Contact ion-pair structure in concentrated cesium chloride aqueous solutions: An extended X-ray absorption fine structure study

The hydration and ion pair structure of Cl⁻ ion in aqueous cesium chloride solutions at 0.5, 6 and 11 mol/kg (H₂O) has been studied using Extended X-ray Absorption Fine Structure (EXAFS) at the Cl K-edge. The co-refinement of multiple EXAFS datasets shows that the hydration structure of the free Cl⁻ ion is consistent with previously reported studies. In concentrated solution the Cl⁻ ion is strongly associated with the cation forming contact ion pairs with 1.6 and 2.8 Cs ions in 6 and 11 mol/kg aqueous CsCl, respectively. Together with ion-pairing structure of RbCl and RbBr, reported in our previous work, these results for the Cs-Cl ion-pair structure provide a more general view of ion-pairing in concentrated aqueous alkali halide solutions. The reported Cs-Cl ion-pair structure provides a basis for the development and testing of improved interaction potentials leading to more realistic structures from molecular dynamics (MD) simulations.