Synthesis, surface properties and biological activity of long chain ammonium herbicidal ionic liquids

Article abstract of DOI:10.5935/0103-5053.20160058

Herbicidal ionic liquids exhibit many advantages compared to conventional chemical plant protection agents, which allow for a reduction of the herbicide dose per hectare and decrease of toxicity. Popular herbicides, such as 4-chloro-2-methylphenoxyacetic

Full text of DOI:10.5935/0103-5053.20160058

http://dx.doi.org/10.5935/0103-5053.20160058
J. Braz. Chem. Soc., Vol. 27, No. 10, 1774-1781, 2016.
Printed in Brazil - ©2016 Sociedade Brasileira de Química
0103 - 5053 $6.00+0.00
Article
Synthesis, Surface Properties and Biological Activity of Long Chain Ammonium
Herbicidal Ionic Liquids
Rafał Giszter,*,a Marta Fryder,a Katarzyna Marcinkowskab and Agata Sznajdrowskaa
aDepartment of Chemical Technology, Poznan University of Technology, 60-965 Poznan, Poland
bInstitute of Plant Protection, National Research Institute, 60-965 Poznan, Poland
Herbicidal ionic liquids exhibit many advantages compared to conventional chemical
plant protection agents, which allow for a reduction of the herbicide dose per hectare and
decrease of toxicity. Popular herbicides, such as 4-chloro-2-methylphenoxyacetic (MCPA) or
2,4-dichlorophenoxyacetic (2,4-D) acids, can be modified by reaction with long chain ammonium
cations in order to obtain novel forms. The aim of this study was to present an alternative method
for the synthesis of herbicidal ionic liquids based on a natural cation known as Ethoquad C/12
and to analyze the physicochemical properties and herbicidal activity of the obtained products.
New ionic liquids comprising MCPA and 2,4-D as anions were obtained and their chemical as
well as thermal stability were evaluated. Furthermore, their biological and surface activity were
investigated. The obtained results confirmed that the herbicidal activity has been preserved.
Keywords: herbicidal ionic liquids, herbicidal activity, wettability
Introduction
farming and, at the same time, reduce the labor necessary
for weed control practices.
Ionic liquids (ILs) are a class of organic salts, which
are liquids below 100 °C. They have attracted much
attention because of their unique properties, such as high
thermal stability, low volatility and non-flammability.1,2
The number of application of ILs has recently increased:
they can be employed as useful solvents in diverse fields,
such as organic synthesis, chemical catalysis, separation
technology, and novel electrolytes for batteries.3-5 Due
to their structural variety, there is also a considerable
number of different ILs resulting from the combination
of diverse cations and anions, which allows to design
them for particular applications. Nowadays, there is an
extensive list of literature reports regarding successive
applications of already known ILs. Additionally, new
possible applications were also discovered since ILs with
pharmaceuticals,6 antifeedants7,8 or fungicidal activity were
recently introduced.8,9
In 2011, a new group of ILs called herbicidal ionic
liquids (HILs) was described.10 This herbicidal group
of ionic liquids may be applied as weed control agents
in crop protection. The initial research included HILs
comprising anion from a single class of herbicides:
phenoxyacids.10 Phenoxyacids are a popular group of
herbicides which have been commercialized since the
1940s and they are still commonly used in weed control.11
The most important commonly used compounds include
4-chloro-2-methylphenoxyacetic acid (MCPA) and
2,4-dichlorophenoxyacetic acid (2,4-D). A phenoxy
herbicide is a member of a family of chemicals related
to the indoleacetic acid (IAA) growth hormone. When
sprayed on broadleaf plants they induce rapid, uncontrolled
growth, eventually killing the weeds.12 Currently, chemical
herbicides have a decisive impact on the protection of crop
plants, because their application is the most effective and
cheapest method.
Modern agriculture depends on effective control of pests
and weeds. Chemicals in agriculture have significantly
influenced the quality and quantity of food available for the
growing world population. The application of herbicides
is critical to increase food production, reduce the cost of
HILs exhibit many advantages compared to chemical
plant protection; agents allow for a reduction of the
herbicide dose per hectare and decrease of toxicity.10,13-18
Reduced volatility contributes to their safer use by operators
and reduction of overall environmental impact of the
employed herbicide.16 Solubility in water can be reduced
*e-mail: rafal.giszter@put.poznan.pl

Vol. 27, No. 10, 2016
Giszter et al.
