Pulsed Field Gradient NMR Research Articles

Physical properties of tofu gel probed by water translational/rotational dynamics

The dynamic behavior of water molecules in tofu gels is observed using dielectric spectroscopy (DS) and pulsed field gradient nuclear magnetic resonance (PFG-NMR) methods. The breaking stress of tofu gels are dependent on the heating time, which affects the water structures characterized by the dielectric relaxation time, τ. On the other hand, the inhomogeneity of the gel structures decreases with increasing coagulant concentration was reflected as decreasing of the dielectric broadening parameter, β, that related for a fluctuation of the dynamic water structure. In addition, a decrease in the translational diffusion coefficient of water molecules is obtained with increasing coagulant concentration suggesting a reduction of the free space for the diffusional motion of water molecules. Negative correlations between the broadening parameter, β, and the inverse of diffusion coefficient, 1/D, are found in tofu gels and soymilk with distinct correlation coefficients of −0.95 and −0.87, respectively. These results suggest that water dynamics analysis and complementary analysis using DS and PFG-NMR measuring techniques can be an effective tool to evaluate food gel physical properties. Further these technique could also explain the molecular mechanisms of gel and liquid structures and properties via dynamic water structures.

Effects of Anion Mobility on Electrochemical Behaviors of Lithium–Sulfur Batteries

The electrolyte is a crucial component of lithium–sulfur (Li–S) batteries, as it controls polysulfide dissolution, charge shuttling processes, and solid-electrolyte interphase (SEI) layer formation. Experimentally, the overall performance of Li–S batteries varies with choice of solvent system and Li-salt used in the electrolyte, and a lack of predictive understanding about the effects of individual electrolyte components inhibits the rational design of electrolytes for Li–S batteries. Here we analyze the role of the counteranions of common Li salts (such as TfO–, FSI–, TFSI–, and TDI–) when dissolved in DOL/DME (1:1 by vol.) for use in Li–S batteries. The evolution of ion–ion and ion–solvent interactions due to various anions was analyzed using 17O NMR and pulsed-field gradient (PFG) NMR and then correlated with electrochemical performance in Li–S cells. These data reveal that the formation of the passivation layer on the anode and the loss of active materials from the cathode (evidenced by polysulfide di...

Quantifying Binding of Ethylene Oxide-Propylene Oxide Block Copolymers with Lipid Bilayers.

Block copolymers composed of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) have been widely used in cell membrane stabilization and permeabilization. To explore the mechanism of interaction between PPO-PEO block copolymers and lipid membranes, we have investigated how polymer structure influences the polymer-lipid bilayer association by varying the overall molecular weight, the hydrophobic and hydrophilic block lengths, and the end-group structure systematically, using 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) unilamellar liposomes as model membranes. Pulsed-field-gradient NMR (PFG-NMR) was employed to probe polymer diffusion in the absence and presence of liposomes. The echo decay curves of free polymers in the absence of liposomes are single exponentials, indicative of simple translational diffusion, while in the presence of liposomes, the decays are biexponential, with the slower decay corresponding to polymers bound to liposomes. The binding percentage of polymer to the liposome was quantified by fitting the echo decay curves to a biexponential model. The NMR experiments show that increasing the total molecular weight and hydrophobicity of the polymer can significantly enhance the polymer-lipid bilayer association, as the binding percentage and liposome surface coverage both increase. We hypothesize that the hydrophobic PPO block inserts into the lipid bilayer due to the fact that little molecular exchange between bound and free polymers occurs on the time scale of the diffusion experiments. Additionally, as polymer concentration increases, the liposome surface coverage increases and approaches a limit. These results demonstrate that PFG-NMR is a simple yet powerful method to quantify interactions between polymers and lipid bilayers.

The application of pulse field gradient (PFG) NMR methods to characterize the efficiency of separation of water-in-crude oil emulsions

Demulsification of water-in-crude oil emulsions is an essential and sometimes challenging procedure for crude oil processing facilities. Pulse field gradient (PFG) NMR techniques are known to monitor the dynamics of emulsion separation. This method has limitations that restrict its application to some crude oils. A comprehensive methodology applicable to all types of crude oil regardless of its viscosity, without assumptions, and providing a large number of data with fast measurements, is proposed in this paper. The coalescence and sedimentation of unstable emulsions was observed through simultaneous measurements of the evolution of the brine profile and droplet size distribution (DSD). Measurements of emulsions after stabilization, with and without the contribution of the free water layer, revealed the residual emulsified water quantity and location in the sample. A new, faster approach to separate the oil and water overlapping T2 relaxation signals was demonstrated on real water-in-crude oil emulsions, using the root mean square displacement (RMSD) measured with the spoiler recovery and a loop of 13-interval pulsed field gradient stimulated echo (PFGSTE) oneshot sequences. The residual water within the crude oils after separation was determined and used to quantify the efficiency of the demulsifier used.

