Synthesis and properties of biodegradable cationic gemini surfactants with diester and flexible spacers

Article abstract of DOI:10.1016/j.molliq.2017.05.092

A series of cationic gemini surfactants with diester and flexible spacers, namely C₁₂-PG-C₁₂, C₁₄-PG-C₁₄ and C₁₆-PG-C₁₆, were synthesized, purified and characterized. The surface properties and aggregation behavior of the gemini surfactants were investigated by surface tension, electrical conductivity, fluorescence and Krafft point. These gemini surfactants possess higher surface activity than the traditional monomeric surfactants. The thermodynamic parameters exhibited that the micellization was a spontaneous and exothermic process in environment. The micellization process became less favorable with the decrease of alkyl chain length and the increase of temperature. Steady-state fluorescence measurements revealed that the micropolarity and aggregation number of micelles decreased with the increase of hydrocarbon chain length. The Krafft points were taken as

Full text of DOI:10.1016/j.molliq.2017.05.092

Accepted Manuscript
Synthesis and properties of biodegradable cationic gemini
surfactants with diester and flexible spacers
Dongqing Xu, Xiaoyue Ni, Congyu Zhang, Jie Mao, Changchun
Song
PII:
DOI:
Reference:
S0167-7322(17)30083-1
doi: 10.1016/j.molliq.2017.05.092
MOLLIQ 7384
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
7 January 2017
25 April 2017
21 May 2017
Please cite this article as: Dongqing Xu, Xiaoyue Ni, Congyu Zhang, Jie Mao, Changchun
Song , Synthesis and properties of biodegradable cationic gemini surfactants with diester
and flexible spacers, Journal of Molecular Liquids (2016), doi: 10.1016/
j.molliq.2017.05.092
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ACCEPTED MANUSCRIPT
Synthesis and properties of biodegradable cationic gemini
surfactants with diester and flexible spacers
Dongqing Xua,c, Xiaoyue Nib, Congyu Zhanga, Jie Maob,c, Changchun Songb,c
a Center of Analysis and Measurement, Anhui Science and Technology University, Fengyang,
Anhui 233100, China.
b School of Chemistry and Material Engineering, Anhui Science and Technology University,
Fengyang, Anhui 233100, China
c Institute of Material Chemistry, Anhui Science and Technology University, Fengyang, Anhui
233100, China
Abstract
A series of cationic gemini surfactants with diester and flexible spacers, namely C12-PG-C12,
C14-PG-C14 and C16-PG-C16, were synthesized, purified and characterized. The surface properties
and aggregation behavior of the gemini surfactants were investigated by surface tension, electrical
conductivity, fluorescence and krafft point. These gemini surfactants possess higher surface
activity than the traditional monomeric surfactants. The thermodynamic parameters exhibited that
the micellization was a spontaneous and exothermic process in environment. The micellization
process became less favorable with the decrease of alkyl chain length and the increase of
temperature. Steady-state fluorescence measurements revealed that the micropolarity and
aggregation number of micelles decreased with the increase of hydrocarbon chain length. The
Krafft points were taken as less than 0℃, which indicated the synthesized gemini surfactants had
good water solubility. The biodegradability of the gemini surfactants were evaluated in river water
using Closed Bottle tested and showed their high biodegradation ratio in the open environment
due to the diester bond inserting in the flexible spacer of surfactant molecules.
Keywords Gemini surfactants, Diester and fexible sacers, Surface properties, Biodegradability
1 Introduction
Gemini or dimeric surfactants, consisting of two single alkyl tails and two polar head groups
covalently linked by a spacer group [1-3], have attracted more and more research attentions
because of the lower critical micellar concentration (CMC), the better adsorption behavior, the
superior aqueous solution, and the tendency to form micelles of different shapes and dimensions
even at low concentration compared to the corresponding monomeric surfactants [4-7]. Among
