The enhanced dissolution of β-cyclodextrin in some hydrophilic ionic liquids

Article abstract of DOI:10.1021/jp907333v

β-Cyclodextrin (β-CD) is difficult to dissolve in water and in many common solvents and searching for the proper solvents is the key step to expand its application. In this work, six kinds of hydrophilic ionic liquids 1-n-butyl-3-methylimidazolium chloride, 1-(2-hydroxyethyl)-3-methylimidazolium chloride, 1-allyl-3-methylimidazolium chloride, 1-n-butyl-3-methylimidazolium dicyanamide, 1-(2-hydroxyethyl)-3- methylimidazolium dicyanamide and 1-allyl-3-methyl-imidazolium dicyanamide have been prepared. The solubilities of β-cyclodextrin in these ionic liquids have been determined in the temperature range from 333.2 to 363.2 K with 5 K intervals. The solution thermodynamic parameters β-cyclodextrin have been calculated from the solubility data. It was shown that solubility of β-cyclodextrin was remarkable in ionic liquids, it was as high as 125.0 g in 100 g of [Amim][N(CN)₂] at 348.2 K. The dissolution process was unfavorable thermodynamically and controlled by the enthalpic term. ¹H NMR and IR spectroscopic measurements were used to study the enhanced dissolution of β-cyclodextrin in the ionic liquids. The results indicated that 1:1 inclusion complexes were formed between β-cyclodextrin and imidazolium cations of the ionic liquids. The differences in the solubility of β-cyclodextrin have been discussed from the interionic interaction between cation and anion of the ionic liquids and the inclusion interaction of the cations of the ionic liquids into the cavities of β-cyclodextrin. The strength of the interionic interactions was found to be predominant for the dissolution of β-cyclodextrin. It is expected that such information may find application in the molecular design of the stationary phases in gas chromatography.

Full text of DOI:10.1021/jp907333v

3
926
J. Phys. Chem. A 2010, 114, 3926–3931
The Enhanced Dissolution of ꢀ-Cyclodextrin in Some Hydrophilic Ionic Liquids†
‡
‡
§
Yong Zheng, Xiaopeng Xuan, Jianji Wang,* and Maohong Fan
School of Chemistry and EnVironmental Science, Henan Key Laboratory of EnVironmental Pollution Control,
Henan Normal UniVersity, Xinxiang 453007, P. R. China, and Department of Chemical and Petroleum
Engineering, UniVersity of Wyoming, Laramie, Wyoming 82071
ReceiVed: July 30, 2009; ReVised Manuscript ReceiVed: September 14, 2009
ꢀ
-Cyclodextrin (ꢀ-CD) is difficult to dissolve in water and in many common solvents and searching for the
proper solvents is the key step to expand its application. In this work, six kinds of hydrophilic ionic liquids
-n-butyl-3-methylimidazolium chloride, 1-(2-hydroxyethyl)-3-methylimidazolium chloride, 1-allyl-3-meth-
1
ylimidazolium chloride, 1-n-butyl-3-methylimidazolium dicyanamide, 1-(2-hydroxyethyl)-3- methylimidazolium
dicyanamide and 1-allyl-3-methyl-imidazolium dicyanamide have been prepared. The solubilities of
ꢀ-cyclodextrin in these ionic liquids have been determined in the temperature range from 333.2 to 363.2 K
with 5 K intervals. The solution thermodynamic parameters of ꢀ-cyclodextrin have been calculated from the
solubility data. It was shown that solubility of ꢀ-cyclodextrin was remarkable in ionic liquids, it was as high
2
as 125.0 g in 100 g of [Amim][N(CN) ] at 348.2 K. The dissolution process was unfavorable thermodynamically
1
and controlled by the enthalpic term. H NMR and IR spectroscopic measurements were used to study the
enhanced dissolution of ꢀ-cyclodextrin in the ionic liquids. The results indicated that 1:1 inclusion complexes
were formed between ꢀ-cyclodextrin and imidazolium cations of the ionic liquids. The differences in the
solubility of ꢀ-cyclodextrin have been discussed from the interionic interaction between cation and anion of
the ionic liquids and the inclusion interaction of the cations of the ionic liquids into the cavities
of ꢀ-cyclodextrin. The strength of the interionic interactions was found to be predominant for the dissolution
of ꢀ-cyclodextrin. It is expected that such information may find application in the molecular design of the
stationary phases in gas chromatography.
