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Aggregation behavior of dodecyltriphenylphosphonium bromide in aqueous solution: Effect of aromatic ionic liquids
Accepted Manuscript Title: Aggregation Behavior of Dodecyltriphenylphosphonium Bromide in Aqueous solution: Effect of Aromatic Ionic Liquids Author: Fei Lu Lijuan Shi Han Yan Xiujie Yang Liqiang Zheng PII: DOI: Reference:
S0927-7757(14)00531-7 http://dx.doi.org/doi:10.1016/j.colsurfa.2014.05.071 COLSUA 19272
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
4-4-2014 22-5-2014 27-5-2014
Please cite this article as: F. Lu, L. Shi, H. Yan, X. Yang, L. Zheng, Aggregation Behavior of Dodecyltriphenylphosphonium Bromide in Aqueous solution: Effect of Aromatic Ionic Liquids, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2014), http://dx.doi.org/10.1016/j.colsurfa.2014.05.071 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Aggregation Behavior of Dodecyltriphenylphosphonium Bromide in Aqueous solution: Effect of Aromatic Ionic Liquids
Key laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan, 250100, P. R. China b Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, China
Ac ce p
te
d
M
an
us
cr
a
ip t
Fei Lua, Lijuan Shib, Han Yana, Xiujie Yanga, and Liqiang Zheng*, a
Corresponding author: Prof. Liqiang Zheng E-mail address: [email protected] Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, China Phone number: +86-531-88361528 Fax number: +86-531-88564750
Page 1 of 29
Abstract The effects of
ionic liquids (ILs), 1-butyl-3-methylimidazolium acetate
([Bmim][OAc]) and 1-butyl-3-methylimidazolium benzoate ([Bmim][PhCOO]) on
ip t
the aggregation behavior of dodecyltriphenylphosphonium bromide (C12TPB) in
cr
aqueous solution were investigated by surface tension measurements, dynamic light
us
scattering (DLS) measurements and 1H NMR spectroscopy. The introduction of benzene rings in the anions of [Bmim][PhCOO] can promote the micellization of
an
C12TPB more efficiently with a smaller CMC value and a bigger micellar size. The 1H NMR spectra revealed that [Bmim][PhCOO] can participate in the micelle formation.
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The combined effect of intermolecular interactions, such as electrostatic attraction, hydrophobic effect and π-π stacking interaction is proposed to be responsible for the
te
d
enhancement in the micellization of C12TPB.
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Keywords: ionic liquids; aromatic anions; micelles; π-π stacking
Page 2 of 29
1. Introduction Ionic liquids (ILs), as a class of compounds composed of ions with melting points at or near room temperature, have unusual physicochemical properties including low
ip t
volatility, high thermal stability, high ionic conductivity and easy recyclability [1,2].
cr
Thus ILs have been applied widely in the range of organic synthesis and catalysis
us
[3-7], inorganic synthesis [8], chromatography [9], analysis systems [10,11] and biochemistry [12]. Perhaps the most unique capability of ILs is to support the
an
self-assembly of amphiphiles [13,14]. The physicochemical properties of ILs can be easily modulated by altering the substituents or anions. Based on this characterization,
M
a large variety of ILs with different structures have been designed for various applications [15-19].
te
d
As is known universally, surfactants can self-assemble into micelles in aqueous solutions when the surfactant concentration is above the critical micelle concentration
Ac ce p
(CMC). The microstructures, shapes, and properties of the micelles in aqueous solutions mainly depend on the structures of the surfactant molecules. In addition, the physicochemical properties of the given surfactant solutions can be modified by the addition of external additives, including inorganic or organic salts, co-solvents, and co-surfactants [20-26]. Recently, the utilization of ILs to modify the properties of aqueous surfactant solutions has attracted extensive attention. The special properties of ILs would play a unique role in altering the aggregation behavior of surfactant solutions. Many works have concentrated on the effects of ILs on the aggregation behavior of
Page 3 of 29
surfactant in aqueous solution. Pandey et al. have systematically investigated the effects of various ILs on the aggregation behavior of a series of surfactants [27-33]. They
have
reported
that
the
addition
of
1-butyl-3-methylimidazolium
ip t
tetrafluoroborate ([Bmim][BF4]) to aqueous Triton X-100 solution results in a
Nagg.
While
the
hydrophobic
ILs
1-butyl-3-methylimidazolium
us
number
cr
decreased micellar size, an increased CMC value, and a decreased aggregation
hexafluorophosphate ([Bmim][PF6]) have no significant change in CMC value. They also
observed
that
the
addition
of
IL
to
aqueous
an
have
N-dodecyl-N,N-diemthyl-3-ammonio-1-propanesulfonate (SB-12) solution results in
M
decreased Nagg; and the decrease in Nagg is significantly drastic for the [Bmim][PF6] addition as compared with that for [Bmim][BF4]. They have explained this fact on the
te
d
basis of simple packing considerations and the differences in the size of the two anions. Recently Pandey et al. have found that the addition of ILs in sodium dodecyl
Ac ce p
benzene sulfonated (SDBS) aqueous solutions can give rise to a sudden aggregate size enhancement. Aromaticity of the IL cation alongside the presence of sufficiently aliphatic (butyl or longer) alkyl chains on the IL appear to be essential for this dramatic critical expansion in self-assembly dimensions within aqueous SDBS solution. The effects of ILs 1-hexyl-3-methylimidazolium bromide ([Hmim][Br]) and co-surfactant n-hexyltrimethylammonium bromide (HeTAB) on the aggregation benhavior of aqueous cetyltrimethylammonium bromide (CTAB) solution were also investigated, and they found that [hmim][Br] appears to be more effective in altering the properties of aqueous CTAB solution. In addition, Sarkar et al. have also
Page 4 of 29
investigated the micellization of CTAB in aqueous solution with the addition of two protic ILs dimethylethanol ammonium hexanoate (DAH) and dimethylethanol ammonium formate (DAF) [34]. The location of the anions is different due to the
ip t
different alkyl chain length, resulting in different micellar size of CTAB. The addition
cr
of functional ILs with special groups was also studied by our group [35]. The
us
introduction of acidic, basic and neutral groups in the IL cations has different effects on the micellization of SDS and the aggregation behavior was also dependent on the
an
pH values. It can be deduced from the previous reports that ILs have significant effects on the aggregation behavior of surfactant in aqueous solution.
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In the present work, we have focused on the aggregation behavior of a cationic surfactant dodecyltriphenylphosphonium bromide (C12TPB) in aqueous solution upon of
an
aromatic
IL
d
addition
1-butyl-3-methylimidazolium
benzoate
te
the
([Bmim][PhCOO]). For comparison, the effect of IL 1-butyl-3-methylimidazolium
Ac ce p
acetate ([Bmim][OAc]) was also investigated. The chemical structures of the surfactant and ILs are shown in Fig. 1. The aim of this work seeks to explore the role of benzene rings of added ILs on the aggregation behavior of C12TPB in aqueous
solution. We expected the introduction of aromatic structures can generate distinct π-π stacking interaction and tune the physicochemical properties of the surfactant solution.
Page 5 of 29
(a) 1
(b)
2 3
4
N
1
8 9
5
6
N
2 3
COO
8
7 9
4
N
8
5
9
[Bmim][PhCOO]
6
CH3COO
N
7
[Bmim][OAc]
a
c
b
d e
P
cr
Br
an
2. Materials and methods
us
C12TPB
Fig. 1
ip t
(c)
2.1. Materials and synthesis
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Triphenylphosphine (99%), 1-bromododecane (97%), 1-methylimidazole (98%), 1chlorobutane (99%), D2O (99.96%) and CDCl3 (99.96%) were purchased from
te
d
Sigma-Aldrich. Sodium benzoate (98%), ammonium acetate (98%), sodium acetate (99%) and sodium bromide (99%) were the products of Sinopharm Chemical Reagent
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Co. Toluene (99%), pentane (99%), acetonitrile (99%) and isopropanol (99%) were obtained from Beijing Chemical Reagent Co. Triply distilled water was used throughout all the experiments. The surfactant dodecyltriphenylphosphonium bromide was synthesized according
to the previous literature [36] and the purity of the product was ascertained by the 1H NMR spectrum in D2O. The ILs ([Bmim][PhCOO]) and ([Bmim][OAc]) were
prepared based on the procedures reported previously [37,38] and the purity of the obtained ILs was ascertained by the 1H NMR spectrum in CDCl3.
