An equation for the isosbestic point wavelength in aqueous solutions of electrolytes

The existence of an isosbestic point in the OH bond vibration band of H2O molecules in solutions of strong electrolytes has been shown by many research groups but its physical meaning has been only recently read as the equilibrium point between two populations of water molecules where either water-ion or water-water interactions respectively prevail. Thus the isosbestic point provides useful information on the bulk displacement of water molecules in presence of solutes. Herein we show that the isosbestic point occurrence is accurately described by an equation depending on the balance of the electrostatic forces of the ion and its counter-ion. Furthermore, its wavelength is also related to the slope of the density vs. solute concentration. This support the view that the effect of the solute can be barely regulated within few hydration shells.