Alkyl(C16, C18, C22)trimethylammonium-Based Herbicidal Ionic Liquids

Article abstract of DOI:10.1021/acs.jafc.6b04528

In the framework of this study a synthesis methodology and characterization of long alkyl herbicidal ionic liquids (HILs) based on four commonly used herbicides (2,4-D, MCPA, MCPP, and dicamba) are presented. New HILs were obtained with high efficiency (>95%) using an acid-base reaction between herbicidal acids and hexadecyltrimethylammonium, octadecyltrimethylammonium, and behenyltrimethylammonium hydroxides in alcoholic medium. Among all synthesized salts, only three compounds comprising the MCPP anion were liquids at room temperature. Subsequently, the influence of both the alkyl chain length and the anion structure on their physicochemical properties (thermal decomposition profiles, solubility in 10 representative solvents, surface activity, density, viscosity, and refractive index) was determined. All HILs exhibited high thermal stability as well as surface activity; however, their solubility notably depended on both the length of the carbon chain and the structure of the anion. The herbicidal efficacy of the obtained salts was tested in greenhouse and field experiments. Greenhouse testing performed on common lambsquarters (Chenopodium album L.) and flixweed (Descurainia sophia L.) as test plants indicated that HILs were characterized by similar or higher efficacy compared to commercial herbicides. The results of field trials confirmed the high activity of HILs, particularly those containing phenoxyacids as anions (MCPA, 2,4-D, and MCPP).

Full text of DOI:10.1021/acs.jafc.6b04528

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Article
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ALKYL(C,C,C)TRIMETHYLAMMONIUM BASED HERBICIDAL IONIC LIQUIDS
Juliusz Pernak, Rafa# Giszter, Agnieszka Biedziak, Micha# Niemczak,
Rados#aw Olszewski, Katarzyna Marcinkowska, and Tadeusz Praczyk
J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.6b04528 • Publication Date (Web): 20 Dec 2016
Downloaded from http://pubs.acs.org on December 21, 2016
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Journal of Agricultural and Food Chemistry is published by the American Chemical
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Graphic abstract
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Structures of used herbicidal acids.
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Viscosity and density values for 1d and 2d.
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Surface tension of obtained salts.
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Efficacy of HILs and reference products against two weed species in spring barley (Winna Gora – western
part of Poland).
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ALKYL(C16,C18,C22)TRIMETHYLAMMONIUM
BASED HERBICIDAL IONIC LIQUIDS
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Juliusz Pernak,†,* Rafał Giszter, † Agnieszka Biedziak, † Michał Niemczak,†
Radosław Olszewski,‡ Katarzyna Marcinkowska, § Tadeusz Praczyk, §
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† Department of Chemical Technology, Poznan University of Technology, Poznan 60-965,
Poland
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‡ ADOB Sp. z o.o. Sp.k., 61-070 Poznan, Poland
§ Institute of Plant Protection – National Research Institute, Poznan 60-318, Poland
Corresponding Author
Juliusz Pernak, Tel: 00148-61-6653682. E-mail: juliusz.pernak@put.poznan.pl
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ABSTRACT. In the framework of this study a synthesis methodology and characterization of
long alkyl herbicidal ionic liquids (HILs) based on four commonly used herbicides (2,4-D,
MCPA, MCPP and dicamba) was presented. New HILs were obtained with high efficiency
(>95%) using an acid-base reaction between herbicidal acids and hexadecyltrimetylamonium,
octadecyltrimetylamonium and behenyltrimethylammonium hydroxides in alcoholic medium.
Among all synthesized salts, only three compounds comprising the MCPP anion were liquids
at room temperature. Subsequently, the influence of both the alkyl chain length and the anion
structure on their physicochemical properties (thermal decomposition profiles, solubility in 10
representative solvents surface activity, density, viscosity and refractive index,) was
determined. All HILs exhibited high thermal stability as well as surface activity, however,
their solubility notably depended on both the length of the carbon chain and the structure of
the anion. The herbicidal efficacy of the obtained salts was tested in greenhouse and field
experiments. Greenhouse testing performed on common lambsquarters (Chenopodium album
L.) and Flixweed (Descurainia sophia L.) as test plants indicated that HILs were characterized
by similar or higher efficacy compared to commercial herbicides. The results of field trials
confirmed the high activity of HILs, particularly those containing phenoxyacids as anions
(MCPA, 2,4-D and MCPP).
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KEYWORDS. Phenoxyacids, Growth regulators, Herbicidal Ionic Liquids, efficacy, long
alkyl.
