A Comprehensive Study on the Synthesis and Micellization of Disymmetric Gemini Imidazolium Surfactants

Article abstract of DOI:10.1007/s11743-016-1830-y

Two groups of disymmetric Gemini imidazolium surfactants, [C₁₄C₄C_mim]Br₂ (m?=?10, 12, 14) and [C_mC₄C_nim]Br₂ (m?+?n?=?24, m?=?12, 14, 16, 18) surfactants, were synthesized and their structures were confirmed by ¹H NMR and ESI–MS spectroscopy. Their adsorption at the air/water interface, thermodynamic parameters and aggregation behavior were explored by means of surface tension, electrical conductivity and steady-state fluorescence. A series of surface activity parameters, including cmc, γ_cmc, π_cmc, pC₂₀, cmc/C₂₀, Γ_max and A_min, were obtained from surface tension measurements. The results revealed that the overall hydrophobic chain length (N_c) for [C₁₄C₄C_mim]Br₂ and the disymmetry (m/n) for [C_mC₄C_nim]Br₂ had a significant effect on the surface activity. The cmc values decreased with an increase of N_c or m/n. The thermodynamic parameters of micellization (ΔG_m ^θ, ΔH_m ^θ, ΔS_m ^θ) derived from the electrical conductivity indicated that the micellization process of [C₁₄C₄C_mim]Br₂ and [C_mC₄C_nim]Br₂ was entropy-driven at different temperatures, but the contribution of ΔH_m ^θ to ΔG_m ^θ was enhanced by increasing N_c or m/n. The micropolarity and micellar aggregation number (N_agg) were estimated by steady-state fluorescence measurements. The results showed that the surfactant with higher N_c or m/n can form larger micelles, due to a tighter micellar structure.

Full text of DOI:10.1007/s11743-016-1830-y

J Surfact Deterg
DOI 10.1007/s11743-016-1830-y
ORIGINAL ARTICLE
A Comprehensive Study on the Synthesis and Micellization
of Disymmetric Gemini Imidazolium Surfactants
Xiaohui Zhao1 Dong An1 Zhiwen Ye1
•
•
Received: 7 January 2016 / Accepted: 26 April 2016
Ó AOCS 2016
Abstract Two groups of disymmetric Gemini imidazo-
lium surfactants, [C14C4Cmim]Br2 (m = 10, 12, 14) and
[CmC4Cnim]Br2 (m ? n = 24, m = 12, 14, 16, 18) sur-
factants, were synthesized and their structures were con-
firmed by 1H NMR and ESI–MS spectroscopy. Their
adsorption at the air/water interface, thermodynamic
parameters and aggregation behavior were explored by
means of surface tension, electrical conductivity and
steady-state fluorescence. A series of surface activity
parameters, including cmc, ccmc, pcmc, pC20, cmc/C20, Cmax
and Amin, were obtained from surface tension measure-
ments. The results revealed that the overall hydrophobic
chain length (Nc) for [C14C4Cmim]Br2 and the disymmetry
(m/n) for [CmC4Cnim]Br2 had a significant effect on the
surface activity. The cmc values decreased with an increase
of Nc or m/n. The thermodynamic parameters of micel-
lization (DGhm, DHmh , DSmh ) derived from the electrical
conductivity indicated that the micellization process of
[C14C4Cmim]Br2 and [CmC4Cnim]Br2 was entropy-driven
at different temperatures, but the contribution of DHhm to
DGhm was enhanced by increasing Nc or m/n. The microp-
olarity and micellar aggregation number (Nagg) were esti-
mated by steady-state fluorescence measurements. The
results showed that the surfactant with higher Nc or m/n can
form larger micelles, due to a tighter micellar structure.
Keywords Disymmetric Gemini imidazolium surfactants Á
Synthesis Á Surface activity Á Thermodynamic parameters Á
Micropolarity Á Aggregation number
Introduction
Gemini surfactants, ‘‘dimers’’ of single-tailed surfactants
linked at the level of, or very close to, the head groups by a
spacer group, have gained much attention in the past few
decades [1–4]. Compared to the conventional single-tailed
surfactants, Gemini surfactants exhibit overwhelming
superiority, such as lower critical micelle concentration
(cmc), higher adsorption efficiency, better wetting, foam-
ing and lime-soap dispersing properties, and unusual rhe-
ological and aggregation properties [5–7], which makes
them widely useful in skin care, petrochemistry, medicine,
life science, and construction of porous materials [8–13].
Considerable effort has been exercised to design and
develop a library of Gemini surfactants with diverse
structure and functionality. The most investigated Gemini
surfactants are the dicationic quaternary ammonium com-
pounds with the general structure [CmH2m?1(CH3)2-
N(CH2)s N (CH3)2CnH2n?1]Br2, abbreviated as CmCsCnBr2
(m = n) [14, 15]. The effect of alkyl tail length and spacer
group length on the surface properties and micellization
process has been systematically studied. Heterocycles
headgroup-based Gemini surfactants, such as pyrroli-
dinium, imidazolium and hexahydropyridine Gemini sur-
factants, have also attracted increasing interest, and
different hydrophilic head groups have a significant effect
on the surface adsorption and aggregation behavior of
Gemini surfactants [16–21]. For example, Gemini imida-
zolium surfactants ([CmCsCnim]Br2, m = n), employing
heterocyclic imidazole as the headgroup and methylene as
& Zhiwen Ye
yezw@mail.njust.edu.cn
1
School of Chemical Engineering, Nanjing University of
Science and Technology, Nanjing 210094, People’s Republic
of China
123

J Surfact Deterg
the spacer group, demonstrate higher surface activity and
lower cmc [22–24]. In addition, owing to the existence of
imidazolium headgroup, as cationic micelle systems, they
show a distinctly greater tendency to self-aggregation and
thus are used as supramolecular templates in the prepara-
tion of functional materials [25]; as cationic reverse-mi-
celle systems, they display greater solubilization ability
compared with cationic Gemini ammonium surfactants
[26]. They can form tighter membranes [27] and thus be
applied widely in the field of biology due to the strong
attraction between the imidazolium headgroup and aro-
matic rings through p–p interaction [28].
activity and micellization process can be discussed com-
paratively and systematically.
Experimental Section
Materials
Imidazole, potassium hydroxide, dimethyl sulfoxide
(DMSO), chloroform, anhydrous magnesium sulfate, ethyl
acetate, acetone, hexane, methanol, and isopropanol were
purchased from Chengdu Kelong Reagent. Alkyl bromide
(CnH2n?1Br, n = 6, 8, 10, 12, 14, 16 and 18), 1, 4-dibro-
mobutane, and pyrene (99 %) were purchased from Alad-
din Reagent. Benzophenone (C.P.) was purchased from
Sinopharm Reagent. Ultrapure water was utilized for
preparing all the surfactant solutions.
