Journal of Molecular Structure 1058 (2014) 244–251

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Journal of Molecular Structure

journal homepage: www.elsevier .com/locate /molstruc

Hydrogen bonding between acetate-based ionic liquids and water: Threetypes of IR absorption peaks and NMR chemical shifts change upondilution

0022-2860/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.molstruc.2013.11.010

⇑ Corresponding author. Tel.: +86 10 62514925; fax: +86 10 62516444.E-mail address: tcmu@chem.ruc.edu.cn (T. Mu).

Yu Chen, Yuanyuan Cao, Yuwei Zhang, Tiancheng Mu ⇑Department of Chemistry, Renmin University of China, Beijing 100872, PR China

h i g h l i g h t s

�Water would affect the structure and property of acetate-based ionic liquids (AcILs) through hydrogen bonds.� There are three types of change in chemical shifts of 1H NMR in AcILs–water systems.� There are three types of change in IR absorption peaks in AcILs–water systems.

a r t i c l e i n f o

Article history:Received 27 September 2013Received in revised form 5 November 2013Accepted 5 November 2013Available online 16 November 2013

Keywords:Acetate-based ionic liquidHydrogen bondingWater

a b s t r a c t

The hydrogen-bonding interaction between acetate-based ionic liquids (AcIL) and water was investigatedby attenuated total reflection infrared (ATR-IR) and 1H NMR. Interestingly, the relative change of chem-ical shift d of 1H NMR upon dilution could be divided into three regions. All the H show an upfield shift inRegions 1 and 2 while a different tendency in Region 3 (upfield, no, and downfield shift classified as Types1, 2, 3, respectively). For ATR-IR, the red, no, or blue shift of mOD (IR absorption peak of OD in D2O) and m±

(IR absorption peak of AcILs) also have three types, respectively. Two-Times Explosion Mechanism(TTEM) was proposed to interpret the dynamic processes of AcILs upon dilution macroscopically, mean-while an Inferior Spring Model (ISM) was proposed to help to understand the TTEM microscopically, Allthose indicate that AcILs present the state of network, sub-network, cluster, sub-cluster, ion pairs andsub-ion pairs in sequence upon dilution by water and the elongation of hydrogen bonding betweenAcILs–water, between cation–anion of AcILs is plastic deformation rather than elastic deformation.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Ionic liquids (ILs) are organic salts with the melting point nor-mally less than 100 �C. ILs are deemed as ‘‘designer solvents’’ dueto their tunability by modifying the cation, anion or both [1,2].They are also regarded as ‘‘green solvents’’ because of their low va-pour pressure [3,4]. Thus ILs have been widely studied in organicsynthesis and reaction [5–10], electrochemistry [11], emulsionsand nanocrystal [12,13], extraction and separation processes[2,14–18], and so on. Among all the ILs investigated, acetate-basedILs (AcILs) are particularly important because they have high disso-lution of biomass (e.g., cellulose, chitosan) [19,20]. AcILs are con-sidered as one of the best choose for dissolving cellulose becauseof their desirable properties (e.g., low toxicity, low melting point,low corrosiveness, low viscosity and favourable biodegradability)[21]. For example, [EMIM][Ac] has already been applied to dissolvecellulose by BASF [22].

ILs (either hydrophobic or hydrophilic) absorbs some waterfrom the moisture air [23–28]. The presence of water affects themicroscopic structure [29–37], physical property [38–41] andchemical reactivity, hence the application of ILs [42–44]. The AcILshave higher hydrophilic anions, which makes the AcILs morehygroscopic than other ILs [25]. Since AcILs are so important in bio-mass dissolution, the investigations of their hygroscopicity andinteraction mechanism with water are necessary. Up to now, thereports on this topic are scare. [BMIM][Ac] and [BPy][Ac] could ab-sorb 15.36 and 11.85 g/g H2O/ILs from the air within 3 h, at a con-trolled temperature 23 �C and relative humidity 52% [25].[BMIM][Ac] is the most hygroscopic ILs ever reported [25].

For other kinds of ILs (excluding AcILs), their interaction(including hydrogen bonds) mechanism with water was enor-mously studied [29–37]. However, the hydrogen-bonding interac-tion between AcILs and water was seldom reported, which wouldbe described as below. The interaction mechanisms betweenAcILs (i.e., [EMIM][Ac], [HMIM][Ac], [P4444][Ac]) and water wereinvestigated by ab initio methods and pulse field gradientNMR spectroscopy [45]. Density functional theory and infrared

0.0 0.2 0.4 0.6 0.8 1.00

2

4

6

8

10

12relative change

C2 C4 C5 C7C6 C8 Cb

xD2O

absolute change

0.0 0.2 0.4 0.6 0.8 1.0-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

Δδ/ p

pm

xD2O

(a)

10.0

10.5

11.0relative change

C2-[EMIM][Ac] C2-[BMIM][Ac]C2-[HMIM][Ac] C2-[OMIM][Ac]

absolute change

-1.0

-0.5

0.0

(b)

δ/ p

pmpm pm

Y. Chen et al. / Journal of Molecular Structure 1058 (2014) 244–251 245

spectroscopy were used to investigated the interaction between[EMIM][Ac] and water [46]. Another study conducted by Rieset al. found that [EMIM][Ac] interacted with water without the for-mation of new compounds by measuring the density, viscosity, dif-fusion and NMR relaxometry [47]. But the concentration of AcILs–water mixtures is limited, i.e., not in the entire range. Moreover, 1HNMR spectra on each H in the AcILs and water were not reported.The hydrogen bonding among cation, anion, and water need to bemore systematically investigated.

