Relationship between structure and biodegradability of gemini imidazolium surface active ionic liquids

Article abstract of DOI:10.1016/j.molliq.2018.12.066

Three ester-containing gemini imidazolium surface active ionic liquids [1,4-bis(1-alkyl-oxypropyl-imidazolium)-butane dichloride], abbreviated as [peC_nBim₂]Cl₂ (n = 12, 14, 16), were synthesized from imidazole, 1,4-dibromobutane, 1-chloro-3-propanol, and various aliphatic carboxylic acids. Chemical structures of [peC_nBim₂]Cl₂ were confirmed by H-Nuclear magnetic resonance, Fourier transform infrared spectroscopy, and elemental analysis. Thermal properties of [peC_nBim₂]Cl₂ were studied with Thermogravimetric analysis and Differential scanning calorimetry. The micellization parameters, adsorption/aggregation properties, and thermodynamic parameters of [peC_nBim₂]Cl₂ were demonstrated using tensiometry, electrical conductivity. Critical micelle concentration values of [peC_nBim₂]Cl₂ aqueous solutions decreased with increasing the alkyl chain length (from C12 to C16). The formation of micelles was spontaneous, exothermic, and entropy-driven. Using OECD 301D, the aerobic biodegradation of nine gemini imidazolium SAILs ([peC_nBim₂]Cl₂, [C_nPim₂]Br₂, and [C_nBim₂]Br₂) and three monoimidazolium SAILs ([C_nIM]Br) were systematically investigated to determine the relationship between biodegradation and structural elements of imidazolium SAILs. Aerobic biodegradation of twelve SAILs followed the decreasing trend: [peC_nBim₂]Cl₂ > [C_nPim₂]Br₂ > [C_nIM]Br > [C_nBim₂]Br₂. Gemini imidazolium SAILs with ester functional group ([C_nPim₂]Br₂ and [peC_nBim₂]Cl₂) showed much higher biodegradation rates than those of [C_nIM]Br and [C_nBim₂]Br₂. The location of ester group played an obvious role on biodegradation. Gemini SAILs containing ester group in the side alkyl chains ([peC_nBim₂]Cl₂) showed much higher biodegradation ratio than those of [C_nPim₂]Br₂ (ester functional group in spacer). [peC_nBim₂]Cl₂ could be classified as readily biodegradable. The biodegradability of gemini imidazolium SAILs increased with increasing the alkyl chain length.

