Burkhard Geil

Relaxation in the glass former acetylsalicylic acid studied by deuteron magnetic resonance and dielectric spectroscopy

Supercooled liquid and glassy acetylsalicylic acid was studied using dielectric spectroscopy and deuteron relaxometry in a wide temperature range. The supercooled liquid is characterized by major deviations from thermally activated behavior. In the glass the secondary relaxation exhibits the typical features of a Johari-Goldstein process. Via measurements of spin-lattice relaxation times the selectively deuterated methyl group was used as a sensitive probe of its local environments. There is a large difference in the mean activation energy in the glass with respect to that in crystalline acetylsalicylic acid. This can be understood by taking into account the broad energy barrier distribution in the glass.

New perspectives of NMR in ultrahigh static magnetic field gradients

Abstract The magnetic field gradient nuclear magnetic resonance (NMR) stimulated echo experiment measures the incoherent (or self) part of the intermediate scattering function S( Q ,t) ∼ 〈 exp [ − i Qr (O)] exp [ i Qr (t)]〉 with a ‘generalized’ scattering vector Q = γ· g ·τ (γ is the gyromagnetic ratio, g is the magnetic field gradient, τ is the evolution time). With ultrahigh static field gradients up to ≈ 180 T/m, a prototype of which has recently been installed in Mainz, Q-values up to > 10−2 A−1 become accessible. The first part of the paper focusses on details of this technical development and points out the close analogy with incoherent neutron scattering. In the second part, the enormous new possibilities of this kind of gradient NMR are demonstrated through a collection of most recent applications: the measurement of small self-diffusion coefficients down to about 10−15 m2s−1 in supercooled liquids and in molecular crystals, long chain polymer dynamics, restricted diffusion in systems of confined mesocopic geometries and anomalous diffusion on fractal structures.

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

With an interest in exploring the limits of relative cation/anion mobilities in nonaqueous electrolyte solutions, we have measured the diffusivities of Li- and F-containing species in 0.5 M solutions of the new lithium salt, lithium bis(perfluoropinacolato) borate, LiBPFPB, which contains a giant anion with 24 fluorine atoms. Using the pulsed field gradient spin echo method on the NMR resonances of 7 Li and 19 F in the temperature range 30-95°C we find, for the first time in nonaqueous salt-in-molecular solvent solutions, lithium diffusivities that arc higher than those of the anion-containing species. Furthermore, solutions in propylene carbonate (PC) appear to be fully dissociated, since the conductivities calculated from the Nernst-Einstein equation exceed the measured conductivities by only 23% at ambient temperature and 41% at 95°C. These values are comparable with those observed for molten salts such as LiNO 3 , NaNO 3 , and aqueous LiCI solutions. Since such deviations are known to be due to interionic friction alone, transport numbers for Li + may be calculated from the diffusivities without correction for neutral species We obtain n a value of 0.55 for PC solutions at 50°C. In the lower dielectric constant 1,2-dimethoxyethane solutions the ratio of calculated to measured conductivity is much higher. Here it would appear that ion association is still a problem and must he corrected for in calculating the transport number, For this case we obtain the value 0.53 We discuss means of increasing this value toward unity and show that this must involve abandoning simple salt solutions as electrolytes.

Reorientational and translational dynamics of benzene in zeolite NaY as studied by one- and two-dimensional exchange spectroscopy and static-field-gradient nuclear magnetic resonance

One- and two-dimensional 2H- and 13C-NMR (nuclear magnetic resonance) echo spectroscopy and 1H static field gradient NMR self-diffusion experiments have been used to study the reorientational and translational dynamics of benzene molecules adsorbed on zeolite NaY as a function of loading. Comparison of the data with model calculations establish that the elementary motional process of the guest molecules is consistently identified as a jump process among well defined adsorption sites inside a supercage and/or a jump between nearby supercages. In cases where the zeolite cavities contain high loadings of guest molecules, each molecular jump is accompanied by concomitant relaxation of the local environment. Molecular jump events between adsorption sites correspond to the elementary processes from which long range translational diffusion evolves.

Colloidal probe microscopy of membrane-membrane interactions: From ligand-receptor recognition to fusion events

A versatile model system to study membrane-membrane interactions in great detail is introduced. Based on colloidal probe microscopy with membrane covered spherical probes attached to tip-less cantilevers the interaction forces and adhesion energies are quantified down to single molecule resolution. Two opposing membranes equipped with ligands on one side and receptors on the other side were brought in contact at a defined load and pulled apart at constant velocity. Ni-NTA functionalized lipids served as receptors on the probe, while lipopeptides displaying short His-tags (CGGH(6) or CGWH(6)) were incorporated in the planar supporting membrane on a silicon substrate. The rather intricate force distance curves were scrutinized in terms of breakthrough events upon contact of the probe with the surface, the overall work of adhesion, maximum adhesion force, as well as formation frequency, lifetime, and force of membrane tethers suggesting that hemifusion of the two opposing bilayers takes place.

