Odd-even effect in the formation and extraction performance of ionic

Subscriber access provided by UNIV AUTONOMA DE COAHUILA UADEC

Separations

Odd-even effect in the formation and extraction performance of ionic-liquid-based aqueous biphasic systems Diana Cléssia Belchior, Mafalda R. Almeida, Tânia E. Sintra, Sonia P.M. Ventura, Iola F Duarte, and Mara G. Freire Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00663 • Publication Date (Web): 18 Apr 2019 Downloaded from http://pubs.acs.org on April 21, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

1

Odd-even effect in the formation and extraction

2

performance of ionic-liquid-based aqueous

3

biphasic systems

4

Diana C. V. Belchior a, Mafalda R. Almeida a, Tânia E. Sintra a, Sónia P. M. Ventura a, Iola F.

5

Duarte a, and Mara G. Freire a*

6

a CICECO

- Aveiro Institute of Materials, Department of Chemistry, University of Aveiro, 3810193 Aveiro, Portugal

7

8

* Corresponding author. Tel: +351-234-370200; Fax: +351-234-370084; E-mail address:

9

[email protected]

10

11

1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 28

1

Abstract

2

Aqueous biphasic systems constituted by ionic liquids (IL-based ABS) are a target of investigation

3

in the separation of high-value biomolecules. However, the identification of the molecular-level

4

mechanisms ruling two-phase formation and extraction performance of these systems is crucial to

5

successfully design effective separation processes. In this work, IL-based ABS formed by K2HPO4

6

and cholinium carboxylate ILs ([Ch][CnCO2] with n = 1 to 7, comprising anions with odd and even

7

alkyl chain length) were investigated. The corresponding ternary phase diagrams, including

8

binodal curves, tie-lengths, tie-line lengths and critical points, as well as the Setschenow salting-

9

out coefficients (ks), which quantitatively describe the two-phase formation ability, were

10

determined at 298 K. The extraction capability of these systems was then evaluated for four amino

11

acids (L-tryptophan, L-phenylalanine, L-tyrosine, L-3,4-dihydroxyphenylalanine/L-dopa). It was

12

found that ILs composed of anions with even alkyl chains display slightly higher ks values,

13

meaning that these ILs are more easily salted-out or more easily phase separate to form ABS,

14

whereas ABS formed by ILs with anions comprising odd alkyl chains lead to slightly higher

15

partition coefficients of amino acids. Beyond the neat ILs odd-even effect resulting from their

16

nanostructuration, being this a well-known phenomenon occurring in a series of their

17

thermophysical properties, it is here shown the existence of an odd-even effect displayed by the

18

IL anion aliphatic moiety in aqueous solution, visible both in the two-phase formation ability and

19

extraction performance of ABS. These findings contribute to elucidate the molecular-level

20

mechanisms governing ABS formation and partitioning of biomolecules, ultimately contributing

21

to the design of proficient separation platforms.

22

Keywords: Aqueous two-phase systems, cholinium carboxylate ionic liquids, salting-out

23

coefficient, extraction, partition coefficient, odd-even effect. 2 ACS Paragon Plus Environment

Page 3 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1


1. Introduction

2

Separation and purification steps within biotechnological processes are still major

3

challenges due to difficulties in purifying and recovering the target compounds from the original

4

complex media in which they are produced.1 Furthermore, these should be cost-effective and

5

should be able to keep the products biological activity. Research on alternative separation

6

techniques has been carried out with Aqueous Biphasic Systems (ABS), which correspond to

7

liquid-liquid extraction systems formed by water and two solutes (e.g. a polymer and a salt, two

8

polymers, two salts, etc.) that undergo phase separation above defined concentrations.2-5 In ABS,

9

both phases are majorly constituted by water, meaning that such media may be compatible with

10

biologically active molecules.2

11

In addition to more conventional ABS formed by polymers, in the last decade, a substantial

12

interest has been placed in ionic-liquid-based ABS, initially demonstrated to be formed by adding

