Synthesis and Surface Properties of Piperidinium-Based Herbicidal Ionic Liquids as a Potential Tool for Weed Control

Abstract

A series of piperidinium-based herbicidal ionic liquids (HILs) were synthesized and investigated. The designed HILs, obtained with high yields, consisted of cation 1-alkyl-1-methylpiperidinium with surface activity and a commercially available herbicidal anion: (3,6-dichloro-2-methoxy)benzoates (dicamba). The above-mentioned compounds were characterized in terms of surface activity and phytotoxicity. Preliminary results were obtained at higher wettability for all HILs when compared to the wettability of commercial Dicash, with HIL having 18 atoms in the carbon chain being the best effectiveness in wetting surfaces (weeds and crop leaves), whereby a drop of HILs with short alkyl chains (C8-C10) could not slide down a leaf. Our findings present that wettability or mobility of HILs drops varied depending on the plant species. Moreover, in this study, by zeta potential and atomic force microscopy measurements, we provide conclusive evidence to demonstrate that alkyl chain elongation plays a significant role in the evolution of surface properties of HILs.

Full text

Synthesis and Surface Properties of Piperidinium-Based Herbicidal
Ionic Liquids as a Potential Tool for Weed Control
Marta Wojcieszak,*Anna Syguda, Aneta Lewandowska, Agnieszka Marcinkowska,
Katarzyna Siwinska-Ciesielczyk, Michalina Wilkowska, Maciej Kozak, and Katarzyna Materna
Cite This: J. Agric. Food Chem. 2023, 71, 4550−4560
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sı Supporting Information
ABSTRACT: A series of piperidinium-based herbicidal ionic liquids (HILs) were synthesized and investigated. The designed HILs,
obtained with high yields, consisted of cation 1-alkyl-1-methylpiperidinium with surface activity and a commercially available
herbicidal anion: (3,6-dichloro-2-methoxy)benzoates (dicamba). The above-mentioned compounds were characterized in terms of
surface activity and phytotoxicity. Preliminary results were obtained at higher wettability for all HILs when compared to the
wettability of commercial Dicash, with HIL having 18 atoms in the carbon chain being the best eectiveness in wetting surfaces
(weeds and crop leaves), whereby a drop of HILs with short alkyl chains (C8−C10) could not slide down a leaf. Our findings present
that wettability or mobility of HILs drops varied depending on the plant species. Moreover, in this study, by zeta potential and
atomic force microscopy measurements, we provide conclusive evidence to demonstrate that alkyl chain elongation plays a
significant role in the evolution of surface properties of HILs.
KEYWORDS: herbicidal ionic liquids, static contact angle, sliding angle, dicamba, zeta potential
■INTRODUCTION
Agriculture is a basic sector of the economy. Many problems
hinder agricultural development, the most important of which
is weed infestations in fields.
1
This infestation is the result of
intensive fertilization of plants, which increases crop yields but
promotes the development of weeds. An eective method of
weed control is the use of chemical preparations in the form of
herbicides.
2
An extremely important aspect of the use of
herbicides in plant production is the possibility that weeds will
become resistant to plant protection agents.
3−5
One of the
most common herbicides is dicamba, (3,6-dichloro-2-
methoxy)benzoic acid, which was developed in the early
1960s as a selective herbicide for pre- and postemergence
control of weeds in cereal crops.
6−9
After application, dicamba
can translocate through all broadleaf weeds, causing their
destruction.
9
Herbicides based on dicamba have high vapor
pressures (2.6 ×10−8atm at 25 °C),
10
suggesting substantial
volatility after application.
7,11,12
In recent years, the eorts of various research groups have
focused on the transformation of herbicides into herbicidal
ionic liquids (HILs) for use in crop production.
13−19
HILs are
organic salts with herbicidal anions that exist in a molten state
at temperatures below 100 °C.
15,16
The main function of HILs
is to protect crops by enhancing the activities of herbicides and
reduce their environmental risk.
9,18,20,21
Ionic liquids in which
the anions exhibit herbicidal activity have been widely reported
in the literature.
22−24
An attractive solution for increasing the
eectiveness of HILs is bifunctional herbicidal ionic liquids,
which are a combination of an anion with herbicidal activity
and a cation with surface properties.
25,26
This combination
produces an increase in the surface area covered by a herbicide
and improved adhesion of the preparation to the surface of
plant leaves. The aforementioned result was observed by our
previous work where we analyzed morpholinium HILs and
their wettability of the leaf surface.
21
The influence of the
length or nature of the alkyl chain on the surface activity of
ionic liquids is well known.
27,28
Studies have shown that the
length of the carbon chain in a surfactant impacts the
interfacial properties, which translates into a significant
reduction in surface tension and excellent wetting ability.
29,30
Continuing this thought, to contribute toward understanding
the wetting phenomenon, the values of contact angle (CA) are
established experimentally. The CA values are very useful for
characterizing droplets deposited on a surface
31−33
and are
determined by interactions in the liquid (forming the
droplets), which spreads not only on the plant surface under
consideration but also on the area covered by the herbicide
spray. The CA values are crucial for determining the type of
interaction between a test surface and a wetting com-
pound.