them, cationic gemini surfactants have been widely developed because their synthetic route from
readily available starting materials is straight-forward. In recent years, cationic gemini surfactants
show their great potential as a next generation surfactant for biomedical and industrial applications
such as effective corrosion inhibitors [8,9], bactericidal agents [10-12], gene delivery agents
[13,14], drug entrapment and release [15], and detergents, etc. However, the applicability of
cationic gemini surfactants is usually hampered by certain environmental concerns and toxicity
Corresponding author.
E-mail address: xudongq@163.com (D.Q. Xu), songcc@ahstu.edu.cn (C.C. Song).
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issues [16]. Therefore, great efforts have been taken to overcome these popular problems that exist
for cationic gemini surfactants. The main approach was to introduce an easily cleavable bond or
readily biodegradable groups into the structure of cationic gemini surfactants.
Most recently, there have been several reports focus on the physical and chemical properties
of cationic gemini surfactants with ester and flexible spacers in literatures [17-19]. It is due to
ester and flexible spacers, cationic gemini surfactants can not only increase the biodegradation
ratio, but also improve the overall performance of gemini surfactants and extend the potential
application. For example, Tehrani-Bagha et al. presented a new type of cationic gemini surfactant
with an ester bond inserted into a short space. The cationic gemini surfactant designed in this way
was observed to be classified as readily biodegradable [17]. Bergstrꢀm et al reported a novel type
of cationic gemini surfactants containing an ester group and found that the growth behavior,
geometrical shape and second CMC of micelles formed were conspicuously differ depending on
the length of the gemini surfactant spacer group [18]. Parikh et al exploited two series (ester/amide
and polymethylene spacers) of cationic gemini surfactants as potential plasmid DNA binding
vectors, and the ester spacer based cationic gemini surfactants showed higher surface activity,
readily biodegradable and lower toxicity to membranes among the synthesized gemini surfactants,
which may become a potential vector for biomedical applications such as drug delivery or gene
transfection/therapy [19]. In our previous work, we have synthesized a series of cationic gemini
surfactants with rigid spacer, and investigated their surface and aggregation properties at different
temperature in organic co-solvents. However, due to the poor solubility in aqueous solution, these
gemini surfactants were difficult to degrade, which limited their potential application.
Enlightened by the advantages of inserting ester bonds in the structure of gemini surfactants,
in this paper, we reported the design and synthesis of a group of cationic gemini surfactants with
diester and flexible spacers containing different alkyl chains. The diester and flexible spacers were
introduced to design our gemini surfactants with the expectation that the problem about hard
degradation can be solved and the properties of gemini surfactants can be further improved. The
gemini surfactants designed in this way were observed to exhibit good biodegradability.
Furthermore, the surface properties of these gemini surfactants were studied, including their
surface activity, thermodynamic parameters, and micropolarity. The chemical structure of the
synthesized gemini surfactants is shown in Scheme 1.
2 Experiment
2.1 Materials
1-bromododecane (>98%), 1-bromotetradecane (>98%), 1-bromohexadecane (>98%),
acetone, dimethylamine (33% aqueous solution), chloroacetyl chloride (≥98%), ethyl acetate,
ethanol were purchased from Sinopharm Chemical Reagent Corporation (Shanghai, China).
Pyrene (99%) and 1,3-dihydroxypropane (98%) were purchased from Sigma-Aldrich and used as
received without further purification. Ultrapure water was used in all experiments.
1H NMR spectra were recorded on a Bruker AV 400 NMR Spectrometer (Bruker Corporation,
