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Nanostructured branched-chain carboxylate ionic liquids: synthesis, characterization and extraordinary solubility for bioactive molecules Yuqi Ke, Wenbin Jin, Qiwei Yang, Xian Suo, Yiwen Yang, Qilong Ren, and Huabin Xing ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01353 • Publication Date (Web): 22 May 2018 Downloaded from http://pubs.acs.org on May 22, 2018

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ACS Sustainable Chemistry & Engineering

Nanostructured branched-chain carboxylate ionic liquids: synthesis, characterization and extraordinary solubility for bioactive molecules Yuqi Ke, Wenbin Jin, Qiwei Yang, Xian Suo, Yiwen Yang, Qilong Ren, Huabin Xing*

Yuqi Ke, Dr. Wenbin Jin, Dr. Qiwei Yang, Xian Suo, Prof, Yiwen Yang, Prof. Qilong Ren, Prof. Huabin Xing Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Department of Chemical and Biological Engineering, Zhejiang University Mailing address: 38 Zheda Road, Hangzhou 310027, China Email address of the corresponding author: [email protected]

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ABSTRACT The design of multifunctional ionic liquids (ILs) that integrate different attractive characteristics, such as strong hydrogen-bonding interactions, good lipophilicity, significant nanoscale organization and low viscosity, in a molecular structure is of great importance for the application of ILs in extraction, biomass conversion and catalysis, but remains challenging. Here we synthesized a family of novel phosphonium ILs featuring branched chain carboxylate anions with carbon number up to 18, and systematically characterized the physicochemical properties of prepared ILs. The branched chain carboxylate ILs (BCC-ILs) exhibit very strong hydrogen-bond basicity (β = 1.49-1.72, 30 °C) and excellent lipophilicity (π* = 0.73-0.95, 30 °C), have obvious nanoscale segregation and moderate viscosity (η = 91.8-200.5 mPa, 40 °C). The d value of BCC-ILs ranged from 10.5 to 15.6 Å when the length of alkyl chain in anions or cation rose. These nanostructured solvents can undergo self-assembly process with typical bioactive molecule of cholesterol to form highly ordered mesoscopic structures through cooperative hydrogen-bond and Van der Waals interactions, which enable unprecedented high solubility of cholesterol in BCC-ILs (molar solubilities 0.31-1.12 at 50 °C). Additionally, we found that n-heptane had the significant synergistic effect on the dissolution of cholesterol molecules and the introduction of n-heptane promoted the formation of highly ordered lamellar liquid crystals in BCC-IL/cholesterol system. The dissolving mechanism of BCC-ILs for bioactive molecules was explored by experimental studies. This work indicates that the multifunctional BCC-ILs are promising solvents for extractions, catalysis, and biomass conversion. KEYWORDS: Ionic liquids, Nanoscale heterogeneity, Hydrogen-bond interaction, Extraction, Bioactive molecules Introduction


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Ionic liquids (ILs) have become one of the most fascinating materials due to their unique characteristics such as ultralow vapor pressure, diversity of physicochemical properties, and multiple solvation interactions.1-3 Among these properties the most fascinating one is the enhanced hydrogen bond (H-bond) interaction and multiple solvation interactions.4-7 Thus, ILs exhibit unprecedented solubility for biomass, polymers, drug molecules, gases, and metal ions, and enable some green and energy-saving processes including the solubilization and fraction of biomass,8,

9

catalytic conversion of biomass,10,

11

separation of bioactive

compounds,12-15 and gas separations.16-18 Moreover, the supramolecular organization of ILs can be finely tuned by changing the structure of cations or anions, and mesoscopic structural heterogeneities were observed in several ILs,19,

20

which have stimulated many studies

including ion conductors with nano-channels,21 nanostructured solvents for separation,22, 23 and dispersion of metal nanoparticles for catalysis.24, 25 Although numerous ILs have been prepared and characterized to date, most were based on halogenated counter anions and their derivatives. Their highly localized charge result in a relatively high melting point and applications of these halide-containing anions are limited by toxicological, ecological and economic issues.3 Furthermore, anions with a relatively strong basicity usually lead to ILs with a high polarity also due to their highly localized charge, which is unfavorable to the affinity of the IL to many organic compounds.26 Over the past three decades, carboxylate ILs have attracted great interests, because carboxylic acids obtained from a wide variety of biological sources are environmentally friendly. Short-chain carboxylate ILs have been reported to possess strong H-bond basicity, low viscosity, moderate thermal and electrochemical stability.8,