34,35
Surface properties are an important influence
factor for the eectiveness of HILs, because the physical and
chemical attributes of HILs can result in dierent eects on a
leaf surface. In the case of herbicides for which the ecacy
depends on the quality of leaf coverage, low product retention
can result in failure of weed control.
36
The spraying process
includes the formation of droplets, the retention and spread of
these droplets over a plant, and the penetration of the active
Received: January 18, 2023
Revised: February 19, 2023
Accepted: February 21, 2023
Published: March 6, 2023
Articlepubs.acs.org/JAFC
© 2023 The Authors. Published by
American Chemical Society 4550
https://doi.org/10.1021/acs.jafc.3c00356
J. Agric. Food Chem. 2023, 71, 4550−4560

substance through relevant parts of the leaf structure until the
target is eectively reached.
33−36
The aim of this study was to focus on a series of novel
piperidinium-based HILs that consist of amphiphilic cations
and anions with potential herbicidal activity. The synthesis,
thermal analysis, surface activity, phytotoxicity, atomic force
microscopy (AFM) analysis, and zeta potential of the studied
HILs are presented. The main purpose of designing
compounds was to develop research on the surface properties
of HILs, in particular, to expand the topic of wettability. To
our knowledge, the wettability of biological surfaces by HILs
has been described for the first time in our previous work.
21
The aforementioned wettability tests were based on the
studied static CA on biological systems. Seeing the potential of
our research, we wanted to go one step further and focus on
the issue of drop mobility of novel piperidinium-based HILs on
the surfaces of weed leaves which to the best of our knowledge
has not been reported thus far. Considering the problem of
formulation may runo from sprayed surfaces, it seems
reasonable to study the mobility of new formulations of
HILs from the weed leaves surface, which is crucial for eective
crop protection.
■MATERIALS AND METHODS
Materials. 1-Methylpiperidine (CAS 626-67-5) 98%, 1-bromooc-
tane (CAS 111-83-1) 98%, 1-bromononane 98% (CAS 98-639-58), 1-
bromodecane (CAS 112-29-8) 98%, 1-bromododecane (CAS 143-15-
7) 97%, 1-bromotetradecane (CAS 112-71-0) 97%, 1-bromohex-
adecane (CAS 112-82-3) 98%, 1-bromooctadecane (CAS 112-89-0)
98%, reagents for two-phase system titration: [dimidium bromide
(CAS 95-518-67-2) 95%, patent blue V sodium salt (CAS 20262-76-
4) 97%, and sodium dodecylsulfate(VI) (CAS 151-21-3) 98%] were
purchased from Sigma-Aldrich. (3,6-Dichloro-2-methoxy)benzoic
acid (dicamba) (CAS 1918-00-9) 95% was purchased from
Organika-Sarzyna (Poland). Acetone (CAS 67-64-1) 99%, ethyl
acetate (CAS 141-78-6) 99%, chloroform (CAS 67-66-3) 98.5%,
silver(I) nitrate(V) (CAS 7761-88-8) 99%, and sodium bicarbonate
(CAS 144-55-8) 99% were purchased from Avantor.
Synthesis of 1-Alkyl-1-methylpiperidinium Bromides. First,
0.05 mol of 1-methylpiperidine was dissolved in 5 cm3of acetone and
placed in a 250 cm3round-bottomed flask, to which 0.0525 mol of the
appropriate alkyl bromide and 10 cm3of acetone were added. The
reaction was carried out in acetone under reflux for 24 h. The flasks
were then placed in a refrigerator for 24 h. The resulting products
were vacuum-filtered and washed with a small quantity of cold ethyl
acetate. The obtained compounds with C8H17 to C10H21 substituents
were dried in a vacuum desiccator over P2O5, and the remaining
products were dried in a vacuum oven at 60 °C for 24 h.
Synthesis of 1-Alkyl-1-methylpiperidinium (3,6-Dichloro-2-
methoxy)benzoates. A reagent mixture consisting of 0.01 mol of
dicamba in acid form, 20 cm3of distilled water, and 0.011 mol of a
10% aqueous solution of sodium bicarbonate was mixed in a round-
bottomed flask equipped with a magnetic stirring bar, a reflux
condenser, and an addition funnel. The mixture was heated at 50 °C
until the solution became clear. Afterward, 0.01 mol of 1-alkyl-1-
methylpiperidinium bromide dissolved in 20 cm3of water was added
to the solution, which was stirred for 30 min at room temperature.
Then, the product was extracted from the aqueous phase with 50 cm3
of chloroform and washed with distilled water until bromide ions were
no longer detected using AgNO3. The chloroform was removed, and
the product was dried under reduced pressure at 60 °C for 24 h.
NMR Analysis. Proton nuclear magnetic resonance (1H NMR)
spectra were recorded using a Bruker Ascend 400 MHz NanoBay
spectrometer operating at 400 MHz with tetramethylsilane as the
internal standard CDCl3as a solvent. Carbon-13 nuclear magnetic
resonance (13C NMR) spectra were obtained with the same
instrument at 100 MHz. All analyses were performed at Adam
Mickiewicz University, Poznan (Poland).
