The “Hidden” Phase Diagram of Water + 3-Methylpyridine at Large Absolute Negative Pressures

Abstract

Liquid−liquid (L−L) phase splitting in mixtures of water (H/D) + 3-methylpyridine (3-MP) at the limit of pure H2O as solvent and under high tension are reported for the first time. The phase diagram is thus encountered at large absolute negative pressure regimes. To the best of the authors' knowledge, these studies constitute the first to report phase transitions in nonpolymeric fluid mixtures at negative pressures, and the values of tension achieved (≈−350 bar) constitute a record for macroscopically sized samples. The overall behavior of mixtures of water (H/D) + 3-MP is successfully interpreted by using a simple gE-model. It is clearly shown that there is an intimate relation between pressures and the isotopic content of the solvent, and, thus, the latter can act as a thermodynamic variable. At critical concentrations, while mixtures of D2O + 3-MP always present phase separation irrespective of the applied pressure, when the solvent is pure H2O a miscibility window with the size of 1600 bar emerges. Th...