Solute-solvent interactions in water-tert-butyl alcohol mixtures. X. Partial molal volumes of alkali and halide lons

Ultrasonic vibration potentials for five alkali metal chlorides and five bromides in water-tert-butanol (t-BuOH) mixtures (t-BuOH mole percent X=2.5; 5.0; 7.5; 10.0 and 13.1%) have been measured as a function of concentration. From these data it has been possible to obtain for the first time the partial molal volumes of the individual ions in solvent mixtures. The partial molal volumes of Cl^– and Br^– ions go through a shallow minimum while those for alkali ions first incease or level off and then decrease as X is increased. A rapid decrease in the partial molal volumes occurs for H⁺ and Li⁺ in a narrow range of composition and these two ions appear to be essentially solvated by t-BuOH at X=13%. The modified Hepler's plots yield a single straight line for anions and cations up to X=5.0%, whereas two distinct lines are found at X7.5%. This change of behavior appears to be related to the maximum structuring effect of water upon addition of t-BuOH, which occurs in the same range.