Thermal Analysis. Thermogravimetric analysis (TGA) was used
to study the thermal stability of the ionic liquids. Measurements were
made on a Tarsus TG 209 F3 analyzer (NETZSCH-Geratebau
GmbH, Germany) in the temperature range of 30−600 °C.
Approximately 10 mg of a sample was placed in a platinum crucible
and heated at a rate of 10 °C/min under a nitrogen atmosphere (the
flow rates of the protective and purge gases were 10 and 20 mL/min,
respectively).
The thermal transition temperatures were determined by dier-
ential scanning calorimetry (DSC) using a DSC1 instrument
(Mettler-Toledo, Greifensee, Switzerland). Before measurements
were performed, 5−10 mg of a synthesized ionic liquid were placed
in an aluminum pan and sealed with a pan lid. Then, the sample was
cooled with an intracooler to −80 °C at a cooling rate of 10 °C/min
and heated at the same rate under a nitrogen atmosphere.
Surface Activity Studies. The surface properties (surface tension
and CA) of the samples were measured using a DSA 100 analyzer
(Kruss, Germany, accuracy ±0.01 mN/m) at 25 °C. The sample
temperature was monitored using a Fisherbrand FBH604 thermo-
static bath (Fisher, Germany, accuracy ±0.1 °C). The surface tension
of the samples was determined based on the shape of an axisymmetric
drop placed at the tip of a needle. An image of the drop was taken and
digitized. The surface tension (γin mN/m) was determined by using
the Laplace equation to analyze the drop profile.
The parameters of the critical micelle concentration (CMC) and
the surface tension at the CMC (γCMC) were determined from the
intersection of two straight lines drawn in the low- and high-
concentration regions of the surface tension curves (γvs log Ccurves)
using linear regression analysis.
Surface excess concentrations at the saturated interface (Γmax), the
minimum surface occupied by a molecule at the interface (Amin),
Gibbs free energy of the adsorption layer (ΔG0ads), CMC/C20 ratio,
and the adsorption eciency, pC20, have been presented in our
previous reports.
22,37,38
The static CA can be determined from the image of a drop on a
test surface. Liquid drops are deposited on a solid hydrophobic
surface. The actual drop shape and contact line are determined, and
the drop shape is fitted to a mathematical model from which the CA is
calculated.
In this study, the sliding angle (SA) was determined using the
tilting plate method (using an angle of inclination of up to 90°with a
resolution of 0.01°, an accuracy of 0.3° ± 0.1°, and a tilt speed range
of 0.5°−50°/s). Initially, the drop was deposited on a test surface
placed on a table. The table was then slowly tilted to increase the
angle of inclination of the surface. At first, the drop did not move but
deformed to an extent that depends on the liquid density, drop
volume, and surface tension. At a particular angle of inclination, the
drop started to move and slid or rolled across the surface. The entire
process was digitalized.
The static CA and SA were performed on the adaxial and abaxial
sides of leaves.
In this study, paran (a model surface used in laboratory) and
biological systems, common wheat (Triticum aestivum L.), cornflower
(Centaurea cyanus L.), winter rapeseed (Brassica napus L.), and white
mustard (Sinapis alba L.), were analyzed as the solid phase.
Zeta Potential Measurement. The zeta potential of the ionic
liquid aggregates in aqueous solution was measured at 25 °C on a
Zetasizer Nano-ZS (Malvern Instrument Ltd.) equipped with an
autotitrator. The zeta potential was estimated using the Smoluchowski
equation. The pH of the ionic liquid aggregates was automatically
adjusted by an automatic titrator using hydrochloric acid (0.02 mol/
L) or sodium hydroxide (0.02 mol/L). The test samples were
solutions of HILs at CMC concentrations.
Statistical Analysis. Statistical analysis was carried out using the
standard error of the mean (SEM) value method; that is, the standard
errors in the mean were estimated. The SEM was calculated using the
equation given below:
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J. Agric. Food Chem. 2023, 71, 4550−4560
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=
s
n
SEM 0.5
where SEM is the standard error of the mean, sis the sample standard
deviation, and nis the number of samples.
Observation of the Microstructure. A digital microscope
(VHX-7000 series, Keyence) was used to observe the morphology
of the leaf surfaces.
Atomic Force Microscopy. The samples of the analyzed
herbicidal ionic liquids were dissolved in water, and small volumes
(3 μL) of the test solutions containing dierent concentrations of HIL
were deposited on freshly prepared mica substrates and dried.
Topographic images were collected using a NanoWizard IV (JPK,
Germany) atomic force microscope and Tap150AL AFM cantilevers
(Ted Pella, Inc., Redding, USA). The experimental AFM data
obtained for the analyzed HILs were processed and analyzed using
Gwyddion v2.58 image processing software.
