658
whereas ionic liquids are synthesized by combination of protonation and complexation reaction. Therefore, ILs require purification step which makes ILs costly.
Abbott et al. described the DESs by a general formula AP
+
B
−
zC, where AP
+
is in
principle any phosphonium- or ammonium-based salt having Lewis base (commonly halide) “B” [107, 108]. The z number of Lewis or Bronsted acid C molecule
forms the complex anionic species with B
−
. Most of the current research concerning
DES is based on quaternary ammonium salts.
4.1 Physicochemical Properties of DESs
Deep eutectic solvents have physicochemical properties similar to that of ionic liquids (ILs). These solvents are non-volatile in nature having high thermal stability
[107, 109]. Their properties can be further altered based on the proper selection of
Fig. 7 Common hydrogen bond acceptor and hydrogen bond donor
H
Choline Cholride
Ethylene Glycol
Deep Eutectic Solvent
O
N
+
Cl
-
H
HOCH 2 CH 2 N (CH 3 ) 3 Cl .2(CH 2 OH) 2
O
O
80 qC
H
2
Fig. 8 Synthesis of reline (deep eutectic solvents formed by ChCL/EG)
M. B. Haider et al.