Regular articleEffects of DC electric field on phase equilibrium and partitioning of ionic liquid-based aqueous two-phase systems

- DC electric field affected ionic liquid-based aqueous two phase system (ILATPS).
- Electrokinetic promotion and inhibition for the ILATPS demixing were studied.
- Partition coefficient of ionic liquid was influenced by electric polarity.
- DC electric field had no significant effect on protein partitioning in ILATPS.

The liquid-liquid equilibrium and partitioning of ionic liquid-based aqueous two phase systems (ILATPS) including [Bmim]Cl-K2HPO4 ATPS, [Bmim]BF4-NaH2PO4 ATPS and [Bmim]BF4-Na3C6H5O7 ATPS in a DC electric field were extensively studied. The demixing rates of three ILATPSs with different tie-line lengths (TLL) were measured at different DC voltage levels with normal polarity (NP) or reverse polarity (RP), respectively, and the partitioning of ILs and model proteins (including lysozyme and bovine serum albumin) were investigated simultaneously. [Bmim]Cl-K2HPO4 ATPS had higher overall salt concentrations (>15 wt%) and lower overall IL concentrations (<20 wt%) compared with [Bmin]BF4-based ATPSs, which led to faster two phase separation rates and higher efficiency of electrokinetic demixing mainly due to the salting-out effect. The effects of TLL and DC voltage on the dispersing and gathering of IL molecules in the three ILATPS collectively gave the electrokinetic promotion (accelerate ILATPS demixing rate) and electrokinetic inhibition (decelerate ILATPS demixing rate) for the ILATPS demixing. In [Bmim]BF4-NaH2PO4 ATPS, the highest promotion demixing were at TLL = 55, 90 V (RP), while the highest inhibition demixing were at TLL = 64, 150 V (RP). The movement and type of organic cations or inorganic anions of IL molecules played an important role in IL partitioning in the three ILATPSs, which generally caused the partition coefficient of IL to decrease in NP and increase in RP to different degrees. There were two different protein partitioning characteristics in the three ILATPS that most model proteins distributed in the IL-rich phase in [Bmim]Cl-K2HPO4 ATPS due to the salting-out effect and distributed in the salt-rich phase in [Bmim]BF4-based ATPS due to hydrophobic repulsion. For the model protein partitioning in the electric field, the DC voltage and electric polarity had no significant effect on the protein partitioning trend.