Synthesis, characterization and physical properties of novel cholinium-based organic magnetic ionic liquids

Accepted Manuscript Synthesis, characterization and physical properties of novel cholinium-based organic magnetic ionic liquids

Li-rong Nie, Shun Yao, Bing Dong, Xin-lu Li, Hang Song PII: DOI: Reference:

S0167-7322(17)31402-2 doi: 10.1016/j.molliq.2017.05.044 MOLLIQ 7336

To appear in:

Journal of Molecular Liquids

Received date: Revised date: Accepted date:

1 April 2017 9 May 2017 10 May 2017

Please cite this article as: Li-rong Nie, Shun Yao, Bing Dong, Xin-lu Li, Hang Song , Synthesis, characterization and physical properties of novel cholinium-based organic magnetic ionic liquids, Journal of Molecular Liquids (2017), doi: 10.1016/ j.molliq.2017.05.044

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ACCEPTED MANUSCRIPT Synthesis, characterization and physical properties of novel cholinium-based organic magnetic ionic liquids Li-rong Nie, Shun Yao, Bing Dong, Xin-lu Li, Hang Song*

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School of Chemical Engineering, Sichuan University, Chengdu 610065, P. R. China

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*Corresponding Authors

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E-Mail:[email protected] (H. Song)

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Tel & Fax: +86-028-85405221

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ACCEPTED MANUSCRIPT ABSTRACT: In this paper, five novel cholinium-based organic magnetic ionic liquids (MILs) were synthesized. The carbon chain links to the nitrogen atom from cationic nucleus. The MILs were comprehensively identified by FT-IR, HPLC, UV-Vis, 1H NMR and ESI-MS. The MILs have interesting performance to form

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aqueous two-phase system and can respond to the external magnetic field after the

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formation of aqueous two-phase system, which can assists magnetically phase

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separation. The magnetic susceptibility of the MILs was measured at 298K. Other physical properties such as conductivities, acidity and density were measured in range

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of 278.15K to 323.15K for further application investigation. Satisfactory results were

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obtained after the data of physical properties were correlated. Based on the measurements and correlations above, some discussions were carried out, including

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the ion association and ion diffusion, the influences of MILs carbon chain and

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temperature on the molar conductivity, the influences of the cation size on the molar volume, acidity variation of the MILs.

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Key words: Magnetic ionic liquids; Synthesis; Conductivity; Density; Acidity

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ACCEPTED MANUSCRIPT 1. Introduction Room-temperature ionic liquids (RTILs) are described as molten salts with various cationic and anionic species near room temperature with many advantages [1]. Magnetic ionic liquids (MILs) are composed of organic cations and inorganic or

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organic anions and considered as a new class of RTILs. In the MILs, the magnetic

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centers are attributed to the cationic groups or anionic groups. Therefore, MILs not

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only have excellent properties of RTILs but also an unexpectedly response to the external magnetic field [2, 3].

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In 2004, Hayashi firstly prepared a MIL [bmim]FeCl4 [4]. Subsequently, MILs

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have been attracting more attention in view of special properties and possible applications. MILs can be divided into metal ion-containing (Fe, Co, Dy, Pd, Mn, etc.)

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MILs [5-10] and non-metal organic MILs based on radical ion [11-13]. The organic

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MIL was discovered in 2007 [11] and recently guanidinium-based organic MIL have been developed [13]. But further study of more organic MILs are scarcely reported so

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far.

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In our paper, five novel cholinium-based organic MILs were firstly prepared and characterized by FT-IR, HPLC, UV-Vis, 1H NMR and ESI-MS. The magnetization and susceptibility of the MILs would be interesting for further research. Additionally, their physical properties such as conductivities, acidity and density were measured for further application investigation, Acidity is an important property which especially for catalysis application [14, 15]. The acidity order of ionic liquids were considered to be accordant with their catalysis activities order in acid-catalyzed reactions [16-18]. 3

ACCEPTED MANUSCRIPT Among the methods for the acidity of ILs, the most well-known one is the Hammett method, which has been successfully applied in the acidity measurement of many ILs, such as imidazolium, pyridinium, benzothiazolium and so on [15, 16, 19]. Electrical conductivity is one of the important indicators of electrochemical application of ionic

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liquids [20]. Moreover, ions interactions play an important role in the potential

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applications of ionic liquids. Many important researches on the ion association and

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transfer can be obtained with conductivity study. The ion association constant (KA) and the limiting molar conductivity (Λ∞) were also well calculated [21]. Finally, the

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density of ILs is also a frequently needed fundamental property [22]. The new MILs

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are likely to be applied to many fields such as catalysis, biotechnology, electrochemistry, especially extraction and separation technology by using the

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aqueous-two phase system. It could provide useful reference to design more new

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organic MILs by introducing more different functional groups.

