PFG NMR method

Studies of flow-induced coalescence are possible with the methods described here. Effects of flow conditions and emulsion properties, such as shear rate, initial droplet size, viscosity and type of surfactant can be investigated in detail. Recently developed, fast (3-10 s) [82, 83] PFG NMR methods of measuring droplet size distributions have provided nearly real-time droplet distribution curves during evolving flows such as emulsification [83], Studies of other destabilization mechanisms in emulsions such as creaming and flocculation can also be performed. 

Every et al. determined diffusion coefficients of Nafion 117 and BPSH 40 using a modified PFG NMR method. Nafion 117 and BPSH 40 were immersed in a MeOH solution of known concentration, and diffusion coefficients were measured as a function of MeOH concentration. For a Nafion... 

Figure 33.10 (a) Distance dependence and (b) time dependence of diffusion coefficient of Alexa 488 and cytc in HA aqueous solution (0.1, 0.9, 1.5 wt%) obtained by PCBR, SVC-FCS, PFG-NMR methods. The values are normalized with those obtained without HA (D0). The figures are reproduced from Refs. [23] and [25],... 

It is noteworthy that, in principle, each of these time constants may be determined by the PFG NMR method Xj-rys,, which is identical with Xi ,ra " and Xpciict directly from the measurement of the coefficients of intracrystalline and of long-range diffusion, and x harr by combining the result of the NMR tracer desorption measurements (Xjn, see Sec. III.C.) with Xj ,ra , using the equation Timra + x barr Since in the NMR measurements a wide range of... 

For studies of these complex systems NMR is very well suited, as few other methods exist that can determine such basic properties as the state of water and the size distribution of internal emulsion droplets. In addition, the PFG NMR method is sensitive to the molecular transport from emulsion droplets, a quantity which is relevant in the context of release mechanisms from such emulsion carriers. 

The temporal development of the displacement pro file reflects the presence of restricted motion, which may be studied in detail. Examples are motion inside compartments, between compartments, and motion of the compartment itself We may anticipate progress in the use of PFG NMR methods to study release mechanisms and kinetics from emulsion carriers. 

The PFG NMR solvent suppression approach as discussed in this work removes all solvent peaks, impurities from the solvents, residual monomers, and small molecules reaction byproducts. As a result, the PFG NMR method permits the acquisition of a clean spectrum of polymers by LC-NMR and GPC-NMR. One can detect a signal such as the methoxy signal of pMMA (3.6 ppm) in THF which also resonates at about the same frequency. This is not possible with other well established techniques such as WET since the suppression is applied at selected solvent frequencies and all signals, solvent and polymer, in the vicinity of those frequencies will be eliminated. 

The advantages of the PFG NMR method include its independence on the chemical environments, feasible under a wide range of internal and external conditions, and without influences and damaging to the sample. Therefore, the PFG NMR is the most reliable technique for self-diffusion... 

The easiest way to determine D+ and D is the pfg-NMR method. The method has the advantage that it is not restricted to binary electrolytes even multicomponent systems are realizable. However, it requires expensive instrumentation, is restricted to NMR-visible nuclei, and cannot be compared to electrochemical methods, because it averages ion pairs and isolated ions. In the hterature, many examples of these measurements are found [460-462], also for ILs [411,463,464]. Transference numbers for lithium salts in IL as solvents have already been studied as well as the improvement of the lithium-ion transference number by the choice of a useful anion [411]. 

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