Page 6 of 29
2.2. Sample preparation Certain amount of additives was dissolved in triplydistilled water in a 250 mL volumetric flask and this additive solution with definite additive concentration was
ip t
used to dissolve an appropriate amount of C12TPB to obtain the stock solution. Next,
cr
the stock solution of C12TPB was diluted in turn by the above additive solution
and a definite additive concentration were prepared.
an
2.3. Methods
us
instead of water. Finally, a series of C12TPB solutions with different concentrations
Surface tension measurements were carried out by a surface tensiometer (Model
M
JYW-200B, Chengde Dahua Instrument Co.) using the ring method. Temperature was controlled at 25 ±0.1°C by a thermostatic bath. The sizes and size distributions of the
te
d
micelles were determined by dynamic light scattering (DLS) using a Nanotrac Particle Size Analyzer (Nanotrac NPA 250) and the microtrac FLEX application software
Ac ce p
program. All measurements were performed with a laser diode (658 nm wavelength, 3 m Wnominal, Class
B at the scattering angle of 90°). The temperature was
controlled with a thermostat (F31C, Julabo). 1H NMR spectra were conducted on a Bruker Advance 400 spectrometer at 25 °C with a frequency of 400.13 Hz. All the samples were dissolved in D2O, and chemical shifts were referred to the center of the
HDO signal (4.700 ppm). 3. Results and discussion 3.1. Effects of ILs on surface properties and micellization
Page 7 of 29
1mM 5mM 10mM 25mM 50mM
50
60
50
(a)
(b)
40
40 1E-7
1E-6
1E-5
1E-4
1E-3
0.01
1E-6
1E-5
C/M
(c)
γ (mN/m)
60
50
0.01
0.1
70
50
(d)
40
1mM 5mM 10mM
60
us
50
1mM 10mM 50mM
70
γ (mN/m)
γ (mN/m)
60
1E-3
C/M
cr
1mM 5mM 10mM
70
1E-4
ip t
60
1mM 5mM 10mM 25mM 50mM
70
γ (mN/m)
γ (mN/m)
70
(e)
40
1E-6
1E-5
1E-4
1E-3
0.01
40
1E-7
1E-6
1E-5
1E-4
C/M
1E-3
0.01
an
C/M
1E-6
1E-5
1E-4
1E-3
0.01
C/M
M
Fig. 2
Surface tension measurements were performed to investigate the effects of ILs with
d
different anions on the surface properties and micellization behavior of C12TPB in
te
aqueous solution. The surface tension (γ) versus concentration (C) plot of C12TPB in
Ac ce p
the presence of the two ILs was determined and shown in Fig. 2(a) and (b). It is clearly that the surface tension decreases initially with the increasing concentration and then a distinct break point appears, indicating the formation of micelles. The critical micelle concentrations (CMC) were determined by the intersection of two straight lines of the surface tension curves (γ-log C curves). In order to access the role of ILs, the effects of two organic salts sodium benzoate (PhCOONa) and sodium acetate (NaOAc) with the same anions of ILs, and an inorganic salt sodium bromide (NaBr) with the same anions of C12TPB, were also investigated. When the salt concentration increases, a sudden increase in turbidity reveals the phase separation of aqueous surfactant solutions as salting-out phenomenon [39]. In particular, only
Page 8 of 29
~25mM PhCOONa or NaBr makes C12TPB salt out. The surface tension curves of C12TPB at some typical concentrations of PhCOONa, NaOAc and NaBr were presented in Fig. 2(c), (d) and (e).The derived CMC values of both IL systems and
cr
ip t
salt systems are plotted against the concentration of additives in Fig. 3.
[Bmim][PhCOO] [Bmim][OAc] PhCOONa NaOAc NaBr
us
0.0020
0.0010
an
CMC (M)
0.0015
0
10
M
0.0005
20
30
40
50
d
Cadditive (mM)
te
Fig. 3
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According to the previous report, the CMC value of pure C12TPB in aqueous solution is 2.0 mM [40]. With the increasing concentration of two ILs, the CMC
values of C12TPB decrease sharply at first and then decrease much more slowly. This is due to the increased ionic strength as IL concentration is increased [41]. A very similar result was obtained in the presence of organic or inorganic salts, indicating that the two ILs affect micellization of C12TPB like electrolytes. The enhanced ionic strength can effectively reduce the electrostatic repulsion among the headgroups of C12TPB and facilitate micelle formation. As the ionic strength is large enough, the electrostatic repulsion among the headgroups may become invariable, resulting in the CMC values becoming constant [21]. Compared with [Bmim][OAc], the addition of
Page 9 of 29
[Bmim][PhCOO] can reduce the CMC values of C12TPB in a larger magnitude. Even the addition of organic salt PhCOONa results in the dramatic reduction of CMC values. This result demonstrated that the introduction of benzene anions effectively
ip t
promotes the micellization of C12TPB. The electrostatic repulsion among the
cr
headgroups of C12TPB can be significantly reduced due to the electrostatic attraction
us
and π-π stacking interaction between the benzoic acid anions and the triphenyl cations [42,43]. The benzene ring enhances the hydrophobicity of the anion and leads to
an
stronger intermolecular interactions. Table 1.
CILs (mM) CMC (mM) γCMC (mN/m) ПCMC (mN/m) pC20 0 2.00 46.0 26.5 3.01 1 1.32 43.9 28.6 3.92 5 0.97 42.4 30.1 4.25 [Bmim][PhCOO] 10 0.33 41.7 30.8 4.45 25 0.28 40.9 31.6 4.65 50 0.19 39.7 32.8 5.01 1 1.83 45.5 27.0 3.39 5 1.58 44.6 27.9 3.51 10 1.23 44.3 28.2 3.72 [Bmim][OAc] 25 0.94 43.3 29.2 3.92 50 0.81 42.4 30.1 4.22
Ac ce p
te
d
M
ILs
To investigate the effects of ILs on the surface activity of C12TPB aqueous solution,
several parameters such as γ CMC , Π CMC and pC 20 were calculated and listed in Table 1.
The ability to reduce the surface tension can be reflected by γ CMC and Π CMC , where Π CMC is the surface pressure at CMC and defined as [44]:
Π CMC = γ 0 − γ CMC
(1)
where γ 0 is the surface tension of pure water and γ CMC is the surface tension at the
Page 10 of 29
CMC. A smaller value of γ CMC or a greater value of ΠCMC means a better ability to reduce the surface tension of a surfactant solution. The obtained γ CMC and Π CMC values are plotted against the concentration of different ILs and shown in Fig. 4(a) and (b). It
ip t
is clear that the addition of [Bmim][PhCOO] can reduce the γ CMC greatly and
cr
the Π CMC values are distinctly larger compared with [Bmim][OAc]. That is to say, the
us
ability of reducing the surface tension of aqueous C12TPB solutions is significantly enhanced by the addition of [Bmim][PhCOO]. Furthermore, the decrease in
an
γ CMC values also indicates that the presence of the phenyl anions in [Bmim][PhCOO]
(a)
44
[Bmim][PhCOO] [Bmim][OAc]
d
γCMC (mN/m)
46
M
can strengthen the close packing of the C12TPB molecules at the air-water interface.
