Evidences of nonideal mixing in poly(ethylene glycol)/organic solvent mixtures by Brillouin scattering

The concentration dependence of the hypersonic properties of solutions of polyethylene glycol of mean molecular mass 600g/mole (PEG600) in benzene and toluene has been investigated by Brillouin scattering. The two solvents are very similar in structure and chemical properties but while benzene is fully non-polar a neat dipole characterizes the toluene molecule. In both solvents, at high concentrations, a high frequency relaxation process has been observed which has been assigned to conformational rearrangements of the polymeric chains, triggered by reorientation of the side groups. In both cases, the concentration dependence of the adiabatic compressibility largely deviates from linearity, indicating the existence of non-ideal mixing phenomena driven by aggregative processes taking place in the systems. However, temperature seems not playing any role when solutions of PEG600 in benzene are taken into consideration; on the contrary, the results obtained for solutions of PEG600 in toluene turned out to be noticeably dependent of the temperature. The comparison of the experimental data with the results of previous experiments on similar systems allows to draw a general picture for weakly interacting mixtures of hydrogen-bonded systems and organic solvents. In particular, in the presence of a non-polar solvent molecule the local structure of the mixture is dominated by solute self-association processes and any resulting hetero correlation is barely induced by excluded volume effects. At enough high dilution the self-aggregation of solute molecules produces a variety of new local topologies that cannot be observed in bulk solute and, as a consequence the concentration evolution of the system is too rich to be described in terms of a linear combination of the same few components over the whole concentration range. The situation seems to become simpler when polar solvent molecules are taken into consideration. In this situation, in fact, a three component model seems able to fit the experimental concentration dependence of the hypersonic velocity. Interpretation of it is realized in terms of the working hypothesis that the interaction between the solvent dipoles and the active sites of the solute produces a relatively stable hetero coordination while the relevance of self association is partially reduced.