1
. Introduction
Cyclodextrins (CDs) are a series of cyclic oligosaccharides,
which are usually composed of more than 5 R-D-glucopyrano-
side units linked by R-1,4 glycosidic bonds. The structures of
cyclodextrins are alike in that each of them contains a molecular
cavity. The hydroxyl groups are on the outside of the cavity so
that the outside surface is hydrophilic, while the inner cavity is
hydrophobic. Owing to their unique structure, cyclodextrins are
able to accommodate a lot of guest molecules and have been
widely applied in pharmaceutical chemistry, food industry, and
other fields.1 Among them, ꢀ-cyclodextrin (ꢀ-CD) which
contains 7 glucose units (Figure 1) is insoluble in water and in
-3
4
many common solvents. Therefore, searching for proper
solvents and increasing its solubility is the key step to expand
the application of ꢀ-cyclodextrin.
As a new kind of green solvents, ionic liquids (ILs) have a
variety of excellent properties, such as negligible vapor pressure,
nonflammability, thermal and chemical stability, chemical
Figure 1. The structure of ꢀ-CD.
5
-9
tunabilities, and recyclability.
This makes ILs attractive as
the potential novel materials in catalysis, organic synthesis,
9-11
most of the studies are focused on the interactions between ꢀ-CD
and ILs in aqueous solutions. Gao et al.16 reported that ꢀ-CD
separation, and electrochemistry.
Recent studies demonstrate
that ILs are capable of dissolving a number of carbohydrates
including glucose, sucrose, starch, and cellulose among others.1
2-14
and [Bmim][PF ] could form 1:1 inclusion complexes in water
6
with the whole imidazolium cation entering the cavity of ꢀ-CD.
In recent years, although investigations on the interactions
between ꢀ-CD and ILs have been reported in literature,15-22
Further study showed that [C mim][PF ], [C mim][PF ], and
12
6
14
6
[
C
16mim][PF
with ꢀ-CD in water.
alkyl chain of imidazolium ring that entered into the cavity of
6
] could also form 1:1 or 2:1 inclusion complexes
17,18
†
For these three ionic liquids, it was the
Part of the special issue “Green Chemistry in Energy Production
Symposium”.
*
To whom correspondence should be addressed. Tel.: +86-373-3325805.
19,20
ꢀ
-CD. He et al.
with [Bmim]Cl, [Bmim][PF
solutions at 298 K. It was also reported that the anions of some
determined the association constants of ꢀ-CD
Fax: +86-373-3326544. E-mail: jwang@henannu.edu.cn.
‡
], and [C12mim][BF ] in aqueous
Henan Key Laboratory of Environmental Pollution Control.
University of Wyoming.
6
4
§
1
0.1021/jp907333v  2010 American Chemical Society
Published on Web 10/07/2009

Dissolution of ꢀ-CD in Hydrophilic Ionic Liquids
J. Phys. Chem. A, Vol. 114, No. 11, 2010 3927
product was obtained as a white waxlike solid at room-
1
temperature. Yield: 87.4%. H NMR: δ ) 9.446 (s, 1H), 7.840
(
d, 1H), 7.765 (d, 1H), 4.177 (t, 2H), 3.856 (s, 3H), 1.747 (m,
2
H), 1.233 (m, 2H), and 0.873 (m, 3H) ppm.
2
Bmim][N(CN) ]. For the synthesis of [Bmim][N(CN) ], the
[
2
15
procedure reported by Liu and co-workers was followed. The
final product was obtained as a colorless liquid. Yield: 89.4%.
1
H NMR: δ ) 9.099 (s, 1H), 7.758 (d, 1H), 7.694 (d, 1H), 4.162
Figure 2. The structures of imidazolium cations studied in this work.
(t, 2H), 3.849 (s, 3H), 1.769 (m, 2H), 1.265 (m, 2H), and 0.902
(
4
m, 3H) ppm. 13C NMR: δ ) 135.79, 122.77, 121.50, 118.37,
8.38, 35.08, 30.90, 18.35, and 12.30 ppm.
