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

5.6 Systems with complex background contributions

is correct. In this context three additional systems has been studied, isobutoxyethanol- water, 2,6-dimethylpyridine-water and triethylamine-water.

5.6.1 Isobutoxyethanol-water

Ultrasonic attenuation spectra of the isobutyric acid-water mixture of critical composi- tion, reveal significant effects of slowing down near the critical temperature [101] not just in the monomer/linear dimer equilibrium of the carboxylic acid, but also in the linear dimer/cyclic dimer equilibrium, the latter being essentially an unimolecular process. The isobutoxyethanol-water mixture belongs to the class of micellar systems of non-ionic sur- factants. In the past, conflicting results have been reported for this systems, namely, non- universal near-critical demixing properties and also universal behavior [102], [103], [104], [105]. Recent broadband ultrasonic studies of a variety of CiEi/water systems [106], [107] suggested the idea of a fluctuation controlled monomer exchange [108], [109]. From the critical micelle mass fractions Ycmc of higher CiEi homologues the critical micelle mass fraction Ycmc = 0.07 has been extrapolated for the system isobutoxyethanol/water (i−C4E1/H2O), corresponding with a critical micelle concentration cmc = 0.6 mol/l at room temperature [108], [109]. The cmc of this short- chain amphiphile is, of course, not a sharply defined concentration but characterizes a transition range from predomi- nantly molecularly dispersed solutions to mixtures containing micellar structures. For the system (i−C4E1/H2O) this transition regime is located well below the lower critical demixing mass fraction: Yc = 0.318 [117] and Yc = 0.330 [110] have been found, accord- ing to the equal-volume criterion. The critical temperature was determined visually as well as from measurements of half-power bandwith of an acoustical resonance at 4000 kHz of a biplanar resonator, Fig.(5.23), and was Tc = 298.10 K. This value is smaller than the critical temperature in previous experimental studies, Tc = 299.60 K [117]. Hence mi- celles and concentration fluctuations exist simultaneously near the critical demixing point. The question whether or not the fluctuations in the local concentration interfere with the micelle formation/decay kinetics has not been answered so far [111]. A broadband ultra- sonic spectrometry study of the (i−C4E1/H2O) system has been performed recently [110] in order to investigate the aggregation kinetics as well as the critical dynamics at the lower demixing point.

View original page 95 (authoritative equations, figures and layout)