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2. Experimental section

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2.1. Materials

All reagents and solvents were of analytical-reagent grade or higher. Ethanol, acetone,

dichloromethane,

n-hexane,

silver

nitrate,

hydrochloric

acid

and

chlorosulfonic acid were purchased from Aladdin biological technology Co., Ltd (Shanghai, China) with a minimum mass fraction purity of 98%. 4-OH-TEMPO (98%) was purchased from Best Reagent Co., Ltd. (Chengdu, China). 1-Bromoethane, 1-bromopropane, 1-bromobutane and 1-bromopentane were all obtained from Aladdin 4

ACCEPTED MANUSCRIPT Chemical Reagent Co., Ltd. (Shanghai, China) with a minimum mass fraction purity of 98%. Ultrapure water was prepared with the ultra-pure water system (0.4-mm filter) from Ulupure Technologh Co. Ltd. (Chengdu, China). 2.2. Apparatus

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ESI-MS were conducted on a GCMS-QP2010 Plus mass spectrometer

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(Shimadzu, Japan) and data were analyzed by GCMS solution Ver.2 workstation.

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Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet 6700 Spectroscopy (Thermo Electron Corporation, USA) to analyze characteristic

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functional groups of five magnetic ILs. 1H NMR spectra of the intermediates were

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performed on a Bruker AV II-600 MHz spectrometer (Bruker, Switzerland). Electrical conductivity were carried out on a DDS-12A digital conductivity meter (Hongyi,

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Shanghai, China) Material mass was determined by an electronic analytical balance

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with an uncertainty of ±0.0001 g (ESJ200-4A, Longteng Electronic Co., Ltd., Shenyang, China). A water thermostat (±0.05 K, CY20A, Boxun Industry &

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Commerce Co., Ltd., Shanghai, China) and a low-temperature thermostat (±0.05 K,

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DC-0506, Hengping Instrument and Meter Factory, Shanghai, China) were applied for keeping constant temperature of the system. HPLC analysis with an uncertainty of ±1% was performed on high-performance liquid chromatographic instrument equipped with an UV1201 detector and the EC2006 workstation (Elite Instrument Co., Ltd., Dalian). The analytical column was a 250-mm Symmetry C18 column (4.6 µm, Welchrom, Shanghai).

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ACCEPTED MANUSCRIPT 2.3. Synthesis and characterization of organic magnetic ionic liquids 2.3.1. Synthesis of [N1 1 n 2OH]Br or [N1 1 H 2OH]Cl Ionic liquids of the alkyl-(2-hydroxyethyl)-dimethylammonium bromides ([N1 1 n 2OH]Br,

n=2,3,4,5) were synthesized and purified refering to the previously reported

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literature [23]. To briefly summarize, alkyl bromide (19.6100 g, 0.22 mol) was

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dissolved in ethanol (20 mL) and the solution was slowly added to another solution of

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2-(dimethylamino) ethanol (17.8272, 0.2 mol) in ethanol (30 mL). The reaction mixture was refluxed at 80oC for 24 h with continuous stirring, and then residual

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solvent (ethanol) was removed under vacuum. Diethyl ether was added and the

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precipitated salt ([N1 1 n 2OH]Br) was filtered and dried under vacuum for 48 h. All the salts, including [N1 1 2 2OH]Br, [N1 1 3 2OH]Br, [N1 1 4 2OH]Br and [N1 1 5 2OH]Br, were

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obtained in 93%, 93%, 97%, 98% yield, respectively. For [N1 1 H 2OH]Cl, a solution of

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hydrochloric acid (0.22 mol) in ethanol (20 mL) was added slowly into the ethanol solution (30 mL) containing 0.2 mol 2-(dimethylamino) ethanol. With the same

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reaction and purification process, a white transparent liquid ([N1

1 H 2OH]Cl)

was

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obtained in 94% yield.

2.3.2. Synthesis of [TEMPO-OSO3]Na The synthesis route of sodium 4-sulfonatooxy-2,2,6,6-tetramethylpiperidine -1-yloxyl ([TEMPO-OSO3]Na) was similar to the procedure in the literatures [12, 24]. The free radical 4-hydroxy-TEMPO (8.6120 g, 0.05 mol) was dissolved in dichloromethane (30 mL) in the range of 0-5oC, equal molar of chlorosulfonic acid 6

ACCEPTED MANUSCRIPT (ClSO3H) was added into above solution dropwise within 1 h. After continuous reaction for 4 h, the mixture was filtered and washed three times with dichloromethane. The yellow solid (H[TEMPO-OSO3]) was dried under vacuum at 40oC. Afterward, [TEMPO-OSO3]Na was prepared by the neutralization of

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H[TEMPO-OSO3] and NaOH in aqueous solution in 96% yield.

The organic magnetic ILs [N

1 1 n 2OH]

[TEMPO-OSO3] were prepared by the

1 n 2OH]Br

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metathesis of [TEMPO-OSO3]Na and [N1

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2.3.3. Synthesis of organic magnetic ILs [N 1 1 n 2OH] [TEMPO-OSO3]

or [N1

1 H 2OH]Cl

in ethanol.

