cg5b00938_si_001.pdf (483.86 kB)
Does the γ Polymorph of Glycine Nucleate Faster? A Quantitative Study of Nucleation from Aqueous Solution
journal contribution
posted on 2015-11-04, 00:00 authored by Laurie J. Little, Richard P. Sear, Joseph
L. KeddieWe quantitatively
study the crystallization of glycine from solution
by following the crystallization of a plate with 96 wells, each with
0.1 mL of supersaturated solution. Our first aim is to address the
difficult problem of obtaining nucleation data that is reproducible.
This problem is difficult because of the extreme sensitivity of nucleation
times. Nucleation is sensitive to factors that include how the crystallizing
system is prepared and even small (1%) variations in the supersaturation.
We discuss the appropriate statistical tests needed to show reproducibility.
Our second aim is to study the competition between the nucleation
of the alpha and gamma polymorphs of glycine. We find that nucleation
appears to be heterogeneous: Some samples crystallize in minutes,
whereas others do not crystallize after days, which indicates that
there is no well-defined nucleation rate for the set of samples. Homogeneous
nucleation gives a well-defined rate. Those samples that crystallize
in minutes mostly yield the metastable alpha polymorph. We speculate
that these crystals may be the result of seed crystals formed by transient
local increases in supersaturation in pipette tips during sample preparation.
However, those that crystallize in hours are largely the equilibrium
gamma polymorph. This is perhaps surprising because typically the
alpha polymorph is obtained from crystallization from aqueous solutions
near neutral pH. We speculate that the nucleation rate of the gamma
polymorph may be higher than that of the alpha form but that in earlier
work with larger solution volumes (increasing the probability of seeding)
and with stirring the alpha polymorph dominates because of its much
faster growth rate.