1775
Scheme 1. Synthesis of quaternary ammonium bromide.
by appropriate selection of the cation, which contributes
to reduced mobility in soil and groundwater.14 Moreover,
HILs display multifunctional properties and may be used in
order to reduce the number of additional chemical such as
adjuvants or surfactants, which are required per application.
The risk of adverse effects of HILs on the environment
is reduced, because they will not react with the metal
ions present in soil. Additionally, they offer significant
advantages and are viable alternatives to other currently
used forms of herbicides.16
HILs mostly exhibit herbicidal activity, however, they may
display additional traits when a herbicidal anion is combined
with a cation, which exhibits appropriate properties, such as
plant growth inhibition19 or fungicidal activity.20 The newest
research reports introduced glyphosate-,21 metsulfuron-
methyl-22 and bis(ammonium)-based HILs.23 The novel
HILs are often based on Ethoquad C/12, a mixture of
bis(2-hydroxyethyl)alkylmethylammonium cations of
natural origin. These cations comprise alkyl constituents
with 8 to 18 carbon atoms and are used as emulsifiers, dye
components, thickeners and as antistatic agents. It is plausible
that HILs based on cations with a specific carbon chain length
may differ in terms of properties and herbicidal activity,
therefore it is imperative to compare the effects of defined
cations, which represent the constituents of Ethoquad C/12,
on the performance of HILs.
operating at 400 MHz with tetramethylsilane as the internal
standard. 13C NMR spectra were obtained using the same
instrument at 75 MHz. CHN elemental analyses were
performed at A. Mickiewicz University, Poznan (Poland).
Preparation of alkylbis(2-hydroxyethyl)ammonium bromide
In a round-bottom flask, 0.1 mol of the appropriate
bromoalkane was dissolved in 75 mL of acetonitrile. Next,
bis(2-hydroxyethyl)methylamine was added with a 10%
excess. The synthesis was conducted at the boiling point
temperature of the solvent for 24 h (Scheme 1).
Then, the solvent was removed by a vacuum evaporator
and 100 mL of ethyl acetate were added. The product,
which precipitated in the form of a white solid, was isolated
by filtration, washed with small portions of ethyl acetate
(2 × 15 mL) and dried under reduced pressure at 70 °C for
24 h (Table 1).
Table 1. Prepared quaternary ammonium bromides
Melting
Run
R
Anion
Yield / %
Purity / %
point / °C
109-110
114-115
115-116
1
2
3
C14H29
C16H33
C18H37
Br–
Br–
Br–
92
91
90
99
98
96
Theaimofthisstudywastoevaluatethephysicochemical
properties as well as surface and herbicidal activity of newly
synthesized HILs based on the alkylbis(2-hydroxyethyl)
ammonium cation and MCPA or 2,4-D anions. This study
attempts to evaluate the influence of cations with specific
alkyl chain length on the herbicidal activity. The length of
the alkyl chain substituent was equal to fourteen, sixteen
and eighteen carbon atoms.
Preparation of ammonium acetates
The appropriate alkylbis(2-hydroxyethyl)ammonium
bromide (0.01 mol) was introduced into a round-bottom
flask and dissolved in 50 mL of anhydrous methanol,
followed by the addition of 0.01 mol of potassium
hydroxide dissolved in 25 mL of methanol. The reaction
was conducted for 30 min at room temperature and the
precipitated inorganic by-product (potassium bromide)
was removed by filtration. Then, the filtrate containing
quaternary ammonium hydroxide was neutralized with
stoichiometric amount of MCPA or 2,4-D (1a-3b). Then
solvent was removed under reduced pressure and the
crude product was obtained. Afterwards, the residue was
Experimental
General
1H nuclear magnetic resonance (NMR) spectra were
recorded using a Mercury Gemini 300 spectrometer

1776
Synthesis, Surface Properties and Biological Activity of Long Chain Ammonium Herbicidal Ionic Liquids
J. Braz. Chem. Soc.
Scheme 2. General synthesis of salts with MCPA and 2,4-D anion.
dissolved in 50 mL of acetone and inorganic salt and
other residues were filtered off. After the evaporation of
acetone, the synthesized salts were dried under vacuum
for 24 h at the temperature of 70 °C (Scheme 2). Purity
was determined by a direct two-phase titration technique
(International Organization for Standardization European
standard (EN ISO) 2871-2-2010) (Table 2).