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Certain amount of [TEMPO-OSO3]Na (8.2274 g, 0.03 mol) was dissolved in ethanol (30 mL), equal molar of the correspondent [N1 1 n 2OH]Br or [N1 1 H 2OH]Cl was added

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dropwisely. Under vigorous stirring, the mixture solution was heated overnight at

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60 ℃ .Then the residual solvent was removed by vacuum rotary evaporation. Afterward the product was washed with acetone until no precipitation (NaBr) was

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observed in the solution and dried under vacuum for 24 h. Finally, reddish or

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dark-reddish liquids were collected with yield of 91% for [N

1 1 H 2OH]

[TEMPO-OSO3], 87% for [N

1 1 2 2OH]

[TEMPO-OSO3], 95% for [N

1 1 3 2OH]

[TEMPO-OSO3], 96% for [N

1 1 4 2OH]

[TEMPO-OSO3], 97% for [N

1 1 5 2OH]

[TEMPO-OSO3]. The residue of bromide anion in these products was not observed with an aqueous solution of AgNO3. Their synthetic procedure was shown in Fig. 1. All magnetic ILs were confirmed by UV-Vis spectra, FT-IR and ESI-MS spectra. Additionally, 1H NMR spectra of the intermediates were measured to determine the 7

ACCEPTED MANUSCRIPT cationic structures and purities of the intermediates. Finally, the purities of five MILs

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were determined by HPLC.

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Fig. 1. General synthetic route for the preparation of five organic magnetic ILs

2.4. Magnetic susceptibility measurement

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The magnetic susceptibilities of five MILs were performed with MPMS (SQUID) system. [N 1 1 H 2OH] [TEMPO-OSO3] (0.0180 g), [N 1 1 2 2OH] [TEMPO-OSO3] (0.0173

[N

1 1 5 2OH]

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g), [N 1 1 3 2OH] [TEMPO-OSO3] (0.0164 g), [N 1 1 4 2OH] [TEMPO-OSO3] (0.0239 g) or [TEMPO-OSO3] (0.0198 g) was accurately weighed in a special capsule

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of nonmagnetic and pressure-proof material (4 cm) and measured at 298 K in the magnetic field range from −20000 Oe to 20000 Oe.

2.5. Conductivity measurement The conductivity meter with the measurement precision of ± 0.5% was calibrated using a standard KCl solution (0.01 mol·L-1). For temperature-dependent measurements, a thermostatic water bath with an uncertainty of ±0.05 K was used to 8

ACCEPTED MANUSCRIPT control the temperature. Before the measurement, conductance cell was washed with ultrapure water and acetone, successively. Then the conductance cell and electrode were blow-dried with cold wind. 2.6. Density measurement

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Densities of five MILs in the 278.15 K to 323.15 K with intervals of 5 K were

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measured by pycnometer method that is same to the reference [25] in principle. A

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thermostatic water bath with an uncertainty of ± 0.05 K was used to control the temperature. More detailed procedure for this measurement can be referred to

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previous report [25]. The maximum uncertainty was found to be ± 0.001 in measuring

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the densities of MILs. 2.7. Acidity measurement

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The Brønsted acidity strength of five magnetic ILs was measured with UV-Vis

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spectrum by Hammett method [26]. The UV-Vis spectrophotometer was used with a

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wavelength accuracy of ± 0.5 nm. The Hammett function (H0) was defined as: H 0  pK a  log([I ] / [HI ])

(5)

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where pKa is constant (pKa = 0.99). [I] and [HI] are the percentages (%) of unprotonated and protonated forms of indicator, respectively. In this experiment, p-nitroaniline was chosen as indicator. More detailed procedure for this measurement can be referred to previous report [26]. According to the Equation (5), the values of H0 for five MILs were obtained with an uncertainty of ± 0.001.

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ACCEPTED MANUSCRIPT 3. Results and discussion 3.1. Characterization of magnetic ILs 3.1.1. FT-IR spectra The FT-IR spectra of the five MILs are shown in Fig. 2. For example of the 1 1 5 2OH]

[TEMPO-OSO3], the peak between 2800cm-1 and 3000 cm-1

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spectrum of [N

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is mainly assigned to stretching vibrations of saturated C–H bonds in the methyl and

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methylene groups. The peak at 1706 cm-1 may be attributed to skeleton stretching vibration of anion. The peak at 1465 cm-1 is the asymmetric in-plane bending

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vibration C–H bonds in the methyl and methylene groups. The peaks at 1380 cm-1 and

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1363 cm-1 are attributed to C-H symmetric in-plane bending vibration in the gem-dimethyl of anion. The peak at 1215 cm-1 is the C–N stretching vibration. The

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peak at 1060 cm-1 belongs to the N–O stretching vibration in the anion. Besides, the

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peaks at 982 cm-1 and 860 cm-1 are attributed to O-S-O stretching and asymmetric and

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symmetric stretching vibration, respectively.

