(170f) Computational Study of Polymorph Selection in Glycine

Controlling
polymorph selection is of great importance for pharmaceutical efficacy and
mechanical properties.  We recently
developed RMSD local order parameters that can distinguish the local
environments of individual molecules in the α, β, and γ-glycine polymorphs.¹
The solvated polar γ-glycine crystallite in aqueous solution forms a thicker
surface melted layer than uncharged α-glycine. The addition of NaCl to the solvent screens the surface charge of the polar
γ-glycine crystallite allowing a refreezing of its surface melted layer. In
contrast, the surface melted layer of the non-polar α-glycine is
unaffected by the NaCl. Experimental results have
shown that α-glycine forms first out of aqueous solution, while
γ-glycine forms first out of salt solution.² Our results
suggest that double layer formation and surface melting are important for
selection between polar and non-polar polymorphs.

References:

1.    
Duff N, Peters B. Polymorph specific RMSD local order parameters for
molecular crystals and nuclei: α-, β-, and γ-glycine. J. Chem.
Phys. 2011;135:134101.

2.     Yang X, Lu J, Wang XJ, Ching CB. Effect of sodium
chloride on the nucleation and polymorphic transformation of glycine. J. Cryst.
Growth. 2008;310:604-611.