39
Phytotoxicity. Phytotoxicity of synthesized 1-alkyl-1-methylpiper-
idinium (3,6-dichloro-2-methoxy)benzoates in relation to dicotyled-
onous plants was measured on the basis of measurements of shoot
and root growth inhibition of the plant. Cornflower (Centaurea cyanus
L.) was used as a model dicotyledonous plant. Commercial product�
Dicash (480 g of dicamba in the form of dimethylammonium salt per
1 L) was used as the reference sample. The tests were carried out
using vertical plastic Phytotoxkit containers (Tigret company,
Belgium). The sand was previously screened and cleaned by washing
it several times with tap water and deionized water and then dried in a
dryer for 24 h at 105 °C. Each container was filled with 130 ±0.1 g of
sand. 0.25 mmols of the tested compound was placed in a 100 mL
graduated flask. Consequently, the initial solutions of the tested
compounds were prepared at a concentration of 2.5 mmol/L. Then a
10-fold dilution was made by taking 10 mL of the initial solution and
diluting it in a 100 mL volumetric flask. In the next stage, a 10-fold
dilution was made again. Finally, 0.025 mmol/L use solutions of the
compounds were obtained. Afterward, 25 mL of the prepared
solutions were taken and used to water the sand in a container, which
corresponded to 0.0048 mmol of the tested compound per kg of dry
sand. One of the containers was prepared as a “0” control, which
contained only deionized water. Next, 10 seeds of tested plants were
planted in each container and incubated for 7 days at 25 °C. Seven
days after sowing, the lengths of the shoots and roots were measured.
The tests were performed according to the PN-ISO 11269-1 (1998)
standard.
■RESULTS AND DISCUSSION
Synthesis. The first step in the preparation of piperidinium
ionic liquids with a (3,6 dichloro-2-methoxy)benzoate anion
(dicamba) was to synthesize the precursors, 1-alkyl-1-
methylpiperidinium bromides. The reaction course is shown
in Scheme 1. The quaternization of 1-methylpiperidine with
alkyl bromides was carried out in a polar solvent to accelerate
the formation of the polar product. The reaction was of the
SN2-type. The substrates dissolved very well in acetone, from
which the resulting product crystallized. The product was
filtered and washed with cold ethyl acetate to elute unreacted
starting materials. White crystalline solids were obtained. The
compounds with C8H17 to C10H21 substituents were highly
hygroscopic and therefore dried in a vacuum desiccator over a
P2O5drying agent, whereas the remaining compounds were
considerably less hygroscopic. The surfactant content of the
precursors of the ionic liquids was determined by a two-phase
titration. The yields and melting points of the bromides are
given in Table S1 (Supporting Information). During the
second stage of the preparation scheme, ionic liquids were
obtained by a metathesis reaction. Long-chain quaternary salts
have been eectively obtained by performing a metathesis
reaction and isolating the synthesis product from the reaction
mixture with chloroform;
9
therefore, the same procedure was
adopted in this study. The sodium salt of dicamba has been
more eectively obtained using sodium bicarbonate than
NaOH
9
because the inertness of NaHCO3precludes the
formation of Homan elimination products from the 1-alkyl-1-
methylpiperidinium cation in the presence of the residual
excess neutralizing agent. The obtained compounds were very
viscous liquids. The yield and surfactant content of these
compounds were determined analogously to those of the
precursors and are given in Table 1.
Proton and carbon nuclear magnetic resonance spectra were
obtained to determine the structures of the prepared
compounds. Tables 2 and 3show the signals from the protons
and carbon atoms for the representative ionic liquid (C10) and
its precursor, and for the rest of the compounds in Tables S2−
S13, whereas all the NMR spectra are presented in Figures
Scheme 1. Two-Step Synthesis of Piperidinium-Based HILs
Table 1. Synthesized 1-Alkyl-1-methylpiperidinium (3,6-
Dichloro-2-methoxy)benzoates
CnR surfactant content (%) yield (%)
C8C8H17 93.0 85
C9C9H19 95.5 90
C10 C10H21 98.5 95
C12 C12H25 98.0 97
C14 C14H29 97.0 96
C16 C16H33 94.5 94
C18 C18H37 93.5 93
Table 2. Chemical Shifts (δ) and Coupling Constants (J) in
the 1H NMR Spectra of 1-Decyl-1-methylpiperidinium
Bromide and (3,6-Dichloro-2-methoxy)benzoate (CDCl3)
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S1−S27 in the Supporting Information. All the expected
signals were found in the spectra, and no signals of organic
pollutants were observed. The only additional peak that
appeared in the proton spectra was that of water. In the 1H
NMR spectra, the chemical shifts of the protons located closest
to the quaternary nitrogen atom diered depending on the
anion type. The chemical shifts of the analogous protons of the
ionic liquid were all lower than those of the precursor. This
result shows higher proton shielding for the dicamba anion
than the bromide anion, as has been previously reported.
21
However, in the 13C NMR spectra, the analogous carbon
atoms for the ionic liquid, except for 10 and 11, had slightly
higher chemical shifts (Table 3) than the precursor.
Thermal Analysis. The thermal properties of the
synthesized ionic liquids were investigated using DSC and
TGA. The phase transition temperatures were determined
from the DSC thermograms. The crystallization (Tc) and
melting (Tm) temperatures were estimated from the maximum
values of the exothermic peak on cooling and the endothermic
peak on heating, respectively. The cold crystallization (Tcc)
temperature was determined from the maximum values of the
exothermic change during the heating cycles, and solid−solid
transition temperature (Ts−s) was determined from the
maximum of the endothermic change during the heating
cycles. The glass transition (Tg) was determined as the
midpoint of the change in the heat capacity during the heating
cycle. The results are presented in Table 4, and representative
DSC thermograms of the investigated HILs are presented in
Figure S29. Glass transition temperatures were observed for all
the synthesized HILs except C18. For the C8−C10 HILs, the
glass transition is the only transition observed in the studied
temperature range. Crystallization in these HILs is restricted
by van der Waals attractive and dispersive interactions between
hydrocarbon chains, as well as by multiple rotational modes.