Herein, the interaction mechanisms of AcILs with water in thewhole concentration range were studied by 1H NMR and attenu-ated total reflection infrared (ATR-IR), which is helpful to under-stand the dynamic solvation processes. There are three types ofchange (i.e., Type 1, 2, 3 for NMR and type 10, 20, 30 and 100, 200, 300

for IR) for the chemical shift of 1H NMR and wavenumber shift ofIR for AcILs upon dilution. A Two-Times Explosion Mechanism(TTEM, for macroscopic analysis) and an Inferior Spring Model(ISM, for microscopic analysis) were proposed to explain the solva-tion processes of AcILs in water. Five AcILs varying in alkyl chainlength ([EMIM][Ac], [BMIM][Ac], [HMIM][Ac], [OMIM][Ac]) andC2 methylation ([BMIM][Ac], [BMMIM][Ac]) (Table S1) wereselected.

0.0 0.2 0.4 0.6 0.8 1.0

8.5

9.0

9.5

xD2O

0.0 0.2 0.4 0.6 0.8 1.0-2.5

-2.0

-1.5

xD2O

δ/ p

Δδ/ p

Fig. 1. Absolute change and relative change in chemical shift d of 1H NMR spectra of[EMIM][Ac]–D2O system for [EMIM][Ac] (a), and C2H as a function of waterconcentration (b).

3200 3000 2800 2600 2400 1600 1560 1520

0.0

0.4

0.8

1.2

1.6C=N

O-D

C2-H Alkyl-C-H

C4/5-H

wavenumber / cm-1

Abs

orba

nce

0.19910.40440.58390.80200.9803

2492

Fig. 2. IR spectra [EMIM][Ac]–D2O system. The blue and red arrows represent blueand red shifts of IR absorption peak, respectively; the green arrow represents thechange of absorbance. (For interpretation of the references to colour in this figurelegend, the reader is referred to the web version of this article.)

2. Experimental section

The five AcILs (Table S1) investigated in this study were pur-chased from Lanzhou Greenchem ILs, LICP, CAS (Lanzhou, China)with a purity >98.5%. They were dried in a vacuum oven at 50 �Cfor 96 h. After the drying, water contents (less than 310 ppm, mea-sured by Karl Fisher titration, ZDJ-400S Multifunctional titrator,Beijing Xianqu Weifeng Company, Beijing, China), halogen ion(undetectable, measured by AgNO3 precipitation) and other impu-rities (undetectable, measured by NMR, Bruker AM 400 MHz spec-trometer) were determined. D2O (99.9 atom% D) was purchasedfrom J & K Chemical Limited and was used as received.

A certain concentration interval of AcIL–D2O binary mixtureswere prepared by weighing methods. The mole fractions of D2Oin AcIL–D2O mixtures vary from 0 to 0.8 at an interval of 0.2 andthen 0.98 mol fractions at last. Afterwards, the ATR-IR Spectra (asnext experimental method mentioned) and 1H NMR spectra weremeasured. 1H NMR spectra was conducted on a Bruker DMX 300NMR spectrometer (300 MHz) at 298 K and was calibrated byTMS, which is dissolved in CCl4 (the volume fraction of TMS isabout 5%) [30,35]. The NMR spectrum was depicted in Figs. 1, S1,and S2.

The IR spectra from 400 to 4600 cm�1 were recorded at roomtemperature (�25 �C) using a Prestige-21 FTIR spectrometer (Shi-madzu, Japan), equipped with a DTGS detector. For each sample,40 scans were made with a resolution of 4 cm�1. Multiple ATR cell(ATR-8200H) employed in the experiments was made of ZnSe crys-tal with incident angles of 45�. The AcIL-D2O mixtures were placedon top of the crystal to cover the 584 mm2 surface. After everymeasurement, ATR correction was made to allow a quantitativeanalysis of the spectra. The IR spectrum was depicted in Figs. 2and S3.

3. Results and discussion

3.1. Proposed mechanisms and models

In this study, the complicated interactions among the cation,anion and water upon dilution were investigated. Two-TimesExplosion Mechanism (TTEM), and Inferior Spring Model (ISM)were proposed to explain the water sorption process.

