Binary liquid concentration fluctuations linked to elementary reactions

Acoustical relaxation spectra of solutions and mixtures exhibiting a micro-heterogeneous liquid structure are discussed in view of non-critical and critical fluctuations in the local concentration. A unifying model of non-critical concentration fluctuations that includes all previous theories is shown to favourably apply to the relevant experimental spectra. This model assumes a parallel development of fluctuations by diffusion as well as by a rate process. Allowance is also made for spatial correlations in the fluctuations. For various aqueous solutions a correlation between the fluctuation correlation length and the hydrophobic properties of the solute becomes obvious. In addition, experimental results are reported which enable the scaling function of critically demixing binary systems to be determined without adjustable parameter. It is found that the Bhattacharjee-Ferrell dynamic scaling theory applies nicely to the experimental findings. Spectra for more complicated systems, such as revealing critical micelle formation are also discussed briefly, in order to indicate couplings between the fluctuations and elementary reactions.