Synthesis, physicochemical and antimicrobial properties of cardanol-derived quaternary ammonium compounds (QACs) with heterocyclic polar head

Article abstract of DOI:10.1016/j.molliq.2019.111669

As new biomass derived raw material, cardanol has attracted the attention of numerous academic and industrial groups due to its renewability and unique structural features. In this work, seven cardanol-derived quaternary ammonium compounds (QACs) were syn

Full text of DOI:10.1016/j.molliq.2019.111669

Journal of Molecular Liquids 294 (2019) 111669
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Synthesis, physicochemical and antimicrobial properties of
cardanol-derived quaternary ammonium compounds (QACs) with
heterocyclic polar head
a
a
c
Jingyi Ma a, Na Liu a, Mengen Huang a, Limin Wang a,b, , Jianwei Han , Hui Qian , Fei Che
⁎
a
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road,
Shanghai 200237, PR China
b
Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, The Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, PR China
Shanghai Bronkow Chemical Co., Ltd., Shanghai 201512, PR China
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 May 2019
Received in revised form 29 August 2019
Accepted 31 August 2019
Available online 02 September 2019
As new biomass derived raw material, cardanol has attracted the attention of numerous academic and industrial
groups due to its renewability and unique structural features. In this work, seven cardanol-derived quaternary
ammonium compounds (QACs) were synthetized with their physicochemical properties and antimicrobial abil-
ity evaluated. The surface tension was measured in aqueous medium by the platinum ring method, 2b has the
optimal surface activities, which is considerably better than commercialized QAC product
cetyltrimethylammonium bromide. Both broth dilution method and agar dilution method were used to obtain
the MIC and MBC values of the targeted QACs. 1a, 1b, 2b inhibit the tested bacterial at a concentration of 32
μg/mL. Scanning electron microscopy (SEM) results intuitively showed that the QACs could interact with the bac-
terial cell membrane, for disrupting the integrity of the membrane. Meanwhile, it was found that the antimicro-
bial activity depended not only on the alkyl chain length, but also on the CMC value. To sum, seven cardanol-
derived QACs were synthetized easily, which showed excellent surface activities and antimicrobial activity, as
a promising alternative to the existing fossil fuel derived cationic surfactants and antiseptics.
Keywords:
New biomass raw material
Cardanol
Quaternary ammonium compounds (QACs)
Surface activities
Antimicrobial activity
Scanning electron microscopy (SEM)
© 2019 Elsevier B.V. All rights reserved.
1. Introduction
unique chemical structure makes cardanol easy for chemical deriva-
tization. Many research groups have synthetized a variety of func-
With the diminishing of fossil fuel resources, surfactants synthetized
from renewable raw materials are gaining increasing attention from the
academic and industrial fields. Up to now, surfactants derived from sus-
tainable resources such as amino acids [1,2], glucose [3], rosin [4] etc.
have been well studied. Meanwhile, agro-industry waste like cardanol
could be adopted as an abundant bio-renewable source for further
chemical derivatization.
CNSL (cashew nut shell liquid) is a by-product of the cashew nut
processing industry [5], accounting for 25% of the total cashew nut
weight [6]. The global yield of CNSL almost approaches one million
tons per year [7,8]. Cardanol could be obtained after distillation of
CNSL [9,10]. There are 3 types of reactive sites on the chemical struc-
ture of cardanol: the phenolic hydroxyl group, aromatic ring and the
unsaturated position of the olefin in the side chain [11–14]. The
tional products based on cardanol, such as epoxy compounds
[15,16], acrylic monomers [17,18], plasticizers [19], coatings [20]
and surfactants [21,22]. Faye et al. [23] synthesized three ionic sur-
factants: a cationic, an anionic and a zwitterionic surfactant, derived
from cardanol, which exhibited good surface activities and solubility.
They could be sustainable alternatives to alkylbenzene surfactants.
Peungjitton et al. [24] synthesized sodium cardanol sulfonate surfac-
tants, of which surface activities and detergency properties were
measured. Compared with commercialized dodecylbenzene sulfo-
nate, the results showed that the sodium cardanol sulfonate could
be used as an alternative to anionic surfactant. Atta et al. [25] studied
syntheses of new nonionic surfactants derived from cardanol. The
good adsorption of as-synthetized surfactants at water/oil surface
were evaluated as asphaltene dispersants and emulsion breakers
for heavy crude oil. Godoy et al. [26] reported syntheses of
cardanol-based cationic surfactants. Wang et al. [27] carried out
ether and quaternization reactions based on cardanol to obtain
cardanol-derived QACs and studied their applications in emulsion
polymerization. The majority of reports regarding cardanol-derived
QACs were involved in physicochemical applications. Meanwhile,
⁎
Corresponding author at: Key Laboratory for Advanced Materials and Institute of Fine
Chemicals, School of Chemistry and Molecular Engineering, East China University of
Science and Technology, 130 Meilong Road, Shanghai 200237, PR China.