Fig. 2. FT-IR spectra (KBr disc) of [N 1 1 H 2OH] [TEMPO-OSO3] (a), [N 1 1 2 2OH] 10

ACCEPTED MANUSCRIPT [TEMPO-OSO3] (b), [N 1 1 3 2OH] [TEMPO-OSO3] (c), [N 1 1 4 2OH] [TEMPO-OSO3] (d) [N 1 1 5 2OH] [TEMPO-OSO3] (e)

3.1.2. ESI-MS analysis

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ESI-MS spectra of the MILs were measured to determine their structures and

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purities. Obviously, the anions of MILs have same chemical structure. So all of MILs

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show one ionic peak at m/z 251 (C9H17NO5S-). And it can be seen that the characteristic ionic peaks appear at m/z 90.0929 (C4H12NO-), m/z 118.1223

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(C6H16NO-), m/z 132.1381 (C7H18NO-), m/z 146.1547 (C8H20NO-), m/z 160.1708

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(C9H22NO-) successively corresponding to the cationic molecular weight of the magnetic ILs. That is to say, the anionic and cationic structures of MILs are definitely

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determined. Additionally, obvious impurity peaks were not observed. Therefore, it can

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be concluded that five MILs have been successfully synthesized with good quality. The experimental data are as follow: Electrospray MS+ m/z: 90.0929 for [N 1 1 H 2OH]+;

[N

+

112

; MS- m/z: 251.0830 for [TEMPO-OSO3]-; Electrospray MS+ m/z: 132.1381 for

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2OH]

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MS- m/z: 251.0823 for [TEMPO-OSO3]-; Electrospray MS+ m/z: 118.1223 for [N

+ 1 1 3 2OH] ;

MS- m/z: 251.0825 for [TEMPO-OSO3]-; Electrospray MS+ m/z:

146.1547 for [N 1 1 4 2OH]+; MS- m/z: 251.0824 for [TEMPO-OSO3]-; Electrospray MS+ m/z: 160.1708 for [N

1 1 5 2OH]

+

; MS- m/z: 251.0834 for [TEMPO-OSO3]-. All spectra

above are shown in the Supporting Information (Fig. S1-S5).

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ACCEPTED MANUSCRIPT 3.1.3. 1H NMR spectra of the intermediates Because the magnetic IL products could not be measured directly by 1H NMR spectra, the cationic structures and purities of the intermediates [N1 1 n 2OH]Br (n=2, 3, 4, 5) and [N1 1 H 2OH]Cl were primarily confirmed. The experimental data are listed as

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follows:

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[N1 1 H 2OH]Cl, 1H NMR (600 MHz, DMSO-d6) δ/ppm: 2.65-2.66 (d, J=5.0 Hz,

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6H), 3.02-3.04 (dd, J=10.6, 5.3 Hz, 2H), 3.61-3.63 (m, 2H), 4.13 (s, 1H), 10.26 (s, 1H).

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[N1 1 2 2OH]Br, 1H NMR (600 MHz, DMSO-d6) δ/ppm: 1.14 (t, J=7.2 Hz, 3H),

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2.97 (s, 6H), 3.27-3.30 (m, 2H), 3.32-3.36 (m, 2H), 3.72 (br. s., 2H), 5.18 (t, J=5.1 Hz, 1H).

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[N1 1 3 2OH]Br, 1H NMR (600 MHz, DMSO-d6) δ/ppm: 0.78 (t, J=7.3 Hz, 3H),

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1.57-1.61 (m, 2H), 2.99 (s, 6H), 3.22-3.24 (m, 2H), 3.28-3.32 (m, 2H), 3.72 (br. s., 2H), 5.18 (t, J=5.1 Hz, 1H).

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[N1 1 4 2OH]Br, 1H NMR (600 MHz, DMSO-d6) δ/ppm: 0.83 (t, J=7.4 Hz, 3H),

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1.17-1.21 (dd, J=14.8, 7.4 Hz, 2H), 1.53-1.59 (m, 2H), 2.99 (s, 6H), 3.23 (m, 2H), 3.28-3.32 (m, 2H), 3.72 (br. s., 2H), 5.18 (t, J=5.1 Hz, 1H). [N1 1 5 2OH]Br, 1H NMR (600 MHz, DMSO-d6) δ/ppm: 0.79 (t, J=7.4 Hz, 3H), 1.12-1.16 (dd, J=15.1, 7.8 Hz, 2H), 1.21-1.25 (dd, J=14.8, 7.4 Hz, 2H), 1.55-1.59 (m, 2H), 2.99 (s, 6H), 3.25-3.28 (m, 2H), 3.31-3.32 (m, 2H), 3.71 (br. s., 2H), 5.17 (t, J=5.1 Hz, 1H). Therefore, it can be concluded that [N1 1 n 2OH]Br (n=2, 3, 4, 5) and [N1 1 H 2OH]Cl

have been synthesized successfully. 12

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3.1.4. HPLC and UV-Vis analysis The UV-Vis spectra of five MILs at 298 K (Fig. 3a) indicate a maximum absorption band at 245 nm. The purities of the prepared five MILs were determined

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by HPLC. 40% methanol aqueous was selected as elution solvent. HPLC analysis of

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temperature 25oC and detection wavelength 245 nm.

