Original English dissertation · scientific equations reproduced directly from the source PDF.

6 Conclusions and Outlook

which n alcohol molecules associate to n-mers. Assuming this relaxation to contribute its low frequency wing to the spectra, where special temperature dependent measurements for the ethanol-dodecane and methanol-hexane mixture of critical composition were per- formed, leads to an excellent agreement of the experimental scaling function with the theoretical form of the Bhattacharjee-Ferrell model. This is a noteworthy result because the evaluation of experimental data needs only one adjustable parameter.

Use of the Bhattacharjee-Ferrell model for the analytical representation of the critical part in the ultrasonic attenuation spectra of more complicated systems allows for a favor- able description of further relaxation terms. With the assumption that the contributions from chemical relaxations contribute additively to the critical contributions the ultrasonic spectra as well as the scaling function of the critical systems can be well represented in terms of the Bhattacharjee-Ferrell model. This was shown for the binary mixtures isobutoxyethanol-water and 2,6-dimethylpyridin-water. The system triethylamine-water, which includes two Debye-type relaxation terms, reflecting noncritical chemical reac- tions in addition to the high-frequency part, indicates a coupling between the critical dynamics and the noncritical relaxation terms. The experimental scaling function agrees with the Bhattacharjee-Ferrell function only, when both Debye relaxation terms are al- lowed slow down near the critical point. This is also true for the term that is assigned to a monomolecular structural isomerization of TEA which is expected to be independent from fluctuations. This finding may be taken to indicate water to play a noticeable role in the essentially uni-molecular isomerization. Water probably participates in the relaxation due to an extensive rearrangement of hydration shells associated with the isomerization of TEA. Unfortunately, it is difficult to study the behavior of the Debye-type relaxations without interferences by the critical term in the attenuation spectra. However, the slowing of critical relaxations near the critical temperature, has been also reported by Procaccia, Krichevskii and Wheeler.

The results for ternary mixtures demonstrate, that the ternary α′, col-point and α′′ systems (nitroethane-3-methylpentane-cyclohexane ) can be also well represented by the empiri- cal Bhattacharjee-Ferrell scaling function with scaled half attenuation frequency Ω1/2 = 2.1. For most quantities, such as the characteristic relaxation rate amplitude Γ0, the am- plitude of the fluctuation correlation length ξ0, as well as the background parameters Aη, Bη, and Tη of the shear viscosity, an evident dependence on the weight fraction of the third component (3-methylpentane) results. Especially, studies of the crossover behavior in dependence of the distance of the reduced temperature from the critical temperature as well from the used model of background viscosity indicate that only by viscosity measure- ments in a wide temperature range the correct determination of the fluctuation correlation length amplitude ξ0 and the cut-off wave numbers qc, qD (and consequently Γ0) is guar- anteed. Furthermore, the results of the low frequency sound attenuation measurements

View original page 132 (authoritative equations, figures and layout)