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INTRODUCTION
Within the last several years, ionic liquids (ILs) have evolved from their application as
solvents in synthesis towards new directions.1 As a result, the third and most recent
generation of ILs was created, which is focused on the production of ILs with biological
activity, mainly in the field of pharmaceuticals.2-5 Several studies confirm that active
pharmaceutical ingredients in the form of ILs can provide new and unique properties
compared to the solid pharmaceutical forms, with the possibility of improved performance,
such as controlled solubility and drug delivery.6-9 Moreover, dual-functional ILs composed of
two active ions may even lead to synergistic effects.6,10 Agrochemistry is another field where
the combination of biological activity and the IL form contributes to many outstanding
operational benefits. The novel fungicides with thiabendazole, imazalil, tebuconazole and
propiconazole as ILs, with increased biological activity against many common plant
pathogens, have already been described.11,12 Moreover, ILs proved to be effective plant
resistance inducers, which allow to stimulate the resistance system of the plant to act against
pathogenic microorganisms.13,14
The insertion of the herbicidal anion into the structure of ionic liquids led to the
development of a novel group of compounds defined as herbicidal ionic liquids (HILs).15
This new group of ILs, described for the first time in 2011, may be applied as anti-weed
agents in crop protection. HILs belong to the third generation of ILs due to the fact that they
simultaneously exhibit targeted biological activity combined directly with selected physical
and chemical properties.2 There are several reports describing HILs which incorporate
commonly used herbicides, such as: 2,4-D,16,17 MCPA,18-20 MCPP,15,20 MCPB,21 dicamba,22
fomesafen,23 glyphosate,24 clopyralid,25 metsulfuron-methyl,26 and bentazone.27 The
combination of herbicidal activity of the anion with a different biological property of the
cation (e.g., plant growth inhibition) leads to dual-function HILs.28 HILs can be also used in
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the protection of crops against pathogenic fungi – transformation of fungicides, such as
tebuconazole and propiconazole, results in efficient fungicidal ILs with improved
physicochemical properties.29
The currently available herbicidal formulations contain active ingredients mainly in the
forms of sodium, potassium or ammonium salts and alkyl esters. Disadvantages of these
forms are high water solubility and high vapor pressure. HILs are one of the recent
discoveries in plant protection, which may bring a lot of advantages and solve many
problems associated with the present agricultural practices. Due to their enhanced efficiency
the required dose of active ingredient per hectare could be significantly reduced.16,19,23 HILs
could also reduce toxicity, hence a toxic herbicidal acid combined with an appropriate cation
may form a nontoxic herbicide.15 The negative influence of toxic vapors can be reduced by
the extraordinary low vapor pressure of HILs – the ionic form of the pesticide does not spread
to the neighboring fields and the potentially negative impact on the environment is
significantly reduced.22
(Long alkyl)trimethylammonium chlorides and bromides are popular cationic surfactants
that are commonly used mainly in the cosmetic industry.30-32 The most representative
compounds from this group possess C16, C18 and C22 straight alkyl substituents attached to the
nitrogen atom.33-36 Low costs as well as good availability of hexadecyltrimethylammonium
chloride (CAS 112-02-7), octadecyltrimethylammonium chloride (CAS 112-03-8) and
behenyltrimethylammonium chloride (CAS 17301-53-0) are the results of a positive opinion
of European Food Safety Authority (EFSA) and U. S. Food and Drug Administration (FDA),
which allowed their use in cosmetic products.37,38 Furthermore, the maximum authorized
concentration of these ammonium salts should not exceed 0.1% when applied as
preservatives or be in range from 0.5 to 3.0% for other, non-preservative functions. However,
the possible occurrence of skin irritating issues limits their use at higher concentrations in
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dermal care products.37 Nevertheless, due to their unique, multifunctional properties,
alkyl(C16,C18,C22)trimetylamonium halides have also been successfully applied as
intercalating agents for layered silicates and their nanocomposites,39-42 drag reducing agents
for anionic polymer solutions,43 emulsion viscosity stabilizers44 and basic hydrolysis catalysts
via cationic mixed micelles.45
This study describes a convenient synthesis methodology of biologically active ILs
possessing a long-alkyl-chain quaternary ammonium cation. Furthermore, HILs comprising
an alkyl substituent longer than C18 in the cation – a behenyl group (C22 alkyl) was presented
for the first time and their properties were compared with their homologues -
alkyltrimethylammonium HILs with a shorter carbon chain (C16 and C18 alkyl). The source of
anion were phenoxyacids (2,4-D, MCPA and MCPP) as well as dicamba, which are systemic,
selective herbicides belonging to the group of growth regulators. The high, unabated
popularity of these compounds is due to the fact that the risk of formation of biotypes
resistant to them is relatively low compared to other classes of herbicides (e.g.,
sulfonylureas). This study involves the evaluation of the influence of length of alkyl chain on
the physicochemical properties (i.e. thermal stability, solubility, surface activity) as well as
results of herbicidal efficiency determined in greenhouse and field experiments.
