PFG NMR self-diffusion measurements

PFG-NMR self-diffusion measurements on Ppep-4 indicate formation of stable tetramers over a range of temperatures from 5 C to 50 C. In fact, even at 40°C and a peptide concentration of 40 pM, ppep-4 maintains an average molecular weight characteristic of a dimer (data not shown). Downfield shifted aH resonances remain, although dimished in relative intensity. In this respect, ppep-4 is not fully dissociated and exhibits an apparent dissociatoin constant in the pM range. Only when 3.5 M urea is added to solution, is Ppep-4 fully dissociated. 

Figure 7 Schematic representation of PFG NMR self-diffusion measurements (a-d) and their application in the spin-echo technique (e-g). Broken lines in (c) and (d) indicate the behavior with molecular migration. (a,e) r.f. pulses, (b,f) field gradient pulses, (c) transverse magnetization of different regions, (d,g) total transverse magnetization, equal to the vector sum of (c). (From Ref. 73.)...

The existence of intracrystalline transport resistances has been confirmed by PFG NMR self-diffusion measurements of short-chain length alkanes in MFI-type zeolites [216,217] with varying observation time. Figure 25 presents the relevant data obtained with n-butane as a probe molecule. Here, the diffusivities are plotted in a way, which is made possible by the special features of PFG NMR, viz. as a function of the displacements over which the molecular diffusion paths (giving rise to the plotted diffusivities) have been measured. This is achieved on the basis of Eq. 7 by which the measured diffusivities maybe transferred into the mean square displacements covered by the molecules during the observation time. Obviously, in the case of ordinary diffusion, i.e. in the original notion of Eq. 7, the diffusivity depends on neither... 

The combined application of PFG NMR self-diffusion and tracer desorption experiments has thus proved to be an effective tool for studying the hydrothermal stability of A-type zeolites with respect to their transport properties [186]. It turns out that with commercial adsorbent samples there are considerable variations in hydrothermal stability between different batches of product and even between different pellets from the same batch. As an example. Fig. 24 shows the distribution curves [A(Tin,ra) versus Ti ,r.j] measured with ethane as a probe molecule at 293 K for two different samples of commercial 5A zeolites. Evidently batch 1 is more resistant to hydrothermal deterioration, because the lengthening of Tjn,ra is less dramatic than with batch 2. Since the intracrystalline diffusivity was the same for all samples, the deterioration can be attributed to the formation of a surface barrier. 

We have shown PFG-NMR-derived diffusion coefficients can be used to calculate molar conductivity and compared with impedance measurements to understand ionicity. In a somewhat reversed experiment, diffusion coefficients measured electrochemicaUy have been compared to PFG-NMR self-diffusion coefficients to investigate proton transfer in protic ILs [19]. Aside from probing ionization dynamics in ILs, PFG-NMR has also been used for probing aggregation as shown in Fig. 5, especially for... 

As a non-invasive method, however, PFG NMR also provides excellent conditions for the measurement of molecular diffusivities under transient conditions. As an example. Fig. 18 shows the results of space- and time-resolved self-diffusion measurements over a bed of activated zeolite Na-X during the uptake of n-hexane [163]. Taking into account that, due to the finite size of... 

A novel one-sided NMR magnet has been developed for self-diffusion measurements in thin samples [7]. Using this technique, researchers have demonstrated measurements in bulk [BMIM][TFSI] and [BMIM][TFSI] confined in nanoporous anodized aluminum oxide membranes, as shown in Fig. 3. This operates at a proton Larmor frequency of 14.08 MFlz, and the expected appHcation is toward in situ measurements in portable energy devices. This method utilizes the fringe field to measure diffusion coefficients instead of conventional PFG-NMR. 

For self-diffusion measurements, the HT PFG NMR probe is a 10 mm liquid probe (Bruker design) equipped with a gradient coil providing 5.5G/cm/A and with a gradient amplifier of 10 A. The thermal shielding of the radiofrequency and gradient coils is insured by several ceramic barriers. The NMR pulse sequence used here combines stimulated echoes and bipolar gradient pulses [17]. 

Fig. 3.1.5 Temperature dependence of the coefficient of long-range self-diffusion of ethane measured by PFG NMR in a bed of crystallites of zeolite NaX (points) and comparison with the theoretical estimate (line). The theoretical estimate is based on the sketched models of the prevailing Knudsen diffusion...

Pulsed field gradient NMR (PFG-NMR) spectroscopy has been successfully used for probing interactions in several research fields.44-53 The method was developed by Stejskal and Tanner more than 40 years ago54 and allows the measurement of self-diffusion coefficient, D, which is defined as the diffusion coefficient in absence of chemical potential gradient. 

Pulsed field gradient (PFG)-NMR experiments have been employed in the groups of Zawodzinski and Kreuer to measure the self-diffusivity of water in the membrane as a function of the water content. From QENS, the typical time and length scales of the molecular motions can be evaluated. It was observed that water mobility increases with water content up to almost bulk-like values above T 10, where the water content A = nn o/ nsojH is defined as the ratio of the number of moles of water molecules per moles of acid head groups (-SO3H). In Perrin et al., QENS data for hydrated Nation were analyzed with a Gaussian model for localized translational diffusion. Typical sizes of confining domains and diffusion coefficients, as well as characteristic times for the elementary jump processes, were obtained as functions of A the results were discussed with respect to membrane structure and sorption characteristics. ... 