N]- and [NfO] , could form inclusion
-
[Amim]Cl. This IL was synthesized according to the litera-
ILs, such as [Tf
2
24
2
1
ture. The final product was obtained as a yellow viscous liquid.
complexes with ꢀ-CD in water as well. Literature survey
1
1
5
Yield: 96.3%. H NMR: δ ) 9.434 (s, 1H), 7.786 (d, 1H), 7.769
reveals that only one report deals with the direct interaction
between ꢀ-CD and ILs in the absence of any molecular solvent.
Actually, only one solubility datum point was reported at 348
(
(
d, 1H), 6.005 (m, 1H), 5.282 (m, 2H), 4.865 (d, 2H), and 3.853
s, 3H) ppm.
1
5
2
[Amim][N(CN) ]. The IL was synthesized according to the
K
for ꢀ-CD in the ionic liquid [Bmim][N(CN)
2
], although
25
procedure described in literature. The final product was
ꢀ
-CD derivatives dissolved in ILs have been used as the
1
2
2
obtained as a pale yellow liquid. Yield: 92.2%. H NMR: δ )
stationary phases in gas chromatography.
9
(
1
3
.287 (s, 1H), 7.792 (d, 1H), 7.750 (d, 1H), 5.491 (t, 1H), 4.241
In this paper, we present our solubility data of ꢀ-CD in the
hydrophilic ILs (their cationic structures were shown in Figure
t, 2H), 3.873 (s, 3H), and 3.695 (m, 2H) ppm. 13C NMR: δ )
35.89, 130.34, 123.13, 121.58, 120.23, 118.49, 50.66, and
5.39 ppm.
2
): 1-n-butyl-3-methylimidazolium chloride ([Bmim]Cl), 1-(2-
hydroxyethyl)-3-methylimidazolium chloride ([Hemim]Cl), 1-allyl-
-methylimidazolium chloride ([Amim]Cl), 1-n-butyl-3-methyl-
imidazolium dicyanamide ([Bmim][N(CN) ]), 1-(2-hydroxyethyl)-
-methylimidazolium dicyanamide ([Hemim][N(CN) ]) and
-allyl-3-methylimidazolium dicyanamide ([Amim][N(CN) ]) in
[
Hemim]Cl. This IL was synthesized based on the procedure
3
26
reported in literature. The final product was obtained as a white
wax-like solid at room temperature. Yield: 80.7%. H NMR: δ
2
1
3
1
2
)
4
9.287 (s, 1H), 7.792 (d, 1H), 7.750 (d, 1H), 5.491 (t, 1H),
.241 (t, 2H), 3.873 (s, 3H), and 3.695 (m, 2H) ppm.
Hemim][N(CN) ]. A 10 g portion of [Hemim]Cl and 5.6 g
of NaN(CN) were added into 20 mL of acetone. The suspension
2
the temperature range from 333.2 to 363.2 K with 5 K intervals.
On the basis of these data, the standard solution Gibbs energy
[
2
∆
G°
S
, standard solution enthalpy ∆H°
S
, and standard solution
have been calculated for the dissolution of ꢀ-CD.
2
entropy T∆S°
S
was stirred at room temperature for 48 h and then filtered
through G4 glass frit. After the solvent was removed by rotary
evaporation, the residual liquid was diluted with methylene
chloride and filtered again. The filtrate was concentrated by
rotary evaporation and dried under vacuum at 343 K for 48 h.
The final product was obtained as a colorless liquid. Yield:
Thermodynamic analysis has been performed to understand the
controlling factor for the dissolution. The enhanced solubility
1
of ꢀ-CD in these ionic liquids was studied through H NMR
and IR spectra. It was found that 1:1 inclusion complexes were
formed between ꢀ-CD and cations of the ILs. The results have
been discussed from the disruption of the attractive interactions
in the ionic liquids and the formation of the inclusion complexes.
1
76.6%. H NMR: δ ) 9.060 (s, 1H), 7.699 (d, 1H), 7.666 (d,
1H), 5.147 (t, 1H), 4.204 (t, 2H), 3.863 (s, 3H), and 3.718 (m,
13
2
H) ppm. C NMR: δ ) 137.20, 123.74, 123.05, 119.47, 59.72,
52.07, and 36.07 ppm.