Full text of DOI:10.1016/j.molliq.2018.12.066

Accepted Manuscript
Relationship between structure and biodegradability of gemini
imidazolium surface active ionic liquids
Guowei Wang, Xiaoqing Xu, Yu Sun, Linghua Zhuang, Cheng
Yao
PII:
S0167-7322(18)34410-6
DOI:
Reference:
https://doi.org/10.1016/j.molliq.2018.12.066
MOLLIQ 10142
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
26 August 2018
25 November 2018
12 December 2018
Please cite this article as: Guowei Wang, Xiaoqing Xu, Yu Sun, Linghua Zhuang, Cheng
Yao , Relationship between structure and biodegradability of gemini imidazolium surface
active ionic liquids. Molliq (2019), https://doi.org/10.1016/j.molliq.2018.12.066
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ACCEPTED MANUSCRIPT
Relationship between Structure and Biodegradability of Gemini Imidazolium Surface Active
Ionic Liquids
Guowei Wang a, Xiaoqing Xu b, Yu Sun a, Linghua Zhuang c,*, Cheng Yao c,*
a College of Food Science and Light Industry, Nanjing Tech University, Nanjing 211800, China
b
College of Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, Nanjing
211800, China
c
College of Chemistry and Molecular Engineering, Nanjing Tech University, Nanjing 211800,
China
∗ Corresponding authors
E-mail address: zhuanglhnj@163.com (Linghua Zhuang)
yaochengnjut@126.com (Cheng Yao)
Linghua Zhuang and Cheng Yao contributed equally to this work.
Abstract
Three
ester-containing
gemini
imidazolium
surface
active
ionic
liquids
[1,4-bis(1-alkyl-oxypropyl-imidazolium)-butane dichloride], abbreviated as [peCnBim2]Cl2
(n=12,14,16), were synthesized from imidazole, 1,4-dibromobutane, 1-chloro-3-propanol, and
various aliphatic carboxylic acids. Chemical structures of [peCnBim2]Cl2 were confirmed by
H-Nuclear magnetic resonance, Fourier transform infrared spectroscopy, and elemental analysis.
Thermal properties of [peCnBim2]Cl2 were studied with Thermogravimetric analysis and
Differential scanning calorimetry. The micellization parameters, adsorption/aggregation properties,
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and thermodynamic parameters of [peCnBim2]Cl2 were demonstrated using tensiometry, electrical
conductivity. Critical micelle concentration values of [peCnBim2]Cl2 aqueous solutions decreased
with increasing the alkyl chain length (from C12 to C16). The formation of micelles was
spontaneous, exothermic, and entropy-driven.
Using OECD 301D, the aerobic biodegradation of nine gemini imidazolium SAILs
([peCnBim2]Cl2, [CnPim2]Br2, and [CnBim2]Br2) and three monoimidazolium SAILs ([CnIM]Br)
were systematically investigated to determine the relationship between biodegradation and
structural elements of imidazolium SAILs. Aerobic biodegradation of twelve SAILs followed the
decreasing trend: [peCnBim2]Cl2＞[CnPim2]Br2＞[CnIM]Br＞[CnBim2]Br2. Gemini imidazolium
SAILs with ester functional group ([CnPim2]Br2 and [peCnBim2]Cl2) showed much higher
biodegradation rates than those of [CnIM]Br and [CnBim2]Br2. The location of ester group played
an obvious role on biodegradation. Gemini SAILs containing ester group in the side alkyl chains
([peCnBim2]Cl2) showed much higher biodegradation ratio than those of [CnPim2]Br2 (ester
functional group in spacer). [peCnBim2]Cl2 could be classified as readily biodegradable. The
biodegradability of gemini imidazolium SAILs increased with increasing the alkyl chain length.
Keywords: Surface active ionic liquids; synthesis; physicochemical properties; aerobic
biodegradation; ester group
Abbreviations
ILs Ionic liquids
SAILs Surface active ionic liquids
[peCnBim2]Cl2 (n=12,14,16) 1,4-bis(1-alkyl-oxypropyl-imidazolium)- butane dichloride
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[CnPim2]Br2
dibromide
(n=12,14,16)
2,2-bis(alkyl-oxymethyl)-1,3-bis(3-methylimidazolium)-propane
[CnBim2]Br2 (n=12,14,16) 1,4-bis(3-alkylimidazolium-1-yl) butane dibromide
[CnIM]Br (n=12,14,16) 1-alkyl-3-methylimidazolium bromide
1HNMR H-Nuclear magnetic resonance
FT-IR Fourier transform infrared spectroscopy
TGA Thermogravimetric analysis
DSC Differential scanning calorimetry
cmc Critical micelle concentration
Amin Minimum surface area per molecule
γcmc Surface tension at critical micelle concentration
Γmax Maximum surface excess
β value Degree of counterion binding to micelles
Ga0ds
Gibbs free energy of adsorption
Gm0
Gibbs free energy change of micellization
Hmo
Standard enthalpy change of micellization
Sm
Standard entropy change of micellization
1. Introduction
Ionic liquid is a kind of molten organic salt, which is widely used as solvents for synthesis and
catalysis, biodegradable composite materials, extraction or separation processes for bioactive
compounds, and greenhouse gas capture systems [1-6].
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Wide application of ionic liquids (ILs) in many industrial fields inevitably results in the release
of ILs into aquatic environments, which may lead to water pollution and related environmental
impact. Environmental hazards (biodegradability rate and toxicity) of ILs are of the highest
importance in designing, synthesis, and application of new ILs. The synthesis and related
environmental impact (aerobic biodegradation) of common ILs were summarized in review
articles [7-11].
Surface active ionic liquid (SAIL), or named as ionic liquid surfactant, is an important new
type of functional ionic liquid with combined properties of ionic liquid and surfactant. SAILs
have diverse cations, such as imidazolium, pyridinium, and piperidinium. In particular, the
synthesis, micellization behavior, and potential application of imidazolium SAILs were studied
[12-17].
Recently, multivalent imidazolium ILs (including di-, tris-, and tetrakis-), some of which were
novel SAILs, showed unique physicochemical properties [18-21] and found attractive application in
high-temperature lubricants, gas chromatography stationary phase, conductive polymer
supercapacitor, gel polymer electrolyte for sodium-ion batteries, fluorescent sensor for detection of
hydroquinone/catechol, etc [22-29].
The influence of structural elements (spacer chain length or functional moiety, terminal alkyl side
chain length of cations, and headgroups of cation) on the physicochemical properties of multivalent
cationic ILs were discussed [30-34], whereas few systematic investigations focused on the aerobic
biodegradation of multivalent cationic ILs [35-40].
Stefan Stolte et al. [37] studied biodegradation of ten dicationic ILs with OECD 301F method.
Al-Mohammed reported the synthesis and biodegradability of tris- and tetrakis-imidazolium or
benzimidazolium ILs using OECD 301D method [38,39].
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Principles for development of biodegradable surfactants can be applied for designing, synthesis
of biodegradable SAILs:
(i) existence of ester and/or amide functional groups as potential sites for enzymatic hydrolysis;
(ii) oxygen introduction as hydroxyl, aldehyde or carboxylic acid groups;
(iii) the existence of unsubstituted long linear alkyl chains as potential sites of oxygenase attack.
Therefore, the main approach for designing biodegradable SAILs is to introduce easily cleavable
bonds or readily biodegradable groups into SAILs.
With our continuous studies on synthesis and application of novel SAILs, three ester-containing
gemini imidazolium SAILs, 1,4-bis(1-alkyl-oxypropyl-imidazolium)-butane dichloride,
