Structural and Physicochemical Characterization of a Solid Solution Produced by Antisolvent Crystallization of a New Phosphoantigen

During the crystallization of the monohydrated disodium salt of hydroxydimethylallyl pyrophosphonate (C-HDMAPP·Na2·H2O) induced by addition of ethanol to an aqueous solution, a synthesis impurity (C-ethylPP2−) can be incorporated into the crystal lattice. Owing to the chemical similarity between the two anionic molecular entities, a solid solution of substitution type is produced and has been characterized by X-ray powder diffraction (XRPD), thermal analysis, and dynamic vapor sorption (DVS). Besides the incidence of the solid solution on diffraction peak positions caused by expansion/contraction of the lattice, XRPD revealed that a distinct crystalline phase is obtained when the proportion of impurity is higher than ca. 20 wt %. The crystal structure of both C-HDMAPP·Na2·H2O and C-ethylPP·Na2 phases were determined, which allowed molecular modeling investigations. A mechanism has been proposed to explain the substitution at a molecular level and is consistent with the correlation observed between the actual water content (nonstoichiometry) and the proportion of impurity in the solid solution of global formula C-HDMAPP1−x·C-ethylPPx·Na2·(H2O)1+2x, (0 ≤ x ≤ 0.26).