The transition from a single phase to two phases (and vice-versa) can be easily demonstrated using a mixture of two suitable liquids.
A mixture of cyclohexane and aniline can exist as two separate phases or as a single phase, depending on the temperature. The thermodynamic transition between these two states can be understood by considering the balance between entropy and enthalpy (see Thermodynamics section in this TLP).
Aniline and cyclohexane are immiscible over a wide range of compositions below about 35 ºC. In this immiscible region there exists an aniline-rich and a cyclohexane-rich phase, separated by a boundary, seen as a meniscus. When this mixture is heated, the volume of one phase increases at the expense of the other. This can be seen as movement of the meniscus, provided the heating is slow enough. At the transition temperature for the particular composition, there will no longer be two discernable phases.
A significant point occurs when the distinction between the two coexisting phases reduces to zero. Here the domains present in the mixture can switch easily between aniline-rich and cyclohexane-rich. The composition variance is on such a scale as to interfere with light passing through it. Light will scatter in proportion to the squared difference in n, the index of refraction, of the two phases. Therefore light scatters more as the number of domain interfaces increases. Hence light is scattered strongly by the mixture across a small temperature range around the transition temperature.
At the critical point, the scattering is so intense that the system becomes opaque. This phenomenon is called critical opalescence. The domains demonstrate some interesting properties, such as fractal shapes, and there is a peak in the heat capacity. This critical transition temperature is a maximum with respect to the composition. Thus it can be determined by interpolating transition temperatures from known compositions.
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