Structure of aqueous electrolyte solutions. Ionic internal pressures and proton chemical shifts

Abstract

The total ionic pressure changes indicate that most ions break the structure of water. The polyvalent cations and the SO^2–₄(and possibly the CO^2–₃) ion are exceptions since they enhance the structure. This agrees with the total proton chemical shift effects. On the other hand, the hydrogen-bond pressures of cations enhance structure whilst the corresponding pressures of anions break the structure (decrease hydrogen bonding).

The ionic pressures ascribed to the first and second cospheres of the cation are proportional to the electric fields at the H₂O centre in the first and second cospheres, respectively. The corresponding components of the proton chemical shift, δ₊, are proportional to these pressures. A third component of δ₊ is proportional to the combined ion–ion and ion–water-orientation pressures. The overall results for the cations are compatible with a distorted interstitial-type arrangement of the cations which bond to an average of 3.5 water molecules, have 6 H₂O molecules in the first cosphere and 11 in the second.

The anions behave quite differently. The effects due to the electric deformation of the water molecules are offset by those due to intermolecular effects. The electrostatic (no charge transfer) hydrogen-bond pressures which are associated with decreases in hydrogen-bonding for most anions (except F^–, SO^2–₄ and CO^2–₃) effectively dominate. Thus the observed shift for anions appears to derive from this type of effect alone. The hydration numbers are 6 (or 8 for ions such as ClO^–₄) for the first cosphere. Unlike the halides the influence of the oxyanions extends beyond the first cosphere so that altogether up to 21 water molecules are affected by the ions. The foregoing applies for solutions at 1 mol (kg H₂O)^–1 at 25 °C and 1 bar pressure. The equations derived for δ₊ and δ_– indicate that the components of the shifts will all be functions of temperature, pressure and concentration.