3.1.1. Three types 1H NMR and ATR-IR of AcILs upon dilutiondCH is defined as the 1H NMR chemical shift of hydrogen teth-

ered at the carbon C on AcILs, which is described as absolute andrelative change with water concentration. The notations of hydro-gen atoms are present in Table S1. dC2,20 ,4,5,6,7,x,bH are 1H NMRchemical shifts of corresponding hydrogen atoms in the AcILs.The relative change of upfield shift of AcILs upon dilution is or-dered as: DdCbH < DdCxH < DdC6,7H < DdC4,5H < DdC20H < DdC2H (Figs. 1,S1 and S2). The change of dCH with water concentration can be di-

246 Y. Chen et al. / Journal of Molecular Structure 1058 (2014) 244–251

vided into three regions, Region 1 (low concentration), Region 2(medium concentration), and Region 3 (high concentration), asshown in Scheme 1a. There are three types of DdCH. All types godownward in Region 1, then go downward faster in Region 2, whilewith different tendency in Region 3: go downward slower (type1),keep almost constant (type 2) and go upward (type 3). According tothe classification, dC2H, dC20H and dC4,5H are type 1, dCbH and dCxH aretype 3 for all AcILs, while dC6,7H shows different types in differentAcILs. For C6H, [EMIM][Ac] and [BMMIM][Ac] are type 3, [BMI-M][Ac], [HMIM][Ac] and [OMIM][Ac] are type 1. For C7H, [EMI-M][Ac] and [BMMIM][Ac] are type 3, [BMIM][Ac] are type 2,

(a)

(b)

(c)

Scheme 1. The relative change of 1H NMR chemical shift Dd and IR absorption spectra ofExplanation for the Dd of AcILs by the interactions. � ,+ , and w indicate anion, cation, awater, anion and water, and cation and anion, respectively. (c) Explanation for the Dm b

[HMIM][Ac] and [OMIM][Ac] are type 1 (Scheme 1a). This classifi-cation of the dCH is also depicted in Table 1.

The wavenumber of IR absorption peak is denoted as m, symme-try stretching and asymmetry stretching are abbreviated as s andas, respectively. For example, mOD is the IR absorption peak of ODin D2O, m± is the IR absorption peak of AcILs. The wavenumber ofIR absorption peak were ordered as: mC4,5H > masC2H > msC2H > -mC6,7xH > mOD > mC@N (Figs. 2 and S3). The wavenumber may havea blue shift (mCH, including mC2H, mC20H, mC4,5H in the imidazoliumring, mC6,7,xH in the alkyl substitute, i.e., the alkyl hydrogen), no shift(mOD), and red shift (mC@N); the peak intensity may increase (mOD,

O–D Dm compared to pure AcIL with water concentration. (a) Three types of Dd. (b)nd water, respectively. f+w, f�w, and f+� denote the interactions between cation andy the interactions.

Table 1Proposed explanation for the change of 1H NMR chemical shift for AcILs upon dilution.

Type H AcILs Water concentration

[EMIM][Ac] [BMIM][Ac] [HMIM][Ac] [OMIM][Ac] [BMMIM] [Ac] Region 1 Region 2 Region 3

Type 1 C2Hp p p p

d;;; d;;; d;;;C4H

p p p p pd;;; d;;; d;;;

C5Hp p p p p

d;;; d;;; d;;;C20H

pd;; d;; d;;

C6Hp p p

d;; d;; d;;C7H

p pd;; d;; d;;

Type 2 C7Hp

d;; d;; d?

Type 3 C6Hp p

d;; d;; d"C7H

p pd;; d;; d"

CbHp p p p p

d; d; d"CxH

p p p p pd; d; d"

Y. Chen et al. / Journal of Molecular Structure 1058 (2014) 244–251 247

mC2H, mC4,5H), keep constant (mC@N), or decrease (mC6,7,xH); and thepeak width may become widen (mOD, mC2H, mC4,5H,), constant (mC@N),or narrow (mC6,7,xH) (Figs. 2 and S3).

In this paper, f is used to denote hydrogen bonding interaction.f+- expresses the hydrogen bonding interaction between the cation(all of its CH, C2H, C4,5H, C6,7H, CxH) and the anion (all of its O).f+w the hydrogen bonding interaction between cation and water(its O). f�w indicates the hydrogen bonding interaction between an-ion (all of its O) and water (its D). fww implies the hydrogen bond-ing interaction between water molecules. fw± is the hydrogenbonding interaction between water and AcILs, which is the sumof f+w and f�w.

DdCH is determined by the overall effects of f+� (dCH;) and f+w

(dCH") for C2, 4, 5, 6, 7, xH in the cation, f+� (dCH;) and f�w (dCH")for CbH in the anion upon dilution by the water (Scheme 1b andTable 1). Specifically, greater hydrogen bonding interaction in-duces a lower electron density of hydrogen, thus a downfield shift(i.e., a high value of chemical shift d), and vice versa. As to the H inthe cation, taking C2H as an example, fC2H� decreases (dC2H;, up-field shift) and fC2Hw increases (dC2H", downfield shift) upon dilu-tion. If the dC2H" is greater than dC2H;, the overall effects of dCH

would be dC2H", and vice verse. As to the H in the anion, due tothe poor proton donor ability of CbH in the methyl of anion, it isassumed existing no hydrogen bonding interactions with water,other anion, or cation, therefore the DdCbH is only determined bythe overall effect of f+� and f�w; or one can say that the dCbH is onlycaused by the electron density of O of the anion. So the f+� de-creases (dCbH;) and the f�w increases (dCbH") upon dilution. If pureAcIL is considered, only the f+� decrease affects dCbH due to the ab-sence of f�w, so dCbH could be used to represent the f+� of pureAcILs, which we will discuss later.