The disymmetric Gemini surfactants consist of two
different hydrophobic chains or two different hydrophilic
head groups, connected by a rigid or flexible spacer group.
Owing to more controlled molecular structures, these sur-
factants have many peculiar properties, such as the
enhanced contribution of enthalpy to the Gibbs free energy
[29], higher efficiency of reducing surface tension and
greater ability to the formation of micelles caused by the
disymmetric Gemini surfactants with ionic–nonionic head
groups [30], counterions-free systems constructed by the
disymmetric Gemini surfactants containing cationic–an-
ionic head groups [31], and various morphologies of
aggregates formed by the dissymmetric Gemini surfactants
making use of their different length alkyl chains [32, 33].
Currently, the synthesis and properties of the dissymmetric
ammonium Gemini surfactants (CmCsCnBr2, m = n) [34,
35] and the disymmetric pyrrolidinium Gemini surfactants
(CmCsCnPB, m = n) have been researched in detail [36].
Their results indicate the disymmetric degree of the
hydrophobic chains have a strong influence on the
physicochemical properties of the surfactants. However, no
works have focused on preparing and exploring the
disymmetric Gemini imidazolium surfactants ([CmCsCn-
im]Br2, m = n). The motivation of the present work is to
further study the effect of disymmetric degree on the
physicochemical properties of these surfactants and build
up the structure–property relationships of them. Therefore,
a series of disymmetric Gemini imidazolium surfactants
with a four-methylene spacer group were synthesized and
characterized by 1H NMR and ESI–MS spectroscopy
(Scheme 1). The investigated surfactants can be divided
into two groups: the disymmetric [C14C4Cmim]Br2
(m = 10, 12, 14) surfactants with 14 carbon atoms in one
hydrocarbon chain, and the disymmetric [CmC4Cnim]Br2
(m ? n = 24, m = 12, 14, 16, 18) surfactants with the
same total carbon number of 24 in the two hydrophobic
chains. Their surface activity, aggregation number and
thermodynamic properties of micellization were obtained
by means of surface tension, steady-state fluorescence and
electrical conductivity measurements. Therefore, the effect
of dissymmetry of the hydrophobic chains on the surface
Synthesis of N-Alkyl Imidazole
A mixture of imidazole (30 mmol, 2.04 g), potassium
hydroxide (30 mmol, 1.68 g) and dimethyl sulfoxide
(10 mL) was stirred for 2 h at room temperature. After
that, alkyl bromide (25.0 mmol of 1-bromohexane, 1-bro-
mooctane, 1-bromodecane, 1-bromododecane, 1-bromote-
tradecane, 1-bromohexadecane, or 1-bromooctadecane)
was dropped in slowly and the mixture was stirred for an
additional 4 h. Upon completion, water (30 mL) was added
to the resulting mixture followed by extraction with chlo-
roform (5 9 30 mL). The combined organic layer was
dried over anhydrous magnesium sulfate and the filtrate
was concentrated under reduced pressure. The residue was
subjected to flash chromatography with ethyl acetate as
eluent to give N-alkyl imidazole. The respective yields of
N-hexyl imidazole, N-octyl imidazole, N-decyl imidazole,
N-dodecyl imidazole, N-tetradecyl imidazole, N-hexadecyl
imidazole and N-octadecyl imidazole are 84.6, 82.3, 81.2,
1
80.5, 80.4, 79.8 and 79.6 %. H NMR (Bruker Avance III
500, 500 MHz, CDCl3) N-hexyl imidazole: d 7.42 (s,
–NCHN–), 7.01 (s, –NCHCHN–), 6.87 (s, –NCHCHN–),
3.88 (t, –NCH2–), 1.73 (m, –NCH2CH2–), 1.25–1.27 (m,
–NCH2CH2(CH2)3–), 0.85 (t, –CH2CH3). N-octyl imida-
zole: d 7.44 (s, –NCHN–), 7.03 (s, –NCHCHN–), 6.88 (s,
–NCHCHN–), 3.90 (t, –NCH2–), 1.75 (m, –NCH2CH2–),
1.24–1.27 (m, –NCH2CH2(CH2)5–), 0.85 (t, –CH2CH3); N-
decyl imidazole: d 7.41 (s, –NCHN–), 6.99 (s, –NCHCHN–),
6.86 (s, –NCHCHN–), 3.88 (t, –NCH2–), 1.74 (m,
–NCH2CH2–), 1.21–1.25 (m, –NCH2CH2(CH2)7–), 0.85 (t,
–CH2CH3); N-dodecyl imidazole: d 7.44 (s, –NCHN–), 7.03
(s, –NCHCHN–), 6.88 (s, –NCHCHN–), 3.90 (t,
–NCH2–), 1.75 (m, –NCH2CH2–), 1.23-1.27 (m,
–NCH2CH2(CH2)9–), 0.88 (t, –CH2CH3); N-tetradecyl imi-
dazole: d 7.43 (s, –NCHN–), 7.01 (s, –NCHCHN–), 6.88 (s,
123

J Surfact Deterg
Scheme 1 Synthesis route of
the disymmetric Gemini
imidazolium surfactants
–NCHCHN–), 3.89 (t, –NCH2–), 1.75 (m, –NCH2CH2–),
1.22–1.26 (m, –NCH2CH2(CH2)11–), 0.85 (t, –CH2CH3); N-
hexadecyl imidazole: d 7.45 (s, –NCHN–), 7.05 (s,
–NCHCHN–), 6.90 (s, –NCHCHN–), 3.91 (t, –NCH2–), 1.76
(m, –NCH2CH2–), 1.25–1.32 (m, –NCH2CH2(CH2)13–),
0.87 (t, –CH2CH3); N-octadecyl imidazole: d 7.44 (s,
–NCHN–), 7.04 (s, –NCHCHN–), 6.89 (s, –NCHCHN–),
3.91 (t, –NCH2–), 1.76 (m, –NCH2CH2–), 1.21–1.30 (m,
–NCH2CH2(CH2)15–), 0.87 (t, –CH2CH3).