27, 28

Short-chain carboxylate ILs have

shown excellent performance in aromatics extraction, separation of bioactive compounds,29-31 catalytic systems,32 CO2 capture and gas separation.33, 34 Long-chain carboxylate ILs with phosphonium cation were prepared and exhibited strong H-bond basicity, excellent



lipophilicity, low viscosity and high thermal stability.26,

35

These LCC-ILs demonstrated

remarkable extraction efficiency for bioactive molecules and excellent metal nanoparticle stability for catalysis.13, 22, 36, 37 A series of long-chain carboxylate ILs have been reported as potential ashless lubricant additives owing to their enhanced solubility in oil.38 Although significant progress has been made, there is still a great challenge to design functional ILs with strong basicity, good lipophilicity, and low viscosity for separation and dissolution of bioactive molecules and other related applications. Furthermore, recent research indicate that ILs with branched anions exhibited, relative to the linear isomers, lower ecological effects and toxicity.39 However, most of studies focused on the linear carboxylate ILs and studies on ILs with branched long-chain carboxylate anion are relatively scarce.40-43 Herein, we first synthesized a class of functional phosphonium ILs featuring branched long-chain carboxylate anion with naturally-derived or inexpensive commercial materials (Scheme 1), which exhibit very strong H-bond basicity (β = 1.49-1.72, 30 °C), good lipophilicity (π* = 0.73-0.95, 30 °C), and interestingly, possess a wide liquid range (－43.2-246.1 °C), relatively low viscosity (η = 91.8-200.5 mPa, 40 °C) but has strong nanoscale segregation at room temperature. These branched chain carboxylate ionic liquids (BCC-ILs) can undergo self-assembly process with typical bioactive molecule of cholesterol to form highly ordered mesoscopic structures at room temperature, which enables unprecedented high solubility for cholesterol molecule (molar 1.12 at 50 °C). The solubility was thousands times of that in organic solvents and more than forty times higher than that in common ILs. The structure-property relationships of BCC-ILs and their solubility for bioactive molecules were comprehensively investigated. The nanoscale segregation structures of pure BCC-ILs and the self-assembly mesoscopic structures of BCC-IL/solute mixtures were explored by small angle X-ray scattering (SAXS) wide angle X-ray scattering (WAXS) and polarized optical microscopy (POM).


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Experimental Section Materials. The 40 wt% aqueous solution of tetrabutylphosphonium hydroxide ([P4444][OH]) was purchased from Tokyo Chemical Industry Co., Ltd. Tributyl(ethyl)phosphonium bromine ([P4442][Br], ≥99.0%) and trihexyl(tetradecyl)phosphonium bromide ([P66614][Br], 97%) were obtained from Green Chemistry and Catalysis, LICP, CAS (China). 2,2-dimethylbutyric acid (99%), 2-ethylbutyric acid (99%), 2-ethylhexnoic acid (99%), 2-n-hexyldecanoic acid (97%), 2,2,4,8,10,10-hexamethylundecane-5-carboxylic acid (>99%, GC) and cholesterol (≥95%, GC) were purchased from J&K Scientific Ltd. 4-ethyloctanoic acid (98%) was purchased from Aladdin Reagent Co., Ltd. 2-butyloctanoic acid and strongly basic anion-exchange resin Dowex Monosphere 550A UPW (OH) were obtained from Sigma-Aldrich Co., Ltd. The indicator probes for the Kamlet-Taft solvatochromic experiments were 4-nitroaniline (>99%, Aladdin Reagent Co., Ltd.) and N, N-diethyl-4-nitroaniline (97%, Oakwood Products, Inc.). Preparation of BCC-ILs. The BCC-ILs were synthesized via neutralization of a phosphonium hydroxide aqueous solution with specific fatty acid as a similar procedure reported in the literature for the synthesis of common carboxylate ILs.41, 44, 45 Equimolar [P4444][OH] (aqueous solution) and branched chain carboxylic acid were stirred at 45 °C for 24 h to produce the aqueous solution of tetrabutylphosphonium-based BCC-ILs. A rotary evaporator was employed to remove most of the water in the product at a temperature of 323 K, and then the oil pump was used to remove residual water. The ILs obtained were frozen and lyophilized (Labconco Free Zone 2.5 Plus, Kansas City, MO) for 48 h, and stored in a desiccator at room temperature until use. The accurate concentration of [P4444][OH] in the original aqueous solution was determined by HCl titration. [P4442][OH] aqueous solution was produced by the anion-exchange of [P4442][Br] with strongly basic anion-exchange resin.44 The preparation of trihexyl(tetradecyl)phosphonium ILs was similar to the above procedures,



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with the ethanolic solution of [P66614][OH] produced by the anion-exchange of [P66614][Br]. Moreover, the water contents of all the BCC-ILs were measured with the Karl-Fisher titration method on a Metrohm 870 KF Titrino plus and the values were less than 0.94 %. Details provided as Supporting Information. The chemical structure of synthesized ILs was confirmed by 1H NMR (Supporting Information).