Switzerland, 400 MHz). Mass spectra were recorded on a GC-MS (Aglient, USA). Elemental
analyses were carried out on a vario EL III Element Analyzer (Elementer Corporation, Germany).
The infrared (IR) spectra were measured on a Japan FT/IR-430 spectrometer as KBr pallets.
2.2 Synthesis
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2.2.1 Propane-1,3-diylbis(chloroacetate) (1)
1,3-dihydroxypropane (0.10 mol, 7.61 g) in dichloromethane (20 mL) was added drop wise
to a stirred dichloromethane (40 mL) solution of chloracetyl chloride (0.22 mol, 24.86 g) at 0℃.
Then the mixture was stirred for 4 h at 40℃, and neutralized by saturated sodium bicarbonate
solution. The organic layer was separated, washed with water, and dried over anhydrous
magnesium sulfate. The solvent was removed under vacuum to give compound 1 as a colorless
1
liquid (22.12 g, 98.56% content for GC-MS, 95.20% yield based on 1,3-dihydroxypropane). H
NMR (400 MHz CDCl3, TMS) δ ppm: 2.05-2.09 (m, 2H), 4.06-4.12 (s, 4H), 4.27-4.31 (m, 4H);
IR (KBr) cm-1: 2956, 1758, 1311, 1183, 789; GC-MS (m/z: 228.9 g mol-1), 135.0 (M-OOCCH2Cl),
107.0 (M-CH2CH2OOCCH2ClC11H23), 77.0 (M-OCH2CH2CH2OOCCH2Cl).
2.2.2 Compounds 2-4
A mixture of 1-bromoalkane (0.1 mol) and dimethylamine (33% aqueous solution, 0.5 mol, 5
equiv) in ethanol (40 mL) was placed in a dried 3-necked flask (150 mL). After stirring for 24 h at
reflux, the reaction mixture was cooled to room temperature. The solvent was removed under
vacuum, and the residue was poured into the saturated sodium bicarbonate solution (60 mL). The
mixture was extracted with ethyl acetate, and the extract phase was washed with water, then dried
over anhydrous magnesium sulfate. The solvent was removed to and the residue was purified by
distillation to give compounds 2-4 as colorless oily products.
N,N-Dimethyldodecylamine (2, 20.15 g, 98.96% content for GC, 93.62% yield based on
1
1-bromododecane). H NMR (400 MHz, CDCl3, TMS) δ ppm: 0.86-0.90 (t, J = 8.0 Hz, 3H),
1.26-1.28 (d, J = 8.0 Hz, 18H), 1.47 (s, 2H), 2.23-2.30 (m, 8H); IR (KBr) cm-1: 2925, 2854, 1464,
721; GC-MS (m/z: 213.3 g mol-1), 58.2 (M-C11H23).
N,N-Dimethyltetradecylamine (3, 22.10 g, 99.32% content for GC, 91.08% yield based on
1
1-bromotetradecane). H NMR (400 MHz, CDCl3, TMS) δ ppm: 0.86-0.90 (t, J = 8.0 Hz, 3H),
1.26 (s, 22H), 1.47 (s, 2H), 2.23-2.30 (m, 8H); IR (KBr) cm-1: 2924, 2853, 1465, 721; GC-MS
(m/z: 241.3 g mol-1), 58.2 (M-C13H27).
N,N-Dimethylhexadecylamine (4, 24.24 g, 99.42% content for GC, 89.59% yield based on
1
1-bromohexadecane). H NMR (400 MHz CDCl3, TMS) δ ppm: 0.86-0.90 (t, J = 8.0 Hz, 3H),
1.26 (s, 26H), 1.47 (s, 2H), 2.21-2.26 (m, 8H); IR (KBr) cm-1: 2924, 2853, 1463, 720; GC-MS
(m/z: 269.3 g mol-1), 58.1 (M-C15H31).
2.2.3 Gemini surfactants
A mixture of propane-1,3-diylbis(chloroacetate) (10 mmol) and N,N-dimethylalkylamine (44
mmol, 2.2 equiv) in ethyl acetate (60 mL) was placed in a dried 3-necked flask (150 mL). After
stirring for 24 h at reflux, the reaction mixture was cooled to room temperature. The precipitate
was collected by filtration and washed with plenty of ethyl acetate, then dried under reduced
pressure. The crude product was recrystallized for two times from acetone/acetonitrile (3:1) to
afford the gemini surfactants as white crystals.
1
C12-PG-C12 (4.93 g, 75.73% yield based on propane-1,3-diylbis(chloroacetate)). H NMR
(400 MHz, CDCl3, TMS) δ ppm: 0.86-0.90 (t, J = 8.0 Hz, 6H), 1.25-1.33 (m, 36H), 1.75 (s, 4H),
2.07-2.11 (m, 2H), 3.54 (s, 12H), 3.75-3.80 (t, J = 8.0 Hz, 4H), 4.34-4.38 (t, J = 8.0 Hz, 4H), 5.32
(s, 4H); IR (KBr) cm-1: 2924, 2854, 1748, 1630, 1467, 1410, 1199, 1040, 898, 722, 602; Anal.
Calcd. for C35H72N2O4Cl2 %: C, 64.09; H, 11.07; N, 4.27. Found %: C, 63.88; H, 11.01; N, 4.31.
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1
C14-PG-C14 (5.85 g, 85.29% yield based on propane-1,3-diylbis(chloroacetate)). H NMR
(400 MHz, CDCl3, TMS) δ ppm: 0.86-0.90 (t, J = 8.0 Hz, 6H), 1.25-1.34 (m, 44H), 1.75 (s, 4H),
2.07-2.10 (m, 2H), 3.54 (s, 12H), 3.75-3.80 (t, J = 8.0 Hz, 4H), 4.34-4.38 (t, J = 8.0 Hz, 4H), 5.32