42
te
40
Ac ce p
0
10
20
30
40
50
CIL (mM)
(b)
[Bmim][PhCOO] [Bmim][OAc]
ПCMC (mN/m)
32
30
28
26
0
10
20
30
40
50
CIL (mM)
(c)
[Bmim][PhCOO] [Bmim][OAc]
5.0
pC20
4.5
4.0
3.5
3.0 0
10
20
30
40
50
CIL (mM)
Fig. 4
Page 11 of 29
The parameter pC 20 can reflect the efficiency to reduce the surface tension of C12TPB and defined as [44]:
pC 20 = − log C 20
(2)
ip t
where C is the molar concentration of surfactant and C20 stands for the concentration
cr
of surfactant to reduce the surface tension of pure solvent by 20 mN/m. The
greater pC 20 means the higher efficiency to reduce the surface tension. It can be seen
us
from Fig. 4(c) that the pC 20 values are larger in the presence of [Bmim][PhCOO]
an
than [Bmim][OAc], suggesting that [Bmim][PhCOO] can enhance the efficiency of reducing the surface tension of C12TPB more significantly than [Bmim][OAc]. As
M
discussed above, the IL [Bmim][PhCOO] with phenyl anions can enhance the surface
te
d
activity of C12TPB more effectively than [Bmim][OAc].
3.2. Effects of ILs on the micellar size
Ac ce p
Dynamic light scattering (DLS) is utilized to further investigate the micellar sizes
and size distributions of aqueous C12TPB solution above the CMC value with the addition of ILs and salts. The sizes and size distributions of 5 mM C12TPB aqueous solution in the presence of varying concentration of different additives are shown in Fig. 5. The obtained peak diameters (D) of the C12TPB aggregates at different
additive concentrations are summarized in Table 2. The DLS data prove that all the systems can form the micelle-like aggregates, and interesting variations of micellar sizes are observed with the addition of different ILs.
Page 12 of 29
0 mM 5 mM 10 mM 25 mM 50 mM
20
(a)
10
0
0 mM 5 mM 10 mM 25 mM 50 mM
20
(b)
10
us
% Intensity
15
5
0 30
an
0mM 5mM 10mM
M
Intensity (%)
(c) 20
10
(d)
10
5
Ac ce p
Intensity (%)
te
15
0 mM 10mM 50mM
d
0
20
ip t
5
cr
% Intensity
15
0
Intensity (%)
20
15
0mM 5mM 10mM
(e)
10
5
0
1
D (nm)
10
Fig. 5
Page 13 of 29
Table 2. ILs
D (nm) 5mM
10mM
25 mM
50 mM
[Bmim][PhCOO]
2.1
3.1
3.6
4.3
4.6
[Bmim][OAc]
2.1
2.9
3.2
3.7
3.9
PhCOONa
2.1
2.6
3.8
-
NaOAc
2.1
-
2.9
-
NaBr
2.1
2.7
3.0
ip t
0 mM
-
cr
3.3 -
us
-
an
When [Bmim][OAc] was added into the C12TPB aqueous solution, the peak diameters (D) of the micelles only increase from 2.1 to 3.9 nm. This result indicates
M
that the anions of [Bmim][OAc] only adsorbed on the surface of micelle, and most of the anions may stay in the bulk solutions and have no effect on the properties of
te
d
micelles. It is noted that the peak diameters of the micelles also slightly increase with the addition of NaOAc and NaBr, as shown in Fig. 5(D) and (E). It is demonstrated
Ac ce p
that the IL [Bmim][OAc] show the electrolytic behavior within aqueous C12TPB solutions. Whereas the addition of [Bmim][PhCOO] leads to a more significant growth of the C12TPB micelles, from 2.1 to 4.6 nm, as well as 10 mM PhCOONa
results in the C12TPB micelles into 3.9 nm. In other words, the aromatic [PhCOO]anions play a significant role in the micellar growth. It has been proved form the above results that both the electrostatic attraction and π-π stacking interaction between the phenyl anions and triphenyl cations can significantly decrease the electrostatic repulsion among the headgroups of C12TPB and facilitate the micellization. With the increasing [Bmim][PhCOO] or PhCOONa concentration, the [PhCOO]- anions
Page 14 of 29
enhance the hydrophobic effect and the π-π stacking interaction with aligned among the phenyl moieties in C12TPB headgroups. As a result, the packing of the C12TPB molecules becomes denser and in the meantime the micellar aggregation number
ip t
increases, which is responsible for the micellar growth. It can be concluded that the IL
cr
[Bmim][PhCOO] is more effective than [Bmim][OAc] to promote the micelle
3.3. Mechanism of IL effect on C12TPB micellization 4
3
(a) 8 a 2 1 9 20mM 15mM 10mM
M
5mM 1mM 8
5
an
b
us
formation.
5
4
3
e
ppm
1
7 5
6 c
b
7
d
e
te
50mM
d
21
d
8
3 a
6
2
4
(b)
c
25mM 10mM
Ac ce p
5mM
1mM
8.0
1
7.5
4
3
2
1
ppm
Fig. 6
H NMR technique was applied to reveal the intermolecular interaction in the
IL-C12TPB systems. The protons on various carbons are labeled as shown in Fig. 1. The C12TPB concentration was chosen to be 10 mM, which is larger than the CMC values of C12TPB in all the IL systems. Therefore, the observed chemical shifts can
reflected the chemical shift variations of the protons in the micelles. Fig. 6 shows the chemical shifts of C12TPB solutions at different concentrations of [Bmim][PhCOO]
Page 15 of 29
and [Bmim][OAc]. The chemical shift changes of the C12TPB molecules in the 1H NMR spectra are different for various ILs.
H-b
[Bmim][OAc] [Bmim][PhCOO]
[Bmim][OAc] [Bmim][PhCOO]
ip t
H-a 7.56
7.52
3.1
cr
δ (ppm)
δ (ppm)
3.2
7.44
0
10
20
30
40
3.0
50
0
10
20
30
40
50
C (mM)
C (mM)
H-c
H-e
[Bmim][OAc] [Bmim][PhCOO]
0.65
[Bmim][OAc] [Bmim][PhCOO]
δ (ppm)
an
1.35
1.30
0.60
M
δ (ppm)
us
7.48
1.25
10
20
30
te
C (mM)
40
50
d
0
0.55
0
10
20
30
40
50
C (mM)
Fig. 7
Ac ce p
The chemical shifts of protons for C12TPB (Ha-He) as a function of IL
concentration are shown in Fig. 7. With the addition of [Bmim][OAc], no significant variations of the chemical shifts of protons for C12TPB were observed, confirming that the C12TPB micelles do not have any remarkable changes upon the addition of
[Bmim][OAc]. This result is in good agreement with the conclusion above that the anions of [Bmim][OAc] are only slightly adsorbed on the surface of the micelles. Differently, the IL [Bmim][PhCOO] causes distinct changes of the chemical shifts of the protons for C12TPB. As shown in Fig. 7, the chemical shifts of protons around the headgroup (Ha-Hc) move upfield, whereas the protons in the terminal methyl (He)
Page 16 of 29
show a downfield shift. This is mainly due to the reason that the addition of [Bmim][PhCOO] generates intermolecular π-π stacking interaction between benzene rings and triphenyl groups. So the protons around the headgroup locate under the ring
ip t
current and resonate in a higher field due to the shielding effect [45,46]. Nevertheless,
cr
due to the relatively far distance to the headgroup, the shielding effect for He is feeble
us
and the medium effect plays the main role. The medium effect is caused by the movement of surfactant molecules from water to micelles, and the decrease in the
an
polarity of the microenvironment leads to a downshift for He [47-49]. Interestingly, the protons in the alkyl chain (Hd) show a distinct splitting with the increasing
M
concentrations of [Bmim][PhCOO]. It is proposed that the protons in alkyl chain are
Ac ce p
te
d
affected by both shielding effect and medium effect.
