The most common property of solvents used for crystallizing/recrystallizing solid substrates is that they dissolve a larger quantity of solid when hot than when cooled and this extra solid very often comes out of solution as crystals.
Binary mixtures of solvents also exhibit this widespread property; more solid is soluble in them hot than cold.
But now suppose that in some instances the mixture of hot solvents that is being used to recrystallize some solid splits into two distinct and separate phases upon cooling? Do crystals still appear? Does more solid crystallize than might be expected from a non-thermotropic solvent? Do the two liquid phases have any distinguishable effect on the crystal form, the crystal size, the crystal purity, or the ease of filtration?
These answers are not known even for isolated cases. The answers would depend upon the particular binary solvent system and the particular substrate to which the treatment is applied. What is very likely is that unless a very substantial solvent-to-solute ratio is required to get a complete dissolution of the substrate no phase separation at all will appear upon cooling. The method might need to be applied only to poorly soluble substrates.
But if two liquid phases still separated, I can imagine two distinct situations.
In the first of these, the substrate is much more soluble in one of the compositions into which the hot thermotropic azeotrope splits than in the azeotrope itself. In this situation, some solid may separate but there is less likelihood of any special enhancement in the recovery.
In the second situation, neither of the new liquid compositions turns out to be superior to the composition of the thermotropic azeotrope even at identical temperatures. Unexpectedly enhanced recoveries may be possible.
In both these outcomes having two different fluid components in intimate contact with the solid during crystal formation and crystal growth stages may give rise to different crystal morphologies or different rates of formation.
Let us as, an exercise, consider a specific binary azeotrope. I propose we look at the lower-boiling, thermotropic azeotropic composition between hexane and methanol. It is an azeotrope combining very non-polar hexane and the much more polar methanol. In this situation, let us further specify that the substrate is not particularly soluble in either hexane or methanol alone. It is most compatible with a mixture of the solvents that perhaps provides a low-energy solvation for an apolar domain and a different more polar domain. But the hot azeotropic composition and the two-component compositions into which the thermotrop splits are all mixtures of methanol and hexane. The thermotrop is 73.1% hexane and 26.9% methanol by volume. The two fluids existing at lower temperatures are 85% hexane/ 15% methanol and 42% hexane/ 58% methanol.
When the temperature is reduced the overall solubilizing capacity will most often be lowered as the temperature effect is likely dominant. At the same time, two phases will be created; one a less polar and the other a more polar one. Neither phase will be anywhere near pure hydrocarbon or pure alcohol, however. The total volume of the two phases taken together will be more or less unchanged.
In my example, if the hexane-methanol separates on cooling the composition of the upper layer will be 85% hexane and 15% methanol with specific gravity of 0.675. The composition of the lower layer will be 42% hexane and 58% methanol with a specific gravity of 0.724 and there will be, by volume, 68% upper phase and 32% lower phase. In the single-phase azeotrope, there will be 73.1% hexane and 26.9% methanol by volume.
The most probable result is that the overall solubility decrease for the main substrate may be not much different than if the solvent had remained homogeneous. Impurities, however, whether very apolar or very polar are likely to migrate substantially to the more polar or less polar phase and stay in solution more than otherwise. The effect might be similar to washing a seriously apolar solid with a little methanol or washing a distinctly polar molecule with a hydrocarbon. The main difference might be that the demixing can be expected to proceed more quickly because everything is in solution.
There could also be a difference in the rate of crystallization. Crystallization depends both upon a nucleation event and a crystal growth event. The rate of crystal nucleation is a function of the solvent composition as is the rate of crystal growth. With the dissolved solute in both liquid phases, there is an increased chance for a satisfactory nucleation rate and a crystal growth rate because there are two available environments for nucleation and two environments for possible crystal growth.
What this is suggesting is that if a solid presents difficulties forming crystals trying an azeotropic thermotropic solvent mixture might prove advantageous.
A solid with a tendency to oil out before crystallizing might produce a better physical form when crystals form from such an azeotropic thermotropic composition where the crystallization could proceed within droplets of one solvent composition suspended in the other.
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