(s, 4H); IR (KBr) cm-1: 2924, 2853, 1748, 1636, 1466, 1409, 1200, 1040, 904, 721; Anal. Calcd
for C39H80N2O4Cl2 %: C, 65.79; H, 11.33; N, 3.94. Found %: C, 65.53; H, 11.33; N, 3.99.
1
C16-PG-C16 (7.60 g, 85.93% yield based on propane-1,3-diylbis(chloroacetate)). H NMR
(400 MHz, CDCl3, TMS) δ ppm: 0.84-0.88 (t, J = 8.0 Hz, 6H), 1.23-1.32 (m, 52H), 1.75 (s, 4H),
2.06-2.09 (m, 2H), 3.50 (s, 12H), 3.72-3.76 (m , J = 8.0 Hz, 4H), 4.33-4.38 (t, J = 10.0 Hz, 4H),
5.36 (s, 4H); IR (KBr) cm-1: 2920, 2850, 1666, 1648, 1540, 1492, 1467, 1326, 1291, 1159, 729;
Anal. Calcd for C43H88N2O4Cl2 %: C, 67.24; H, 11.55; N, 3.65. Found %: C, 66.92; H, 11.48; N,
3.69.
Scheme 1. Synthetic route of the gemini surfactants C12-PG-C12, C14-PG-C14 and C16-PG-C16.
2.3 Surface tension measurement
The surface tensions of the gemini surfactant solutions at water-air interface were
investigated by using a Wilhelmy plate technique (Krüss K20 Tensiometer, Germany) [20] at
298.15 ± 0.1 K, and the surface tension values were measured at least three times. The surface
tension of pure water was obtained as being 71.99 ± 0.05 mN m-1 at 298.15 K. All solutions were
prepared with ultrapure water.
2.4 Electrical conductivity measurement
The conductivity measurements were carried out using a direct reading conductometer,
model S320 (Mettler-Toledo GmbH, Switzerland) at different temperatures, KCl solutions (0.1 M
and 0.01 M) were used for calibration of conductivity cell. The conductivity was recorded when
its value fluctuated less than 1% over a 5 min interval. The tested solution was added to a
container kept in a thermostat (THD 0506, NingBo TianHeng Instrument Factory, China) with the
temperature accuracy of ± 0.01℃. The critical micelle concentration (CMC) of gemini surfactants
was obtained at the break point of the specific conductivity versus concentration curve [21].
2.5 Fluorescence Measurement
Steady-state fluorescence measurements were performed using an F-4600 fluorescence
spectrophotometer (Hitachi High-Technologies Corp, Tokyo Japan). Pyrene was used as a
fluorescence probe and cetylpyridinium chloride (CPC) as a quencher. The excitation wavelength
was 336 nm, the emission spectra were scanned over the spectrum range of 345-460 nm, and the
slit widths of excitation and emission were fixed at 5.0 and 2.5 nm, respectively. The ratio of the
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intensities of the first peak (around 373 nm) to the third peak (around 384 nm) was used as an
index of the polarity of the pyrene-solubilizing medium [22]. The intensity ratio of the first peak
to the third peak (I1/I3) can be taken as a measure of the microenvironment polarity, which is high
in polar media and low in hydrophobic environments.
An aliquot of the ethanolic pyrene solution was transferred into a volumetric flsk and the
solvent was evaporated. The fixed concentration of gemini surfactant solution (20 CMC) was
added and the pyrene concentration was kept at 1 × 10-5 M. The quencher concentration was
varied from 0 to 9 × 10-5 M. The fluorescence intensities of the peaks decrease with the increase of
the quencher concentration without appearance of any new peak.
2.6 Krafft Temperature Measurements
A clear aqueous solution of the gemini surfactant (0.5 wt %, above the CMC ) was prepared
by dissolving surfactant in water and placed in a refrigerator at 273.15 K for at least 24 h, so that
solid surfactant-hydrate crystals appear. The temperature of the cooled gemini surfactant solution
was then raised at a heating rate of 0.1℃ min-1 under constant stirring in a thermostat with a
temperature accuracy of ± 0.01℃. The conductance (κ) was measured every few minutes by a
S320 conductometer (Mettler-Toledo GmbH, Switzerland). The Krafft temperature was taken as
the temperature at which the conductance versus temperature plot showed an abrupt change in
slope [23].
2.7 Biodegradability
Biodegradability of the prepare gemini surfactants was measured by the Closed Bottle test.
The test method (OECD 301D) has been given in detail in literatures [24,25] and will be described
only briefly in this paper. The experiment adopted 3.0 mg L-1 of test substance existing in river