CH3), 1.26 (s, 30H, CH2), 1.76 (d, 2H, J 5.5 Hz, CH2), 3.34
(m, 3H, CH3), 3.56 (m, 4H, CH2), 3.75 (m, 4H, CH2), 4.10
(s, 4H, CH2), 4.80 (s, 2H, OH); 13C NMR (100 MHz, CDCl3)
d 14.0, 22.5, 26.3, 29.2 (6), 29.4 (4), 29.5 (5), 31.7, 50.3,
55.6, 63.6, 63.9; anal. calcd. for C23H46BrNO2: C, 61.59;
H, 10.34; N, 3.12; found: C, 61.02; H, 10.92; N, 3.71.
Bis(2-hydroxyethyl)methyltetradecylammonium (4-chloro-
2-methylphenoxy)acetate (1a)
Table 2. Prepared salts
1H NMR (400 MHz, CDCl3) d 0.88 (t, 3H, J 6.7 Hz,
CH3), 1.25 (s, 22H, CH2), 1.55 (d, 2H, J 5.5 Hz, CH2), 2.21
(s, 3H, CH3), 3.03 (s, 3H, CH3), 3.24 (m, 2H, CH2), 3.42 (m,
4H, CH2), 3.91 (s, 4H, CH2), 4.41 (s, 2H, OH), 5.27 (s, 2H,
CH2), 6.65 (d, 1H, J 5.5 Hz, Ar–H), 7.05 (m, 2H, Ar–H);
13C NMR (100 MHz, CDCl3) d 14.1, 16.2, 22.4, 22.6, 26.3,
29.2 (3), 29.3 (2), 29.5 (2), 29.6 (3), 31.8, 49.8, 55.5, 63.6,
63.9, 67.5, 112.5, 124.8, 126.2, 128.7, 130.2, 155.5, 174.1;
anal. calcd. for C28H50ClNO5: C, 65.16; H, 9.76; N, 2.71;
found: C, 65.67; H, 9.22; N, 3.22.
Yield /
%
Melting
temperature / °C
Purity /
%
Salt
R
Anion
1a
2a
3a
1b
2b
3b
C14H29
C16H33
C18H37
C14H29
C16H33
C18H37
MCPA
MCPA
MCPA
2,4-D
2,4-D
2,4-D
99
98
99
96
97
95
69-71
52-54
59-61
liquid
49-51
69-71
99
97
98
97
96
96
MCPA: 4-chloro-2-methylphenoxyacetic anion; 2,4-D: 2,4-dichloro-
phenoxyacetic anion.
Hexadecylbis(2-hydroxyethyl)methylammonium (4-chloro-
2-methylphenoxy)acetate (2a)
Bis(2-hydroxyethyl)methyltetradecylammonium bromide (1)
1H NMR (400 MHz, CDCl3) d 0.88 (t, 3H, J 6.7 Hz,
CH3), 1.26 (s, 22H, CH2), 1.76 (d, 2H, J 5.5 Hz, CH2), 3.34
(m, 3H, CH3), 3.56 (m, 4H, CH2), 3.75 (m, 4H, CH2), 4.10
(s, 4H, CH2), 4.80 (s, 2H, OH); 13C NMR (100 MHz, CDCl3)
d 14.0, 22.5, 26.3, 29.2 (4), 29.4 (4), 29.5 (3), 31.7, 50.3,
55.6, 63.6, 63.9; anal. calcd. for C19H42BrNO2: C, 57.56;
H, 10.68; N, 3.53; found: C, 57.99; H, 10.20; N, 4.10.
1H NMR (400 MHz, CD3OD) d 0.89 (t, 3H, J 6.7 Hz,
CH3), 1.28 (s, 26H, CH2), 1.76 (d, 2H, J 5.5 Hz, CH2), 2.25
(s, 3H, CH3), 3.15 (s, 3H, CH3), 3.41 (m, 4H, CH2), 3.52 (s,
4H, CH2), 3.96 (s, 4H, CH2), 4.39 (s, 2H, OH), 4.90 (s, 2H,
CH2), 6.72 (d, 1H, J 5.5 Hz, Ar–H), 7.08 (m, 2H, Ar–H);
13C NMR (100 MHz, CD3OD) d 14.5, 16.6, 23.4, 23.8, 27.5,
30.3 (2), 30.5 (4), 30.6 (3), 30.7 (2), 30.9, 33.1, 49.7, 56.7,
65.0, 65.1, 68.9, 113.5, 125.9, 127.2, 130.2, 131.2, 157.2,
176.4; anal. calcd. for C30H54ClNO5: C, 66.21; H, 10.00;
N, 2.57; found: C, 66.77; H, 9.49; N, 3.12.
