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Doubt regarding meta-stable and labile region of a supersaturated solution

Engineering Asked by Devil on October 4, 2021

I have read in a paper that the for formation of new crystals in a solution the energy released due to formation of solid from solution should be more than the energy needed to overcome solid-liquid interactions. As in the meta-stable this is not favorable, no new crystals form in the meta-stable region i.e. low supersaturation state. My doubt is, if it is thermodynamically unfavorable to form new crystals in meta-stable region then why does crystals grow in meta-stable region. In a way, it is also the formation of new crystals.

The exact lines from the text is: “The formation of stable nuclei only happens if the increase in interfacial energy resulting from the formation of a solid{liquid interface is offset by a decrease in energy released by the formation of a (thermodynamically favored) solid. At low supersaturations, the energy required to form a solid{liquid interface is bigger than the energy released upon formation of a cluster which are consequently unstable resulting in a very low nucleation rate. Only when the supersaturation is high enough, i.e. when we enter the labile region, stable solid particles will form.”

One Answer

What the text is saying is that there are two different processes at work.

  1. The creation of a solid-liquid interface. In other words, going from not having a crystal to having a crystal at all.

  2. The growth of a crystal when there is already something solid for it to grow on.

It is possible (the book says) for 2 to be thermodynamically favoured while 1 is not. In this régime new crystals will not spontaneously form out of the liquid but if a solid surface or particle is present then a crystal will grow on it.

In other words: nucleation.

There are further distinctions to be made based on the nature of the solid-liquid interface. A speck of dust is better than the smooth wall of a vessel, and a tiny crystal of the substance in question is better than a random speck of dust. The principle is the same, though: it is easier to add molecules to something that is already the right shape than to create “the right shape” out of nothing.

Answered by Martin Kochanski on October 4, 2021

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