Capillary electrophoresis instrumentation for determination of viscosity of deep eutectic solvents

Deep eutectic solvents (DES), and especially natural deep eutectic solvents (NADES), are advantageous alternatives over non-environmentally friendly organic solvents traditionally employed for sample preparation in analytical chemistry. Moreover, the use of DES has recently experienced remarkable growth to fulfill the green analytical chemistry principles. One of their most influential characteristics, which should be determined for each new DES, is viscosity. Manual or automatic viscometers are usually employed to this aim, but their use is sometimes hampered by characteristics of DES such as their variable water content or their wide viscosity range. In this study a proof-of-concept of the feasibility of employing a capillary electrophoresis (CE) instrument to determine this physicochemical property is shown. No change in the commercial instrument is required, and advantages such as applying controlled pressure and temperature, online UV detection at several wavelengths, use of extremely low volumes of solvents and viscosity markers, automation, etc., allow the easy and affordable adaptation of this method to different DES. As example of application, CE methods were used for viscosity measurement of ten different DES, including hydrophilic (based on either choline chloride or betaine) and hydrophobic ones (based on thymol), within two orders of magnitude (6 to 500 mPa⋅s). Agreement between the experimental results obtained with the CE instrument and data gathered by a commercial viscometer adequate for solvents without water or with published data (when available), is shown. Contribution of using DES to the green analytical chemistry principles is reinforced by the characterization for the first time of these solvents with the more sustainable and high applicability CE method here proposed.