On Water Structure in Concentrated Salt Solutions

In concentrated salt solutions the average distances between the ions, dav=1.1844⋅(∑νici)−1/3 nm, are commensurate with the sizes of the solvated ions, so that no ‘bulk solvent’ remains. This is illustrated with two saturated aqueous solutions, where 16.67 mol⋅dm−3 CsF at 75 °C has dav(Cs–F)=0.368 nm and 14.54 mol⋅dm−3 LiI at 80 °C has dav(Li–I)=0.385 nm. The minimal distance required for the bare ions (sum of their radii) are 0.303 nm for CsF and 0.289 nm for LiI. Hence no water molecule, diameter 0.276 nm, can be fitted between the ions to form linear or slightly bent hydrogen bonds. Some recent work ignoring such constraints, even in 3–6 mol⋅dm−3 solutions, is criticized on this account.