The effect of crowder charge in a model polymer-colloid system for macromolecular crowding: Polymer structure and dynamics.

We have examined the effect of crowder particle charge on macromolecular structure, studied via small-angle neutron scattering, and translational dynamics, studied via pulsed-field gradient NMR, in addition to bulk viscosity measurements, in a polymer macromolecule (polyethylene glycol)-nanoparticle crowder (polysucrose, Ficoll70) model system, in the case where polymer size and crowder size are comparable. While there are modest effects of crowder charge on polymer dynamics at relatively low packing fractions, there is only a tiny effect at the high packing fractions that represent the limit of molecular crowding. We find, via different measures of macromolecular mobility, that the mobility of the flexible polymer in the crowding limit is 10-100 times larger than that of the compact, spherical crowder in spite of their similar size, implying that the flexible polymer chain is able to squeeze through crowder interstices.

NMR investigation of water diffusion in different biofilm structures.

Mass transfer in biofilms is determined by diffusion. Different mostly invasive approaches have been used to measure diffusion coefficients in biofilms, however, data on heterogeneous biomass under realistic conditions is still missing. To non-invasively elucidate fluid-structure interactions in complex multispecies biofilms pulsed field gradient-nuclear magnetic resonance (PFG-NMR) was applied to measure the water diffusion in five different types of biomass aggregates: one type of sludge flocs, two types of biofilm, and two types of granules. Data analysis is an important issue when measuring heterogeneous systems and is shown to significantly influence the interpretation and understanding of water diffusion. With respect to numerical reproducibility and physico-chemical interpretation, different data processing methods were explored: (bi)-exponential data analysis and the Γ distribution model. Furthermore, the diffusion coefficient distribution in relation to relaxation was studied by D-T2 maps obtained by 2D inverse Laplace transform (2D ILT). The results show that the effective diffusion coefficients for all biofilm samples ranged from 0.36 to 0.96 relative to that of water. NMR diffusion was linked to biofilm structure (e.g., biomass density, organic and inorganic matter) as observed by magnetic resonance imaging and to traditional biofilm parameters: diffusion was most restricted in granules with compact structures, and fast diffusion was found in heterotrophic biofilms with fluffy structures. The effective diffusion coefficients in the biomass were found to be broadly distributed because of internal biomass heterogeneities, such as gas bubbles, precipitates, and locally changing biofilm densities. Thus, estimations based on biofilm bulk properties in multispecies systems can be overestimated and mean diffusion coefficients might not be sufficiently informative to describe mass transport in biofilms and the near bulk.

Hydrodynamic characteristics of branched polystyrenes with varying content of a highly branched fraction

The branched polymers containing different amounts of the highly branched fraction are synthesized by the radical copolymerization of styrene and divinylbenzene under conditions of the reversible inhibition by 2,2,6,6-tetramethylpiperidine-1-oxyl. The branched polystyrenes are studied by size-exclusion chromatography combined with static light scattering, viscometry, and pulsed-field gradient nuclear magnetic resonance. The branched polymers prepared by living radical polymerization (in the presence of 2,2,6,6-tetramethylpiperidine-1-oxyl) feature reduced intrinsic viscosities and increased self-diffusion coefficients compared with their linear analogs. As the content of the highly branched fraction in the synthesized polymers grows, the Zimm contraction factor in toluene solution decreases to g′ = 0.13. The Kuhn-Mark-Houwink parameters for these polymers in toluene solution (a = 0.43) confirm the nonlinear architecture of macromolecules.