10.39 (s, –NCHN–), 7.63 (s, –NCHCHN–), 7.40 (s,
–NCHCHN–), 4.47 (t, –N?–CH2–), 4.28 (t, –NCH2–), 3.45
(t, –CH2Br), 2.06–2.14 (m, –N?–CH2CH2–), 1.90 (m,
–NCH2(CH2)2–), 1.24 (m, –N?–CH2CH2(CH2)5), 0.82 (t,
–CH2CH3); 1-(4-bromobutyl)-3-decylimidazolium bro-
mide: d 10.31 (s, –NCHN–), 7.70 (s, –NCHCHN–), 7.46 (s,
–NCHCHN–), 4.45 (t, –N?–CH2–), 4.29 (t, –NCH2–), 3.43
(t, –CH2Br), 2.08 (m, –N?–CH2CH2–), 1.92 (m, –NCH2
(CH2)2–), 1.26 (m, –N?–CH2CH2(CH2)7–), 0.85 (t, –CH2
CH3); 1-(4-bromobutyl)-3-dodecylimidazolium bromide: d
10.49 (s, –NCHN–), 7.62 (s, –NCHCHN–), 7.40 (s,
–NCHCHN–), 4.48 (t, –N?–CH2–), 4.28 (t, –NCH2–), 3.45
(t, –CH2Br), 2.11 (m, –N?–CH2CH2–), 1.91 (m, –NCH2
(CH2)2–), 1.17–1.33 (m, –N?–CH2CH2(CH2)9–), 0.84 (t,
–CH2CH3).
Synthesis of 1-(4-Bromobutyl)-3-Alkylimidazolium
Bromide
1,4-dibromobutane (36.0 mmol, 7.70 g) was dissolved in
dry acetone (50 mL) and the obtained N-alkyl imidazole
(9.0 mmol of N-hexyl imidazole, N-octyl imidazole, N-
decyl imidazole, or N-dodecyl imidazole) was added
gradually. The reaction mixture was refluxed at 50 °C
under nitrogen atmosphere and the reaction was monitored
by TLC analysis. After the reaction finished, the solvent
was removed under reduced pressure, and subsequently the
un-reacted 1, 4-dibromobutane was washed thoroughly
with hexane. The collected viscous liquid was purified by
silica column chromatography with mixtures of acetone/
methanol (10:1, by vol) as eluent to afford 1-(4-bro-
mobutyl)-3-alkylimidazolium bromide. The yields of 1-(4-
Synthesis of the Disymmetric Gemini Imidazolium
Surfactants
An amount of 5.0 mmol of the obtained 1-(4-bromobutyl)-
3-alkylimidazolium bromide (in which the alkyl were
hexyl, octyl, decyl, or dodecyl, respectively) was dissolved
in isopropanol (20 mL), followed by addition of 7.5 mmol
of N-alkyl imidazole (in which the alkyl were octadecyl,
hexadecyl, tetradecyl, or tetradecyl, correspondingly). The
mixture was stirred at 60 °C for 48 h under nitrogen
atmosphere. After removal of isopropanol, the residue was
washed with ethyl acetate, purified three times by recrys-
tallization in acetone and then dried under vacuum for
48 h, to yield the disymmetric Gemini imidazolium sur-
factants as white powders. The respective yields of [C18-
C4C6im]Br2, [C16C4C8im]Br2, [C14 C4C10im]Br2 and
[C14C4C12im]Br2 are 85.5, 87.3, 88.2 and 89.0 %. 1H NMR
(500 MHz, CDCl3) Take [C14C4C10im]Br2 as an example:
d 10.27 (s, 2H, –NCHN–), 7.96 (s, 2H, –NCHCHN–), 7.15
(s, 2H, –NCHCHN–), 4.58 (t, 4H, –N?–CH2–), 4.24 (t, 4H,
–NCH2–), 2.22 (m, 4H, –N?–CH2CH2–), 1.90 (m, 4H,
bromobutyl)-3-hexylimidazolium
bromide,
1-(4-bro-
mobutyl)-3-octylimidazolium bromide, 1-(4-bromobutyl)-
3-decylimidazolium bromide and 1-(4-bromobutyl)-3-do-
decylimidazolium bromide are 72.5, 74.1, 76.8 and 80.3 %,
respectively. 1H NMR (500 MHz, CDCl3) 1-(4-bro-
mobutyl)-3-hexylimidazolium bromide:
d
10.32 (s,
–NCHN–), 7.68 (s, –NCHCHN–), 7.45 (s, –NCHCHN–),
4.43 (t, –N?–CH2–), 4.25 (t, –NCH2–), 3.41 (t, –CH2Br),
2.01–2.09 (m, –N?–CH2CH2–), 1.79–1.94 (m, –NCH2
(CH2)2–), 1.21 (m, –N?–CH2CH2(CH2)3–), 0.79 (t, –CH2
CH3); 1-(4-bromobutyl)-3-octylimidazolium bromide: d
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J Surfact Deterg
–NCH2CH2–), 1.19–1.37 (m, 36H, –N?–CH2CH2CH2–),
0.87 (t, 6H, –CH2CH3); MS (Finnigan TSQ Quantum ultra
AM, ESI) [M–2Br]2?: 264.32. [C14C4C12im]Br2: d 10.31
(s, 2H, –NCHN–), 7.98 (s, 2H, –NCHCHN–), 7.15 (s, 2H,
–NCHCHN–), 4.58 (t, 4H, –N?–CH2–), 4.24 (t, 4H,
–NCH2–), 2.23 (m, 4H, –N?–CH2CH2–), 1.89 (m, 4H,
–NCH2CH2–), 1.21–1.36 (m, 40H, –N?–CH2CH2CH2–),
0.87 (t, 6H, –CH2CH3); MS (ESI) [M-2Br]2?: 278.30.
Electrical Conductivity Measurements
The electrical conductivity of surfactant solutions at five
different temperatures was obtained by a conductivity
analyzer (DDS-11A; Shanghai Leici TRONY Instrument).
The measurements were repeated three times for each
temperature and each temperature was controlled with a
precision of 0.1 °C.
Steady-State Fluorescence Measurements
Synthesis of the Symmetric Gemini Imidazolium
Surfactants ([CmC4Cmim]Br2, m 5 12, 14)
Fluorescence spectra were recorded on a FluoroMax-4
spectrofluorometer with a 1-cm2 quartz cuvette. Pyrene
was used as the fluorescence probe and benzophenone as
the quencher. The concentration of pyrene was kept at
1.0 9 10-6 mol L-1. The excitation wavelength was fixed
at 335 nm and the emission spectra range was selected
from 350 to 500 nm. The slit widths of excitation and
emission were set at 8 and 2 nm, respectively.
A mixture of 10 mmol of N-dodecyl imidazole or N-
tetradecyl imidazole, 1,4-dibromobutane (4 mmol,
0.856 g) and isopropanol (20 mL) was stirred for 48 h at
60 °C under nitrogen atmosphere. After removal of
isopropanol, the residue was washed with ethyl acetate,
further purified four times by recrystallization in acetone
and then dried under vacuum for 48 h. [C12C4C12im]Br2
and [C14 C4C14im]Br2 as the white powder were gained.
