Complex coacervates as extraction media

Abstract

Various solvents such as ionic liquids, deep eutectic solvents, and aqueous two phase systems have been suggested as greener alternatives to existing extraction processes. We propose to add macroscopic complex coacervates to this list. Complex coacervates are liquid-like forms of polyion condensates and consist of a complex of oppositely charged polyions and water. Previous research focussing on the biological significance of these polyion-rich phases has shown that polyion condensates have the ability to extract certain solutes from water and back-extract them by changing parameters such as ionic strength and pH. In this study, we present the distribution coefficients of five commonly used industrial chemicals, namely lactic acid, butanol, and three types of lipase enzymes in poly(ethylenimine)/poly(acrylic acid) complex coacervates. It was found that the distribution coefficients can vary strongly upon variation of tunable parameters such as polyion ratio, ionic strength, polyion and compound concentrations, and temperature. Distribution coefficients ranged from approximately 2 to 50 depending on the tuning of the system parameters. It was also demonstrated that a temperature-swing extraction is possible, with back-extraction of butanol from complex coacervates with a recovery of 21.1%, demonstrating their potential as extraction media.

Fig. 1. A schematic representation of the difference between ATPS and CC phase separation. In ATPS (left), the added constituents form separate aqueous phases. In CC (right), two oppositely charged polyions form a polyion-rich aqueous phase.

Fig. 2. Analysis of CC formation properties. (A) Total CC formed as a function of F⁻ at a polyion concentration of 20 g L⁻¹ and a NaCl concentration of 10 mM. Values are represented as average with standard deviations from triplicate experiments. (B) Photographs of different F⁻ ratios with consistent amounts of total polyions. (C) The water content of the formed CCs was determined using TGA for F⁻ = 0.26 and 0.36 at a polyion concentration of 10 g L⁻¹ and a NaCl concentration of 10 mM. The left Y-axis shows the remaining mass fraction of the CC as the temperature presented on the right Y-axis is increased and water is vaporized. Values in (A) represent the average with standard deviations from triplicate experiments.

Fig. 3. Distribution coefficients – K_D for CALA (A, D and G), CALB (B, E and H), and PPL (C, F and I) as a function of CC composition (A–C), NaCl concentration (D–F), and polyion concentration (G–I). Unless otherwise specified, total polyion concentration is 5 g L⁻¹, enzyme concentration is 67 μM, NaCl concentration is 10 mM, and F⁻ is 0.36 for CALB and 0.26 for CALA and PPL. Measurements are shown as average with standard deviation for n = 3, except (D), which is shown as individual measurements.

Fig. 4. Lactic acid partitioning in PEI/PAA CC as a function of (A) CC composition, (B) NaCl concentration, (C) polyion concentration, (D) initial LA concentration, and (E) temperature. Unless otherwise specified, the experiments took place at approximately 20 °C, a polyion concentration of 5 g L⁻¹, a LA concentration of 100 mM, a NaCl concentration of 10 mM, pH = 7, and F⁻ = 0.26. Results are shown as average with standard deviations for n = 3.

Fig. 5. Interaction between butanol and PEI/PAA CCs. (A) Distribution coefficients of butanol in PEI/PAA CCs as a function of CC composition shown as an average with standard deviation with n = 2. (B) Distribution of butanol as a function of temperature for F⁻ = 0.26 and 0.36. For (A) and (B), the total polyion concentration was 5 g L⁻¹, the NaCl concentration was 10 mM, and the butanol concentration was 400 mM. (C) Image of PEI/PAA CC at 50 g L⁻¹ with (left) and without (right) the aqueous supernatant. (D) Butanol remaining in the CC at F⁻ = 0.26 during RT to 70 °C extraction and 70 °C to RT back-extraction. Data are shown as average with n = 4.

Fig. 6. Distribution of LA as a function of NaCl concentration for PEI/PAA CC containing (A) CALA, (B) CALB, and (C) PPL lipase enzymes. Initial LA concentration was 100 mM, polyion concentration was 5 g L⁻¹, and F⁻ = 0.26 for CALA, and 0.36 for CALB and PPL. Results are shown as individual independent experiments.

Fig. 7. Distribution of butanol as a function of NaCl concentration for PEI/PAA CC containing (A) CALA, (B) CALB, and (C) PPL lipase enzymes. Initial butanol concentration was 400 mM, polyion concentration was 5 g L⁻¹, and F⁻ = 0.26 for CALA, and 0.36 for CALB and PPL. Results are shown as individual independent samples.