.3. Determination of Solubilities. The procedure used for
2
. Experimental Section
.1. Materials. All the chemicals used in this work were
purchased commercially with purity higher than 99.5%. ꢀ-CD,
-methylimidazolium, acetone, and methylene chloride were
obtained from Shanghai Chem. Co., Ltd. 1-Chlorobutane,
-chloroethanol, and sodium dicyanamide were purchased by
Beijing Chem. Co., Ltd., and allyl chloride was from Alfa Aesar.
The liquid solvents were distilled twice before use. Sodium
dicyanamide and ꢀ-CD were recrystallized twice in distilled
water and dried under vacuum for 24 h at 333 K.
2
2
the determination of ꢀ-CD solubilities was similar to that
reported in literature.27 For this purpose, ꢀ-CD (1.00 wt % of
the IL) was added into a 20 mL colorimetric tube which
contained 2-3 g of the dried IL, and the tube was sealed with
parafilm. The colorimetric tube was immersed in an oil bath
(DF-101S, Gongyi Yingyu Instrument Factory, China), and the
bath temperature was controlled to be T ( 0.5 K. The mixture
was heated at a given temperature and stirred under argon
atmosphere. Additional ꢀ-CD (1.00 wt % of the IL) was added
until the solution became completely clear under polarization
1
2
2
.2. Synthesis of the ILs. [Bmim]Cl. The IL was synthesized
2
3
according to the procedure described in literature. The final
TABLE 1: Solubilities of ꢀ-CD in the ILs at Different Temperatures
solubility (gram per 100 g of the IL)
[Amim]Cl [Bmim][N(CN)
T (K)
33.2
38.2
43.2
48.2
53.2
58.2
63.2
[Bmim]Cl
[Hemim]Cl
2
]
[Hemim][N(CN)
2
]
2
[Amim][N(CN) ]
3
3
3
3
3
3
3
a
79.4
87.8
73.0
81.1
90.5
98.4
106.2
114.0
121.5
102.3
109.8
117.4
125.0
132.2
78.4
86.1
94.5
102.5
110.4
69.7
79.4
90.6
99.3
106.7
97.3
89.5
105.9
112.3
97.4
105.1
a
The solubility was not measured at the given temperature because of the high viscosities of the system.

3
928 J. Phys. Chem. A, Vol. 114, No. 11, 2010
Zheng et al.
TABLE 2: Solution Thermodynamic Parameters of ꢀ-CD in
the ILs at Different Temperatures
θ
(kJ mol-1
θ
(kJ mol-1
T∆Sθ
(kJ mol-1
)
T (K)
∆H
S
)
∆G
Bmim]Cl
6.7
S
)
S
[
3
3
3
3
3
43.2
48.2
53.2
58.2
63.2
18.4
19.0
19.5
20.1
20.6
11.7
12.6
13.3
14.1
14.7
6.4
6.2
6.0
5.9
[
Hemim]Cl
6.4
3
3
3
3
3
38.2
43.2
48.2
53.2
58.2
13.7
14.1
14.5
14.9
15.4
7.3
7.8
8.3
8.9
9.5
6.3
6.2
6.0
5.9
[
Amim]Cl
6.4
3
3
3
3
3
33.2
38.2
43.2
48.2
53.2
14.0
14.5
14.9
15.3
15.8
7.6
8.3
8.8
9.3
9.9
6.2
6.1
6.0
5.9
Figure 3. Plots of ln x versus T for ꢀ-CD in the ILs.
microscope (Nanjing Jiangnan Novel Optics Co. Ltd., China).
When ꢀ-CD became saturated, judged by the fact that ꢀ-CD
could not be dissolved further within 1 h, its solubility
[Bmim][N(CN)2]
333.2
14.8
15.2
15.7
16.1
16.6
6.0
5.8
5.6
5.5
5.4
8.8
9.4
10.1
10.6
11.2
3
3
3
3
38.2
43.2
48.2
53.2
(
expressed by gram per 100 g of ionic liquid) at the given
temperature could be calculated from the amounts of the IL
and ꢀ-CD added. For each IL, the solubility values of ꢀ-CD
were measured at five temperatures with 5 K intervals. The
initial temperature for the dissolution was set at 333.2, 338.2,
and 343.2 K according to the melting points of different ILs.