In Situ NMR Imaging of Ion Transport in Li-Ion Batteries

The operation of battery management systems requires that the transient conditions experienced by a battery in electrified vehicle applications can be accurately described through battery modeling. Reliable data on the transport properties of electrolyte solutions, as functions of both salt concentration and temperature, are a prerequisite for such a task. The accurate characterization of the transport parameters is a complex problem, because the values of ionic diffusion coefficients in an electrolyte solution depend strongly on the salt concentration. At the same time, the salt concentration inside a battery is neither stationary nor homogeneous, during battery operation at high charge/discharge rates. It is, rather, a function of current density, time and distance from the electrodes. Recently, MRI was used for in operando characterization of materials used in Li-ion batteries. The main advantage of MRI over commonly used methods, such as NMR and electrochemical techniques, is its ability to provide spatially resolved details regarding the chemical and dynamic features of solution species. Therefore, such data enable a more accurate representation of the processes occurring in an electrolyte solution under an applied electrical potential, instead of values averaged over the entire volume of a sample cell. Using in situ MRI, we have unambiguously mapped the large concentration polarization experienced by an electrolyte solutions during battery operation.1 The concentration gradient created in the electrolyte solution of the cell under galvanostatic conditions will increase until a steady-state (time invariant) condition is achieved, when the diffusion flux compensates for the migration flux of the ions. Our results confirm that the concentration gradient developed under such conditions is proportional to the applied current (Fig. 1b) as described by the diffusion-migration equation (Eqn. 1), where i is the current density and F is the Faraday constant.2 We have shown previously that, while the degree of ion pairing is quite high (up to 67%) in electrolyte solutions under normal operating conditions, it is almost constant during a steady-state NMR gradient measurements (with salt concentrations varying from 0.8 to 1.2 M).3 Therefore, one can use averaged diffusion coefficients of charged and neutral species measured by pulsed field gradient NMR (PFG-NMR) in Eqn. 1. Also the total salt concentration, obtained directly by MRI should be used, instead of concentration of free ions.2 The inverse modeling of evolution of salt concentration profiles measured by in situ MRI under applied electric current also supported this conclusion.4 Therefore, by combining the PFG-NMR and the in situ MRI techniques into one experiment, one can simultaneously determine the distribution of salt concentration and the salt diffusivity in the electrolyte solution during a galvanostatic experiment (Fig. 2). It can be readily seen that the diffusion coefficient has a significant dependence on the salt concentration, with the values of D measured at opposite ends of the cell varying by more than 60%. Such changes obviously must be taken into account in order to provide an accurate description of mass-transport. The Li+ transference number (t+ ) can then be obtained by means of Eqn.(1). A slight decrease in t+ with concentration was observed, which can be explained by an increase in ion association. However, with the exception of data point at the lowest concentration, all of values of the Li+ transference numbers thus obtained fall within the experimental error and t += 0.31 ± 0.03. The formed concentration gradient affects not only the mass transport in electrolyte solution, but also the electrode lithiation. The electrolyte depletion prevents the active material in the solid from being lithiated in regions where electrolyte has been depleted. Furthermore, when the electrolyte concentration approaches zero, its conductivity drops, significantly hampering the process. The current tends to flow into the particles nearest the separator until they are completely lithiated, causing the intercalation reaction to occur preferentially at the front of the electrode. Implementation of pure phase-encoding MRI methods allows us to visualize non-uniform lithiation of the graphite electrode during cycling of Li-ion cell (Fig. 1c), leading to the next level of battery performance characterization.

Multi-Temperature in Situ Magnetic Resonance Imaging of Polarization and Salt Precipitation in Li-Ion Battery Electrolytes

