Calorimetric search for a tricritical point in mixtures of water, 3-methylpyridine and sodium bromide

Abstract

We investigated the specific heat capacity near the demixing transition of ternary mixtures of water, 3-methylpyridine and sodium bromide as a function of salt concentration. In this system, a crossover from Ising to tricritical behavior caused by the appearance of a new “microheterogeneous” phase was reported from light-scattering measurements [Jacob et al., Phys. Rev. E 58, 2811 (1998)]. We used adiabatic scanning calorimetry, a very sensitive technique for detecting and characterizing phase transitions, to investigate this system and found no specific heat anomaly to be associated with the formation of a new phase. In the one phase region, the critical exponent α is consistent with the Ising value and is incompatible with the tricritical value for all the investigated samples. In the phase-separated region, the values of the exponent are substantially lower and even negative. This is probably caused by the change in salt concentration of the two phases close to the critical point.