The respective yields of [C12C4C12im]Br2 and [C14 C4-
1
Results and Discussion
C14im]Br2 are 92.4 % and 93.3 %. H NMR (500 MHz,
CDCl3) [C12C4C12im]Br2: d 10.19 (s, 2H, –NCHN–),
8.05 (s, 2H, –NCHCHN–), 7.21 (s, 2H, –NCHCHN–),
4.58 (t, 4H, –N?–CH2–), 4.24 (t, 4H, –NCH2–), 2.19 (m,
4H, –N?–CH2CH2–), 1.88 (m, 4H, –NCH2CH2–),
1.20–1.35 (m, 36H, –N?–CH2CH2(CH2)9–), 0.86 (t, 6H,
–CH2CH3); MS (ESI) [M-2Br]2?: 264.27. [C14C4
C14im]Br2 d 10.30 (s, 2H, –NCHN–), 7.99 (s, 2H,
–NCHCHN–), 7.16 (s, 2H, –NCHCHN–), 4.58 (t, 4H,
–N?–CH2–), 4.24 (t, 4H, –NCH2–), 2.22 (m, 4H, –N?
–CH2CH2–), 1.89 (m, 4H, –NCH2CH2–), 1.20–1.37 (m,
44H, –N?–CH2CH2(CH2)11–), 0.87 (t, 6H, –CH2CH3);
MS (ESI) [M-2Br]2?: 292.32.
Adsorption at Air/Water Interface
The adsorption at air/water interface for Gemini imida-
zolium surfactants, including [C12C4C12im]Br2, [C14C4-
C10im]Br2,
[C16C4C8im]Br2,
[C18C4C6im]Br2,
[C14C4C12im]Br2 and [C14C4C14im]Br2, was evaluated by
the equilibrium surface tension as a function of surfactant
concentration (Fig. 1). The surface tension gradually
decreased with the increase of surfactant concentration and
then reached a plateau. The concentration of the clear
The Krafft temperatures of [CmC4Cnim]Br2 (m ? n =
24, m = 12, 14, 16, 18) were all lower than 15 °C. For
[C14C4C12im]Br2 and [C14C4C14im]Br2, their solubility
was not better than those of [CmC4Cnim]Br2 (m ? n = 24,
m = 12, 14, 16, 18), but when their concentration was not
higher than 2 mmol/L, they can still remain as a clear
aqueous solution at 15 °C. Therefore, in our experimental
range of the prepared surfactant concentration, all the
experiments can be performed at or above 15 °C.
Equilibrium Surface Tension Measurements
The equilibrium surface tension was estimated by using the
pendant drop method, performed on an optical contact
angle measuring instrument (OCA 40; Beijing Eastern
Dataphy Instruments). All measurements were taken at
25.0 0.1 °C and the surfactant solutions were kept still
for 30 min in order to achieve the system equilibrium.
Fig. 1 Plots of surface tension vs. log c at 25 °C for the disymmetric
Gemini imidazolium surfactants in aqueous solution. The error of
surface tension value is 0.2 mN/m
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J Surfact Deterg
Table 1 Surface activity of the disymmetric Gemini imidazolium surfactants at 25 °C
Surfactant
m/n cmc (mmol/L)
ccmc
(mN/m) (mN/m)
pcmc
pC20 cmc/ Cmax Amin
C20
(lmol/m2) (nm2)
Nagg
Surface
tension
Electrical
conductivity
Steady-state
fluorescence
[C12C4C12im]Br2
1
0.73
0.69
0.63
0.50
0.738
0.717
0.649
0.565
0.316
0.132
0.73
0.70
0.64
0.55
0.29
0.13
37.50
38.34
39.36
42.59
36.77
35.31
34.47
33.63
32.61
29.38
35.20
36.66
3.54 2.50
3.59 2.68
3.65 2.81
3.76 2.88
4.02 2.72
4.42 3.16
1.76
1.56
1.49
1.29
1.88
1.96
0.94
1.06
1.11
1.29
0.88
0.85
16
18
21
27
20
22
[C14C4C10im]Br2 1.4
[C16C4C8im]Br2
[C18C4C6im]Br2
2
3
[C14C4C12im]Br2 1.17 0.26
[C14C4C14im]Br2 0.12
1
breakpoint in the c-log c curves corresponded to the critical
micelle concentration (cmc), indicating the formation of
micelles, and the surface tension at the cmc was defined as
c0 is the surface tension of water and ccmc is the surface
tension of surfactant solution at the cmc. The larger the
value of pcmc, the higher the effectiveness of the surfactant
to lower the surface tension of water. The value of cmc/C20
is related with the structural factors in the adsorption and
micellization process. The surfactant with larger cmc/C20
value a tends more to adsorb at the interface than to form
micelles. Based on the c-log c curves, the above-described
physicochemical parameters for [C14C4Cmim]Br2 and
[CmC4Cnim]Br2, such as cmc, ccmc, pcmc, pC20, cmc/C20,
Cmax and Amin, are listed in Table 1.
c
cmc. In addition, the absence of a minimum near the
breakpoint suggested a low concentraction of surface
chemical impurities for the investigated Gemini surfactants
[20]. In this work, the classic Gibbs adsorption theory was
used to study the surface properties of the disymmetric
Gemini imidazolium surfactants.
The maximum surface excess concentration, Cmax, and
the minimum average area occupied by a single surfactant
molecule at the air/water interface, Amin, can be estimated
according to the Gibbs adsorption equation [37]:
From Fig. 2a, the values of cmc for the disymmetric
[C14C4Cmim]Br2 series decreased with the increasing total
carbon number in two hydrocarbon chains regardless of
the degree of disymmetry, implying that the longer
overall hydrophobic chain length (Nc), the higher the
aggregation abilityas. The contribution of each additional
methylene unit to the cmc values of [C14C4Cmim]Br2 was
higher than that of classic single-tailed surfactant homo-
logues [Cn mim]Br [39, 40]. It can be noticed that the
ccmc values of [C14C4Cmim]Br2 decreased with the
increasing Nc, indicating that [C14C4C14im]Br2 possessed
higher surface activity, which could also be found from
the changes of pcmc and pC20 values. For the disymmetric
[C14C4Cmim]Br2 series, the changes of cmc, ccmc, pcmc
and pC20 values with Nc were similar to those of the
disymmetric Gemini pyrrolidinium surfactants (CmC3C14-
PB, m = 10, 12, 14) [36]. The cmc/C20 values increased
with the increase of Nc, which suggested that, in com-
parison with micellization process, the adsorption at the
air/water interface for [C14C4Cmim]Br2 with longer
hydrophobic chains was much easier. However, the Amin
values of [C14C4Cmim]Br2 decreased with the increasing
Nc, meaning that [C14C4C14im]Br2 had a higher packing
density. The hydrophobic interactions were strengthened
with the increasing Nc, which could promote the surfac-
tant molecules to pack much more densely, and conse-
quently the Amin values will decrease. This result was
different from CmC3C14PB, and the possible reason was
1
dc
2:303nRT dlogc
Cmax ¼ À
Â
ð1Þ
1024
Amin
¼
ð2Þ
NACmax
where Cmax is in lmol/m2, R is the gas constant
(8.314 J mol-1 K-1), T is the absolute temperature in
Kelvin, c is the surface tension in mN m-1, c is the sur-
factant concentration and (d c/d logc) is the slope of the
linear part in the c-log c curves where the surfactant con-
centration is below the cmc. In aqueous solutions of the
investigated Gemini imidazolium surfactants, n is taken as
3, considering one dimeric ion and two counterions in the
surfactant molecule [38]. NA is Avogadro’s number and
Amin is in nm2/molecule.