40
HILs with substituents containing long alkyl chains (C12−C18)
exhibit additional melting and crystallization transitions, such
that the structural order increases with the hydrocarbon chain
length. This order is related to microphase separation between
the ionic and hydrophobic parts of the HIL.
40
The C12 and C14
HILs undergo transitions during heating that can be identified
Table 3. Chemical Shifts (δ) in the 13C NMR Spectra of 1-
Decyl-1-methylpiperidinium Bromide and (3,6-Dichloro-2-
methoxy)benzoate (CDCl3)
Table 4. Glass Transition Temperature (Tg), Cold
Crystallization Temperature (Tcc), Solid−Solid Transition
Temperature (Ts−s), Melting Temperature (Tm), and
Crystallization Temperature (Tc) of the Obtained ILs
Determined from DSC Thermograms (at a Heating/
Cooling Rate of 10 °C/min)
CnTg(°C) Tcc
(°C) Ts−s1
(°C) Ts−s2
(°C) Tm
(°C) Tc1
(°C) Tc2
(°C) Tc3
(°C)
C8−37.7
C9−39.2
C10 −48.6
C12 −46.2 25.8 42.8
C14 −39.8 8.7 31.1
C16 −29.0 −0.1 12.9 43.4 −7.3
C18 −1.7 37.3 61.2 −1.0 29.6 36.2
Table 5. Surface Activity of Synthesized HILs
CnCMC (mmol/L) CMC/C20 γCMC (mN/m) pC20 Γmax ×106(mol/m2)Amin ×1019 (m2)ΔG0ads (kJ/mol)
C860.25 6.5 34.4 2.03 6.26 2.65 −13.5
C924.73 5.9 32.0 2.37 9.82 1.69 −13.6
C10 18.50 5.9 36.1 2.50 7.30 2.28 −15.8
C12 4.43 4.9 37.5 3.04 6.62 2.51 −18.9
C14 1.27 4.7 38.2 3.57 6.82 2.43 −21.9
C16 0.42 4.1 37.5 4.00 8.75 1.90 −23.3
C18 0.08 1.8 39.8 4.35 8.25 2.01 −25.3
Figure 1. Surface tension versus log concentration of the synthesized
HILs at 25 °C.
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as cold crystallization at 24 and 10 °C with melting points at
42 and 31 °C, respectively. The higher homologues (C16 and
C18) melt during heating and crystallize upon cooling. Solid−
solid polymorphic transitions appear before the melting
transition that are associated with changes in the conformation
of the long hydrocarbon chain of the substituent. These
conformational changes cause a change in the density of the
sample before the melting transition.
41
Additionally, the longer
the alkyl substituent chain is, the higher the melting
temperature of the HIL is.
The beginning of the thermal decomposition of the studied
HILs was determined as the temperature at which a 5% mass
loss occurred. The temperature at which a 50% mass loss
occurred was also estimated. The results are presented in Table
S14. Generally, all the synthesized HILs are stable over the
temperature range of 173−180 °C and are thermally degraded
by a multistep process (Figure S29). The HIL thermal stability
increases with the length of the hydrocarbon chain of the
substituent. An opposite trend has generally been reported in
the literature, that is, the temperature at which thermal
decomposition occurs decreases with increasing alkyl chain
length. There are, however, some reports of trends similar to
that presented here.
42,43
The authors of these latter studies
suggest that this trend is caused by progressive fragmentation
of hydrocarbon substituents, which leads to less volatile
decomposition products. The thermal degradation of the C8
Figure 2. Static CA values on the adaxial surface of leaves (more details are provided in Table S3, Supporting Information).
Figure 3. Digital microscope images for the adaxial side of leaves. The images were obtained under magnification. The bar represents 100 μm.
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4554

and C9HILs was studied. The first step of the degradation of
these compounds ends at approximately 250 °C. The exact
temperatures are at 252 °C for C8with an 87% mass loss and
253 °C for C9with an 87% mass loss. The second step ends at
420 °C with an 8% mass loss, 430 °C with a 9% mass loss and
420 °C with a 14% mass loss. The C10, C12, and C14 HILs
exhibit similar two-step degradation: the first step ends at 251
°C with a 73% mass loss (C10), 250 °C with a 72% mass loss
(C12) and 270 °C with a 74% mass loss (C14), followed by a
second step in the range of 251−432 °C (23% mass loss),
250−445 °C (20% mass loss), and 270−450 °C (22% mass
loss). The thermal stability of the HIL with a longer alkyl
chain, i.e., C16, is dierent, i.e., higher, from the other
herbicidal ionic liquids. The first step in thermal decom-
position ends at the highest temperature of 303 °C with a 70%
mass loss, and the second step ends at 435 °C with a 26% mass
loss. The thermal decomposition of C18 with the longest alkyl
chain exhibits similarities to that of C14. The first stage of
decomposition ends at 285 °C with a 79% mass loss, and the
second step ends at 443 °C with a 17% mass loss.