The parameterization and validation of robust electrochemical models for lithium-ion batteries in automotive applications, which are intended to address both the state of charge (SOC) and state of health (SOH) estimation problems, is an important milestone in their widespread deployment. Much effort has been focused on the mass transport characteristics of the electrode domains, but a significant fraction of overall polarization stems from concentration gradients that form in the electrolyte domain and their attendant effect on the mass transport parameters, which have recently been shown by in situ MRI to be quite significant, even within modest operational regimes.1 The effect of temperature on these concentration gradients remains to be explored by in situ MRI. The mass transport in the electrolyte domain is typically modelled as being governed by material balance of a binary salt in a concentrated solution, which can be realized by modifying the Nernst-Planck equation to have non-constant, concentration-dependent coefficients (Equation 1).2 This system has a steady-state concentration gradient (Equation 2) when migration flux driven by the potential and diffusion flux against the concentration gradient are in equilibrium (see Figure 1A). Using in situ MRI, the evolution of this gradient toward the steady-state upon constant-current charging can be monitored (Figure 1B). A surprising outcome of these studies was that at 10°C, an equal parts (v/v) mixture of EC/DEC with 1.00 M LiPF6 exhibited anomalous concentration gradient formation (Figure 2) with a current density corresponding to 6 A·m-2 (or ~C/4), whereas the same mixture and cell construction could be polarized without difficulty at room temperature. On comparing raw MR images from before the polarization and after the relaxation of the disrupted concentration gradient, it is clear that the overall signal intensity is lower whilst the shape of the profile is retained, which is a direct indicator that salt precipitation has occurred. There are two routes by which this salt precipitation phenomenon can be avoided: lowering the salt concentration, so that the absolute concentration at the upper portion of the gradient is decreased proportionately for an equivalent current density, and the selection of an electrolyte with better low-temperature solvation characteristics. Successful polarization and visualization of the electrolyte domain in situ was achieved with a 0.85 M variant of the equal-parts EC/DEC mixture utilized above, at a series of temperatures. This constitutes a means of avoiding the critical concentration at the low temperature where the 1.00 M variant exhibited precipitation, but it would come with a trade-off of reduced ionic conductivity. More significantly, a ‘full-strength’ 1.00 M LiPF6 in EC/PC/DMC, 5:2:3 (v/v) mixture was also successfully polarized at low, room, and elevated temperatures, with the same 6 A·m-2 applied current density. Previous ex situ experiments using the pulsed-field gradient NMR technique, in conjunction with conductivity measurements, had revealed that ion pairing at low temperature was significantly reduced in electrolyte mixtures where the fraction of high-dielectric solvent was maximized.3 A quantitative comparison of the steady-state concentration gradients at a series of inter-electrode distances and temperatures was also conducted with this electrolyte formulation, to assess how the magnitudes of the gradients varied with temperature, and to test the hypothesis inherent in Equation 2 that the inter-electrode spacing does not play a role in determining the magnitude of the steady-state gradient. Within 3σ confidence intervals, the inter-electrode spacing was not found to affect the inter-electrode spacing, which directly confirms that the in situ MRI technique can generate steady-state mass transport parameters which are relevant to real battery modelling efforts. A significant effect of the temperature on the magnitude of the steady-state concentration gradient is also illustrated in these multi-temperature imaging experiments, demonstrating the importance of characterizing the spatial variation and temperature-dependence of electrolyte mass transport characteristics for electrochemical model parameterization and validation. References (1) Krachkovskiy, S. A.; Bazak, J. D.; Werhun, P.; Balcom, B. J.; Halalay, I. C.; Goward, G. R. Visualization of Steady-State Ionic Concentration Profiles Formed in Electrolytes during Li-Ion Battery Operation and Determination of Mass-Transport Properties by in Situ Magnetic Resonance Imaging. J. Am. Chem. Soc. 2016, 138 (25), 7992–7999. (2) Sethurajan, A. K.; Krachkovskiy, S. A.; Halalay, I. C.; Goward, G. R.; Protas, B. Accurate Characterization of Ion Transport Properties in Binary Symmetric Electrolytes Using In Situ NMR Imaging and Inverse Modeling. J. Phys. Chem. B 2015, 119 (37), 12238–12248. (3) Krachkovskiy, S. A.; Bazak, J. D.; Fraser, S.; Halalay, I. C.; Goward, G. R. Determination of Mass Transfer Parameters and Ionic Association of LiPF 6 : Organic Carbonates Solutions. J. Electrochem. Soc. 2017, 164 (4), A912–A916. Figure 1

Assessing Guest-Molecule Diffusion in Heterogeneous Powder Samples of Metal-Organic Frameworks through Pulsed-Field-Gradient (PFG) NMR Spectroscopy.