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five MILs were performed in the condition of flow rate 1.0 mL/min, column

Fig. 3. HPLC chromatograms of 4-OH-TEMPO (1), [N 1 1 H 2OH] [TEMPO-OSO3] (2),

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[N 1 1 2 2OH] [TEMPO-OSO3] (3), [N 1 1 3 2OH] [TEMPO-OSO3] (4), [N 1 1 4 2OH] [TEMPO-OSO3] (5) [N 1 1 5 2OH] [TEMPO-OSO3] (6)

Fig. 3b presents the HPLC spectra of five MILs and demonstrates all MILs with the purity 99% for [N 1 1 5 2OH] [TEMPO-OSO3], 99% for [N 1 1 5 2OH] [TEMPO-OSO3], 97% for [N

1 1 5 2OH]

[TEMPO-OSO3], 96% for [N

[N 1 1 5 2OH] [TEMPO-OSO3]. 13

1 1 5 2OH]

[TEMPO-OSO3], 99% for

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3.1.5. Magnetic property analysis of the MILs The relationship of magnetization vs. magnetic field of these MILs is shown in Fig. 4. Among the MILs, [N

1 1 5 2OH]

[TEMPO-OSO3] exhibits the largest magnetization

[TEMPO-OSO3] < [N 1 1 4 2OH]

[TEMPO-OSO3] < [N

[TEMPO-OSO3] < [N

1 1 3 2OH]

1 1 5 2OH]

[TEMPO-OSO3]. The

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[TEMPO-OSO3] < [N

1 1 2 2OH]

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1 1 H 2OH]

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intensity at 298 K. The magnetic susceptibilities of the five MILs follow the order: [N

interdependencies between magnetic moment and magnetic field indicate that the

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MILs are paramagnetic. Moreover, the five MILs can form the MILs aqueous-two

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phase system (MIL-ATPs) by mixing with inorganic salts such as K3PO4, K2HPO4, K2CO3, Na3C6H5O7 and K3C6H5O7. The experimental process was described as follow: 1 1 5 2OH]

[TEMPO-OSO3] solution of known mass fraction was taken into

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the MIL [N

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a colorimetric tube. A salt solution (K3PO4) of known mass fraction was then added dropwise to the colorimetric tube under magnetic stirring condition until the mixture

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became cloudy. The first binodal data point was obtained by noting the composition

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of this mixture. Then, double-distilled water was added dropwise to the colorimetric tube to get a clear, one-phase mixture. The above procedure was repeated multiple times to obtain a complete binodal curve. The binodal curve of MIL [N

1 1 5 2OH]

[TEMPO-OSO3] with K3PO4 at 298.15 K is representatively shown in Fig. 5. The binodal curve was correlated according to the empirical nonlinear expression proposed by Merchuk [27]. The experimental data can fit very well with the calculated

values

with

R2=0.9989, 14

which

proves

that

this

method

ACCEPTED MANUSCRIPT is practicable and reliable. MIL-rich phase of the aqueous-two phase system can respond to the external magnetic field after the formation of aqueous two-phase system. The magnet-assisted phase separation is demonstrated in Fig. 6. The aqueous two-phase system was composed of 6% of MIL [N 1 1 5 2OH] [TEMPO-OSO3] and 30%

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of the salt K3PO4. The experiment was carried out at room temperature (298.15 K). It

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was observed that the MIL aqueous-two phase system could reached more rapidly to

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phase separation by magnetic effect than by still standing. That is to say, compared with the liquid-liquid biphase separation, the MIL aqueous-two phase system can

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realize the phase-separation more rapidly under the influence of external magnetic

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field. This is an advantage for its application in extraction separation, especially

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microextraction.

Fig. 4. Magnetic susceptibility of five MILs as a function of applied magnetic field at 298 K

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Fig. 5. Binodal curve of MIL [N 1 1 5 2OH] [TEMPO-OSO3] with K3PO4 at 298.15 K

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Fig. 6. Magnetic response of IL in aqueous two-phase system to neodymium magnet

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3.2. Conductivities of (water + MIL) binary system The electrical conductivity of solution can be transformed into the molar conductivity by the following formula:  m  103 k / c

(6)

where Λm (S·cm2·mol−1) is the molar conductivity, k (S·cm-1) is the electrical conductivity of solution, c (mol·L−1) is the molar concentration of magnetic ILs. The data of measured molar conductivity of the five MILs aqueous solutions in series of 16

ACCEPTED MANUSCRIPT different molar concentrations and temperature are presented in Table S1. The molar conductivity of five MILs are calculated by using the Arrhenius–Ostwald model [28, 29], and it is defined as: 1 1 c K     A2  m  m ( m )

(7)

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where Λm (S·cm2·mol−1) is the molar conductivity at the concentration of c (mol·L−1),

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Λm∞(S·cm2·mol−1) is the limiting molar conductivity, and ΚA (L·mol−1) is the ion

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association constant.

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The relationship curves between electrical conductivity and its concentration are shown in Fig. S6-S10. Based on the experimental data, the linear fitting were done

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using least square. Then, the least squares linear fitting curves of 1/Λm against cΛm at different temperature were prepared. The values of Λm∞ and ΚA can be calculated

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according to the slope and intercept of the fitted curves. The obtained results for Λm∞ and ΚA are summarized in Table 1. As shown in Fig. S6-S10, it can be seen that the

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electrical conductivity increases remarkably with increasing concentration of ionic liquids. The change trends of electrical conductivity for all MILs are consistent.

