Thermal Motion in Water + Electrolyte Solutions According to Quasi-Elastic Incoherent Neutron Scattering Data

The main attention of this article is focused on the study of the physical mechanisms of thermal motion in water and water + electrolyte solutions that lead to the broadening of the incoherent neutron scattering peak. It is taken into account that the neutron peak has a diffusion nature and is described by a Lorentzian line shape only for wave vectors k having magnitudes |k| ≡ k ≪ 1/a, where a is the interparticle spacing. A modified version of the theory developed by Singwi and Sjolander ( Phys. Rev. 1960, 119, 863 ) for the description of the Lorentzian half-width is proposed. It is shown that for k > 1/a, the neutron peak is described by a Gaussian line shape whose half-width is proportional to the average thermal velocity of the Lagrange particles. The relevant theoretical parameters can be determined by fitting experimental data for the half-width of the neutron peak. In such a way, the self-diffusion coefficients of water molecules, their collective parts, and the residence times as well as the radii of the Lagrange particles for the pure water and water + electrolyte solutions were determined. It is established that the specificity of the self-diffusion process in water + electrolyte solutions is mainly determined by the relation between a and the radius r I+ of the cations I+. The hydrated shell becomes more stable as the inequality r I+ < a/2 becomes stronger. In the opposite case, its stability decreases. It is shown that the sizes of the Lagrange particles determined by different independent methods are consistent with each other. This fact is very important, since it testifies to the self-consistency of the obtained results.