Sound absorption mechanism of an aqueous solution in nonelectrolyte

Carboxylic acids in aqueous solutions: Hydrogen bonds, hydrophobic effects, concentration fluctuations, ionization, and catalysis

In the frequency range between 100 kHz and 2 GHz, ultrasonic absorption spectra have been measured for a series of carboxylic acids from formic to enanthic acid, including constitutional isomers. Also investigated have been the spectra for mixtures with water of short-chain formic, acetic, propionic, butyric, and isobutyric acid, in each case covering the complete composition range. The neat carboxylic acids feature two Debye-type relaxation terms with relaxation times between 5.6 and 260 ns as well as 0.14 and 1.4 ns, respectively, at room temperature. Depending on the composition, mixtures with water reveal an additional Debye relaxation term in the intermediate frequency range (acetic acid) or a term subject to a relaxation time distribution (propionic, butyric, and isobutyric acid). The relaxations of the neat acids are assigned to the equilibrium between monomers and single-hydrogen-bonded linear dimers and between linear and twofold-hydrogen-bonded cyclic dimers. The latter equilibrium is considerably catalyzed by hydronium and carboxylate ions. Several mixtures with water indicate one of the up to three Debye relaxations to reflect the protolysis of the organic acid. The term with underlying relaxation time distribution is due to noncritical fluctuations in the local concentrations. The Debye relaxations are evaluated to yield the parameters of the relevant elementary chemical reactions, such as the rate and equilibrium constants and the isentropic reaction volumes. A comparison of the correlation length of concentration fluctuations with data for other aqueous systems confirms the idea that the hydrophobic part of the organic constituent promotes the formation of a micro-heterogeneous liquid structure, whereas the hydrophilic moiety is of minor importance in this respect. The high-frequency limiting absorption suggests the equilibrium between conformers of linear dimers to contribute to the spectra well above the frequency range of measurements.

Fluctuations near the critical micelle concentration. II. Ultrasonic attenuation spectra and scaling

Based on a thermodynamic model of amphiphile solutions derived in the first part of the paper, the ultrasonic attenuation of such systems has been considered theoretically, including fluctuations of local concentrations and micelle sizes. At amphiphile concentrations smaller than the critical micelle concentration (cmc), scaling behavior in terms of the concentration distance to the cmc is predicted by theory, in fair agreement with experimental evidence. The scaling function in the sound attenuation below the cmc reveals the unsymmetric broadening in the spectra that clearly emerges from measurements when approaching the cmc. The shape of the scaling function corresponds to the experimental spectra of solutions with comparatively large cmc as well as with the relaxation spectral function of the unifying model of non-critical concentration fluctuations. Above the cmc, an additional relaxation term is predicted in correspondence with the Landau-Khalatnikov term in the sound attenuation of superfluid helium. This term is difficult to verify by measurements because, in the relevant frequency range, other processes may also contribute the ultrasonic attenuation spectra.

Crossover behavior in micellar solutions with lower critical demixing point: broadband ultrasonic spectrometry of the isobutoxyethanol-water system

The aggregation kinetics of isobutoxyethanol-water mixtures with lower critical demixing point has been investigated. Two types of kinetics have been observed, a diffusion-controlled formation of micellar species and the formation of a microheterogeneous liquid structure, governed by fluctuations in the local concentration. Ultrasonic attenuation spectra of isobutoxyethanol-water mixtures have been measured between 100 kHz and 2 GHz at 25 degrees C and at several concentrations, covering the complete composition range. With the mixture of critical composition measurements have been performed at some temperatures near the critical temperature (T(c)=299.51 K). In addition to the asymptotic high frequency background contribution, the broadband spectra reveal a Bhattacharjee-Ferrell relaxation term due to critical concentration fluctuations, a restricted Hill term reflecting the monomer exchange between micelles and the suspending phase, and two Debye-type relaxation terms that are assigned to chemical relaxations. The relaxation rates of the Bhattacharjee-Ferrell term exceed those from static and dynamic light scattering (amplitude Gamma(0)=5.3 x 10(9) s(-1)), likely due to the effect of a second parallel pathway of relaxation in the ultrasonic field. The adiabatic coupling constant following from the amplitude in the ultrasonic spectrum agrees with that from a thermodynamic relation (g=1.3). The restricted Hill term displays the features of an extended Teubner-Kahlweit-Aniansson-Wall model of the micelle formation and decay kinetics in surfactant solutions with high critical micelle concentration (C=0.6 mol/l). The idea of a fluctuation controlled monomer exchange in aqueous solutions of poly(ethylene glycol) monoalkyl ether-water mixtures near the critical point is briefly discussed.