Polar Dissolution and Regrowth of (±)-Modafinil Twins Grown in Gels

Although (±)-modafinil form I crystallizes in the centrosymmetric space group P21/a, a significant proportion of twins crystallized in polymerized TMOS gel exhibit a polar dissolution. In a slightly undersaturated solution, one body of the twin dissolves smoothly. leading to a progressive disappearance of every face; the second body dissolves heterogeneously, leading to lamellar fragments. This phenomenon could be explained by the insertion of (001) layers of form IIIa metastable noncentrosymmetric modification (SG Pna21)inside a single body of the twinned form I. Because the slice (001) is common to polymorphs I and III, the interlacing of the two forms is compatible with the formation of well-shaped crystals. Regrowth experiments on partially dissolved twins in slightly supersaturated solutions confirms the sustainability of the heterogeneous character for a single body only of the twins.