abbreviated as [peCnBim2]Cl2 (n=12,14,16), were synthesized using imidazole, 1,4-dibromobutane,
1-chloro-3-propanol, and various aliphatic carboxylic acids. Chemical structures of [peCnBim2]Cl2
1
were confirmed by HNMR, FT-IR, and elemental analysis. Thermal properties of [peCnBim2]Cl2
were studied with TGA and DSC. The surface activity and thermodynamic properties of
[peCnBim2]Cl2 were determined by means of surface tension and electrical conductivity
measurements.
Meanwhile, the aerobic biodegradation of [peCnBim2]Cl2, other six gemini imidazolium SAILs
([CnPim2]Br2 and [CnBim2]Br2), and three monoimidazolium SAILs ([CnIM]Br) were
systematically investigated to determine the relationship between aerobic biodegradation and
structural elements of different imidazolium SAILs.
1) functional groups of side chain in cation,
2) headgroups of cation
3) ester group position
The biodegradability ratio of nine gemini imidazolium SAILs was reported for the first time.
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This study is helpful in facilitating the selection of environmentally favourable structural elements
and designing of inherently safer gemini imidazolium SAILs.
2. Materials and methods
2.1 Materials
1-chloro-3-propanol, 1-methylimidazole, imidazole, 1,4-dibromobutane, dodecanoic acid,
tetradecanoic acid, hexadecanoic acid, 1-bromododecane, 1-bromotetradecane, and
1-bromohexadecane, all analytical reagent grade, were purchased from Aladdin reagent Co. Ltd.,
China.
Isopropanol, ethanol, ethyl acetate, acetone, dimethyl sulphoxide (DMSO), acetonitrile, sulphuric
acid, sodium hydroxide (NaOH), and sodium carbonate, were purchased from Aladdin reagent
Co. Ltd., China.
2, 2-bis (bromomethyl)-1, 3-propanediol was purchased from Leadsh Chemical Co. Ltd.,
China. Sodium dodecyl sulfate (SDS,＞99.0%) was purchased from Sigma Aldrich (China).
These chemicals were used as received. Water used in this work was distilled water.
Above mentioned nine gemini imidazolium SAILs were divided into three types:
gemini imidazolium SAILs 1-3: 1,4-bis(1-alkyl-oxypropyl-imidazolium)-butane dichloride, as
[peCnBim2]Cl2 (n=12,14,16); gemini imidazolium SAILs 4-6: 2,2-bis(alkyl-oxymethyl)-1,3-bis
(3-methylimidazolium)-propane dibromide, as [CnPim2]Br2 (n=12,14,16); gemini imidazolium
SAILs 7-9: 1,4-bis(3-alkylimidazolium-1-yl) butane dibromide, as [CnBim2]Br2 (n=12,14,16).
Nine gemini imidazolium SAILs and three monoimidazolium SAILs were shown in Fig 1.
The structures and chemical identifiers of all imidazolium SAILs were summarized in Table 1.
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Fig 1 Molecular structure of various imidazolium SAILs
2.2 Synthesis
Gemini imidazolium SAILs [peCnBim2]Cl2 (1-3) were prepared according to the following
procedure (as shown in Fig 2).
Fig 2 Synthesis of ester containing gemini imidazolium SAILs [peCnBim2]Cl2
2.2.1 Synthesis of intermediate diester
In 250 mL bottomed flask, equipped with thermometer, Dean-Stark separator, condenser pipe,
and mechanical stirring device, 7.56g 1-chloro-3-propanol (80mmol) and catalytic amount of
sulphuric acid (5%w/w relative to the aliphatic carboxylic acid) were added to a solution of
12.26g dodecanoic acid (60mmol) in 50 mL toluene. The mixture was reflux. The water
formed was removed by reactive distillation and collected in Dean-Stark. The reaction was
stopped until no water was generated. After cooling, the solution was transferred into a
separating funnel, and washed three times with 45mL Na2CO3 aqueous solution (1%
concentration) and water, respectively. The toluene layer was collected, then dried with
anhydrous calcium chloride. Toluene was evaporated, and a pale yellow transparent liquid
product was obtained, which was dodecanoic acid-3-chloropropanol ester (yield 96%).
Tetradecanoic acid 3-chloropropanol ester and hexadecanoic acid 3-chloropropanol ester were
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synthesized according to the above method from tetradecanoic acid and hexadecanoic acid,
respectively.
2.2.2 Synthesis of 1,4-bis(imidazole-1-yl)butane
Using imidazole, NaOH, and 1,4-dibromobutane as raw materials, 1,4-bis(imidazole-1-yl)butane
was prepared according to reported literatures with minor modification [41-42]. Imidazole (13.64 g,
0.2 mol) and NaOH (8.48 g, 0.21mol) was added to DMSO (70mL) and stirred under nitrogen at
100℃ for 2 h. After that, 1,4-dibromobutane (0.095mol)was added slowly into the above reaction
solution and the resulting mixture was stirred vigorously for 4 h at 65℃. When the reaction was
over, DMSO was evaporated, the white solid crude products were purified with Millipore water
(30mL×5). Yield was 91%.
2.2.3 Synthesis of gemini imidazolium SAILs (1-3)
In 100 mL flask, equipped with thermometer, condenser pipe, and mechanical stirring device,
6.91g（24.99 mmol）dodecanoic acid-3-chloropropanol ester (compound in section 2.2.1) and
2.106g (10.9 mmol) 1,4-bis(imidazole-1-yl)butane (compound in section 2.2.2) were added and
stirred vigorously for 10 h at 110℃. White solid crude product was filtered after ethyl acetate
addition, which was further recrystallized twice with ethyl acetate and acetonitrile. The white
solid was 1,4-bis(1-dodecanoyloxypropyl- imidazolium)- butane dichloride (1, [peC12Bim2]
Cl2).
Compound
2
and
3
([peC14Bim2]Cl2 and [peC16Bim2]Cl2) were prepared using
1,4-bis(imidazole-1-yl)butane, tetradecanoic acid 3-chloropropanol ester, and hexadecanoic acid
3-chloropropanol ester.
Gemini imidazolium SAILs 4-6 (2,2-bis (alkyl-oxymethyl)-1,3-bis (3-methylimidazolium)-propane
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dibromide, abbreviated as [CnPim2]Br2) were prepared according to the method in literature [19].
Gemini imidazolium SAILs 7-9 (1,4-bis(3-alkylimidazolium-1-yl) butane dibromide, abbreviated as
[CnBim2]Br2) were synthesized according to the method in literature [43-45]. Common
monoimidazolium SAILs 10-12 (1-alkyl-3-methylimidazolium bromide, abbreviated as [CnIM]Br)
were synthesized according to References [13, 46-48].
2.3 Measurements
Melting point was determined using micro-melting point apparatus X-4 (Shanghai Precision
& Scientific Instrument Co. Ltd., China).
1HNMR was recorded on the AVANCE AV-500 (Bruker Co. Ltd., Germany) operating at 500
MHz and chemical shifts were given in ppm units relative to tetramethylsilane (TMS). The
splitting patterns are designated as follows: s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet.
Element analysis was recorded on Flash EA-1112A element analyzer (Thermo Finhigan, Italy).
FT-IR was recorded using a Nicolet 5700 FT-IR spectrometer (Thermo Electron Corporation,
Madison, USA). For each spectrum, 64 scans were recorded over the range of 4000-525cm−1 at
25℃ at a resolution of 2 cm−1. The background spectrum was recorded on air and subtracted from
the sample spectrum.
Thermogravimetric analysis (TGA) and Differential scanning calorimetry (DSC) measurements
were performed with a SDT Q600 DSC-TGA analyzer. The samples were heated from 50 to 300℃
(for TGA test) and 40-250℃ (DSC measurement) at the heating rate of 10K/min. The experiments
were carried out on an alumina sample pan by using a nitrogen flow rate of 100mL/min.
Krafft point was determined by heating the surfactant solution until a clear solution. All SAILs
solution concentrations were 1.0 wt. % [19,49].