MATERIALS AND METHODS
Materials
Quaternary ammonium chlorides with long alkyl groups were purchased from Stockmeier
and Brenntag. Herbicidal acids (2,4 D, MCPA, MCPP and dicamba) were supplied by
Organika-Sarzyna S.A. Poland and PESTINOVA Poland. All solvents (methanol, DMSO,
acetonitrile, acetone, isopropanol, ethyl acetate, chloroform, toluene, hexane) and KOH were
purchased from Aldrich (European market, Poznan, Poland) and used without further
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purification. Water for solubility and surface activity measurements was deionized with the
conductivity below 0.1 mS cm-1 from demineralizer HLP Smart 1000 (Hydrolab).
Synthesis of alkyltrimethylammonium salts
After evaporation of solvent from an aqueous solution of appropriate precursor, each
alkyltrimethylammonium chloride (0.1 mol) was dissolved in 50 cm3 of anhydrous methanol,
and then mixed with solution of 0.1 mol of potassium hydroxide dissolved in 25 cm3 of
methanol. The reaction was conducted for 10 minutes and the precipitated inorganic by-
product was removed by filtration. Next, the filtrate containing alkyltrimethylammonium
hydroxide was neutralized with the selected herbicidal acid. All neutralization reactions were
conducted in a Mettler Toledo semi-automated reactor system - EasyMax™. In order to
prevent the degradation of intermediate products, the temperature of the reactor contents was
thermostatted at 25 °C. The connected SevenMulti Mettler Toledo instrument equipped with
a calibrated glass electrode allowed for precise measurement of the decrease in pH of the
mixture as a result of proceeding reaction. Then, the solvent was removed under reduced
pressure and the crude product was obtained. Afterwards, the residue was dissolved in 50 cm3
of acetone and the inorganic salts (by-products) were filtered off. After the evaporation of
acetone, the compound was dried under vacuum for 24 hours at the temperature 65 °C. NMR
spectra descriptions, elemental analyses for all compounds are provided in the ESI.
General
The structures of obtained salts were confirmed by the analysis of the nuclear magnetic
13
resonance spectrum (1H and C). 1H NMR spectra were recorded on a Mercury Gemini 300
and Varian VNMR-S 400 MHz spectrometers operating at 300 MHz and 400 MHz with TMS
as the internal standard. 13C NMR spectra were obtained with the same instruments at 75 and
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100 MHz, respectively. CHN elemental analyses were performed at A. Mickiewicz
University, Poznan (Poland). The water content was determined by using an Aquastar
volumetric Karl-Fischer titration with Composite 5 solution as the titrant and anhydrous
methanol as a solvent.
Density measurements
Density was determined using an Automatic Density Meter DDM2911 with a mechanical
oscillator method. The density of the samples (approx. 2.0 cm3) were measured with respect
to temperature controlled conditions via Peltier, from 20 to 80 °C. The apparatus used for
density measurements was calibrated using deionized water as the reference substance. After
each series of measurements, the densimeter was washed by two kinds of solvents (methanol
and acetone) and dried.
Viscosity measurements
Viscosity was determined using a rheometer (Rheotec RC30-CPS) with cone-shaped
geometry (C50-2). The viscosity of the samples (approx. 1.5 cm3) was measured with respect
to temperature, from 20 to 80 °C. The uncertainty of the viscosity measurement was
estimated to be less than 10-4 Pa∙s.
Refractive index measurements
Refractive index was determined using Automatic Refractometer J357 with electronic
temperature control in range from 20 to 80 °C. The accuracy of the temperature stabilization
was 0.02 °C and uncertainty of the refractive index measurement was less than 0.00005.
The cationic active substance content
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The cationic active substance content was assayed by a direct two-phase titration according
to EN ISO 2871-1:2010. The method is based on the titration of the ammonium salt in the
water-chloroform biphasic system (or water-methanol-chloroform in the case of compounds
with limited solubility in water) by a standard solution of sodium dodecylsulphate(VI) in the
presence of the mixed indicator (dimidium bromide indicator (CAS: 518-67-2) is used for
determination of cationic active substances, and sulfan blue indicator (CAS: 129-17-9) is
employed for determination of anionic active substances.
Thermal analysis
Differential scanning calorimetry (DSC) was performed on a Mettler Toledo Stare
TGA/DSC1 unit (Leicester, UK) under nitrogen. Samples between 5 and 15 mg were placed
in aluminum pans and were heated from 25 to 160 °C at a heating rate of 10 °C∙min-1 and
cooled at a cooling rate of 10 °C∙min-1 to -100 °C. Thermogravimetric analyses were
performed on a Mettler Toledo Stare TGA/DSC1 unit (Leicester, UK) under nitrogen.
Samples between 2 and 10 mg were placed in aluminum pans and were heated from 30 to
500 °C with a heating rate of 10 °C∙min-1.
