Liquid–liquid critical point


A liquid–liquid critical point is the endpoint of a liquid–liquid phase transition line ; it is a critical point where two types of local structures coexist at the exact ratio of unity. This hypothesis was first developed by H. Eugene Stanley to obtain a quantitative understanding of the huge number of anomalies present in water.
Near a liquid–liquid critical point, there is always a mixture of two alternative local structures. For instance, in supercooled water, two types of local structures exist: a low-density liquid and a high-density liquid, so above the critical pressure, a higher fraction of HDL exists, while below the critical pressure a higher fraction of LDL is present. The ratio r = LDL / of phase amounts is determined according to the thermodynamic equilibrium of the system, which is often governed by external variables such as pressure and temperature. A discontinuity is present in r when crossing the liquid–liquid phase transition, which separates the LDL-rich phase from the LDL-poor phase. At any point of the liquid–liquid phase transition, including the associated liquid–liquid critical point, the ratio of LDL to HDL is exactly one.
The liquid–liquid critical point theory can be applied to all liquids that possess the tetrahedral symmetry. The study of liquid–liquid critical points is an active research area with hundreds of articles having been published, though only a few of these investigations have been experimental since most modern probing techniques are not fast and/or sensitive enough to study them.