Diffusion, NMR method

Figure 2. Spin-echo -NMR spectra from a diffusion experiment with a cubic phase of dDAVP (10%), MO (60%) and 2H20 (40%). Temperature 40 C, t=20 ms, A=24 ms, g=l 19 gauss/cm and 8=1.0,2.0..., 9.0 ms. The inset shows the aromatic region originating from dDAV P at a higher amplification. Also shown is the pulse sequence used in the NMR-diffusion method (see text for details).

It is this relative insensitivity that is usually considered as the major drawback of NMR spectroscopy. However, the flexibility of the NMR technique, with the ability to obtain structural information, quantitative data (e.g. kinetic parameters), as well as an indication of molecular volume, using pulsed gradient spin echo (PGSE) NMR diffusion methods [6], makes NMR a most valuable tool. 

To eonelude this seetion we summarize the main advantages of the NMR diffusion method as applied to emulsion droplet sizing. It is nonperturbing, requiring no sample manipulation (such as dilution with the continuous phase) and, as noted above, it is nondestructive. It is insensitive to the physical appearance of the sample, and can be applied to nontransparent samples. It requires small amounts of sample (typically of the order of a few hundred milligrams) and is normally quite rapid (of the order of 10 min per sample). 

We end this section by summarizing the areas where we feel that the NMR diffusion method will prove important in future studies of emulsions and refer to a more detailed account presented in Chapter 10 of this book. As theories describing emulsion stability become more refined, there will be a need for data on droplet size distribution and also on total emulsion droplet area and how these quantities evolve with time. As outlined above, NMR is eapable of providing sueh data. Another important question pertains to the mi-crostrueture of the continuous phase, which can be studied both in the emulsion phase and also in the phase-separated systems which yield the emulsion. Finally, we note that one important class of emulsions, namely, multiple emulsions, is practically virgin territory with regard to NMR studies. In the characterization and understanding of important features of these systems NMR will most likely play an important role. 

The discussion carried out in Sec. Ill applies to the case where the molecules are confined to the droplets on the time scale of the experiment. This is a reasonable assumption for many emulsions, and it can in fact be tested by the NMR diffusion method by varying A. However, there are some interesting emulsion systems where this is not always the case. These are the so-called highly concentrated emulsions (often termed high internal phase emulsions) (32,33), which may contain up to (and in some cases even more than) 99% dispersed phase. Here, the droplets are separated by a liquid film which may be very thin (of the order of 100 A), and which may in some instances be permeable to the dispersed phase. 

Chymotripsin (ChT) is a serine protease which was already shown to be inhibited by a specific family of calix[4]arene receptors [26, 31]. P NMR diffusion methods were exploited to probe the molecular interaction of ChT with organo-phosphorus compounds which are known to block the enzyme. In such cases, covalent conjugates with the serine active site were detected on the basis of the relevant decreases of diffusion coefficients of the enzyme bound species, simultaneously enabling the determination of the diffusion coefficient of the enzyme [27]. 

P. S. Pregosin, NMR Diffusion Methods in Inorganic and Organometallic Chemistry, in Spectroscopic Properties of Inorganic and Organometallic Compounds, ed. J. Yarwood, R. Douthwaite and S. Duckett, Royal Society of Chemistry, 2012, vol. 42, p. 248. 

While the nuclear magnetic resonance (NMR) technique has widely been used to study diffusion processes of normal liquids, solids, or colloidal systems, there are only a few applications to molten salts. The spin echo self-diffusion method with pulsed field gradients was applied to molten salts by Herdlicka et al. "" There is no need to set up or maintain a concentration gradient. 

R M. Cotts, M. J. R. Hoch, T. Sun, J.T. Markert 1989, (Pulsed field stimulated echo methods for improved NMR diffusion measurements in heterogeneous systems), J. Magn. Reson. 83, 252. 

B. Balinov, B. Jonsson, P. Linse, O. So-derman 1993, (The NMR self-diffusion method applied to restricted diffusion -simulation of echo attenuation from molecules in spheres and between planes),/. Mag. Reson. A 104, 17. 