E-mail address: wanglimin@ecust.edu.cn (L. Wang).
https://doi.org/10.1016/j.molliq.2019.111669
0167-7322/© 2019 Elsevier B.V. All rights reserved.

2
J. Ma et al. / Journal of Molecular Liquids 294 (2019) 111669
few papers paid attention to the antimicrobial application of
cardanol-derived QACs.
2. Materials and methods
Here, we synthesized mono-heterocyclic quaternary ammonium
compounds (QACs) and asymmetric Gemini quaternary ammonium
compounds with double bond or carbon chain containing hydroxyl
linked group derived from cardanol. Meanwhile, the physicochemi-
cal properties of cardanol-derived QACs, such as surface tension,
Krafft temperature (Tk), wettability, were studied. In addition, we in-
vestigated antimicrobial activity by broth microdilution method,
agar dilution method and scanning electron microscopy (SEM) mon-
itoring. As-synthetized cardanol-derived QACs in this work possess
excellent physicochemical properties and antimicrobial efficacy,
making them as highly desirable alternative of fossil fuel derived
counterparts.
2.1. Materials
Cardanol (3-pentadecylphenol) (industrial grade 90%), SIGMA-
ALDRICH, Co., Ltd.; (E)-1,4-Dibromobut-2-Ene (≥98%), Shanghai Titan
Scientific Co., Ltd.; ( )-Epichlorohydrin (AR), Shanghai Aladdin Bio-
chemical Technology Co., Ltd.; 1,4-Diazabicyclo[2.2.2]Octane (99%),
Adamas Reagent Co., Ltd.; 1-Methylpyrazole (≥98%), Shanghai Dibo
Chemical Technology Co., Ltd.; N-Methylmorpholine (99%), Saan Chem-
ical Technology (Shanghai) Co., Ltd.; Thiazole (99%), Beijing Bailingwei
Technology Co., Ltd.; n-Butyl bromide (N99%), Shanghai Macklin Bio-
chemical Co., Ltd.; Hexyl Bromide (99%), Adamas Reagent Co., Ltd.; Hy-
drobromic acid (AR), Shanghai Lingfeng Chemical Reagent Co., Ltd.;
Scheme 1. Syntheses of cardanol-derived mono-heterocyclic QACs and asymmetric Gemini QACs with double bond linked group.

J. Ma et al. / Journal of Molecular Liquids 294 (2019) 111669
3
Yeast extract powder (BR), Shanghai Macklin Biochemical Co., Ltd.;
Tryptone (OXIDE), Shanghai Macklin Biochemical Co., Ltd.; Sodium
Chloride (AR, ≥99.5%), Shanghai Titan Scientific Co., Ltd.
2.2. Synthesis
2.2.1. Syntheses of cardanol-derived mono-heterocyclic QACs and asym-
metric Gemini QACs with double bond linked group (Scheme 1)
2.2.2. Syntheses of cardanol-derived mono-heterocyclic QACs with carbon
chain contained hydroxyl linked group (Scheme 2)
2.3. Interface properties measurement
Fig. 1. Surface tension of cardanol-derived QACs at 20 ( 2) °C.
The surface tension of cardanol-derived QACs was measured by the
platinum ring method using JK99C automatic tension meter at 20 (
2) °C. 20 mL of deionized water solution was placed in a clean cup crys-
tallizing dish with a diameter of 60 mm. By using the step-by-step
dilution-extraction method [28], an aqueous solution (0.001 mol/L) of
QAC was dropwise added to crystallizing dish, and then the surface ten-
sion was measured. The platinum was thoroughly cleaned and dried be-
fore measurement, and the surface tension was considered as an
average of three replicates [29]. The critical micelle concentration
(CMC) is determined by the intersection of the falling line and the
steady line after CMC on the γ-c curve [30].