The maximal error in the solubility measurements was in the
[
11.4
2
Hemim][N(CN) ]
3
3
3
3
33.2
38.2
43.2
48.2
5.6
5.5
5.4
5.3
5.2
5.8
6.3
6.7
7.2
7.7
11.8
12.1
12.5
12.9
range from 0.75 to 1.4% depending on the solubility magnitudes.
353.2
1
2
.4. Measurements of NMR and IR Spectra. The H NMR
[
2
Amim][N(CN) ]
and 13C NMR experiments of the ILs were performed on a
Bruker AV-400 MHz spectrometer in DMSO and D O, respec-
tively, at 298 K. In the measurements of H NMR for the
samples of ꢀ-CD in ILs, acetone was used as an external to
suppress the interference from other solvents. It should be
3
33.2
10.0
10.3
10.6
10.9
11.2
5.4
5.3
5.2
5.1
5.0
4.6
5.0
5.4
5.8
6.2
2
338.2
343.2
348.2
1
3
53.2
1
pointed out that for the H NMR investigation of the interaction
between ꢀ-CD and the ILs, only the dicyanamide-based ILs +
increasing temperature. These results indicate that structure of
the cation, nature of the anion, interactions between cations and
anions of the ionic liquids, and the temperature affect the
solubility of ꢀ-CD significantly.
ꢀ
-CD systems were studied since their low viscosity facilitated
1
the H NMR measurements.
IR spectra of ILs with and without ꢀ-CD were recorded on
a Nicolet Nexus IR/Raman spectroscopy at 298 K with a
On the basis of the solubility data, the standard solution Gibbs
-
1
θ
resolution of 2 cm . To reduce the error in the calculation of
vibrational intensity, the ꢀ-CD + IL samples with the highest
concentration of ꢀ-CD were used to study the interaction
between ꢀ-CD and the ILs. The molar ratio of ꢀ-CD to the ILs
was in the range from 0.15 to 0.22.
energy, ∆G , for ꢀ-CD in the ILs can be calculated by the
S
following equation:
θ
S
∆
G ) -RT ln x
(1)
where x stands for the solubility expressed by the mole fraction
of ꢀ-CD, while R and T have their usual meanings. According
to the Gibbs-Helmholtz equation and eq 1, we can obtain the
following equation:
3
. Results and Discussions
.1. Solubilities and the Standard Solution Thermody-
3
namic Parameters of ꢀ-CD in the ILs. Data in Table 1 show
some interesting features for the dissolution of ꢀ-CD in the ILs
under the same experimental conditions: (i) Compared with the
1
5
[
Bmim][N(CN)
of ꢀ-CD have been observed in [Amim]Cl, [Hemim][N(CN)
and [Amim][N(CN) ]. For example, solubility of ꢀ-CD in
is about 32% higher than that in
] at the temperature range investigated. (ii)
2
] used in literature, much higher solubilities
θ
2
d ln x
dT
∆
H ) RT
(2)
S
(
)
2
],
2
[
[
Amim][N(CN)
Bmim][N(CN)
2
]
By plotting ln x versus T, good straight lines are observed in
θ
2
all the cases as shown in Figure 3. Values of ∆H
have been
S
Solubilities of ꢀ-CD in the ILs decrease in the order,
calculated from the slopes of the resulting straight lines and eq
[Amim][N(CN)
2 2 2
] > [Hemim][N(CN) ] > [Bmim][N(CN) ], and
28
θ
2
.
