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

6 Conclusions and Outlook

In this thesis dynamic light scattering and shear viscosity measurements have been per- formed and broad band ultrasonic spectrometry was carried out in the frequency range be- tween 180 kHz and 500 MHz. Evaluating the experimental data for liquid mixtures of crit- ical composition particular attention has been given to the applicability of Bhattacharjee- Ferrell dynamic scaling model and to corrections for the effects from the crossover from Ising to mean-field behavior. Three types of critical liquids have been considered: bi- nary mixtures without complex background contributions in their ultrasonic spectra (n- pentanol-nitromethane, nitroethane-cyclohexane, nitroethane-3-methylpentane, methan- ol-hexane, and ethanol-dodecane), binary mixtures with additional relaxations in the time domain of critical fluctuations (2,6-dimethylpyridin-water, isobutoxyethanol-water, tri- ethylamine-water), and ternary mixtures with concentrations selected along the plait-point line (nitroethane-3-methylpentane-cyclohexane). With the latter system interest has been particulary directed to the dependence of critical parameters up on the concentration of a constituent.

In the homogenous region near a consolute point the critical dynamics of binary liq- uid mixtures without additional sonic relaxations can be consistently represented by the Bhattacharjee-Ferrell theory. The scaling function of the more recent Onuki and Folk- Moser theories exhibit systematic deviations from the experimental data. Using the for- mer theory the scaled half-attenuation frequency Ω1/2 nicely agrees with the value 2.1 as predicted by Bhattacharjee and Ferrell. An exception is the system n–pentanol-nitro- methane for which the slightly smaller Ω1/2 = 1.86 is found. A noteworthy result is the finding that relaxation rate data from dynamic light scattering and shear viscosity mea- surements can be used in the theoretical description of the ultrasonic spectra.

In the case of the critical binary mixture nitroethane-cyclohexane an anomalous varia- tion in the amplitude of the critical contribution SBF has been found. This temperature dependence is obviously due to a temperature dependence in the adiabatic coupling con- stant g which results from the predominance of the T dependent thermal expansion coef- ficient. If this effect is disregarded scaling function data from different temperature runs do not condense onto one curve. In the critical binary liquids ethanol-dodecane as well as methanol-hexane a relaxation process, with relaxation time in the nanosecond range τD = 19.1 ns, and τD = 2 ns, respectively, has been found. It was assigned to a reaction in

View original page 131 (authoritative equations, figures and layout)