# What is the reason for phase separation? [closed]

I have seen phase separation as a phenomenon taking place. However I do not understand why this phenomenon occurs as a counter example to the natural laws of diffusion along the concentration gradient. During phase separation the richer side becomes richer with the atoms which it already has and becomes more and more deficient with the poorer atoms and vice-versa. Similar things will occur in the opposite case as well. How can phase separation be explained in detail, for example with factors like the chemical potential?How do you quantify whether 2 liquid interactions will be unfavorable or not?

• The 2 liquids' interactions are unfavourable, so they simply go for minimal contact policy. Jul 18, 2019 at 13:18
• @WilliamR.Ebenezer Yes but how do you quantify whether 2 liquid interactions will be unfavourable or not? Jul 18, 2019 at 14:49
• What do you mean by "in detail?" That makes this question very broad and unspecific, and therefore unsuitable for the site. It would help if you could narrow down the question, perhaps by including in your post the last question in your comment, "how do you quantify whether 2 liquid interactions will be unfavourable or not? " Jul 18, 2019 at 16:57
• @BuckThorn edited Jul 18, 2019 at 18:40

The change in Gibbs energy has to be negative for a process to happen (at constant pressure in the absence of work). Gibbs energy is a combination of entropy and enthalpy.

I do not understand why this phenomenon occurs as a counter example to the natural laws of diffusion along the concentration gradient.

Diffusion is driven by entropy changes, specifically the entropy of mixing (in an ideal solution, the enthalpy of mixing is zero, so the entropy term is the only consideration). The entropy goes up when a solute moves from high to low concentration.

Phase separation occurs when the enthalpy term dominates. If substances A and B are initially mixed and then undergo phase separation, some change occurred (change of temperature, added third component, etc.) so that now phase separation more exothermic (lower energy for [A to interact with A and B interact with B] than [A interacting with B]). Under these circumstances, phase separation can happen.

• What will be the typical shape of the G vs X curve for a single phase two component system? Jul 18, 2019 at 19:44
• How will the G vs X curve look like in such circumstances?There is a typical "common tangent" method that we can apply to find out the equilibrium condition for a multi-phase system.Now that works pretty well when I have two concave upwards shapes.What if I had concave downwards?How would I find the minimum free energy?Common tangents could not be drawn then isn't it? Jul 18, 2019 at 19:50
• @user586228 See chemistry.stackexchange.com/a/113061/72973 for the equilibrium.
– Karsten
Jul 18, 2019 at 21:21
• – Karsten
Jul 18, 2019 at 21:47