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Because the high ionic liquids concentration could increase the amount of cation and anion per unit volume, the contribution of the amount of ions to the electrical conductivity is larger than the solution viscosity. Moreover, the effect of temperature on the electrical conductivity was also investigated in the temperature range from 278.15K to 328.15K. When the concentration of MILs is the same, the electrical conductivity increases slightly with rising temperature; this phenomenon is largely attributed to the greater number of the free state-free charges at the higher temperature. 17

ACCEPTED MANUSCRIPT And elevated temperature will lead to the reduction of solution viscosity and strenuous movement of cation and anion in solution, which is another reason for the increased conductivity.

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Table 1 Limiting Molar Conductivities (Λm∞ / S·cm2·mol−1), Association Constants (ΚA / L·mol−1) for Aqueous Solutions of Five MILs at Different Temperature ∞

Λm

KA

∞

Λm

KA

∞

Λm

KA

T/K MIL 3

47.551

220.10

73.314

153.61

66.225

288.15

66.007

371.25

102.04

202.00

85.470

298.15

100.60

597.65

124.84

233.79

308.15

121.95

812.02

166.67

318.15

163.40

1144.1

196.85

328.15

210.53

1366.4

93.746

58.823

65.398

134.41

82.645

116.80

79.365

95.112

113.64

178.20

107.53

137.59

105.26

118.56

283.33

137.55

182.39

135.14

162.53

123.46

131.08

168.63

198.21

156.25

167.24

149.25

140.79

411.62

191.57

211.76

186.22

186.91

175.75

159.07

[TEMPO-OSO3]; MIL 2= [N

1 1 2 2OH]

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339.84

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1 1 H 2OH]

MIL 5

[TEMPO-OSO3]; MIL 3= [N

1 1 3 2OH]

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MIL 1= [N

KA

62.112

235.85

122.36

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278.15

MIL 4

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MIL 2

∞

Λm

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MIL 1

KA

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∞

Λm

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[TEMPO-OSO3]; MIL 4= [N 1 1 4 2OH] [TEMPO-OSO3]; MIL 5= [N 1 1 5 2OH] [TEMPO-OSO3].

It can be seen from Table 1 that nearly all the values of KA for the investigated MILs are greater than 100 L·mol−1. Compared with the values of KA for imidazolium-based ionic liquids (12–17 L·mol−1) [30] and tropine-based ionic liquids (5–7 L·mol−1) [31], the values of KA for five MILs are very large, which suggests that the coulomb attraction between ions of MILs in water is greater than that of imidazolium-based or tropine-based ionic liquids. Moreover, MILs in water can form 18

ACCEPTED MANUSCRIPT ion-pairs by ion association. Ions from MILs in water have the high surface-charge density, which can result in the strong interactions of ion-pairs. On the basis of molecular structure, there are hydroxyl groups on their cations. Intermolecular hydrogen bonding interaction is a main driving force for the ion association.

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Interestingly, the values of KA decreased with the elongation of carbon chain at the

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same temperature. Studies have shown that cation-anion interactions for ionic liquids

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decrease with the elongation of alkyl chain on the cation [32, 33]. The trend is consistent with the change trend of ion association constant. Therefore, the

1 1 H 2OH]

[TEMPO-OSO3], the values of KA

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association of MILs. Additionally, for [N

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electrostatic interaction between anion and cation can used to explain the ion

is the highest of all, which indicate that the ion association is stronger than that of

1 1 H 2OH]

[TEMPO-OSO3], which causes greater intermolecular hydrogen bonding

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[N

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other MILs. There exists one hydrogen atom on the nitrogen atom from the cation of

interaction. In addition, the value of KA increases as the temperature rises, which

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reveals that the association of magnetic ILs in water is an endothermic process.

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Furthermore, the limiting molar conductivity (Λm∞) of five MILs in water were investigated in the temperature range of 278.15 K to 328.15K, and the plots of Λm∞ vs the length of the carbon chains were shown in Fig. 7. The difference of Λm∞ at the same temperature is only influenced by the structure of cation and solvent. The results showed that the values of Λm∞ decreased with the elongation of carbon chain except the organic magnetic ILs [N 1 1 H 2OH] [TEMPO-OSO3], which has one hydrogen atom on the nitrogen atom of its cation. The existence of the hydrogen atom is easier to 19

ACCEPTED MANUSCRIPT form intermolecular hydrogen-bond interactions which lead to the active volume of cation greater and the decrease of Λm∞. The values of Λm∞ decreases with the elongation of alkyl chain on cation. The reason is that the active volume and hydration ability of cation increase with the elongation of alkyl chain. The conclusion

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is consistent with the result of the literature [31]. However, the value of Λm∞ increases

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as the temperature rises, which reveals that high temperature is more advantageous to

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ion diffusion.

