c-Methylcalix[4]resorcinarene/choline salt/water ternary systems as novel bio-based, macrocycle-containing deep eutectic solvents

Building on recent advances in cyclodextrin-based deep eutectic systems [1], we present herein the mechanochemical synthesis and study of novel supramolecular deep eutectic solvents (DESs) derived from the proper combinations of C-methylcalix[4]resorcinarene (1) with various ammonium salts, such as choline chloride (ChCl), acetylcholine chloride (AChCl), and (2-chloroethyl)trimethylammonium chloride (ClChCl), in the presence of water. The integration of powder X-ray diffraction (PXRD) and optical microscopy analyses in the synthetic protocol helped identify the more promising formulations. Thermogravimetric analysis (TGA) provided accurate quantification of water content [2] and revealed differences in the thermal stability of the mixtures. Further chemical-physical characterizations agree with the obtaining of mostly homogenous viscous fluids. While the mechanochemical synthesis of the mixtures is conducted at room temperature, a post-synthetic one-pot heat treatment seems to be needed for their structural organization and stability. Overall, the collected results support the enormous potential of calix[4]resorcinarene/ammonium (choline-like) salt/water ternary systems as candidates for unconventional macrocycle-containing DES formulations while also pointing out to inherent synthetic as well as analytical challenges, particularly when working with hydrated supramolecular media.