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

TL;DR

This paper derives an equation linking the isosbestic point wavelength in water-electrolyte solutions to electrostatic interactions and density changes, providing insights into water molecule behavior near solutes.

Contribution

It introduces a new equation that accurately describes the isosbestic point wavelength based on electrostatic force balance and density slope, advancing understanding of water-solute interactions.

Findings

Isosbestic point wavelength depends on electrostatic forces.

The wavelength correlates with the density vs. concentration slope.

Water molecule behavior is confined within few hydration shells.

Abstract

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.