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Surface tension was performed using a du Nouy Ring method at 298.15K with Kino A101
automatic surface tension meter (Kino, US). The temperature was maintained with
low-temperature thermostat bath (DC-1006, Shanghai HengPing Instruments Co., LTD) with
accuracy of ±0.05 K. All the measurements were repeated three times until the data were found to
be accurate within ±0.2 mN.m-1.
Electrical conductivity was measured by DDS-307A Conductivity Meter (Shanghai INESA
scientific instrument Co., LTD) with platinum black electrode at 20℃, 25℃, 30℃, and 40℃.
Before each measurement, the platinum black electrode was washed by Millipore water, and
dried.
Critical micelle concentrations (cmc) were determined from surface tension and electrical
conductivity test.
2.3 Biodegradation tests using Closed Bottle Test
The biodegradation test was performed according to modified version of ‘Closed Bottle Test’
(OECD 301D) [50]. The test method was based on biochemical oxygen demand (BOD) due to
SAIL microbial degradation. The BOD values were derived from the quantified respirometric
dissolved oxygen (DO) in a culture containing either SAIL or sodium dodecyl sulfate (SDS) as a
reference sample [38-39, 50, 51].
Capped Scotch bottles containing 100 mL solution of 100 mg L-1 concentration IL or reference
sample in distilled water, were used to prepare test samples that inoculated with 1 mL of microbial
effluent collected from the domestic wastewater treatment plant (Pukou, Nanjing, China).
Three groups of samples were prepared: inoculum and SAIL samples in Group 1, Group 2
contained only the inoculum (no IL samples), while Group 3 contained the inoculum and SDS
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reference sample. The bottles of solutions were incubated in the dark at 25℃ for 28 days under
continuous shaking (200 rpm).
JPSJ-605 dissolved oxgen meter (Shanghai INESA Scientific Instrument Co. Ltd., China) was used
for DO measurements at seven days intervals (168 h) over 28 days incubation. Three parallel DO
measurements for each IL samples, blank sample (without test substance) and reference substance
(SDS) were performed, and the DO values were the average of three DO measurements.
BOD values were calculated using equation (1):
O2(s) O2(b)
BOD 
M
(1)
where O2(s) is uptake of oxygen in the sample, mg·L-1; O2(b) is uptake of oxygen in the blank sample,
mg·L-1 ; M is the concentration of substance present in the sample, mg·L-1.
The theoretical oxygen demand (ThOD) of compound (CcHhClclNnNanaOoPpSs), without nitrification,
can be calculated using equation (2)
1
5
1
16[2c  (h  cl  3n)  3s  p  na  o]
2
2
2
ThOD 
MW
(2)
where MW is relative molecular weight of the surfactants. Then, the biodegradation percentage is
given by equation (3):
BOD
%Biodegradation 
100%
ThOD
(3)
3. Results and discussion
3.1 Characterization of compounds 1-3
1HNMR of gemini imidazolium SAILs [peCnBim2]Cl2 (1-3) were shown in Fig S1, Fig S2, Fig
S3.
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FT-IR of three gemini imidazolium SAILs [peCnBim2]Cl2 (1-3) were shown in Fig S4, S5, S6.
Wavenumbers at 3135 cm-1 and 3044cm-1 were assigned to the C-H stretching vibration peaks of
imidazole ring, 2954cm-1 was ascribed to the C-H stretching vibration peak of methyl group, 2921
and 2850 cm-1 were assigned to C-H stretching vibration peaks of methylene group, 1727 cm-1
was C=O stretching vibration peak, C-N stretching vibration peak located at 1627 cm-1, 1565 cm-1
was ascribed to C=C stretching vibration peak, The deformation vibration of C-H located at 1471
cm-1 and 1376 cm-1, 1200 cm-1 and 1177 cm-1 were assigned to the C-O stretching vibration peaks,
vibrational absorption peak of -(CH2)n- skeleton located at 720 cm-1. FT-IR showed typical ester
group and imidazole ring, which was consistent with ester containing gemini imidazolium
surfactants [19].
3.2 TGA/DSC analysis
Thermogravimetric analysis of [peC12Bim2]Cl2 and [peC14Bim2]Cl2 were shown in Fig 3.
As revealed in Fig 3, the decomposition temperature of [peC12Bim2]Cl2 was higher that of
[peC14Bim2]Cl2. [peC12Bim2]Cl2 started decomposition at 291.3℃, while [peC14Bim2]Cl2 started
decomposition at 279.3℃. The decomposition temperatures were higher than those of conventional
quaternary ammonium gemini surfactants [52-53], and other gemini imidazolium surfactants
[18,35]. The decomposition temperatures of [peC12Bim2]Cl2 and [peC14Bim2]Cl2 were higher than
those other ester-bonded gemini imidazolium surfactants [19]. It indicated that incorporating of
ester bond increased the thermal stability of these gemini SAILs.
As shown in Fig 4, [peC14Bim2]Cl2 and [peC16Bim2]Cl2 showed two phase transition
temperatures, while [peC12Bim2]Cl2 showed only one phase transition temperature. The phase
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transition peak temperatures and corresponding enthalpy were summarized in Table 2.
Transition peak temperature (T1) of [peC12Bim2]Cl2, [peC14Bim2]Cl2 and [peC16Bim2]Cl2 were
54.8℃, 63.5℃, and 67.4℃ respectively, which were close to monocationic imidazolium SAILs
(melting points of [C12mim]Br, [C14mim]Br, and [C16mim]Br were 45-47℃ , 54-58℃, and
71-74℃ respectively) [54]. The phase transition temperature (T1) increased with the increase of
hydrophobic chain length, and the corresponding transition enthalpy increased with the increase
of hydrophobic chain length (54.8℃, 63.5℃, and 67.4℃).
T1 of [peCnBim2]Cl2 (n=12, 14, and 16) were much lower than those of [Cnim]2 (106-114 ℃)[19],
which was ascribed to the different lattice forces and intermolecular accumulation compactness.
The C4H8 spacer in [peCnBim2]Cl2 makes the distance between the two hydrophilic headgroups
and hydrophobic alkyl groups farther away. The electrostatic interaction between hydrophilic
imidazolium headgroups are relative small, which results in lower phase transition temperatures.
Fig 3 TGA of [peC12Bim2]Cl2 and [peC14Bim2]Cl2
a, [peC12Bim2]Cl2; b, [peC14Bim2]Cl2
Fig 4 DSC of gemini imidazolium SAILs
a, [peC12Bim2]Cl2; b, [peC14Bim2]Cl2; c, [peC16Bim2]Cl2
3.3 Krafft point
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Krafft points of [peCnBim2]Cl2 were determined and found to be less than 0℃ for [peC12Bim2]Cl2
and [peC14Bim2]Cl2, 15℃ for [peC16Bim2]Cl2. It was obvious that Krafft point increased with the
increase of the hydrophobic alkyl chain length in [peCnBim2]Cl2, which followed same trend as
[Cnim]2 [19]. While Krafft points of [peCnBim2]Cl2 were lower than those of [Cnim]2 with same
alkyl chain length. The reason relies on the rigid C3H6 spacer group in [Cnim]2, which makes the
distance between the two hydrophilic imidazolium headgroups of [Cnim]2closer, resulting in lower
Krafft points of [peCnBim2]Cl2. At the same time, two hydrophobic groups (ester alkyl chain)
derived from same middle carbon in C3H6 spacer group, made the hydrophobic effect stronger,
more compact arrangement, and larger lattice forces.
3.4 Surface tension and conductivity measurements
The surface tension measurement was shown in Fig 5. The critical micelle tension (cmc) value
and surface tension at cmc (γcmc) were determined from the breakpoint of the curve. Maximum
surface excess (Γmax) at the air-water interface was calculated by using Gibbs adsorption isotherm
equation [18,19,55]:


1
d




(1)
max  
2.303nRT d log C

T
where R=8.314 J·mol−1·K−1, T = 298.15 K, γ represents the surface tension in mN/m, C is the
surfactant concentration (mol/L), n=2.
The area occupied per surfactant molecule (Amin) at the air/water interface expressed in nm2 is
obtained by using following equation [18,19,55]
1018
Amin

(2)
Nmax
NA is Avogadro’s number (6.02 × 1023 mol−1).
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The surface activity parameters of [peCnBim2]Cl2 were summarized in Table 3.
Fig 5 Surface tension measurement of [peCnBim2]Cl2 at 25℃
a, [peC12Bim2]Cl2; b, [peC14Bim2]Cl2; c, [peC16Bim2]Cl2
As shown in Fig 5 and Table 3, cmc of [peC12Bim2]Cl2, [peC14Bim2]Cl2, and [peC16Bim2]Cl2
were 0.410mM, 0.112mM, and 0.305mM, respectively. The cmc values decreased when alkyl
chain length increased from C12 to C14, while increased surprisingly to 0.305 mM of
[peC16Bim2]Cl2.
The cmc values of [peC12Bim2]Cl2 and [peC14Bim2]Cl2 were about one order of magnitude lower
than those of monoimidazolium surfactants ([CnIM]Br) [14].
The cmc values of [peC12Bim2]Cl2 and [peC14Bim2]Cl2 were close to those values of gemini
imidazolium SAILs ([CnBim2]Br2 , n=12, 14). γcmc values of [peC12Bim2]Cl2 and [peC14Bim2]Cl2
were close to those values of monoimidazolium SAILs ([C12IM]Br, 39.4 mN/m; [C14IM]Br, 39.2
mN/m). These results demonstrated that the surface activity of [peC12Bim2]Cl2 and [peC14Bim2]Cl2
were superior to those of monoimidazolium SAILs ([CnIM]Br).
Amin of [peCnBim2]Cl2 decreased with the increase of alkyl chain length, which followed the same
trend with monoimidazolium SAILs [14,47].
However, Amin trend of [peCnBim2]Cl2 was different from that of Gemini imidazoliu SAILs [18,
43]. The reason was related to the different hydrophobic chain length in [peCnBim2]Cl2. Another
15