Correlation of primary relaxations and high-frequency modes in supercooled liquids

The question regarding a possible correlation of the time scales of primary and secondary relaxations in supercooled liquids is formulated quantitatively. It is shown how this question can be answered using spin-lattice relaxation weighted stimulated-echo experiments, which are presented in an accompanying paper [A. Nowaczyk, B. Geil, G. Hinze, and R. Böhmer, Phys. Rev. E 74, 041505 (2006)]. General theoretical expressions relevant for the description of such experiments in the presence of correlation effects are derived. These expressions are analyzed by Monte Carlo integration for various correlation scenarios also including exchange processes, which are the hallmark of dynamical heterogeneity. The results of these numerical simulations provide clear signatures that allow one to distinguish uncorrelated from differently correlated cases. Since modified spin-lattice relaxation effects occur in the presence of nonexponential magnetization recovery, it is shown how to correct for them to a good approximation.

Resolving single membrane fusion events on planar pore-spanning membranes

Even though a number of different in vitro fusion assays have been developed to analyze protein mediated fusion, they still only partially capture the essential features of the in vivo situation. Here we established an in vitro fusion assay that mimics the fluidity and planar geometry of the cellular plasma membrane to be able to monitor fusion of single protein-containing vesicles. As a proof of concept, planar pore-spanning membranes harboring SNARE-proteins were generated on highly ordered functionalized 1.2 μm-sized pore arrays in Si3N4. Full mobility of the membrane components was demonstrated by fluorescence correlation spectroscopy. Fusion was analyzed by two color confocal laser scanning fluorescence microscopy in a time resolved manner allowing to readily distinguish between vesicle docking, intermediate states such as hemifusion and full fusion. The importance of the membrane geometry on the fusion process was highlighted by comparing SNARE-mediated fusion with that of a minimal SNARE fusion mimetic.

Deuteron spin lattice relaxation in amorphous ices.

Temperature-dependent deuteron spin lattice relaxation times T(1) have been obtained from water in its three amorphous states at ambient pressure: low density amorphous (LDA), high density amorphous (HDA), and very high density amorphous (VHDA). It is found that in all of these states the magnetization recovery is essentially monoexponential and that T(1) of LDA is significantly longer than that of the higher density forms. Thus, T(1) can be used as a monitor parameter to study the kinetics of the transitions from HDA to LDA and from VHDA to LDA. During the transformation of VHDA to LDA an intermediate state is formed, which, according to its T(1) at low temperature, is clearly determined to be HDA-like. However, and most significantly, the transition from VHDA to this HDA-like state and further on to LDA occurs at temperatures significantly above the kinetic stability limit of native HDA produced at 77 K. These findings contribute to the current discussion on the nature of HDA and VHDA by strengthening the view that the annealing of VHDA at ambient pressure produces a relaxed HDA-like state.

Water dynamics on the hydrate lattice of a tetrabutyl ammonium bromide semiclathrate

Deuteron nuclear magnetic resonance (NMR) and dielectric spectroscopy are utilized to investigate the dynamics of the water molecules in the semiclathrate (tetra-n-butyl ammonium bromide) 26 H(2)O. Stimulated-echo spectroscopy reveals a nonexponential correlation function predominantly due to rotational motion with jump angles that are broadly distributed around the tetrahedral angle. The reorientational correlation times from this technique agree excellently with those from dielectric measurements, both resulting in an activation energy of (43+/-1) kJ/mol. Large, spatially varying electrical dipolar fields, set up by the Br(-) and the N(+) ions located on the hydrate lattice, are held responsible for the pronounced stretching of the correlation functions. Solid-echo spectra were acquired over a broad temperature range. They exhibit an apparent two-phase character discussed in terms of various scenarios. Two-dimensional NMR spectra and four-time stimulated echoes were recorded, but an exchange of slow and fast subensembles could not be detected. Spin-lattice relaxation does not directly reflect the local reorientational motion and its nonexponentiality is interpreted with reference to the translational dynamics of the water molecules.

Absence of molecular mobility on nano-second time scales in amorphous ice phases

High-resolution neutron backscattering techniques are exploited to study the elastic and quasi-elastic response of the high-density amorphous (HDA), the low-density amorphous (LDA) and the crystalline ice Ic upon temperature changes. Within the temperature ranges of their structural stability (HDA at T < or = 80 K, LDA at T < or = 135 K, ice Ic at T < or = 200 K) the Debye-Waller factors and mean-square displacements characterise all states as harmonic solids. During the transformations HDA --> LDA (T approximately 100 K), LDA --> Ic (T approximately 150 K) and the supposed glass transition with Tg approximately 135 K no relaxation processes can be detected on a time scale t < 4 ns. It can be concluded from coherent scattering measurements (D2O) that LDA starts to recrystallise into ice Ic at T approximately 135 K, i.e. at the supposed Tg. In the framework of the Debye model of harmonic solids HDA reveals the highest Debye temperature among the studied ice phases, which is in full agreement with the lowest Debye level in the generalised density of states derived from time-of-flight neutron scattering experiments. The elastic results at low T indicate the presence of an excess of modes in HDA, which do not obey the Bose statistics.