Cocrystals of ammonium perchlorate with a series of crown ethers: preparation, structures, and properties

Abstract

Three cocrystals of ammonium perchlorate (AP) with a series of crown ethers have been prepared successfully through the solvent/antisolvent method, including 18-crown-6 (18C6), benzo-18-crown-6 (B18C6), and dibenzo-18-crown-6 (DB18C6). Crystal structures of the three cocrystals characterized by single crystal X-ray diffraction (SXD) reveal that all the cocrystals belong to the monoclinic system, in which the space group of the 1 : 1 cocrystal of AP/18C6 and AP/B18C6 is P2₁/n, P2₁/c for the 1 : 1.5 cocrystal of AP/DB18C6. The main intermolecular interactions in the three cocrystals are hydrogen bonds and π–π stacking. Powder X-ray diffraction (PXRD) analyses have been performed to scrutinize the purity of all the crystals. The water contact angles of the three cocrystals were measured to be 23.3° (AP/18C6), 25.4° (AP/B18C6), and 75.4° (AP/DB18C6), increasing with the number of phenyl groups connected to the crown ether, and the surface energy of the three cocrystals are greatly decreased compared to AP. The thermal properties of the cocrystals were investigated by thermogravimetry-differential scanning calorimetry (TG-DSC) and hot-stage optical microscopy (HSOM). The results indicate that the decomposition peak temperatures of the three cocrystals are greatly decreased compared to that of pure AP. In particular, the heat release increases from 475.5 J g⁻¹ of AP to 1304.2 J g⁻¹ for AP/B18C6 and 1488.4 J g⁻¹ for AP/DB18C6. So, through carefully choosing the co-former of AP, cocrystallization can comprehensively tune the hygroscopicity and the thermal decomposition property of AP.