Solubilities
The solubilities of the prepared salts were determined according to the protocols in Vogel’s
Textbook of Practical Organic Chemistry.46 Ten representative protic and aprotic solvents
with a diverse polarity range were selected for analyses and arranged in order of decreasing
polarity expressed as the Snyder polarity index: water, 9.0; methanol, 6.6; DMSO, 6.5;
acetonitrile, 6.2; acetone, 5.1; isopropanol, 4.3; ethyl acetate, 4.3; chloroform, 4.1; toluene,
2.3; hexane, 0.0. A 0.1 g sample of each salt was added to a certain volume of the solvent and
the samples were thermostatted in Water Bath MEMMERT Model WNB 7 at 25 °C. Based
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on the volume of solvent used, 3 types of behaviors were recorded: ‘soluble’ applies to
compounds which dissolved in 1 cm3 of solvent, ‘limited solubility’ – applies to compounds
that dissolved in 3 cm3 of solvent, and ‘not soluble’ – applies to the compounds which did not
dissolve in 3 cm3 of solvent.
Surface activity
Surface tension measurements were carried out by the use of a DSA 100 analyzer (Krüss,
Germany, accuracy 0.01 mN m-1), at 25 °C. Temperature was controlled using a Fisherbrand
FBH604 thermostatic bath (Fisher, Germany, accuracy 0.1 °C). The surface tension was
determined using the shape drop method. The principle of this method is to form an
axisymmetric drop at the tip of a needle of a syringe. The image of the drop is taken from a
CCD camera and digitized. The surface tension (in mN m-1) is calculated by analyzing the
profile of the drop according to the Laplace equation. 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 (γCMC vs logC curves) using a linear regression analysis method. The basis
for the determination of the contact angle is the image of the drop on the examined surface
(paraffin). After determination of the 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). Complete drop
contour is evaluated. After successful fitting of the Young–Laplace equation, the contact
angle is determined as the slope of the contour line at the 3-phase contact point (solid–liquid
and liquid–air). The measurements were carried out by the use of DSA 100 analyzer, Krüss.
Greenhouse experiment
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The test were conducted using two species of weeds: common lambsquarters
(Chenopodium album L.) and flixweed [Descurainia Sophia (L.) Webb ex Prantl]. The seeds
were sown into plastic pots filled with commercial peat-based potting material. All ILs were
dissolved in a mixture of water and ethanol (1:1 v/v) in an amount corresponding the dose of
400 g MCPA, MCPP, 2,4-D per 1 ha or 200 g dicamba per 1 ha. The commercial herbicides
Chwastox Extra 300 SL (300 g of sodium and potassium salts of MCPA in 1 dm3),
Aminopielik Standard 600 SL (600 g dimethylammonium salts of 2,4 D in 1 dm3) and
Dikamba 480 SL (480 g dicamba in 1 dm3) were dissolved in water at the same doses of
active ingredient as in the case adequate ILs. The treatments were applied at 4-6 leaf stage
using a moving sprayer occupied with Tee Jet 110/02 flat-fan nozzles delivering 200 dm3 of
spray solution per 1 ha at 0.2 MPa pressure. The plants were placed in a greenhouse at the
temperature of 20 °C, humidity of 60% and photoperiod (day/night hours) of 16/8. The study
was carried out in four replications in a completely randomized setup. After 2 weeks, the
plants were cut to soil level and weighed (at 0.01 g accuracy). The reduction of plant fresh
weight was determined in comparison to corresponding control: commercial herbicides
treatments to the water treatment (control 1); ILs treatments – to the water:etanol treatment
(control 2).
The linear models with only treatment were used as a fixed effect to analyze the influence of
treatments on fresh weight of both investigated plants. In the next step the assumptions
through inspection of diagnostic plots were checked for deviations from homoscedasticity
and normality of residuals.47,48 The post hoc pairwise comparisons were performed with
Tukey’s adjustment on all possible combinations of treatments. All analyses were performed
in R 3.3.2 (R Core Team 2012) with lsmeans 2.25 package.49,50
Field experiment
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The field experiment was performed in 2015 at the Experimental Station of the Institute of
Plant Protection in Winna Gora (Poland) in spring barley. The individual plot size was 16 m2.
All treatments were applied using a small plot spraying equipment with AXR 110/03 flat-fan
nozzles (TeeJet Technologies, Wheaton, IL, USA) delivering 200 dm3 ha-1 of spray solution
at 0.3 MPa of operating pressure. The synthesized ILs were applied at the same dose as in
greenhouse experiments. Weed control was evaluated visually using a scale of 0 (no effect) to
100% (complete weed destruction). The target weeds were common lambsquarters
(Chenopodium album L.) and field pennycress (Thlaspi arvense L.). The study was carried
out in 4 replications in a randomized blocks setup. The study was carried out in four
replications in a randomized setup. Each error margin range represents standard errors of the
mean (SEM). The SEM values were calculated according to equation 1:
Equation 1. Calculation of SEM values.
