Kritisches Verhalten im ternären System 3-Methylpyridin/Wasser/NaBr

Anisimov :i:et al.:/i: (ref. 34-36) reported the existence of a tricritical point for the ternary system 3-methylpyridine/water/NaBr at about 16.5-17 mass% NaBr. This was based on several experimental observations: (a) a dip was found in the critical locus, (b) neutron scattering showed a transition between a homogenouos and a micro-heterogenous liquid phase and (c) light scattering investigations of the critical samples indicated a mean-field behaviour in the mentioned region, expected for a tricritical point.:p:The critical points in the region 10-19 mass% NaBr were redetermined in this work and no dip could be found. The analysis of the viscosity measurements indicated clearly Ising critical behaviour for all investigated samples. The same behaviour could be concluded by analysing the refractive index measurements, too. Using a 3D-fit of the visually obtained phase diagrams, the concentration of the three components in the coexisting phases could be estimated from the refractive index data. The obtained concentrations are in good agreement with the results of the phase diagrams.:p:The light scattering measurements showed at first a background intensity which disappeared after about 90 hours. This background intensity could be responsible for the mean-field behaviour observed by Anisimov :i:et al.:/i:. The temperature dependence of the scattering intensity was measured for all samples after a waiting time of about 8 days. The intensity was corrected for turbidity loss. In order to correct it for multiple scattering influences, two methods were used: (a) Monte-Carlo simulations and (b) 3D-crosscorrelation technique. Using both methods, Ising criticality could be found for all samples. Analysis of the dynamic light scattering data gave the same behaviour. The diffusion constant of the multiple scattering :i:D:sub:m:/sub::/i: could be also determined.:p:No evidence of a tricritical point in the region 16.5-17 mass% NaBr could be found.