S
The remaining T∆S can be obtained by
[
Amim]Cl > [Hemim]Cl > [Bmim]Cl; (iii) solubilities of ꢀ-CD
in dicyanamide-based ILs are much higher than those in
θ
S
θ
S
θ
S
T∆S ) ∆H - ∆G
(3)
chloride-based ones; and (iv) solubilities of ꢀ-CD increase with

Dissolution of ꢀ-CD in Hydrophilic Ionic Liquids
J. Phys. Chem. A, Vol. 114, No. 11, 2010 3929
∆δ (ppm)
TABLE 3: 1H NMR Chemical Shifts of ꢀ-CD in [Bmim][N(CN)
]
2
δ (ppm)
a
b
c
proton
H-1 (outer)
H-2 and H-4 (outer)
H-3 (inner)
H-5 (inner)
H-6 (inner)
w ) 10%
w ) 15%
w ) 20%
∆δ
1
∆δ
2
4.072
2.598
2.863
4.051
2.574
2.841
not observed
2.789
4.037
2.570
2.823
appear
2.771
4.755
3.428
-0.021
-0.024
-0.022
-0.014
-0.004
-0.018
not observed
2.807
-0.018
-0.040
-0.024
-0.018
-0.047
-0.055
OH-2 and OH-3 (outer)
OH-6 (outer)
4.842
3.507
4.802
3.483
a
]. b ∆δ
c
w stands for the mass fraction of ꢀ-CD in [Bmim][N(CN)
2
1
) δ(w ) 15%) - δ(w ) 10%). ∆δ ) δ(w ) 20%) - δ(w ) 15%).
2
The results have been listed in Table 2. It can be seen that
(OH-2, OH-3,and OH-6) outside the cavity of ꢀ-CD were
found to shift upfield because of the interactions of these
ꢀ-CD protons with anions of the ILs. On the other hand, it
θ
θ
θ
values of ∆G
studied. The positive ∆G
S
, ∆H
S
and ∆S
S
are positive for all the systems
values suggest that interactions
θ
S
+
between the ILs are stronger than those between ꢀ-CD and ILs.
The dissolution of ꢀ-CD in the ILs is unfavorable thermody-
namically. In addition, the positive entropy changes and the
can be seen from Table 4 that all the protons of [Bmim]
shift upfield as the mass fraction of ꢀ-CD increases.
Compared to the protons of the butyl chain, those of the
imidazolium cation have larger chemical shifts under the
same conditions. This supports the conclusion that some
imidazolium cations have entered into the cavities of ꢀ-CD
and have stronger interaction with ꢀ-CD than the protons of
butyl chain. At the same time, the electron densities of
protons of the imidazolium cations decrease due to the partial
disruption of the interionic interactions between cation and
θ
θ
relation |∆H
S
S
| > |T∆S | are always found for the dissolution
process, which indicates that the dissolution processes in the
ILs are entropically favorable and controlled by the enthalpy
term. To enhance the solubility, the increase of temperature is
necessary.
3
.2. The Interactions between ꢀ-CD and the ILs. The
excellent dissolution capabilities of ꢀ-CD in the ILs inspired
us to study the interactions between them. As stated in the
introduction, the formation of inclusion complexes between
2
anion of [Bmim][N(CN) ] by the inclusion of some imida-
zolium cations into the cavity of ꢀ-CD. This also makes the
chemical shifts to change toward upfield.
ꢀ
-CD and ILs has been reported in aqueous solutions. To
examine if the inclusion complexes were also formed in the
The dissolution of ꢀ-CD in the ILs was further investigated
by IR spectra. It is known that infrared absorbance ratio reflects
the relative content of different chemical groups. If the inclusion
complexes between ꢀ-CD and cation of the ILs could be formed,
the relative absorbance of imidazolium ring would decrease
dissolution process, the interaction of ꢀ-CD with
1
[
Bmim][N(CN)
2
] was studied by H NMR as an example.
1
The H NMR chemical shifts for ꢀ-CD in [Bmim][N(CN)
2
]
+
ꢀ-CD were listed in Table 3. Owing to the high viscosity
of the mixture, the peaks of H-2 and H-4 as well as OH-2
and OH-3 protons turned into one broad peak, respectively.
With the increase of the mass fraction of ꢀ-CD in
compared with the anion. Additionally, since the absorbance
-
of the [N(CN)
2
]
anion is intensive and can be easily distin-
2
] and [Bmim][N(CN) ]
guished, IR spectra of [Bmim][N(CN)
2
[
Bmim][N(CN)
peak was observed in the mixture of 20% ꢀ-CD + 80%
Bmim][N(CN) ] (see Figure 4). This suggests that inclusion
complexes between ꢀ-CD and [Bmim][N(CN) ] were really
formed in such a case. The H chemical shift of the protons
H-3, H-5, and H-6) in the inner cavity of ꢀ-CD was
2
], these protons shifted upfield, and the H-5
+ ꢀ-CD (molar ratio of ꢀ-CD to the IL ) 0.19) have been
selected for the study (see Figure 5).
