Analytical parameters have been recalculated for atomic scattering factors and incoherent scattered intensities of the elements of atomic numbers 2 to 36, based on the tables of Tavard, Nicolas & Rouault [J. Chim. Phys. (1967), 64, 540]. The fit has also been checked with the total scattered intensities.
The first neighbour model, FNM, used frequently for the description of hydration has been modified by dropping the assumption that the structure of the "free" solvent is identical with that of the pure solvent. The modified model, FNM2, reproduces the experimental X-ray structure functions of alkali chloride solutions quite well and enables the study of perturbed solvent-solvent interactions. The refined structural parameters reasonably indicate the dependence of the hydration of the alkali cations on their radius and concentration. The hydration of Cl- has been found to be almost independent of both concentration and the type of counter cation.
The model assumes regular symmetry for the first neighbour coordination sphere, but the high rms deviations of the water-water distances within the shells of the aggregates indicate con-siderable individual asymmetries.
A further modification of the FNM will be discussed in a following paper.
The scattering of amorphous solid or liquid samples with moderate absorption coefficients is advantageously measured in the symmetrical transmission arrangement. For this case, the steps of data processing, converting the raw intensity data into the reduced intensity function, are discussed. The correcting functions are expressed as explicit functions of the angle (0 or 20), and of s (= sin 0]2) as well. The advantages of the latter formulae are demonstrated.
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