Investigation of guest diffusion in porous metal-organic frameworks (MOFs) is of major importance, because many porosity-related properties of MOFs are influenced by diffusion effects. The diffusion of dimethyl sulfoxide (DMSO) in the MOF MIL-53-NH2 (Al) was investigated through pulsed-field-gradient (PFG) NMR spectroscopy. The microporous material was synthesized in small crystallites (under 500 nm), which agglomerated in a large range of particle sizes (from hundreds of nanometers to tens of micrometers), giving a morphologically very heterogeneous sample. No special agglomeration pattern could be observed, which makes a PFG NMR investigation very challenging, yet it represents a realistic situation for the diffusion of guest molecules in porous materials. We were able to distinguish between two diffusion regimes existing in parallel with each other over the total range from 15 to 200 ms of observation times as accessible in the experiments: In the large crystal agglomerates (diameters above 20 μm), guest movement was found to be subdiffusive, with a time exponent κ =0.8 (rather than one as for normal diffusion). Guest diffusion in the remaining, smaller host particles followed the pattern of normal diffusion within a bed of spheres of impenetrable external surfaces, with a size distribution in good agreement with that of the material under study. Diffusion in a rather complex system could thus be referred to a two-region model with new potentials for application to systems of intricate topology.

Molecular Dynamics Phenomena of Water in the Metalorganic Framework MIL-100(Al), as Revealed by Pulsed Field Gradient NMR and Atomistic Simulation

Measured, via pulsed field gradient (PFG) NMR, and computed molecular dynamics (MD) were utilized for the study of the phase equilibrium and kinetics of water sorbed in a bed of MIL-100(Al) crystallites. The computations rely on our recent methodology for modeling water equilibria and dynamics in the Fe-homologue MIL-100 crystal; in that sense, the particular NMR technique serves also as a validation tool of the previous simulation work which is adapted to the current system. In addition, a computational scheme for assigning partial charges on the host framework atoms was devised; it involves density functional theory (DFT) combined with electronegativity equalization method (EEM) calculations. The derived this way electronegativity, hardness, and gamma parameters for the specific MIL-100(Al) atoms can be used in EEM calculations of other aluminum metalorganic frameworks (MOF) bearing similar atom types. The thermodynamics predictions obtained via MD, comprising equilibria, enthalpies, adsorbate probabili...

Alkane/alkene mixture diffusion in silicalite-1 studied by MAS PFG NMR

The diffusivity of n-alkanes and n-alkenes (C2 to C6) and their mixtures in silicalite-1 was studied by magic-angle spinning pulsed field-gradient nuclear magnetic resonance (MAS PFG NMR) at the temperatures of 273 K, 313 K and 373 K. It could be proved that there is no significant difference between the diffusivities of alkanes, alkenes and their mixtures for equal carbon numbers and equal total loading. The diffusivities of the alkanes, alkenes and their mixtures are found to monotonically decrease with increasing carbon number, in agreement with the results obtained in MD simulations with n-alkanes in silicalite-1.

Photoresponsive Foams Generated by a Rigid Surfactant Derived from Dehydroabietic Acid.

Innovation in the structure of surfactants is crucial to the construction of a surfactant-based system with intriguing properties. With dehydroabietic acid as a starting material, a nearly totally rigid azobenzene surfactant (R-azo-Na) was synthesized. The trans-R-azo-Na formed stable foams with half-lives of 636, 656, 976, and 872 min for 0.3, 1, 2, and 4 mmol·L-1 aqueous solutions, respectively. Under UV light irradiation, a fast collapse of the foams was observed, showing an in situ response. The excellent foam stability of trans-R-azo-Na leads to the extremely high photoresponsive efficiency. As revealed by dynamic surface tension and pulsed-field gradient NMR methods, an obvious energy barrier existed in the adsorption/desorption process of trans-R-azo-Na on the air/water interface. The foams formed by trans-R-azo-Na are thus stable against coarsening processes. The results reveal the unique photoresponsive behavior of a surfactant with a rigid hydrophobic skeleton and provide new insights into the structure causing aggregation of surfactants.

The role of water in methane adsorption and diffusion within nanoporous silica investigated by hyperpolarized 129Xe and 1H PFG NMR spectroscopy

Understanding the properties and behavior of water molecules in restricted geometries, such as the nanopores of rocks, is of interest for shale gas exploitation. We present herein ex situ and in situ nuclear magnetic resonance (NMR) studies on the effects of water on the adsorption and diffusion of methane in nanopores. Silica materials with one-dimensional pores of ZSM-22, MCM-41, and SBA-15, with pore sizes ranging from 0.5 to 6 nm, were chosen as models. Hyperpolarized (HP) 129Xe NMR results show that water adsorption does not affect the pore sizes of ZSM-22 and MCM-41 but reduces that of SBA-15. The presence of water suppresses methane adsorption; this suppression effect is stronger in smaller pores. The self-diffusion coefficients of methane within ZSM-22 and MCM-41 are not significantly influenced by the presence of water, as measured by 1H pulsed field gradient (PFG) NMR. However, within SBA-15, which has a pore size of 6 nm, the diffusion coefficient of methane increases as the amount of water adsorption increases, peaks, and then decreases to a constant value with further water adsorption. These experiments reveal the effects of the pore size and the presence of water on methane adsorption and diffusion in constrained spaces, which could have important implications for flow simulations of methane in shales.