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C3H6 groups between the ester group and polar imidazolium group made the whole hydrophobic
chain length longer, resulted in obvious change of Amin.
Moreover, the surface tension of [peC12Bim2]Cl2 and [peC14Bim2]Cl2 aqueous solutions
decreased rapidly with the increase of surfactant concentration at low concentration, and the tension
remained constant when the concentration reached a certain value. But [peC16Bim2]Cl2 showed
abnormal behavior. Surface tension of [peC16Bim2]Cl2 did not change obviously with the increase
of surfactant concentration at low concentration, and started to decrease when the concentration
increased to certain value.
The cmc of [peC16Bim2]Cl2 increased to higher cmc value (0.305 mM) and higher γcmc (53.0
mN/m), while [peC16Bim2]Cl2 showed relative lower Amin value (0.67 nm2). This abnormal
surface behavior was found in other gemini surfactants [47, 56-57].
This particular phenomenon is attributed to the formation of small, non-surface-active aggregates
(dimmers, trimers, etc.) of surfactant prior to micelle formation. Premicellar aggregates at
air/water interface resulted in much lower Amin value of [peC16Bim2]Cl2 than those of the other
two surfactants [56-57].
The cmc values of [peCnBim2]Cl2 were evaluated by the conductivity method and summarized in
Fig 6 and Table 3.
Fig 6 Conductivity measurements of [peCnBim2]Cl2
a, [peC12Bim2]Cl2; b, [peC14Bim2]Cl2; c, [peC16Bim2]Cl2
The cmc values of [peCnBim2]Cl2 from electrical conductivity were in good agreement with the
16

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values determined from surface tension measurements. The cmc value decreased first and then
increased with the increase of hydrophobic chain length from C12 to C16. Moreover, cmc values
measured by conductivity method were higher than those measured by surface tension method.
β values (degree of counterion binding to micelles) were determined from electrical conductivity,
which were shown in Table 3. β values of [peCnBim2]Cl2 were smaller than those of
monoimidazolium SAILs, which were 0.76, 0.78 and 0.77 [54] for [C12IM]Br, [C14IM]Br, and
[C16IM]Br respectively. β values of [C12Bim2]Br2 and [C14Bim2]Br2 were 0.70, 0.57
respectively[43], which were higher than those of corresponding [peCnBim2]Cl2. This suggested
that the ability of counterion in [peCnBim2]Cl2 binding to micelles was somewhat weaker than that
of counterion in [CnBim2]Br2 and [CnIM]Br.
The standard Gibbs free energy change of micellization was calculated from following equation
[43]
Gm0  RT(0.5 )ln Xcmc
(3)
where Xcmc refers to cmc in molar fraction, Xcmc = cmc/55.4, 55.4 mol derives from weight of 1L
water at 25℃. β is the degree of counterion binding to micelles.
G0
Gibbs free energy change of adsorption ( ads ) was determined with the following equation [47]:
0 cmc
Gaods  Gmo 
(4)

where γo and γcmc refer to surface tensions of water and surfactants solution at cmc, respectively.
H o
The standard enthalpy change for the micellization process,
m , can be determined using the
Gibbs–Helmholtz equation:
17

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o












Gm




T
o
Hm 
(5)
1


T




Ho  RT2 0.5 ln X / dT


(6)
m
cmc
So
m is obtained using the following equation:
Thus, the standard entropy of micelle formation,
TSo  Ho Go
(7)

m 
m
m
Go
Gmo
Go Gmo
ads and
The
ads and
values of [peCnBim2]Cl2 were summarized in Table 3. All
values of [peCnBim2]Cl2 were negative, which indicated the formation of micelles is spontaneous
Gmo
for [peCnBim2]Cl2. The more negative
indicated more favored micellization.
Gmo
of [peC16Bim2]Cl2 (-29.72kJ mol-1) was higher than those of [peC12Bim2]Cl2 (-31.09kJ mol-1)
and [peC14Bim2]Cl2 (-31.82kJ mol-1). The tendency of micelle formation was lower, and resulted in
higher cmc value (0.305mM).
Go
ads and
Go
m of different surfactants demonstrated more
The higher energy difference between
Go Go
m ) of [peC12Bim2]Cl2 and
favorable adsorption at interface. The energy difference (
-
ads
[peC14Bim2]Cl2 were higher than that of [peC16Bim2]Cl2. As for [peC16Bim2]Cl2, the energy
Gaods Gmo
difference between
and
was smaller, which formed premicellar aggregates more
readily and showed lower Amin value.
Electrical conductivity of different [peCnBim2]Cl2 (n=12, 14, 16) concentrations at different
temperatures (20 , 25℃, 30℃, 40℃) were tested and shown in
℃
.
Fig S7, S8, S9
18

