Tendencies of Solid Phase Formation in Deep Eutectic Solvents of Betaine and Xylitol

Deep eutectic solvents (DES), formed from Lewis or Brønsted acids and bases [1], often have excellent fluidity. Natural DES from betaine and xylitol, which are solids when pure, may turn liquid if mixed and then show promising biocatalytic performance [2]. This report studies the effects of their ratios on phase separation.Mixtures were heated to 160°C or until melting, but those with 50% wt. or more betaine did not melt fully. Molten samples were brought to room temperature for 1 hr and, if still clear, cooled down to ~-20°C. No clouding could be observed in 4 hrs of freezing. Then samples were thawed to room temperature and inspected for phase separation daily. Gradually, solids began to form on the surface of all mixtures. Approximate volume of solids was evaluated visually and durations were recorded when they exceeded 5% and 20% vol, fig. 1.

Evidently, betaine (m.p. 300°C+) and xylitol (m.p. ~95°C) mixtures with 60-65% wt. of latter can liquefy without any chemical reaction and retain fluidity quite long. Mixtures outside this ratio are not as stable and might form excessive solids