In recent years, considerable progress has been made in the development of novel porous materials with controlled architectures and pore sizes in the mesoporous range. An important feature of these materials is the phenomenon of adsorption hysteresis: for certain ranges of applied pressure, the amount of a molecular species adsorbed by the mesoporous host is higher on desorption than on adsorption, indicating a failure of the system to equilibrate. Although this phenomenon has been known for over a century, the underlying internal dynamics responsible for the hysteresis remain poorly understood. Here we present a combined experimental and theoretical study in which microscopic and macroscopic aspects of the relaxation dynamics associated with hysteresis are quantified by direct measurement and computer simulations of molecular models. Using nuclear magnetic resonance techniques and Vycor porous glass as a model mesoporous system, we have explored the relationship between molecular self-diffusion and global uptake dynamics. For states outside the hysteresis region, the relaxation process is found to be essentially diffusive in character; within the hysteresis region, the dynamics slow down dramatically and, at long times, are dominated by activated rearrangement of the adsorbate density within the host material.

The mechanism of charge transport in the imidazolium-based ionic liquid 1,3-dimethylimidazolium dimethylphosphate is analyzed by combining broadband dielectric spectroscopy (BDS) and pulsed field gradient nuclear magnetic resonance (PFG NMR). The dielectric spectra are dominated-on the low-frequency side-by electrode polarization effects while, for higher frequencies, charge transport in a disordered matrix is the underlying physical mechanism. Using the Einstein and Einstein-Smoluchowski equations enables one to determine-in excellent agreement with direct measurements by PFG NMR-the diffusion coefficient of the charge carriers. By that, it becomes possible to extract from the dielectric spectra separately the number density and the mobilities of the charge carriers and the type of their thermal activation. It is shown that the observed Vogel-Fulcher-Tammann (VFT) dependence of the dc conductivity can be traced back to a similar temperature dependence of the mobility while for the number density an Arrhenius-type thermal activation is found. Extrapolating the latter to room temperature indicates that nearly all charge carriers are participating in the conduction process.

Broadband dielectric and terahertz spectroscopy (10(-2)-10(+12) Hz) are combined with pulsed field gradient nuclear magnetic resonance (PFG-NMR) to explore charge transport and translational diffusion in the 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid. The dielectric spectra are interpreted as superposition of high-frequency relaxation processes associated with dipolar librations and a conductivity contribution. The latter originates from hopping of charge carriers on a random spatially varying potential landscape and quantitatively fits the observed frequency and temperature dependence of the spectra. A further analysis delivers the hopping rate and enables one to deduce--using the Einstein-Smoluchowski equation--the translational diffusion coefficient of the charge carriers in quantitative agreement with PFG-NMR measurements. By that, the mobility is determined and separated from the charge carrier density; for the former, a Vogel-Fulcher-Tammann and for the latter, an Arrhenius temperature dependence is obtained. There is no indication of a mode arising from the reorientation of stable ion pairs.

The PFG NMR technique is applied to investigate the intracrystalline diffusion of methane and n-butane molecules in MFI-type zeolites at several temperatures (from 123 up to 383 K) and over a wide range of diffusion times (from 2 to 35 ms). The intracrystalline self-diffusion coefficients of the guest molecules recorded at low temperatures were observed to decrease with increasing root-mean-square displacements. The comparison of the experimental results with the results of the Monte Carlo simulations of diffusion allowed us to rule out the restriction of diffusion by crystal boundaries as a possible explanation of the observed dependencies of the diffusivities on the root-mean-square displacement. These dependencies are tentatively attributed to the existence of intracrystalline transport barriers in MFI-type crystals. The intersections between the elementary building blocks of the crystals and/or intergrowth sections of MFI crystals are suggested as the possible candidates for the transport barriers.

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By applying pulsed-field gradient nuclear magnetic resonance (PFG NMR) in comparison to quasi-elastic neutron scattering (QENS), we sense by measurement of the diffusion of n-octane on different length scales, transport resistances in faujasite type X (which is isostructural with type Y and differs by the lower Si/Al ratio only) with mutual distances of less than 300 nm. Direct observation of the real structure of zeolite X by transmission electron microscopy identifies them as stacking faults of mirror-twin type on (111)-type planes of the cubic framework. Thus, direct experimental proof is given that, in general, nanoporous host systems such as zeolite crystals cannot be considered as a mere arrangement of cavities. It is rather the presence of structural defects that dominates their properties as soon as transport phenomena with practical relevance, including chemical conversion by heterogeneous catalysis and chemical separation by molecular sieving and selective adsorption, become relevant.

The size of free-volume holes in neat poly[(ethylene glycol)23dimethacrylate] [poly((EG)23DMA)] and in the same polymer doped with 0.6 mol/kg LiCF3SO3 have been studied as a function of temperature in the range between 100 and 370 K using positron annihilation lifetime spectroscopy. The results are compared with differential scanning calorimetry and ionic conductivity measurements. In both systems, the hole volume νh shows a typical glass-transition behavior, i.e., a small linear increase with temperature below the glass transition temperature Tg and a steeper increase above Tg. From these measurements Tg was estimated to be 233 K (neat polymer) and 240 K (polymer with salt) and the coefficients of the thermal expansion of the hole volume were determined. The fractional free volume (f=0.080) and the number density of holes (Nh=0.6 nm−3) were also estimated. Below Tg the average hole volume of the polymer electrolyte is larger than in the neat polymer. This is consistent with the bulky character of the CF3SO3− anion. Above Tg the salt-doped system shows the lower hole volume of the two systems, probably caused by a reduced segmental mobility as a consequence of the interactions of the Li+ ions with the ethylene oxide units of the polymer. Based on the free-volume theory of Cohen–Turnbull the ionic conductivity σ is correlated with the mean hole volume νh. A linear relation between log(σT 0.5) and 1/νh was observed to be valid for variations of the conductivity over several orders of magnitudes. From these plots critical hole sizes of γν*=0.65 nm3 (neat polymer) and 0.87 nm3 (polymer-salt system) were estimated. The parameters B and T0 of the Vogel–Tamman–Fulcher equation were also determined, as well as the apparent activation volume ΔVapp by pressure-dependent conductivity measurements. The cationic transference number in the polymer-salt system was determined by pulsed field gradient-nuclear magnetic resonance to be t+≈0.3.