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The cmc values and thermodynamic micellization parameters of [peCnBim2]Cl2 at different
temperatures (20℃, 25℃, 30℃, 40℃) were determined and summarized in Table 4.
The values of standard enthalpy change for micellization of [peCnBim2]Cl2 were negative, which
Hmo
indicated that the micelle formation process was exothermic. Besides,
values of
[peCnBim2]Cl2 varied insignificantly at different temperatures, indicating no significant change in
the environment surrounding the hydrophobic chain of [peCnBim2]Cl2 molecule with increasing
temperatures.
Hmo
As shown in Table 4,
of [peCnBim2]Cl2 decreased in multiples with the increase of
hydrophobic chain length, which indicated the enthalpy-driven trend was greater with the increase
of hydrophobic chain length in [peCnBim2]Cl2.
Hmo
As shown in Table 4, for [peC12Bim2]Cl2 and [peC14Bim2]Cl2, absolute value of
TSo
was much
smaller than
m , therefore, the micellization process was entropy-driven, and the driving force
for the micellization process was the tendency of the hydrophobic group of the surfactants transfer
from the solvent environment to the interior of the micelle. As for [peC16Bim2]Cl2, absolute value
Hmo
TSo
was much larger than that of m , indicating that the micellization process of
of
[peC16Bim2]Cl2 was enthalpy-driven. The micelle formation was most affected by micellization
temperature [43].
3.5 Biodegradation Results
Molecular structure, molecular weight, and ThOD of different SAILs were summarized in Table 5.
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The aerobic biodegradation percentages of gemini imidazolium SAILs 1-9, mono-imidazolium
SAILs 10-12, and reference substance (SDS) were summarized in Table 6. Each biodegradation
percentage value was the average of three parallel tests.
As shown in Table 6, the biodegradation percentage of gemini imidazolium-based SAILs
([C14Bim2]Br2) was lower than that of mono imidazolium-based SAILs ([C14IM]Br) with same
alkyl chain length.
Gemini SAILs with ester functional group ([CnPim2]Br2 and [peCnBim2]Cl2) showed much higher
biodegradation percentages than common gemini SAILs and mono imidazolium-based SAILs.
With regard to OECD 301D, if 60% biodegradation percentage is reached within 28 d, the
compound is classified as “readily biodegradable”. Based on the measurements, all
[peCnBim2]Cl2 (n=12, 14, 16) and [C16Pim2]Br2 can be classified as readily biodegradable.
3.5.1 Different biodegradation rate of monoimidazolium and gemini imidazolium SAILs
In this study, the biodegradation potential of monoimidazolium SAILs and different gemini
imidazolium-based SAILs were compared.
As presented in Table 6 and Fig 7, mono imidazolium-based SAILs ([CnIM]Br) exhibited higher
biodegradation rate than those of common gemini imidazolium-based SAILs ([CnBim2]Br2) with
same side chain length within 28 days. For example, biodegradation rate of [C14Bim2]Br2 was
13% after 28 days, which was lower than that of mono imidazolium-based SAIL [C14IM]Br
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(18%).
Fig 7 Biodegradation ratio of mono imidazolium and gemini imidazolium SAILs
a, [C12IM]Br; b, [C14IM]Br; c, [C16IM]Br; d, [C12Bim2]Br2; e, [C14Bim2]Br2; f, [C16Bim2]Br2
As presented in Table 6 and Fig 8, gemini imidazolium-based SAILs with ester functional group
([CnPim2]Br2 and [peCnBim2]Cl2) showed much higher biodegradation rates than those of mono
imidazolium-based SAILs. [C14Pim2]Br2, [peC14Bim2]Cl2, and [C14IM]Br, which have same
hydrophobic chain C14, exhibited the following biodegradation rates: [peC14Bim2]Cl2 ＞
[C14Pim2]Br2＞[C14IM]Br.
Two gemini imidazolium-based SAILs containing ester functional group ([CnPim2]Br2 and
[peCnBim2]Cl2) exhibited different biodegradation rates. [peCnBim2]Cl2 showed much higher
biodegradation rate than those of [CnPim2]Br2 with same side chain length within 28 days. For
example, biodegradation rate of [peC16Bim2]Cl2 was 92% after 14 days, which was much higher
than that of [C16Pim2]Br2 (52%).
Fig 8 Biodegradation ratio of ester-containing gemini imidazolium and mono imidazolium SAILs
a, [CnIM]Br; b, [CnPim2]Br2; c, [peCnBim2]Cl2
As above mentioned, the introduction of an ester functional group, position of ester group will
influence the biodegradation ratios of different gemini imidazolium SAILs.
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3.5.2 Effect of ester functional group on biodegradation
As previously reported, the introduction of an ester or amide functional group improves the
biodegradation rate of gemini ammonium surfactants, because ester-bond is easy to be attacked by
microorganisms [36, 40, 58-62].
As presented in Table 6 and Fig 9, gemini imidazolium SAILs with ester functional group
([CnPim2]Br2 and [peCnBim2]Cl2) showed much higher biodegradation rates than those of common
gemini ILs and mono imidazolium-based SAILs.
[peCnBim2]Cl2 were degraded up to 66%-80% within one week, and they reached nearly 100%
biodegradation rate after 21 days, which showed comparable biodegradation ratio to SDS. The
results further clarified the introduction of an ester functional group improved the biodegradation
rate of gemini imidazolium SAILs.
The location of ester group also had an obvious influence on biodegradation of gemini imidazolium
SAILs. [peCnBim2]Cl2 with ester functional group in side chain showed much higher
biodegradation rates than those of [CnPim2]Br2 (ester functional group in spacer).
Fig 9 Biodegradation ratio of gemini imidazolium SAILs with ester functional group
A, [peCnBim2]Cl2; B, [CnPim2]Br2
The biodegradation ratios of [peCnBim2]Cl2 and [CnPim2]Br2 (with same imidazolium headgroup
and ester group) showed remarkable difference.
For example, the biodegradation ratios of [CnPim2]Br2 ranged from 44% to 58% after 21 days,
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which were much lower than those of [peCnBim2]Cl2 (from 92% to 100%). [peCnBim2]Cl2 was
classified as readily biodegradable SAILs after 7 days.
The results showed that ester group position played important role on biodegradation ratios of
gemini SAILs with ester groups. Gemini imidazolium SAILs with ester groups in alkyl side chains
([peCnBim2]Cl2) showed much higher biodegradation ratios than those of gemini imidazolium ILs
with ester groups in spacers ([CnPim2]Br2). This may be due to the presence of the steric hindrance
of the neopentane group in [CnPim2]Br2, which affects the hydrolysis of the microorganism on the
ester group.
This above mentioned biodegradation property was different from gemini ammonium surfactants
[58-62]. So not all gemini surfactants containing ester bond are readily biodegradable, ester group
location and head group type as well as alkyl chain length can affect biodegradability of surfactants.
3.5.3 Effect of headgroups on biodegradation
The biodegradation potential of imidazolium-based SAILs (mono and gemini SAILs) were
compared with reported ammonium-based surfactants (mono and gemini quaternary surfactants)
[63-64].
As presented in Table 6, the biodegradation ratios of mono imidazolium SAILs [CnIM]Br (n=12,
14,16) ranged from 10% to 20% after 28 days, which were different from those of relative mono
quaternary ammonium surfactants [63-64]. It suggests that mono quaternary ammonium displays
better biodegradability than mono imidazolium with same side chain.
As shown in Table 6, the biodegradation percentage of imidazolium-based gemini SAILs
([CnBim2]Br2) ranged from 9% to 18% after 28 days, which were lower than those of cationic
gemini ammonium surfactants with same alkyl chain length and same C4H8 spacer (59% for
23