Page 17 of 29
7.34
[Bmim][OAc] [Bmim][PhCOO]
H-1
[Bmim][OAc] [Bmim][PhCOO]
H-2
δ (ppm)
δ (ppm)
7.38
7.32
7.36
7.34
0
10
20
30
40
0
50
10
H-4
[Bmim][OAc] [Bmim][PhCOO]
H-3
δ (ppm)
4.08
3.78
3.76 4.06 30
40
0
50
H-5
[Bmim][OAc] [Bmim][PhCOO]
1.24
δ (ppm)
1.74
1.72
20
30
40
50
[Bmim][OAc] [Bmim][PhCOO]
H-6
10
20
30
40
1.20
1.16
50
d
0
C (mM)
0
10
20
30
40
50
C (mM)
[Bmim][OAc] [Bmim][PhCOO]
te
H-7
0.82
Ac ce p
δ (ppm)
10
M
δ (ppm)
1.76
0.84
50
C (mM)
C (mM)
an
20
us
δ (ppm)
4.10
10
40
[Bmim][OAc] [Bmim][PhCOO]
3.80
0
30
cr
4.12
20
C (mM)
C (mM)
ip t
7.30
0.80
0
10
20
30
40
50
C (mM)
7.6
Fig. 8 1.90
8 COO
9
(b)
1.85
9
7.4
8 9
7.2
δ (ppm)
δ (ppm)
H-8
(a)
1.80
8
H-8 H-9
7.0
0
CH3 COO 5
10
15
20
1.75
0
10
20
30
40
50
C (mM)
C (mM)
Fig. 9
Page 18 of 29
The changes in chemical shifts of protons for ILs in the C12TPB aqueous solution can also reflect their interactions with the C12TPB molecules. Fig. 8 shows the chemical shifts of various protons for the IL cations as the function of IL
ip t
concentrations. It is obvious that all the protons of the imidazolium cations show
cr
downfield shifts, especially for [Bmim][PhCOO]. As discussed above, the decrease in
us
the polarity of the microenvironment can lead to a downshift for the protons. So it is reasonable to assume that the imidazolium cations of ILs may participate in the
an
micelle formation. When micelles are formed, the butyl chains of imidazolium cations are immersed in a hydrophobic core and the hydrophilic headgroups are solvated by
M
the interfacial water whose polarity is lower than the bulk water. Interestingly, all the chemical shift changes exhibit a distinct break point when the IL concentration
te
d
reaches 10 mM, indicating the equilibrium of [Bmim]+ into the micelles. For the protons in IL anions (Fig. 9), the same downfield chemical shifts can be observed on
Ac ce p
basis of the medium effect, and the H9 in the phenyl anions of [Bmim][PhCOO]
exhibits the extreme changes in chemical shifts. Interestingly, the H8 shows an opposite trend and shifts to upfield. We contribute the upfield shift to the shielding effect of ring current between the headgroups of C12TPB. This result indicates that the phenyl anions of [Bmim][PhCOO] not only simply adsorb on the micelle surface but also penetrate into the palisade layer of the micelle due to the π-π stacking interaction. The distinct break points around 10 mM of the IL concentration indicate that the adsorption of the anions has been saturated with 10 mM concentration, and the excess ions are free in the bulk water.
Page 19 of 29
(a)
bulk water
Br
N N
N N
CH 3COO
N N Br
Br
N
CH3COO
Br
N
CH3COO
CH3COO
P
ip t
P
us
cr
P
an
micelle bulk water
(b) OOC
N
COO
N
N Br N
M
Br
d
P
Br N
OOC
COO
P
Ac ce p
te
OOC
N
N
P Br
N
micelle
Scheme 1.
According to the analysis above, the possible arrangement and different location of
ILs in the C12TPB micelles are depicted in Scheme 1. When [Bmim][OAc] or [Bmim][PhCOO] was added in the C12TPB solution, the imidazolium cations of the both ILs may participate in the micelle formation with the butyl chains of imidazolium cations immersing in a hydrophobic core. While the anions of the two
Page 20 of 29
ILs affect in the different ways. Both [OAc]- and [PhCOO]- can be adsorbed at the micellar surfaces due to the electrostatic interaction. In the case of [Bmim][PhCOO], the [PhCOO]- can also aligned into the C12TPB micelles with the enhanced
ip t
hydrophobic effect and π-π stacking interaction, resulting in a smaller CMC values
cr
and micellar growth.
us
4. Conclusions
The present work investigated the effects of aromatic imidazolium ILs on the
an
micellization of C12TPB in aqueous solution. Both [Bmim][PhCOO] and [Bmim][OAc] can facilitate the micelle formation of C12TPB, and a better surface
M
activity of aqueous C12TPB solution can be obtained with the addition of [Bmim][PhCOO]. DLS data demonstrate that [Bmim][PhCOO] makes C12TPB to
te
d
pack more densely, resulting in a bigger micellar size. Compared with [Bmim][OAc], the introduction of benzene rings in the anions of [Bmim][PhCOO] can promote the
Ac ce p
micellization of C12TPB more efficiently due to the enhanced hydrophobic effect and
π-π stacking interaction. This work is helpful to understand the effects of ILs with aromatic structures on the aggregation behavior of surfactant in aqueous solution. Acknowledgments
The authors are grateful to the National Natural Science Foundation of China (No.91127017), the National Basic Research Program (2013CB834505), Specialized Research Fund for the Doctoral Program of Higher Education of China (No.20120131130003) and the Shandong Provincial Natural Science Foundation, China (ZR2012BZ001).
Page 21 of 29
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ip t
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cr
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te
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te
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[46] L. J. Shi, N. Li, H. Yan, Y. A. Gao, L. Q. Zheng, Aggregation Behavior of Long-Chain N-Aryl-Imidazolium Bromide in Aqueous Solution, Langmuir 27 (2011) 1618–1625. [47] X. Huang, Y. C. Han, Y. X. Wang, W. L. Wang, Aggregation Behavior of
ip t
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cr
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[49] E. B. Tada, L. P. Novaki, O. A. El Seoud, Solvatochromism in Cationic Micellar
M
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Ac ce p
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d
the Surfactant Headgroup, Langmuir 17 (2001) 652–658.
Page 26 of 29
ip t
cr
Figure and table captions:
Fig. 1 The chemical structures and 1H NMR signal assignment of the surfactant and
us
ILs.
Fig. 2 Surface tension curves of C12TPB plotted against the surfactant concentration
an
in the presence of different additive concentrations at 25 °C: (a) [Bmim][PhCOO]; (b)[Bmim][OAc]; (c) PhCOONa; (d) NaOAc; (e) NaBr.
M
Fig. 3 CMC values of C12TPB plotted against the concentrations of different additives.
concentration of different ILs.
d
Fig. 4 Plots of γ CMC (a), Π CMC (b) and pC 20 (c) values of C12TPB as a function of the
te
Fig. 5 Size distributions of 5 mM C12TPB at different additive concentrations at 25°C:
Ac ce p
(a) [Bmim][PhCOO]; (b) [Bmim][OAc]; (c) PhCOONa; (d)NaOAc; (e)NaBr Fig. 6 Proton assignments and 1H NMR spectra of 10 mM C12TPB with various concentrations of ILs at 25°C: (a) [Bmim][PhCOO]; (b) [Bmim][OAc]. Fig. 7 Chemical shifts of protons for C12TPB against different IL concentrations.
Fig. 8 Chemical shifts of protons for IL cations against different IL concentrations. Fig. 9 Chemical shifts of protons for IL anions against different IL concentrations: (a)
[Bmim][PhCOO]; (b) [Bmim][OAc]. Scheme 1. Cartoon depicting segments the C12TPB micelles in the presence of (a) [Bmim][OAc] and (b) [Bmim][PhCOO]. Table 1. Surface properties and micellization parameters of aqueous C12TPB solutions in the presence of different concentrations of ILs at 25°C. Table 2. Micellar sizes of 5 mM C12TPB aqueous solution in the presence of different concentrations of ILs at 25°C.
