Evidence of heterogeneous aggregation in methanol/CCl4 mixtures:: A Brillouin scattering investigation

By means of a Brillouin scattering experiment in methanol/CCl4 mixtures, deviations of the hyperacoustic parameters from the ideal behavior are studied as a function of concentration and of temperature. The experimental results are compared with the indication from a literature model describing the dependence of hypersonic velocity in weakly interacting liquid mixtures, assuming the system consists of three components, i.e., bulky methanol, bulky CCl4, and a third unknown component described as by an (hetero-) associated species. The agreement with the model turns out to be qualitatively satisfactory, the observed concentration and temperature dependencies of the hyperacoustic parameters allowing to infer the existence of heteroaggregates, whose nanostructure seems to be more connected with volume effects than with the stoichiometry of the system. The deviations, at low methanol contents, from the three component model prediction are interpreted as being originated by the distribution of oligomeric methanol aggregates, including low-energy configurations, not present in the mixtures for volume fractions of methanol above 0.3.