Hexadecylbis(2-hydroxyethyl)methylammonium bromide (2)
1H NMR (400 MHz, CDCl3) d 0.88 (t, 3H, J 6.7 Hz,
CH3), 1.26 (s, 26H, CH2), 1.76 (d, 2H, J 5.5 Hz, CH2), 3.34
(m, 3H, CH3), 3.56 (m, 4H, CH2), 3.75 (m, 4H, CH2), 4.10
(s, 4H, CH2), 4.80 (s, 2H, OH); 13C NMR (100 MHz, CDCl3)
d 14.0, 22.5, 26.3, 29.2 (4), 29.4 (4), 29.5 (5), 31.7, 50.3,
55.6, 63.6, 63.9; anal. calcd. for C21H44BrNO2: C, 59.70;
H, 10.50; N, 3.32; found: C, 59.21; H, 10.98; N, 2.71.
Bis(2-hydroxyethyl)methyloctadecylammonium (4-chloro-
2-methylphenoxy)acetate (3a)
1H NMR (400 MHz, CD3OD) d 0.89 (t, 3H, J 6.7 Hz,
CH3), 1.28 (s, 30H, CH2), 1.76 (d, 2H, J 5.5 Hz, CH2),
2.25 (s, 3H, CH3), 3.14 (s, 3H, CH3), 3.41 (m, 4H, CH2),
3.52 (s, 4H, CH2), 3.95 (s, 4H, CH2), 4.39 (s, 2H, OH),
4,90 (s, 2H, CH2), 6.72 (d, 1H, J 5.5 Hz, Ar–H), 7.07 (m,
Bis(2-hydroxyethyl)methyloctadecylammonium bromide (3)
1H NMR (400 MHz, CDCl3) d 0.88 (t, 3H, J 6.7 Hz,
13
2H, Ar–H); C NMR (100 MHz, CD3OD) d 14.6, 16.6,

Vol. 27, No. 10, 2016
Giszter et al.
1777
23.4, 23.8, 27.5, 30.3 (2), 30.5 (4), 30.6 (3), 30.7 (2),
30.8 (2), 30.9, 33.1, 50.3, 56.7, 65.0, 65.1, 68.9, 113.5,
125.9, 127.2, 130.2, 131.2, 157.2, 176.4; anal. calcd.
for C32H58ClNO5: C, 67.16; H, 10.22; N, 2.45; found:
C, 67.67; H, 9.74; N, 2.00.
between 5 and 15 mg were placed in aluminum pans and
heated from 25 to 120 °C at a heating rate of 10 °C min-1,
and cooled with an intracooler at a cooling rate of
10 °C min-1 to –100 °C. Thermogravimetric analysis (TGA)
was performed using a Mettler Toledo Star TGA/DSC1
unit, under nitrogen. Samples between 2 and 10 mg were
placed in aluminum pans and heated from 30 to 450 °C at
a heating rate of 10 °C min-1.
Bis(2-hydroxyethyl)methyltetradecylammonium
(2,4-dichlorophenoxy)acetic (1b)
1H NMR (400 MHz, CD3OD) d 0.89 (t, 3H, J 6.7 Hz,
CH3), 1.28 (s, 22H, CH2), 1.78 (d, 2H, J 5.5 Hz, CH2), 3.18
(s, 3H, CH3), 3.44 (m, 2H, CH2), 3.55 (m, 4H, CH2), 3.98 (s,
4H, CH2), 4.48 (s, 2H, OH), 4.88 (s, 2H, CH2), 6.9 (d, 1H,
J 5.5 Hz,Ar–H), 7.21 (m, 1H,Ar–H), 7.36 (d, 1H, J 5.7 Hz,
Ar–H); 13C NMR (100 MHz, CD3OD) d 14.6, 23.4, 23.7,
27.4, 30.5 (3), 30.6 (2), 30.7 (3), 30.8 (2), 33.1, 50.4, 56.7,
65.0, 65.1, 69.4, 115.9, 124.5, 126.4, 128.6, 130.6, 154.9,
175.1; anal. calcd. for C27H47Cl2NO5: C, 60.44; H, 8.83;
N, 2.61; found: C, 59.92; H, 9.34; N, 2.08.