The reduction of the long-range diffusivity, Di by a factor of four with respect to bulk water can be attributed to the random morphology of the nanoporous network (i.e., effects of connectivity and tortuosity of nanopores). For comparison, the water self-diffusion coefficient in Nafion measured by PFG-NMR is = 0.58 x 10 cm s at T = 15. Notice that PFG-NMR probes mobilities over length scales > 0.1 /rm. Comparison of QENS and PFG-NMR studies thus reveals that the local mobility of water in Nafion is almost bulk-like within the confined domains at the nanometer scale and that the effective water diffusivity decreases due to the channeling of water molecules through the network of randomly interconnected and tortuous water-filled domains. ... 

Self-difFusion coefficients were measured with the NMR spin-echo method and mutual diffusion coefficients by digital image holography. As can be seen from Figure 4.4-3, the diffusion coefficients show the whole bandwidth of diffusion coefficient values, from 10 m s on the methanol-rich side, down to 10 on the [BMIM][PFg]-rich side. The concentration dependence of the diffusion coefficients on the methanol-rich side is extreme, and shows that special care and attention should be paid in the dimensioning of chemical processes with ionic Hquids. 

The self-diffusion of benzene in PIB [36], cyclohexane in BR [37] and toluene in PIB [38-40] has been investigated by PFG NMR. In addition more recently Schlick and co-workers [41] have measured the self-diffusion of benzene and cyclohexane mixtures in polyisoprene. In the first reported study of this kind, Boss and co-workers [36] measured the self-diffusion coefficients of benzene in polyisoprene at 70.4 °C. The increase in Dself with increasing solvent volume fraction could be described by the Fujita-Doolittle theory which states that the rate of self-diffusion scales with the free volume which in turn increases linearly with temperature. At higher solvent volume fractions the rate of selfdiffusion deviates from the Fujita-Doolittle theory, as the entanglement density decreased below the critical value. 

In principle, any of the techniques described for studying self-diffusion may be applied to both single- and multicomponent systems. So far, however, most selfdiffusion measurements of multicomponent systems have been carried out by PFG NMR. Such measurements are possible by applying a set of samples with deuterated compounds with only one species (namely that to be measured) in the protonated form [98], by simultaneously considering different nuclei (e.g. H and 19F [79]), and by Fourier transform PFG NMR [91,92]. 

Pulsed field gradient NMR (PFG-NMR) is a powerful, nondestructive technique of measuring self-diffusion coefficients in a colloidal dispersion [69-71]. Molecules associated with an aggregate or a particle will diffuse more slowly than their free dissolving state. More specifically, when a water-soluble species is partially adsorbed onto an abrasive particle, the measured overall diffusion coefficient (D ) of the species is decreased. If the diffusion coefficient of the free dissolving species can be measured in the absence... 

First we will review the relevant NMR techniques and their most recent developments (1) pulsed-field gradient (PFG) NMR (315-18) for the measurement of translational molecular self-diffusion and (2) (6,7,10,19,20)... 

Figure 6 represents typical plots of the spin-echo intensity in PFG NMR experiments. Comparing the slopes of these representations with those of standard liquids, one obtains the mean self-diffusivities, which are found to decrease with increasing sorbate concentration (5,12,16,59,60). It appears from Fig. 6 that within the accuracy of the measurement no deviation from a single exponential decrease may be observed. A comparison of the experimental spin-echo attenuation (Fig. 6) with the results of numerical calcula-... 

This result is in agreement with recent PFG NMR measurements with oriented ZSM-5 crystals, where the mean self-diffusivity of methane averaged over the straight and sinusoidal channels (i.e., in the x, y plane) has been found to be larger by a factor of 3-5 than the self-diffusivity in the c direction, perpendicular to those 42,43). For the self-diffusion of methane in ZSM-5/silicalite-I, the MD simulations were in very satisfactory agreement with the experimental PFG NMR and QENS self-diffusion data (cf. Refs. 34-38). Furthermore, the degree of a mass transport anisotropy in the MFI framework as predicted by MD simulations (34-38) is compatible with the experimental findings (42,43,49). 

Fig. 18. Self-diffusion coefficients of benzene in NaX at 458 K PFG NMR, O (97) and (92) (JENS, A (13) deduced from NMR lineshape analysis, (10). Comparison with nonequilibrium measurements T, sorption uptake with piezometric control (93) , zero-length column method (96) o, frequency-response and single-step frequency-response technique (98). The region of the results of gravimetric measurements with different specimens (92) is indicated by the hatched areas. Asterisked symbols represent data obtained by extrapolation from lower temperatures with an activation energy confirmed by NMR measurements.

A reduction of the intracrystalline self-diffusion coefficient, Dintra, of a probe molecule (e.g., methane) as measured by PFG NMR has to be regarded as proof of the existence of coke, modifiers, etc. in the volume phase of the zeolite crystal. 

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