Liquid–liquid critical point

A liquid-liquid critical point (or LLCP) is the endpoint of a liquid-liquid phase transition line (LLPT); 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 (LDL) and a high-density liquid (HDL), so above the critical pressure, a higher percentage of HDL exists while below the critical pressure a higher percentage of LDL is present. The ratio r = LDL/(LDL + HDL) 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 (r = ½).

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 papers 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.