The shift of mOD in water upon dilution is also determined by theoverall effects of two opposite hydrogen bonding interactions, i.e.,fww (mOD;, red shift) and fw± (mOD", blue shift). A stronger hydrogenbonding interaction induces a red shift of mOD and vice versa. Spe-cifically, fww increases and fw± decreases upon dilution. The shift ofmOD is determined by the dominating interaction. Type 10 (redshift), type 20 (no shift), and type 30 (blue shift) of mOD refer tothe situation where mOD; > mOD", mOD; � mOD", and mOD" > mOD;,respectively (Scheme 1c).

The change of m± (i.e., m except for mOD of AcILs) is also caused bythe overall effects of f+� and f+w for the absorption bands in the cat-ion, and of f+� and f�w for the absorption bands in the anion. Redshift, no shift, and blue shift of m± are classified into type 100, 200,300, respectively (Scheme 1c).

3.1.2. Two-Times Explosion Mechanism macroscopicallyA Two-Times Explosion Mechanism (TTEM) (Scheme 2) was

proposed to explain the solvation process of AcIL. The first explo-sion refers to the explosion of AcIL network to clusters. The second

explosion is the explosion of AcIL cluster to ion pairs with morewater. It is indicated by DdCH of the 1H NMR at the water concen-tration around 80 mol.% (Figs. 1, S1 and S2). This process is also evi-denced with the two-dimensional correlation IR spectra and excessproperties [48,49].

The relative change of chemical shift d of AcILs upon dilutionshows two turning points (Figs. 1, S1 and S2) and could be summa-rized into three types (Scheme 1a). Each type has three regions(Regions 1–3) and two turning points (turning points 1 and 2).The two turning points correspond to the two explosions; the Re-gions 1, 2, 3 correspond to the network, clusters and ion pairs,respectively. The three types have a similar turning point 1, indi-cating that the network of the AcILs have similar stability, sobreaking the network needs similar force. For the turning point2, a significant difference occurs. Type 1 remains downward ten-dency while with slower speed; type 2 keeps constant after turningpoint 2; type 3 goes upward. It indicates that significant differencesexist among the clusters of the five AcILs.

The differences among these types could be explained as fol-lows in Scheme 1b. Region 1 is the expansion of network, whichis caused by more reducing of f+� (dCH;) than the increasing offw± (dCH") upon dilution. The overall chemical shift is expected tobe lower, which is in consistent with the TTEM and experiments.Simply, dCH; overpowers dCH" in this region. At turning point 1,the network explodes to clusters, and the expansion of the clustersresults in Region 2, where the overall chemical shift is also lowerwith steeper slope. Namely, the dCH; still overpowers in this regionwith more dominating effect. At turning point 2 the cluster ex-plodes to ion pairs, the ion pairs continued to expand with theincreasing of water, leading to Region 3.

The AcILs in Region 3 are extremely diluted. The chemical shiftsin Region 3 may present downward (in type 1), constant (in type2), or upward (in type 3) (Scheme 1a and Table 1). For type 1 in Re-gion 3, the chemical shift keeps decreasing with the increasing ofwater concentration but with a slower speed (e.g., dC2H, dC4,5H), itis because not enough f+� (dCH;) exists to reduce than the less di-luted AcILs and fw± (dCH") keeps increasing but at a slower rate.In the case that the two effects match well, a constant chemicalshift occurs, i.e., type 2 in Region 3 (e.g., dC7H of [BMIM][Ac]). Ifthe increase of fw± is more than that of f+�, an upward chemicalshift appears, which is the type 3 in Region 3 (e.g., dCbH, dCxH,

dC6,7H), where dCH" dominates.As AcIL is extremely diluted, the chemical shift of the same H for

different ILs converges, indicating the same water environment(Fig. S2). Actually, in this case, the cations are far away from the an-ions, resulting in a similar interaction. However, several discrepan-cies also exist. (i) for [BMMIM][Ac], the chemical shifts of dC4,5H,dC6,7H, dCbH (except dC8H) in very diluted solution do not convergewith the same hydrogen of other AcILs ([EMIM][Ac], [BMIM][Ac],[HMIM][Ac], [OMIM][Ac]) (Fig. S2). This discrepancy suggests the

