminent alcohol/water mixture; however, they do indicate the existence of long lived but dynamic molecular associates of water and methanol molecules. Analysis of the structural relaxation shows that the lifetime of molecular association through hydrogen bonding determines the fact that viscosity of the mixtures at intermediate concentrations is higher than that of both pure components.

Relevance of hydrogen bonded associates to the transport properties and nanoscale dynamics of liquid and supercooled 2-propanol

2-Propanol was investigated, in both the liquid and supercooled states, as a model system to study how hydrogen bonds affect the structural relaxation and the dynamics of mesoscale structures, of approximately several Ångstroms, employing static and quasi-elastic neutron scattering and molecular dynamics simulation. Dynamic neutron scattering measurements were performed over an exchanged wave-vector range encompassing the pre-peak, indicative of the presence of H-bonding associates, and the main peak. The dynamics observed at the pre-peak is associated with the formation and disaggregation of the H-bonded associates and is measured to be at least one order of magnitude slower than the dynamics at the main peak, which is identified as the structural relaxation. The measurements indicate that the macroscopic shear viscosity has a similar temperature dependence as the dynamics of the H-bonded associates, which highlights the important role played by these structures, together with the structural relaxation, in defining the macroscopic rheological properties of the system. Importantly, the characteristic relaxation time at the pre-peak follows an Arrhenius temperature dependence whereas at the main peak it exhibits a non-Arrhenius behavior on approaching the supercooled state. The origin of this differing behavior is attributed to an increased structuring of the hydrophobic domains of 2-propanol accommodating a more and more encompassing H-bond network, and a consequent set in of dynamic cooperativity.

Neutron Spin-Echo Studies of the Structural Relaxation of Network Liquid ZnCl2 at the Structure Factor Primary Peak and Prepeak

Using neutron spin-echo spectroscopy, we studied the microscopic structural relaxation of a prototypical network ionic liquid ZnCl₂ at the structure factor primary peak and prepeak. The results show that the relaxation at the primary peak is faster than the prepeak and that the activation energy is ∼33% higher. A stretched exponential relaxation is observed even at temperatures well-above the melting point T_m. Surprisingly, the stretching exponent shows a rapid increase upon cooling, especially at the primary peak, where it changes from a stretched exponential to a simple exponential on approaching the T_m. These results suggest that the appearance of glassy dynamics typical of the supercooled state even in the equilibrium liquid state of ZnCl₂ as well as the difference of activation energy at the two investigated length scales are related to the formation of a network structure on cooling.

Coupling between Structural and Dielectric Relaxations of Methanol and Ethanol Studied by Molecular Dynamics Simulation

The microscopic origin of the fast dielectric relaxation modes and the integrated dielectric relaxation times of methanol and ethanol was investigated by means of cross-correlation analysis of molecular dynamics simulation. Random force on the fluctuation of collective dipole moment was correlated with the two-body density mode in both real and reciprocal spaces. A strong coupling was observed with the OH alternation mode at 30 nm^-1, suggesting that alternating switching of the hydrogen bond within a hydrogen-bonding chain is the principal origin of the retarded friction on the collective dipole moment. The relaxation of the coupling was much slower than that of the partial intermediate scattering functions at the corresponding wavenumber, which suggests the breakdown of the factorization approximation employed in the mode-coupling theory. Although the prepeak structure is strongly coupled to the viscoelastic relaxation, its coupling with the dielectric relaxation is relatively weak. The difference between the viscoelastic and the dielectric relaxations was discussed in terms of the different symmetries of the shear stress tensor and the collective dipole moment.

Energy Relaxation and Thermal Diffusion in Infrared Pump-Probe Spectroscopy of Hydrogen-Bonded Liquids

Infrared pump-probe spectroscopy provides detailed information about the dynamics of hydrogen-bonded liquids. Due to dissipation of the absorbed pump pulse energy, thermal equilibration dynamics also contributes to the observed signal. Disentangling this contribution from the molecular response remains a challenge. By performing non-equilibrium molecular dynamics simulations of liquid-deuterated methanol, we show that faster molecular vibrational relaxation and slower heat diffusion are decoupled and occur on different length scales. Transient structures of the hydrogen bonding network influence thermal relaxation by affecting thermal diffusivity over a length scale of several nanometers.

Picosecond self-diffusion in ethanol-water mixtures

We report the self-diffusion in ethanol-water mixtures as a function of the water-ethanol ratio measured at different temperatures using quasi-elastic neutron spectroscopy (QENS). For our protiated samples, QENS is mainly sensitive to the dominant ensemble-averaged incoherent scattering from the hydrogen atoms of the liquid mixtures. The energy range and resolution render our experiment sensitive to the picosecond time scale and nanometer length scale. These observation scales complement different scales accessible by nuclear magnetic resonance techniques. Subsequent to testing different models, we find that a simple jump-diffusion model averaging over both types of molecules, water and ethanol, best fits our data.

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.

Local dielectric response in 1-propanol: α-relaxation versus relaxation of mesoscale structures

Understanding how the local dielectric response is affected by the supramolecular Debye process in 1-propanol.

Analysis of shear viscosity and viscoelastic relaxation of liquid methanol based on molecular dynamics simulation and mode-coupling theory

The role of the prepeak structure of liquid methanol in determining its shear viscosity was studied by means of molecular dynamics (MD) simulation and mode-coupling theory (MCT). The autocorrelation function of the shear stress and the intermediate scattering functions at both the prepeak and the main peak were calculated from the MD trajectories. Their comparison based on MCT suggests that the viscoelastic relaxation in the ps regime is affected by the slow structural dynamics at the prepeak. On the other hand, the MCT for molecular liquids based on the interaction-site model (site-site MCT) fails to describe the coupling between the prepeak dynamics and shear stress. The direct evaluation of the coupling between the two-body density and the shear stress reveals that the viscoelastic relaxation is actually affected by the prepeak dynamics, although the coupling is not captured by the site-site MCT. The site-site MCT works well for a model methanol without partial charges, suggesting that the failure of the site-site MCT originates from the existence of a hydrogen-bonding network structure.

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.

Glycerol Hydrogen-Bonding Network Dominates Structure and Collective Dynamics in a Deep Eutectic Solvent

The deep eutectic solvent glyceline formed by choline chloride and glycerol in 1:2 molar ratio is much less viscous compared to glycerol, which facilitates its use in many applications where high viscosity is undesirable. Despite the large difference in viscosity, we have found that the structural network of glyceline is completely defined by its glycerol constituent, which exhibits complex microscopic dynamic behavior, as expected from a highly correlated hydrogen-bonding network. Choline ions occupy interstitial voids in the glycerol network and show little structural or dynamic correlations with glycerol molecules. Despite the known higher long-range diffusivity of the smaller glycerol species in glyceline, in applications where localized dynamics is essential (e.g., in microporous media), the local transport and dynamic properties must be dominated by the relatively loosely bound choline ions.