Micellization, surface activities and thermodynamics study of pyridinium-based ionic liquid surfactants in aqueous solution

Article abstract of DOI:10.1039/c9ra04226a

The micellization and surface activity properties of long-chain pyridinium ionic liquids n-Alkyl-3-methylpyridinium bromide ([C_nmpy][Br], n: The carbon numbers of hydrophobic tails, n = 12, 14, 16) in aqueous solution were systematically investigated through electronic conductivity measurement, surface tension, and ultraviolet-Absorption spectra. The surface chemical parameters and thermodynamics parameters were obtained. The [C_nmpy][Br] ionic liquids exhibit higher surface activities than conventional surfactants with corresponding alkyl chain lengths. The effects of inorganic salts (LiBr, NaBr, MgBr₂), organic alcohols (C₂H₅OH, C₃H₇OH, C₄H₉OH, C₅H₁₁OH) and temperature on the critical micelle concentration (CMC) values of [C_nmpy][Br] aqueous solutions were also investigated. The CMC values remarkably decreased with the addition of inorganic salts. The CMC values increased slightly in the presence of ethanol, but decreased gradually as the chain length of the alcohol increased. The CMC values assumed a trend of decreasing and then increasing with the increase of temperature. The calculation results of thermodynamic parameters show that both adsorption and micellization processes of [C_nmpy][Br] are spontaneous; the enthalpy of [C₁₂mpy][Br] is negative at 293.15 K and becomes negative with temperature increasing. For [C₁₄mpy][Br] and [C₁₆mpy][Br] this transition occurs at 288.15 K and the micellization process is entropy-driven in the investigated temperature range.

Full text of DOI:10.1039/c9ra04226a

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PAPER
Micellization, surface activities and
thermodynamics study of pyridinium-based ionic
Cite this: RSC Adv., 2019, 9, 28799
liquid surfactants in aqueous solution†
a
Dong Fu,
Xiaoru Gao,b Bo Huang,a Jue Wang,a Yao Sun,a Weijun Zhang,a
a
a
Kan Kan, Xiaochen Zhang,
Yang Xiea and Xin Suia
*
*
The micellization and surface activity properties of long-chain pyridinium ionic liquids n-alkyl-3-
methylpyridinium bromide ([Cnmpy][Br], n: the carbon numbers of hydrophobic tails, n ¼ 12, 14, 16) in
aqueous solution were systematically investigated through electronic conductivity measurement, surface
tension, and ultraviolet-absorption spectra. The surface chemical parameters and thermodynamics
parameters were obtained. The [Cnmpy][Br] ionic liquids exhibit higher surface activities than
conventional surfactants with corresponding alkyl chain lengths. The effects of inorganic salts (LiBr,
NaBr, MgBr2), organic alcohols (C2H5OH, C3H7OH, C4H9OH, C5H11OH) and temperature on the critical
micelle concentration (CMC) values of [Cnmpy][Br] aqueous solutions were also investigated. The CMC
values remarkably decreased with the addition of inorganic salts. The CMC values increased slightly in
the presence of ethanol, but decreased gradually as the chain length of the alcohol increased. The CMC
values assumed a trend of decreasing and then increasing with the increase of temperature. The
calculation results of thermodynamic parameters show that both adsorption and micellization processes
of [Cnmpy][Br] are spontaneous; the enthalpy of [C12mpy][Br] is negative at 293.15 K and becomes
negative with temperature increasing. For [C14mpy][Br] and [C16mpy][Br] this transition occurs at 288.15 K
and the micellization process is entropy-driven in the investigated temperature range.
Received 5th June 2019
Accepted 1st September 2019
DOI: 10.1039/c9ra04226a
rsc.li/rsc-advances
transfer catalysts,9 oil recovery,10 drug/gene delivery systems,11
dispersants of petroleum asphaltenes,12 green solvents of
carbohydrate,13 cosmetics14 and textile industry.15 Therefore,
micellization behavior of ILs has become one of the intriguing
objects in the eld of colloid and interface chemistry. The
analysis of physical and thermodynamic behavior of ILs in
aqueous solution is very important to help us comprehend their
applicability in different industries.
Micellization behavior of ILs based on the imidazolium,
pyrrolidinium, piperidinium has been intensively explored.16–24
Some papers have reported the aggregation behavior of
pyridinium-based ILs in water.25–27 These researches mainly
focused on the N-substitution pyridinium and anions. Only few
studies on the colloidal chemical aspects of amphiphilic
methylpyridinium based ILs were reported.16,25,28–32 Fayyaz et al.
synthesized ve new surfactants [Cnmpy][Br] (n ¼ 6, 8, 10, 12,
14) and investigated the micellization behavior of [Cnmpy][Br]
in ethanol.33,34 The results showed that [Cnmpy][Br] had lower
CMC value in ethanol and signicant antimicrobial activities.