Influence of membrane-type and flow field design on methanol crossover on a single-cell DMFC: An experimental and multi-physics modeling study

The performance of a 5 cm2 single-cell direct methanol fuel cell (DMFC) was evaluated experimentally by using two different electrolyte membranes (Fumapem® F-1850 and Nafion® N-117) for assembling the electrodes and three different types of flow field design (a unique serpentine, four parallel serpentines, four inlet serpentines). A 3D multi-physics, multi-component, two-phase, and not-isothermal model was computed with Comsol® Multiphysics v4.4 platform, to analyze and understand the behavior of the various configuration tested. The model consists of Maxwell-Stefan, Stokes-Brinckman, extended two-phase Darcy Law, modified Butler-Volmer and Tafel equations to simulate the performance of the DMFC. Pulse Field Gradient (PFG) NMR spectroscopy was used to get a direct measurement of the diffusion coefficients of water and methanol through the membranes. These values were then implemented in the multi-physics model. The model well reproduces the cell performance of all the MEA tested regarding polarization curves obtained under various experimental conditions (varying the inlet mass flows, the methanol concentration, the type of oxidant, the temperature). Thus, the model was used as a tool to investigate anodic overpotentials, water and methanol crossover flow rates, current density distribution at the catalyst layer/membrane interface, understanding the relationship between flow fields and cell performance. At similar specific power density, and similar anodic overpotentials, the methanol crossover flow rate is one order of magnitude lower for Fumapem® F-1850 than for Nafion® N-117, notwithstanding the much lower thickness of the F-1850 membrane.

Properties of High Na-Ion Content N-Propyl-N-Methylpyrrolidinium Bis(Fluorosulfonyl)Imide -Ethylene Carbonate Electrolytes

Sodium-based batteries have emerged as promising alternatives to Li-based batteries for future safe, high energy-density energy storage. They are expected to be cheaper, due to the greater abundance of Na and likely reduction in raw material costs. In this study, we investigate the properties of superconcentrated sodium bis(fluorosulfonyl)amide (NaFSI) mixtures with the ionic liquid (IL) methylpropylpyrrolinium (C3mpyr) FSI in the presence of ethylene carbonate (EC) in the liquid and gel states. Ionic conductivity and thermal stability are evaluated through electrochemical impedance spectroscopy (EIS) and differential scanning calorimetry (DSC), respectively. NaFSI is soluble in the IL up to 55 mol% Na; adding EC (30wt.%) to the IL almost doubles the ionic conductivity at ambient temperature. The temperature dependence of conductivity is well described by the Vogel-Tamman-Fulcher equation. NMR spectroscopy and Pulse Field Gradient NMR diffusion were employed to investigate transport in these electrolyte systems, while the chemical interactions were also studied using ATR-FTIR. Stable plasticized gel electrolytes were observed, even at 30wt. % EC; the formation of the gel does not significantly affect the liquid-like ion dynamics in these materials, as shown by DSC and FTIR analysis. The Na+ transference number of Na0.55[C3mpyr]0.45[FSI]+30wt.% EC was up to 0.32, and deposition and dissolution of sodium metal were observed in cyclic voltammetry around 0V vs. Na/Na+. Moreover, the suitability of the prepared electrolyte is preliminarily verified in half-cells at room temperature using Na3V2(PO4)3 as a cathode. The cells delivered capacity of 52.4 mAhg−1 at C/20.

