Patrick Judeinstein

Proton Diffusion in the Hexafluorophosphoric Acid Clathrate Hydrate

The hexafluorophosphoric acid clathrate hydrate is known as a super-protonic conductor: its proton conductivity is of the order of 0.1 S/cm at ca. room temperature. The long-range proton diffusion and the associated mechanism have been analyzed with the help of incoherent quasi-elastic neutron scattering (QENS) and proton pulsed-field-gradient nuclear magnetic resonance ((1)H PFG-NMR). The system crystallizes into the so-called type I clathrate structure (SI) at low temperature and into the type VII structure (SVII) above ca. 230 K with a melting point close to room temperature. While, in the SI phase, no long-range proton diffusion is observed (at least faster than the present measurement capabilities, i.e., 10(-7) cm(2)·s(-1)) with respect to the probed time scale, both techniques evidence a long-range proton diffusion process in the SVII phase (3.85 × 10(-6) cm(2)·s(-1) at 275 K with an activation energy of 0.19 ± 0.04 eV). QENS experiments lead to modeling the microscopic mechanism of the long-range proton diffusion by means of a Chudley-Elliot jump diffusion model with a characteristic jump distance of 2.79 ± 0.17 Å. In other words, the long-range diffusion occurs through a Grotthus mechanism with proton jumping from one water-oxygen site to another. Moreover, the analysis of the proton diffusion for hydration numbers greater than 6 (i.e., in the SVII structure) reveals that the additional water molecules coexisting with the SVII structure act as a structural defect barrier for the proton diffusivity, responsible for the conductivity.

A PGSE-NMR Study of Molecular Self-Diffusion in Lamellar Phases Doped with Polyoxometalates

Using pulsed gradient spin-echo NMR, we studied molecular self-diffusion in aligned samples of a hybrid lyotropic lamellar L(alpha) phase. This composite organic-inorganic material was obtained by doping the lamellar phase of the nonionic surfactant Brij-30 with the [PW(12)O(40)](3-) polyoxometalate (POM). Both water and POM self-diffusion display a large anisotropy, as diffusion is severely restricted along the normal to the bilayers. Water diffusion in planes parallel to the bilayers does not depend on the POM concentration but depends on the lamellar period, which is due to a variable fraction of bound water molecules. POM diffusion in the hybrid L(alpha) phase is almost 2 orders of magnitude slower than in aqueous solution. Moreover, it is not at all affected by the thickness of the aqueous medium separating the bilayers. This proves that the POM nanoparticles do not freely diffuse in the interbilayer aqueous space but adsorb onto the PEG brushes that cover both sides of the surfactant bilayers.

Multiscale NMR investigation of mesogenic ionic-liquid electrolytes with strong anisotropic orientational and diffusional behaviour

A new class of mesogenic ionic-liquid electrolytes made of long-chain imidazolium cations mixed with lithium salts is described. Fast, homogeneous and uniform orientational alignment of these systems can be obtained by cooling the mixture from the isotropic melt under magnetic constraint (magnetic fields of 9.4 and 14.1 T). The local orientational behaviour of the different species inside the electrolyte and the ionic diffusional properties in oriented phases of these new materials are investigated by NMR of 2H, 7Li and 11B quadrupolar nuclei and PFG-NMR measurements, respectively. Interestingly, the local orientation of C–H internuclear directions along the alkyl chains of the imidazolium cations is examined using natural abundance deuterium (NAD) 2D-NMR spectroscopy, thus avoiding any isotopic enrichment of material. The large anisotropy of self-diffusion coefficients measured from mobile ions (Li+ and BF4−) evidences the long-range alignment of ionic slabs of these materials.

Competing coexisting phases in 2D water

The properties of bulk water come from a delicate balance of interactions on length scales encompassing several orders of magnitudes: i) the Hydrogen Bond (HBond) at the molecular scale and ii) the extension of this HBond network up to the macroscopic level. Here, we address the physics of water when the three dimensional extension of the HBond network is frustrated, so that the water molecules are forced to organize in only two dimensions. We account for the large scale fluctuating HBond network by an analytical mean-field percolation model. This approach provides a coherent interpretation of the different events experimentally (calorimetry, neutron, NMR, near and far infra-red spectroscopies) detected in interfacial water at 160, 220 and 250u2009K. Starting from an amorphous state of water at low temperature, these transitions are respectively interpreted as the onset of creation of transient low density patches of 4-HBonded molecules at 160u2009K, the percolation of these domains at 220u2009K and finally the total invasion of the surface by them at 250u2009K. The source of this surprising behaviour in 2D is the frustration of the natural bulk tetrahedral local geometry and the underlying very significant increase in entropy of the interfacial water molecules.