248 Y. Chen et al. / Journal of Molecular Structure 1058 (2014) 244–251

importance of C2 methylation at the imidazolium cation on theproperties of AcILs and its interaction with water. (ii) The chemicalshifts of the terminal H (including C8H of [EMIM][Ac], C10H of[BMIM][Ac], and C12H of [HMIM][Ac]) do not converge with thatof other AcILs (Fig. S2). (iii) The dCH of AcILs with the relative longchain length are similar: dC9H � dC10H � dC11H (for [HMIM][Ac]), anddC9H � dC10H � dC11H � dC12H � dC13H (for [OMIM][Ac]) (Fig. S2). Itmakes dC9H of [OMIM][Ac] different from the others (Fig. S2);dC10H of [OMIM][Ac] in extremely diluted solution is different fromthat of [HMIM][Ac], which are further different from that of [BMI-M][Ac] and [BMMIM][Ac] (Fig. S2); dC12H of [OMIM][Ac] is differentfrom that of [HMIM][Ac] (Fig. S2). Fig. S2 also shows that in Region3 the terminal H has greatest dCH" and [EMIM][Ac] has the drasticdCH".

3.1.3. Inferior Spring Model (ISM) microscopicallyThe Inferior Spring Model (ISM, Scheme 3) is proposed to ex-

plain the TTEM microscopically. It implies that the cation–anionresembles a big inferior spring, which is composed by many smallinferior springs, such as CH–anion hydrogen bonding interaction.Taking a cue from the principle of superposition, it could be de-scribed by Eq. (1):

Sþ� ¼ w1SC2H� þw2SC4;5H� þw3SC6;7H� þw4SCxH� ð1Þ

where S+� and SCH� denote the big spring and the partial spring,respectively; wi denotes the contribution of the partial spring tothe big spring. The water–water hydrogen bonding interaction fww

could also be deemed as inferior spring. We call it inferior becauseit do not comply with the Hooke’s law strictly. This inferior springimplies that it is easier to have plastic deformation when it extends,

Scheme 2. Proposed two-times explosion mechanism for the interaction of the AcIL–D2Oto colour in this scheme legend, the reader is referred to the web version of this article

then a more drastic plastic deformation when it extends longer(Scheme 3). It could be corroborated by the semi-empirical investi-gations of AcILs as more water is added (Scheme S1).

Here we take cation–anion spring as an example. At low waterconcentration, strong external force (i.e., little water) is needed toelongate the spring to expand the AcIL network. It could be vindi-cated by the low slope k1 for the line of f+� (Region 1+�, k1+� < 0).Turning point 1+- occurs when the spring elongates to some extent,when it suffers from the first plastic deformation from network toclusters. After that point, fewer efforts are needed to make thespring continue to stretch, i.e., expansion of clusters. Thus we cansee a steeper region with a higher slope k2+- for the line of f+- (Re-gion 2+�, k2+� � 0). After a second plastic deformation (turningpoint 2+�), only minor interaction between the cation and anionexists. The slope k3+- thus shows a slower downward tendencyor flat (Region 3+�, k3+� � 0) (Scheme 1a and 3).

For the Inferior Spring Model, the spring could resist moreexternal forces when the spring is strong enough. For example,the C2H–anion spring is very strong, which could be corroboratedby optimal geometry of the five AcILs conducted by semi-empiricalway (Scheme S2). In this case, Region 3 takes place only when theexternal force is strong enough, i.e., extremely water concentra-tion. This spring would stretch in most of the cases, or we cansay that it is difficult to be totally broken. Thus the slope of Region3 keeps a similar slope as Region 2.

More importantly, the springs between cation–anion and cat-ion–water, cation–anion and anion–water, or water–water andwater–ILs, are different. For example, if the cation–anion spring(e.g., C2H–anion spring) is less inferior to the cation–water spring(C2H–water spring), then different types of change in relative 1H

system. The filled green circle represents water. (For interpretation of the references.)

Scheme 3. Proposed Inferior Spring Model (ISM) for the change in hydrogen bonding strength between cation and anion of AcILs formed by C2 (a), C4, 5 (b), C6, 7 (c), and C6,7, b, x (d), upon dilution. C2, 4, 5, 6, 7, x form hydrogen bonds with the anion; Cb forms hydrogen bond with the cation. Cx indicates the remaining hydrogen atoms in thecation expect for C2, 4, 5, 6, 7, b, i.e., C8, 9, 10, 11, 12, 13, and 14.

Y. Chen et al. / Journal of Molecular Structure 1058 (2014) 244–251 249

NMR chemical shift or IR absorption peak may occur. The cation–anion interaction decreases at almost the whole water concentra-tion, while cation–water interaction increases to be flat when littlewater is added. So the overall interaction decreases because therate changed less with the increase of water concentration.

3.2. Interaction mechanism of AcIL with water

3.2.1. Water�AcIL interactionThe steady-state water sorption capacity indicates an overall

water–AcIL interaction including hydrogen bond and van derWaals force. Greater water sorption capacity means strongerwater–AcIL interactions. In this sense, the order of overall water–AcIL interaction is: [EMIM][Ac] > [BMMIM][Ac] > [BMIM][Ac] > [H-MIM][Ac] > [OMIM][Ac]. While hydrogen bond interaction indi-cated by 1H NMR chemical shift and IR absorption spectra is justone part of the overall interaction [30,34,35].