Surface tension
The surface tension was determined using the drop
shape method. The surface tension (γ in mN m-1) is
calculated by analyzing the profile of the drop according
to the Laplace equation.
Surface tension measurements were carried out by
the use of a drop shape analyzer (DSA) (Krüss DSA
100, accuracy 0.01 mN m-1), at 25 °C. Temperature
was controlled using a Fisherbrand FBH604 thermostatic
bath (Fisher, accuracy 0.1 °C). The values of the critical
micelle concentration (CMC) and the surface tension at the
CMC (γCMC) were determined from the intersection of
the two straight lines drawn in low and high concentration
regions in surface tension curves (γ-log C curves) using a
linear regression analysis method.
Hexadecylbis(2-hydroxyethyl)methylammonium
(2,4-dichlorophenoxy)acetic (2b)
1H NMR (400 MHz, CD3OD) d 0.89 (t, 3H, J 6.7 Hz,
CH3), 1.28 (s, 26H, CH2), 1.78 (d, 2H, J 5.5 Hz, CH2), 3.18
(s, 3H, CH3), 3.44 (m, 2H, CH2), 3.55 (m, 4H, CH2), 3.98 (s,
4H, CH2), 4.48 (s, 2H, OH), 4.88 (s, 2H, CH2), 6.9 (d, 1H,
J 5.8 Hz,Ar–H), 7.21 (m, 1H,Ar–H), 7.36 (d, 1H, J 5.7 Hz,
Ar–H); 13C NMR (100 MHz, CD3OD) d 14.6, 23.5, 23.8,
27.5, 30.4 (2), 30.5 (4), 30.6 (3), 30.7 (3), 33.1, 50.4, 56.7,
65.0, 65.1, 69.4, 115.8, 124.6, 126.4, 128.6, 130.6, 154.9,
175.2; anal. calcd. for C29H51Cl2NO5: C, 61.69; H, 9.10;
N, 2.48, found: C, 61.18; H, 9.61; N, 2.99.
Wettability
Surface active agents tend to prevent drops from
running off the leaf and improve wettability of a herbicide
formulation. The basis for the wettability is the drop
shape analysis and contact angle measurements. Values of
contact angle were determined from the image of the drop.
After determination of actual drop shape and the contact
line, the drop shape is adapted to fit a mathematical model
used to calculate the contact angle. The most exact method
to calculate this value is Young-Laplace fitting (sessile
drop fitting), where complete drop contour is evaluated.
After successful fitting of theYoung-Laplace equation, the
contact angle was determined as the slope of the contour
line at the 3-phase contact point (solid-liquid, liquid-air
and air-solid). The solid phase was paraffin. This surface
is a common wrap material used for a variety of purposes
in laboratories, and is often used as a standard model for
a waxy leaf surface, due to its high contact angle with
water of 120°. The contact angle for water on waxy leaf
surfaces varies by type from 90 to 150° in extreme cases,
but in general, the value of 120° is a median, making
paraffin very suitable as a model. The measurements were
carried out by the use of DSA 100 analyzer described
earlier (Figure 1).
Bis(2-hydroxyethyl)methyloctadecylammonium
(2,4-dichlorophenoxy)acetic (3b)
1H NMR (400 MHz, CD3OD) d 0.89 (t, 3H, J 6.7 Hz,
CH3), 1.28 (s, 30H, CH2), 1.78 (d, 2H, J 5.5 Hz, CH2),
3.18 (s, 3H, CH3), 3.44 (m, 2H, CH2), 3.55 (m, 4H, CH2),
3.98 (s, 4H, CH2), 4.48 (s, 2H, OH), 4.88 (s, 2H, CH3),
6.91 (d, 1H, J 9.8 Hz, Ar–H), 7.21 (m, 1H, Ar–H), 7.36
13
(d, 1H, J 9.7 Hz, Ar–H); C NMR (100 MHz, CD3OD)
d 14.6, 23.5, 23.8, 27.5, 30.3 (2), 30.4 (2), 30.5 (4), 30.6
(3), 30.7 (3), 33.1, 50.4, 56.8, 65.0, 65.1, 69.3, 115.9,
124.6, 126.5, 128.6, 130.7, 154.9, 175.1; anal. calcd. for
C31H55Cl2NO5: C, 62.82; H, 9.35; N, 2.36; found: C, 63.38;
H, 9.81; N, 2.89.