[
2
If the infrared intensity of imidazoliun cation and dicy-
anamide anion of the [Bmim][N(CN) ] is I and I , respec-
2 1 2
tively. According to Lambert-Beer’s law, they can be written
as
2
1
6
1
(
attributed to the shielding effect of cations of the ILs, which
is an indication for the entry of cation of the ILs into the
cavity of ꢀ-CD. Also, the signals of the hydroxyl protons
I ) A b c
(4)
(5)
1
1
1
1
2
I ) A b c
2
2 2
where c
1 2
and c stand for the concentrations of free
imidazolium cation and dicyanamide anion, respectively. It
is known that under the same experimental conditions, the
values of A
and b
, respectively,29 for the identical molecules, and the
ratio a () I
1 1 2
and b are approximately equal to those of A
2
1
/I
2
) can be expressed by
a ) I /I ) c /c
2
(6)
1
2
1
On the basis of the IR spectra of [Bmim][N(CN)
2
] and ꢀ-CD
+
[Bmim][N(CN) ] mixture, the vibrational intensities of CdN
2
group of imidazolium ring (VCdN) and C≡N group of dicyana-
mide anion (VCtN) have been calculated and given in Table 5.
According to the data in Table 5, it could be concluded that the
Figure 4. 1H NMR spectra of ꢀ-CD in [Bmim][N(CN)
mass fractions of ꢀ-CD: (1) 10%, (2) 15%, (3) 20%.
2
] at different

3
930 J. Phys. Chem. A, Vol. 114, No. 11, 2010
Zheng et al.
TABLE 4: 1H NMR Chemical Shifts of [Bmim]+ in ꢀ-CD + [Bmim][N(CN)
2
] Mixture
δ (ppm)
∆δ (ppm)
a
b
c
proton
H-2
w ) 10%
w ) 15%
w ) 20%
8.041
6.736,6.675
3.287
3.004
0.900
∆δ
-0.033
1
∆δ
-0.025
2
8.099
6.787,6.723
3.328
8.066
6.759,6.696
3.306
3.023
0.917
0.388
-0.044
H-4 and H-5
-0.028, -0.027
-0.022
-0.023, -0.021
-0.019
N-CH
2
-
N-CH
3
3.045
0.936
0.404
-0.028
-0.022
-0.019
C-CH
CH
C-CH
2
-C
-0.019
-0.017
2
-Me
0.371
-0.061
-0.016
-0.017
3
-0.016
-0.017
a
]. b ∆δ
c
w stands for the mass fraction of ꢀ-CD in [Bmim][N(CN)
2
1
) δ(w ) 15%) - δ(w ) 10%). ∆δ
2
) δ(w ) 20%) - δ(w ) 15%).
ratio of cation to anion in the pure IL (a
mixture (a
0
) and IL + ꢀ-CD
into two main steps: (i) partial destruction of the attractive
interactions between cation and anion of the ILs; and (ii)
formation of the inclusion complexes between ꢀ-CD and cation
of the ILs. Therefore, the standard dissolution enthalpies of
m
)
a0 ) VCdN/VCtN ) 0.11
(7)
(8)
ꢀ
-CD may be expressed as the sum of two contributions:
am ) VCdN/V
CtN
) 0.09
θ
θ
C-A
θ
∆
H ) ∆H
+ ∆H
(11)
S
C-CD
Therefore, the following equations could be obtained:
where ∆H θC -A represents the enthalpy change for the breaking
of the attractive interactions between cations and anionstions
a /a ≈ 0.82
(9)
m
0
and anions in the ILs, and ∆H θC -CD stands for the enthalpy
change caused by formation of the inclusion complexes between
am
cations of the ILs and ꢀ-CD, that is, the enthalpy of inclusion.
1
- a0 ≈ 0.18
(10)
It is expected that the first term is positive, whereas the second
θ
term is negative. Because the ∆H
S
values determined experi-
mentally are positive, the ∆H Cθ -A term would be predominant.
This suggests that the enthalpies of inclusion are not negative
enough to overcome the positive contribution from the destruc-
tion of the attractive interaction in the ILs.
