We had previously studied [1] interaction of tetraalkyl ammonium iodide with mixed ethanol-water solvents using ultrasonic techniques. Ultrasonic techniques may be advantageously used to measure parameters such as ultrasonic velocity, isentropic compressibility, and apparent/partial molal compressibility. From a study of ultrasonic measurements it was concluded that the larger R4N+ ions exhibit a higher ion-solvent interaction; such interaction was found to follow the trend: Bu4N+ > Pr4N+ > Et4N+ > Me4N+.

More recently it was reported [2] that association of opposite ions in aqueous tert-Butanol solutions of tetraalkyl ammonium iodides is weak. This suggests an important influence of the solvent structure. It may thus be concluded that ternary system, water+ t-BuOH+Bu4NI, can serve as a model system to study both hydrophobic phenomena, the hydrophobic hydration of t-BuOH molecules and the hydrophobic association of Bu4N+ ions.

Our previous results and [2] are consistent in that the data in both cases indicates a higher level of ion-solvent interaction with larger R4N+ ions. A number of interesting issues remain open; e.g., (1) Are there important differences in the ion-solvent interactions depending upon whether ethanol-water or tert-butanol-water solvent combination is used? (2) Since tert-Butanol find application in combination with ethanol as a denaturant it is interesting to explore the characteristics of quad water+ethanol+tert-Butanol+Bu4NI systems, (3) What role temperature plays on tertiary and quad systems like the ones above. We believe that ultrasonic measurements offer significant new insights and flexibility with respect to these issues. Several set of data and analysis from comparative ultrasonic studies of such systems will be presented.

References [1] "Interaction of Tetraalkylammonium Iodides in Some Mixed Solvents by Ultrasonic Studies," Ph.D. Thesis, Dr. Anandita Rajat, University of Lucknow, Lucknow, India, 1996. [2] "Hydration and association in solutions of tetraalkylammonium Iodides," Agnieszka Kuba and Ewa Hawlicka, Phys. Chem. Chem. Phys., 2003, 5, 4858�4863.