The structural parameters of rigid matrix of saccharide-based gel described by short and long diffusion regime analysis

The tortuosity–porosity relation and internal structure diameters in a selected organogel are studied. The diffusion coefficient measured by pulsed field gradient NMR at short and long observation times can be used to estimate the tortuosity and the pore surface-to-volume (S/Vpore) ratio of a porous material (Latour et al. in J Magn Res A 112:83–91, 1995; Mair et al. in Magn Res Imaging 19:345–351, 2001; Mitra et al. in Phys Rev B 47(14):8565–8574, 1993; Valiullin and Skirda in J Chem Phys 114:452–458, 2001). The value of S/Vpore allows to calculate the average pore and barrier diameters. In the experiment, the time-dependent diffusion coefficient of toluene in a low molecular weight gelator methyl-4,6-O-(p-nitrobenzylidene)-α-d-glucopyranoside (Gluco-NO2) matrix was measured. The results revealed the relation between the diffusion coefficient of toluene and the concentration of gelator Gluco-NO2 in the system. With increasing Gluco-NO2 compound in the sample, the diffusion coefficient of toluene decreases. On the basis of the diffusion coefficient behaviour as a function of time and gelator concentration, the fibrous structure of the gel matrix was identified and the structural parameters of the system were investigated.

Relation between the porosity and tortuosity of a membrane formed by disconnected irregular pores and the spatial diffusion coefficient of the Fick-Jacobs model.

In this work, we provide a theoretical relationship between the spatial-dependent diffusion coefficient derived in the Fick-Jacobs (FJ) approximation and the macroscopic diffusion coefficient of a membrane that depends on the porosity, tortuosity, and the constriction factors. Based on simple mass conservation arguments under equilibrium as well as in nonequilibrium conditions, we generalize previous expressions for the effective diffusion coefficient of an irregular pore, originally obtained by Festa and d'Agliano for horizontal and periodic pores, and then extended by Bradley for tortuous periodic pores, to the case of pores with arbitrary geometry. Through a formal definition of the constrictivity factor in terms of the geometry of the pore, our results provide very clear physical interpretation of experimental measurements since they link the local properties of the flow with macroscopic quantities of experimental relevance in the design and optimization of porous materials. The macroscopic diffusion coefficient as well as the spatiotemporal evolution of the concentration profiles inside a pore have been recently measured by using pulse field gradient NMR techniques. The advantage of using the FJ approach is that the spatiotemporal concentration profile inside a pore of irregular geometry is directly related to the pore's shape and, therefore, that the macroscopic diffusion coefficient can be obtained by comparing the spatiotemporal concentration profiles from such experiments with those of the theoretical model. Hence, the present study is relevant for the understanding of the transport properties of porous materials where the shape and arrangement of pores can be controlled at will.

Cation Dynamics in Supercooled and Solid Alkyl Methylimidazolium Bromide Ionic Liquids

The molecular dynamics of alkyl methylimidazolium bromide ionic liquids with different side groups of the cation are studied over a wide range of temperatures, covering the supercooled and crystalline states. Nuclear magnetic resonance relaxation dispersion (NMRD) at different magnetic field strengths was combined with NMR pulsed field gradient (PFG) diffusion measurements in order to obtain a description of the temperature dependence of the cationic mobility. While an Arrhenius dependence of the correlation times was found at high temperatures, a deviation is observed below a critical temperature of Tdyn ∼ 275 K which corresponds to about 1.25 Tg for two of the substances. The macroscopic diffusion coefficient, on the other hand, is best described by a VFT dependence down to a similar temperature, and a much weaker temperature dependence below. Measurements carried out in the crystalline state of 1-butyl-3-methylimidazolium bromide (Bmim Br) exhibit a dramatically increased self-diffusion coefficient in agreement with earlier reports of strong dynamic heterogeneity in the presence of minute amounts of water.

Possible role of molecular clustering in single-file diffusion of mixed and pure gases in dipeptide nanochannels

Single-file diffusion (SFD) of CO, CO2, and CH4 gas molecules confined in L-Ala-L-Val nanochannels as both pure and binary mixtures is investigated using pulsed field gradient (PFG) NMR at the high field of 17.6 T. A unique SFD mobility is observed for each of the sorbates as pure gases. As binary mixtures, both sorbates exhibit identical SFD mobilities. The mixture mobilities were observed to be larger than those of the slowest-diffusing component in its pure form under the conditions when the total concentration of sorbate molecules in the channels was the same or comparable. It is proposed that molecular clustering in the studied channels is responsible for the relationship experimentally observed between the one-component and mixture mobilities. It is demonstrated that molecular clustering can also explain the lack of the experimental observation of a transition from the SFD mechanism to the center-of-mass diffusion mechanism, viz. diffusion mechanism characterized by correlated movements of all molecules in any particular channel.