The strength of hydrogen bond between AcIL and water couldalso be derived from the following equation, i.e., Eq. (2) [23].

DH ¼ �80ðmvapourasOH � mILs

asODÞ=mvapourasOD ð2Þ

Fig. S4 shows that the strength of hydrogen bond between AcILand water follows the order: [BMIM][Ac] > [HMMIM][Ac] > [BMMI-M][Ac] > [EMIM][Ac] � [OMIM][Ac], which is obtained from mea-suring IR absorption spectra ofmOD and calculating followingEq.(2). It is different from the overall interaction between AcILand water, indicating that hydrogen bond is only part of the overallinteraction.

It is shown that [BMMIM][Ac] absorbs more water than [BMI-M][Ac] at the same conditions. From the point view of hydrogenbond, [BMIM][Ac] has a C2H while [BMMIM][Ac] has a methyl H(C20H) instead. The C2H provides more opportunity for [BMIM][Ac]to absorb water by the hydrogen bond between C2H and waterthan C20H and water. However, the result is opposite. The possiblereason is that [BMMIM][Ac] with C2 methylation might have morefree volume to load water compared to the non-C2 methylationcounterpart [BMIM][Ac]. In other words, the fragile [BMMIM][Ac]could be attacked by water molecule easily, thus higher hygroscop-icity occuring. The conclusion is useful for designing ILs: decreas-ing interaction between cation and anion tends to obtainhydrophilic ILs, while increasing that interaction favours hydro-phobic ILs for the water-proof materials which could be safely usedin the moisture air.

Hydrogen bond interaction between AcIL and water could alsobe observed by the absorption wavenumber and intensity of IRspectra (Figs. 2 and S3). C2H and C4,5H are slightly blue shift with

less broadening peak width and increasing peak height upon dilu-tion, indicating the formation of less stronger hydrogen bond withwater, mainly the m" during the dilution process. CalklyH is slightlyblue shift with narrowing peak width and decreasing peak heightupon dilution, it means that less hydrogen bonding is formed. Interms of C@N, only red shift upon dilution, which might be causedby the slacking of pack of the imidazolium ring.

3.2.2. Water�water interactionAccording to the TTEM, in case the water concentration is extre-

mely low (x ? 0) (Region 1), the water molecules do not formwater clusters or network themselves while interact with AcIL. Inthat sense, the water–water interaction (fww) goes to minimum.In case the water concentration is extremely high (x ? 1) (Region3), the water molecules form network, the maximum of fww exists.In case the water concentration is moderate (Region 2), the watermolecules form clusters themselves and interact with AcILsimultaneously.

The process could be described as follows. In this case we mayassume ILs is the solvent which is the opposite process of addingwater to ILs. Namely, it is hard to break hydrogen bond networkof water with little AcIL. As more AcIL is added, the water networksare expanded (Region 10) to water clusters (turning point 10), thenwater clusters would keep expanding (Region 20) until they arebroken by the AcIL (turning point 20). The water molecules con-tinue to be separated (Region 20) as they are surrounded by moreAcIL.

However, mOD measured by IR is the overall effect of fww and fw±.The mOD might show three types of shifts: red shift (type 10), noshift (type 20), or blue shift (type 30). Those shifts have been re-ported by other researchers too. The mOD of [EMIM][BF4] showeda red shift upon dilution [48]. The mOH of ILs such as [BMIM][Tf2N]with minimum water presented no shift when exposed to moistureair [23]. The mOH of [BMIM][TFA] [30], [BPy][BF4] [34] and[EMIM][ES] [35] showed blue shift upon dilution. The differenceof the IR absorption shift of mOD reflects that the fww and fw± of dif-ferent systems are different. For the five AcILs investigated, no shiftof mOD occurs when water is added. It means that for AcILs thechange of fww (increasing) and fw± (decreasing) is equally matchedwith water concentration (type 20).

3.2.3. Cation–anion interactionThe cation–anion interaction (f+�) is deemed as a big spring,

including several small springs, as suggested in Inferior SpringModel. The five AcILs investigated have a same anion and the cat-ion is slightly different by varying in alkyl chain length or C2 meth-

250 Y. Chen et al. / Journal of Molecular Structure 1058 (2014) 244–251

ylation. Only one kind of H acceptor dCbH in the anion exists for thepure AcIL. In other words, dCH of the cation is determined by thecation–cation interaction and cation–anion interaction, while dCH

of the anion dCbH is only affected by the cation. This feature impliesthat we can choose dCbH (expressed in absolute change) as an indi-cator of the overall cation–anion interaction in the pure AcIL, asshown in Fig. S4.

Fig. S4 shows that the order of cation–anion interaction is:[EMIM][Ac] > [BMIM][Ac] > [HMIM][Ac] > [OMIM][Ac]. It is be-cause the alkyl chain is hydrophobic and the acetate is hydrophilic;the longer the alkyl chain length, the more hydrophobic of the cat-ion. Another possible reason is that AcIL with longer alkyl chainlength tends to form a more stable network by the stronger vander Waals force among the chains.

