NMR Relaxometry Accessing the Relaxation Spectrum in Molecular Glass Formers

Abstract

It is a longstanding question whether universality or specificity characterize the molecular dynamics underlying the glass transition of liquids. In particular, there is an ongoing debate to what degree the shape of dynamical susceptibilities is common to various molecular glass formers. Traditionally, results from dielectric spectroscopy and light scattering have dominated the discussion. Here, we show that nuclear magnetic resonance (NMR), primarily field-cycling relaxometry, has evolved into a valuable method, which provides access to both translational and rotational motions, depending on the probe nucleus. A comparison of 1H NMR results indicates that translation is more retarded with respect to rotation for liquids with fully established hydrogen-bond networks; however, the effect is not related to the slow Debye process of, for example, monohydroxy alcohols. As for the reorientation dynamics, the NMR susceptibilities of the structural (α) relaxation usually resemble those of light scattering, while the dielectric spectra of especially polar liquids have a different broadening, likely due to contributions from cross correlations between different molecules. Moreover, NMR relaxometry confirms that the excess wing on the high-frequency flank of the α-process is a generic relaxation feature of liquids approaching the glass transition. However, the relevance of this feature generally differs between various methods, possibly because of their different sensitivities to small-amplitude motions. As a major advantage, NMR is isotope specific; hence, it enables selective studies on a particular molecular entity or a particular component of a liquid mixture. Exploiting these possibilities, we show that the characteristic Cole–Davidson shape of the α-relaxation is retained in various ionic liquids and salt solutions, but the width parameter may differ for the components. In contrast, the low-frequency flank of the α-relaxation can be notably broadened for liquids in nanoscopic confinements. This effect also occurs in liquid mixtures with a prominent dynamical disparity in their components.

Dynamical Susceptibilities of Confined Water from Room Temperature to the Glass Transition

“…Therefore, the 1 H and 2 H T 1 (ω) dispersions provide straightforward access to spectral densities J 2 (ω) associated with water reorientation. For detailed analyses, it proved to be advantageous − to exploit the fluctuation–dissipation theorem and convert J 2 (ω) data to NMR susceptibilities χ NMR ″ ω T 1 ( ω ) = C [ ω J 2 false( ω false) + 4 ω J 2 false( 2 ω false) ] = C [ χ 2 ″ false( ω false) + 2 χ 2 ″ false( 2 ω false) ] ≡ χ NMR ″ ( ω ) which are readily obtained by multiplying the measured SLR rates 1/ T 1 (ω) by the respective angular frequencies ω; see the Supporting Information.…”

“…65 The slope of the high-frequency flank, −α HN β HN , amounts to −0.34 for H 2 O and −0.40 for D 2 O, which also indicates a somewhat larger broadening than in most bulk supercooled liquids. 62,64 We emphasize that temperature-dependent changes of the low/high frequency slope at lower/higher than the studied temperatures are not excluded. We will return to this aspect below.…”

“…The broadening of the low-frequency flank, which has a slope of ca. + 0.85, does not exist in bulk supercooled liquids, , and thus, it is caused by enhanced spatial heterogeneity of water dynamics inside the confinements. Consistently, molecular dynamics simulations traced such low-frequency broadening to slower water dynamics near the pore walls than in the pore center .…”

The Nature of the Low-Temperature Crossover of Water in Hard Confinement

“…Here, we assume a Havriliak–Negami (HN) function and determine its shape parameters in a two-step process. In additional 1 H field-cycling relaxometry (FCR) studies, we first measure 1 H T 1 (ω L ) at low frequencies and temperatures T ≥ 250 K and calculate the NMR susceptibility χ NMR ″ (ω) ≡ ω/ T 1 from these data . The results in the Supporting Information (Figure S-2) reveal power laws ω L 0.84 in the available frequency and temperature ranges, which correspond to the low-frequency flank of the loss peak associated with the HN susceptibility (eq ).…”

“…1 H field-cycling relaxometry (FCR) studies were conducted to measure the frequency dependence of the 1 H spin–lattice relaxation time. The used setup and the data analysis were described in some detail in previous works. , Finally, we used 1 H static field gradient (SFG) NMR to measure the self-diffusion coefficients D of water. These measurements were performed at a Larmor frequency of ω L of 91 MHz and a strength of the magnetic field gradient g of 139 T/m.…”