13

ionic liquids to inorganic salts aqueous solutions.6 Ionic liquids (ILs) are poorly coordinated

14

organic salts, and hence may be liquid at or close to room temperature. Besides other relevant

15

properties, the major advantage associated to IL-based ABS relies on these compounds designer

16

ability, resulting in a virtually endless opportunity of tuning the ILs chemical structures for specific

17

applications.7 As a result, IL-based ABS have been successfully applied in the separation of a wide

18

number of biomolecules, such as amino acids, proteins, antioxidants, among others.8 Still, most of

19

the ILs studied up to date are imidazolium-based, most of the times combined with [BF4]-, which

20

may raise some toxicity and biodegradability concerns.8 The ample flexibility of ILs should

21

however permit the preparation of new compounds with an acceptable environmental footprint and

22

biocompatibility to be used in the creation of novel ABS. Cholinium-based ILs are a promising

23

family of ILs, which can be prepared from natural sources, and may present high biodegradability 3 ACS Paragon Plus Environment


Page 4 of 28

1

and, in some cases, marginal (eco)toxicity.9-11 In particular, cholinium chloride is an essential

2

nutrient with an important role, namely as precursor in the synthesis of vitamins (e.g., thiamine)

3

and enzymes that participate in the carbohydrates metabolism.12-13 Taking into account their

4

advantageous features, several works have been published reporting the use of cholinium-based

5

ILs to form ABS with salts. Xie et al.14 investigated novel ABS formed by four cholinium-based

6

ILs comprising carboxylate anions and K3PO4 to extract tryptophan, phenylalanine, and caffeine.

7

The authors reported novel ABS liquid-liquid phase diagrams and showed their enhanced

8

extraction performance, as well as the presence of liquid crystal structures in ABS formed by ILs

9

with long alkyl chain anions. Shahriari et al.15 determined the phase diagrams of novel ABS formed

10

by a series of cholinium-based ILs with different anions and K3PO4, having evaluated their

11

performance to extract antibiotics. This type of ABS has also been used to extract antibiotics from

12

real fermentation broths,16 and it was latter demonstrated that they represent a more economical

13

alternative to recover tetracycline from the fermentation broth than more traditional ABS formed

14

by polymers.17

15

To design effective ABS for separation processes, it is relevant to determine and

16

characterize their phase diagrams. The respective ternary phase diagrams provide valuable data on

17

the phase-forming components compositions required to form two-phase systems, and on the

18

composition of each phase for a given overall mixture. Accordingly, a significant number of works

19

have focused on the determination of novel IL-based ABS ternary phase diagrams.8 Furthermore,

20

it is well known that ILs comprising long alkyl side chains tend to self-aggregate in aqueous media,

21

a trend that was further demonstrated to occur in ABS18 and that may lead to unfavorable extraction

22

results.19 In a previous work, the impact of the IL cation alkyl side chain length in two-phase

23

separation and creation of ABS has been comprehensively investigated by employing the 1-alkyl4 ACS Paragon Plus Environment

Page 5 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

3-methylimidazolium chloride series, with both odd and even alkyl side chains.20 An odd-even

2

effect was identified on the Setschenow salting-out coefficient (ks), which quantitatively addresses

3

the two-phase formation capacity, as a function of the number of methylene groups at the longest

4

IL cation alkyl side chain.20 ILs comprising cations with even alkyl side chains have higher molar

5

volumes and are thus more easily salted-out, meaning that more easily undergo phase separation

6

in aqueous media. Although this trend was detected for the IL cation, the effect of the IL anion on

7

ABS formation is scarcely studied. Some reports exist on the effect of the IL anion alkyl chain

8

length on the formation of ABS,14,21–24 but most of these studies employed ILs with chemically

9

different anions or only with even alkyl chains at the anion. In general, it is understood that ILs

10

with longer aliphatic moieties at the anion are more easily salted-out and need less inorganic salt

