Viscometry, volatility and stability of deep eutectic solvents from betaine

Deep eutectic solvents can be formed from solids without any reaction or ionic exchange. Recently,
natural deep eutectic solvents (NADES) are gaining popularity in extraction, lubrication or cooling, in particular
betaine-based ones. A series of betaine formulations with xylitol and glycerol as H-bond donors were prepared
and tested. A blend of betaine and glycerol at 1:2 wt. ratio recorded 456 mm²/s and 25.2 mm²/s at 40°C and
100°C, while that of betaine and xylitol was much more viscous with 234 mm²/s at 100°C. Their volatility was
studied by exposing thin films of NADES on steel and aluminum surfaces at 90°C. Vaporization rates depended
on the molecular weight of H-bond donors and their thermal/oxidative resistance. Initially, both combinations
showed similar lack of volatility, but in long term testing xylitol-based NADES turned out significantly less
volatile than those of glycerol. Although viscosities were higher than those of conventional organic solvents,
broad viscometric range and satisfactory stability imply many promising applications for NADES.