Betaine and L-carnitine ester bromides: Synthesis and comparative study of their thermal behaviour and surface activity

Six esters of both betaine and L-carnitine bromides, featuring alkyl groups ranging from C8 to C18 in length, have been synthesized. The thermal behaviour of these twelve bio-based salts has been analyzed and compared by thermal gravimetric analysis and differential scanning calorimetry. The L-carnitine alkyl ester bromides melted below 100 °C and can hence be considered ionic liquids (ILs) with full rights. Conversely, the betaine alkyl ester bromides, with the exception of the shortest member of the series, melted slightly above this upper limit. Also, they resulted less thermally stable when compared to their L-carnitine analogues. Moreover, the self-aggregation behavior of these structurally related betaine and L-carnitine alkyl ester bromides, inspected through surface tension, conductivity measurements and isothermal titration calorimetry, varied significantly. The critical micelle concentration (CMC) values for the L-carnitine series were lower than those of the betaine series, and the former ILs were also capable of adsorbing more efficiently at the air/water interface as well as of reducing surface tension. A potential use of these organic salts as components of deep eutectic solvents is envisaged, thus showcasing the applicability of this natural trimethylammonium cation.