11

to undergo phase separation. However, no studies on the self-aggregation impact of long alkyl

12

chain anion ILs and on the possibility of an odd-even effect have been performed. Furthermore, to

13

the best of our knowledge, the impact of the odd-even effect in the partitioning of target molecules

14

in ABS was never investigated. It should be remarked that although the odd-even effect is scarcely

15

studied in aqueous solutions, the ILs odd-even effect was already reported in a series of neat ILs

16

properties, such as molar volumes, viscosities and entropy and enthalpy of vaporization.25-28 Santos

17

and co-workers26-28, by studying several series of imidazolium-based ILs with cations of different

18

alkyl side chain length, demonstrated that the odd-even effect is a result of the ILs nanosegregation

19

and consequent impact in these fluids cohesive energy.

20

Aiming at better understanding the influence of the IL anion alkyl chain length on the

21

creation of ABS, in this work, seven cholinium-based ILs with carboxylate anions, from acetate to

22

octanoate and comprising both even and odd alkyl side chains ([Ch][CnCO2], n = 1 to 7), were

23

synthesized and used in ABS formation with dipotassium hydrogen phosphate (K2HPO4). The 5 ACS Paragon Plus Environment


Page 6 of 28

1

corresponding ternary phase diagrams were determined at (298 ± 1) K, including tie-lines and

2

critical points, and further evaluated in terms of their extraction capability for amino acids, namely

3

L-tryptophan, L-phenylalanine, L-tyrosine and L-3,4-dihydroxyphenylalanine/L-dopa. The

4

Setschenow coefficient of each ABS was determined, and the odd-even effect was addressed in

5

both ABS formation and amino acids partitioning between the coexisting phases.

6

7

2. Materials and methods

8

2.1. Materials

9

ABS were prepared using aqueous solutions of potassium phosphate, K2HPO4 (>98% of

10

purity), purchased from Sigma–Aldrich, and aqueous solutions of each IL. The investigated

11

cholinium-based ILs comprise anions derived from carboxylic acids, corresponding to cholinium

12

acetate ([Ch][C1CO2], > 98 wt % pure) from Iolitec, and cholinium propanoate ([Ch][C2CO2]),

13

cholinium butanoate ([Ch][C3CO2]), cholinium pentanoate ([Ch][C4CO2]), cholinium hexanoate

14

([Ch][C5CO2]), cholinium heptanoate ([Ch][C6CO2]), and cholinium octanoate ([Ch][C7CO2]) that

15

were synthetized in our laboratory following protocols previously described.29 All ILs synthesized

16

showed high purity (>97 wt%) (1H and 13C NMR results are given in the Supporting Information).

17

The required precursors, namely cholinium hydroxide solution (46 wt%, Sigma-Aldrich),

18

propanoic, butanoic, pentanoic, hexanoic and heptanoic acids (99 %, Acros Organics), and

19

octanoic acid (98 %, Sigma-Aldrich), were commercially acquired. Briefly, the corresponding

20

acids were added drop wise into an aqueous solution of [Ch][OH] (1.1 equivalents of acid) at 298

21

K, being the resultant mixture stirred at room temperature and under inert atmosphere for at least

22

12 h to produce the cholinium-based ILs. The mixtures were then dried for 4 h under vacuum. The


Page 7 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ACS Paragon Plus Environment


Page 8 of 28

1

where A, B, and C are constants obtained by the regression, and [IL] and [Salt] correspond to the

2

IL and salt weight fraction percentages, respectively.

