Phase Separation

George DeTitta detitta at
Tue Sep 10 10:58:00 EST 1996

Phase separation is a generic term for the transformation from one phase to another, and so can mean many things.  In the context of protein crystallization it is often meant to imply phase resolution:  the protein is initially in one phase (in solution) and then is in two phases (in solution and in crystal).  But perhaps what you mean is phase separation of the solution.  This is common in PEG/salt systems.  PEG in aqueous solution is highly soluble but the addition of salt (ammonium sulfate, sodium chlor
ide) lowers the solubility of PEG.  As the droplet dehydrates in response to a vapor pressure gradient between it and the reservoir, the concentrations of all non-volatile solutes increases.  Once the concentrations of PEG and salt reach critical values there is a spontaneous phase separation.  Where there was one liquid phase there are now two; one is rich in salt, the other in PEG.  It is often observed that crystals grow at the interface separating these two liquid phases.  To read further about this ph
enomenon see Ray and Bracker (1986) J. Crystal Growth 76, 562-576

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