Phase Equilibrium in Complex Liquids under Negative Pressure

Abstract

Liquids under some specific external condition are referred as complex liquids. One of these specific conditions is the stretching of liquids, where the pressure can be negative. There are several studies of the behaviour of these extended liquids, but only a very few of them are concerned with the phase equilibrium. Because no gas phase can exist under negative pressure, these equilibria can be only liquid-liquid or liquid-solid ones.Here we present experimental and theoretical results concerning liquid-liquid equilibrium in liquids under negative pressure. There are three groups of liquids which have been studied experimentally, namely binary solution of small molecules, binary polymeric liquids and liquid crystalsThe phase diagrams of these systems close to zero pressure are usually smooth extension of the region of the phase diagram at positive pressure, however, at large negative pressures unexpected behaviour can appear: the liquids can reach the liquid-vapour instability line (spinodal), i.e. the liquid-liquid phase diagrams cannot be extended to arbitrarily low pressures, they have to terminate before crossing the spinodal. We present results of some calculations showing that in some binary liquids the liquid-liquid phase equilibrium at negative pressure is limited by an unstable critical curve.