Tricriticality in the ternary system 3-methylpyridine/water/NaBr? The coexistence curves

The existence of a tricritical point was postulated recently for the ternary system water/3-methylpyridine/NaBr (J. Jacob et al., Phys. Rev. E, 1998, 58, 2188) because light-scattering measurements showed at high concentrations of the salt purely mean field behaviour with a critical exponent of the susceptibility γ = 1 instead of the Ising value γ = 1.24, which is found at low salt concentrations. However, in a preceding paper (Wagner et al., Phys. Chem. Chem. Phys., 2002, 4, 5300) we have reported measurements of the viscosities, which for all salt concentrations showed normal Ising criticality with a crossover to non-critical behaviour at large separations from the critical temperatures. In this paper we continue the research on this system and report refractive index measurements of the coexisting phases. For all salt concentrations (10–17 mass%) the critical exponents are consistent with Ising critical behaviour where the Lorenz–Lorentz function is taken as order parameter. The effective value of the critical exponent β is found to depend on the concentration of the salt. In the region of high salt concentration mean field tricriticality was expected with an exponent β = 1 while a value somewhat below the Ising value is found. The effective value of β is reduced at lower salt content. Essentially the same results are obtained when the mole, mass or volume fractions of the different components are taken as order parameter, where the compositions in the two phases are estimated on the basis of the separation temperatures determined for previously given concentrations.