Hydrophobic-Like Solvation in Non-Aqueous Solutions: Positron Annihilation in and Compressibility of Tert-butanol and Tetramethylurea in Ethylene Glycol

The pioneer experimental results of positron annihilation experiments in non-aqueous solutions of tetramethylurea and tert-butanol were compared to those of ultrasonic velocity. For tert-butanol, the positron annihilation results exclude existence of hydrophobic-like interactions and formation of weak, labile crystalline-like solvates in solution. For tetramethylurea solute the changes of annihilation parameters with concentration are minor and only limited conclusions can be drawn on this system, while they are stronger for tert-butanol solute. However, all the above is consistent with an assumption that the more hydrophobic solute (tert-butanol) should affect the original glycol structure stronger than that interacting with glycol via hydrogen bondings only. On the other hand, the ultrasonic data for the systems tested, although very precise, hardly confirm formation of any specific structure. It is possible, however, that the solvates do not differ in their compressibility from the pure liquids, and/or are formed only in very small amounts. Thus, usefulness of the positron annihilation method in investigation of liquid structure was proven.