ACCEPTED MANUSCRIPT
II-12-2(butane) after 14 days) [59].
Gemini imidazolium SAILs with ester groups in alkyl side chains ([peCnBim2]Cl2) can be classified
as readily biodegradable. The biodegradation ratios of [peCnBim2]Cl2 ranged from 80% to 92%
after 14 days, which were higher than bis(ester-ammonium) chloride II-10-2, II-12-2, II-14-2, and
II-12-2(butane) in references [58-59]. This two series gemini ammonium surfactants with ester
group in alkyl side chain also showed good biodegradability (52% to 59% after 14 days) [58-59].
For gemini ammonium surfactants with ester group in alkyl side chain in references [60, 62], which
did not reached the threshold value of 60% after 28 days, and cannot be classified as readily
biodegradable.
The biodegradation ratios of [CnPim2]Br2 (with ester group in the spacer) ranged from 50% to 65%
after 28 days, which were lower than those of gemini ammonium surfactants with ester group in the
spacer [40, 61]. The biodegradation ratio of all gemini ammonium surfactants Cn-PG-Cn exceeded
70% after 28 days (76.54% for C12-PG-C12 and 73.50% for C14-PG-C14), which could be classified
as readily biodegradable surfactants. The biodegradation ratio of gemini ammonium surfactants
G2C-12 exceeded 60% after 28 days [40, 61].
3.5.4 Effect of alkyl chain length on biodegradation
S. Steudte [37] and Al-Mohammed [38-39] studied the biodegradation of dicationic ionic liquids,
tris-imidazolium, and tetrakis-imidazolium ILs respectively, their results showed that a longer linear
alkyl side chains, the introduction of terminal hydroxyl groups, the introduction of ester groups in
the linkage chain enhanced biodegradability. Our experiments further supported similar
biodegradation trends [37-39, 65].
As shown in Table 6 and Fig 10, for all imidazolium SAILs, [CnBim2]Br2, [CnIM]Br,
24

ACCEPTED MANUSCRIPT
[peCnBim2]Cl2 and [CnPim2]Br2, with different alkyl side chain length, the biodegradation ratio of
these gemini SAILs increased when the alkyl hydrophobic side chain changed from dodecyl to
hexadecyl.
Fig 10 Biodegradation ratio of different imidazolium SAILs
a, [peCnBim2]Cl2; b, [CnPim2]Br2; c, [CnBim2]Br2; d, [CnIM]Br
For example, the biodegradation ratios of [C12Bim2]Br2, [C14Bim2]Br2, and [C16Bim2]Br2 ranged
from 9% to 18% after 28 days. The biodegradation ratios of [peCnBim2]Cl2 exceeded 80% as
readily biodegradable ILs after 14 days.
Due to the adaptation processes of microorganisms to imidazolium SAILs, further detailed
investigations should be carried out. The degradation mechanism, the analysis and determination
of possible hydrolysis products of these gemini imidazolium SAILs ([peCnBim2]Cl2, [CnPim2]Br2,
and [CnBim2]Br2) were ongoing.
4. Conclusions
Three ester-containing gemini imidazolium SAILs, abbreviated as [peCnBim2]Cl2 (n=12,14,16)
were synthesized from imidazole, 1,4-dibromobutane, 1-chloro-3-propanol, and various aliphatic
1
carboxylic acids. Chemical structures of [peCnBim2]Cl2 were confirmed by HNMR, FT-IR, and
elemental analysis. Thermal properties of [peCnBim2]Cl2 were studied with TGA and DSC analyses.
The physicochemical properties, micellization parameters, adsorption/aggregation properties, and
thermodynamic parameters of [peCnBim2]Cl2 were demonstrated using tensiometry, electrical
conductivity. The formation of micelles was spontaneous, exothermic, and entropy-driven.
25

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Using OECD 301D, the aerobic biodegradation of nine gemini imidazolium SAILs
([peCnBim2]Cl2, [CnPim2]Br2, and [CnBim2]Br2) and three monoimidazolium SAILs ([CnIM]Br)
were systematically investigated to determine the relationship between biodegradation and
structural elements (different spacers, alkyl or ester-containing side chains) of SAILs.
Aerobic biodegradation of twelve SAILs followed the decreasing trend: [peCnBim2]Cl2 ＞
[CnPim2]Br2＞[CnIM]Br＞[CnBim2]Br2.
Gemini imidazolium SAILs with ester functional group ([CnPim2]Br2 and [peCnBim2]Cl2) showed
much higher biodegradation rates than those of [CnIM]Br and [CnBim2]Br2.
The location of ester group played an obvious role on biodegradation. Gemini SAILs containing
ester group in the side alkyl chains ([peCnBim2]Cl2) showed much higher biodegradation ratio
than those of [CnPim2]Br2 (ester functional group in spacer). [peCnBim2]Cl2 could be classified as
readily biodegradable.
The biodegradability of gemini imidazolium SAILs increased with increasing the alkyl chain
length.
The biodegradation property of gemini imidazolium SAILs was different from gemini ammonium
surfactants with same alkyl side chain, functional group, and spacer length.
The results revealed the relationship between the structures and the biodegradability of
imidazolium SAILs to a certain extent, and will help to design and develop environmentally
favourable gemini imidazolium SAILs.
Acknowledgements
This work was supported by National Natural Science Foundation of China (No. 21706127), and
Natural Science Foundation of Jiangsu Province (BK20140939). The authors also gratefully
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appreciate the support from National/Jiangsu Students Innovation and Entrepreneurship Training
Program (No. 201710291015, 201710291016, 2018DC661). The financial support from
Postgraduate Research & Practice Innovation Program of Jiangsu Province (SJCX18-0345) was
also gratefully appreciated.
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