*3.3. Preparation of NADESs*

Three NADESs having different properties were used. NADES-A—choline chloride urea 1:2; NADES-B protonated by HCl L-proline-xylitol 5:1; NADES-C, protonated by H2SO4 L-proline-xylitol 5:1.

Two synthesis routes were used.

NADES-A was prepared by simple mixing of compounds at 60 ◦C. The synthesis of the deep eutectic solvents based on protonated L-proline (NADES-A and -B) involved dissolving L-proline in an acid solution (the amount of acid with respect to L-proline was equimolar). Next, a predetermined amount of xylitol was added to the solution. Thus, prepared solution was placed in a rotary evaporator model Rotavapor R-300 (Buchi, Flawil, Switzerland) and water was distilled off under reduced pressure. Studies on the synthesis of this type of DESs and their physicochemical characteristics were the subject of a separate paper [47].
