Rheology based estimates of self- and collective diffusivities in viscous liquids.

The self-diffusion coefficient of viscous liquids is estimated on the basis of a simple analysis of their rheological shear spectra. To this end, the Almond-West approach, previously employed to access single-particle diffusivities in ionic conductors, is generalized for application to molecular dynamics in supercooled liquids. Rheology based estimates, presented for indomethacin, ortho-terphenyl, and trinaphthylbenzene, reveal relatively small, yet systematic differences when compared with diffusivity data directly measured for these highly viscous liquids. These deviations are discussed in terms of mechanical Haven ratios, introduced to quantify the magnitude of collective translational effects that have an impact on the viscous flow.

Communication: Correlation of terminal relaxation rate and viscosity enhancement in supramolecular small-molecule liquids.

Monohydroxy alcohols with a large supramolecular Debye-type dielectric process often exhibit a significant decoupling between the Debye mode and the structural relaxation. Using shear rheology, a technique that is sensitive to both processes as well, the current work reveals a widely applicable correlation in terms of the dynamical onset and the viscosity enhancement of the supramolecular shear mode with respect to the structural relaxation. Rheological data from an array of about 50 oligomers, associating polymers, (polymerized) ionic liquids, and aqueous solutions corroborate this correlation which thus appears to be generic to many classes of complex fluids.

Communication: Nonadditive dielectric susceptibility spectra of associating liquids.

Highly unusual linear-response spectra involving contributions from hydrogen-bonded supramolecular processes and from structural relaxations are found in 4-methyl-3-heptanol mixed with 2-ethyl-1-hexylbromide. Although the mean time scales of the underlying relaxations are separated by more than 3 decades, the overall spectra cannot be decomposed into a sum of these processes. This finding challenges the ubiquitous practice of disentangling susceptibility spectra of Debye liquids by adding suitable subspectra. The spectral shape of the studied viscous mixtures is excellently described using the Williams ansatz, here a necessary approach and not as previously considered merely an alternative to additive analyses.

Supramolecular x-ray signature of susceptibility amplification in hydrogen-bonded liquids.

Mixing two nonconducting hydrogen-bonded liquids, each exhibiting a low dielectric relaxation strength, can result in a highly electrically absorbing fluid. This susceptibility amplification effect is demonstrated for mixtures of monohydroxy alcohols. Whereas in the pure liquids a tendency to form ringlike low-dipole moment clusters prevails, in the mixtures such supramolecular structures are disfavored leading to an up to tenfold enhancement of the dielectric loss. The compositional evolution of density and mean cluster-cluster separation is traced using x-ray scattering and indicates deviations from ideal mixing with decreased C-C but simultaneously increased O-O correlation lengths. Thus, the variation in the supramolecular absorption strength could be tracked using a static scattering technique. These observations are in harmony with volume exclusion and ring open effects that predict an optimized susceptibility amplification for mixtures in which the two components occupy equal volume fractions as experimentally observed.