Page 27 of 29
Graphical abstract
60
50
cr
γ (mN/m)
ip t
1mM 5mM 10mM 25mM 50mM
70
1E-7
1E-6
1E-5
1E-4
1E-3
us
40 0.01
an
C/M
M
Surface tension curves of C12TPB / [Bmim][PhCOO] system and the
Ac ce p
te
d
proposed mechanism.
Page 28 of 29
Highlights A kind of room‐temperature ILs with aromatic anions was successfully synthesized.
ip t
The aromatic ILs affect the micellization of a surfactant with aromatic headgroups.
Ac ce p
te
d
M
an
us
cr
The enhanced π‐π stacking interaction and hydrophobic interaction play a key role.
Page 29 of 29
S0927-7757(14)00531-7 http://dx.doi.org/doi:10.1016/j.colsurfa.2014.05.071 COLSUA 19272
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
4-4-2014 22-5-2014 27-5-2014
Please cite this article as: F. Lu, L. Shi, H. Yan, X. Yang, L. Zheng, Aggregation Behavior of Dodecyltriphenylphosphonium Bromide in Aqueous solution: Effect of Aromatic Ionic Liquids, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2014), http://dx.doi.org/10.1016/j.colsurfa.2014.05.071 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Aggregation Behavior of Dodecyltriphenylphosphonium Bromide in Aqueous solution: Effect of Aromatic Ionic Liquids
Key laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan, 250100, P. R. China b Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, China
Ac ce p
te
d
M
an
us
cr
a
ip t
Fei Lua, Lijuan Shib, Han Yana, Xiujie Yanga, and Liqiang Zheng*, a
Corresponding author: Prof. Liqiang Zheng E-mail address: [email protected] Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, China Phone number: +86-531-88361528 Fax number: +86-531-88564750
Page 1 of 29
Abstract The effects of
ionic liquids (ILs), 1-butyl-3-methylimidazolium acetate
([Bmim][OAc]) and 1-butyl-3-methylimidazolium benzoate ([Bmim][PhCOO]) on
ip t
the aggregation behavior of dodecyltriphenylphosphonium bromide (C12TPB) in
cr
aqueous solution were investigated by surface tension measurements, dynamic light
us
scattering (DLS) measurements and 1H NMR spectroscopy. The introduction of benzene rings in the anions of [Bmim][PhCOO] can promote the micellization of
an
C12TPB more efficiently with a smaller CMC value and a bigger micellar size. The 1H NMR spectra revealed that [Bmim][PhCOO] can participate in the micelle formation.
M
The combined effect of intermolecular interactions, such as electrostatic attraction, hydrophobic effect and π-π stacking interaction is proposed to be responsible for the
te
d
enhancement in the micellization of C12TPB.
Ac ce p
Keywords: ionic liquids; aromatic anions; micelles; π-π stacking
Page 2 of 29
1. Introduction Ionic liquids (ILs), as a class of compounds composed of ions with melting points at or near room temperature, have unusual physicochemical properties including low
ip t
volatility, high thermal stability, high ionic conductivity and easy recyclability [1,2].
cr
Thus ILs have been applied widely in the range of organic synthesis and catalysis
us
[3-7], inorganic synthesis [8], chromatography [9], analysis systems [10,11] and biochemistry [12]. Perhaps the most unique capability of ILs is to support the
an
self-assembly of amphiphiles [13,14]. The physicochemical properties of ILs can be easily modulated by altering the substituents or anions. Based on this characterization,
M
a large variety of ILs with different structures have been designed for various applications [15-19].
te
d
As is known universally, surfactants can self-assemble into micelles in aqueous solutions when the surfactant concentration is above the critical micelle concentration
Ac ce p
(CMC). The microstructures, shapes, and properties of the micelles in aqueous solutions mainly depend on the structures of the surfactant molecules. In addition, the physicochemical properties of the given surfactant solutions can be modified by the addition of external additives, including inorganic or organic salts, co-solvents, and co-surfactants [20-26]. Recently, the utilization of ILs to modify the properties of aqueous surfactant solutions has attracted extensive attention. The special properties of ILs would play a unique role in altering the aggregation behavior of surfactant solutions. Many works have concentrated on the effects of ILs on the aggregation behavior of
Page 3 of 29
surfactant in aqueous solution. Pandey et al. have systematically investigated the effects of various ILs on the aggregation behavior of a series of surfactants [27-33]. They
have
reported
that
the
addition
of
1-butyl-3-methylimidazolium
ip t
tetrafluoroborate ([Bmim][BF4]) to aqueous Triton X-100 solution results in a
Nagg.
While
the
hydrophobic
ILs
1-butyl-3-methylimidazolium
us
number
cr
decreased micellar size, an increased CMC value, and a decreased aggregation
hexafluorophosphate ([Bmim][PF6]) have no significant change in CMC value. They also
observed
that
the
addition
of
IL
to
aqueous
an
have
N-dodecyl-N,N-diemthyl-3-ammonio-1-propanesulfonate (SB-12) solution results in
M
decreased Nagg; and the decrease in Nagg is significantly drastic for the [Bmim][PF6] addition as compared with that for [Bmim][BF4]. They have explained this fact on the
te
d
basis of simple packing considerations and the differences in the size of the two anions. Recently Pandey et al. have found that the addition of ILs in sodium dodecyl
Ac ce p
benzene sulfonated (SDBS) aqueous solutions can give rise to a sudden aggregate size enhancement. Aromaticity of the IL cation alongside the presence of sufficiently aliphatic (butyl or longer) alkyl chains on the IL appear to be essential for this dramatic critical expansion in self-assembly dimensions within aqueous SDBS solution. The effects of ILs 1-hexyl-3-methylimidazolium bromide ([Hmim][Br]) and co-surfactant n-hexyltrimethylammonium bromide (HeTAB) on the aggregation benhavior of aqueous cetyltrimethylammonium bromide (CTAB) solution were also investigated, and they found that [hmim][Br] appears to be more effective in altering the properties of aqueous CTAB solution. In addition, Sarkar et al. have also
Page 4 of 29
investigated the micellization of CTAB in aqueous solution with the addition of two protic ILs dimethylethanol ammonium hexanoate (DAH) and dimethylethanol ammonium formate (DAF) [34]. The location of the anions is different due to the
ip t
different alkyl chain length, resulting in different micellar size of CTAB. The addition
cr
of functional ILs with special groups was also studied by our group [35]. The
us
introduction of acidic, basic and neutral groups in the IL cations has different effects on the micellization of SDS and the aggregation behavior was also dependent on the
an
pH values. It can be deduced from the previous reports that ILs have significant effects on the aggregation behavior of surfactant in aqueous solution.
M
In the present work, we have focused on the aggregation behavior of a cationic surfactant dodecyltriphenylphosphonium bromide (C12TPB) in aqueous solution upon of
an
aromatic
IL
d
addition
1-butyl-3-methylimidazolium
benzoate
te
the
([Bmim][PhCOO]). For comparison, the effect of IL 1-butyl-3-methylimidazolium
Ac ce p
acetate ([Bmim][OAc]) was also investigated. The chemical structures of the surfactant and ILs are shown in Fig. 1. The aim of this work seeks to explore the role of benzene rings of added ILs on the aggregation behavior of C12TPB in aqueous
solution. We expected the introduction of aromatic structures can generate distinct π-π stacking interaction and tune the physicochemical properties of the surfactant solution.