The overall water–AcIL interaction ([EMIM][Ac] > [BMMI-M][Ac] > [BMIM][Ac] > [HMIM][Ac] > [OMIM][Ac]) is consistentwith the cation–anion interaction ([EMIM][Ac] > [BMIM][Ac] > [H-MIM][Ac] > [OMIM][Ac]). It indicates that a stronger overall cat-ion–anion interaction results in a lower aggregation of cation,thus a stronger overall water–AcIL interaction, hence a greatersteady-state water sorption capacity. This is inconsistent with Fre-ire’s report [50]. It indicates that water tends to attack the fragileILs network with less aggregation (e.g. [EMIM][Ac]), which hasmore hydrophilic free volume and less hydrophobic free volume,thus could be easier exploded (the first explosion, as proposed byTTEM, expressed in relative change). Conversely, the more stableILs network is hard to be exploded. For the AcIL with the most sta-ble network (i.e., [OMIM][Ac]) even no explosion of network exists(the slopes of Region 1 and Region 2 are identical, turning point 1does not exist.). Another evidence lies in Region 3. The slope of Re-gion 3 is ordered as: [EMIM][Ac] > [BMMIM][Ac] > [BMI-M][Ac] > [HMIM][Ac] > [OMIM][Ac]. A steeper slope in Region 3means a stronger interaction between AcIL and water after the sec-ond explosion. Obviously, this rule is not applicable for the com-parison of the hygroscopicity of [BMIM][Ac] and [BMMIM][Ac],which was mentioned above.

In addition to the overall interaction of the big spring, the par-tial interactions of those small springs were also investigated.Those changes in cation–anion interaction upon dilution could bewell understood by combining the TTEM and ISM. Below we focuson the relative change of dCbH, dCxH, dC6,7H, dC4,5H, and dC2H.

The 1H NMR spectra of the AcILs show that dCbH has the leastupfield shift at little to moderate water concentration (within Re-gions 1 and 2) but has the biggest downfield shift at very highwater concentration (cal. 0.98 mol.%, Region 3). The reason maybe that the dCbH" force is less than the dCbH; in Regions 1 and 2,while overpowers dCbH; in Region 3. The overall dCbH" is the mostpowerful compared to other H at the imidazolium cation becausewater prefers to interact with the anion of ILs.

dCxH (including dC8, 9, 10, 11, 12, 13, 14H), have the second least up-field shift in Regions 1 and 2, and the second highest downfieldshift in Region 3 compared to that of dCbH. The overall effect isthe dCxH; of CxH compared to CbH. The two kinds of force contrib-ute to the second position of CxH among all the CH. Another inter-esting finding is that the farther of H at the alkyl chain from theimidazolium cation (e.g., C14H), the greater of the chemical shiftoccurs. It is due to the higher extent of aggregation of terminal Hthat has less space hindrance of this aggregation [51], hence a moredCxH; upon dilution. Also, the aggregation of terminal H hinderstheir interaction with anion, making another kind of dCxH; negligi-ble upon dilution. Water would interact with CxH, contributing toa dCxH" upon dilution. Those two converse effects might be offsetequally for all the dCxH, which is consistent with the observationthat the first two regions of the CxH are almost the same and witha nearly constant slope. Water tends to interact with the terminalH in the last region where the network or cluster is exploded and

has less hydrogen bond competition from the anion. For the chem-ical shift of terminal H in dCxH, terminal H (i.e., C8H) of the shortestchain ([BMIM][Ac] and [BMMIM][Ac]) is the most dCxH" (Figs. 1, S1,and S2). Terminal H (i.e., C10H) of the moderate chain [HMIM][Ac]is the moderate dCxH" (Figs. 1, S1, and S2. Terminal H of the longestchain [HMIM][Ac] and [OMIM][Ac] is the most dCbH; (Figs. 1, S1,and S2). It indicates that longer chain length results in higheraggregation of AcIL, thus resulting in less dCxH", since higher aggre-gation is not favourable to form hydrogen bond with water.

C6,7H is the third position, less than CxH and CbH (Figs. 1, S1,and S2). Since C6,7H belong to the H in the alkyl chain lengthand differing in their adjacency to imidazolium ring, and that posi-tion of C6,7H is not favourable for their aggregation. For [HMI-M][Ac] and [OMIM][Ac] with longer alkyl chain, C6H shifts toupfield more than C7H in the Regions 1 and 2 but shifts to down-field more than C7H in Region 3. The aggregation of C6H is morestable than that of C7H in the first two regions. It is because theaggregation of C7H is prohibited by the aggregation of CxH,whereas C6H is not. It is also because that C6 has more negativeelectron due to the lack of electron supply by the long alkyl chain,hence has greater van der Walls interaction. There are no aggrega-tions in Region 3, so water mainly interacts with C6H rather thanC7H, because there are more competition from the H nearby theC7H. While for [EMIM][Ac] and [BMIM][Ac] with shorter alkylchain, C6H shifts to upfield more than C7H in all regions becausethe difference between C6H and C7H is minor in the last region,i.e., their interactions with water are more similar than that ofAcILs with longer chain length. For [BMMIM][Ac] with C2 methyl-ation and less alkyl chain length, C6H, C7H, along with C20H havealmost same change of chemical shifts upon dilution, it indicatesthat the environment of those three H is almost the same both toanion and to water. It also implies that the aggregation of AcIL withC2 methylation is less stable, which makes the three H moresimilar.

