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.