2.4. Krafft temperature (Tk) measurement
The Krafft temperature is the critical solution temperature of the sur-
factant. Surfactants do not dissolve appreciably in aqueous solutions
below Tk. The concentration of the surfactant solution was set to
1.0 wt%, when the temperature rose to Tk, solubility of the surfactant
sharply rose. After heating the surfactant solution until the solution
was clarified, the clear solution was gradually cooled in ice-bath. The
temperature at which the clarified surfactant solution appeared turbid
was recorded as the Tk. The experiment was repeated three times and
averaged [31].
2.5. Contact angle measurement
The wettability of cardanol-derived QACs was determined by using a
Biolin contact angle meter to measure the contact angle of the film of
surfactant with water molecule. The experiment was repeated three
times and averaged. The smaller contact angle signifies greater wetta-
bility with water molecule [32].
2.6. Antimicrobial activity measurement
The tested microorganisms were Gram-positive bacterium, Staphy-
lococcus aureus (ATCC 25923), Bacillus subtilis (ATCC 6633); Gram-
negative bacterium, Escherichia coli (ATCC 25922).
2.6.1. Determination of minimum inhibitory concentration (MIC) - broth
dilution method
Prepared a 1280 μg/mL aqueous solution of cardanol-derived QAC
solution using sterile water, aqueous solution was filtered with a 0.45
μm membrane, and then diluted to get QAC solution concentrations:
Table 1
Adsorption parameters of cardanol-derived QACs at 20 ( 2) °C.
Compound CMC
γCMC
C20
pC20 CMC/C20 Γmax
Amin
(mM) (mN/m) (10−4
mol/L)
(μmol/m2) (nm2)
1a
1b
1c
1d
1e
2a
2b
0.009 17.12
0.005 17.74
0.090 17.54
0.027 19.00
0.022 19.43
0.032 13.98
0.002 11.32
0.045
0.018
0.256
0.054
0.040
0.121
0.010
5.350 2.01
5.728 2.74
4.592 3.52
5.265 4.88
5.402 5.65
4.916 2.69
6.005 2.47
10.90
6.82
4.32
1.22
3.06
7.90
10.12
0.15
0.24
0.38
1.36
0.54
0.21
0.16
Scheme 2. Syntheses of cardanol-derived mono-heterocyclic QACs with carbon chain
contained hydroxyl linked group.

4
J. Ma et al. / Journal of Molecular Liquids 294 (2019) 111669
Table 2
microorganisms suspension was placed in a 2 mL centrifuge tube,
which was used to obtain experimental sample. The mixture of 100 μL
of sterilized deionized water and 900 μL of 105 cfu/mL microorganisms
suspension was used as a control group. The mixture was placed in a
37 °C incubator for 24 h. Then, the mixture was fixed with 2.5% glutar-
aldehyde, washed three times with phosphate buffered saline,
dehydrated with 20%, 40%, 60%, 80%, 95%, 100% alcohol. After that, the
as-treated mixture (10 μL) was dropped on the glass and dried at
clean room, finally, sputtered coated. The SEM images of the bacteria
were recorded on a scanning electron microscope [38].
Krafft temperature (Tk) and the contact angles of cardanol-derived QACs.
Compound
Tk (°C)
Average angle (°)
1a
1b
1c
1d
1e
2a
2b
b0
3
8
7.59
18.89
27.30
38.06
48.79
26.19
5.23
3
N90
b0
b0
640, 320, 160, 80, 40, 20, 10, 5, 2.5, 1.25 μg/mL. Different concentrations
of cardanol-derived QACs (10 μL) were regularly added to the sterilized
a 96-well plate. Meanwhile, 90 μL of 105 cfu/mL bacterial suspension
was added to each well. After the 96-well plate was cultured in a 37
°C incubator for 24 h [33,34], it was observed whether or not the bacte-
ria grew to obtain the minimum concentration at which no bacterial
growth was observed by analyzing the absorbance of bacteria suspen-
sion at 600 nm, which was considered as the MIC.
3. Results and discussion
3.1. Evaluation of surface activity by surface tension measurement
In this work, the surface tension of aqueous solution of as-
synthesized cardanol-derived QACs was measured to evaluate surface
activities. At 20 ( 2) °C, the relationship between the surface tension
(γ) and the logarithm concentration (log C) of the aqueous solution of
cardanol-derived QACs are shown in Fig. 1. With the concentration of
the aqueous solution of QACs increased, the surface tension decreased,
reached at a steady value, finally. The critical micelle concentration
(CMC) is determined by the intersection of the falling line and the
steady line on the γ-log C curve. When the critical micelle concentration
is exceeded, the solution is dissolved to saturation by the amphiphilic
molecules. There is no longer any change in surface tension [39–41].
