Two Herbicides in a Single Compound: Double Salt Herbicidal Ionic

Research Article pubs.acs.org/journal/ascecg

Two Herbicides in a Single Compound: Double Salt Herbicidal Ionic Liquids Exemplified with Glyphosate, Dicamba, and MCPA Hemant Choudhary,† Juliusz Pernak,*,‡ Julia L. Shamshina,§ Michał Niemczak,‡ Rafał Giszter,‡ Łukasz Chrzanowski,‡ Tadeusz Praczyk,∥ Katarzyna Marcinkowska,∥ O. Andreea Cojocaru,⊥ and Robin D. Rogers*,†,§,⊥ †

Department of Chemistry, McGill University, 801 Sherbrooke St. West, Montreal, QC H3A 0B8, Canada Department of Chemical Technology, Poznan University of Technology, Poznan 60-965, Poland § 525 Solutions, Inc., 720 Second Street, Tuscaloosa, Alabama 35401, United States ∥ Institute of Plant Protection − National Research Institute, Poznan 60-318, Poland ⊥ Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487, United States ‡

S Supporting Information *

ABSTRACT: Herbicidal ionic liquids (HILs) have been demonstrated to have potential as highly effective herbicides that may also have different modes of action that their neutral precursors. Here, double salt herbicidal ionic liquids (DSHILs) containing at least two herbicidal anions selected from glyphosate, dicamba, or 4-chloro-2-methylphenoxyacetate (MCPA) paired with ammonium or phosphonium cations are reported along with their post-emergence herbicidal activity against several plant species, from both greenhouse and field study-based bioassays. The novel DSHILs were shown to integrate the advantages of two different herbicides into a single HIL, enhance herbicidal efficacy, and reduce the risk of weed resistance due to the various modes of action of the applied treatment. The formation of the DSHILs dramatically reduced melting points and modified the compound solubilities compared to the parent herbicides. Statistical analyses for the greenhouse efficacy tests demonstrated that DSHILs had significant positive differences against winter wheat (Triticum aestivum L.) and white mustard (Sinapis alba L.) as compared to commercial formulations. Biodegradability studies were also performed on selected DSHILs, and the compounds were found to be not readily biodegradable. KEYWORDS: Herbicidal ionic liquids (HILs), Double salt herbicidal ionic liquids (DSHILs), Glyphosate, Dicamba, MCPA, Herbicidal effectiveness, Dual action, Herbicide resistance

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INTRODUCTION Herbicides, widely used agrochemicals, are the chemical substances used to restrict weed growth. Significant attention to weed control has resulted in the development of herbicides that either are weed species specific (e.g., 4-chloro-2methylphenoxyacetic acid, MCPA) or have a broad spectrum herbicidal activity (e.g., 2-[(phosphonomethyl)amino]acetic acid, glyphosate). Major issues with common herbicides in their current form include their loss from the site of action by either physical drift, volatilization, or runoff due to high water solubility.1 To address these issues, we introduced an herbicidal ionic liquids (HILs, organic salts with melting point below 100 °C possessing an herbicidal anion) approach in 2011 that allowed transforming herbicides into their liquid salts and helped resolve some of the above-mentioned concerns to a major extent.2 Similarly to other biologically active ionic liquids (ILs), HILs were prepared based on the strategies and tools developed for active pharmaceutical ingredient ionic liquids (API-ILs).3 Initially, an “anticrystal engineering” approach was adapted, © 2017 American Chemical Society

where cations were chosen simply for frustration of crystallinity and liquefaction of the products. Later, the strategy evolved to control the physical properties of these salts through the choice of appropriate counterions (e.g., water solubility could be decreased by selection of hydrophobic ammonium cations). Finally, dual-active HILs were synthesized by choosing cations with secondary biological function (e.g., antimicrobial, plantgrowth regulation, or fungicidal activity) and pairing these with herbicidal anions.4−6 Using the approaches above, several new HILs with different herbicides were reported such as those with 2,4-dichlorophenoxyacetic acid (2,4-D),7,8 4-chloro-2-methylphenoxyacetic acid (MCPA),9,10 3-(4-chloro-2-methylphenoxy)propanoic acid (MCPP), 11 4-(4-chloro-2-methylphenoxy)butanoic acid (MCPB),12 3,6-dichloro-2-methoxybenzoic acid (dicamba),13,14 fomesafen,15 bentazon,16 3,6-dichloro-2-pyridinecarboxylic acid Received: April 19, 2017 Revised: May 21, 2017 Published: May 24, 2017 6261

