The Mixing of Two Pure Liquids

We examine next the stability of a binary system that reaches equilibrium at constant temperature and pressure. To this purpose consider two liquid compounds 1 and 2 that are mixed at a specified temperature T and some low pressure P, and examine the variation of its Gibbs free energy with composition. 

Assuming, for simplicity, that the molar excess Gibbs free energy is given by the two-suffix Margules equation  

Consider now a mixture of a specified composition ro- We notice that while for =0 (ideal solution) and = 4 the corresponding values of G are the minimum ones for this composition as required by Eq. 12.3.8, this is not the case for A =1. Here the system can assume a lower value (G, ) by splitting into two phases - in equilibrium with each other - with compositions given by the points where the common tangent line touches the G curve c, andxi and this splitting will occur for all compositions in the range JCi Xj JCj but not outside it. 

In terms of molecular behavior, the very nonideal behavior of the mixture - manifested by the large value of /4 - Indicates that the molecules of the two species do not feel very comfortable with each other, thus do not mix in all proportions. 

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The present volume deals with the mixing of two pure liquid components to form a binary homogeneous (single-phase) liquid system or a heterogeneous (two-phase) liquid-liquid system (Case a). All the components are well-defined pure substances. Only heat of mixing data obtained by direct calorimetric measurements are considered. 

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