According to the graph of relationship between the surface tension γ
2.6.2. Determination of minimum bactericidal concentration (MBC) - agar
dilution method
Prepared a 2048 μg/mL aqueous solution of cardanol-derived QAC
solution using sterile water, aqueous solution was filtered with a 0.45
μm membrane, and then diluted to get antimicrobial solution concen-
trations: 1028, 512, 256, 128, 64, 32, 16 μg/mL. Sterilizing the liquid
agar media by autoclave at temperature of 120 °C for 20 min [35],
followed 15 mL of liquid agar medium at high temperature mixed
with different concentrations of cardanol-derived QAC solution
(1 mL). And then, the mixture was poured into the culture dish to be so-
lidified to obtain antibacterial agar medium as samples, the agar me-
dium without cardanol-derived QAC as control group. 100 μL of a
bacterial suspension of 105 cfu/mL was added to the culture dish of
above agar medium and spread it with an applicator [36]. The culture
medium was cultured in a 37 °C incubator for 24 h. The number of bac-
terial strains in all samples and control groups were recorded. The min-
imum bactericidal concentration (MBC) is the minimum concentration
at which the growth of live bacteria is reduced by ≥99.9% [37].
and log C, adsorption parameters, such as CMC, γCMC, C20, pC20, Γmax
,
Amin and CMC/C20, were obtained by calculating. The surface tension
of CMC is γCMC, which is the minimum surface tension value that can
be reached by increasing the concentration of the aqueous solution of
QAC; C20 is the concentration of QAC at which the surface tension of de-
ionized water is reduced by 20 mN/m. The adsorption efficiency pC20, is
the negative logarithm of C20 [42].
pC20 ¼ − log C20
ð1Þ
Γmax is the maximum excess surface concentration, the difference
between the amount of the substance containing the solute in the sur-
face layer per unit surface area and the amount of the substance con-
taining the solute in the bulk of the solution. Amin is the smallest area
occupied by adsorbed amphiphilic molecule at the air/water interface
when the gas-liquid interface reaches saturation. Γ and A reflect the ar-
rangement of amphiphilic molecules at the air/water interface. Γmax and
2.6.3. Scanning electron microscope (SEM)
Scanning electron microscopic study of the influence of MIC concen-
tration cardanol-derived QAC on the microorganisms of S. aureus, E. coli
have been performed. The mixture of 100 μL of cardanol-derived QAC
solution with good antibacterial effect and 900 μL of 105 cfu/mL
Fig. 2. The contact angles of the film of cardanol-derived QACs (on glass sheet) with water molecules.

J. Ma et al. / Journal of Molecular Liquids 294 (2019) 111669
5
Table 3
Amin reflect the denser arrangement of amphiphilic molecules in solu-
tion. The value of CMC/C20 reflects the relative adsorption activity of
the surfactant at the interface and the ability to form micelles in aqueous
solution.
Antimicrobial activity results of cardanol-derived QACs (μg/mL) on Gram-positive bacteria
(S. aureus and B. subtilis) and Gram-negative bacteria (E. coli).
Compound
S. aureus
B. subtilis
E. coli
MIC
MBC
MIC
MBC
MIC
MBC
Γmax ¼ −ð∂γ=∂ln CÞ=2:3nRT
ð2Þ
1a
1b
1c
1d
1e
2a
2b
32
32
128
64
64
128
32
64
64
8
16
64
16
16
64
8
32
64
128
64
32
N128
32
16
32
N128
16
16
N128
16
32
128
N128
64
64
N128
32
Amin ¼ 1024=ðNAΓmax
Þ
ð3Þ
ð4Þ
N128
N128
128
N128
64
CMC=C20 ¼ CMC=C20
where γ is the surface tension of cardanol-derived QACs. R is the gas
constant, R is equal to 8.315 × 107. T is the absolute temperature of
gas. (∂γ/∂ln C) is the slope of the linear simulation diagram of the sur-
face tension before CMC. The parameter n is the number of ionic species.
NA is the Avogadro constant (6.02 × 1023) [43].
1a, 2a, 2b show excellent solubility at 0 °C, and enable them to be
used in low temperature, which have a wide range of applications.
