Change of translational-rotational coupling in liquids revealed by field-cycling 1H NMR

Applying the field-cycling nuclear magnetic resonance technique, the frequency dependence of the H-1 spin-lattice relaxation rate, R-1(omega) = T-1(-1) (omega), is measured for propylene glycol (PG) which is increasingly diluted with deuterated chloroform. A frequency range of 10 kHz-20 MHz and a broad temperature interval from 220 to about 100 K are covered. The results are compared to those of experiments, where glycerol and o-terphenyl are diluted with their deuterated counter-part. Reflecting intra- as well as intermolecular relaxation, the dispersion curves R-1(omega, x) (x denotes mole fraction PG) allow to extract the rotational time constant tau(rot)(T, x) and the self-diffusion coefficient D(T, x ) in a single experiment. The Stokes-Einstein-Debye (SED) relation is tested in terms of the quantity D(T, x)tau(rot)(T, x) which provides a measure of an effective hydrodynamic radius or equivalently of the spectral separation of the translational and the rotational relaxation contribution. In contrast to o-terphenyl, glycerol and PG show a spectral separation much larger than suggested by the SED relation. In the case of PG/chloroform mixtures, not only an acceleration of the PG dynamics is observed with increasing dilution but also the spectral separation of rotational and translational relaxation contributions continuously decreases Finally, following a behavior similar to that of oterphenyl already at about x = 0.6; i.e., while D(T, x)tau(rot)(T, x) in the mixture is essentially temperature independent, it strongly increases with x signaling thus a change of translational-rotational coupling. This directly reflects the dissolution of the hydrogen-bond network and thus a change of solution structure. (c) 2015 AIP Publishing LLC.