The adsorption efficiency at the air–water interface,
pC20, and the surface pressure at cmc, pcmc, are obtained
from Eqs. (3, 4):
pC20 ¼ ÀlogC20
pcmc ¼ c0 À ccmc
ð3Þ
ð4Þ
where C20 represents the surfactant concentration at which
the surface tension of water is reduced by 20 mNÁm-1
.
Generally, the larger the value of pC20, the higher the
adsorption efficiency of the surfactant.
123

J Surfact Deterg
Fig. 2 Effect of the overall hydrophobic chain length (a) and the disymmetry (b) on cmc
that the longer hydrophobic chains were more prone to
bending.
made the surface area per molecule larger, and therefore
resulted in the larger ccmc value. The ccmc and Amin values
showed the opposite relation with the disymmetry in
comparison with CmC3CnPB (m ? n = 24, m = 12, 14,
16). Furthermore, the increase in cmc/C20 indicated that the
adsorption at the air/water interface was more favored over
the micellization process for those surfactants with the
higher disymmetry. The same conclusion could also be
speculated from the variation in pC20 values.
The typical effect of the disymmetry on the cmc can be
found from the disymmetric [CmC4Cnim]Br2 series
(Fig. 2b), those surfactants with the same total carbon
number of 24 in two hydrocarbon chains. Higher structural
disymmetry resulted in a lower cmc by employing m/n as
the degree of disymmetry and a similar conclusion was also
observed in other disymmetric Gemini surfactants [29, 34,
36]. Clearly, the introduction of disymmetry to the
hydrophobic chains can strengthen the aggregation
behavior, and adding two methylene groups to one longer
chain was more efficient in lowering the cmc than adding
each of them to two chains separately. The hydrophobic
interactions had two contributions to the aggregation
behavior, intermolecular and intramolecular. The latter will
be weaker because the spacer groups kept the two
hydrophobic chains apart. For the symmetric Gemini sur-
factants, the hydrophobic interactions will be minimized
due to the equal number of hydrophobic units for both
intramolecular and intermolecular interactions. However,
for the disymmetric Gemini surfactants, as m/n increased,
the ratio of hydrophobic units interacting intermolecularly
to those intramolecularly will increase [41]. Therefore, the
overall hydrophobic interactions were gradually improved
with the increasing m/n, which made a positive contribu-
tion to the formation of micelles. On the other hand, both
the ccmc and Amin values increased with the increase of m/n,
implying that the Gemini surfactants with a higher
disymmetry were packed more loosely at the air/water
interface. This might be attributed to the fact that the longer
hydrocarbon chain for [C18C4C6im]Br2 with the highest
disymmetry was more prone to bend at the interface, which
Degree of Counterion Binding to Micelles
The degree of counterion binding to micelles (b), a fun-
damental feature of the ionic surfactant micelles, is certain
to have an important role in the micellization process and
various aggregation behaviors in aqueous solutions. An
understanding of b can provide insight into the specific
properties of ionic surfactants and a further investigation of
new ionic Gemini surfactants. The electrical conductivity
measurements are often used to determine the cmc and b
values. For each of [C14C4Cmim]Br2 or [CmC4Cnim]Br2
series, Fig. 3 gives the plots of the conductivity (j) versus
surfactant concentration (c) at five different temperatures.
It was evident that all plots had breakpoints, and that the
concentration corresponded to the cmc. This was a result of
the reduction of effective charges in the solution, due to the
binding of some counterions to the micelles above the cmc.
Moreover, the degree of counterion dissociation (a) can be
taken as the ratio of the slopes of two fitted lines above and
below the cmc. Thus, the b values could be obtained by
using b = 1 - a. The cmc and b values of [C14C4Cm-
im]Br2 and [CmC4Cnim]Br2 series at five different tem-
peratures are listed in Table 2. It can be seen that the cmc
123

J Surfact Deterg
Fig. 3 Plots of electrical conductivity vs. surfactant concentration at five different temperatures. The error of electrical conductivity value
is 0.2 lS/cm. [C12C4C12im]Br2 (a), [C14C4C10im]Br2 (b), [C16C4C8im]Br2 (c), [C18C4C6im]Br2 (d), [C14C4C12im]Br2 (e), [C14C4C14im]Br2 (f)
values for all the studied surfactants increased upon raising
the temperature, and that the cmc values estimated from the
electrical conductivity at 25 °C were in accordance with
those obtained from the surface tension. A reasonable
explanation was that the ordered water molecules around
the hydrophobic chains may be broken at a higher
123

J Surfact Deterg
Table 2 Thermodynamic parameters of micellization for the disymmetric Gemini imidazolium surfactants at different temperatures
Surfactant
m/n
Temperature (°C)
cmc (mmol/L)
b
DGhm (kJ/mol)
DHhm (kJ/mol)
TDShm (kJ/mol)
[C12C4C12im]Br2
1
15
20
25
30
35
15
20
25
30
35
15
20
25
30
35
15
20
25
30
35
15
20
25
30
35
15
20
25
30
35
0.706
0.723
0.738
0.758
0.765
0.682
0.687
0.717
0.731
0.740
0.619
0.631
0.649
0.672
0.693
0.525
0.542
0.565
0.599
0.639
0.284
0.297
0.316
0.335
0.367
0.107
0.116
0.132
0.142
0.156
0.745
0.742
0.730
0.726
0.709
0.768
0.757
0.756
0.737
0.715
0.751
0.743
0.739
0.731
0.716
0.813
0.810
0.807
0.802
0.800
0.700
0.696
0.682
0.680
0.679
0.658
0.638
0.635
0.628
0.593
-47.12
-47.75
-48.14
-48.72
-49.00
-47.88
-48.38
-48.99
-49.19
-49.31
-47.83
-48.36
-48.95
-49.39
-49.63
-50.24
-50.88
-51.48
-51.93
-52.43
-49.61
-50.17
-50.34
-50.88
-51.30
-52.26
-52.20
-52.47
-52.83
-52.16
-5.059
-5.227
-5.370
-5.539
-5.667
-5.858
-6.026
-6.229
-6.370
-6.499
-6.769
-6.974
-7.197
-7.406
-7.586
-12.52
-12.93
-13.36
-13.77
-14.21
-15.02
-15.51
-15.91
-16.43
-16.97
-22.20
-22.70
-23.44
-24.13
-24.40
42.06
42.52
42.77
43.18
43.33
42.03
42.35
42.76
42.82
42.81
41.06
41.38
41.75
41.98
42.04
37.72
37.95
38.13
38.16
38.22
34.59
34.66
34.43
34.45
34.33
30.06
29.50
29.03
28.70
27.76
[C14C4C10im]Br2
[C16C4C8im]Br2
[C18C4C6im]Br2
[C14C4C12im]Br2
[C14C4C14im]Br2
1.4
2
3
1.17
1
DGhm ¼ RTð1 þ bÞ ln Xcmc
ð5Þ
temperature, which would be unfavorable for the formation
of micelles [42]. All the values of b showed a monotonous
decrease with the increasing temperature, which indicated
the decrease of charge density on the micelle surface. The
same trends of cmc and b values with temperature have
also been observed for the disymmetric Gemini pyrroli-
dinium surfactants CmC3C14PB and CmC3CnPB.