However, the adsorption of [Cnmpy][Br] at the air/water inter-
face and the mechanism of micelle formation in water have not
yet been investigated.
1 Introduction
Ionic liquids (ILs), which melt at or close to room temperature,
are a class of organic molten electrolytes composed of an
organic cation and an inorganic anion. They have been
receiving much attention over recent years due to their specic
chemical and physical properties such as nonammability,
negligible vapor pressure, low volatility, high thermal stability,
high ionic conductivity, and easy recyclability.1,2 These unique
properties make ILs suitable candidates for a large range of
applications.3 ILs bearing long alkyl chains have shown obvious
amphiphilic characters and could possess surface active prop-
erties similar to the conventional cationic surfactants. Owing to
the inherent molecular nature, the long-chain IL molecules can
self-assemble in aqueous media to form micelles, vesicles, lyo-
tropic liquid crystals, and gels, etc.4 The self-assembly of ILs
make it suitable for a range of application such as liquid–solid
extraction,5 corrosion protection,6 synthesis of functionalized
carbon nanotubes,7 synthesis of CuO nano-structures,8 phase-
aHeilongjiang Academy of Sciences Institute of Advanced Technology, Harbin, 150020,
China. E-mail: kankan.has@foxmail.com; xc_zhang.has@hotmail.com
bHarbin FRP Research Institute, Harbin, Heilongjiang 150029, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra04226a
In this work, we studied the micellization behavior and
surface activities of [Cnmpy][Br] (n ¼ 12, 14, 16) in aqueous
solution in detail by conductivity, surface tension and UV
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absorption spectra. In addition, the counterion and alcohol
have strong inuence on CMC value and shape of the aggre-
gates of the micelle system.35,36 Thus, we also investigated the
effects of the additives (inorganic salts and organic alcohol) on
the micellization behavior of [Cnmpy][Br] in aqueous solution.
As an extension of the previous study, we also analyzed
adsorption and micellization processes of [Cnmpy][Br] aqueous
solution through thermodynamic parameters. These thermo-
dynamic parameters were calculated from the temperature
dependence of CMC value and the degree of dissociation of
micelles (b), which obtained by approach in the conductivity
data analysis based on a nonlinear t. This research will
promote our basic understanding of the aggregation behavior of
[Cnmpy][Br] in water.
Fig. 1 The synthetic pathways of [Cnmpy][Br] (n ¼ 12, 14, 16).
ESI, Fig. S1† shows (1H, 13C) NMR and FTIR spectra of syn-
thesised [Cnmpy][Br].
2 Experimental
2.1 Materials
3-methylpyridinium (97%), 1-bromododecane (97%), 1-bromo-
tetradecane (97%), 1-bromohexadecane (97%), toluene (99%),
ethyl acetate (97%), ethanol (99%), n-propanol (99%), n-butanol
(99%), and n-pentanol (99%), pyrene (97%), lithium bromide
(LiBr analytical reagents, $99%), sodium bromide (NaBr
analytical reagents, $99%), magnesium bromide (MgBr2
analytical reagents, $99%), was obtained from Macklin
2.4 Surface tension measurements
The surface tension of ILs solutions were measured with a Du
¨
¨
Nouy tensiometer (Kruss K100, Germany) by applying the plat-
inum ring technique. The surface tension recorded was related
to the force required to li the ring from the surface of the air–
liquid interface.37 Measurements were performed at 298.15 ꢂ
0.10 K until successive values agreed to within 0.2 mN mꢁ1
.
Reagent. Dilute stock solutions of pyrene (1 ꢀ 10ꢁ3 mol Lꢁ1
)
were prepared by dissolving pyrene in methanol in a volumetric
ask. Samples for UV were prepared as follows: equivalent
amounts of probe stock solutions were transferred into test
tubes and evaporated with nitrogen. Then the ILs solution was
added.
2.5 Conductivity measurements
The conductivity of the ILs solutions was measured by con-
ductometer (DDS-307, Shanghai Precision & Scientic Instru-
ments, China). A super-thermostatic cistern (temperature
uctuation: ꢂ0.01 K, SC-30, Shanghai Guning Industrial Co.,
Ltd, China) was used to control the temperature. The conduc-
tivity measurements were carried out at different temperatures
for each ILs system.
