CO2 Sequestration by Bile Salt Aqueous Solutions and Formation of

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CO2 Sequestration by Bile Salt Aqueous Solutions and Formation of Supramolecular Hydrogels Meng Zhang, Zhiyuan Ma, Kaojin Wang, and X.X. (Julian) Zhu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05112 • Publication Date (Web): 21 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019

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ACS Sustainable Chemistry & Engineering

CO2 Sequestration by Bile Salt Aqueous Solutions and Formation of Supramolecular Hydrogels Meng Zhang, Zhiyuan Ma, Kaojin Wang and X. X. Zhu* Département de Chimie, Université de Montréal, C.P. 6128, Succ. Centre-ville, Montreal, QC, H3C 3J7, Canada. *E-mail: [email protected]

KEYWORDS: CO2-induced hydrogel, Bile salts, CO2 capture and storage, Hydrogen bond, Salting-out effect

ABSTRACT: Bubbling carbon dioxide (CO2) into aqueous solutions of bile salts such as sodium deoxycholate caused a gelation of the solution, forming a hydrogel made of entirely natural biological molecules and providing a convenient storage reservoir of CO2 in water. The carboxylate group of the bile salt becomes protonated in the aqueous solutions to make the bile acid only marginally soluble in water, which induces the formation of a supramolecular hydrogel with nanofibrous structures. Such hydrogels show convenient gel-sol transition by the desorption of CO2. The mechanical properties of the hydrogels may be varied by the amounts of CO2 in the media, reaching a peak value of the storage modulus of the hydrogel. Bubbling CO2 initially yielded a transparent hydrogel which upon continued purging became mechanically stronger and optically opaque. The bile salt aqueous solutions absorb CO2 effectively and may potentially serve as an alternative material for CO2 capture and storage.

Introduction Carbon dioxide (CO2) is a key metabolite in cells and shows good biocompatibility and membrane permeability.1 It has been widely used in cell incubators to support cell

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growth2 and as a food additive especially in carbonated beverages including beer and soft drinks. CO2 is a constituent in the earth atmosphere and has gained much attention due to its environmental importance as one of the greenhouse gases.3-4 Its atmospheric concentration has been increasing significantly in the past decades due to the massive consumption of fossil fuels, leading to a serious environmental problems particularly global warming. CO2 capture technologies have drawn widespread attention to reach the global demand for reducing the CO2 content in the atmosphere.5-6 Various materials, such as basic compound aqueous solutions,7-8 ionic liquids9 and porous solid adsorbents,10-12 have been tested for use in CO2 sequestration. To date, chemical absorption using aqueous alkanolamine solutions remains to be the leading technique for the CO2 capture process, while such a technique suffers from a major drawback of amine group degradation8 and the lack of potential applications of the final products. CO2 may interact with either primary or secondary amine groups in non-aqueous solutions to form carbamates, or decrease the pH of aqueous solutions and protonate certain basic functional groups such as amine, amidine and carboxylate groups.13-14 Therefore, it has been widely used as a trigger to change the properties of certain smart materials.15-16 Bubbling CO2 into solutions of a series of aromatic compounds bearing an amine group in organic solvents could significantly improve their fluorescence intensity upon the formation of carbamates.17 The presence of CO2 in aqueous solutions may also induce a size growth of vesicles from a polymer bearing amidine groups, whereas exposure of the solution to an Ar environment made the vesicles shrink back to the original size.1, 18 The self-assembly behaviors of certain polymers and molecular surfactants in aqueous solutions may also be varied by purging CO2.19-22 Hydrogels are soft materials possessing porous structures and high water content, potentially useful in biomedical areas such as cell culture, drug delivery and tissue engineering.2, 23-24 An amine-induced polymeric hydrogel may also be used to capture CO2 with advantages such as simple preparation, high performance and low cost.7 Some


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hydrogels may be used as “smart” materials by responding to certain stimuli with a reversible sol-gel transition process, for instance, temperature, pH and light.25-26 CO2 was reported to induce the sol-gel and gel-sol phase transition in aqueous solutions of certain polymers, respectively, and the transition processes can be recovered upon heating and purging N2.27-29 Several low-molecular-weight compounds with long alkyl chains bearing amine groups were reported to interact with CO2 to form supramolecular hydrogels with worm-like micelles in aqueous solutions.30-32 These worm-like micelles may interpenetrate with each other to yield weak hydrogels. The principle of CO2induced phase transition in an aqueous solution is a decrement of pH; the use of CO2 rather than other Bronsted acids has advantages since it is clean and biocompatible at a low concentration. Such CO2-responsive systems are generally reversible since CO2 may be conveniently removed by purging inert gases such as N2 and Ar or by gentle heating, without the accumulation of any side products over repeated treatment cycles.33 Bile acids are naturally occurring compounds in the body of humans and most animals and, in their salt form, help in the digestion of fats and fat-soluble nutrients. Though amphiphilic in nature, bile acids are generally hydrophobic and their solubility in water is limited.34 In their sodium salt form, however, they are quite soluble in water (solubility > 40 wt%) to form micelles or lyotropic liquid crystals depending on their concentration.3539