Page 5 of 29
(a) 1
(b)
2 3
4
N
1
8 9
5
6
N
2 3
COO
8
7 9
4
N
8
5
9
[Bmim][PhCOO]
6
CH3COO
N
7
[Bmim][OAc]
a
c
b
d e
P
cr
Br
an
2. Materials and methods
us
C12TPB
Fig. 1
ip t
(c)
2.1. Materials and synthesis
M
Triphenylphosphine (99%), 1-bromododecane (97%), 1-methylimidazole (98%), 1chlorobutane (99%), D2O (99.96%) and CDCl3 (99.96%) were purchased from
te
d
Sigma-Aldrich. Sodium benzoate (98%), ammonium acetate (98%), sodium acetate (99%) and sodium bromide (99%) were the products of Sinopharm Chemical Reagent
Ac ce p
Co. Toluene (99%), pentane (99%), acetonitrile (99%) and isopropanol (99%) were obtained from Beijing Chemical Reagent Co. Triply distilled water was used throughout all the experiments. The surfactant dodecyltriphenylphosphonium bromide was synthesized according
to the previous literature [36] and the purity of the product was ascertained by the 1H NMR spectrum in D2O. The ILs ([Bmim][PhCOO]) and ([Bmim][OAc]) were
prepared based on the procedures reported previously [37,38] and the purity of the obtained ILs was ascertained by the 1H NMR spectrum in CDCl3.
Page 6 of 29
2.2. Sample preparation Certain amount of additives was dissolved in triplydistilled water in a 250 mL volumetric flask and this additive solution with definite additive concentration was
ip t
used to dissolve an appropriate amount of C12TPB to obtain the stock solution. Next,
cr
the stock solution of C12TPB was diluted in turn by the above additive solution
and a definite additive concentration were prepared.
an
2.3. Methods
us
instead of water. Finally, a series of C12TPB solutions with different concentrations
Surface tension measurements were carried out by a surface tensiometer (Model
M
JYW-200B, Chengde Dahua Instrument Co.) using the ring method. Temperature was controlled at 25 ±0.1°C by a thermostatic bath. The sizes and size distributions of the
te
d
micelles were determined by dynamic light scattering (DLS) using a Nanotrac Particle Size Analyzer (Nanotrac NPA 250) and the microtrac FLEX application software
Ac ce p
program. All measurements were performed with a laser diode (658 nm wavelength, 3 m Wnominal, Class
B at the scattering angle of 90°). The temperature was
controlled with a thermostat (F31C, Julabo). 1H NMR spectra were conducted on a Bruker Advance 400 spectrometer at 25 °C with a frequency of 400.13 Hz. All the samples were dissolved in D2O, and chemical shifts were referred to the center of the
HDO signal (4.700 ppm). 3. Results and discussion 3.1. Effects of ILs on surface properties and micellization
Page 7 of 29
1mM 5mM 10mM 25mM 50mM
50
60
50
(a)
(b)
40
40 1E-7
1E-6
1E-5
1E-4
1E-3
0.01
1E-6
1E-5
C/M
(c)
γ (mN/m)
60
50
0.01
0.1
70
50
(d)
40
1mM 5mM 10mM
60
us
50
1mM 10mM 50mM
70
γ (mN/m)
γ (mN/m)
60
1E-3
C/M
cr
1mM 5mM 10mM
70
1E-4
ip t
60
1mM 5mM 10mM 25mM 50mM
70
γ (mN/m)
γ (mN/m)
70
(e)
40
1E-6
1E-5
1E-4
1E-3
0.01
40
1E-7
1E-6
1E-5
1E-4
C/M
1E-3
0.01
an
C/M
1E-6
1E-5
1E-4
1E-3
0.01
C/M
M
Fig. 2
Surface tension measurements were performed to investigate the effects of ILs with
d
different anions on the surface properties and micellization behavior of C12TPB in
te
aqueous solution. The surface tension (γ) versus concentration (C) plot of C12TPB in
Ac ce p
the presence of the two ILs was determined and shown in Fig. 2(a) and (b). It is clearly that the surface tension decreases initially with the increasing concentration and then a distinct break point appears, indicating the formation of micelles. The critical micelle concentrations (CMC) were determined by the intersection of two straight lines of the surface tension curves (γ-log C curves). In order to access the role of ILs, the effects of two organic salts sodium benzoate (PhCOONa) and sodium acetate (NaOAc) with the same anions of ILs, and an inorganic salt sodium bromide (NaBr) with the same anions of C12TPB, were also investigated. When the salt concentration increases, a sudden increase in turbidity reveals the phase separation of aqueous surfactant solutions as salting-out phenomenon [39]. In particular, only
Page 8 of 29
~25mM PhCOONa or NaBr makes C12TPB salt out. The surface tension curves of C12TPB at some typical concentrations of PhCOONa, NaOAc and NaBr were presented in Fig. 2(c), (d) and (e).The derived CMC values of both IL systems and
cr
ip t
salt systems are plotted against the concentration of additives in Fig. 3.
[Bmim][PhCOO] [Bmim][OAc] PhCOONa NaOAc NaBr
us
0.0020
0.0010
an
CMC (M)
0.0015
0
10
M
0.0005
20
30
40
50
d
Cadditive (mM)
te
Fig. 3
Ac ce p
According to the previous report, the CMC value of pure C12TPB in aqueous solution is 2.0 mM [40]. With the increasing concentration of two ILs, the CMC
values of C12TPB decrease sharply at first and then decrease much more slowly. This is due to the increased ionic strength as IL concentration is increased [41]. A very similar result was obtained in the presence of organic or inorganic salts, indicating that the two ILs affect micellization of C12TPB like electrolytes. The enhanced ionic strength can effectively reduce the electrostatic repulsion among the headgroups of C12TPB and facilitate micelle formation. As the ionic strength is large enough, the electrostatic repulsion among the headgroups may become invariable, resulting in the CMC values becoming constant [21]. Compared with [Bmim][OAc], the addition of
Page 9 of 29
[Bmim][PhCOO] can reduce the CMC values of C12TPB in a larger magnitude. Even the addition of organic salt PhCOONa results in the dramatic reduction of CMC values. This result demonstrated that the introduction of benzene anions effectively
ip t
promotes the micellization of C12TPB. The electrostatic repulsion among the
cr
headgroups of C12TPB can be significantly reduced due to the electrostatic attraction
us
and π-π stacking interaction between the benzoic acid anions and the triphenyl cations [42,43]. The benzene ring enhances the hydrophobicity of the anion and leads to
an
stronger intermolecular interactions. Table 1.
CILs (mM) CMC (mM) γCMC (mN/m) ПCMC (mN/m) pC20 0 2.00 46.0 26.5 3.01 1 1.32 43.9 28.6 3.92 5 0.97 42.4 30.1 4.25 [Bmim][PhCOO] 10 0.33 41.7 30.8 4.45 25 0.28 40.9 31.6 4.65 50 0.19 39.7 32.8 5.01 1 1.83 45.5 27.0 3.39 5 1.58 44.6 27.9 3.51 10 1.23 44.3 28.2 3.72 [Bmim][OAc] 25 0.94 43.3 29.2 3.92 50 0.81 42.4 30.1 4.22
Ac ce p
te
d
M
ILs
To investigate the effects of ILs on the surface activity of C12TPB aqueous solution,
several parameters such as γ CMC , Π CMC and pC 20 were calculated and listed in Table 1.
The ability to reduce the surface tension can be reflected by γ CMC and Π CMC , where Π CMC is the surface pressure at CMC and defined as [44]:
Π CMC = γ 0 − γ CMC
(1)
where γ 0 is the surface tension of pure water and γ CMC is the surface tension at the
Page 10 of 29
CMC. A smaller value of γ CMC or a greater value of ΠCMC means a better ability to reduce the surface tension of a surfactant solution. The obtained γ CMC and Π CMC values are plotted against the concentration of different ILs and shown in Fig. 4(a) and (b). It
ip t
is clear that the addition of [Bmim][PhCOO] can reduce the γ CMC greatly and
cr
the Π CMC values are distinctly larger compared with [Bmim][OAc]. That is to say, the
us
ability of reducing the surface tension of aqueous C12TPB solutions is significantly enhanced by the addition of [Bmim][PhCOO]. Furthermore, the decrease in
an
γ CMC values also indicates that the presence of the phenyl anions in [Bmim][PhCOO]
(a)
44
[Bmim][PhCOO] [Bmim][OAc]
d
γCMC (mN/m)
46
M
can strengthen the close packing of the C12TPB molecules at the air-water interface.