For some years an objective of this laboratory has been the development of methods for isolating and characterizing the last-mentioned size of natural product. Countercurrent distribution has been a chief tool but more recently we have been developing a membrane-diffusion method which we have called thin-film dialysis (1). It has considerable potential for studying molecular size, conformation, molecular interactions, and other solution behavior, particularly when combined with the rapidly developing possibilities suggested by high resolution NMR. 

Fleisher et al. [12] studied the self-diffusion of oil and water in rape seeds. The selfdiffusion of oil was found to be completely restricted. The experiments could be explained in toms of the model of diffusion within spherical droplets and a Gaussian mass distribution of the droplet radii. At the same time Van den Enden et al. [9] introduced the technique described above. It is a rapid method for the determination of water droplet size distributions in spreads by using low resolution pulsed field gradient NMR. Their method was based on the recognition that a set of echo attenuation values (R) as a function of the field gradient pulsed width, obtained under conditions where R is independent of the time allowed for diffusion, contains all the necessary information on the water droplet size distribution (see above). A log-normal distribution of water droplet sizes was assumed. 

A more sophisticated method which has found wide application in the study of intracrystalline diffusion in zeolites is the nuclear magnetic resonance (NMR) pulsed field gradient self-diffusion method. The method, which is limited to hydrocarbons and other sorbates with a sufficient density of unpaired nuclear spins, depends on measuring directly the mean square distance traveled by molecules,... 

Chapter 7 of this text details the use of NMR imaging methods to determine structure and function of crosslinked rubbers. In this section we review the NMR imaging of rubbers swollen with small molecules. Two sets of experiments are described, namely the study of diffusion of small molecules into a previously unswollen rubber (macroscopic diffusion), and the measurement of images of the solvent and/or polymer after equilibrium has been achieved. 

Zawodzinski et al. [64] have reported self-diffusion coefficients of water in Nafion 117 (EW 1100), Membrane C (EW 900), and Dow membranes (EW 800) equilibrated with water vapor at 303 K, and obtained results summarized in Fig. 36. The self-diffusion coefficients were deterinined by pulsed field gradient NMR methods. These studies probe water motion over a distance scale on the order of microns. The general conclusion was the PFSA membranes with similar water contents. A, had similar water self-diffusion coefficients. The measured self-diffusion coefficients in Nafion 117 equilibrated with water vapor decreased by more than an order of magnitude, from roughly 8 x 10 cm /s down to 5 x 10 cm /s as water content in the membrane decreased from A = 14 to A = 2. For a Nafion membrane equilibrated with water vapor at unit activity, the water self-diffusion coefficient drops to a level roughly four times lower than that in bulk liquid water whereas a difference of only a factor of two in local mobility is deduced from NMR relaxation measurements. This is reasonably ascribed to the additional effect of tortuosity of the diffusion path on the value of the macrodiffusion coefficient. For immersed Nafion membranes, NMR diffusion imaging studies showed that water diffusion coefficients similar to those measured in liquid water (2.2 x 10 cm /s) could be attained in a highly hydrated membrane (1.7 x 10 cm /s) [69]. 

In 2003, Watanabe reported the development of proton-conducting Brpnsted acid-base ILs [32], With the help of the PSE pulse sequence they could demonstrate fast proton-exchange processes between protonated imidazolium cations and imidazole. Recently, Judeinstein also published two papers on the investigation of proton-conducting ILs, here based on amines and perfluorinated acids [33,34], With a combination of diffusion NMR and varios NMR correlation methods, interionic spatial correlations have been obtained. Additionally, three different types of ion association were found for three different ILs (a) mostly associated ion-pairs, (b) dissociated ions and (c) fully dissociated protons (Fig. 4). 

The practical applicability of such a system is quite clear nanospheres or nanoparticles may be produced by the emulsion-diffusion method and stored in the gel state of the particle framework. At any later point of time, an active ingredient may be added to the dispersion which easily diffuses into or through the porous gel matrix. By a simple freezing step, the particles are sealed and the active ingredient is trapped inside the spheres or capsules. The whole process is easily monitored by solid-state NMR which, in this case, could hardly be replaced by any other analytical approach. 

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