Molecular Arrangement and X-Ray Diffraction in Ionic Solutions

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Abstract
Solutions containing heavy ions in water were examined with Cu Kα radiation, care being taken to avoid effects of general radiation. The diffraction patterns of saturated solutions of Th(NO3)4 and UO2(NO3)2 consist mainly in a pronounced ring, shifting to smaller angles and getting fainter with increasing dilution. This indicates a more or less regular arrangement of the ions in the liquid, for which the name ``superarrangement'' will be used to distinguish it from the arrangement with smaller periodicity of the water molecules. Both arrangements are of course of the ``liquid‐type.'' The above results show that the ``super‐arrangement'' expands and gets more diffuse with increasing dilution, as might be expected. Another kind of pattern is exhibited by solutions of AgNO3, Pb(NO3)2, Ba(NO2)2 and lead acetate. Here the intensity does not show pronounced maxima and minima. More especially it does not decrease markedly when the diffraction angle tends to zero. An explanation is offered, assuming the existence of a large fraction of undissociated molecules, with a gaseous distribution. A third class is formed by solutions of LiI, RbBr, LiBr and other alkali‐halides. These yield a broad ring the position of which is independent of concentration. It is shown that these facts may be accounted for by ascribing this ring mainly to the interference between the scattering from a heavy ion and that from the water molecules around it. To clear up some of the above points a few experiments are made with solutions of analogous light ions. Finally a general theory on diffraction by solutions is formulated and applied to some of the experimental cases.

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