Aqueous solutions of bile salts may show a sol-gel phase transition upon addition of

acidic compounds such as HCl and certain organic acids.39-40 These hydrogels, however, were rather weak and did not show a convenient reversible sol-gel transition process, which may limit their applications. In this work, we found that bubbling CO2 into the aqueous solutions of bile salts such as sodium deoxycholate (NaDC) yielded hydrogels with varying mechanical strengths. The appearance and mechanical strength of the hydrogels depend strongly on the amount of CO2 introduced. The addition of inorganic salts such as NaCl may improve the strength of these hydrogels significantly. The bile salt aqueous solutions manifested good chemical stability, easy and convenient CO2


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absorption with a reasonably high capacity, making them promising alternative CO2 capturing materials by a process of gelation and the hydrogels formed can be potentially useful as soft materials.

Results and discussion Gelation of bile salts with CO2 capture. We have tested several bile salts for their gelation behavior in water with CO2. The ease of gelation is found to depend on the relative hydrophilicity of the bile salt. Sodium cholate (NaC), the most hydrophilic one, does not form a hydrogel after purging with CO2 (Fig. S1A). The most hydrophobic bile salt, sodium lithocholate (NaLC, Fig. 1A), however, readily forms a hydrogel without CO2 when its concentration in water is sufficiently high (3 wt%, Fig. S1E). At a lower concentration (2 wt%, Fig. S1D), a bluish viscous solution is obtained, which turns into an opaque hydrogel upon purging with CO2 (Fig. S1C), and the gelation is thermally irreversible, which may be due to the strong hydrophobic interaction between the lithocholates. NaDC and sodium chenodeoxycholate (NaCDC) (Fig. 1A) are intermediate in terms of hydrophilicity among the bile acids and may be dissolved in water to form transparent solutions, which can then turn into hydrogels upon bubbling of CO2 (Figs. 1B and S1B). A hydrogel based on NaDC can be obtained within several minutes after bubbling CO2, while the formation of a stable hydrogel by NaCDC requires a longer equilibration time (more than 24 h). Therefore, the NaDC solution showed advantages of fast gelation and thermo-reversibility and was used as a representative example to study the CO2 sequestration behavior and the properties of the resulting hydrogels. Bubbling CO2 initially for about 1 or 2 s into the aqueous solutions of NaDC yields a weak transparent hydrogel in a sealed vial after storing the mixture without disturbance for several minutes (Fig. 1C), and a longer bubbling of ca. 5 s prompted the formation of an opaque hydrogel under the same conditions (Fig. 1D). The formation of transparent and opaque hydrogels without any gas bubbles was observed after purging CO2 for a few


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seconds, indicating the fast kinetics of CO2 sequestration. Both the transparent and opaque hydrogels revert to transparent solutions upon the desorption of CO2 by heating and by bubbling with N2. The sol-gel transition process is reversible and repeatable (Fig. 1).

Figure 1. (A) The chemical structure of NaLC, NaCDC, NaDC, and NaC. (B) An aqueous solution of 2 wt% NaDC. (C) Bubbling CO2 for 2 s yields a transparent hydrogel, (D) Bubbling CO2 for 5 s yields an opaque hydrogel. Hydrogels form after equilibration of several minutes. All samples contain 2 wt% NaDC. The vials containing solutions and gels were sealed and stored under ambient atmosphere.

Bile salts form micelles in aqueous solutions above the critical micelle concentration (CMC), which may vary with temperature, ionic strength, and the presence of additives such as a dimeric bile salt.41-43 Pyrene may be used as a fluorescent probe to study the micellization of lipids by monitoring the changes of the intensity ratio of the third to the first peak (I3/I1) of its fluorescence spectrum.44 A two-step increment of the I3/I1 ratio (Fig. S2) indicates the formation of primary and secondary micelles at concentrations of approximately 3 and 5 mM, respectively.41, 45 The morphological transition of NaDC in aqueous solutions with various amounts of CO2 was studied by transmission electron microscope (TEM). NaDC (at 10 mM) forms nonspherical micelles in aqueous solutions with an average diameter of about 10 nm (Fig.


