UNCOMMON ULTRASONIC-ABSORPTION SPECTRA OF TETRAALKYLAMMONIUM BROMIDESIN AQUEOUS-SOLUTION

Ultrasonic absorption coefficients and sound velocities of aqueous sol utions of symmetric tetraalkylammonium bromides have been measured at 25 degrees C as a function of frequency nu (300 kHz less than or equal to nu 5 GHz) and molal concentration m of salt (0 less than or equal to m less than or equal to 6 mol/kg). The hydrophobic chains of the ca tions (CnH2n+1N+ have been varied from n = 1 to n = 5. The absorption spectra for solutions of Me(4)NBr (n = 1) did not show contributions i n excess to the classical absorption, while those for solutions of lar ger hydrophobic cations revealed two relaxation regions. One of these regions can be represented by a Debye-type relaxation process with a r elaxation time tau(D) (tau(d) approximate to 20 ns) which is almost in dependent of the solute concentration and the length of the cation alk yl groups. The process is attributed to an intramolecular mechanism of rotational isomerization. The other relaxation region reflects a rela xation time distribution. Its principal relaxation time tau(max) adopt s values between 15 and 230 ps. This relaxation appears to be due to a microheterogeneous structure of the salt solutions. It can be well re presented by the Romanov-Solov'ev model of concentration fluctuations if this model is extended to also consider effects of correlations. Th e values for the correlation length are found to nearly agree with the particle radius that can be calculated from the mutual diffusion coef ficient and the shear viscosity of the solutions according to the Stok es-Einstein relation. A noticeable result is the finding that the exte nded Romanov-Solov'ev model meets with the unusual concentration depen dence in the relaxation amplitude. The volume viscosity data derived f rom the classical part of the sound absorption and data for the isentr opic compressibility as resulting from the sound velocity are also dis cussed in terms of structural properties of the organic salt solutions .