2.2 Instrumentation
Conrmation of the structures of [Cnmpy][Br] was obtained by
infrared spectroscopy (FTIR) and nuclear magnetic resonance
(NMR). FTIR spectra (KBr) were recorded in the 4000–400 cmꢁ1
range on a Lambda 7600 s FT-IR spectrometer (Lambda, Aus-
2.6 Ultraviolet absorption spectra
1
tralia). H, 13C NMR spectra were recorded on a Bruker Avance
The UV absorption of the ILs solutions were measured on a T-6
ultraviolet-visible spectrophotometer (Beijing Purkinje General
Instrument Co. Ltd. China the spectra of the samples were
400 nuclear magnetic resonance (Bruker, Switzerland) oper-
ating at 400 and 100 MHz respectively with CDCl3 as a solvent,
and tetramethylsilane (TMS) as an internal standard.
scanned from 220 nm to 400 nm). Pyrene (1.0 ꢀ 10ꢁ4 mol Lꢁ1
)
was used as absorption probe. These spectra were recorded
against a blank of ILs solutions, and the CMC values can be
obtained from the curves of absorption intensity versus
concentration of ILs solutions.
2.3 Synthesis
[C12mpy][Br] were synthesized according to a previously re-
ported procedure.33 Briey, equimolar amounts (0.1 mol) of 3-
methylpyridinium and 1-bromododecane were mixed in 30 mL
dry toluene in a 100 mL three-necked ask and reuxed for 6 h
under nitrogen atmosphere. Aer the reaction, toluene was 2.7 Surface chemical parameter calculation
removed under vacuum and the pure [C12mpy][Br] was recrys-
Adsorption efficiency (pC20). The adsorption efficiency (pC20)
tallized in fresh ethyl acetate at least four times. The syntheses
of [C14mpy][Br] and [C16mpy][Br] are very similar to the above
procedure, in which 1-bromotetradecane and 1-bromohex-
adecane were used, respectively. The synthetic pathways of the
surfactants are shown in Fig. 1.
is calculated using the following equation:
pC20 ¼ ꢁlg Cp¼20
(1)
In this equation, pC20 is the negative log of the ILs surfac-
These synthesized surfactants were characterized by 1H, 13
C
tants concentration which reduce the surface tension of the
NMR and FTIR spectra, the detailed spectroscopic data were in pure solvent by 20 mN mꢁ1
.
22,23,38
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Effectiveness of surface tension reduction (Pcmc). To show
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the ability to decrease surface tension of solutions, the effec-
tiveness of the surface tension reduction (Pcmc) is dened as:
Pcmc ¼ g0 ꢁ gcmc
(2)
where g0 is the surface tension of bidistilled water and gcmc is
the surface tension of the solutions at CMC.22,23,38
Maximum surface excess concentration (Gmax). The
maximum surface excess concentration (Gmax) at the air/water
interface was estimated by applying Gibbs adsorption isotherm:
ꢀ
ꢁ
1
dg
Gmax ¼ ꢁ
(3)
2:303nRT dlog C
T;P
where, R is the gas constant (8.314 J molꢁ1 Kꢁ1), T is the abso-
lute temperature and C is the surfactant concentration. As there
is one counter ion associated with one ionic head group, the
value of n is taken to be 2.18,20,38
Fig. 2 Conductivity dependence of concentration for [Cnmpy][Br] in
aqueous solution at 298.15 K.
Minimum surface area (Amin). Minimum surface area (Amin
)
rapidly in the pre-micellar region. This is due to the concen-
tration of current carriers increased with the rising of ILs
concentration. Due to binding of a fraction of the counterions to
the micellar surface resulting in an effective loss of ionic
charges, and a slower rise in the isotherm area of electrical
conductivity aer micellization.26 Moreover, the proposed
function will accurately represent the physical model to which
the conductivity data would adhere. This method avoids any
data processing and thus avoids the introduction of noise. By
using nonlinear t simply search for the best correlation, the
evaluation of the degree of counterion binding b could be per-
formed, avoiding artifacts due to the individual selection of
points to be included in the linear t of the linear regimes.
The surface tension measurement is a classical method in
studying the CMC of surfactants. The variations of the surface
tension (g) with the ILs concentration (C) at 298.15 K are shown
in Fig. 3. The plots of g versus log C show a breakpoint
concentration corresponding to the CMC value of the three
surfactants. The CMC values are listed in Table 1.
