Structure-physical properties relationship of eutectic solvents prepared from benzyltriethylammonium chloride and carboxylic acids

Abstract

Deep eutectic solvents (DESs) have attracted the attention of the researchers as alternative solvents due to desirable properties such as easy preparation and high thermal stability. In this work, it was reported the preparation of five DESs from benzyltriethylammonium chloride (BTEAC) as quaternary ammonium salt (QAS) and carboxylic acids (oxalic, malonic or benzoic) as hydrogen bonding donor (HBD), which four were not reported so far. Furthermore, it was presented the first type III liquid DES at room temperature from benzoic acid and shown by interaction energy calculations why the formation of benzyltriethylammonium chloride:benzoic acid (BTEAC:BA) is favored over choline chloride:benzoic acid (ChCl:BA). The optimized geometries showed the Cl···π interaction is determining for decreasing the freezing point of benzyltriethylammonium chloride:benzoic acid mixture. Infrared spectra showed evidence for the existence of hydrogen bonds in DESs. Physical (freezing point, density and refractive index), thermal (thermogravimetric analysis) and rheological characterizations were performed for all solvents. The structure of HBDs affected the results on all evaluated properties and proved that it could be tunable. The prepared DESs were stable thermically and can be used in a wide range of temperature. Finally, rheological studies showed that Newtonian or non-Newtonian behavior can be observed by the nature of HBD.