X-Ray and Neutron Scattering by Aqueous Solutions of Electrolytes
The composition of aqueous solutions* can be thought of as either (a) salt and water, or (b) anions, cations, and water, or (c) anions, cations, oxygen atoms, and hydrogen atoms. Thus, depending on circumstances, aqueous solutions can be thought of as either two-component, three-component, or four-component liquids. Macroscopic physical chemistry (e.g., measurements of solubility) uses the description implicit in (a), while in the microscopic Debye—Hückel theory, in its primitive form, the solution is regarded as a three-component system. When, however, we consider the scattering of radiation, solutions must be thought of in terms of description (c). Each “entity” scatters incident radiation in a characteristic way. In neutron scattering, for example, the primary scattering mechanism is the interaction of neutrons with each of the nuclei of the four components. In X-ray scattering, the situation is not quite so clear cut (Section 2), but for practical purposes we can ascribe to each chemical element a distribution of electrons that interacts as a whole with the primary beam. Formally, then, aqueous solutions must be thought of as four-component systems, each component of which has a characteristic scattering factor, which depends on the nature of the radiation used. For convenience we will call each component an “ion type”; thus, the four ion types in a solution of a salt MX n ,, dissolved in water H2O are M, X, H, and O.
KeywordsNeutron Diffraction Neutron Scattering Radial Distribution Function Partial Molar Volume Interference Term
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