ꢀ
ꢁ
oðDGhm=TÞ
oð1=TÞ
d lnðXcmcÞ
DHmh ¼
DHmh ¼ ÀRT2ð1 þ bÞ
ð6Þ
dT
DHmh À DGhm
DShm ¼
ð7Þ
T
where R is the gas constant (8.314 J mol-1 K-1), T is the
absolute temperature in K, b is the degree of counterion
binding to micelles which was obtained previously, and
Xcmc is the cmc expressed in mol fractions.
Thermodynamics Parameters
The thermodynamic parameters for the micellization pro-
cess of [C14C4Cmim]Br2 and [CmC4Cnim]Br2 series were
investigated by employing the mass action model [43]. The
standard Gibbs free energy change (DGhm), the standard
enthalpy change (DHhm), and standard entropy change
(DShm) for the micellization process can be determined from
Eqs. (5–7):
The corresponding thermodynamic parameters of micel-
lization for all the investigated surfactants at five different
temperatures are shownin Table 2. The values of DGhm are all
negative, meaning that the aggregation for each surfactant in
aqueous solution is spontaneous. Notably, both the overall
hydrophobic chain length and the disymmetry degree greatly
123

J Surfact Deterg
Micropolarity and Micellar Aggregation Number
affected the thermodynamic parameters, or, in other words,
the micellization process. The observed more negative DGmh
values for [C14C4Cmim]Br2 series with longer overall
hydrophobic chains indicated a stronger tendency to micel-
lization due to greater hydrophobic interactions. Although
the changes of DGmh values for [CmC4Cnim]Br2 series with an
increaseof the disymmetry were not so large, it demonstrated
that the more negative DGhm of [C18C4C6im]Br2 with a higher
disymmetry was more beneficial to micellization than that of
[C12C4C12im]Br2. Simultaneously, the values of the stan-
dard enthalpy (DHhm) for each surfactant were also negative,
indicating that the formation process of micelles was
exothermic, which can be attributed to possible surfactant–
solvent interactions [17]. Additionally, all the DHhm values
changed little as the temperature increased, suggesting no
significant changes in the environment around the
hydrophobic chains of surfactant molecule. The data from
Table 2 indicated the DShm values were positive in all cases
and the values of TDShm were much larger than that of |DHhm|.
The negative values of DGmh were mainly due to the larger
positive DShm, implying that the micellization for [C14C4-
Cmim]Br2 or [CmC4Cnim]Br2 series was entropy-driven. It
can be concluded that the driving force of micellization
process was the tendency of hydrophobic chains to transfer
from the solvent to the interior of micelle. In general, there
are mainly four contributions to DHhm for the Gemini sur-
factants: the van der Waals interaction between the alkyl
chains, the hydrophobic interaction, the head group repul-
sion and the configuration of spacer chains [44]. Among
these interactions, the van der Waals interaction and the
hydrophobic interaction will lead to the changes in the DHmh
values for the present investigated surfactants. As was pre-
viously discussed in our work, the hydrophobic interactions
increased significantly with increasing Nc and m/n, which
made the DHhm values more negative. Consequently, the
overall hydrocarbon chain length and the degree of disym-
metry had a significant effect on the contribution of DHhm to
DGhm. For the [C14C4Cmim]Br2 series, the contribution of
DHhm to DGhm increased when the overall hydrocarbon chain
length was increased, which was similar to those of the
symmetric [CmC4Cmim]Br2 and the disymmetric CmC3C14-
PB. The |DHhm| values for [C14C4C14im]Br2 were nearly
equal to the values of TDSmh with the increase in the tem-
perature. On the other hand, for the [CmC4Cnim]Br2 series, a
higher degree of disymmetry can result in a higher contri-
bution of DHhm to DGhm, being consistent with those of the
disymmetric CmC6CnBr2 [34] and the disymmetric CmC3-
CnPB. From the data in Table 2, the changes of DHhm values
at 25 °C were from -5.370 to -13.36 kJ/mol as the
disymmetry increased. The introduction of disymmetry to
the hydrophobic chain shed new light on the possibility of
adjusting micellization process further.
It is well known that pyrene is used as a fluorescence probe
to investigate the cmc and polarity of the micellar
microenvironment. Normally, the fluorescence emission
spectra of pyrene have five vibration bands after the exci-
tation at 335 nm. The ratio of fluorescence intensity (I1/I3)
between the first vibronic band (373 nm) and the third
vibronic band (384 nm) is strongly dependent on the
position of the pyrene in micelles and thus can be seen as a
measure for the polarity of the micellar microenvironment
[45]. Once the surfactant molecules begin to form micelles,
the extreme hydrophobic pyrene molecules will transfer
from the water environment into the interior hydrophobic
region, which was reflected as a sharp drop in the I1/I3
value. The concentration corresponding to the sharp drop
was taken as the cmc. Figure 4 gives the relationship
between the I1/I3 ratio and the concentration of all the
investigated surfactants. For the [C14C4Cmim]Br2 or [Cm-
C4Cnim]Br2 series, the values of cmc determined from the
fluorescence plots shown in Table 1 were consistent with
those estimated by the methods of electrical conductivity
and surface tension. The values of I1/I3 were independent
of the surfactant concentration above cmc and almost the
same for the [C14C4Cmim]Br2 or [CmC4Cnim]Br2 series,
suggesting that the hydrophobic chains length and the
disymmetry have little effect on the micropolarity that was
sensed by the pyrene. Moreover, the low I1/I3 values
indicated that the pyrene was solubilized in the palisade
layer near the polar head groups [22]. On the one hand, as
the Nc or m/n ratio increased, the hydrophobic interaction
was gradually optimized. This was expected to result in a
decrease of micropolarity, owing to the tighter packing of
Fig. 4 I1/I3 ratio of pyrene as a function of the concentration for the
disymmetric Gemini imidazolium surfactants in aqueous solution
123

J Surfact Deterg
increasing Nagg values with increasing m/n has been found
in the disymmetric CmC6CnBr2. However, the opposite
relationship of decreasing Nagg values with increasing Nc or
m/n was observed in the disymmetric CmC3C14PB or Cm-
C3CnPB, and this might result from the formation of
premicelles. The more premicelles that could be formed,
the lower the Nagg
.