provides information on the degree of packing and orientation
of the adsorbed surfactant molecule. The minimum area
occupied by a single amphiphilic molecule at the air–water
interface (Amin) was calculated by applying the Gibbs adsorption
isotherm:
1
Amin
¼
(4)
NAGMax
where NA is the Avogadro constant.20,38
3 Results and discussion
3.1 The CMC of [Cnmpy][Br]
The aggregation behavior of the three surfactants aqueous
solutions was studied via conductivity measurements. As far as
the conductivity measurements are concerned, the CMC values
were obtained at the point of intersection of the linear portions
of conductivity versus concentration plots above and below the
breakpoint. Here, we used a different approach in the conduc-
tivity data analysis based on a nonlinear t introduced by Car-
pena et al.39 to determine common micellization parameters of
ILs, such as CMC value and the degree of dissociation of
micelles (b). This method was successfully applied to analyze
conductivity data on several surfactants.10,30,40,41 The conduc-
tivity data were tted by the function reported in eqn (5), which
represents the integral function of a sigmoid:
ꢀ
ꢁ
1 þ eðcꢁc Þ=Dc
0
kðcÞ ¼ kð0Þ þ A1c þ DcðA2 ꢁ A1Þln
(5)
1 þ eꢁc =Dc
0
where k0 is the initial conductivity of water, A1 and A2 are the
limiting slopes for low and high concentration respectively, C0
is the central point of the transition (i.e. CMC value) and DC is
the width of the transition. The b value can be deduced from the
ratio A2/A1. The variations of conductivity against concentration
of ILs at 298.15 K are shown in Fig. 2.
As shown in Fig. 2, it is obvious that a trend changes of rapid
rise and then slow rise appeared in the curves with the increase
Fig.
3 Surface tension vs. concentration plot of [Cnmpy][Br] in
of ILs concentration. The conductivity of the solution rising aqueous solution at 298.15 K.
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Table 1 The values of CMC determined by conductivity measure-
ment, surface tension and UV absorption spectra for the [Cnmpy][Br] in
aqueous solution at 298.15 K
3.2 Surface activity of [Cnmpy][Br]
The surface properties of the surfactants including the CMC,
surface tension (gcmc), the maximum surface pressure (Pcmc),
adsorption efficiency (pC20), maximum surface excess concen-
tration (Gmax), and the minimum surface area (Amin) are listed in
Table 2, together with the data reported for dodecyltrimethyl
ammonium bromide (DTAB), tetradecyltrimethyl ammonium
bromide (TTAB), cetyltrimethyl ammonium bromide (CTAB)
and n-alkyl-pyridinium bromide ([Cnpy][Br], n ¼ 12, 14, 16).
From the obtained results, all the synthesized surfactants
show moderate surface activity. It is obvious that surface
tension (gcmc) decrease from 39.2 to 37.6 mN mꢁ1, which is due
to the increased hydrophobic nature of the ILs. The CMC values
of ILs are compared with those of DTAB, TTAB, CTAB and [Cnpy]
[Br] (n ¼ 12, 14, 16) in Table 2. It is observed that CMC values of
ILs are lower in comparison to these conventional cationic
surfactants and [Cnpy][Br]. The CMC values of ILs gradually
decrease as the hydrophobic chain length increases from 12 to
16. This indicates a relationship between CMC value and alkyl
chain length of ILs, that is, the longer the alkyl chain is, the
lower CMC value will be. Two important parameters of surfac-
CMC (mmol Lꢁ1
)
Conductivity
Ultraviolet
ILs
measurements
Surface tension absorption
[C12mpy][Br] 9.74
[C14mpy][Br] 2.40
[C16mpy][Br] 0.567
9.83
2.26
0.508
10.04
2.33
0.520
As shown in Fig. 3, surface tension values decrease rapidly
with increasing surfactant concentration due to the adsorption
of molecules at the air–liquid interface. Then a distinct break
point appears, indicating the formation of micelles.19 The CMC
value was determined from the break point of the plot. Aer the
break point, the surface tension remains nearly constant with
increasing ILs concentrations. This is due to the saturation of
the air–liquid interface with ILs molecules.37
The UV spectrometric technique is also an easy method to
determine the CMC of surfactants.42,43 In this method, pyrene
was used as absorption probe. The maximum characteristic
absorption wavelength (lmax) of pyrene was determined by
plotting the absorption intensity of pyrene vs. wavelength,
which was 338 nm. Subsequently, the CMC values were
measured by plotting absorption intensity vs. various concen-
trations of ILs solutions containing pyrene at 338 nm. Fig. S2†
shows the absorbance plots affected by varying concentrations
of [Cnmpy][Br] (n ¼ 12, 14, 16) containing pyrene in aqueous
solution. In polar region, the intensity of the peaks enhanced
slowly, while the absorption intensity enhanced dramatically
when the microenvironment changed. Thus, the abrupt change
on the curve of absorption intensity vs. concentration indicated
the formation of micelles.44 Fig. 4 shows the plots of absorption
intensity vs. concentration of [Cnmpy][Br] at 338 nm. The CMC
values of [Cnmpy][Br] with different chain length obtained from
the three methods mentioned above are listed in Table 1.