Surface Properties. Surface tension measurements were
carried out to investigate the interactions at the water−air
interface and used to determine several parameters, including
the CMC, surface tension at the CMC (γCMC), adsorption
eciency (pC20), CMC/C20 ratio, Gibbs free energy of the
adsorption layer (ΔG0ads), excess concentration (Γmax), and
minimum surface area occupied by a molecule at the interface
(Amin), as will be described later. The values of these surface
properties for the series of piperidinium ionic liquids are
summarized in Table 5.
The surface tension curve for the synthesized HILs is
presented in Figure 1. In theory, the surface tension first
gradually decreases as the ionic liquid concentration increases.
This region is called premicellar,
44
and the concentration at
which premicellar aggregates form is called the critical
aggregation concentration. Beyond this region, the surface
tension stops changing at a well-defined concentration, known
as the CMC. When the concentrations exceed the CMC, the
surface tension remains stable and does not change
significantly with increasing surfactant concentration. This
regime is called the postregion.
45
In this study, the surface tension of aqueous solutions
(γCMC) of the analyzed HILs decreased from the value for
water (72.8 mN/m) to plateau at a minimum ranging from
32.0 to 39.8 mN/m. Surface tension measurements for HILs
are important as an ecient way of obtaining information on
the intrinsic energy involved in the interactions between
cations and anions.
46−48
At constant HIL concentrations, the
surface tension increases with the alkyl chain length. The
critical surfactant concentration associated with the micelliza-
tion process is a crucial parameter that can be determined from
the surface tension−concentration profile in aqueous solution.
The critical micelle concentrations of the analyzed HILs are
summarized in Table 5. The CMC appears to decrease with
increasing alkyl chain length. The micellization process is the
result of two forces. One is the attraction force between
compounds with surface activity and water molecules. This
force increases the tendency of ionic liquid molecules to
localize in the bulk of the solution. Above observation was also
observed in 2012 by Tariq et al.
46
The second force is the
repulsion between cyclic groups and water molecules. This
force promotes the presence of compound molecules at the
air/water interface. The CMCs range from 0.08 to 60.25
mmol/L. The C18 HIL has the highest tendency to form
micelles at low concentrations. Similarly, to conventional
surfactants, increasing the number of carbon atoms in the
molecules of the analyzed ionic liquids increases their tendency
to form in a water solution. Therefore, increasing the
hydrophobicity of the molecules promotes their adsorption
at the air−aqueous interface until a saturated state is reached.
Note that all the relationships presented above between the
CMC and the length of the alkyl substituent follow the Stau−
Klevens rule.
49
The parameter used to evaluate the surface activity of HILs
is the adsorption eciency (pC20). In the literature,
46,50
higher
pC20 values have been reported to indicate a higher anity of
HIL molecules for adsorption at the air−water interface. As
expected, increasing the length of the alkyl chains causes an
increase in the hydrophobicity of the compound molecules.
This result is reflected in the increase of the pC20 values.
Correspondingly, the highest pC20 of 4.35 was obtained for
compound C18. In general, the adsorption process can also be
characterized using the CMC/C20 ratio.
48
Increasing the
length of the alkyl group caused the CMC/C20 ratio to
decrease from 6.5 to 1.8. The negative values of ΔG0ads
presented in Table 5 show that micelle formation is a
Figure 4. SAs of HILs on the adaxial side of the weed leaf surface and
paran.
Figure 5. Zeta potential of aggregates of the analyzed HILs.
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spontaneous process. A similar situation was noted in 2012 by
Zdziennicka et al.
50
ΔG0ads decreases with the increase in the
length of the alkyl chain for all the ionic liquids, showing that
the aggregation is driven by the hydrophobic eect.
The values of Amin for the HILs increase from 1.69 to 2.65 ×
10−19 m2, whereas Γmax ranges from 6.26 to 9.82 ×10−6mol/
m2. Interestingly, high values of the maximum surface excess
and low values of the minimum surface area suggest that
surface-active ionic liquid molecules have a high tendency to
adsorb at various interfaces, i.e., at air−water or solid−water
interfaces. Consequently, knowledge of the tendency of HILs
to adsorb at dierent interfaces can improve their utility for
many applications, for example, as agricultural herbicides.
51,52
The wettability of a plant surface is assessed by the CA.
Observation of static CA values is essential for the application
of appropriate chemical formulations of spray solutions. CA
values depend on factors that are primarily related to the
morphology of leaves (the adaxial and abaxial sides of a leaf).
53
All the synthesized HILs were analyzed in terms of the
behavior of spray solutions on the leaf surfaces of three popular
species of weeds that are widespread in Polish fields:
cornflower (Centaurea cyanus L.), winter rapeseed (Brassica
napus L.) and white mustard (Sinapis alba L.). The leaves of
common wheat (Triticum aestivum L.) were also tested. Dicash
(dicamba dimethylammonium salt, 480 g/L) contains an active
ingredient analogous to the investigated HILs and was used as
a reference.
