Mixed Debye-type liquids studied by dielectric, shear mechanical, nuclear magnetic resonance, and near-infrared spectroscopy

Abstract

Monohydroxy alcohols have been in the focus of scientific research for a long time and their Debye (-like) process was studied predominantly using dielectric spectroscopy. However, a number of other techniques are useful to unravel the dynamics of these supramolecular liquids. For a recent review on neat monohydroxy alcohols, see R. Böhmer, C. Gainaru, R. Richert, Phys. Rep. (accepted). On this background the present article deals mostly with mixtures involving monohydroxy alcohols using various experimental methods. Examples given include dielectric spectroscopy on a mixture which shows a small Debye process well separated from the structural relaxation. For another mixture it is demonstrated that the time scales of the dielectric and the rheological signatures of the Debye process coincide. Isotope labeling is exploited to map out the rotational dynamics of both components in binary mixtures of 1-butanol (BuOH) and 1-bromobutane (BuBr) using spin–lattice relaxation nuclear magnetic resonance (NMR) spectroscopy. While the hydroxyl motion in BuOH becomes faster upon admixture of BuBr, the alkyl bromide dynamics is virtually independent of composition. Two-time and four-time stimulated-echo NMR experiments show that the dynamic exchange in a BuOH–BuBr mixture is similar to that of supercooled liquids devoid of a Debye process and hence it does not provide a rationale to understand the symmetric broadening of the structural dielectric loss peak in this and related mixtures. Finally, a wavelength dependent derivative analysis of near-infrared spectra recorded for pure and mixed monohydroxy alcohols over wide temperature ranges shows that the rearrangement of the hydrogen network differs below and above about 250K.