DOI: 10.1021/acssuschemeng.7b01224 ACS Sustainable Chem. Eng. 2017, 5, 6261−6273

Research Article

ACS Sustainable Chemistry & Engineering

Scheme 1. Synthetic Scheme for Glyphosate-Based DSHILs Including the Chemical Structure of Anions and Cations Employed in This Study

(clopyralid),17 metsulfuron-methyl,18 and glyphosate.19 As the field of API-ILs advanced, new tools and strategies appeared such as the concept of oligomerization of ions20 that enabled the liquefaction of solid API-ILs simply by changing the stoichiometry of the synthetic precursors. These tools were also adapted to HILs, and examples have been reported with salts of dicamba and MCPA, where this strategy solved some issues of weed resistance and facilitated a higher herbicidal loading per cation.21,22 Recently, systems containing three or more ions have been reported in the API-IL field, where pure ion combinations with three or more unique ions have been termed “double salt ionic liquids” (DSILs).23 Inspired by this new tool, we initiated the preparation of DSILs from glyphosate, MCPA, and dicamba as model anionic herbicides. It is worth mentioning that upon formation of DSILs the identities of the individual constituents these salts are composed of are not preserved.23 (Thus, the obtained resultant compounds cannot be termed “mixtures” but new and unique compounds.) As a result, DSILs often possess unexpected physicochemical properties due to changes at the nanoscopic level and demonstrate nonideal behavior, which could provide different and tunable properties in DSILs.24 If applied to HILs, such systems would allow not only the abovementioned modification of physicochemical characteristics but also allow for management of widely distributed herbicideresistant weeds through applying DSILs (abbreviated here as DSHILs to emphasize their herbicidal nature) made of

herbicides with different modes-of-action currently fought using herbicidal mixtures.25 In this study, we hypothesized that the DSILs prepared from an EPSP inhibitor (glyphosate) and plant growth regulator rate intensifiers (dicamba or MCPA) paired with quaternary tetraalkylammonium/phosphonium cations would result in increased herbicidal efficiency of the DSHILs since they have two herbicides with different mechanisms of action. We also expected to find an influence on the physical properties such as volatility and solubility and on biodegradability. To the best of our knowledge, this is the first strategic report to enhance the efficacy of herbicidal anions through this double salt approach. We would like to note, though, that if successful the commercialization of DSILs would require additional studies by herbicides producers since these compounds need to be properly formulated and regulated before being sold. Indeed, this is extremely similar to recent FDA-posed questions where the treatment of multicomponent solids has already triggered concerns from the research community in the field of pharmaceutical cocrystals.26 The FDA has suggested that a cocrystal would require much less stringent testing than salts that have been classified as new compounds and thus require more extensive regulation.

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RESULTS AND DISCUSSION Synthesis. We have previously shown that a variety of HILs could be prepared using the dicamba, glyphosate, or MCPA herbicides by either metathesis of the corresponding

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ACS Sustainable Chemistry & Engineering Table 1. Prepared Glyphosate-Based DSILs #

Abbreviation

Cationa

Formula [Cat][Herb1]x[Herb2](1−x)b

Gly I mol % Activec

Dic II mol % Activec

MCPA III mol % Activec

Color

Physical State

Yield

1 2 3 4 5 6 7 8 9 10

A-I-II B-I-II C-I-II D-I-III E-I-III F-I-III B-I-III C-I-III B-I-III(H)e C-I-III(H)e

A B C D E F B C B C

[P66614][Gly]0.848[Dic]0.152 [Etq-O][Gly]0.848[Dic]0.152 [CET][Gly]0.848[Dic]0.152 [BA][Gly]0.848[MCPA]0.152 [Arq C/35][Gly]0.848[MCPA]0.152 [DDA][Gly]0.848[MCPA]0.152 [Etq-O][Gly]0.848[MCPA]0.152 [CET][Gly]0.848[MCPA]0.152 [Etq-O][Gly][MCPA-H]0.179 [CET][Gly][MCPA-H]0.179