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Fig. 7. The relationship between limiting molar conductivity and length of alky chain

3.3. Density of magnetic ILs The densities of five MILs and water were summarized in Table 2. The temperature dependence of density was shown in Fig. 8. It is not difficult to find that densities of magnetic ILs decrease with the increasing temperature and elongation of carbon chain. Thus, these results show that the temperature has an inverse effect on density. The volume of MILs expands to a greater size with the increase of 20

ACCEPTED MANUSCRIPT temperature, which results in lower density. At the same temperature, the density of MILs in the order of [N

1 1 H 2OH]

[TEMPO-OSO3] (MIL2) > [N

[TEMPO-OSO3] (MIL1) > [N

1 1 3 2OH]

1 1 2 2OH]

[TEMPO-OSO3] (MIL3)> [N

1 1 4 2OH]

[TEMPO-OSO3] (MIL4) > [N 1 1 5 2OH] [TEMPO-OSO3] (MIL5). It is because that ion

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size and steric hindrance increase as the elongation of carbon chain, which leads to

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the smaller Coulomb forces. Moreover, the variation trend of density with the change

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in temperature and carbon chain is in good agreement with the literature [34].

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Table 2 Density of Five MILs at Different Temperatures ρ/g·cm-3

283.15

288.15

293.15

298.15

303.15

308.15

313.15

318.15

323.15

MIL1

1.2357

1.2252

1.2095

1.1918

1.1755

1.1625

1.1481

1.1331

1.1193

1.1075

MIL2

1.2014

1.1836

1.1753

1.1585

1.1418

1.1291

1.1141

1.1005

1.0896

1.0715

MIL3

1.1801

1.1696

1.1509

1.1391

1.1223

1.1109

1.0993

1.0824

1.0650

1.0537

MIL4

1.1617

1.1527

1.1333

1.1212

1.1091

1.0929

1.0788

1.0648

1.0535

1.0364

MIL5

1.1481

1.1169

1.1035

1.0944

1.0777

1.0618

1.0484

1.0311

1.0203

0.9991

0.9982

0.9970

0.9956

0.9940

0.9922

0.9902

0.9881

[TEMPO-OSO3]; MIL 2= [N

1 1 2 2OH]

CE 1.1352

AC

H2O

1.0000

MIL 1= [N

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278.15

D

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MILs

0.9997

1 1 H 2OH]

[TEMPO-OSO3]; MIL 4= [N

1 1 4 2OH]

[TEMPO-OSO3]; MIL 3= [N

1 1 3 2OH]

[TEMPO-OSO3]; MIL 5= [N 1 1 5 2OH] [TEMPO-OSO3]. The

maximum uncertainty of density was ± 0.001.

21

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ACCEPTED MANUSCRIPT

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Fig. 8. Temperature dependence of density of five MILs.

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3.3.1. Molecular volume

The molar volume (Vm) of these series ILs was calculated according to the

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following equation [35]:

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Vm  M / ( N   )

(8)

where M (g·mol-1) represents the molar mass of MILs. ρ (g·cm-3) represents the

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density of MILs. N (mol-1) represents Avogadro's constant (N = 6.022×10²³ mol-1).

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The data of molar volume at T = 298.15 K were summarized in Table 3. The plot of Vm against the number of carbon atoms in substituent groups is presented in Fig. 9. At 298.15 K, the slope of the linear regression equation of y=0.0289 x+0.4798 （R²=0.9999）is 0.0289 nm3, which reveals the average value of per methylene contributes to cation volume. The above results were similar with the previous reports of alkyl ionic liquids by Glasser, L. [35].

22

ACCEPTED MANUSCRIPT Table 3 Density ρ, Molecular Volume Vm, Lattice Energy UPOT, Standard Entropy S0 at 298.15 K and Thermal Expansion Coefficient αp for Five MILs M/(g·mol-1)

ρ/(g·cm-3)

Vm/(nm3)

UPOT/(kJ·mol-1)

S0(J·moL-1·K-1)

αp(10-3K-1)

MIL1

341.17

1.1755

0.4820

403.04

630.25

2.51

MIL2

369.21

1.1418

0.5370

392.45

698.83

2.50

MIL3

383.22

1.1223

0.5670

387.25

736.31

MIL4

397.24

1.1091

0.5947

382.78

MIL5

411.25

1.0944

0.6240

378.35

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MILs

2.54

770.84

2.54

807.34

2.65

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The uncertainty was ± 0.01 for the values of UPOT, S0, αp and ± 0.001 for values of Vm.

Fig. 9. Plot of molecular volume (Vm) against the number of carbon atoms (nc) in substituent groups of the MILs

3.3.2. Thermal expansion coefficient Thermal expansion coefficient (αp) is used to measure the volume expansion coefficient with the change of temperature. The definition is as following [35]: 23

ACCEPTED MANUSCRIPT  P  (1/ V )(V / T )P   ( ln  / T )P

(9)

Where ρ (g·cm-3) and V (mL) are the density and volume of MILs, represively. T (K) is the thermodynamic temperature. The plot of lnρ against T for five MILs were shown in Fig. 10, and the

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relationship between lnρ and T is approximately linear with the good linear

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correlation (R2>0.99). The slope of the fitting curve is thermal expansion coefficient

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(αp). The obtained experimental data were summarized in Table 3. It is obvious that MILs with different length of side carbon chain have close values of αp but poor

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regularity. Moreover, all of the values of αp are greater than those of the traditional

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organic solvents (αp of pyridine= 10.7×10-4 K-1; αp of n-butanol = 11.07×10-4 K-1)

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[36].