tants, i.e. the effectiveness of surface tension reduction (PCMC
)
and the adsorption efficiency (pC20) were obtained from the
surface tension plots. The maximum reduction in surface
tension caused by the dissolution of amphiphilic molecules has
been indicated by PCMC and as a result PCMC becomes a symbol
for the effectiveness of the amphiphile to lower the surface
tension of the water.50 Pyridinium-based surfactants synthe-
sized in present work have greater ability to reduce surface
tension of the aqueous system. pC20, generally used to indicate
the efficiency of surfactant to decrease surface tension of water
or other solvent, is found to increase in the order of [C12mpy]
[Br] < [C14mpy][Br] < [C16mpy][Br]. Comparing with conven-
tional surfactants, [Cnmpy][Br] show better surface activity. The
ratio of CMC/C20 can evaluate the structural factors and various
microenvironments in the process of adsorption or micelliza-
tion.37 The greater the CMC/C20 is, the greater tendency of the
amphiphile has to reduce surface tension of the system. Thus,
[C12mpy][Br] have a strong ability to reduce surface tension of
aqueous system, while the corresponding ability of [C14mpy][Br]
and [C16mpy][Br] is weaker. The maximum surface excess
concentration (Gmax) and the minimum surface area (Amin), two
additional important parameters of surfactants which reect
the molecular arrangement of ILs molecules at the air–liquid
interface, were estimated by Gibbs adsorption isotherm. Table 2
shows an increasing trend in Gmax and decreasing Amin values.
This indicates that the ILs with longer alkyl chains have higher
hydrophobicity, leading to greater adsorption of more mole-
cules at the interface. The greater the adsorption, the smaller
the effective area (Amin) of the ILs molecules at the surface is. ILs
molecules with longer alkyl chains form more compact mono-
layers that occupy less area.37 It is interesting to nd that the
trend for Gmax value of [Cnmpy][Br] is opposite to [Cnpy][Br],
although [Cnmpy][Br] possess a much larger headgroup. This
result may be due to the more dispersed charge of the [Cnmpy]+
cation, which could weaken the electrostatic repulsion among
Fig. 4 The plots of absorption intensity vs. concentration of [Cnmpy]
[Br] at 338 nm.
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Table 2 Surface active properties of the synthesized surfactants
ILs
CMC (mmol Lꢁ1
)
pC20
gcmc (mN mꢁ1
)
Amin (nm2)
Gmax (mmol mꢁ2
)
Pcmc (mN mꢁ1
)
CMC/C20
[C12mpy][Br]
[C14mpy][Br]
[C16mpy][Br]
[C12py][Br]
[C14py][Br]
[C16py][Br]
9.83
2.26
0.508
10.0 (ref. 46)
2.70 (ref. 46)
0.64 (ref. 46)
2.8
3.1
3.8
2.3 (ref. 47)
—
39.2
38.8
37.6
39.3 (ref. 26)
38.0 (ref. 26)
44.0 (ref. 48)
45.0 (ref. 49)
—
0.903
0.794
0.697
0.71 (ref. 26)
0.86 (ref. 26)
1.26 (ref. 48)
1.84
2.09
2.38
2.34 (ref. 26)
1.93 (ref. 26)
1.32 (ref. 48)
32.9
33.3
34.5
32.93 (ref. 47)
—
6.22
2.86
3.22
2.53 (ref. 47)
—
—
2.7 (ref. 48)
28.5 (ref. 48)
DTAB45
TTAB45
CTAB45
12.45
3.60
0.89
2.1
2.9
3.2
0.476
0.514
0.459
3.49
2.09
3.62
32.4
32.9
28.1
1.57
2.86
1.41
—
—
head groups.19 In addition, for [Cnmpy][Br], the relationship of arising from a need to minimize the exposure of the hydro-
minimum surface area per amphiphilic molecule versus the phobic core to water.28 Increasing amount of counterions will
number of carbon atoms in the alkyl chain is linear, which can alter the two forces. As a result, adsorption of the counterions
be described by a0 ¼ 1.519 ꢁ 0.0515n.
onto the aggregates surface can reduce the repulsive between
pyridine head group of [Cnmpy][Br], thereby lowering the CMC
values of the [Cnmpy][Br]. Moreover, it is found that cations had
an insignicant effect on the CMC value.
3.3 Effect of additives on micellization of [Cnmpy][Br]
As is well known, the aggregation formation of surfactants in
water can be observably affected by the nature of additives
presented in solution.51 Therefore, it is very important to study
the effect of additives on the CMC value of surfactants in both
theory and practice.
As can be seen from Table 3, the effect of inorganic salts and
organic alcohols on the CMC value of ILs show the same trend.