ꢃ
ꢀꢁꢂ =
ꢄ0.5
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where:
SEM – standard error of the mean,
s – sample standard deviation,
n – number of samples.
RESULTS AND DISCUSSION
Synthesis and characterization of ILs
All salts presented in Table 1 were synthesized via the neutralization reaction between
alkyltrimethylammonium hydroxides and different herbicidal acids, such as (4-chloro-2-
methylphenoxy)acetic acid (MCPA), (2,4-dichlorophenoxy)acetic acid (2,4-D), 3,6-dichloro-
2-methoxybenzoic (dicamba) and ( )2-(4-chloro-2-methylphenoxy)propionic acid (mecoprop
- MCPP). Structures of the used herbicides are presented in Figure 1.
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Figure 1. here.
Obtained compounds have been divided into four groups: group a refers to salt with MCPA
anion, b to salts with 2,4-D anion, c to salts with dicamba anion and d to salts with MCPP
anion. The acid-base reaction (Scheme 1) was carried out at 25°C in semi-automatic reactor
EasyMax™ equipped with a glass electrode, where a decrease in pH has been controlled
precisely.
Scheme 1. here.
Efficient neutralization and purification of obtained products allowed to achieve high yields
between 90 and 99% (Table 1). The purity of 1-3 was determined via the direct two-phase
titration technique. According to this method, the synthesized salts were characterized by
high purities exceeding 98%. The selected syntheses methodology proved to be highly
effective and repeatedly faster than the conventional anion exchange reaction performed in an
aqueous solution. Therefore, the difficulties associated with the long time of separation of
phases via two phase extraction (caused by the presence of highly surface-active cation) as
well as foam formation during evaporation of the solvent have been easily eliminated. It was
noted that the increase in the length of the alkyl chain did not deteriorate the reaction yields
or the purity of the obtained products.
Table 1. here.
Structures of the obtained new salts were confirmed by H and 13C NMR spectroscopy.
1
NMR spectra descriptions as well as elemental analysis results are provided in the Supporting
Information. The water content of 1-3 measured via Karl-Fischer titration was below 0.5%.
Moreover, the synthesized salts were stable in air as well as in contact with water and
common organic solvents during all the performed analyses. They could be made anhydrous
by heating at 70 °C in vacuum and storing them over P4O10.
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All salts with MCPA (1a-3a), 2,4-D (1b-3b) and dicamba (1c-3c) anion were solids at
room temperature. In case of these three groups, the length of alkyl chain did not significantly
affect their state at 25 °C. Furthermore, only salts comprising the MCPP anion (1d-3d) were
liquids at 25 °C, which demonstrates that additional methyl groups in the molecule of
herbicide caused a considerable reduction of melting points. Moreover, concerning the low
melting points (< 25 °C) of 1d, 2d and 3d, they may be described as room temperature ionic
liquids (RTILs).
Basic physicochemical properties
Basic physicochemical properties, such as viscosity, density and refractive index were
determined for 1d and 2d in range from 20 to 80 °C and presented in Fig. 2 (the collected
physicochemical data are provided in the Supporting Information in Table S1).
Figure 2. here.
At 20 °C the values of viscosity for 1d and 2d notably increased with the increase of the
alkyl chain length. Therefore, the measured values for RTILs with hexadecyl (1d) and
octadecyl (2d) group were equal to 10.898 Pa∙s and 6.535 Pa∙s, respectively. Further
elongation of the alkyl chain caused a subsequent, rapid increase of this parameter. As a
result, the IL with behenyl (3d) substituent was a grease at room temperature. Generally, the
obtained data is consistent with several reports which established that the viscosity of IL
would increase as the size of the non-polar part in the cation becomes larger.51,52 It was also
noted that the viscosity of both RTILs is comparable with values described in other reports
regarding HILs.18,20,21 As expected, the viscosity of the synthesized salts decreased
nonlinearly with increasing temperature. Hence, an increase in the temperature from 20 to 80
°C caused a rapid reduction of the analyzed parameter, which ultimately reached 0.091 for
(1d) and 0.103 Pa∙s for (2d). Fig. 2 shows that the density of the RTIL with hexadecyl group
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(1d) was higher compared to the density of pure water, however, the elongation of alkyl
chain caused a reduction of this parameter for 2d. As a result, the values at 20 °C collected
were equal to 1.065 g∙cm-3 for 1d and 0.990 g∙cm-3 for 2d, respectively. The observed
tendency was similar to other previously described HILs.18,21 Moreover, the density values of
the obtained ILs were temperature-dependent and decreased linearly with the increase in
temperature. Hence, a temperature increase from 20 to 80 °C resulted in the reduction of this
parameter by approx. 0.04 g∙cm-3 for both analyzed salts. The linear decrease of density was
expressed as the equation y = a·x + b (R2>0.99), where a was equal to approx. -0.0007 while
b was at 1.08 g∙cm-3 for 1d and 1.00 g∙cm-3 for salt 2d.