Acknowledgments We are grateful for the financial support from the
Six Talent Peaks Program of Jiangsu Province, China (Project No.
JNHB-001).
References
1. Song LD, Rosen MJ (1996) Surface properties, micellization, and
premicellar aggregation of Gemini surfactants with rigid and
flexible spacers. Langmuir 12:1149–1153
2. Zana R (2002) Dimeric and oligomeric surfactants, behavior at
interfaces and in aqueous solution: a review. Adv Colloid Inter-
face Sci 97:205–253
Fig. 5 ln (I0/I) of pyrene as a function of concentration of the
quencher benzophenone in the surfactant aqueous solution
the surfactant alkyl chains. On the other hand, the tighter
packing structure may make the pyrene slightly closer to
the micellar surface and thus cause an increase of
micropolarity, which compensated the first effect just dis-
cussed. As a result, the micropolarity of these investigated
surfactants micelles that was sensed by the pyrene was
observed to be almost the same.
3. Chevalier Y (2002) New surfactants: new chemical functions and
molecular architectures. Curr Opin Colloid Interface Sci 7:3–11
4. Zhong X, Guo JW, Feng LJ (2014) Cationic Gemini surfactants
based on adamantane: synthesis, surface activity and aggregation
properties. Colloids and Surfaces A 441:572–580
5. Zheng O, Zhao JX (2006) Solubilization of pyrene in the aqueous
micellar solutions of Gemini surfactant C12-s-C12Á2Br. J Colloid
Interface Sci 300:749–754
6. Lu T, Lan Y, Liu C, Huang J, Wang Y (2012) Surface properties,
aggregation behavior and micellization thermodynamics of a
class of Gemini surfactants with ethyl ammonium head groups.
J Colloid Interface Sci 377:222–230
The micellar aggregation number, Nagg, was measured
by fluorescence quenching experiments [46] and calculated
by Eq. (8):
7. Ding YS, Zha M, Zhang J, Wang SS (2007) Synthesis of a kind of
geminal imidazolium ionic liquid with long aliphatic chains. Chin
Chem Lett 18:48–50
ꢂ ꢃ
I0
I
NaggCQ
ln
¼
ð8Þ
CS À cmc
8. Tookazu Y, Miri B, Keisuke M (2009) Surface properties and
aggregate morphology of partially fluorinated carboxylate-type
anionic gemini surfactants. J Colloid Interface Sci 339:230–235
9. Zhou LM, Chen H, Jiang XH, Lu F, Zhou YF (2009) Modifica-
tion of montmorillonite surfaces using a novel class of cationic
gemini surfactants. J Colloid Interface Sci 332:16–21
10. Song CP, Wu DQ, Zhang F, Liu P, Lu QH, Feng XL (2012) Gem-
ini surfactant assisted synthesis of two-dimensional metalnanopar-
ticles/graphene composites. Chem Commun 48:2119–2121
11. Zhou Y, Cheng Y, Han CY, Lai JL, Sui L, Luo GX (2014)
Synthesis and surface properties of anionic Gemini surfactants
having N-acylamide and carboxylate groups. J Surfact Deterg
17:727–732
12. Jain T, Tehrani-Bagha AR, Shekhar H, Crawford R, Johnson E,
Norgaard K (2014) Anisotropic growth of gold nanoparticles
using cationic gemini surfactants: effects of structure variations
in head and tail groups. J Mater Chem C 2:994–1003
13. Sharma VD, Lees J, Hoffman NE, Brailoiu E, Madesh M,
Wunder SL, Ilies MA (2014) Modulation of pyridinium cationic
lipid–DNA complex properties by pyridinium Gemini surfactants
and its impact on lipoplex transfection properties. Mol Pharma-
ceutics 11:545–559
where I0 and I are the fluorescence intensities of pyrene in
the absence and presence of the quencher at a specific
wavelength of 373 nm, and CQ and CS are the molar
concentration of quencher and surfactant, respectively.
Figure 5 shows the variation of ln (I0/I) as a function of
CQ for [C14C4Cmim]Br2 and [CmC4Cnim]Br2 series. The
concentrations were all 2.0 mmol L-1 for the [CmC4Cn-
im]Br2 series, and 1 mmol L-1 for the [C14C4Cmim]Br2
(m = 12, 14) series. Each plot was fitted into a straight line
and the slope was obtained. The Nagg values of [C14C4-
C10im]Br2, [C14C4C12im]Br2 and [C14C4C14im]Br2 were
found to be 18, 20 and 22, respectively, implying that Nagg
increased with increasing Nc for the disymmetric [C14C4-
Cmim]Br2 series. For the disymmetric [CmC4Cnim]Br2
series, Nagg were 16, 18, 21 and 27 for m = 12, 14, 16 and
18, respectively, as listed in Table 1. Obviously, the Nagg
values were dependent on m/n, increasing from 16 to 27 as
m/n increased from 1 to 3. The possible explanation was
that the surfactant with the higher Nc or m/n can form a
more compact micellar structure, because of the enhance-
ment in the hydrophobic interaction. Such a trend of
14. Alami E, Beinert G, Marie P, Zana R (1993) Alkanediyl-.alpha.,
omega.-bis(dimethylalkylammonium bromide) Surfactants. 3.
Behaviour at the Air-Water Interface. Langmuir 9:1465–1467
15. Silva SMC, Sousa JJS, Marques EF, Pais AACC, Michniak-Kohn
BB (2013) Structure activity relationships in alkylammonium
123

J Surfact Deterg
C12-Gemini surfactants used as dermal permeation enhancers.
AAPS J 15:1119–1127
16. Zhou L, Jiang X, Li Y, Chen Z, Hu X (2007) Synthesis and
properties of a novel class of Gemini pyridinium surfactants.
Langmuir 23:11404–11408
17. Ao MQ, Xu GY, Zhu YY, Bai Y (2008) Synthesis and properties
of ionic liquid-type Gemini imidazolium surfactants. J Colloid
Interface Sci 326:490–495
18. Liu GY, Gu DM, Liu HY, Ding W, Li Z (2011) Enthalpy-entropy
compensation of ionic liquid-type Gemini imidazolium surfac-
tants in aqueous solutions: a free energy perturbation study.
