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Entry by Emily Redston, AP 225, Fall 2011

Please see phase separation for an introduction to the regular solution model phase diagram we will be focusing on.

Figure 1 Schematic free energy curve (Jones, Fig. 3.4)
Figure 2 Schematic diagram of stability (http://en.wikipedia.org/wiki/Metastability)

Figure 1 shows the free energy of mixing as a function of composition for one portion of the two-phase region. Let's look at composition <math>\phi_a</math> with free energy <math>F_a</math>. Small fluctuation in composition create a higher energy state <math>F_{a'}</math>, and this compostion is therefore considered metastable. On the other hand, composition <math>\phi_b</math> is unstable since small fluctuations in composition create a lower energy state. A quick way to visualize stability can be seen in Figure 2. Point (1) is metastable because small fluctuations lead to higher energy states, but if it gets enough energy to make it over the barrier, it can move to a lower energy state. Point (2) is unstable because any fluctuations will cause it to move to a lower energy state. Point (3) is stable.

The description just given can be repeated for <math>\chi </math> values greater than two. For each <math>\chi </math> value, we find four characterisitic points: the two concentrations with minumum energy (the outer concentrations) and two concentrations in the 2-phase region corresponding to the transition from metastable concentrations to unstable concentrations, that is, the inflection points on the energy versus concentration curve.

The curve on the right above is magnified and replotted here. This is the phase diagram of a two-component liquid system whose free energy of mixing is described by the regular solution model. When the <math>\chi </math> parameter is greater than two, the system phase separates. The outer curve is the phase boundary. The inner curve is the spinodal - the energy vs concentration relation where the homogeneous mixture passes from a unstable to metastable state.

Figure 2 Temperature versus composition phase diagram showing phase separation (made by Emily Redston)

As a brief aside, the name spinodal, which means "thorn" in Latin, refers to their shape in one-component systems [1].

The Science of Chocolate: interactive activities on phase transitions, emulsification, and nucleation