Isotope effects on the hydrodynamic fluctuations of self-associating fluids. A comparison between the Brillouin scattering of 1-octanol and its hydrocarbon chain perdeuterated analogue

In this study we extend a previously developed theoretical model for concentration fluctuations of one-component fluids containing both associated and unassociated species. The model has been developed in order to explain the non-Landau behavior observed in self-associating fluids in the hydrodynamic regime. The complex dynamics for the different species formation is treated in the approximation of a pseudo two-components fluid formed by unbound molecules coexisting with a unique associated specie, the mean aggregation number of which is dictated by thermodynamic requirements. The model can be applied to rationalize the behavior of chain-forming hydrogen-bonding systems. Here, we report Rayleigh-Brillouin scattering data of 1-octanol and its chain perdeuterated analogue over a wide range of temperatures and wave vectors. The model explains, albeit qualitatively, most of the observed deviations from the Landau behavior observed in the Rayleigh-Brillouin spectra at different values of the k-vector.