Diffusion coefficient spectral measurement

The result of a typical diffusion measurement is shown in Figure 2. In the 1H-NMR spectrum of a cubic phase of monoolein and 2H20 with 10% Desmopressin, the signals from the aromatic residues (Tyr and Phe) in Desmopressin, appear in a spectral region which does not contain any signals from the lipid. Therefore, the peptide and lipid diffusion coefficients could be determined separately (Table II), and in Figure 2 the spectra from such an experiment are shown. The lipid diffusion coefficient was also determined in a cubic phase in the absence of Desmopressin. 

Molecular diffusion coefficients are parameters that are not related directly to NMR spectral intensities under normal conditions. However, molecular diffusion can cause NMR signal intensity changes when pulsed field gradients are applied during the FT NMR experiment. A number of pulse sequence developments, particularly the LED sequence, have meant that measurement of diffusion coefficients is relatively routine. The editing of H NMR spectra of biofluids based on diffusion alone or on a combination of spin relaxation and diffusion has been demonstrated. This has been termed the Diffusion and Relaxation Editing (DIRE) pulse sequence. This approach is... 

Fig. 23 Temperature dependence of the (apparent) diffusion coefficients measured with PFG-NMR obtained by fitting the integrals of the most shielded methyl signal for the cooling and the heating regime. The error bars correspond to the statistical error of D obtained during the fitting procedure. For the other spectral regions (amide and aliphatic region) similar behaviour was observed. Reproduced from Brand et al. (2006) with kind permission from American Chemical Society.

We have presented a method to calculate the mean frequency and effective diffusion coefficient of the numbers of cycles(events) in periodically driven renewal processes. Based on these two quantities one can evaluate the number of locked cycles in order to quantify stochastic synchronization. Applied to a discrete model of bistable dynamics the theory can be evaluated analytically. The system shows only 1 1 synchronization, however in contrast to spectral based stochastic resonance measures the mean number of locked cycles has a maximum at an optimal driving frequency, i.e. the system shows bona fide resonance [6]. For the discrete model of... 

By following electronic spectral changes of Fe(TIM) In BN upon reaction with CO as a function of time the rates and equilibrium constants for the complexatlon reaction were determined (18). The diffusion coefficients for the carrier cuid the CO-complex as well as the equilibrium constant were measured using cyclic voltammetry and rotating disk voltammetry. These physical constants were incorporated In the optimization model which predicted a facilitation factor F- 1.12. This data is summarized In Table I. 

In a QELS experiment, a monochromatic beam of light from a laser is focused on to a dilute suspension of particles and the scattering intensity is measured at some angle 0 by a detector. The phase and the polarization of the scattered light depend on the position and orientation of each scatterer. Because molecules or particles in solution are in constant Brownian motion, scattered light will result that is spectrally broadened by the Doppler effect. The key parameter determined by QELS is the diffusion coefficient, D, or particle di sivity which can be related to particle diameter, d, via the Stokes-Einstein equation ... 

Self-diffusion coefficients of distinct mobile species measured using PFG-NMR are based on spectral selectivity. In the context of IL selfdiffusion, PFG-NMR measures the time-averaged (miUisecond timescale) diffusion coefficients. Since ion-pair (cation-anion) interactions take place on a timescale faster than that, the measured diffusion coefficients are a weighted average over charged and neutral species. This is the reason for the variation between conductivity calculated from diffiasion coefficients determined by the Nemst-Einstein equation [15] (Eq. 1) and conductivity measured using impedance analysis ... 

In some adsorption systems, as an additional method, pulsed field gradient (PFG) diffusion measurements were applied to dilute colloidal dispersions, these methods are described in detail elsewhere [18, 19]. Briefly, by this method the self-diffusion coefficients for all spectrally resolvable liquid resonances can be obtained. Usually the method is applied to protons, and can in coated colloid dispersions monitor mobile species such as the solvent or free surfactants. 

Adsorption dynamics. IH NMR was employed beyond spectral studies by performing diffusion measurements Applying IH PFG-NMR diffusion experiments to surfactants adsorbed to latex particles in dilute dispersions, a method was developed for the investigation of surfactant adsorption dynamics [23, 26]. Since surfactant molecules were occurring in two sites, i.e. in solution and as adsorbed surfactant, each site exhibited a different diffusion coefficient and was distinguished in a PFG experiment. This offered a convenient way to vary the relevant experimental time scale, which is determined by the spacing of the gradient pulses A... 

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