Scheme 1 The structures, full names and corresponding abbreviations of BCC-ILs. Characterizations. The Kamlet-Taft dipolarity/polarizability π* and H-bond basicity β were measured

by

solvatochromic

N-diethyl-4-nitroaniline

as

experiments,

probe.46

Differential

using

4-nitroaniline

scanning

calorimetry

and

N,

(DSC)

measurements were performed with a TA Q200 differential scanning calorimeter under a nitrogen atmosphere, within a temperature range from －90 to 150 °C at a scanning rate of 10 °C min-1. Thermal gravimetric analysis (TGA) was conducted with a PerkinElmer Pyris 1 TGA instrument at a heating rate of 10 °C min-1 from 50 to 650 °C under a nitrogen atmosphere. The viscosities of ILs were measured by Brookfield LVDV-II+Pro Cone/Plate programmable viscometer, with an uncertainty of ±2%. The nanoscale heterogeneity was observed in these BCC-ILs using small


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angle X-ray scattering (SAXS). SAXS experiments were performed on a SAXS system of Xenocs France equipped with a semiconductor detector (Pilatus 100 K, Swiss) using CuKa radiation (wavelength = 0.15418 nm). All the BCC-ILs were injected into quartz capillary for measurements and the sample-to-detector distance was 185 mm. Exposure times were typically 600 s for individual measurements. Solubility Study. The solubilities of drugs in various solvents were measured using the static analytical method and the quantity of drug in the saturated solutions was determined by using the gravimetric analysis method. The apparatus and detailed procedure of the solubility experiments have already been described in previous work.23 Several groups of solubility experiments were performed in triplicates to evaluate the repeatability of this method. We found that it was a reliable and valid measurement and regarded a 5% uncertainty as acceptable. Fourier-transform infrared (FT-IR),

1

H NMR, polarized optical microscopy (POM) and wide angle X-ray

scattering (WAXS) were employed to study the key mechanism of the solubilization of cholesterol. Results and discussion Nine

BCC-ILs

were

synthesized

by

neutralizing

symmetric

and

asymmetric

tetraalkylphosphonium hydroxide with branched long-chain carboxylic acids (carbon number of 6 to 18), and their physiochemical properties were investigated. Thermal Analysis. All of the BCC-ILs were obtained as transparent liquids at room temperature. As shown in Table 1, it could be safely concluded that all the BCC-ILs exhibited the glass transition temperature (Tg) below －40 °C but did not have a melting point (Tm). The gentle heat flux versus temperature curves in DSC measurement (Figure S1) indicated that the solid phase was unstable. The branched carboxylate chain anion played an



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important role in reducing the Tg/Tm of ILs, which could lead to asymmetric ion pairs and incompact microstructures. A representative case in point is that the Tg of [P4444][DMBA] and [P4444][2-EBA] were － 63.1 °C and －71.7 °C, which were significantly lower than isomeric linear carboxylate IL, namely tetrabutylphosphonium hexanoate (40.7 °C). Ethyl substituted [P4444][2-EBA] presented a lower Tg than dimethyl substituted [P4444][DMBA]. It mirrored that the different branching way influenced the Tg of BCC-ILs. Besides, asymmetrical cation could lead to lower Tg. For instance, Tg of [P4442][2-HDA] (－72.8 °C) is 7.7 °C lower than [P4444][2-HDA] (－65.1 °C). As listed in Table 1, all the onset temperature of decomposition (Tonset) of BCC-ILs were above 246 °C, indicating that BCC-ILs have excellent thermostability. Considering their relative low Tg and high Tonset, BCC-ILs have a broad

liquid

range

(250-350

°C)

attributed

to

the

high

asymmetry

of

the

tetraalkylphosphonium cations and the branched-chain carboxylic acid anions, and could be used as solvents in many chemical processes. Table 1 The thermoanalysis data, solvation parameters and viscosities of BCC-ILs.

