Rayleigh–Brillouin light scattering from multicomponent mixtures: The Landau–Placzek ratio

In one-component fluids Rayleigh–Brillouin light scattering can be used to determine the equation of state of the fluid: A combination of the Brillouin shift and the Landau–Placzek ratio yields the isothermal compressibility. The same technique has been applied to multicomponent mixtures. We derive an expression for the Landau–Placzek ratio in a multicomponent mixture and show that it contains additional terms compared to the one-component case due to concentration fluctuations in the mixture. When we apply the expression to recent experiments on a five-component mixture of hydrocarbons, we show that the additional contribution cannot be neglected.