water spiked with mineral salts. Ammonium chloride was omitted from the mineral salts which
were prescribed to prevent nitrification. The river water was aerated for 8-10 days before using
and particles were removed by sedimentation. The experiments were carried out in 300 mL BOD
(biological oxygen demand) bottles with glass stoppers, using 3 bottles containing only river water,
and series of 3 bottles containing the respective test substances and river water. The bottles were
incubated in the dark at a temperature of 25 ± 0.5℃. The biodegradation was measured by
following the course of the dissolved oxygen decrease in the bottles [25]. The amount of dissolved
oxygen was monitored with an oxygen electrode (InPro 6850i) and an oxygen meter. The
biodegradability was evaluated as the percentage of the biological oxygen demand to the
theoretical oxygen demand.
3 Results and Discussion
3.1 Synthesis
The synthetic routes for cationic gemini surfactants with diester and flexible spacers
(C12-PG-C12, C14-PG-C14, C16-PG-C16) in this work are shown in Scheme 1. They were easily
synthesized through a three-step procedure from the starting materials. First, the commercially
available 1,3-dihydroxypropane (1 equiv) was reacted with chloracetyl chloride (0.22 equiv) to
generate the important intermediate propane-1,3-diyl bis(chloroacetate) (1). Then, the alkylation
of 1-bromoalkane (1-bromododecane, 1-bromotetradecane, 1-bromohexadecane, 1 equiv) with an
excess molar ratio of dimethylamine (5 equiv) generated the compounds 2, 3, and 4. Finally, the
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gemini surfactants C12-PG-C12, C14-PG-C14 and C16-PG-C16 were prepared at a high yield of
75.73-85.93% by quaterisation of propane-1,3-diyl bis(chloroacetate) (1) with
1
N,N-dimethylalkylamine (2, 3, 4), respectively. Their chemical structures were confirmed by H
NMR spectroscopy, infrared spectroscopy, and elemental analysis.
3.2 Surface activity of the gemini surfactants
The surface tension (γ) of the investigated gemini surfactant aqueous solutions to the
logarithm of concentration (C) at 298.15 K have been plotted in Fig. 1. The surface tension of
aqueous solutions initially decreased with an increase in the concentration of gemini surfactants
and then reached a plateau. The intersection point of surface tension curve is usually described as
the critical micelle concentration (CMC) of a gemini surfactant and the plateau always means the
formation of micelles. The surface activity parameters of the gemini surfactants are listed in Table
1. The CMC values of C12-PG-C12, C14-PG-C14 and C16-PG-C16 are 1.649, 0.2689 and 0.0560
mmol L-1 at 25 ℃, respectively. It can be found that the CMC value gradually decreases with
increasing hydrophobic chain length from 12 to 16, due to the enhanced hydrophobic interaction
between the longer alkyl chains. For comparison, the CMC values of two single-chain cationic
surfactants,
CTAB
(hexadecyltrimethylammonium
bromide)
and
DTAB
(dodecyltrimethylammonium bromide), are also calculated as 0.846 and 15.31 mmol L-1 under the
same experimental conditions. Evidently, the CMC values for the gemini surfactants are much
lower than the traditional single-chain surfactants with the same alkyl chain in the hydrophobic
groups. It demonstrates that the surface activities of the synthesized gemini surfactants are
superior to that of single-chain cationic surfactants, indicating an excellent micelle forming ability.
The important physicochemical parameters including the surface tension value at CMC
(γCMC), the minimum surface area (Amin) occupied by the gemini surfactant molecule, and the
surface excess concentration (Γmax), are summarized in Table 1. The surface area (Amin) occupied
by the gemini surfactant molecules at the air-water interface is a measure of their packing density
[26]. The surface excess concentration (Γmax) and the area occupied by a surfactant molecule (Amin
at the air-water interface were calculated using the Gibbs adsorption equations (1) and (2) [27,28]
as following:
)