3

Tie-lines (TLs) associated to each binodal curve, i.e the composition of each phase for a

4

target mixture composition, were determined by a gravimetric method.30. Several ternary mixtures

5

constituted by IL + K2HPO4 + water at the biphasic region were prepared by weight and vigorously

6

mixed. Each mixture was centrifuged for 30 min at 298 K aiming for the separation of the two

7

phases. Both phases were carefully separated and weighed. Each TL was determined by the lever-

8

arm rule30 according to the following equations (2-5):

9

[IL]T = exp [( × [Salt]0.5 T )

( × [Salt]3T)]

(2)

10

[IL]B = exp [( × [Salt]0.5 B )

( × [Salt]3B)]

(3)

11

[IL]T =

12

[Salt]T =

13

where the subscripts M, T and B represent the initial mixture, the top and bottom phases,

14

respectively. K corresponds to the ratio between the weight of the top phase and the total mass of

15

the system. In all systems, the top phase is enriched in IL (IL-rich phase), whereas the salt is

16

majorly enriched in the bottom phase.

17

18

[IL]M

1

[Salt] M

(4)

× [IL]B 1

(5)

× [Salt]B

Each tie-line length (TLL) was determined by the following equation: TLL = ([Salt]T

[Salt]B)2 + ([IL]T

[IL]B)2

(6)


Page 9 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

The critical point of each system, i.e. when the composition of the two aqueous phases is

2

identical, was determined through the application of the Sherwood method using the TLs slopes,31

3

according to equation (7),

4

(7)

!" = # $%&' + (

5

where f and g are obtained from the fitting, and [IL] and [salt] are the weight fraction percentages

6

of IL and Salt.

7

The magnitude of salting-out may be quantified by correlating solubility values with the

8

empirical equation of Setschenow.32 In this work the modified version of the original Setschenow

9

equation proposed by Hey et al.33, previously used to characterize polymer-based ABS34 and of

10

IL-salt-based ABS34-35, was applied:

11

&)[IL]B = *!"([IL]B

12

where T and B designate the top (IL-rich) and bottom (salt-rich) phases, respectively, and [IL] and

13

[Salt] correspond to the molality of IL and salt. kIL and ks parameters are related with the IL activity

14

coefficient to its concentration and the salting-out coefficient, respectively.

15

[IL]T

([IL]T) + *$([Salt]B [Salt]T)

(8)

2.3. Cation-anion interaction energies

16

The cation-anion total interaction energies of each IL, EInt, were determined using the

17

COSMO-RS thermodynamic model. This model combines quantum chemistry, based on the

18

dielectric continuum model known as COSMO (COnductor-like Screening MOdel for Real

19

Solvents), with statistical thermodynamic calculations. The process employed in this work to

20

determine the interaction energies was described by Kurnia et al.36 The COSMO quantum 9 ACS Paragon Plus Environment


Page 10 of 28

1

chemical calculations were performed with the TURBOMOLE 6.1 program package at the density

2

functional theory (DFT) level. The BP functional B88-P86 with a triple-z valence polarized basis

3

set (TZVP) and the resolution of identity standard (RI) approximation were applied.37 The

4

COSMOthermX program and the parameter file BP_TZVP_C20_0111 (COSMOlogic GmbH &

5

Co KG, Leverkusen, Germany) were used.38

6

2.4. Extraction of amino acids

7

ABS containing amino acids aqueous solutions (5 g.L-1 for L-tryptophan, 15 g.L-1 for L-

8

phenylalanine, 2.5 g.L-1 for L-tyrosine and 1.5 g.L-1 for L-dopa, defined according to the amino

9

acids solubility in water), were prepared and used to study the amino acids partitioning between

10

the ABS coexisting phases. The compositions of the ternary systems applied in the partitioning

11

experiments were defined according to the determined phase diagrams and TLs. To circumvent

12

inconsistencies in results, which could result from the different phase compositions and, with the

13

goal of addressing the odd-even effect in the amino acids partitioning, all extraction studies were

14

carried out at a constant TLL (ca. 58 ± 2). Each mixture was vigorously stirred to allow

15

equilibration and then centrifuged for 30 min at 298 K to reach complete separation between the

16

coexisting phases. Both phases were separated, and the amount of amino acid in each phase was

17

quantified by UV-spectroscopy, using a BioTeck Synergy HT microplate reader using calibration

18

curves previously determined (at a wavelength of 280 nm for L-tryptophan, L-tyrosine and L-

19

dopa, and at 260 nm for L-phenylalanine). At least three ABS with each amino acids were

20

prepared, and at least three samples of each phase were quantified.