The measured refractive index values for RTILs with the MCPP anion at 20 °C were
similar and amounted to 1.503 for 1d and 1.493 for 2d. All collected data are presented in
Supporting Information (Table S1). The obtained results indicated that the elongation of the
alkyl substituent from C16 to C18 caused a decrease of refractive index values. Moreover,
heating both samples caused a slight, linear decrease of this parameter. This phenomenon was
already observed and described in literature for many HILs.18,21 At 80 °C the refractive
indices were approx. 0.006 lower and reached values equal to 1.497 for 1d and 1.486 for 2d,
respectively. The linear regression equations for both salts were characterized by high
correlation coefficients (R2 > 0.99), where a was equal to approx. -0.0001 while b amounted
to 1.504 for 1d and 1.495 for 2d.
Thermal Properties of synthesized ILs
Thermal transitions and decomposition temperatures are presented in Table 2. All the
obtained ILs with the MCPP anion (1d-3d) were liquids even at very low temperatures
reaching -100 °C. Therefore neither melting nor crystallization events were observed for
them in the analysed temperature range. The lowest melting point was noted for 2b (Tm = 55
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°C) and the highest for 1c (Tm = 55 °C), which additionally possessed a crystallization event
on cooling at 117.0 °C.
Table 2. here.
The obtained Tm values did not allow to determine the effect of the alkyl chain length on
this parameter, however, the influence of the structure of the anion could be established. The
ability of the anion to reduce the melting point of the synthesized salts increases in the
following order:
dicamba > 2,4-D ≈ MCPA > MCPP
According to data presented in Table 2, all the prepared salts with the sole exception of 1c
could be classified as ILs. Furthermore, the crystallization temperatures (Tcryst) could not be
determined for the majority of the obtained ILs. In consequence, 1a-1c, 1b-3b, 2c and 3c
exhibited a supercooling behavior and a melting transition was observed on the first heating
but it could not be reproduced within several heating/cooling cycles. Similar findings were
reported for other tetraalkyl ammonium HILs with phenoxyacids as anions.17,22,26 The glass
transition temperatures (Tg) were not determined in the analyzed temperature range for all
synthesized ILs. The measured Tonset5% values occurred at a range from 181 °C for 1c to 199
°C for 3b. ILs comprising the behenyl (C22) substituent (3a-d) exhibited a higher thermal
stability, since their Tonset5% values were higher by 2 to 5 °C compared to ILs with the
hexadecyl (C16) group (1a-d). These differences mean that enhancement of the thermal
stability slightly depends on the elongation of the alkyl chain. Therefore, values collected for
ILs with the octadecyltrimethylammonium cation (2a-d) were similar and varied from 186 to
197 °C. The influence of the anion on the decrease of Tonset5% may be described by the
following order:
2,4-D > MCPA > MCPP > dicamba
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The phenoxy acids (2,4-D, MCPA and MCPP) increased the thermal stability of the obtained
salts compared to dicamba. This observation corresponds well with the previous reports
regarding other HILs.19,20 The temperature values which caused a 50% decomposition of the
sample ranged from 214 °C for 1a to 232 °C for 3a. Similarly as in case of the Tonset5%
parameter, the lowest values were generally observed for salts comprising the hexadecyl
group (1a-d), whereas the highest were observed for ILs with the behenyl substituent (3a-d).
The extension of the alkyl chain by six carbon atoms resulted in a slight increase of the Tonset
parameter form approx. 6 °C for MCPP-based ILs (1d-3d) to approx. 18 °C for 2,4-D-based
ILs (1a-3a). Although the influence of the anion structure on the Tonset values was small, the
increase of thermal stability caused by the alkyl chain elongation is consistent with data
regarding homologous series of HILs.18,21
Solubilities
The solubility of obtained salts (Table 3) was determined in selected solvents, such as
water, DMSO, acetonitrile, acetone, 2-isopropanol, ethyl acetate, chloroform, toluene and
hexane, according to Vogel's Textbook of Practical Organic Chemistry.46
Table 3. here.
Due to the presence of an ionic bond and apolar long alkyl chain, the ILs were soluble in
some polar protic solvents as well as aprotic solvents. Thus, all tested compounds were
soluble in methanol and chloroform. Moreover, the majority of them were poorly soluble in
solvents characterized by the highest polarity - water and DMSO. Interestingly, the
elongation of alkyl chain caused a deterioration of ILs solubility. Therefore, ILs with the
hexadecyl substituent (1a, 1b and 1d) exhibited good affinity for both solvents, in contrast to
ILs with behenyl group (3a-3d).