Thermal analysis
Thermaltransitionsofthepreparedsaltsweredetermined
by differential scanning calorimetry (DSC), with a Mettler
Toledo Star TGA/DSC1 unit, under nitrogen. Samples

1778
Synthesis, Surface Properties and Biological Activity of Long Chain Ammonium Herbicidal Ionic Liquids
J. Braz. Chem. Soc.
Statistical analysis
All the experiments were conducted in triplicate and all
error bars presented on Figures represent standard errors of
the mean (with respect to the number of replicates).
Results and Discussion
Prepared quaternary ammonium salts
The first experimental step was focused on the
preparation of alkylbis(2-hydroxyethyl)ammonium
bromides by conducting the Menschutkin reaction using
bis(2-hydroxyethyl)methylamine and an appropriate
quaternizing agent, which was 1-bromotetradecane in case of
salt 1, 1-bromohexadecane (2) and 1-bromooctadecane (3).
Quaternary ammonium bromides were solids with a purity
higher than 96% (surfactant content).
Figure 1. Drop of (a) the studied IL 1a and (b) water.
Afterwards, 6 new salts were obtained in neutralization
reaction using a quaternary bromide as the source of cation
and different herbicidal acids, such as MCPA and 2,4-D.
The acid-base reaction occurred immediately at room
temperature (25 °C) and allowed to obtain compounds with
yields between 90 and 95% (Table 1), which correspond
well with the yield of other alkylammonium herbicidal ionic
liquids.10 The prepared salts were characterized by 1H and
13C NMR and elemental analysis. The 1H NMR spectra of
synthesized salts indicated the presence of hydroxyl groups
(3.65 ppm). A characteristic signal from protons of alkyl
chain occurred in the range from 1.19 to 1.32 ppm. Every
compound presented more impurity than shown in Table 2,
since prepared salts were hygroscopic and easily absorbed
water from air. It can influence on the elemental analysis
results. Subsequent studies focused on investigation of the
solubility of the obtained compounds. All of the obtained
salts exhibited a melting temperature below 100 °C,
therefore they may be classified as ionic liquids.
Biological activity
The herbicidal efficacy of the obtained HILs was
tested using common lambsquarters (Chenopodium
album L.) and white mustard (Sinapis alba L.) as the test
plants. All HILs were dissolved in a mixture of water
and ethanol (1:1 v/v). Commercial products Chwastox
Extra 300 SL (300 g of sodium and potassium salts of
MCPA in 1 L) and Aminopielik Standard 600 SL (600 g
of dimethylammonium salts of 2,4-D in 1 L) dissolved
in water and ethanol (1:1 v/v) were used as reference
treatments. All herbicides were used at the sub-lethal dose
of 400 g active ingredient per 1 ha, for better evaluation
of the differences in the effectiveness of their action on
weeds.
Plant seeds were sown into plastic pots (0.5 L) and
kept in a greenhouse at a temperature of 20 °C, humidity
of 60% and photoperiod of 16/8 h day/night. The plants
were thinned to five per pot 10 days after emergence, and
watered as needed. The plants were treated by herbicides
at 4-6 leaf growth stage. The treatments were applied
using a moving sprayer (APORO) with TeeJet 110/02
flat-fan nozzles (TeeJet Technologies) delivering 200 L of
spray solution per 1 ha at 0.2 MPa operating pressure. The
distance from the nozzles to the tips of the plant was 40 cm.
Efficacy was evaluated using the fresh weight reduction
method (at 0.01 g accuracy) two weeks after treatment
(2 WAT). Data were expressed as percent of fresh weight
reduction compared with untreated plants. The reduction of
plant fresh weight was measured and compared to control
plants, which were not sprayed with herbicide.
Solubility analysis
The obtained ILs were stable in air and in contact
with water or common organic solvents. The solubility
of the obtained salts in selected solvents (such as water,
dimethylsulfoxide (DMSO), acetonitrile, acetone,
2-isopropanol, ethyl acetate, chloroform, toluene and
hexane) was determined according to Vogel and Furniss.24
All of the synthesized salts were soluble in methanol,
chloroform and poorly in DMF. With one exception (1a),
the synthesized salts were soluble in DMSO. Salts 1-3 and
1b-3b were soluble in 2-isopropanol. Limited solubility
in water was observed when using 2,4-D as an anion.

