Seawater: an explanation of differential isothermal compressibility measurements in terms of hydration and ion-water interactions

Hydration, ion-water interactions, and water structure effects in seawater were studied by determining differences (Δβ) between the compressibilities of test salt solutions and the compressibilities of reference solutions. The reference solutions were distilled water and seawater (35%0), and the test salt solutions were either 0.13 m or 0.26m with respect to one of the following test salts: LiCl, NaCl, KCl, CsCl, NaF, NaI, MgCl₂, CaCl₂, BaCl₂, Na₂SO₄, K₂SO₄, and MgSO₄. The compressibility measurements (to 900 bars) were carried out at 2°C and also at 15°C using a differential method in which a pressure increase or a temperature increase causes Δβ to become less negative. At 1 bar and 15°C, the Δβ (0.26 m, distilled water reference) values ranged from ?1.14 × 10^?6 bar^? for NaI to ?3.84 × 10^?6 bar^?1 for Na₂SO₄, and the Δβ (0.26 m, seawater reference) values ranged from ?1.30 × 10^?6 bar^?1 for NaCl to ?3.04 × 10^?6 bar^?1 for Na₂SO₄. The Δβ values were used to calculate hydration numbers. Entropy of transfer, excess hydrogen bond breaking (determined by NMR), and effective radii of ions are properties which can be used to describe the influence of ions on water structure. The extent to which these properties correlate with Δβ values depends upon whether the ion is an anion or a cation, and this correlation forms the thesis that anions alter water structure in a different way than do cations.