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2A), which are regarded as secondary micelles of NaDC with aggregation numbers of 10-100.46 When the concentration of CO2 reaches 4 mM in the solution, NaDC selfassembles to form nanofibers with diameters of about 10-20 nm (Fig. 2B), and a transparent hydrogel is obtained with the water molecules immobilized in the selfassembled 3-D fibrillar network. When the concentration of CO2 further increases to 40 mM, the hydrogel becomes opaque, and a mixture of thin nanofibers and thicker fiber bundles is obtained (Fig. 2C). The formation of thick fiber bundles (100-200 nm in width) should be responsible for the opaque appearance of hydrogel due to visible light scattering. Amplified TEM images of the micelles, nanofibers and fiber bundles are shown in Fig. S4. The optical microscopic image of an opaque hydrogel sample shows no agglomerated microcrystallites (Fig. S5). During the preparation of TEM samples, the basic NaDC retains the absorbed CO2, and the trace amount of water is evaporated quickly in a few seconds, a time too short to allow for morphology variation of the NaDC gel. Therefore, the dry samples containing the aggregates may be somewhat smaller in dimension, but the TEM images are representative of the morphology of the solutions and the hydrogels.


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Figure 2. TEM images of (A) NaDC aqueous solution at 10 mM, (B) transparent hydrogel with 10 mM NaDC and 4 mM CO2, and (C) opaque hydrogel with 10 mM NaDC and 40 mM CO2. (D) Schematic representation of the mechanism for the formation of hydrogels from aqueous solutions of NaDC and further aggregation with increasing contents of CO2.

It is known that the carboxylate and hydroxyl groups of bile salts face the aqueous environment to stabilize the micelles, and the hydroxyl groups on the surface of micelles may interact with each other to yield secondary micelles.47 The pKa of the carboxylic acid group in DCA is reported to be about 5.0.48-49 When CO2 is introduced into the aqueous solutions containing NaDC micelles, and the carboxylate groups may be protonated due to the low pH of the solution. The micelles may be linked through hydrogen bonding between the protonated carboxylate groups, yielding transparent hydrogels containing nanofibers with a thickness close to the size of the NaDC micelles (Fig. 2B). At a higher content of CO2, more of the carboxylate groups become protonated, facilitating hydrogen bonding between the nanofibers, yielding opaque hydrogels with thicker fiber bundles. The process is illustrated in Fig. 2D. Mechanical properties of the gels. The mechanical properties of the hydrogels depend on the content of CO2 in the solutions. The resulting hydrogels showed decreasing pH values when more CO2 was captured. At a low concentration of CO2 (8 mM), the NaDC solution (23 mM, 1 wt%) remained transparent and no hydrogel formed. A weak transparent hydrogel (storage modulus G' = 16 Pa) was obtained at a CO2 concentration of 12 mM. This gel became opaque and stronger (G' about 46 Pa) when the CO2 concentration reached 40 mM. Beyond this point, the hydrogel became more turbid and weaker (Fig. 3A), probably caused by the formation of thicker fiber bundles (Fig. 2C) that may reduce the homogeneity and the density of the junction points in the 3-D network.50 This is in agreement with the observation that aqueous solutions of NaDC


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formed hydrogels when the pH value of the solution is reduced to around 7,51-54 and a further decrease in pH causes the weakening of the hydrogels or precipitation.53, 55 The pH of each CO2-NaDC hydrogel sample showed no change over time in a sealed vial, suggesting that the CO2 was effectively sequestrated and remained stable without loss of CO2 in the presence of bile salts. The G' value reached a plateau eventually in the time-sweep rheological experiment in which the sample was protected to prevent evaporation by a solvent trap (Fig. S3). It is noted that all the hydrogel samples on the rheometer plate become viscous liquid after oscillatory stress measurements when the stress applied is much higher than their yielding stress. Such a transition may be a result of the nanofiber alignment due to the applied shearing force.56 The mechanical strengths of the NaDC-CO2 hydrogels may be improved by the addition of inorganic salts such as NaCl, due to the salting-out effect (Fig. 3B). In a hydrogel with 23 mM NaDC and 40 mM CO2, at low concentrations of NaCl ( 40 wt%), they may serve as alternative CO2 sequestration materials to replace the conventional alkanolamines marred by oxidative degradation, reactivity with CO2, low vapor pressure and high surface tension.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Materials and method, characterizations, images for various bile salts aqueous solutions upon bubbling CO2, fluorescence spectra of pyrene in an aqueous solutions, oscillatory stress and frequency sweep spectra of a typical NaDC-CO2 hydrogel, TEM images of showing the structures of the CO2 hydrogels.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS Financial support from NSERC of Canada and FQRNT of Quebec is gratefully acknowledged. We thank Mr. Hu Zhu for his help with AFM measurements. Meng Zhang thanks the Chinese Scholarship Council (CSC) for a scholarship.

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TOC/Abstract Graphic:

Synopsis: Bile salt aqueous solutions to be used as a novel CO2 capture and storage material with chemical stability, no volatility and low toxicity.


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CO2 Sequestration by Bile Salt Aqueous Solutions and Formation of

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