42
te
40
Ac ce p
0
10
20
30
40
50
CIL (mM)
(b)
[Bmim][PhCOO] [Bmim][OAc]
ПCMC (mN/m)
32
30
28
26
0
10
20
30
40
50
CIL (mM)
(c)
[Bmim][PhCOO] [Bmim][OAc]
5.0
pC20
4.5
4.0
3.5
3.0 0
10
20
30
40
50
CIL (mM)
Fig. 4
Page 11 of 29
The parameter pC 20 can reflect the efficiency to reduce the surface tension of C12TPB and defined as [44]:
pC 20 = − log C 20
(2)
ip t
where C is the molar concentration of surfactant and C20 stands for the concentration
cr
of surfactant to reduce the surface tension of pure solvent by 20 mN/m. The
greater pC 20 means the higher efficiency to reduce the surface tension. It can be seen
us
from Fig. 4(c) that the pC 20 values are larger in the presence of [Bmim][PhCOO]
an
than [Bmim][OAc], suggesting that [Bmim][PhCOO] can enhance the efficiency of reducing the surface tension of C12TPB more significantly than [Bmim][OAc]. As
M
discussed above, the IL [Bmim][PhCOO] with phenyl anions can enhance the surface
te
d
activity of C12TPB more effectively than [Bmim][OAc].
3.2. Effects of ILs on the micellar size
Ac ce p
Dynamic light scattering (DLS) is utilized to further investigate the micellar sizes
and size distributions of aqueous C12TPB solution above the CMC value with the addition of ILs and salts. The sizes and size distributions of 5 mM C12TPB aqueous solution in the presence of varying concentration of different additives are shown in Fig. 5. The obtained peak diameters (D) of the C12TPB aggregates at different
additive concentrations are summarized in Table 2. The DLS data prove that all the systems can form the micelle-like aggregates, and interesting variations of micellar sizes are observed with the addition of different ILs.
Page 12 of 29
0 mM 5 mM 10 mM 25 mM 50 mM
20
(a)
10
0
0 mM 5 mM 10 mM 25 mM 50 mM
20
(b)
10
us
% Intensity
15
5
0 30
an
0mM 5mM 10mM
M
Intensity (%)
(c) 20
10
(d)
10
5
Ac ce p
Intensity (%)
te
15
0 mM 10mM 50mM
d
0
20
ip t
5
cr
% Intensity
15
0
Intensity (%)
20
15
0mM 5mM 10mM
(e)
10
5
0
1
D (nm)
10
Fig. 5
Page 13 of 29
Table 2. ILs
D (nm) 5mM
10mM
25 mM
50 mM
[Bmim][PhCOO]
2.1
3.1
3.6
4.3
4.6
[Bmim][OAc]
2.1
2.9
3.2
3.7
3.9
PhCOONa
2.1
2.6
3.8
-
NaOAc
2.1
-
2.9
-
NaBr
2.1
2.7
3.0
ip t
0 mM
-
cr
3.3 -
us
-
an
When [Bmim][OAc] was added into the C12TPB aqueous solution, the peak diameters (D) of the micelles only increase from 2.1 to 3.9 nm. This result indicates
M
that the anions of [Bmim][OAc] only adsorbed on the surface of micelle, and most of the anions may stay in the bulk solutions and have no effect on the properties of
te
d
micelles. It is noted that the peak diameters of the micelles also slightly increase with the addition of NaOAc and NaBr, as shown in Fig. 5(D) and (E). It is demonstrated
Ac ce p
that the IL [Bmim][OAc] show the electrolytic behavior within aqueous C12TPB solutions. Whereas the addition of [Bmim][PhCOO] leads to a more significant growth of the C12TPB micelles, from 2.1 to 4.6 nm, as well as 10 mM PhCOONa
results in the C12TPB micelles into 3.9 nm. In other words, the aromatic [PhCOO]anions play a significant role in the micellar growth. It has been proved form the above results that both the electrostatic attraction and π-π stacking interaction between the phenyl anions and triphenyl cations can significantly decrease the electrostatic repulsion among the headgroups of C12TPB and facilitate the micellization. With the increasing [Bmim][PhCOO] or PhCOONa concentration, the [PhCOO]- anions
Page 14 of 29
enhance the hydrophobic effect and the π-π stacking interaction with aligned among the phenyl moieties in C12TPB headgroups. As a result, the packing of the C12TPB molecules becomes denser and in the meantime the micellar aggregation number
ip t
increases, which is responsible for the micellar growth. It can be concluded that the IL
cr
[Bmim][PhCOO] is more effective than [Bmim][OAc] to promote the micelle
3.3. Mechanism of IL effect on C12TPB micellization 4
3
(a) 8 a 2 1 9 20mM 15mM 10mM
M
5mM 1mM 8
5
an
b
us
formation.
5
4
3
e
ppm
1
7 5
6 c
b
7
d
e
te
50mM
d
21
d
8
3 a
6
2
4
(b)
c
25mM 10mM
Ac ce p
5mM
1mM
8.0
1
7.5
4
3
2
1
ppm
Fig. 6
H NMR technique was applied to reveal the intermolecular interaction in the
IL-C12TPB systems. The protons on various carbons are labeled as shown in Fig. 1. The C12TPB concentration was chosen to be 10 mM, which is larger than the CMC values of C12TPB in all the IL systems. Therefore, the observed chemical shifts can
reflected the chemical shift variations of the protons in the micelles. Fig. 6 shows the chemical shifts of C12TPB solutions at different concentrations of [Bmim][PhCOO]
Page 15 of 29
and [Bmim][OAc]. The chemical shift changes of the C12TPB molecules in the 1H NMR spectra are different for various ILs.
H-b
[Bmim][OAc] [Bmim][PhCOO]
[Bmim][OAc] [Bmim][PhCOO]
ip t
H-a 7.56
7.52
3.1
cr
δ (ppm)
δ (ppm)
3.2
7.44
0
10
20
30
40
3.0
50
0
10
20
30
40
50
C (mM)
C (mM)
H-c
H-e
[Bmim][OAc] [Bmim][PhCOO]
0.65
[Bmim][OAc] [Bmim][PhCOO]
δ (ppm)
an
1.35
1.30
0.60
M
δ (ppm)
us
7.48
1.25
10
20
30
te
C (mM)
40
50
d
0
0.55
0
10
20
30
40
50
C (mM)
Fig. 7
Ac ce p
The chemical shifts of protons for C12TPB (Ha-He) as a function of IL
concentration are shown in Fig. 7. With the addition of [Bmim][OAc], no significant variations of the chemical shifts of protons for C12TPB were observed, confirming that the C12TPB micelles do not have any remarkable changes upon the addition of
[Bmim][OAc]. This result is in good agreement with the conclusion above that the anions of [Bmim][OAc] are only slightly adsorbed on the surface of the micelles. Differently, the IL [Bmim][PhCOO] causes distinct changes of the chemical shifts of the protons for C12TPB. As shown in Fig. 7, the chemical shifts of protons around the headgroup (Ha-Hc) move upfield, whereas the protons in the terminal methyl (He)
Page 16 of 29
show a downfield shift. This is mainly due to the reason that the addition of [Bmim][PhCOO] generates intermolecular π-π stacking interaction between benzene rings and triphenyl groups. So the protons around the headgroup locate under the ring
ip t
current and resonate in a higher field due to the shielding effect [45,46]. Nevertheless,
cr
due to the relatively far distance to the headgroup, the shielding effect for He is feeble
us
and the medium effect plays the main role. The medium effect is caused by the movement of surfactant molecules from water to micelles, and the decrease in the
an
polarity of the microenvironment leads to a downshift for He [47-49]. Interestingly, the protons in the alkyl chain (Hd) show a distinct splitting with the increasing
M
concentrations of [Bmim][PhCOO]. It is proposed that the protons in alkyl chain are
Ac ce p
te
d
affected by both shielding effect and medium effect.