Vol. 27, No. 10, 2016
Giszter et al.
1779
None of the prepared salts were soluble in acetonitrile.
Thermal stability of the obtained ILs was studied in next
experimental step.
In the homologous series of compounds 1b-3b comprising
the 2,4-D anion, the surface tension was reduced to
34.4 mN m-1. After comparing the designated CMC and
γCMC values for obtained ILs with two different anions
(MCPA and 2,4-D) no significant differences between
these two groups were observed. Both demonstrated high
surface activity and lowered the surface tension of water
to a similar range. In comparison, the surface tension for
[Ethoquad C/12][2,4-D], which is a mixture of cations
with defined long alkyl substituent, ranged from 30.4 to
32.5 mN m-1.10,25
Thermal analysis
Thermal analysis (DSC and TGA) of selected ionic
liquids (1a, 2a, 1b, and 2b) is presented in Table 3.Analysis
of phase transitions in the examined temperature range
indicates glass transition temperature, temperature of
crystallization and melting point for analyzed ILs. They
were thermally stable between 225 and 230 °C and showed
glass transition temperature in the range of –37 to –5 °C.
Thermal stability values are in accordance with the Tonset
value obtained for [Ethoquad C/12][2,4-D], which was
equal to 222 °C.10 For all samples the melting point was
below 100 °C and 1b reached a value of melting point
approximately equal to room temperature (24 °C). After
analysis of thermal stability the obtained ILs were subjected
to investigation of surface activity.
Table 4. Surface activity parameters of synthesized Ils
b
ILs
1a
2a
3a
1b
2b
3b
CMCa / (mmol dm-1)
γ / (mN m-1)
CAc / degree
65.8
0.45
0.16
0.06
0.50
0.13
0.06
35.1
34.0
64.3
38.5
70.7
34.4
64.1
35.2
68.6
37.2
71.6
Table 3. Thermal analysis (DSC and TGA) of the synthesized Ils
b
aCMC: critical micelle concentration; γ: surface tension at CMC;
cCA: contact angle. ILs: ionic liquids.
ILs
1a
2a
1b
2b
Tga / °C
–37
Tcb / °C Tmc / °C
Tonset5%d / °C
225
Tonsete / °C
332
–6
0
36
40
24
The aqueous solutions of the studied ILs reduced
contact angle of water from approximately 120 to
64.3-70.7° for 4-chloro-2-methylphenoxyacetates, and
64.1-71.6° for 2,4-dichlorophenoxyacetates (Table 4).
According to literature reports, the contact angle value for
[Ethoquad C/12][2,4-D] ranged from 51.80 to 54.32°.10,25
Paraffin, which is strongly hydrophobic, was the solid
phase. The lowest contact angles on the hydrophobic
surface were exhibited by the liquids with the lowest surface
tensions. Good wettability of a solid material (paraffin),
which imitates the real surface of leaves, is useful for
employing the new compounds as herbicides in future
applications.
–28
230
350
–36
–1
6
225
272
–5
40
228
282
b
aTg: glass transition temperature; Tc: temperature of crystallization;
cTm: melting point; dTonset5%: decomposition temperature of 5% of sample;
eTonset: decomposition temperature of 50% of sample. ILs: ionic liquids.
Surface activity
Table 4 presents parameters of surface activity:
CMC, surface tension at CMC and contact angle (CA).
It is observed that the length of the alkyl chain was, as
expected, decisive for the CMC of aqueous solutions
of obtained ILs. The elongation of alkyl chain length
caused a decrease from 0.45 to 0.06 mmol dm-3 for 1a-3a
(MCPA anion), and 0.5 to 0.06 mmol dm-3 for 1b-3b
(2,4-D anion). This is due to enhanced hydrophobic
interaction between the counter ion and micelle. These
CMC values are comparable with typical ionic surface
active agents. Surface activity of the synthesized ILs can
also be evaluated by the surface tension (γCMC). Surface
tension of aqueous solutions of studied ILs decreased
from water value to a minimum located from 34 to
38.5 mN m-1, where it reached a plateau. In the group of
compounds 1a-3a with MCPA anion, the surface tension
was reduced to a minimum for 2a, reaching 34.0 mN m-1.