1
d






Γmax 


(1)
(2)
2.303nRT d logC


T
1016
Amin 
NAΓmax
In the equation, R is the gas constant (8.314 J mol-1 K-1), T is the absolute temperature (K), C
is the surfactant concentration (mol/L), dγ/dlogC is the slope of the surface tension vs the
logarithm of concentration curve when the concentration is near the CMC, NA is Avogadro’s
number (6.02×1023 mol-1). The values of n = 3 (the number of species at the interface whose
concentration changes with the surfactant concentration) are used for calculating the Γmax values
[5,29,30]. As shown in Table 1, the values of Γmax of these gemini surfactants decrease, but the
Amin values increase with increasing the length of the hydrophobic chains from 12 to 16, which
indicates that the gemini surfactant with the longer hydrophobic chains has lower packing
densities at the air-water interface. A possible explanation is that the longer hydrophobic chains
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are more prone to bend at the air-water surface and cause the Amin value to become larger [31].
Other important surface active parameters, such as the adsorption efficiency (pC20) and the
effectiveness of surface tension reduction (ΠCMC) for these gemini surfactants, can also be
obtained from the surface tension curve. The former parameter is obtained by using the equation
(3):
(3)
pC20 logC20
In the equation, C is the molar concentration of gemini surfactant (mol/L), and C20 represents
the value of logarithm surfactant concentration required to reduce the surface tension of pure
water by 20 mN m-1. The pC20 value indicates the adsorption efficiency of gemini surfactants
molecules at the air-water interface. The larger the pC20 value, the less gemini surfactant adsorbed
at the air-water interface. ΠCMC is the surface pressure at CMC, which is defined by using the
equation (4):
Π γ0 -γcmc
(4)
cmc
where γ0 is the surface tension of pure solvent and γCMC is the surface tension of gemini surfactant
solution at CMC. ΠCMC represents the maximum reduction of surface tension which can be used to
evaluate the effectiveness of gemini surfactant to reduce the surface tension of water [26]. So the
larger the value of ΠCMC is, the higher effectiveness of gemini surfactant is. As shown in Table 1,
the variation of pC20 and ΠCMC for the gemini surfactants exhibited that both the pC20 and ΠCMC
increased with increasing length of the hydrophobic chain.
In addition, the ratio of CMC/C20 reflects the relative tendency to adsorption and
micellization of a surfactant, the larger the values of the CMC/C20, the greater the tendency of the
surfactant to adsorb at the air/water interface, relative to its tendency to form micelles. As shown
in Table 1, it can be seen that these gemini surfactants were easier to self-assemble than adsorb at
the air-water interface in aqueous solution. The values of γCMC for the gemini surfactants are 47.87,
45.47 and 43.24 mN m-1 at 298.15 K, respectively. The results showed that the γCMC values
decreased from 47.87 to 43.24 mN m-1 with increasing the number of carbon atom in the
hydrophobic chain from 12 to 16.
C16-PG-C16
70
60
50
40
C14-PG-C14
C12-PG-C12
1E-6
1E-5
1E-4
1E-3
0.01
C / mol.L-1
Fig. 1. Surface tension (γ) vs the concentration (C) plots for the gemini surfactants at 298.15 K.
Table 1
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Surface active properties and parameters of the micellization for the gemini surfactants at 298.15 K.
γCMC
Γmax
Amin
ΠCMC
(mN/m)
24.12
Compounds
CMCa (mM)
CMCb (mM)
pC20
CMC/C20
(mN/m) (μmol/m2)
(nm2)
1.649 ± 0.04
0.2689 ± 0.02
1.858 ± 0.02
0.3144 ± 0.01
47.87
45.47
43.24
2.26
1.34
1.06
0.74
1.24
1.57
3.01
3.55
4.44
1.70
0.95
1.54
C12-PG-C12
C14-PG-C14
C16-PG-C16
26.52
28.75
0.0560 ± 0.005 0.0676 ± 0.004
aMeasured by tensiometry, bMeasured by conductometry.
3.3 Thermodynamic Properties of Micellization
The electrical conductivity measurement was applied in order to get a further investigation on
the micellization behavior of the gemini surfactants in aqueous solution at four different
temperatures (298.15, 308.15, 318.15 and 328.15) K. As shown in Fig. 2, S1 and S2, the CMC
values were determined as sharp break points in the plots of specific conductance (κ) versus
gemini surfactants concentration (C) [21], and the degree of counterion dissociation (α) was
determined from the pre-micellar slope (S1) to the post-micellar slope (S2), α = S2/S1 [32, 33]. As
shown in Table 1, the values of CMC for C12-PG-C12, C14-PG-C14 and C16-PG-C16 measured by
the electrical conductivity measurement are 1.858, 0.3144 and 0.0708 mmol L-1 at 298.15 K,
which are slightly larger than the CMC values measured by surface tension measurements. The
results show that the values of CMC follow the similar trend as those investigated by surface
tension measurements. In addition, the α values decrease significantly with increasing
hydrophobic tail length for the gemini surfactants. The long hydrophobic tails are favorable to
form micelles and result in decreases in CMC values.
298.15 K
900
308.15 K
318.15 K
328.15 K
750
600
450
300
150
0.5
1.0
1.5
2.0
2.5
3.0
3.5
C / mmol.L-1
Fig. 2. Electrical conductivity (κ) versus the concentration (C) for C12-PG-C12.
The thermodynamic parameters of micellization (ΔGo and ΔGoads) for the gemini
mic
surfactants have also been listed in Table 2. The standard Gibbs free energy of the aggregation
(ΔGomic) was calculated by the following equation (5):
ΔGmoic  RT(3-2)ln xcmc
(5)
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where xCMC is the CMC in mole fraction, R is the gas constant, and T is temperature on the Kelvin
scale. The xCMC is calculated by using the equation of xCMC = CMC / (CMC + number of moles for
the solvent). The Gibbs free energy of adsorption (ΔGoads), which represents the free energy of
transfer of 1 mol of gemini surfactant in solution to the interface, is calculated by the following
equation (6):
Πcmc
Γmax
ΔGo ΔGmoic -
ads
(6)
As shown in Table 2, the negative values of ΔGo indicate that the micellization is a
mic
spontaneous process and its calculated absolute values increase with the increase of alkyl chain
length, which have been found to be hydrophobically driven force for the formation of micelles.
Furthermore, as compared to ΔGomic, the higher negativity values of ΔGo suggest that the
ads
process of adsorption at the air-water interface is more spontaneous than micellization process in
the bulk because the preference of adsorption is governed by the thermodynamic stability of the
molecules at the air-water interface for the gemini surfactants. By rising the temperature from
298.15 to 328.15 K, the absolute values of ΔGo are increased due to the less stability of the
mic
micellized gemini surfactant molecules than the freely dispersed in the aqueous phase. The
standard enthalpy change for micellization process (ΔHomic) can be determined using the
Gibbs-Helmholtz equation (7):
 ln x
T



P
ΔHmoic RT2(32)
cmc 
(7)

