Ultrasonic-Absorption Measurements in Mixed Solvents: Water—Dioxane

Ultrasonic-absorption measurements in water—dioxane solutions have been carried out at 25° and 10° over the entire composition range in the frequency region 10 to 200 Mc/sec. The data can be described quantitatively in terms of two relaxation processes. A theory is presented for the absorption of ultrasonic energy in mixed solvents which can be approximated as ideal solutions. Deviations from ideality and excess absorption of ultrasonic energy are attributed to the formation of intermolecular complexes. If only a single relaxation process is observed, the composition at which the amplitude of the relaxational part of the absorption goes through a maximum and the concentration dependence of the relaxation time can be used to determine the stoichiometry of the aggregates, as well as associated kinetic and thermodynamic constants. For a system in which multiple relaxation processes occur, a normal-mode analysis must be used that yields a spectrum of relaxation times and normal thermodynamic variables. In the case of water—dioxane, all of the ultrasonic data are consistent with the formation of intermolecular hydrogen-bonded complexes according to the mechanism 2W+D⇌DW2, D+DW2⇌D2W2, where D represents dioxane and W water. Associated thermodynamic and kinetic constants have been evaluated. The standard volume and enthalpy changes for both reactions are quite small (about ±1 cc/mole and ±1 kcal/mole). The thermodynamic stability of the aggregates and their slow rate of breakdown relative to the rate of hydrogen bond breaking in pure water can be attributed to the formation of ``hydrophobic'' clusters of water around the hydrocarbon portion of dioxane.