Page 17 of 29
7.34
[Bmim][OAc] [Bmim][PhCOO]
H-1
[Bmim][OAc] [Bmim][PhCOO]
H-2
δ (ppm)
δ (ppm)
7.38
7.32
7.36
7.34
0
10
20
30
40
0
50
10
H-4
[Bmim][OAc] [Bmim][PhCOO]
H-3
δ (ppm)
4.08
3.78
3.76 4.06 30
40
0
50
H-5
[Bmim][OAc] [Bmim][PhCOO]
1.24
δ (ppm)
1.74
1.72
20
30
40
50
[Bmim][OAc] [Bmim][PhCOO]
H-6
10
20
30
40
1.20
1.16
50
d
0
C (mM)
0
10
20
30
40
50
C (mM)
[Bmim][OAc] [Bmim][PhCOO]
te
H-7
0.82
Ac ce p
δ (ppm)
10
M
δ (ppm)
1.76
0.84
50
C (mM)
C (mM)
an
20
us
δ (ppm)
4.10
10
40
[Bmim][OAc] [Bmim][PhCOO]
3.80
0
30
cr
4.12
20
C (mM)
C (mM)
ip t
7.30
0.80
0
10
20
30
40
50
C (mM)
7.6
Fig. 8 1.90
8 COO
9
(b)
1.85
9
7.4
8 9
7.2
δ (ppm)
δ (ppm)
H-8
(a)
1.80
8
H-8 H-9
7.0
0
CH3 COO 5
10
15
20
1.75
0
10
20
30
40
50
C (mM)
C (mM)
Fig. 9
Page 18 of 29
The changes in chemical shifts of protons for ILs in the C12TPB aqueous solution can also reflect their interactions with the C12TPB molecules. Fig. 8 shows the chemical shifts of various protons for the IL cations as the function of IL
ip t
concentrations. It is obvious that all the protons of the imidazolium cations show
cr
downfield shifts, especially for [Bmim][PhCOO]. As discussed above, the decrease in
us
the polarity of the microenvironment can lead to a downshift for the protons. So it is reasonable to assume that the imidazolium cations of ILs may participate in the
an
micelle formation. When micelles are formed, the butyl chains of imidazolium cations are immersed in a hydrophobic core and the hydrophilic headgroups are solvated by
M
the interfacial water whose polarity is lower than the bulk water. Interestingly, all the chemical shift changes exhibit a distinct break point when the IL concentration
te
d
reaches 10 mM, indicating the equilibrium of [Bmim]+ into the micelles. For the protons in IL anions (Fig. 9), the same downfield chemical shifts can be observed on
Ac ce p
basis of the medium effect, and the H9 in the phenyl anions of [Bmim][PhCOO]
exhibits the extreme changes in chemical shifts. Interestingly, the H8 shows an opposite trend and shifts to upfield. We contribute the upfield shift to the shielding effect of ring current between the headgroups of C12TPB. This result indicates that the phenyl anions of [Bmim][PhCOO] not only simply adsorb on the micelle surface but also penetrate into the palisade layer of the micelle due to the π-π stacking interaction. The distinct break points around 10 mM of the IL concentration indicate that the adsorption of the anions has been saturated with 10 mM concentration, and the excess ions are free in the bulk water.
Page 19 of 29
(a)
bulk water
Br
N N
N N
CH 3COO
N N Br
Br
N
CH3COO
Br
N
CH3COO
CH3COO
P
ip t
P
us
cr
P
an
micelle bulk water
(b) OOC
N
COO
N
N Br N
M
Br
d
P
Br N
OOC
COO
P
Ac ce p
te
OOC
N
N
P Br
N
micelle
Scheme 1.
According to the analysis above, the possible arrangement and different location of
ILs in the C12TPB micelles are depicted in Scheme 1. When [Bmim][OAc] or [Bmim][PhCOO] was added in the C12TPB solution, the imidazolium cations of the both ILs may participate in the micelle formation with the butyl chains of imidazolium cations immersing in a hydrophobic core. While the anions of the two
Page 20 of 29
ILs affect in the different ways. Both [OAc]- and [PhCOO]- can be adsorbed at the micellar surfaces due to the electrostatic interaction. In the case of [Bmim][PhCOO], the [PhCOO]- can also aligned into the C12TPB micelles with the enhanced
ip t
hydrophobic effect and π-π stacking interaction, resulting in a smaller CMC values
cr
and micellar growth.
us
4. Conclusions
The present work investigated the effects of aromatic imidazolium ILs on the
an
micellization of C12TPB in aqueous solution. Both [Bmim][PhCOO] and [Bmim][OAc] can facilitate the micelle formation of C12TPB, and a better surface
M
activity of aqueous C12TPB solution can be obtained with the addition of [Bmim][PhCOO]. DLS data demonstrate that [Bmim][PhCOO] makes C12TPB to
te
d
pack more densely, resulting in a bigger micellar size. Compared with [Bmim][OAc], the introduction of benzene rings in the anions of [Bmim][PhCOO] can promote the
Ac ce p
micellization of C12TPB more efficiently due to the enhanced hydrophobic effect and
π-π stacking interaction. This work is helpful to understand the effects of ILs with aromatic structures on the aggregation behavior of surfactant in aqueous solution. Acknowledgments
The authors are grateful to the National Natural Science Foundation of China (No.91127017), the National Basic Research Program (2013CB834505), Specialized Research Fund for the Doctoral Program of Higher Education of China (No.20120131130003) and the Shandong Provincial Natural Science Foundation, China (ZR2012BZ001).
Page 21 of 29
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ip t
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Figure and table captions:
Fig. 1 The chemical structures and 1H NMR signal assignment of the surfactant and
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ILs.
Fig. 2 Surface tension curves of C12TPB plotted against the surfactant concentration
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in the presence of different additive concentrations at 25 °C: (a) [Bmim][PhCOO]; (b)[Bmim][OAc]; (c) PhCOONa; (d) NaOAc; (e) NaBr.
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Fig. 3 CMC values of C12TPB plotted against the concentrations of different additives.
concentration of different ILs.
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Fig. 4 Plots of γ CMC (a), Π CMC (b) and pC 20 (c) values of C12TPB as a function of the
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Fig. 5 Size distributions of 5 mM C12TPB at different additive concentrations at 25°C:
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(a) [Bmim][PhCOO]; (b) [Bmim][OAc]; (c) PhCOONa; (d)NaOAc; (e)NaBr Fig. 6 Proton assignments and 1H NMR spectra of 10 mM C12TPB with various concentrations of ILs at 25°C: (a) [Bmim][PhCOO]; (b) [Bmim][OAc]. Fig. 7 Chemical shifts of protons for C12TPB against different IL concentrations.
Fig. 8 Chemical shifts of protons for IL cations against different IL concentrations. Fig. 9 Chemical shifts of protons for IL anions against different IL concentrations: (a)
[Bmim][PhCOO]; (b) [Bmim][OAc]. Scheme 1. Cartoon depicting segments the C12TPB micelles in the presence of (a) [Bmim][OAc] and (b) [Bmim][PhCOO]. Table 1. Surface properties and micellization parameters of aqueous C12TPB solutions in the presence of different concentrations of ILs at 25°C. Table 2. Micellar sizes of 5 mM C12TPB aqueous solution in the presence of different concentrations of ILs at 25°C.
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Graphical abstract
60
50
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γ (mN/m)
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1mM 5mM 10mM 25mM 50mM
70
1E-7
1E-6
1E-5
1E-4
1E-3
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40 0.01
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C/M
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Surface tension curves of C12TPB / [Bmim][PhCOO] system and the
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proposed mechanism.
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Highlights A kind of room‐temperature ILs with aromatic anions was successfully synthesized.
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The aromatic ILs affect the micellization of a surfactant with aromatic headgroups.
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te
d
M
an
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The enhanced π‐π stacking interaction and hydrophobic interaction play a key role.
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