Tricriticality in the ternary system 3-methylpyridine/water/NaBr? The light-scattering intensity

The existence of a tricritical point was postulated for the ternary system water/3-methylpyridine/NaBr (J. Jacob et al., Phys. Rev. E, 1998, 58, 2188) because light-scattering measurements yielded the mean-field critical exponent γ = 1 of the susceptibility at high concentrations of the salt, while the usual Ising value γ = 1.24 was found at low salt concentrations. However, in the preceding papers we have reported measurements of the viscosities and of the coexistence curves, which for all salt concentrations showed normal Ising criticality with corrections to scaling at large separations from the critical temperatures. In this paper, we continue the research on this system and report now measurements of the scattering intensity in the homogeneous phase approaching the critical temperature from below. Micro-heterogeneities causing a background were observed, which required waiting times up to 8 days to decay. The data are corrected for multiple scattering using a Monte Carlo simulation of the photon-paths in the sample and also by using the 3D cross-correlation method that enables to separate the singly scattered light from the other contributions. The scattering intensities of the equilibrated samples are described by the Ornstein–Zernike expression. For all salt concentrations, (10–17 mass%) the critical exponents are consistent with Ising critical behaviour. No systematic variation with the concentration of the salt is found for the various properties determined as the effective values of the critical exponents γ and ν for the susceptibility and the correlation length, respectively, and the corresponding amplitudes.