Crossover critical phenomena in an aqueous electrolyte solution: Light scattering, density and viscosity of the 3-methylpyridine+water+NaBr system

Abstract

We have measured the light scattered by critical mixtures of 3-methylpyridine+water+NaBr, at three different salt concentrations, as a function of temperature and wave vector, in the one-phase region. From the data, we have calculated the susceptibility χ and the correlation length ξ. We have also measured the density and the shear viscosity for the same critical mixtures over a broad temperature range. The analysis of the χ and ξ data in terms of the Wegner expansion lead to negative values for the amplitudes of the first correction-to-scaling terms. This is consistent with the nonmonotonic crossover from Ising to mean-field critical behavior. The analysis of the light scattering data in terms of the crossover theory of Anisimov et al. [Phys. Rev. Lett. 75, 3146 (1995)] leads to a good fit of the data, and allows one to describe accurately the behavior of the effective critical exponents γ and ν. The thermal expansivity calculated from the density measurements is consistent with a (1-α) critical anomaly, with α=0.11 for the three critical mixtures studied. Finally, the shear viscosity has been analyzed in terms of the dynamic crossover function and the ξ values calculated from the theory of Anisimov et al. [Phys. Rev. Lett. 75, 3146 (1995)]. The values of the critical exponent z are consistent with the theoretical predictions.