Rayleigh linewidth and turbidity measurements near a multicomponent fluid tricritical point

Abstract

Rayleigh linewidth measurements have been performed near the tricritical point of an ethanol–water–benzene–ammonium sulfate mixture. We approached the tricritical temperature Tt along various paths of constant composition using several different samples. The linewidth always varies linearly with temperature in both two phase and three phase regions near Tt (but in the hydrodynamic regime). Since the linewidth is inversely proportional to the correlation length ξ in this regime, we find an exponent ν=1.0 in both two and three phase regions, as expected from mean field theory for thermodynamic paths of constant composition. Because of the strong temperature dependence of ξ, the hydrodynamic regime extends only to T−Tt?1 K. Measurements of the turbidity τ indicate that the product Γτ1/2 is constant within our experimental uncertainties, from which we infer that the susceptibility exponent γ?2 along paths of constant composition near the tricritical point.