Metastable critical lines in (acetone + polystyrene) solutions and the continuity of solvent-quality states

Double critical behavior of two distinct types was found in (acetone + polystyrene) liquid solutions. A quasi-monodisperse polystyrene of M_w∼ 16 600 in perprotio-acetone presented a T-p extremum in the critical locus at low-temperature and high-pressure and a metastable p-T one at a higher temperature and a lower (and negative) pressure. The first case corresponds to a pressure-double critical point (DCP), while the latter is a metastable temperature-DCP. At the p-DCP one expects a change in the sign of the excess volume as pressure is varied, and at the T-DCP neighboring region a change in the sign of the excess enthalpy should occur with temperature change. A Berthelot-type method was used to achieve experimental absolute negative pressures of the order of -60 bar. To the best of the authors' knowledge, this constitutes the first experimental observation of a polymer solution with two types of double critical points with one penetrating the metastable region of negative pressures. A review of the pertinent literature suggests that acetone changes by virtue of applied pressure from a "non-solvent" to a "poor-solvent" and finally to a "θ-solvent" for polystyrene.