BCC-ILs

Tg/° C

T2%/° C

T5%/° C

Tonset/° C

β

η/mPa·s

π* 25°C

30°C

35°C

40°C

45°C

50°C

[P4444][DMBA]

-63.1

212.1

226.4

246.1

1.49

0.95

381.9

281.0

199.1

144.2

106.1

79.7

[P4444][2-EBA]

-71.7

216.9

233.6

262.4

1.51

0.94

299.1

210.4

150.1

113.6

85.7

66.1

[P4444][2-EHA]

-64.0

227.2

242.3

277.6

1.56

0.92

418.7

295.6

212.7

156.4

115.9

88.3

[P4444][4-EOA]

-76.1

237.8

252.6

277.9

1.52

0.91

435.9

292.6

204.3

146.5

106.8

79.0

[P4444][2-BOA]

-68.1

233.2

247.7

270.3

1.55

0.85

504.0

354.7

259.9

192.1

142.6

108.8

[P4444][2-HDA]

-65.1

241.2

254.1

279.6

1.60

0.82

502.3

356.8

263.7

200.5

151.9

118.4

[P4444][ISA]

-43.2

233.4

250.1

287.4

1.66

0.75

4021

2518

1569

1011

670.1

461.6

[P4442][2-HDA]

-72.8

221.7

248.5

269.0

1.72

0.81

365.5

258.0

187.7

137.5

102.1

77.3

[P66614][2-HDA]

-63.1

245.9

259.6

297.3

1.53

0.73

205.2

153.0

117.7

91.8

73.2

59.3

Nanoscale heterogeneity. Nanoscale heterogeneity of ILs plays a significant role in the processes of dissolution of biomolecules, dispersion and stabilization of nanoparticle, and


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construction of nano-channels.21, 23, 47 Understanding the relationships of molecular structure and shape with nanoscale segregation is also crucial to effectively designing nanostructured ILs.48 SAXS Experiments were carried out at ambient temperature to study the nanoscale organization of these pure isotropic molten salts.49 As shown in Figure 1, obvious Qlow diffraction peaks were detected in BCC-ILs, indicating the existence of nanoscale segregation of the alkyl tails in these ILs. According to Bragg’s Law, the spatial correlation size d equals to 2π/qmax. The d value of tetrabutylphosphonium BCC-ILs ranged from 10.5 to 13.7 Å when the length of alkyl chain in anions rose from 6 to 16 in Figure 1 a. For example, the spatial nanoscale segregation size of [P4444][2-EHA] (11.1 Å) was relatively smaller than [P4444][2-EOA] (13.3 Å) due to the shorter length of the alkyl chain in anion. The length increase of alkyl chain in cation also caused convincing changes of d (figure 1 b). The d value of [P4444][2-HDA] is 13.6 Å. When the length of the alkyl chain in cation was increased, the d value of [P66614][2-HDA] is 15.3 Å. Therefore, we believed that the degree of order of these nanostructured ILs was substantially dependent on the length of the alkyl chain both in cation and anion. It is well know that increase of alkyl chain length of the cation results in more segregation, however, these ILs usually comes along with undesired features, such as high melting point or glass transition temperature, and high viscosity, which limit the application.50 Interestingly, BCC-ILs possess relatively low viscosity and Tg, but have strong nanoscale segregation at room temperature. These structural features have been rationalised in terms of nanoscale-segregated morphology in BCC-ILs as a consequence of their inherent amphiphilicity, which leads to a polar vs. apolar domains segregation. Benefiting from all these unique properties, BCC-ILs have shown great potential in various application including biomolecule solubilization and nanoparticles dispersion for catalysis.



(a) 80

(b) [P4444][DMBA]

Intensity (A.U.)

Intensity (A.U.)

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

[P4444][2-EBA]

60

[P4444][2-EHA] [P4444][4-EOA] [P4444][2-HDA]

40 20 0

3

4

5

6

q (nm-1)

7

8

Page 10 of 35

80 [P4444][2-HDA] [P4442][2-HDA] [P66614][2-HDA]

60 40 20 0

3

4

5

6

q (nm-1)

Figure 1 SAXS patterns of (a) Tetrabutylphosphonium-based BCC-ILs (b) Tetraalkylphosphonium 2-n-hexyldecanoate ILs at ambient temperature.