Communication : Investigation of ion aggregation in ionic liquids and their solutions with lithium salt under high pressure

X-ray scattering measurements were utilized to probe the effects of pressure on a series of ionic liquids, N-alkyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr1A-TFSI) (A = 3, 6, and 9), along with mixtures of ionic liquid and 30 mol. % lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt. No evidence was found for crystallization of the pure ionic liquids or salt mixtures even at pressures up to 9.2 GPa. No phase separation or demixing was observed for the ionic liquid and salt mixtures. Shifts in the peak positions are indicative of compression of the ionic liquids and mixtures up to 2 GPa, after which samples reach a region of relative incompressibility, possibly indicative of a transition to a glassy state. With the application of pressure, the intensity of the prepeak was found to decrease significantly, indicating a reduction in cation alkyl chain aggregation. Additionally, incompressibility of the scattering peak associated with the distance between like-charges in the pure ionic liquids compared to that in mixtures with lithium salt suggests that the application of pressure could inhibit Li+ coordination with TFSI- to form Li[TFSI2]- complexes. This inhibition occurs through the suppression of TFSI- in the trans conformer, in favor of the smaller cis conformer, at high pressures.

Low-field single-sided NMR for one-shot 1D-mapping: Application to membranes

Many single-sided permanent magnet NMR systems have been proposed over the years allowing for 1D proton-density profiling, diffusion measurements and relaxometry. In this manuscript we make use of a recently published unilateral magnet for low-field NMR exhibiting an extremely uniform magnetic field gradient with moderate strength and cylindrical symmetry, allowing for a well-defined sweet spot. Combined with a goniometer, our system is used to characterize precisely the uniformity of its gradient and to achieve micrometric precision 1D profiling, as well as spatially localized relaxometry and diffusometry on thick (∼150μm) membrane samples. Profiling with this magnet did not require repositioning of the samples with respect to the 1D tomograph.

Ionic Liquids: evidence of the viscosity scale-dependence

Ionic Liquids (ILs) are a specific class of molecular electrolytes characterized by the total absence of co-solvent. Due to their remarkable chemical and electrochemical stability, they are prime candidates for the development of safe and sustainable energy storage systems. The competition between electrostatic and van der Waals interactions leads to a property original for pure liquids: they self-organize in fluctuating nanometric aggregates. So far, this transient structuration has escaped to direct clear-cut experimental assessment. Here, we focus on a imidazolium based IL and use particle-probe rheology to (i) catch this phenomenon and (ii) highlight an unexpected consequence: the self-diffusion coefficient of the cation shows a one order of magnitude difference depending whether it is inferred at the nanometric or at the microscopic scale. As this quantity partly drives the ionic conductivity, such a peculiar property represents a strong limiting factor to the performances of ILs-based batteries.

What use for polysilsesquioxane lithium salts in lithium batteries

Aryl-containing lithium perfluorosulfonates form unquestionably a family of salts having fairly good electrochemical performances, especially high cation transference numbers. Here, this family was extended to polysilsesquioxanes in which every silicon atom bears one lithium perfluorosulfonate group, via a sol–gel synthesis involving an original organotrialkoxysilane salt precursor. Despite their high molecular weights, these macro polyanions were found to be highly soluble in polar solvents such as acetonitrile or ethanol. NMR spectroscopy and SEC analysis were consistent with high molecular weight species and a random arrangement of cyclic and/or branched structures. These salts featured a wide electrochemical stability window up to 4 V in acetonitrile solution, while PFG NMR spectroscopy disclosed that the self diffusion coefficient of the macro polyanion decreased significantly with respect to a parent monomer salt chosen as a reference. The potential of high molecular weight polysilsesquioxane salts as lithium source nanofillers in liquid and polymer electrolytes was then discussed in light of the calculated lithium cation transference number.