Introduction to the Theory of Soft Matter by Jonathan V. Selinger

Author:Jonathan V. Selinger
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

Figure 8.5 shows the generic phase diagram in terms of pressure and temperature. The crystal phase normally occurs at a lower temperature than the liquid phase because the crystal has more order. At high temperature, entropy favors the more disordered liquid phase; at low temperature, energy favors the more ordered crystal phase. Likewise, the crystal normally occurs at a higher pressure than the liquid because the crystal has a higher density (lower volume per particle) in most materials.3 At low temperatures and pressures, the gas, liquid, and crystal phases come together at the triple point. The triple point must not be confused with the critical point where the distinction between liquid and gas vanishes; it is a completely different type of point. Below the triple point , there is no liquid phase; rather, there is a direct transition between crystal and gas.

At high temperatures and pressures, the transition between crystal and liquid goes on forever; it does not terminate in a critical point. This absence of a critical point is an important consequence of symmetry: The distinction between crystal and liquid is a fundamental symmetry difference—the liquid has translational and rotational symmetries that the crystal lacks. This symmetry is either present or absent. For this reason, there must always be a phase transition between crystal and liquid, where the symmetry is broken; this phase transition cannot terminate in a critical point . By contrast, liquid and gas are the same type of phase, with the same symmetry; the distinction between them is a quantitative difference in the density. Because this difference is only quantitative, it can vanish at the critical point.4

Fig. 8.6Generic phase diagram for the crystal, liquid, and gas phases, in terms of volume per particle and temperature

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