Hydrophobic hydration displayed by the molal shift in the isentropic compressibility

Aspects of ion hydration. Adiabatic compressibility compared to the dielectric properties of aqueous electrolyte solutions

Adiabatic compressibility data and principal dielectric relaxation times for aqueous solutions of 1:1 and 2:1 valent electrolytes are evaluated to yield their relative molal shifts Bκ and Bd, respectively, at low solute concentration. Cationic (Bx(+)) and anionic (Bx(–)) contributions to these quantities are calculated and compared to one another. For some ions also the correspondent relative molal shifts (Bm(±)) in the intramolecular proton magnetic relaxation rates are considered. Clear correlations between Bκ(±) and Bd(±) values are found for most series of ions. Within the series of halide ions, for example, Bκ(–) increases, whereas Bd(–) decreases with anion radius. For large hydrophobic cations the opposite is true; that is, Bκ(+) increases and Bd(+) decreases with molar volume of ion. In general, the magnitudes |Bk(±)| in the changes of the compressibility coefficient are smaller than in the shifts of the dielectric relaxation time. The situation is more complicated with dielectrically saturated small ions. Since the apparently irrotationally bound water molecules around such ions do not contribute to the dielectric spectra by reorientation, comparison of compressibility changes with changes in the proton magnetic relaxation rate rather than in the dielectric relaxation time is more appropriate. Some composite ions, such as BF4(–), show special features which can, however, be explained by a nonspherical charge distribution at the ion surface.