Contrast variation experiments

In addition to travel delays, a day may be spent in dialyzing the sample against D20 buffers for contrast variation experiments. 

In contrast to the flexible dendrimers, those of the forth generation with a rigid molecular scaffold expectedly exhibit a dense shell structure [49], in which all the end groups are located in the periphery (Fig. 7.6). This could be demonstrated in contrast variation experiments together with solid state NMR spectroscopic studies on polyphenylene dendrimers [50]. 

Fig. 2 Schematic picture of a set of contrast variation experiments (A) hydrogenous hydrophobe and head group, deuterated solvent (drop contrast) (B) deuterated hydrophobe hydrogenous head group and deuterated solvent, (shell contrast) and (C) deuterated hydrophobe and hydrogenous head group and hydrogenous solvent (core contrast).

In anomalous contrast variation experiments based on the solute, scattering densities are written as (see Section 2.7) ... 

Fig. 18, Anomalous contrast variation experiments using ferritin with synchrotron radiation of varying wavelengths [17], Left Dependence of the apparent of ferritin ( + ) on the wavelength near the K-absorption edge of iron at Xr = 0.1743 nm. The Rq dependence follows that of the dispersion curve...

In order to determine the aggregation number n and the volume Pin, we performed a contrast-variation experiment at constant dC(6)PC concentration (50 mM). The contrast, = ( m/Kn) — Ps> was changed by various D2O/H2O mixtures. Figure 4 presents the Guinier plots for dC(6)PC micelles in the presence of 2 M sorbitol with various contrasts. Equation (1) also indicates that and n can be determined from the position of the zero intensity and the slope of the plot of 7(0) as a function of contrasts, respectively. Figure 5 is a plot of the square root of 7(0) vs. the volume fraction of H20.7(0) vanishes in a solvent containing 16.1 vol% D2O. This corresponds to = 6.93 X 10 A . From this value and the slope of the straight line in Fig. 5, we found n = 32 and Fm = 640 A. In this calculation, the value of the micellar concentration estimated from the above measurement was also taken into consideration. 

Finally, the contrast variation experiments yield the polymer-solid cross structure factor ... 

The SANS observations also show the interest of performing full contrast variation experiments. Indeed, the information given by the two partial structure factors of the polymer are not equivalent because of the very singular nature of the profile at close distances from the wall. The experimental data yield values of the surface excess F. They correspond roughly to one full monolayer of dense polymer. It must be stressed, however, that the chains are not adsorbed in a flat configuration on the solid surface, as demonstrated as well by the EWIF method. 

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