The results presented in Figure 2 and Table S15 show that
the wettability of paran and the leaf surfaces by the studied
HILs depended on the alkyl chain length. In general,
Figure 6. AFM results for piperidinium-based HILs deposited on mica surfaces, showing the surface coverage by piperidinium-based molecules
with dierent alkyl chain lengths (C9and C18). (A, E) Topography of selected areas for piperidinium-based HILs with dierent alkyl chains. (B, F)
Five profile curves for selected round deposits. (C, G) 3D view of the test surfaces. (D, H) Topography of selected areas for piperidinium-based
HILs with small surface areas.
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compound C8had the lowest ability to wet the analyzed
surfaces (the CA values were as follows: common wheat,
119.0°; winter rapeseed, 103.0°; cornflower, 86.4°; white
mustard, 90.3°; and paran, 100.0°). The highest wettability
was recorded for the C18 HIL (the CA values were as follows:
common wheat, 85.9°; winter rapeseed, 75.6°; cornflower,
57.1°; white mustard, 65.4°; and paran, 73.0°).
Special attention should be given to the biological aspects of
the wettability analysis of the leaf surfaces (see Figure 3).
Generally, the variation in the CA values of the HILs was due
to dierences in the plant morphologies, which are determined
by the plant chemical compositions.
54,55
Common wheat
belongs to the Poaceae family,
56
cornflower belongs to the
Asteraceae family, whereas white mustard and winter rapeseed
are part of the Brassicaceae family.
57−59
It should be
emphasized that the CA for a given substance is related to
the morphological structure of the plant, that is, the adaxial and
abaxial surfaces of the leaf. Theoretically, the two sides of a leaf
are morphologically dierent.
60,61
That is, the sides of a leaf
have dierent degrees of hydrophilicity and hydrophobicity.
The exact CA values on the adaxial and abaxial leaf surfaces are
summarized in the Supporting Information (Table S15). The
Figure 7. AFM studies of morpholinium-based HILs (A, B) and piperidinium-based HILs (C, D) deposited on mica surfaces. (E) Height
distribution of selected samples with dierent cations (black curve�morpholinium-based HILs, blue curve�piperidinium-based HILs). (F)
Selected profile curves for topography D with round deposits.
Figure 8. Average root and shoot length for seedlings cornflower
(Centaurea cyanus L.) in sand with the addition of 1-alkyl-1-
methylpiperidinium (3,6-dichloro-2-methoxy)benzoates and Dicash.
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CA values only diered slightly between the leaf sides. This
result indicates comparable performance for piperidinium-
based herbicidal ionic liquids for the two leaf sides.
Paran has a hydrophobic surface, which can however serve
as a model surface to simulate biological systems. The values of
CA for all the solutions on paran ranged from 73.0 to 100.0°.
Thus, we concluded that paran is not an ideal surface for
simulating leaf surfaces.
21,38
A comparison of the values of CA recorded for water and the
Dicash solution showed a dierence in the wettability of the
leaf surfaces of weeds and paran. Compared to the CA values
recorded for the C18 HIL solution, the values of CA of the
Dicash solution were markedly higher on the surfaces of
common wheat or winter rapeseed (by up to 40°) and
cornflower or white mustard (by up to 20°), that is, the Dicash
solution exhibited worse wetting properties. It follows that the
presence of a cation with surface properties increased the
wettability of the surfaces analyzed. A similar relationship was
observed when pure aqueous solutions were used.
Static CA values have been commonly used as a measure of
surface hydrophobicity but are not sucient for evaluating the
sliding ability of drops on surfaces.
62−64
Thus, SA values are
determined to analyze the behavior of a drop on a leaf and,
more specifically, drop mobility on a biological surface.
65
Figure 4 shows the measured SA values on the adaxial side of
the weed leaf surface and paran.
A distinct trend cannot be identified in the SA values of the
HILs on cornflower, winter rapeseed, and white mustard
leaves. However, a drop of HILs with short alkyl chains (C8−
C10) could not slide down a leaf even at 90°, which was the
maximum limit of the tilt angle of the sampling stage.
66
Thus,
the surface microstructure, i.e., the roughness of a micropillar-
structure, undoubtedly plays a significant role in the sliding
behavior of liquid drops on plant leaves.
65
Moreover, there is
an important distinction between paran and the surfaces of
weeds. The data (Table S16) showed that the drops slid on
paran, regardless of the quantity of HILs used to wet the test
surface.
These results are preliminary indications of the applicability
of HILs to agriculture by identifying a crucial issue that, to the
best of our knowledge, has not been suciently reported in the
literature.
Characterization of Aggregates: Zeta Potential
Measurement. The zeta potential (ζ) is a useful parameter
for determining the chemical charge of micelles formed in IL
solutions. Positive zeta potentials were measured for the
studied HILs (except for compounds C8and C9, which
exhibited negative ζs). Therefore, the micelles possessed a
positive surface electric charge at the CMC.
In the literature, zeta potentials have been reported to
depend on the length of the alkyl chain of HILs (Figure 5). In
general, negative zeta potentials are observed for short alkyl
chains (C8−C10), and positive zeta potentials are observed for
long chains (C12−C18).
67
The piperidinium-based HILs
followed this trend, except for the compound with an alkyl
substituent containing 10 carbon atoms.
