Uncommon Ultrasonic Absorption Spectra of Tetraalkylammonium Bromides in Aqueous Solution

Ultrasonic absorption coefficients and sound velocities of aqueous solutions of symmetric tetraalkylammonium bromides have been measured at 25 °C as a function of frequency ν (300 kHz ≤ ν ≤ 5 GHz) and molal concentration m of salt (0 ≤ m ≤ 6 mol/kg). The hydrophobic chains of the cations (C_nH_2n+1)₄N⁺ have been varied from n = 1 to n = 5. The absorption spectra for solutions of Me₄NBr (n = 1) did not show contributions in excess to the classical absorption, while those for solutions of larger hydrophobic cations revealed two relaxation regions. One of these regions can be represented by a Debye-type relaxation process with a relaxation time τ_D (τ_D ≈ 20 ns) which is almost independent of the solute concentration and the length of the cation alkyl groups. The process is attributed to an intramolecular mechanism of rotational isomerization. The other relaxation region reflects a relaxation time distribution. Its principal relaxation time τ_max adopts values between 15 and 230 ps. This relaxation appears to be due to a microheterogeneous structure of the salt solutions. It can be well represented by the Romanov?Solov'ev model of concentration fluctuations if this model is extended to also consider effects of correlations. The values for the correlation length are found to nearly agree with the particle radius that can be calculated from the mutual diffusion coefficient and the shear viscosity of the solutions according to the Stokes?Einstein relation. A noticeable result is the finding that the extended Romanov?Solov'ev model meets with the unusual concentration dependence in the relaxation amplitude. The volume viscosity data derived from the classical part of the sound absorption and data for the isentropic compressibility as resulting from the sound velocity are also discussed in terms of structural properties of the organic salt solutions.