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It was also noted that the dicamba anion notably decreased the solubility in both water and
DMSO in comparison to other anions (MCPA, 2,4-D and MCPP), which represent
phenoxyacids derivatives. Only 1d was soluble in acetonitrile, whereas slightly less polar
acetone dissolved all ILs with the MCPA anion (1a-3a) as well as two ILs (1d and 2d) with
MCPP as the anion. Isopropanol, similarly as methanol, dissolved all salts except 1a and 1c,
which comprised the shortest alkyl substituent. Hence, the chain extension may improve the
affinity for this solvent. In case of nonpolar solvents, all tested compounds with 2,4-D (1b-
3b) and dicamba (1c-3c) anions were insoluble in toluene and hexane. The substitution of the
aromatic ring with a methyl group in MCPA-based salts (1a-3a) and MCPP-based salts (1d-
3d) may improve the affinity for these solvents. Among all selected solvents, only ethyl
acetate and hexane did not dissolve all tested salts.
Surface properties
The surface activity parameters of ILs: critical micelle concentration (CMC), surface
tension at CMC (γCMC), efficiency of surface adsorption on an air-water interface (pC20),
maximum surface excess concentration (Γmax), surface area occupied by salt molecules (Amin
and contact angle (CA) were determined and presented in Table 4.
Table 4. here.
)
The relations between the surface tension values of aqueous solutions of the synthesized
salts and the log of concentration were plotted in Fig. 3.
Figure 3. here.
It can be observed that the surface tension of the aqueous solutions of ILs showed a
progressive decrease with increasing concentration and remained constant above the CMC.
The CMC value is a break point of surface tension curve and the plateau zone reflects the
formation of micelles. Based on these plots, the surface active parameters were calculated for
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the obtained salts and the corresponding chlorides. Moreover, the results obtained for halides
were compared with the literature data. The measured CMC values for
hexadecyltrimethylammonium chloride and octadecyltrimethylammonium chloride were
1.412 and 0.338 mmol·dm-3, respectively. Similar values were presented by Cepeda et al. and
were at 1.6 mmol·dm-3 for hexadecyltrimethylammonium chloride and 0.3 mmol·dm-3 for
octadecyltrimethylammonium chloride (obtained by conductivity method).49
As the derivatives of cationic surfactants – all the obtained salts were characterized by good
surface-active and wetting properties. A decrease of the CMC value was observed with the
increase in the alkyl chain length. For example, in case of ILs with the MCPA anion the
increase of the alkyl chain length resulted in the decrease of the measured CMC value from
0.459 mmol·dm-3 for 1a to 0.043 mmol·dm-3 for 3a. An interesting aspect of obtained results
in the present study, which should be remarked, was the lowest values of CMC in case of ILs
containing the longest alkyl chain (3a-3d). However these ILs were characterized by
significantly higher values of the contact angle than the other tested ILs. The contact angle
value ranged from 77.5° for 3a to 96.7° for 3b. Among all the synthesized salts, the lowest
observed value of contact angle was equal to 55.7° for 2a. Generally, the results obtained for
salts 1a-1d and 2a-2d are similar to the results obtained for HILs comprising C16 and C18
alkyl groups.18,21
Moreover, the area per molecule values (Amin) presented in Table 4 were lowest for ILs
comprising the C22 substituent (3a-3d). The obtained data indicate that alkyl chain elongation
facilitates a more tightly packed arrangement of the synthesized ILs at the water–air interface.
Analysis of the obtained results indicates that the influence of the structure of the anion on
the surface-active properties of ILs is notable only in case of salts belonging to group 1. In
this case the CMC was decreased in the following order:
MCPA > dicamba > MCPP > 2,4-D
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Comparison of surface activity of salt 1a (with the MCPA anion) and salt 1b (with the 2,4-D
anion) suggests that the CMC value was decreased by approx. 4 times. However, the
influence of the anion is notably lower in case of ILs comprising shorter alkyl groups (2a-2d
and 3a-3d), since the obtained results are similar.
Herbicidal Activity
The herbicidal effect of 1-3 on two weed species was tested both under greenhouse and
field conditions. The results of greenhouse experiments are shown in Tables 5-7. ILs with
MCPA (1a-3a), MCPP (1d-3d), 2,4-D (1b-3b) and dicamba (1c-3c) anions showed varying
effectiveness.
Table 5. here.
The efficacy against common lambsquarter ranged from 38% to 82%. Flixweed
demonstrated a greater sensitivity to the tested compounds (69-82%). In most cases the tested
ILs were more active compared to the reference herbicide. The herbicidal effect was lower in
case of ILs containing the 2,4-D anion, especially in case of common lambsquarters which
(efficacy at 33-42%).