The values of lnxCMC in aqueous solutions were plotted against the temperature and the slopes
of these plots were taken as the values of dlnxCMC/dT. The calculated ΔHo values at four
mic
temperatures in aqueous solutions are summarized in Table 2. As shown in Table 2, the values of
standard enthalpy of micellization (ΔHomic) are negative for all the systems. It means that the
micellization process is exothermic in aqueous solutions. The value of the ΔHo is mostly
mic
becoming more negative with increase in alkyl chain length of the gemini surfactants. It means
that the micellization process in C16-PG-C16 is more exothermic than C14-PG-C14 and C12-PG-C12
at the same temperature. In addition, the standard entropy of micelle formation (ΔSomic) can be
calculated by the following equation (8) and the values are listed in Table 2.
(Hmoic Gmoic)
ΔSmoic 
(8)
T
From the observed data, it is obviously that the ΔSo values for the gemini surfactants are
mic
positive in all cases and decrease with the increasing temperature in range from 298.15 K to
328.15 K in aqueous solutions. The positive of ΔSomic can be attributed to the destruction of water
structure around the hydrophobic chain of the surfactant molecules. With the decrease of
temperature, the surrounding iceberg structures of surfactant molecules diminished in size or
became less rigid as temperature increased, the formation of micelles is difficult at higher
temperature. Therefore, the micelle formation process of this system is a thermodynamically
spontaneous process driven by entropy.
Table 2
Critical micellar concentration (CMC), degree of counter ion dissociation (α), standard Gibbs energy of
micellization (ΔGomic), standard Gibbs free energy of adsorption (ΔGoads), standard enthalpy of micellization
9

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(ΔHomic) and standard entropy of micellization (ΔSomic) of the gemini surfactants in aqueous solution at various
temperatures (T).
CMC
(mM)
ΔGo
ΔGo
ΔHo
ΔSo
mic
mic
ads
mic
Surfactant
T/K
α
(kJ mol-1)
-47.66
(kJ mol-1)
-54.78
(kJ mol-1)
-6.819
(kJ K-1 mol-1)
0.1370
298.15 1.858 ± 0.02
308.15 1.952 ± 0.02
318.15 2.028 ± 0.02
328.15 2.105 ± 0.02
298.15 0.3144 ± 0.01
308.15 0.3464 ± 0.01
318.15 0.4326 ± 0.01
328.15 0.5371 ± 0.01
0.5670
0.5942
0.6528
0.7076
0.5966
0.6358
0.7011
0.7680
C12-PG-C12
-47.57
-45.75
-43.96
-54.10
-53.04
-49.67
-46.06
-63.43
-62.98
-62.65
-60.69
-6.419
-6.399
-6.367
-24.90
-25.45
-25.08
-24.44
-14.12
-14.56
-15.02
-15.28
0.1335
0.1237
0.1146
0.0979
0.0895
0.0773
0.0659
0.1654
0.1571
0.1497
0.1384
-67.23
-81.51
C14-PG-C14
298.15 0.0676 ± 0.004 0.5604
308.15 0.0714 ± 0.004 0.5934
318.15 0.0760 ± 0.004 0.6223
328.15 0.0956 ± 0.004 0.6611
C16-PG-C16
3.4 Microenviroment and Aggregation
1.84
1.76
1.68
1.60
1.52
1.44
1.36
C12-PG-C12
C14-PG-C14
C16-PG-C16
1E-5
1E-4
1E-3
0.01
C / mol L-1
Fig. 3. Variation of the pyrene intensity ratio I1/I3 vs the concentrations (C) for the gemini surfactants at 298.15 K.
The micellization for the gemini surfactants was also investigated by a pyrene fluorescence
probe through the steady-state fluorescence technique, which is useful for monitoring the
self-aggregation in aqueous solution [34]. The shape and intensity for the photoluminescence (PL)
spectra of pyene is sensitive to its microenvironment at the site of fluorophore solubilization.
When surfactants begin to form micelles in aqueous solutions, pyrene molecules will penetrate
into the interior hydrophobic region of micelles from the water, which will cause an abrupt change
in I1/I3. Then the I1/I3 value keeps constant as the concentration of surfactant increases in aqueous
solutions. Therefore, the values can be utilized to analyze the micropolarity of the micelle system
in polar media, reflecting the compactness of aggregates. Fig. 3 shows the variations of the pyrene
polarity ratio I1/I3 with the change of the surfactant concentration. The values of I1/I3 change
slightly as the initial surfactant concentration increases, suggesting that pyrene has no interaction
with surfactant at the outset. However, with increasing the gemini surfactant concentration close to
10

ACCEPTED MANUSCRIPT
the CMC value, there is an obvious decrease in I1/I3, because a spot of micelles have formed. Then
the I1/I3 value keeps constant as the concentration of surfactant increases in aqueous solutions,
which indicates that the aggregate structures are compact and the microenvironment does not
change. The minimum I1/I3 values are decreased with the increase of the alkyl chain length,
indicating that the longer hydrophobic tail makes the palisade layer more compact and leads to the
reduction of polarity.
Further, the aggregation number (Nagg) of the gemini surfactant solutions was determined by
steady-state fluorescence quenching method [35]. The relation between the fluorescence
intensities at 373 nm of pyrene in the absence (I0) and presence (I) of the quencher CPC is
determined by total gemini surfactant concentration ([S]) and the quencher concentration in
micelles ([Q]) according to the following equation (9):
Nagg[Q]
I
0
 
