Extraction and Enrichment of n-3 Polyunsaturated Fatty Acids and Ethyl Esters through Reversible π-π Complexation with Aromatic Rings Containing Ionic Liquids

The present study investigated the potentials of ionic liquids (ILs) containing aromatic rings in extracting and enriching n-3 polyunsaturated compounds. The relationship between extraction efficiency, selectivity, and structure of ILs was studied and elucidated. Ionic liquids containing aromatic rings such as the N,N'-dialkylimidazolium and N-alkylpyridinium are found to selectively extract and enrich n-3 polyunsaturated compounds. The IL extraction process reached equilibrium relatively quickly (<30 min), and the extraction efficiency can be further enhanced by increasing the volume ratio of IL to solvent, addition of silver tetrafluoroborate, and usage of double bond containing solvents such as 1-hexene as stripping solvent. At optimal conditions, 1-buty1-3-methylimidazolium hexafluorophosphate [bmim]PF6 and 1-butyl-3-methylpyridinium dicyanamide [BuMePyr]DCN had increased extraction capabilities of n-3 polyunsaturated fatty acids (PUFA) (16.19%) and n-3 polyunsaturated fatty acid ethyl esters (PUFAEE) (144.54%), respectively. Multiple-step reverse extraction whereby n-3 PUFAs were first enriched through IL/hexane extraction followed by addition of fresh hexane repeatedly up to three times to the IL phase to desorb the saturated FM resulted in a higher purity of n-3 PUFAs. Following this reverse extraction operation, the purity of n-3 PUPA and n-3 PUFAEE in the IL phase increased from the initial 15.88 to 38% (step 1) and further to 45% (step 3) and from the initial 72.56 to 82% (step 1) and further to 89% (step 3), respectively. The results from this study strongly suggested the correlation of n-3 PUFA extraction with aromatic/delocalized cation structure of ILs, which provides the basis for future research in designing novel ILs task-specified for efficient long-chain PUFA concentration.