In case of salts 1c-3c with the dicamba anion the obtained results indicated that their
herbicidal efficacy was similar to the reference herbicide was similar (in case of flixweed) or
notably higher. The obtained results suggest that the synthesized ILs display a similar or
better efficacy compared to commercial herbicidal products. It is worth noticing that the
length of the aliphatic alkyl chain in the cation did notably influence the herbicidal activity of
ILs in the tested range of C16 to C22.
Visual inspection of diagnostic plots did not reveal any obvious deviations from
homoskedascity or normality in the case of both plant species. Both models were statistically
significant (F(16,34) = 4.671, p < 0.001 in the case of common lambsquarters and F(16,34) =
8.272, p < 0.001 in the case of significant flixweed), and presented moderately high values of
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adjusted R squared – 0.5402 and 0.6994 (for common lambsquarters and flixweed
respectively). Significant differences (p < 0.05) between treatments pairs are provided in
Table 6 for common lambsquarters and Table 7 for flixweed.
Table 6. and 7. here.
The synthesized ILs may be classified as ammonium HILs with surface activity, which is
associated with the presence of the long alkyl group in the cation (with 16, 18 or 22 carbon
atoms). No significant correlations between the surface activity (reflected by the pC20
parameter given in Table 4) and the herbicidal activity could be established, in contrast to the
previously reported homologues of HILs comprising shorter alkyl substituents.18,21 The
general values of the pC20 parameter were at 3.9-4.0 (Table 4) contributed to an enhancement
of the herbicidal activity in comparison to the commercial herbicide. This is also confirmed
by literature data,18,21 which suggest that an increase of the efficacy of HILs occurred at pC20
values > 3.0. An additional advantage of the employed cations is associated with the fact, that
they
can
be
obtained
from
cheap
and
commercially
available
alkyl(C16,C18,C22)trimethylammonium chlorides and bromides. Furthermore, most of the
obtained HILs (with the exception of 1a, 1b and 2d) displayed hydrophobic properties, which
may reduce their mobility in the terrestrial environment and minimize the risk of groundwater
contamination. Since positive results were obtained under greenhouse conditions, additional
field studies were carried out.
The field studies focused on the evaluation of herbicidal efficacy based on the visual
method. The results are presented in Figure 4. HILs with MCPA (1a-3a), 2,4-D (1b-3b),
dicamba (1c-3c) and MCPP (1d-3d) anions exhibited a very high herbicidal efficacy against
common lambsquarters and field pennycress. HILs with the behenyl group (3) and MCPA,
MCPP or dicamba anions exhibited lower herbicidal efficacy compared to HILs with shorted
alkyl substituents.
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Figure 4. here.
HILs with the 2,4-D anion and the reference herbicide exhibited a very high efficacy in the
range of 80-100 % in case of common lambsquarters and 90-98% in case of field pennycress.
HILs with the dicamba anion exhibited a similar efficacy to the reference herbicide against
common lambsquarters, while the results against field pennycress were more diversified. In
this case only HIL 3c displayed a slightly lower efficacy compared to the reference herbicide.
The results obtained during field studies confirm the high herbicidal efficacy of ammonium
HILs with long alkyl substituents (hexadecyl, octadecyl or even behenyl). To the best of our
knowledge, this is the first report regarding HILs with the behenyl (C22) group.
ABBREVIATIONS
C16
Hexadecyl substituent;
C18
Octadecyl substituent;
C22
Behenyl substituent;
MCPA
2,4-D
Dicamba
MCPP
IL
(4-Chloro-2-methylphenoxy)acetic acid;
(2,4-Dichlorophenoxy)acetic acid;
3,6-Dichloro-2-methoxybenzoic acid;
2-(4-Chloro-2-methylphenoxy)propionic acid;
Ionic liquids;
HILs
pH
Herbicidal ionic liquids;
Acidity of solution;
NMR
TGA
DSC
Tg
Nuclear Magnetic Resonance Spectroscopy;
Thermal Gravimetric Analysis;
Differential Scanning Calorimetry;
Glass transition temperature;
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500
501
Tonset5%
Tonset
Tm
Decomposition temperature of 5% of the sample;
Decomposition temperature of 50% of the sample;
Melting point;
502 CMC
503 γCMC
Critical micelle concentration;
Surface tension at CMC;
504 pC20
505
Efficiency of surface adsorption on an air-water
interface;
506 Γmax
Maximum surface excess concentration;
Contact angle;
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
CA
EN ISO
International Standard, in English;
FUNDING SOURCES
Studies were supported by Grant PBS2/A1/9/2013, The National Centre for Research and
Development, Warszawa, Poland.
1
13
SUPPORTING INFORMATION. H and C NMR spectra, elemental analyses results of
all the compounds as well as results of density, viscosity and refractive index values for ILs
with MCPP anion are provided in Electronic Supporting Information.
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