Ionic liquids and non-ionic surfactants: a new marriage for aqueous segregation

Abstract

The aqueous nature of aqueous biphasic systems has boosted their use in downstream stages in biotechnological processes. Since aqueous solutions of non-ionic surfactants are widely used for different metabolites' purification, we have demonstrated for the first time their segregation capacity in the presence of ionic liquids.

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Since 2003, when Rogers and collaborators¹ first addressed the ability of ionic liquids to be salted out in aqueous solutions by high charge density inorganic salts, great interest has been devoted to ionic liquid-based aqueous biphasic systems (ABS).² These molten salts have posed undoubted benefits in a diversity of fields, be it in electrochemistry, analytical chemistry, surface science, catalysis, nanotechnology or biotechnology.³ This interest emerges from their appealing features. Among them, their structural modularity allows tuning the cation and anion to design millions of combinations for each desired application or task.⁴ In this sense, the application of ionic liquids-based ABS has shown an enormous potential for the separation of metabolites with industrial interest, since very often, the requirements for their extraction are very tough (temperature, solvent properties, pH, etc.). The development of new downstream strategies, which usually represents more than 50–80% of the total processing cost, urges the investigation of more competitive alternatives to maximize product recovery and foster the economic feasibility and robustness of biotechnological and chemical processes.⁵

Miscibility control in aqueous solutions of ionic liquids has been basically tackled by using different inorganic and organic salts, although they often entail problems regarding metabolites stability. These handicaps have made us to hypothesize that the use of non-ionic surfactants, widely employed in bioprocessing operations, could be a suitable strategy for achieving phase separation in the presence of ionic liquids, the latter acting as salting out agents. In this way, liquid–liquid equilibrium is yielded after a complex competition between the non-ionic surfactant and the ionic liquid for the water molecules.

In previous research works, we have demonstrated the ability of surface active compounds belonging to the most commonly used families (Triton and Tween), to be salted out by inorganic and organic salts aiming at applying them for the separation of metabolites and pollutants.⁶ In this present case, given the advantages provided by surfactant-based ABS such as lower interface tension, economical reasons (low cost of the reagents and rapid phase segregation), greater immiscibility windows, null flammability, and commercial availability of all components at bulk quantities,⁶ we have bet in Triton family, due to its relevance in different biotechnological applications.⁷ Thus, Triton X-100 and Triton X-102, composed by an 8-carbon tertiary alkyl chain and 9–10 ethylene oxide units or 12–13 ethylene oxide units, respectively, have been cherry-picked for this work (structure shown in Scheme 1).

Scheme 1 Structure of the ionic liquids and non-ionic surfactants used.

In relation to the salting out agent, we have selected 1-ethyl-3-methyl imidazolium ethylsulfate (C₂MIMC₂SO₄) since it is already produced at an industrial scale (more than one ton per annum), which ensures its availability when implemented at high scale. Besides, it can be easily synthesized in an atom-efficient and halide-free way, at a reasonable cost, it shows high chemical and thermal stability, low melting points and relatively low viscosities.⁸ Its biocompatibility with enzymes has also been reported in previous works for the separation of lipases.⁹ Moreover, 1-ethyl-3-methyl imidazolium butylsulfate (C₂MIMC₄SO₄) and 1-ethyl-3-methyl imidazolium hexylsulfate (C₂MIMC₆SO₄) were selected in order to evaluate the influence of the hydrophobicity of the ionic liquid at 25 °C.

Thus, in this work both the hydrophobicity of the components (ionic liquids and surfactants) and the operation temperature to map the immiscibility region have been screened. The variation of the alkyl chain length in the anion (C₂SO₄, C₄SO₄ and C₆SO₄) revealed that just ethylsulfate-based ionic liquid led to phase segregation, as shown in Fig. 1.

Fig. 1 Immiscibility regions for ABS composed of C₂MIMC₂SO₄ and Triton X-102 (upper) and Triton X-100 (down) at different temperatures: () 25 °C; () 40 °C; () 50 °C; () 60 °C. Dots represent experimental data and solid lines represent the data obtained from correlation.

Usually, as demonstrated previously,¹ phase segregation in aqueous solutions involving ionic liquids and inorganic salts are made up by an upper ionic liquid-rich phase and a bottom inorganic salt-rich phase. In this particular case, the competition of the ionic liquid and non-ionic surfactant for the water molecules is won by C₂MIMC₄SO₄. Notwithstanding the fact that longer alkyl chain lengths in the anion were reported to be beneficial for increased immiscibility windows,¹⁰ this just happens when the phase segregation in aqueous solutions of ionic liquids is triggered by high charge density salts. In this case, ionic liquids are playing the role of salting out agents, and therefore, the more hydrogen bonding capacity the ionic liquid shows, the more interaction with water molecules it displays, thus leading to an easier phase disengagement.

