Decoupling of lithium and proton self-diffusion in supercooled LiCl:7H2O: A nuclear magnetic resonance study using ultrahigh magnetic field gradients

Abstract

Self-diffusion coefficients of lithium ions and water protons ͑D Li and D H ͒ in the glass-forming electrolyte LiCl:7H 2 O have been measured by nuclear magnetic resonance spin-echo experiments using ultrahigh static magnetic field gradients up to 184 T m Ϫ1 . The measurements were complemented by measurements of 7 Li and 1 H spin-lattice relaxation times. The data cover the temperature range from 313 K down to 173 K, i.e., 34 K above the glass transition temperature T g ϭ139 K. In this range D Li and D H change over five orders of magnitude. The self-diffusion data exhibit a strong non-Arrhenius temperature dependence which is typical for fragile glass formers. In the supercooled regime the ratio of the self-diffusion coefficients D H /D Li increases gradually with decreasing temperature, reflecting a decoupling of these diffusive modes. These results are discussed in relation to the behavior of the viscosity, electrical conductance and reorientational correlation time of water in this temperature range. It is found that lithium ion diffusion is closely coupled to these other transport processes, while proton diffusion begins to decouple at TϽ1.5 T g . Additionally, an analysis of 1 H and 7 Li magnetic relaxation rates 1/T 1 is given. It is found that the intermolecular modes causing 1 H-1 H dipolar relaxation and 7 Li quadrupolar relaxation also decouple from the viscosity. The results are discussed in the framework of similar phenomena observed with other fragile glasses and, more specifically, of structural changes known to occur in supercooled LiCl:H 2 O systems.

High Li[sup +] Self-Diffusivity and Transport Number in Novel Electrolyte Solutions

“…An example of other recent diffusivities determined in this manner is given in Ref. 7. Figure 1 shows the raw data of the echo attenuations for lithium and fluorinated species of LiBPFPB in 0.5 M PC solution.…”

Translational Self-Diffusion Measurements and Analysis of a Calamitic Smectogen

“…The method, that exploits the high static gradient (typically of strength 25-80 T=m) present in the fringe field of commercial superconducting magnet has been used by Kimmich et al to measure diffusion in polymer melts [9]; by Feweier et al in super-cooled fluids [10] and Karakatsanis and Bayerl in phospholipids bilayers [11]. In liquid crystals, it has been used by Vilfan and co-workers to measure self diffusion of the nematic phase of 4-n-pentyl-4 0 -cyanobiphenyl (5CB) both in the bulk and confined in controlled porous glasses [12], and by the author to measure diffusion in the nematic and smectic A phase of 4-n-octyloxy -4'-cyanobiphenyl (8OCB) [7] and 4-4 0 -di-noctyloxy-azoxybenzene (OAB) [8].…”

5 NMR studies of self-diffusion in liquids

“…Of course, 1 H is the nucleus of choice, but experiments are also possible with some other nuclei such as 7 Li. 76 Even the 17 O resonance of isotopically enriched H 2…”

“…O has been exploited. 76 Feiweier et al 83 have estimated that, by such ultrahigh field gradients, one should be able to detect displacements down to the 10 nm scale. Using gradients of 180 T m Ϫ1 , they were indeed able to confirm a lower experimental limit of 7 nm.…”

“…In this context, 17 O NMR may also be of use, but with T 2 being of the order of some milliseconds, experiments are certainly borderline. Nevertheless, Feiweier et al 76 were able to perform self-diffusion measurements in ultrahigh field gradients using 17 O-enriched water. Such measurements may provide valuable information, whenever self-diffusion coefficients of protons may be different from that of the water molecules, for example in solutions containing H ϩ ions.…”