Direct experimental verification of negative deviations from ideal mixing in an archetypal deep eutectic solvent

The thermodynamic activity of urea in the liquid solution of the urea–choline chloride DES at the eutectic composition was measured for the first time through direct tensimetric experiments using Knudsen effusion mass spectrometry. In the explored temperature range (319–343 K), the urea activity coefficients (with respect to pure liquid urea) were found to lie in the 0.25–0.44 range, thus indicating a clear negative deviation from the Raoultian behavior. From the temperature dependence of activity, evidence for negative values of partial enthalpy and entropy of mixing was found, indicating strong attractive interactions between urea and choline chloride, with a significant ordering of urea molecules compared to pure liquid urea.