The development of ionic liquids
(ILs) with strong hydrogen-bond
basicity is of great importance for the application of ILs in liquid–liquid
extractions, but a long-standing problem is that the increase in hydrogen-bond
basicity of ILs usually comes along with undesired changes in other
properties such as reduced lipophilicity, raised melting point, and
difficulty of forming biphasic systems with water. Herein, we provided
a promising solution to this problem by synthesizing a class of functional
phosphonium ILs with the use of biocompatible saturated/unsaturated
long-chain fatty acids (carbon number up to 20) as anion precursors,
and we also carried out systematic investigations on their physicochemical
properties. These long-chain fatty acid ILs (LCFA-ILs) featured very
strong hydrogen-bond basicity, up to the top level of all reported
solvents, along with good lipophilicity and a wide liquid range. Moreover, the viscosity of LCFA-ILs
exhibited only a slight increase with increasing chain length, and
the hydrophilicity/hydrophobicity could be tuned by a change in the
side chain of the phosphonium cation. Excellent extraction efficiency
was achieved by applying LCFA-ILs to both aqueous and nonaqueous extractions.
The distribution coefficients of specific solutes reached unprecedented
values up to 3000–5000, which were 17–650 times higher
than those by common ILs and molecular solvents.
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