Herbicidal activity
The greenhouse experiments were carried out in order
to establish the biological activity of the synthesized ILs.
The activity of HILs 1a-3b was tested against common
lambsquarters and white mustard. The efficacy of the tested
herbicides was at a low level (18-60%), which resulted from
the application of sub-lethal doses. The data presented in
Figure 2 indicate that the synthesized ILs have herbicidal
activity and may be classified as HILs.
Results show that the chain length ranging from 14 to
18 carbon atoms in alkylbis(2-hydroxyethyl)ammonium
cation does not have a significant relevance to the herbicidal

1780
Synthesis, Surface Properties and Biological Activity of Long Chain Ammonium Herbicidal Ionic Liquids
J. Braz. Chem. Soc.
Figure 2. Fresh weight reduction as influenced by tested HILs and commercial products for (a) common lambsquarters, (b) white mustard.
activity of tested ILs towards weeds. The biological
activity of HILs depended mainly on plant species. For
example HIL 3a, which contained bis(2-hydroxyethyl)
methyloctadecylammonium (4-chloro-2-methylphenoxy)
acetate, was the most effective treatment against common
lambsquarters, but it was less active against white mustard
plants compared with other tested compounds. Among the
HILs containing the 2,4-D anion, the most effecive treatment
was observed for the compound with hexadecylbis(2-
hydroxyethyl)methylammonium cation used against white
mustard, but its efficacy against common lamsquarters was
similar to other tested compounds (Figure 3).
Conclusions
Commonly used herbicides can be modified by the
addition of a long chain ammonium cation. The acid-base
reaction occurred immediately at room temperature and
allowed to obtain compounds with yields between 90 and
95%. All the obtained salts exhibited melting points below
100 °C. New forms of MCPA and 2,4-D as HILs were stable
in air as well as in contact with water and organic solvents.
Length of the alkyl substituent in cation affected the surface
activity, thermal stability and solubility. All HILs were
thermally stable below 225 °C. Surface activity depended
on the length of substituent and on the type of herbicidal
anion. The surface tension was reduced to a minimum for
HILs 2a and 1b, which confirms the influence of the anion
on surface activity of HILs. Good wettability of a solid
material (paraffin), which imitates the surface of leaves, may
be useful for employing the new compounds as herbicides
in future applications. The highest efficacy of fresh weight
reduction in the comparison to the commercial formulation
was observed for HILs 2a against common lambsquaters
and 3b against white mustard. Their herbicidal activity has
been preserved and it was observed that the chain length
of carbon atoms in alkylbis(2-hydroxyethyl)ammonium
cation had a relevant influence on the performance of the
tested ILs towards weeds.
In most cases, new forms of MCPA and 2,4-D showed
similar effectiveness as reference products. However, other
advantages such as low volatility are important from the
point of view of operator’s safety aerial drift of herbicide
molecules to the not-target plants. Therefore, HILs may
be an interesting alternative to commercial pesticides.
HILs are a group of ionic liquids, which is still developing
Figure 3. Control of common lambsquarters by tested treatments: (a)
MCPA, and (b) 2,4-D.

Vol. 27, No. 10, 2016
Giszter et al.
1781
and may be an alternative to the cultivation of genetically
modified plants.
12. Grossmann, K. In Herbicides and Their Mechanisms of Action;
Cobb, A. H.; Kirkwood, R. C., eds.; Sheffield Academic Press:
Sheffield, 2000, p. 181.
Supplementary Information
13. Pernak, J.; Niemczak, M.; Materna, K.; Marcinkowska, K.;
Praczyk, T.; Tetrahedron 2013, 69, 4665.
Supplementary information is available free of charge
at http://jbcs.sbq.org.br as PDF file.
14. Pernak, J.; Syguda, A.; Materna, K.; Janus, E.; Kardasz, P.;
Praczyk, T.; Tetrahedron 2012, 68, 4267.
15. Praczyk, T.; Kardasz, P.; Jakubiak, E.; Syguda,A.; Materna, K.;
Pernak, J.; Weed Sci. 2012, 60, 189.
Acknowledgments
16. Cojocaru, O. A.; Shamshina, J. L.; Gurau, G.; Syguda, A.;
Praczyk, T.; Pernak, J.; Rogers, R. D.; Green Chem. 2013, 15,
2110.
This work supported by grant No. PBS2/A1/9/2013.
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Submitted: December 18, 2015
Published online: February 18, 2016