Study of the structure-breaking effect in aqueous CsCl solutions based on H2O/D2O isotope effects on transport coefficients and microdynamical properties

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Abstract
We have studied the effect of H2O/D2O isotopic substitution upon various transport and microdynamical properties in aqueous solutions of structure-breaking salts, using aqueous CsCl in the concentration range up to 6 mol kg–1 as a representative example. We report on isotope effects upon the self-diffusion coefficients of water and of Cs+ and Cl– ions, upon the reorientational correlation times of water deduced from 2H magnetic relaxation rates, and upon the magnetic relaxation rates of the quadrupolar relaxing ionic nuclei 133Cs+ and 35Cl– in these solutions. Comparison is made with the isotope effect upon the viscosity, and some subtle differences are outlined. The most substantial one is related to the isotope effects upon 133Cs+ and 35Cl– relaxation which selectively probe motions in the firsts cosphere of the ions. These are substantially lower than those observed with the other dynamical quantities, showing that dynamical isotope effects in the first cosphere may differ substantially from those in the bulk. It is concluded that the structure-breaking effect extends to the first co-sphere of the ions.