The Study Of Thermodynamics On The Amino Acid Ionic Liquid-based Aqueous Two-phase Systems

Ionic liquid-based aqueous two-phase systems (ATPSs), which were found in2003, are a newseparation technique. Ionic liquids (ILs) have strong ability to dissolve many chemicals, non vapor pressure,and fine-tuning ability for the physical and chemical properties, this made ATPSs to have wide applicationsin extraction and separation science for biomaterials. As a part of the project supported by the NationalNatural Science Foundation of China (No.20873036), in this work, we have synthesized two series ofamino acid ionic liquids, and studied the relationship between the the structure of amino acid ionic liquidsand the formation ability of their ATPSs, and examined the effect of the structure of amino acid ionicliquids on the phase formation ability, the relative hydrophobicity of the ionic liquid-rich phase relative tothe salt-rich phase and the polarity of ionic liquid-rich phase. The extraction performence and driving forceof the ionic liquid ATPSs for L-tryptophan, L-tyrosine and cytochrome-c were also investigated. The majorcontent is as follows.1. The amino acid ionic liquids1-butyl-3-methylimidazole serine [C4mim][Ser],1-butyl-3-methylimidazole glycine [C4mim][Gly],1-butyl-3-methylimidazole analine [C4mim][Ala],1-butyl-3-methylimidazole leucine [C4mim][Leu], and glycine tetraethylammonium [N2222][Gly], glycinetetrabutylammonium [N4444][Gly], glycine tetrabutylphosphine [P4444][Gly], glycine tetrapentylammonium[N5555][Gly] have been designed, synthesized and characterized.2. It was found that the amino acid ionic liquids could form ATPSs with inorganic salts, such as K3PO4,K2HPO4and K2CO3, the top phase is ionic liquid-rich, and the bottom phase is salt-rich. The phasebehavior of [C4mim][AA](AA=Ser, Gly, Ala, Leu)+water+inorganic salts and their temperature effectwere studied. It was shown that phase formation ability of the ILs with amino acid as anions decreases inthe order:[C4mim][Leu]>[C4mim][Ala]>[C4mim][Gly]>[C4mim][Ser], which could be explained bythe hydrophobicity of the amino acid anions. The phase formation ability of the inorganic salts was foundto be: K3PO4＞K2HPO4≈K2CO3＞KH2PO4, this order can be predicted by their Gibbs free energy ofhydration.3. Phase diagrams of the glycine ILs ([N1111][Gly],[N2222][Gly],[N4444][Gly],[P4444][Gly],[N5555][Gly])+water+inorganic salt were determined, and the effects of cationic structure of the ILs and the temperature on the phase behavior were studied. The results showed that the phase formation ability ofthe glycine ILs decreased in the order:[N5555][Gly]＞[P4444][Gly]＞[N4444][Gly]＞[N2222][Gly]＞[N1111][Gly]. This order is identical with the hydrophobicity order of the amino acid ionic liquids.4. Partition coefficients of5kinds of2,4-Dinitrophenylamino acids (DNP-amino acids) in theATPSs were determined and used to calculate Gibbs transfer energy of methylene (CH2) from salt-richphase to ionic liquid-rich phase of the ATPSs. Based on these data, relative hydrophobicity between theionic liquid-rich phase and salt-rich phase was chacterized, and the modulation of the structure of the ILsfor the relative hydrophobicity was analyzed. It was shown that all the Gibbs transfer energy values ofmethylene were negative, and they increased in the order:[C4mim][Leu]<[C4mim][Ala]<[C4mim][Gly]<[C4mim][Ser]，and [N5555][Gly]<P4444][Gly]<[N4444][Gly]<[N2222][Gly]. These results showed that thehydrophobicity of the ionic liquid-rich phase is stronger than that of the salt-rich phase. The relativehydrophobicity is dependent closely on the hydrophobicity of the ionic liquids themselves, althoughmicrostructure of the top phase and the bottom phase is very complicated.5. Polarity of the ionic liquid-rich phase of ATPSs was determined with2,6-dichloro-4-(2,4,6-triphenyl-N-pyridino)-phenolate(ET(33)) as probe, and the effect of ILs structure onthe polarity of the ionic liquid-rich phase was examined. It was found that polarity of the ionic liquid-richphase increased in the order:[C4mim][Leu]<[C4mim][Ala]<[C4mim][Gly]<[C4mim][Ser]，and[N5555][Gly]<P4444][Gly]<[N4444][Gly]<[N2222][Gly]. These orders are relavent to the hydrophilicity ofthe ionic liquids.6. The partition coefficients of L-tryptophan, L-tyrosine and cytochrome-c were determined in ionicliquid ATPSs, and the correlation between the partition coefficients and the relative hydrophobicity wasdiscussed. It was shown that the extraction capacity of the ATPSs for biomolecules decreased in the order:[C4mim][Ala]＞[C4mim][Gly]＞[C4mim][Ser], and [P4444][Gly]＞[N4444][Gly]＞[N2222][Gly]. Also, thefollowing partition coefficient order was observed: L-tryptophan＞L-tyrosine. These results suggest thatthe hydrophobicity interaction between the biomolecule and the ionic liquid-rich phase is the main drivingforce for the extraction. The relative hydrophobicity between the top phase and the bottom phase can beenhanced to increase the partition performence by designing the structure of the ionic liquids.7. The conductivity, dynamic light scattering, NMR and ATR-IR techniques were used to study the microstructure of aqueous ionic liquid solutions. It was shown that ionic liquid aggregates were formed inionic liquid-rich phases. The critical aggregate concentration of the ionic liquids decreases with the increaseof their hydrophobicity. This would be useful for understanding the main driving force for the extraction ofbiomolecules.8. The phase diagrams of PPG400(PPG1000)+ionic liquid ([Amim][Cl],[C4mim][CH3COO] and[C4mim][Cl])+water, and PPG1000+ionic liquid ([Amim][Cl]，[C4mim][CH3COO] and [C4mim][Cl])were determined, and the possibility for the recovery of the ionic liquids by the polymer was explored. Itwas found that91.5%of [Amim][Cl] and86.5%of [C4mim][CH3COO] could be recoveried by PPG400,and the polymer can then be recovered by heating. This suggests a new way to recovery ionic liquids fromaqueous solutions.