Evidence of Heterogeneous Aggregation in Methanol/CCl 4 Mixtures: A Brillouin Scattering Investigation F. Aliotta,* ,² M. Musso, ‡ R. Ponterio, ² F. Saija, ² and G. Salvato ² Istituto per i Processi Chimico-Fisici, Sezione di Messina, Consiglio Nazionale delle Ricerche, Via La Farina 237, I-98123 Messina, Italy, and Department of Molecular Biology, DiVision of Physics and Biophysics, UniVersity of Salzburg, Hellbrunnerstrasse 34, A-5020 Salzburg, Austria ReceiVed: May 17, 2004 By means of a Brillouin scattering experiment in methanol/CCl 4 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 CCl 4 , 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 heteroag- gregates, 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. 1. Introduction During the past few decades, a number of experimental and theoretical works have been devoted to the investigation of the static and dynamical properties of methanol in mixtures with carbon-tetrachloride. The interest is driven by the fact that since methanol on one hand is the simplest organic molecule able to exhibit a self-associative behavior through hydrogen bonding, it is an almost perfect model system for the investigation of clustering effects in H-bonded system. On the other hand, the solvent carbon tetrachloride is in principle an apolar molecule and therefore one is led to assume that it behaves as an inert solvent for methanol. Triggered by self-aggregation phenomena induced by hydrogen bonding in the alcoholic component, a number of deviations from the ideal behavior should be observed in methanol/CCl 4 mixtures. These mixtures are therefore a very well-suited prototype for the study of concentration dependent effects in the local structure of hydrogen-bonded networks. Accurate measurements of density and enthalpy of mixing 1-3 already indicated, several years ago, that the system is not a perfect ideal mixture. A way to rationalize such deviations from ideality was found in a molecular dynamics experiment on Lennard-Jones and Stockmayer fluids 4,5 (such systems can be considered as the simplest models for polar/apolar fluid mixtures) and its extension to a methanol/CCl 4 mixture, 6 in which the results coming from the adoption of a polarizable CCl 4 model were compared with those obtained when polar- izability is not taken into account. In particular, these simulations were able to determine the values of excess energy and free energy of mixing as a function of the system composition, as well as to reproduce the positive asymmetry of the excess energy experimentally observed at low concentrations of the polar component. The radial distribution functions of like molecules show that they have a tendency to cluster when their concentra- tion is low, this effect being much stronger for polar molecules in a apolar solvent than vice versa. Additionally, in mixtures where the polar component is highly diluted, an alignment phenomenon of the dipolar molecules is observed. In systems that are highly concentrated in the polar component, the distribution function of the Stockmayer fluid particles turns out to be insensitive to the changes in the strengths of the interactions between dipoles, due to frustration effects, i.e., the impossibility to achieve the energetically most favorable orientations when more than two dipoles are in each other’s vicinity. Therefore, the local structure of the system remains close to the one observed in the bulk liquid. This frustration effect also provides a molecular explanation for the asymmetry in structural and excess mixing properties, explaining why deviations from random mixing are more pronounced at the lower mole fractions. In the methanol/CCl 4 mixture, 6 the radial distribution functions show a strong tendency of methanol to preserve local order similar to the one in the pure fluid. In fact, throughout the composition range, a majority of the methanol molecules are found to be engaged in two hydrogen bonds leading, as in the simulation of the pure fluid, 7-10 to a pattern of hydrogen bonded chains. Upon dilution, the degree of cross- linking between the chains diminishes whereas the free mono- mer fraction rises; furthermore, a significant number of cyclic polymers exists due to energetic reasons (see also refs 11 and 12). The indications from these simulations allowed us to rational- ize a number of experimental results. The observed temperature and pressure dependence of the proton chemical shift in mixtures at x M e 0.2 (φ M e 0.09), obtained by 1 H NMR spectroscopy, 13 can be accounted for by * Corresponding author. E-mail. aliotta@mail.its.me.cnr.it. ² Consiglio Nazionale delle Ricerche. ‡ University of Salzburg. 12972 J. Phys. Chem. B 2004, 108, 12972-12977 10.1021/jp0478918 CCC: $27.50 © 2004 American Chemical Society Published on Web 07/27/2004