Mutual Solubility in the Solid Phase of Simple Molecular Systems at High Pressure

Abstract

Considerable effort has been put in the development of theories for the description of the behaviour of gas-liquid and liquid-liquid phase equilibria. Literature about the mutual solubility of simple molecular systems in the solid phase is rather limited. More work has been done on metallic systems. It is generally believed that solubility at high density, in particular in solid systems, is mainly governed by geometrical effects. For instance, the well known Hume-Rotary rule, often used by metallurgists, states that a binary mixed solid is only obtained if the diameters of the molecules differ by less than 15 %, otherwise the mixture will separate into the pure solids. We will investigate whether such a geometrical rule is also valuable for molecular systems. Nowadays, the geometrical effects can be accurately calculated by means of computer simulations as well as analytical theories such as density functional theory. We will first consider in section 2 the case of binary hard sphere mixtures, since it is to be expected that the behaviour of those mixtures will give some insight into the effects of the difference in molecular size on the phase behaviour of real binary mixtures of simple molecular systems at high density. Moreover, we will deal with the phase behaviour of colloidal mixtures which is often in close agreement with that of hard sphere mixtures. In section 3 the experimental results on disordered solid solutions are examined. In section 4 we discuss a new class of compounds, the van der Waalscompounds, characterised by only weak intermolecular forces. In section 5 the differences between the experimental behaviour of molecular and hard spheres systems will be analysed in more detail and some conclusions will be drawn. Finally in section 6 a summary will be given.