It is obvious that the CMC value decrease when inorganic salts
were presented in ILs aqueous solution. Fig. S3 to S9† showed
the electrical conductivities plots affected by inorganic salts and
different organic alcohols in varying concentrations of [Cnmpy]
[Br] (n ¼ 12, 14, 16) aqueous solution. This phenomenon can be
explained as the counterion effect. In general, aggregation
formation for surfactants is dictated by a balance between the
repulsive headgroup interactions and the attractive forces to
The CMC value of ILs tends to increase slightly with added
ethanol, and decrease gradually when n-propanol, n-butanol
and n-pentanol are added. In general, the polar organic
molecular affects the properties of surfactants by protecting or
destroying the structure of water surrounding the hydrophobic
chains. Ethanol is solubilized mainly in water and “palisade
layer” of micelle aggregates formed by ILs molecules,52 which
destroying the “iceberg structure” surrounding the hydrophobic
chains of surfactant molecules, weakening the hydrophobic
effect and the ability of micellization of surfactants. Thereby the
CMC value tend to increase slightly. However, with the increase
of alkyl chain length of additive alcohol, the CMC values
decrease gradually because of two reasons: (i) the solubility of
alcohol in water decreases, forming the “iceberg structure”
Table 3 The values of CMC for system in the solution of water added inorganic salt and organic alcohols (the molarity of inorganic salt is
0.20 mmol Lꢁ1, MgBr2 is 0.10 mmol Lꢁ1, the mass fraction of alcohols is 1%) at different temperatures
CMC (mmol Lꢁ1
No additive
)
ILs
T (K)
LiBr
9.18
9.30
9.58
10.03
10.30
1.56
1.67
1.88
2.08
2.17
0.502
0.513
0.531
0.541
0.570
NaBr
MgBr2
C2H5OH
C3H7OH
C4H9OH
C5H11OH
[C12mpy][Br]
298.15
303.15
308.15
313.15
318.15
298.15
303.15
308.15
313.15
318.15
298.15
303.15
308.15
313.15
318.15
9.74
9.77
10.02
10.54
11.47
2.40
2.45
2.51
2.81
2.84
0.567
0.590
0.597
0.647
0.668
9.17
9.23
9.32
9.70
10.56
1.53
1.68
1.82
2.08
2.23
0.474
0.513
0.536
0.597
0.609
9.23
9.42
9.79
10.11
10.80
1.77
1.88
1.93
2.15
2.26
0.348
0.349
0.354
0.362
0.384
9.83
9.92
10.14
10.56
11.82
2.48
2.56
2.66
2.84
2.91
0.575
0.596
0.613
0.665
0.690
9.38
9.46
9.70
9.98
10.48
2.30
2.38
2.49
2.67
2.78
0.476
0.503
0.591
0.657
0.668
8.24
8.26
8.47
8.64
8.84
2.10
2.18
2.29
2.36
2.48
0.466
0.498
0.541
0.568
0.596
5.94
5.80
5.91
5.95
6.26
1.48
1.42
1.50
1.69
1.93
0.453
0.473
0.528
0.572
0.576
[C14mpy][Br]
[C16mpy][Br]
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DGqads ¼ DGmq ꢁ PCMC/Gmax
(7)
where DGqm and DGaqds are the standard Gibbs free energy
changes of micellization and adsorption, respectively. b is the
degree of dissociation, the gas constant R ¼ 8.314 J molꢁ1 Kꢁ1, T
is the absolute temperature, Xcmc is CMC in mole fraction; Xcmc
¼ CMC/55.4, where CMC is in mol Lꢁ1 and the factor 55.4
accounts for the fact that 1 L of water corresponds to 55.4 mol of
water at 298.15 K.
The enthalpy of micellization (DHqm) can be determined by
using the Gibbs–Helmholtz equation:21,38
ꢂ
ꢃ
vðlog cCMC
vT
Þ
DHmq ¼ ꢁ2:3 ꢀ ð2 ꢁ bÞRT2
(8)
Fig. 5 Schematic illustration of [Cnmpy][Br] micelle in the presence of
inorganic salts and organic alcohols in water.
The term (vlog Xcmc/vT)P was calculated by tting a second
order polynomial to the log Xcmc versus temperature plots and
by taking the corresponding temperature derivative.57 The
coefficients of polynomials are given in Table 1S in ESI.†
Finally, the standard entropy change during the micelle
formation, DSqm is calculated using eqn (9):
around the long chain alcohol molecules. This effect is bene-
cial to the participation of long chain alcohols in the micelli-
zation process. (ii) The long chain alcohol molecules can also
insert into the palisade layer, which can decrease the electro-
static repulsion forces of ionic head-groups in the micelle,21 this
effect caused by long chain alcohols corresponded to that of
cosurfactants. Both of above reasons justied the lower CMC
values when the additives were n-propanol, n-butanol and n-
pentanol. According to the analysis above and literature
reports,53–55 the possible structure of micelle in the presence of
inorganic salts and organic alcohols in water was depicted and
shown in Fig. 5.