It is evident that the (1 - a
the molar ratio of ꢀ-CD (0.19) in [Bmim][N(CN)
that some cations of [Bmim][N(CN) ] have entered into the cavities
m
/a
0
) value of 0.18 is very close to
2
]. This indicates
2
of ꢀ-CD completely to form 1:1 inclusion complexes so that the
relative infrared absorbance decreases for the imidazole rings.
Similar conclusions can be obtained for the other ꢀ-CD +
dicyanamide-based ionic liquids systems. Considering the fact that
the chloride-based ILs used in the present work have the same
cations as the corresponding dicyanamide-based ILs, it can be
inferred that ꢀ-CD can form inclusion complexes with all cations
of the studied ILs in the dissolution process.
To form inclusion complex, the geometric compatibility of
ꢀ-CD with the imidazolium cations is a key factor. It has been
+
reported that volumes of the [Bmim] and the cavity of ꢀ-CD
3
1,30
are about 0.196 and 0.262 nm , respectively.
The cavity
diameter of ꢀ-CD is between 0.6 and 0.65 nm, and the width
+
31
of [Bmim] is about 0.58 nm. That is to say that the size of
+
+
[Bmim] and cavity of ꢀ-CD is close to each other, [Bmim]
can be included into the cavity of ꢀ-CD to form the complexes.
+
3.3. The Effects of the Inclusion Complex Formation and
To the best of our knowledge, no size data for [Hemim] and
+
the Interactions between Cation and Anion of the ILs on
the Solubility of ꢀ-CD. From the viewpoint of chemical
thermodynamics, the dissolution process of ꢀ-CD can be divided
[Amim] cations have been reported in literature. However, it
+
can be inferred from their structures that [Hemim] and
+
+
[Amim] should have similar diameters as [Bmim] . Therefore,
it is appropriate to state that no significant differences would
be found in the inclusion enthalpies and entropies for the ILs
investigated, and the main difference in the dissolution of ꢀ-CD
is attributed to the difference in the destruction of the attractive
interactions in these ILs.
It has been reported from density function theory calculations
that the absolute values of interaction energies in some studied
ILs follow the order3
[
2-34
[Bmim]Cl > [Bmim][N(CN)
], so that [Bmim]Cl and [Amim]Cl
are solid, but [Bmim][N(CN) ] and [Amim][N(CN) ] are liquid
2
] and
Amim]Cl > [Amim][N(CN)
2
2
2
at room temperature. This indicates that the attractive interac-
tions in the chloride-based ILs are stronger than those in the
dicyanamide-based ILs. Accordingly, more energies are needed
to break those interactions in the chloride-based ILs so that the
dissolution of ꢀ-CD can be successfully processed. This is a
possible reason why the solubilities of ꢀ-CD in the dicyanamide-
based ILs are higher than in the chloride-based ILs at the same
temperature.
As an approximation, melting points of ionic liquids can be
used for the rough estimation of the relative order of the
Figure5. Infraredspectraof[Bmim][N(CN)
2
]andꢀ-CD+[Bmim][N(CN)
2
]:
(
1) [Bmim][N(CN) ]; (2) ꢀ-CD + [Bmim][N(CN)
2
2
].

Dissolution of ꢀ-CD in Hydrophilic Ionic Liquids
J. Phys. Chem. A, Vol. 114, No. 11, 2010 3931
TABLE 5: Infrared Intensity of the WCdN and WCtN
Vibrational Modes
phases in gas chromatography, the above findings may be useful
for the molecular design of such stationary phases.
intensity
Acknowledgment. This work was supported financially by
the National High Technology Research and Development
Program (863 Program, No. 2007AA05Z454), the National
Natural Science Foundation of China (No. 20873036) and the
Innovation Scientists and Technicians Troop Construction
Projects of Henan Province (No. 084200510015).
vibrational modes
and their ratio
[Bmim][N(CN)
2
]
2
ꢀ-CD + [Bmim][N(CN) ]
a
V
V
V
CdN
15.1
134.4
0.11
11.1
119.9
0.09
b
C ≡ N
CdN/VCtN
a
The peak of VCdN is at around 1570 cm-1. b The peak of VCtN is
at around 2200 cm-1.
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(
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(
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(
2 2 2
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(
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(
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θ
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(
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JP907333V