Partial molal enthalpies, excess partial molal heat capacities and structural effects of Bu₄NBr and NaBPh₄ in aqueous binary solvent mixtures

Integral heats of solution of Bu₄NBr in aqueous binary solvent mixtures (containing 0.0 to 0.30 mol fraction of consolvent) of t-butanol, acetone, dioxan and ethylene glycol were determined at 15, 25, and 35°C. Integral heats of solution of NaBPh₄ in the same aqueous binary solvents containing 20 % by volume (about 0.05 mol fraction) of cosolvent were also determined at 15, 25 and 35°C. Corresponding excess partial molal heat capacities ΔC^°_p were derived at 20 and 30°C. The partial molal enthalpies ΔH^°_s of Bu₄ NBr increased (sharply in aqueous t-butanol up to 0.06 mol fraction) with the addition of increasing amount of cosolvent to water till the maximum value was attained at 0.1–0.3 mol fraction of cosolvent. The endothermicity maxima ΔH^°_smax were in the decreasing order of t-butanol > acetone > ethylene glycol > dioxan. H^°_smax decreased with increasing temperatures in all aqueous binary solvents. C^°_p results indicate that addition of small amounts of t-butanol and acetone stabilize the water structure, the maximum stabilization occurring around 0.04 mol fraction of acetone and t-butanol. Further addition of t-butanol to 0.06 mol fraction results in complete collapse of the water structure, whereas further addition of acetone results in gradual breakdown of the water structure which continues up to 0.3 mol fraction of acetone. Dioxan and ethylene glycol seem to have net structure breaking influence on the water structure.