(9)
ln

 
I
[S]cmc
 
The plots for the logarithm of pyrene intensity (I0/I) versus the concentration of quencher
CPC [Q] at 298.15 K are shown in Fig. S3 in supplementary data. All plots are fitted straight lines
and slops are obtained. Thus, using the slope of each straight line, the Nagg values can be obtained
from Eq (9), and given in tabular form (Table 3). The aggregation numbers of C12-PG-C12,
C14-PG-C14 and C16-PG-C16 are found to be 23, 21 and 14, respectively. Obviously, the Nagg
values decreased with the increase of alkyl chain length, which can be attributed that the gemini
surfactant with longer hydrocarbon chain was easier to absorb at air/water interface than to form
bigger aggregation in aqueous solutions [36].
Table 3
The aggregation number (Nagg), correlation coefficient (R), krafft temperature (Kt), and biodegradation ratio of the
gemini surfactants at 298.15 K.
Biodegradation (%)
Kt
Surfactant
Nagg
R
(℃)
10d
20d
28d
23
21
14
0.9957
0.9974
0.9977
< 0
< 0
< 0
37.28
34.37
29.39
64.32 76.54
58.53 73.50
49.63 71.83
C12-PG-C12
C14-PG-C14
C16-PG-C16
3.5 Krafft temperature
The krafft temperature (Kt) is considered as the minimum temperature at which surfactants
form micelles. Below the krafft temperature, the conductance of surfactant solution would
increase slowly while during the temperature transition stage, the conductance increases sharply.
These phenomena are attributed to the large dissolution of surfactant [37]. The abrupt change in
the conductance (κ) versus temperature plots was not observed and the gemini surfactant solution
did not show any sign of precipitation after standing in a refrigerator for 24 hours. The values of
the krafft points for all the gemini surfactants with diester and flexible spacers are assigned to be
below 0℃, which indicates the surfactants have good water solubility.
3.6 Biodegradability
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ACCEPTED MANUSCRIPT
It is well know that cationic gemini surfactants have their toxicity to the cell membranes of
microorganisms. Therefore, the Closed Bottle test, a standardized method (OECD 301D), was
applied to evaluate the biodegradability of these cationic gemini surfactants. From the data in
Table 3, it is obvious that the biodegradation ratio of all the gemini surfactants can reach more
than 70% biodegradation by exposing to the microorganisms after 28 days, which are close to the
data reported for gemini surfactants with similar molecular structures in literatures [17,38,39]. As
compared to C16-PG-C16 at 71.83%, the higher biodegradation ratio is gained in case of
C12-PG-C12 and C14-PG-C14 at 76.54% and 73.50%, respectively. According to OECD’s
guidelines 60% biodegradation, the theoretical value should be achieved in drain water within 28
days in order for a surfactant to be classified as readily biodegradable [40]. Therefore, all the three
gemini surfactants can be classified as readily biodegradable, which should benefit from the
ability to degrade by the action of the environmental microorganisms due to the presence of the
diester and flexible spacers [17]. Furthermore, the biodegradation ratio of these gemini surfactants
decreases with increasing the length of the alkyl chain from 12 to 16.
4 Conclusions
In summary, three cationic gemini surfactants with diester and flexible spacers were designed
and synthesized. Interestingly, the surface and thermodynamic properties of these gemini
surfactants can be influenced by the molecule structures and spacer chain length. They have much
lower CMC values than the traditional single-chain surfactants. The CMC values decreased with
increasing hydrophobic chain length from 12 to 16. From the thermodynamic point of view,
micelle formation was a spontaneous and exothermic process. The micellization process became
less favorable with the decrease of alkyl chain length and the increase of temperature. The
fluorescence measurement demonstrated that the synthesized gemini surfactants formed into
micelles, which changed the microenvironment in solution and formed loosely bound aggregates
in aqueous solutions. Besides, the Nagg decreased with the increase of alkyl chain length. All the
three gemini surfactants possess good water solubility and can be classified as readily
biodegradable. This should benefit from the ability to degrade by the action of the environmental
microorganisms due to the presence of the diester and flexible spacers.
Acknowledgments
fi
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Research highlights
The gemini surfactants with diester and flexible spacers were designed and synthesized.
The surface properties and aggregation behavior of surfactants were investigated.
They have higher surface activity than the traditional monomeric surfactants.
The micellization is a spontaneous and exothermic process in environment.
They possess good water solubility and can be classified as readily biodegradable.

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14