DHm ꢁ DGm
DSmq
¼
(9)
T
The thermodynamic parameters for ILs at different temper-
atures are listed in Table 4. As can be seen from Table 4, the
changes of the free energy of adsorption (DGqads) and micelli-
zation (DGqm) are all negative, indicating a spontaneous process
of micellization and adsorption in water. A higher magnitude of
DGqads is observed as compared to DGqm values, showing that
adsorption process favorably occurs until all the air/solution
interface is occupied, and then surfactants molecules begin to
aggregate and form micelles in the bulk solution.58,59 DGmq and
DGqads become more negative with increasing alkyl chain length
or solution temperature. This may be due to the increased
hydrophobicity of ILs, which can destroy the structure of water
and increase free energy of the system, and improve the
propensity of ILs molecules to migrate to the interface. This
behavior can make energy of the surfactant system decrease.
Thus the values of DGmq and DGaqds are more negative.59 Rising
temperature can cause the decrease of hydration around
hydrophilic groups, which results in the increase of the system
3.4 Thermodynamics of micellization
Conductivities of ILs solutions are studied to estimate their
micellization behavior. The variation values of CMC and b in
different temperatures may be utilized to calculate the ther-
modynamic parameters of micelle formation. The variations of
conductivity against concentration of ILs solutions at different
temperatures are shown in Fig. 6. For cationic surfactants, the
free energy of micellization and adsorption may be calculated
by following equation:37,38,56
DGqm ¼ (2 ꢁ b)RT ln Xcmc
(6)
Fig. 6 Conductivity dependence of concentration for [Cnmpy][Br] in aqueous solution at different temperature.
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Table 4 The values of CMC and b, and thermodynamic parameters for [Cnmpy][Br] in aqueous solution at different temperatures
CMC (mmol
ILs
T (K)
Lꢁ1
)
b
DGqm (kJ molꢁ1
)
DHqm (kJ molꢁ1
)
DSqm (kJ molꢁ1 Kꢁ1
)
TDSqm (kJ molꢁ1
)
DGqads (kJ molꢁ1
)
[C12mpy][Br] 283.15
288.15
9.93
9.82
9.76
9.74
9.77
0.303 ꢁ34.46
0.300 ꢁ35.18
0.287 ꢁ36.09
0.268 ꢁ37.12
0.274 ꢁ37.60
0.270 ꢁ38.20
0.258 ꢁ38.85
0.240 ꢁ39.49
0.311 ꢁ39.99
0.309 ꢁ40.79
0.295 ꢁ41.78
0.295 ꢁ42.46
0.301 ꢁ42.93
0.302 ꢁ43.51
0.285 ꢁ44.16
0.285 ꢁ44.81
0.324 ꢁ45.62
0.322 ꢁ46.52
0.317 ꢁ47.33
0.297 ꢁ48.50
0.307 ꢁ48.86
0.303 ꢁ49.73
0.298 ꢁ50.33
0.295 ꢁ51.08
6.194
2.918
0.1436
0.1322
0.1210
0.1095
0.09583
0.08197
0.07073
0.06223
0.1504
0.1411
0.1320
0.1215
0.1101
0.09865
0.08657
0.07458
0.1629
0.1556
0.1541
0.1503
0.1439
0.1390
0.1330
0.1273
40.65
38.10
35.47
32.65
29.05
25.26
22.15
19.80
42.58
40.65
38.70
36.22
33.37
30.40
27.11
23.73
46.12
45.75
45.18
44.85
43.62
42.85
41.95
40.50
293.15
298.15
303.15
308.15 10.02
313.15 10.54
318.15 11.47
ꢁ0.616
ꢁ4.472
ꢁ8.550
ꢁ12.94
ꢁ16.70
ꢁ19.69
2.585
ꢁ0.138
ꢁ3.082
ꢁ6.237
ꢁ9.560
ꢁ13.11
ꢁ17.05
ꢁ21.08
0.5005
ꢁ0.7750
ꢁ2.148
ꢁ3.653
ꢁ5.237
ꢁ6.884
ꢁ8.681
ꢁ10.58
ꢁ55.00
[C14mpy][Br] 283.15
2.37
2.35
2.38
2.40
2.45
2.51
2.81
2.84
0.527
0.523
0.540
0.567
0.590
0.597
0.647
0.668
288.15
293.15
298.15
303.15
308.15
ꢁ58.39
ꢁ63.00
313.15
318.15
[C16mpy][Br] 283.15
288.15
293.15
298.15
303.15
308.15
313.15
318.15
energy, improves protably to adsorption in the interface and greatly affects the associated micellization process, and that the
formation of micelles in order to decrease energy of the micellization for the ILs in aqueous solution is entropy-
surfactant system.16,37,38 The enthalpy of [C12mpy][Br] is negative driven.17,21,37,58 In addition, we have also found that the
at 293.15 K and becomes negative with temperature increase, temperature dependence of CMC shows typical U-shaped
for the [C14mpy][Br] and [C16mpy][Br] this transition occurs at reaching the minimum value CMC* at the temperature T*
288.15 K, implying that the micelle formation process is endo- form (Fig. S10) in ESI.† These values, obtained from the corre-
thermic at lower temperatures and exothermic at higher sponding derivative of the polynomial CMC ¼ A + BT + CT2 are
temperatures. This behavior can be attributed to the delicate listed in Table 1S in ESI.† As can be seen from Table 1S,† T* is
balance between hydrophobic hydration of nonpolar parts close to temperature T0, where DHqm ¼ 0.60,61,66 The variation of
(endothermic) and counterion binding (exothermic) upon CMC values of [Cnmpy][Br] with temperature could be attrib-
micellization.60,61 The hydrophobic effect is related to the water uted to that temperature increase causes decreased hydration of
removal from the nonpolar surface in order to diminish the hydrophilic ionic head-groups, which facilitates micellization.
