A simple quantum statistical thermodynamics interpretation of an impressive phase diagram pressure shift upon (H/D) isotopic substitution in water + 3-methylpyridine.
In a previous work (J. Phys. Chem. B 2003, 107, 9837), we reported liquid-liquid-phase splitting at negative pressures in mixtures of H2O + D2O + 3-methylpyridine (3-MP) at the limit of pure H2O as the solvent, thus extending for the first time the L-L phase diagrams to this metastable region. We showed that there is an intimate relation between pressure and solvent deuterium content. Isotopic substitution (H/D) in water provokes subtle entropic effects that, in turn, trigger a significant pressure shift, opening a pressure-wide miscibility window of as much as 1600 bar. Isotope effects are quantum in origin. Therefore, a model that is both pressure-dependent and considers quantization constitutes a necessary tool if one wishes to fully describe the p, T, x critical demixing in these systems. In the current work, the statistical-mechanical theory of isotope effects is combined with a compressible pressure-dependent model. This combination enabled us to predict successfully the overall L-L phase diagram via differences in the vibrational mode frequencies of water on its transfer from the pure state to that of dilution in 3-MP: each of the three librational modes undergo a calculated red-shift of -(250 +/- 30) cm(-1), while the overall internal frequencies contribution is estimated as a total +(400 +/- 25) cm(-1) blue-shift.
Visak
Zoran P
ZP
Instituto de Tecnologia Quimica e Biologica, Universidade Nova de Lisboa, Av. da Republica, Apartado 127, 2780-901 Oeiras, Portugal.
Szydlowski
Jerzy
J
Rebelo
Luís P N
LP
D017666:Deuterium Oxide / Q000737:chemistry*; D044367:Phase Transition; D010847:Picolines / Q000737:chemistry*; D011312:Pressure; D011789:Quantum Theory; D013223:Statistics as Topic; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*
D010847:Picolines; D014867:Water; C053603:3-methylpyridine; D017666:Deuterium Oxide