The above discussion of the five AcILs suggests that the shorterof the alkyl chain length and the more substitution in the imidazo-lium ring (e.g., C2 methylation), the harder for AcILs to aggregate.The longer chain AcILs ([HMIM][Ac] and [OMIM][Ac]) keep thechemical shift of C6,7H upfield in all Regions, whereas the shorterchain AcILs ([EMIM][Ac] and [BMIM][Ac]) keep their chemical shiftupfield only in Regions 1 and 2 while downfield in the Region 3. Itis because water tends to attack the aggregation formed by AcILwith the shorter chain, thus the turning point 2 changes fromdownfield to upfield. C2 methylation AcIL [BMMIM][Ac] has a moreobvious downfield in Region 3 than that of non C2 methylation[BMIM][Ac], which is caused by higher affinity with water due tohigher fragility of its aggregation. Only the data of Region 3 areavailable for comparison because [BMMIM][Ac] cannot be totallymiscible with water in the first two regions. Moreover, C6H is morehydrophilic than C7H. It might be caused by the following reasons:(i) the long alkyl chain and its aggregation are hydrophobic, pre-venting adjacent C7H from interacting with water; (ii) the long al-kyl chain competes with adjacent C7H to interact with water; (iii)C7H interacts less with anion, indicating it interacts less withwater.

C4, 5H is the fourth position, characterizing in moving upfieldin all regions (Figs. 1, S1, and S2) because the reduce of C4,5H–anion interaction is more than the enhance of C4,5H–water inter-action. C4,5H–anion interaction reduces so much because theinteraction between C4,5H–anion is stronger. For the C4,5Hchemical shift of the AcILs, the shortest alkyl chain ([EMIM][Ac])and the C2 methylation ([BMMIM][Ac]) show more obvious turn-ing point 2 because their C4,5H–anion interactions are strongerthan the other three AcILs, and thus could be easier explodedby water. Moreover, C5H is more hydrophilic than C4H similarto that of C6, 7H.

Y. Chen et al. / Journal of Molecular Structure 1058 (2014) 244–251 251

C2H is the last position; it has the most upfield shift in all re-gions (Figs. 1, S1, and S2). It can also be interpreted as inScheme 1b but with a larger difference between the reduce of itsinteractions with anion and with water, than C4, 5H. It reflectsthe greater interaction of C2H–anion and also high sensitivitywhen exposed to water. During the dilution process, the anionmight prevent water from interacting with C2H, or the water haslarger space resistance to interact with C2H. One feature of C2Hchemical shift for the four AcILs ([BMMIM][Ac] has no C2H) is thatthe shortest chain [EMIM][Ac] suffers from a more drastic changein the turning point 2. It indicates the C2H–anion interaction of[EMIM][Ac] is stronger hence could be easier exploded by water.Like the C4, 5H–anion interaction, the C2H–anion interaction isalso partially the reason for the weaker of the overall interactionsbetween cation and anion.

4. Conclusion

The change of chemical shift of H in AcILs was found to followthree types in Region 3. Region 3 is the most important region.More importantly, the interaction mechanism between AcIL andwater is explained by the comparison of two opposite force, whichis well summarized the change of dCH, mOD, and m±. dCH may haveupfield, no, or downfield shift. mOD, and m± may have red, no, or blueshift. The overall shift is determined by the competition of the re-verse effects. The solvation process of AcILs upon dilution is de-scribed by Two-Times Explosion Mechanism. Namely, AcIL formsnetwork first, and then turns to sub-network, cluster, sub-cluster,ion pairs, and sub-ion pairs in sequence as the water concentrationincreasing. Microscopically, Two-Times Explosion Mechanismcould be well understood by the Inferior Spring Model (ISM), whichstresses the importance of plastic deformation.

This conclusion may give hint in the application of ILs-watersystem such as biomass dissolution. Moderate water might behelpful for dissolving biomass due to the decrease of viscosity ofILs through breaking the hydrogen bond between cation and anion.However, if water is excessive, water may form hydrogen bondwith cation or anion, which may compete with biomass. Becausedissolving biomass is mainly through breaking the original struc-ture of biomass by forming hydrogen bond between biomass andILs. Therefore, optimal water contents is needed for IL to dissolvebiomass, which is further study for us.

Acknowledgements

This work was supported by the National Natural Science Foun-dation of China (21173267) and the Fundamental Research Fundsfor the Central Universities and the Research Funds of Renmin Uni-versity of China (12XNLL05).

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.molstruc.2013.11.010.

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