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

Structural Evolution in a Glass-Forming Liquid Alcohol by X-Ray Scattering: Contrasting Behaviors of Main Peak and Prepeak Structures

X-ray scattering of liquid 2-ethyl-1-hexanol (2E1H) has been measured from its liquid state to its glassy state with focus on the main scattering peak and the prepeak. The main peak, associated with the packing of the alkyl chains, shifts to higher angle and sharpens in a manner consistent with closely packed spheres, until kinetic arrest at the glass transition temperature Tg (146 K). In contrast, the prepeak, associated with the correlation of the hydroxyl groups separated by the hydrocarbon chains, shows a transition near 220 K, below which its width is nearly frozen and insensitive to the passage of Tg. This transition coincides with a similar transition in the Kirkwood factor gK which reports the orientational correlation of the OH dipoles, and with the transition reported previously as the "250 K anomaly" based on other observables. This transition arises from the increased hydrogen bonding between the hydroxyl groups and the resulting improvement of the regularity of the alcohol bilayers.

Radical Diffusion Crossover Phenomenon in Glass-Forming Liquids

We studied the diffusivities of a nitroxide radical at various temperatures in six glass-forming molecular liquids by electron spin resonance. By comparing the radical diffusivities and solvent self-diffusivities, we found that the radical diffusivities are lower than the self-diffusivities at high temperatures and approach them at low temperatures in all liquids. This crossover behavior was considered as evidence that a single-molecule diffusion process transforms into a collective process with temperature lowering. The crossover phenomenon was analyzed by a novel, simple diffusion model, combining collective and single-molecule diffusion processes, and it was compared to the Arrhenius crossover phenomenon. The obtained results suggest that future studies of tracer diffusion could contribute to a better understanding of diffusion mechanisms in glass-forming liquids. The proposed diffusion model could be used to study the crossover phenomena of tracer diffusion measured by other techniques, and it could serve as a base for developing more advanced models.

Q-dependent collective relaxation dynamics of glass-forming liquid Ca0.4K0.6(NO3)1.4 investigated by wide-angle neutron spin-echo

The relaxation behavior of glass formers exhibits spatial heterogeneity and dramatically changes upon cooling towards the glass transition. However, the underlying mechanisms of the dynamics at different microscopic length scales are not fully understood. Employing the recently developed wide-angle neutron spin-echo spectroscopy technique, we measured the Q-dependent coherent intermediate scattering function of a prototypical ionic glass former Ca_0.4K_0.6(NO₃)_1.4, in the highly viscous liquid state. In contrast to the structure modulated dynamics for Q < 2.4 Å⁻¹, i.e., at and below the structure factor main peak, for Q > 2.4 Å⁻¹, beyond the first minimum above the structure factor main peak, the stretching exponent exhibits no temperature dependence and concomitantly the relaxation time shows smaller deviations from Arrhenius behavior. This finding indicates a change in the dominant relaxation mechanisms around a characteristic length of 2π/(2.4 Å⁻¹) ≈ 2.6 Å, below which the relaxation process exhibits a temperature independent distribution and more Arrhenius-like behavior.

Length scale dependence is important for understanding the collective relaxation dynamics in glass-forming liquids. Here, the authors find in liquid Ca_0.4K_0.6(NO₃)_1.4 a change in the dominant relaxation mechanisms around 2.6 Å, below which the relaxation process exhibits a temperature independent distribution and more Arrhenius-like behavior.

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.