Oligoether Carboxylates: Task-Specific Room-Temperature Ionic Liquids

Abstract

Recently, a new family of ionic liqs. based on oligoether carboxylates was introduced. 2,5,8,11-Tetraoxatridecan-13-oate (TOTO) was shown to form room-temp. ionic liqs. (RTILs) even with small alkali ions such as lithium and sodium. However, the alkali TOTO salts suffer from their extremely high viscosities and relatively low conductivities. Therefore, we replaced the alkali cations by tetraalkylammonium (TAA) ions and studied the TOTO salts of tetraethyl- (TEA), tetrapropyl- (TPA), and tetrabutylammonium (TBA). In addn., the environmentally benign quaternary ammonium ion choline (Ch) was included in the series. All salts were found to be ionic liqs. at ambient temps. with a glass transition typically at around -60 °C. Viscosities, conductivities, solvent polarities, and Kamlet-Taft parameters were detd. as a function of temp. When using quaternary ammonium ions, the viscosities of the resulting TOTO ionic liqs. are >600 times lower, whereas conductivities increase by a factor of up to 1000 compared with their alkali counterparts. Solvent polarities further reveal that choline and TAA cations yield TOTO ionic liqs. that are more polar than those obtained with the, per se, highly polar sodium ion. Results are discussed in terms of ion-pairing and structure-breaking concepts with regard to a possible complexation ability of the TOTO anion.