Additionally, a complete characterization of the immiscibility region was carried out. In general, ABS ternary phase diagrams are plotted in an orthogonal representation, where pure water is located in the origin of the axes.² This is due to the fact that as the concentration of the salt is increased up to the saturation limit, the coexistence of a precipitate should be taken into account. Hence, this kind of systems is always “incomplete”. In this particular case, the presence of a liquid salt (ionic liquid) and the liquid surfactant allows to completely characterize the immiscibility gap. The data shown in Fig. 1 reveals that the immiscibility window occurs only in the ternary region, while binary mixtures involved in the system are completely miscible. Therefore, these systems fall into an island-type ternary system (type 0 in Treybal classification).¹¹

In addition, the Othmer–Tobias¹² correlation equation, which relates the tie line mass concentration of the top phase with the bottom phase to obtain a linear function, was used to fit the experimental tie line data obtained for each ABS system (listed in Table S3†):

where a and b are the fitting parameters, w is the mass fraction, subscripts 1 and 2 refer to surfactant and ionic liquid, respectively, and superscripts I and II indicate the surfactant-rich phase and ionic liquid-rich phase, respectively. The values of the model parameters are presented in the ESI (Table S4†), together with the correlation coefficient R². The data obtained evidences a high degree of thermodynamic consistency since the values of R² are all higher than 0.9.

In this study, the tunability is a term not only associated with the ionic liquid, but also with the non-ionic surfactant, since it provides some degree of structural modularity, by modifying its hydrophobicity degree. A valuable tool to ascertain the hydrophobicity of a surfactant is by using the hydrophilic–lipophilic balance (HLB), which is an empirical number varying from 0 (low hydrophilicity) to 20 (high hydrophobicity). Data from the supplier reveals lower HLB values for Triton X-100 (13.5) than for Triton X-102 (14.4). Taking this into account, it should be expected that the use of surfactants with higher degree of hydrophobicity would entail greater immiscibility windows. From the experimental data illustrated in Fig. 1, it seems that this hypothesis is confirmed and Triton X-100 shows weaker interactions with water molecules than Triton X-102 in the presence of C₂MIMC₂SO₄, thus easing phase disengagement.

Regarding the operation temperature, a visual inspection of the results obtained at temperatures ranging from 25 °C to 60 °C (also shown in Fig. 1) evidences a greater liquid–liquid demixing capacity at higher temperatures. The reason behind this behavior lies in the different nature of the main components existing in the ABS. Thus, the non-ionic surfactant becomes more hydrophobic at increased temperatures, thus weakening hydrogen bond interactions and easing phase segregation. On the contrary, C₂MIMC₂SO₄ becomes more hydrophilic, which leads to a greater interplay with water molecules. These trends are in agreement with available literature data tackling the phase segregation in aqueous solutions of liquid polymers in the presence of organic and inorganic salts.¹³ However, the ionic liquid-based ABS carried out in the presence of inorganic salts,¹⁴ exhibit an inverse trend, which confirms the importance of a conscious selection for a successful ABS composed of ionic liquids and non-ionic surfactants. The analysis of literature data on the effect of temperature on immiscibility gaps for aqueous systems reflects that these trends can be generalized, as shown in Table S5 in the ESI.†

In summary, the present work has demonstrated the suitability of ionic liquids and surfactants for achieving liquid–liquid demixing in aqueous solutions. This combination opens up new opportunities for the separation of biotechnologically relevant biomolecules from aqueous culture broths, where they are usually produced, given the relevance of non-ionic surfactants in both upstream and downstream operations. A highly hydrophobic surfactant combined with a very hydrophilic ionic liquid, operating at elevated temperatures, will thus be a perfect scenario for maximizing the immiscibility region. In this sense, the proposed strategy would perfectly suit to implement an efficient separation process in the extraction of biomolecules from thermophilic microorganisms, where the operation temperatures are usually higher than 50 °C.

Acknowledgements

This work has been supported by the Spanish Ministry of Economy and Competitiveness and EDERF funds (project CTM2012-31534). F. J. Deive acknowledges Xunta de Galicia for funding through an Isidro Parga Pondal contract.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Materials and methods, and tables containing solubility data. See DOI: 10.1039/c4ra04996a

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