The Tk of 1b, 1c, 1d are all below 10 °C, being known as good water sol-
uble surfactant. As mentioned by Mahantesh et al. [54], incorporating
hydroxyl, heterocyclic polar head group into QACs could lower the sol-
ubility. However, the Tk of 1e is above 90 °C. S. Ban et al. [55] found that
the presence of hydrophobic chain led to the aggregation of molecules
and increased hydrophobicity, thereby resulting in the decrease of solu-
bility and the increase of Tk value. Therefore, as the alkyl chain increased,
the Tk of cardanol-derived asymmetric Gemini QACs were increased,
solubility decreased as shown in Table 2.
As is indicated in Table 1, the CMC values of cardanol-derived QACs
are lower 2–3 orders of magnitude than commercialized QAC product
cetyltrimethylammonium bromide (CMC = 1 mmol/L), the γCMC values
are lower than that of cetyltrimethylammonium bromide (γCMC = 35.3
mN/m) [44]. 2b has the optimal the surface activities, the critical micelle
concentration = 0.002 mmol/L, the surface tension = 11.31mN/m.
Khadidja et al. [45] investigated a phenylene on the hydrophobic
chain contributed to micellization. Due to hydrophobic effect and π-π
interactions among the amphiphilic molecules, which may be one rea-
son for better surface activities of cardanol-derived QACs. Yao et al.
[46] adding double bond into surfactants confirmed that, the surface ac-
tivities such as CMC value, emulsifying property, foaming property and
foam stability would improve a lot. Pei et al. [47] studied the adsorption
and micellization behaviors of QACs containing hydroxyl group in aque-
ous medium. The results clearly exhibited that QACs can form intermo-
lecular hydrogen bonds in aqueous medium, which could boost
interfacial adsorption and the formation of micelle directly. Grosmaire
et al. [48,49] mentioned that surfactants with heterocyclic head groups
have higher surface activities and more abundant aggregation behav-
iors. Therefore, introducing the double bond, hydroxyl, heterocyclic
polar head group into QACs could reinforce the solubility, surface activ-
ities and aggregation behaviors in the aqueous medium, resulting a rise
in CMC value, which may be the other reason for better surface activities
of cardanol-derived QACs. For cardanol-derived asymmetric Gemini
QACs, as the alkyl chain increased, the CMC values decreased and the
3.3. The contact angles of the film of cardanol-derived QACs (on glass sheet)
with water molecules
Good wettability of cardanol-derived QACs enriches the application
as surfactants. The contact angle clearly reflects the wetting ability of
QAC and the surface affinity of water molecules and the amphiphilic
molecules shown in Fig. 2. A smaller contact angle means more excel-
lent wettability and spreadability on the film of cardanol-derived
QACs. As can be seen in Table 2, the contact angle of 2b almost reached
to 0°, which is lower than the contact angle of glass with water mole-
cules, indicating that water molecules have excellent spreading proper-
ties on the film of 2b, and it also shows that 2b has good wettability.
γCMC rose. Since the longer alkyl chain lead to the stronger hydrophobic
effect between the QAC molecules, the easier micellization, the smaller
CMC value [50]. At the same time, γCMC showed an opposite trend. Geng
et al. [51] investigated that more methylene groups are exposed due to
the longer free hydrophobic chains and the possibility of free rotation
and curling, resulting in an increase in surface energy and γCMC
.
The low flexibility of short spacer chains [52] of cardanol-derived
asymmetric Gemini QACs lead to a larger value of area per adsorbed am-
phiphilic molecule than cardanol-derived mono-heterocyclic QACs,
higher Amin can be anticipated. The data conformed that the adsorption
efficiency pC20 values are inversely related to the CMC values as men-
tioned by Pinazo et al. [52]. The adsorption efficiency increased with a
higher hydrophobic effect. Lower C20 and higher pC20 indicate higher
adsorption efficiency of QAC, 2b is optimum, having highest pC20. Mean-
while, the larger values of Γmax and smaller values of Amin and CMC/C20
indicate that 1a, 2b have a more dense alignment of molecules and bet-
ter micellization tendency.
3.2. Krafft temperature (Tk) measurement
For surfactants, in the lower temperature range, the solubility rises
very slowly with increasing temperature. When the temperature rises
to a certain value, the solubility increases rapidly, which is regarded as
Krafft temperature (Tk) [53]. A lower Tk results in the better solubility
of the surfactant at low temperature. As is seen from Table 2, the Tk of
Fig. 3. SEM images for: (a) S. aureus untreated; (b) S. aureus treated with 32 μg/mL 2b;
(c) E. coli untreated; (d) E. coli treated with 16 μg/mL 2b. The scale bar is marked in the
figures.