AFM Topographic Analysis. The potential application of
HILs to plant protection is determined by both the physical
properties and adherence to the leaf surface of HILs. AFM
imaging was applied to characterize these properties, whereby
the topography of the tested ionic liquids deposited on the
mica surface was visualized. Selected topographic images of
deposits of the studied HILs are presented in Figures 6 and 7.
To investigate the dierences resulting from the alkyl chain
length in the investigated piperidinium-based HILs, AFM
imaging was performed on HILs containing C9and C18 chains
with the same dicamba anion. Additionally, the coverage of the
dried mica surface of the piperidinium-based HILs was
compared against those of morpholinium-based HILs that
were measured in a previous study.
21
The topography of a large surface (see Figure 6A and E)
covered with a piperidinium derivative of a short alkyl chain
(C9) at the same concentration used for the leaf tests shows a
dense distribution of symmetric round deposits (micelles) on
the surface and is compared against the distribution of similar
structures observed for a HIL with a longer alkyl chain (C18).
By comparison, the smaller field of view of the test area for
the same samples (Figure 6D,H) shows more precise and
homogeneous coverage of the mica surface. The surface
coverage is almost uniform. Thus, HILs with a long alkyl chain
provide more eective surface coverage than compound with a
short chain (C9).
The results of the AFM studies on piperidinium-based HILs
were compared with those described in our previous work.
21
Figure 7 shows the results for herbicides based on dicamba
anions with similar alkyl chain lengths (C9and C10) and
dierent cations. The morpholinium derivatives containing the
dicamba anion show irregular surface coverage due to island
formation and the absence of micellar deposits. The observed
height distribution values suggest similar heights for the
structures formed from both the morpholinium and piper-
idinium derivatives. These results show that the piperidinium
derivatives enable more homogeneous surface coverage than
the morpholinium derivatives.
21
Phytotoxicity. To determine the phytotoxicity of synthe-
sized HILs, germination tests were performed. Cornflower
(Centaurea cyanus L.) was used, which is a popular weed and a
model dicotyledonous plant. Sterilized sand was used as a
substrate to eliminate the impact of additional soil
components. Plants could only get their nutrients from their
reserve materials. Deionized water was used as a control, while
a commercial product�Dicash�was used as a comparative
herbicide. The eects of the ionic liquids and the comparative
herbicide are shown in Figure 8 and in Figures S30−S36.
All the synthesized piperidinium-based ionic liquids retained
their phytotoxicity, although the activity of the ionic liquids
containing 8, 10, 14, 16, and 18 carbon atoms in the alkyl
substituent of the cation is significantly better than that of the
comparative preparation. The 1-hexadecyl-1-methylpiperidi-
nium (3,6-dichloro-2-methoxy)benzoate) (C16) proved to be
the best in inhibiting the growth of the root, while the 1-
methyl-1-octadecylpiperidinium (3,6-dichloro-2-methoxy)-
benzoate) (C18) was the best in inhibiting the growth of the
shoot.
■ASSOCIATED CONTENT
*
sı Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.jafc.3c00356.
1H and 13C NMR spectra with characterization, the
results of thermal stability of obtained HILs; results of
the static contact angles for the adaxial and abaxial sides
of leaves and for paran, the sliding angles for the
adaxial and abaxial sides of leaves; the figures of TG and
DTG curves of the investigated HILs (PDF)
Journal of Agricultural and Food Chemistry pubs.acs.org/JAFC Article
https://doi.org/10.1021/acs.jafc.3c00356
J. Agric. Food Chem. 2023, 71, 4550−4560
4558

■AUTHOR INFORMATION
Corresponding Author
Marta Wojcieszak −Faculty of Chemical Technology, Poznan
University of Technology, Poznan 60-965, Poland;
orcid.org/0000-0003-0385-6886;
Email: marta.d.wojcieszak@doctorate.put.poznan.pl
Authors
Anna Syguda −Faculty of Chemical Technology, Poznan
University of Technology, Poznan 60-965, Poland;
orcid.org/0000-0003-0145-7478
Aneta Lewandowska −Faculty of Chemical Technology,
Poznan University of Technology, Poznan 60-965, Poland
Agnieszka Marcinkowska −Faculty of Chemical Technology,
Poznan University of Technology, Poznan 60-965, Poland
Katarzyna Siwinska-Ciesielczyk −Faculty of Chemical
Technology, Poznan University of Technology, Poznan 60-
965, Poland
Michalina Wilkowska −Department of Biomedical Physics,
Faculty of Physics, Adam Mickiewicz University in Poznan,
Poznan 61-614, Poland
Maciej Kozak −Department of Biomedical Physics, Faculty of
Physics, Adam Mickiewicz University in Poznan, Poznan 61-
614, Poland; orcid.org/0000-0003-3312-6518
Katarzyna Materna −Faculty of Chemical Technology,
Poznan University of Technology, Poznan 60-965, Poland
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.jafc.3c00356
Funding
This study was funded by the Ministry of Education and
Science in Poland as a subsidy to the Poznan University of
Technology (0912/SBAD/2308).
Notes
The authors declare no competing financial interest.
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Journal of Agricultural and Food Chemistry pubs.acs.org/JAFC Article
https://doi.org/10.1021/acs.jafc.3c00356
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