Fig. 10. Plot of lnρ against T for five MILs

24

ACCEPTED MANUSCRIPT 3.3.3. Standard entropy and lattice energy Standard entropy (S0) and lattice energy (UPOT) of magnetic ILs at 298.15 K were obtained according to Glasser empirical equation [35]: (10)

U POT (298.15K) / kJ  mol-1  1981.2(  / M )1/3  103.8

(11)

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S 0 / J  K 1  mol1  1246.5  Vm  29.5

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where Vm (nm3), ρ (g·cm-3) and M (g·mol-1) are the molar volume, density and molar

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mass of magnetic ILs, respectively.

The values of S0 and UPOT at 298.15 K are listed in Table 3. As can be seen in

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Table 3, the values of S0 are between 630 J·moL-1·K-1 to 810 J·moL-1·K-1. Moreover,

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the average contribution value of per methylene to standard entropy is deduced, in this case, 35.42 J·moL-1·K-1. The values of UPOT decrease with the increasing number of

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carbon atoms in substituent groups. The values of UPOT are between 370 kJ·moL-1 to

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410 kJ·moL-1, which leads to a liquid state of MILs around room temperature.

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3.4. Acidity analysis of the MILs

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As can be seen in Table 4, the Brønsted acidity strength of five MILs follows the order of [N

1 1 H 2OH]

[TEMPO-OSO3] > [N

[TEMPO-OSO3] > [N

1 1 3 2OH]

1 1 5 2OH]

[TEMPO-OSO3] > [N

[TEMPO-OSO3] > [N 1 1 2 2OH]

1 1 4 2OH]

[TEMPO-OSO3]. The

anion of five MILs is the same. So the acidities of five MILs depend on their cations. Except for [N

1 1 H 2OH]

[TEMPO-OSO3], the Brønsted acidity strength shows a slight

increase with the elongation of carbon chain linked on nitrogen atom of the cholinium cation. The electron-donating ability of α-carbon decreases with the elongation of 25

ACCEPTED MANUSCRIPT alkyl chain, which results in the increase of positive charge density on the nitrogen. That is to say, the MIL with the longer alkyl chain can release protons easily. Among them, the acidity of [N 1 1 H 2OH] [TEMPO-OSO3] is the strongest. In its structure, one hydrogen atom is linked on the nitrogen atom of the cation, which indicates that it has

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PT

a stronger electron-donating ability and relatively higher Brønsted acidity.

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Table 4 Brønsted Acid Strength of Cholinium-Based MILs at 298.15 K. Amax(AU)

[I](%)

[HI](%)

H0

p-nitroaniline

1.493

100

0

-

[N 1 1 H 2OH] [TEMPO-OSO3]

1.291

13.53

1.796

[N 1 1 2 2OH] [TEMPO-OSO3]

1.401

93.84

6.16

2.173

[N 1 1 3 2OH] [TEMPO-OSO3]

1.394

93.37

6.63

2.139

1.386

92.83

7.17

2.102

1.374

92.03

7.97

2.052

D

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86.47

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[N 1 1 4 2OH] [TEMPO-OSO3]

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MILs

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[N 1 1 5 2OH] [TEMPO-OSO3]

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4. Conclusions

A series of novel cholinium-based MILs were first successfully synthesized and identified by FT-IR, HPLC, UV-Vis,

1

susceptibility presents in the order of [N

H NMR and ESI-MS. Their magnetic 1 1 H 2OH]

[TEMPO-OSO3] < [N

1 1 2 2OH]

[TEMPO-OSO3] < [N 1 1 3 2OH] [TEMPO-OSO3] < [N 1 1 4 2OH] [TEMPO-OSO3] < [N 1 1 5 2OH]

[TEMPO-OSO3]. These organic MILs can respond to an external magnetic

field after the formation of aqueous two-phase system and are expected to be applied 26

ACCEPTED MANUSCRIPT in the more fields, especially extraction and separation by using the aqueous-two phase system. Basic data of their physical prosperities could provide useful reference for the design of more new organic MILs and more different functional groups can be

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used to tune these prosperities.

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Acknowledgements

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Preparation of this paper was supported by the National Natural Science

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Foundation of China (No. 81673316).

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Supplementary material

ESI-MS spectra of the five MILs and the relationship curves between electrical

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conductivity and its concentration are presented in the Supplementary material. In addition, the data of measured molar conductivity of the five MILs aqueous solutions

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References

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ACCEPTED MANUSCRIPT

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Graphical abstract

33

ACCEPTED MANUSCRIPT Highlights: 1. A novel series of cholinium-based organic MILs were firstly synthesized. 2. The structures of these MILs were identified by FT-IR, HPLC, UV-Vis, 1H NMR and ESI-MS.

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3. The MILs can respond to an external magnetic field after the formation of aqueous

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two-phase system.

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4. The conductivities, acidities and densities of MILs were measured and discussed.

34