6
J. Ma et al. / Journal of Molecular Liquids 294 (2019) 111669
According to Table 2, other cardanol-derived QACs also have certain
wettability and spreadability. As the increasing of alkyl chain of
cardanol-derived asymmetric Gemini QACs, the contact angle increased
and wettability decreased [37].
4. Conclusion
Two series of seven quaternary ammonium compounds (QACs) de-
rived from renewable biomass raw material cardanol have been synthe-
sized with excellent surface activity, lower critical micelle concentration
(CMC), Krafft temperature (Tk) and contact angle, compared with tradi-
tional surfactants. Furthermore, cardanol-derived QACs are found to
have better antimicrobial efficacy for varieties of bacteria at a lower con-
centration. The damaging cell images of SEM were clearly observed that
cardanol-derived QACs have effects on pathogenic bacteria. Undoubt-
edly, the syntheses of cardanol-derived QACs promote development of
cationic surfactants.
3.4. Antimicrobial activity
By using both broth dilution method and agar dilution method, the
MIC and MBC values of cardanol-derived QACs were determined. The
MIC and MBC values were measured to evaluate antimicrobial activity
in Table 3, which revealed that most of cardanol-derived QACs exhibited
inhibition activities against the tested bacterial. Sharma et al. [56] found
out that a rise in polarity of the head group in surfactants promoted an-
timicrobial property. The polarity of head group can be improved by
adding the double bond or hydroxyl. It can be seen from the Table 3
that the antimicrobial effect of QACs 1a, 1b, 2b are found to be more
prominent and superior to the traditional single surfactant, which can
inhibit or even kill the tested bacterial at a concentration of 32 μg/mL
or lower. The bacterial inhibition effects are found to be remarkable
when the concentrations of QAC 2b are in the range from 32 to 64
μg/mL for S. aureus, from 8 to 32 μg/mL for B. subtilis, and from 16 to
32 μg/mL for E. coli, respectively.
Acknowledgment
This research was financially supported by the National Natural Sci-
ence Foundation of China (21772039, 21272069) and the Fundamental
Research Funds for the Central Universities and Key Laboratory of
Organofluorine Chemistry, Shanghai Institute of Organic Chemistry,
Chinese Academy of Sciences.
Appendix A. Supplementary data
It is known from the literature [58,61] that the length of hydrophobic
chain has a prominent effect on the antimicrobial activity of cationic
surfactants. Too long or too short of alkyl chain adverse to antimicrobial
activity of cationic surfactants. As the increasing of alkyl chain of
cardanol-derived asymmetric Gemini QACs, the antimicrobial property
increased due to the more hydrophobic environment. Hence, the anti-
microbial activity of 1e is stronger than 1d. At concentrations below
CMC values, QAC molecules are evenly dissolved in the culture medium,
easily exhibit antimicrobial behaviors by electrostatic and hydrophobic
interactions with bacterial cell membranes. Hydrophobicity of alkyl
chain and electron density of ammonium nitrogen atom are the key fac-
tors affecting the antimicrobial activity of surfactants. Hence, Asadov
et al. [57] mentioned that the antimicrobial activity depend not only
on the length of alkyl chain but also on the CMC value. As the CMC
values decrease, the antimicrobial activities are improved [57]. The ex-
perimental data show that QACs 1a, 1b, 2b exhibited significant inhibi-
tory effect against three pathogenic bacteria, whose CMC values exactly
are also lowest. 1c and 2a showed poor antimicrobial properties, prob-
ably, due to the smaller thiazole heterocycle molecule than 1-
methylpyrazole and n-methylmorpholine, and the higher CMC value.
For cardanol-derived asymmetric Gemini QACs 1d and 1e, the CMC
values are higher, thus, the antimicrobial properties is relatively weaker.
Scanning electron microscopy (SEM) as an intuitive means was used
to obtain a visual insight into the bacterial killing of cardanol-derive
QAC. SEM images of S. aureus and E. coli treated with QAC 2b (16 and
32 μg/mL, respectively) for 12 h and without treatment with QAC (as
a control) were dis-played in Fig. 3.
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.molliq.2019.111669.
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