Dielectric relaxation and proton field-cycling NMR relaxometry study of dimethyl sulfoxide/glycerol mixtures down to glass-forming temperatures

Abstract

Mixtures of glycerol and dimethyl sulfoxide (DMSO) are studied by dielectric spectroscopy (DS) and by ¹H field-cycling (FC) NMR relaxometry in the entire concentration range and down to glass-forming temperatures (170–323 K). Molecular dynamics is accessed for 0 < x_DMSO ≤ 0.64, at higher concentration phase separation occurs. The FC technique provides the frequency dependence of the spin–lattice relaxation rate which is transformed to the susceptibility representation and thus allows comparing NMR and DS results. The DS spectra virtually do not change with x_DMSO and T, only the relaxation times become shorter. This is in contrast to the non-associated mixture toluene/quinaldine for which strong spectral changes occur. The FC relaxation spectra of glycerol in solution with DMSO or (deuterated) DMSO-d₆ display a bimodal structure with a high-frequency part reflecting rotational and a low-frequency part reflecting translational dynamics. Regarding the rotational contribution in the glycerol/DMSO-d₆ mixtures, no spectral change with x_DMSO and T is observed. Yet, the non-deuterated mixture reveals a broader relaxation spectrum. Time constants τ_rot(T) probed by the two techniques complement each, a range 10⁻¹¹ s < τ < 10 s is covered. The glass transition temperature T_g(x_DMSO) is determined, yielding T_g = 149.5 ± 1 K of pure DMSO by extrapolation. Analysing the low-frequency FC NMR spectra allows to determine the diffusion coefficient D_trans. Its logarithm shows a linear x_DMSO-dependence as does lg τ_rot. The ratio D_trans/D_rot is independent of x_DMSO and its low value indicates large separation of translation and rotation. The corresponding unphysically small hydrodynamic radius indicates strong failure of Stokes–Einstein–Debye relation. Such anomaly is taken as characteristics of a 3d hydrogen-bonded network. We conclude, although DMSO is an aprotic liquid the molecule is continuously incorporated in the hydrogen network of glycerol. Both molecules display common dynamics, i.e., no decoupling of the component dynamics is found in contrast to quinaldine/toluene with a similar T_g difference of its components.