H-bond Basicity and Dipolarity/Polarizability. Generally, H-bond basicity was used to describe the ability of a solvent to serve as H-bond acceptor. All of the BCC-ILs exhibited extremely strong hydrogen-bond basicity, with β values of 1.49-1.72, 30 °C (Table 1). The value of β was considered to be at the highest level among reported solvents (Table S2). ILs with common anions, like NTf2-, PF6-, BF4-, have weak H-bond basicity (β = 0.21-0.36, 25 °C). Imidazolium-based chloride or acetate salts, well known as H-bond basic ILs, have β values lower than 1.18, much lower than those of BCC-ILs. We found that increased alkyl chain length of anion led to a larger β because the long alkyl chain in the anion has a stronger electron-donating effect on the charged headgroup, thus enhancing the H-bond accepting ability of the anion. Often, dipolarity/polarizability π* could indicate the lipophilicity of a solvent. Although BCC-ILs had the highest H-bond acceptor ability, they surprisingly showed relatively low π* values (0.73-0.95, 30 °C) compared to most ILs (0.90-1.10). In general, increased alkyl chain length of anion led to a smaller π*, which means better lipophilicity, thus better affinity with weakly polar organic bioactive molecules. Compared with isomeric linear carboxylate ILs, all these BCC-ILs possessed enhanced H-bond accepting ability and better lipophilicity (Figure S5) because the basicity could be effectively enhanced by reducing cation-anion interactions and the branched chain in anions improved


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the lipophilicity. Admittedly, it is generally believed that branching reduces lipophilicity based on the log P (o/w) value in medicinal chemistry.51 However, Kamlet-Taft π* value as a widely used parameter in describing the dipolarity/polarizability of ILs.46 In our study, branched chain in anions improved the lipophilicity of ILs. The conclusion might be slightly different based on different methods while the solubility study demonstrated that BCC-ILs showed enhanced dissolving capabilities for weakly polar organic bioactive molecules，which is in conformity with the conclusion that braching increases lipophilicity from the solvatochromic experiments. All these branched long chain carboxylate ILs have strong H-bond accepting ability and good lipophilicity, showing favorable interaction with molecules possessing both strong H-bond donors and hydrophobic segments. Due to these unique physicochemical properties, BCC-ILs could perform as efficient extractants for separation and dissolution of bioactive molecules, as well as considerable potential for other related applications. Viscosity. The viscosity of a solvent is crucial to mass transfer, which is a key factor in dissolution, extraction and absorption process. In our study, It has been shown in Table 1 that the viscosities of BCC-ILs were at the range of 91.8 to 200.5 mPa·s at 40 °C. Notably, the viscosities of [P4442][2-HDA], [P4444][2-HDA] and [P66614][2-HDA] were 102.1, 200.5 and 73.2 mPa·s respectively at 40 °C, which showed that the viscosity decreased remarkably when the cation [P4444]+ was replaced by [P4442]+ or [P66614]+. This phenomenon could be attributed to the increase in entropy of ILs.52-54 Compared with [P4442]+ or [P66614]+, [P4444]+ cation was more symmetric and the rotation of alkyl chains in cation was more difficult, which might cause the reduction of entropy (possibly facilitating alkyl chain aggregation and leading to a more regular arrangement of ions) and then resulted in an increase of viscosity. Therefore, the introduction of asymmetry cation of [P4442]+ or [P66614]+ significantly decreased the viscosity of BCC-ILs.55 In addition, there was a remarkable drop in viscosities of the



BCC-ILs when the temperature went up from 25 to 50 °C (Table 1). However, with increasing length of the alkyl chain of anion, the viscosity went up due to the stronger Van der Waals interactions. For example, the viscosities of [P4444][2-EBA], [P4444][2-EHA] and [P4444][2-HDA] were 113.6, 156.4 and 200.5 mPa·s, respectively, at 40 °C. Moreover, the viscosity rose when there were more branched chains in anion. For instance, the viscosity of [P4444][DMBA] (144.2 mPa·s, 40 °C) is higher than its isomer [P4444][2-EBA] (113.6 mPa·s, 40 °C). Solubility study. In virtue of extremely strong H-bond accepting ability and relatively good lipophilicity, as well as nanoscale segregation, we evaluated the dissolving ability of BCC-ILs for bioactive molecules. Cholesterol is an essential structural component of all animal cell membranes to maintain both membrane structural integrity and fluidity and also serves as a precursor for the biosynthesis of steroid hormones and bile acids. Consisting of both the hydrophobic segments like long alkyl chain and multi-alicyclic ring and the polar hydroxyl group, cholesterol exhibits sparingly aqua- and lipo-soluble property and is very poorly soluble in water, organic solvents and conventional ionic liquids (molar solubility 0.000001-0.03, Figure 2a).23 It is noteworthy that the unprecedented high solubility of cholesterol was achieved in BCC-ILs. The molar solubilities were 0.31-1.12 at 50 °C. To the best of our knowledge, this value represents the highest solubility reported for cholesterol. At 50 °C, the solubility of cholesterol in [P4444][2-HDA] was 1.12，which was millions times of that in water (

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