J Colloid Interface Sci 358:521–526
19. Jiang Z, Li X, Yang G, Cheng L, Cai B, Yang Y, Dong J (2012)
pH-responsive surface activity and solubilization with novel
pyrrolidone-based Gemini surfactants. Langmuir 28:7174–7181
20. Cai B, Dong JF, Cheng L, Jiang Z, Yang Y, Li XF (2013)
Adsorption and micellization of gemini surfactants with pyrroli-
dinium head groups: effect of the spacer length. Soft Matter
9:7637–7643
34. Wang XY, Wang JB, Wang YL (2003) Micellization of a series
of disymmetric Gemini surfactants in aqueous solution. J Phys
Chem B 107:11428–11432
´
ˇ ˇ
´
35. Sikiric M, Primozic I, Filipovic-Vincekovic N (2002) Adsorption
and association in aqueous solutions of disymmetric Gemini
surfactant. J Colloid Interface Sci 250:221–229
36. Zou M, Dong JF, Yang GF, Li XF (2015) A comprehensive study
on micellization of disymmetric pyrrolidinium headgroup-based
gemini surfactants. Phys Chem Chem Phys 17:10265–10273
37. Kamboj R, Singh S, Chauhan V (2014) Synthesis, characteriza-
tion and surface properties of N-(2-hydroxyalkyl)-N0-(2-hydrox-
yethyl) imidazolium surfactants. Colloids and Surfaces
441:233–241
A
38. Wettig SD, Verrall RE (2001) Thermodynamic studies of aque-
ous m-s-m Gemini surfactant systems. J Colloid Interface Sci
235:310–316
39. Dong B, Li N, Zheng LQ, Inoue T (2007) Surface adsorption and
micelle formation of surface active ionic liquids in aqueous
solution. Langmuir 23:4178–4182
21. Cai B, Li XF, Yang Y, Dong JF (2012) Surface properties of
Gemini surfactants with pyrrolidinium head groups. J Colloid
Interface Sci 370:111–116
22. Ao MQ, Huang PP, Xu GY, Yang XD, Wang YJ (2009)
Aggregation and thermodynamic properties of ionic liquid-type
gemini imidazolium surfactants with different spacer length.
Colloid Polym Sci 287:395–402
40. Dong B, Zhao XY, Zheng LQ, Zhang J, Li N, Inoue T (2008)
Aggregation behavior of long-chain imidazolium ionic liquids in
aqueous solution: micellization and characterization of micelle
microenvironment. Colloids Surf A 317:666–672
41. Wang XD, Li QT, Chen X, Li ZH (2012) Effects of structure
disymmetry on aggregation behaviors of quaternary ammonium
Gemini surfactants in a protic ionic liquid EAN. Langmuir
28:16547–16554
23. Ao MQ, Xu GY, Pang JY, Zhao TT (2009) Comparison of
aggregation behaviors between ionic liquid-type imidazolium
´
´
42. Ruiz CC, Dıaz-Lopez L, Aguiar J (2007) Self-assembly of
tetradecyltrimethylammonium bromide in glycerol aqueous
mixtures: a thermodynamic and structural study. J Colloid
Interface Sci 305:293–300
Gemini surfactant [C12-4-C12im]Br
2 and its monomer
[C12mim]Br on silicon wafer. Langmuir 25:9721–9727
24. Ren CC, Wang F, Zhang ZQ, Nie HH, Li N, Cui M (2015)
Synthesis, surface activity and aggregation behavior of Gemini
imidazolium surfactants 1,3-bis(3-alkylimidazolium-1-yl) pro-
pane bromide. Colloids Surf A 467:1–8
25. Antonietti M, Kuang DB, Smarsly B, Zhou Y (2004) Ionic liquids
for the convenient synthesis of functional nanoparticles and other
inorganic nanostructures. Chem Int Ed 43:4988–4992
43. Tan JL, Zhao PJ, Ma DP, Feng SY, Zhang CQ (2013) Effect of
hydrophobic chains on the aggregation behavior of cationic silicone
surfactants in aqueous solution. Colloid Polym Sci 291:1487–1494
44. Fan YR, Li YJ, Cao MW, Wang JB, Wang YL, Thomas RK
(2007) Micellization of disymmetric cationic Gemini surfactants
and their interaction with dimyristoylphosphatidylcholine vesi-
cles. Langmuir 23:11458–11464
45. Keyes-Baig C, Duhamel J, Wetting S (2011) Characterization of
the behavior of a pyrene substituted Gemini surfactant in water
by fluorescence. Langmuir 27:3361–3371
26. Paul BK, Motra R (2005) Water solubilization capacity of mixed
reverse micelles: effect of surfactant component, the nature of the
oil, and electrolyte concentration.
288:261–279
J Colloid Interface Sci
27. Leodidis EB, Hatton TA (1990) Amino acids in AOT reversed
micelles. 1. Determination of interfacial partition coefficients
using the phase-transfer method. J Phys Chem 94:6400–6411
28. Fry AJ (2003) Strong ion-pairing effects in a room temperature
ionic liquid. J Electroanal Chem 546:35–39
46. Wang XQ, Liu J, Yu L, Jiao JJ, Wang R, Sun LM (2013) Surface
adsorption and micelle formation of imidazolium-based zwitte-
rionic surface active ionic liquids in aqueous solution. J Colloid
Interface Sci 391:103–110
29. Bai GY, Wang JB, Wang YJ, Yan HK (2002) Thermodynamics
of hydrophobic interaction of disymmetric Gemini surfactants in
aqueous solutions. J Phys Chem B 106:6614–6616
30. AIami E, HoImberg K, Eastoe J (2002) Adsorption properties of
novel Gemini surfactants with nonidentical head groups. J Col-
loid Interface Sci 247:447–455
Xiaohui Zhao is a doctoral candidate of applied chemistry at Nanjing
University of Science and Technology, P. R. China. Her current
research involves the synthesis and properties of Gemini imidazolium
surfactants.
31. Peresypkin AV, Menger FM (1999) Zwitterionic geminis, coac-
ervate formation from a single organic compound. Org Lett
1:1347–1350
32. Menger FM, Peresypkin AV (2001) A combinatorially-derived
structural phase diagram for 42 zwitterionic geminis. J Am Chem
Soc 123:5614–5615
33. Menger FM, Peresypkin AV (2003) Strings of vesicles: flow
behavior in an unusual type of aqueous gel. J Am Chem Soc
125:5340–5345
Dong An is a doctoral candidate of applied chemistry at Nanjing
University of Science and Technology. His research involves the
design, synthesis and performance of sugar esters nonionic
surfactants.
Zhiwen Ye is a professor of School of Chemical Engineering at
Nanjing University of Science and Technology. He is engaged in
research on the synthesis, properties and application of surfactants.
123