21

The ABS capability to extract different amino acids to the IL-rich phase was evaluated by

22

their partition coefficient (KAA). This parameter corresponds to the ratio of the concentration of


Page 11 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

each amino acid in the phase enriched in IL to that in the salt-rich phase, according to equation

2

(9).

3

+AA =

!"

(9)

,%&'

4

where [AA]IL and [AA]Salt correspond to the concentration of each amino acid (AA) in the phases

5

enriched in IL and salt, respectively.

6

The pH of the IL- and K2HPO4-rich aqueous phases was determined at 298 K using a

7

Mettler Toledo S47 SevenMulti™ dual meter pH/conductivity equipment (data shown in the

8

Supporting Information).

9

3. Results and Discussion

10

3.1. Phase diagrams and critical points

11

New ternary phase diagrams of ABS composed of water, K2HPO4 and cholinium-based

12

ILs with carboxylate anions of different alkyl chain length were determined in this work at (298 ±

13

1) K. Their orthogonal representations, both in percentage weight fraction and in molality units,

14

are shown in Figure 2. The weight fraction detailed results are given in the Supporting Information

15

(Table S1-S7). Experimental binodal data were fitted by Equation (1), whose fitting is given in

16

Figure 2. The fitted constants A, B and C were estimated by the least squares regression, being teir

17

values given in the Supporting Information (Table S8). At least 4 TLs and respective length (TLL)

18

were determined for each system by Equations (2) to (6), with the detailed results shown in the

19

Supporting Information (Table S9). In addition, the critical point of each system was determined,

20

being provided in Figure 2 (cf. the Supporting Information for detailed data - Table S10).



Page 12 of 28

1

Since all ILs are composed of a common cholinium cation, the results depicted in Figure 2

2

mirror the effect of the IL anion on the formation of ABS with K2HPO4. The two-phase region is

3

located above the solubility curve, and the larger this region is, the more prone the IL is to be

4

salted-out by K2HPO4 and to form ABS. It is well documented39 that small IL anions derived from

5

organic acids are excellent hydrogen bond acceptors and exhibit high capability to establish

6

hydrogen bonds with water. Furthermore, the hydroxyl group at the cholinium cation can establish

7

strong hydrogen bonds or dipole-dipole interactions with water molecules.40,41 Therefore,

8

cholinium carboxylate ILs display a high affinity for water and only moderate to strong salting-

9

out salts able to create ABS. Accordingly, it was previously shown that these ILs form ABS with

10

K3PO4,14 and in this work we show that they form ABS with K2HPO4. However, the studied

11

cholinium carboxylate ILs do not form ABS with weaker salting-out salts, such as KH2PO4 or

12

K2HPO4/KH2PO4 at pH 7 (as experimentally attempted by us in this work - data not shown).

13

The investigated ILs capacity to form ABS at ~ 22 wt% of K2HPO4 follows the order:

14

[Ch][C1CO2] S [Ch][C7CO2] < [Ch][C6CO2] < [Ch][C2CO2] < [Ch][C5CO2] < [Ch][C3CO2] S

15

[Ch][C4CO2]. When weight fraction is used [Figure 2(a)], a straight relationship of the ILs’

16

aptitude to be salted-out cannot be straightforwardly assessed. When using molality units, the

17

differences arising from different molecular weights of ILs are circumvented, consenting a more

18

accurate analysis of the effect of the several phase-forming components on endorsing liquidWliquid

19

demixing. The binodal curves represented in molality units, shown in Figure 2(b), prove that the

20

ILs performance to create ABS, e.g. at 2.0 mol·kgW1 of K2HPO4, follows the rank: [Ch][C1CO2]

Odd-even effect in the formation and extraction performance of ionic

Recommend Documents