extent of less favorable hydrophobic hydration in comparison to Whereas, temperature increase also causes disruption of the
clustered water in the bulk.62 In the case of micellization, water structured water surrounding the hydrophobic alkyl chains, an
molecules are organized around nonpolar surfactant tails at low effect that hinders micellization.21
temperature (they form an iceberg structure, or their dynamics
is slowed down signicantly at least).63 Therefore, the destruc-
tion of this water structure is a highly endothermic process.64
4 Conclusions
In the present work, the micellization behavior and surface
activity of [Cnmpy][Br] (n ¼ 12, 14, 16) in aqueous solution were
investigated by conductivity, surface tension and UV absorption
spectra in detail. It can be concluded that [Cnmpy][Br] have
better surface activities than those conventional surfactants
with the same alkyl chain length. The addition of inorganic salts
and organic alcohol was found to exert a remarkable inuence
on the micellization behavior of [Cnmpy][Br] in aqueous solu-
tions. The CMC values of all the systems decreased drastically in
the presence of inorganic salts. The CMC values increased
slightly in the presence of ethanol, but decreased gradually with
long chain alcohols. The CMC values assumed a trend of
Although the formation of favorable H-bond in the bulk, ion
condensation etc. counteract the endothermic contribution of
the hydrophobic dehydration, the results of micellization are
a slightly positive overall enthalpy change. In addition, water
structure at the hydrophobic surfaces is at least partially
disturbed at high temperature, therefore, the endothermic
contribution of the hydrophobic dehydration to the micelliza-
tion is smaller and ultimately the micellization becomes an
exothermic process.65 For [Cnmpy][Br], the value of the standard
entropy of micellization (TDSqm) is larger than the value of
enthalpy changes (ꢁDHm), indicating that the entropy gain
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decreases and then increases with the increase of temperature. 17 N. V. Sastry, N. M. Vaghela and V. K. Aswal, Fluid Phase
The analysis of thermodynamic parameters indicated that both Equilib., 2012, 327, 22–29.
processes of adsorption and micellization of [Cnmpy][Br] are 18 J. Bowers, C. P. Butts and P. J. Martin, Langmuir, 2004, 20,
spontaneous, the micelle formation process is endothermic at 2191–2198.
lower temperatures and exothermic at higher temperatures. and 19 L. J. Shi, N. Li, H. Yan, Y. A. Gao and L. Q. Zheng, Langmuir,
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entropy-driven in the investigated temperature range. Both 20 J. J. Jiao, B. Dong, H. N. Zhang, Y. Y. Zhao, X. Q. Wang,
adsorption and micellization of [Cnmpy][Br] are inclined to
occur with the increase of alkyl chain length or temperature.
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21 Y. Wei, F. Wang, Z. Q. Zhang, C. C. Ren and Y. Lin, J. Chem.
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Conflicts of interest
There are no conicts to declare.
23 M. W. Zhao and L. Q. Zheng, Phys. Chem. Chem. Phys., 2011,
13, 1332–1337.
24 C. L. Dai, M. Y. Du, Y. F. Liu, S. L. Wang, J. H. Zhao, A. Chen,
D. X. Peng and M. W. Zhao, Molecules, 2014, 19, 20157–
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Acknowledgements
We gratefully acknowledge the support of this research by the
´
Youth Innovation Foundation of Heilongjiang Academy of 25 I. Bandres, S. Meler, B. Giner, P. Cea and C. Lafuente, J.
Sciences (CXMS2019GJS01) and the Natural Science Foundation
of Heilongjiang Province (LH2019B030).
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