42.4 42.4 42.4 42.4 42.4 42.4 42.4 42.4 45.9 45.9

7.6 7.6 7.6 − − − − − − −

− − − 7.6 7.6 7.6 7.6 7.6 8.2 8.2

Yellow Yellow White Yellow Yellow Colorless Yellow White Yellow White

Liquid Wax Solidd Liquid Wax Liquid Wax Solidd Wax Wax

95 97 97 95 96 97 96 97 95 98

a Cations: A - trihexyl(tetradecyl)phosphonium ([P66614]+), B - oleylmethylbis(2-hydroxyethyl)ammonium ([Etq-O]+), C - cetyltrimethylammonium ([CET]+), D - benzalkonium ([BA]+), E - cocotrimethylammonium ([Arq C/35]+), F - didecyldimethylammonium ([DDA]+). bFormula represents the mole ratio of the corresponding ion in the DSIL(1 Cation, 2 Herbicidal Anions). cActives: Gly I - Glyphosate or 2-[(phosphonomethyl)amino]acetic acid, Dic II - Dicamba or 3,6-dichloro-2-methoxybenzoic acid, MCPA III - 4-chloro-2-methylphenoxyacetic acid. dNoncrystalline low melting solid, as prepared, although crystallized in a DSC cell. ePresence of an extra amount of herbicide (MCPA, III) in DSHILs 9 and 10 is highlighted by a neutral MCPA-H.

Table 2. Thermal Properties of Synthesized DSHILs (1−10) #

DSHIL

Physical State

T5%a (oC)

T50%b (oC)

Tgc (oC)

Ts‑s (oC)

Tcrystd (oC)

Tme (oC)

1 2 3 4 5 6 7 8 9 10

A-I-II B-I-II C-I-II D-I-III E-I-III F-I-III B-I-III C-I-III B-I-III(H) C-I-III(H)

Liquid Wax Solid Liquid Wax Liquid Wax Solid Wax Wax

177 213 188 159 155 160 215 150 218 200

377 326 266 250 234 278 337 249 337 267

− − − −22 −31 −19 − − − −

− − 27, 49 (heatf) 20, 38 (coolg)

− − 77 − −21 − − 74 − −

− − 85 − −11 − − 83 − −

15, 40 (heat) 8, 36 (cool)

a

Decomposition temperature of 5% sample. bDecomposition temperature of 50% sample. cGlass transition temperature. dTemperature of crystallization. eMelting point. The DSC curves were recorded in the temperature range of −100 to 105 °C. fTransition seen on heating. gTransition seen on cooling.

bottom flask rather than an ion exchange column). In the second step, ammonium/phosphonium hydroxides [A− F][OH] underwent a direct acid−base reaction with herbicidal acids for which two methods have been used. In Method A, ammonium/phosphonium hydroxides were reacted with a mixture of herbicidal acids both added at once with the total molar amount of the acids being equal to that of the hydroxide salts resulting in formation of the herbicidal DSHILs 1−8. In Method B, the herbicidal acids were added consecutively, and an excess of the acids with respect to the desired stoichiometry was used resulting in formation of the herbicidal DSHILs 9 and 10 (Scheme 1). All of the glyphosate-based compounds 1−10 were obtained in high yields (95−98%). Among all synthesized compounds, two were solids (3 and 8), three were liquids (1, 4, and 6), and five others were waxes (2, 5, 7, 9, and 10; Table 1). The chemical structure of each compound was identified by 1H, 13C, and 31P nuclear magnetic resonance spectroscopy (NMR; SI). Thermal Properties. Thermal stability of the synthesized compounds was evaluated by thermogravimetric analysis (TGA) and the thermal behavior (melting points and thermal transitions) by differential scanning calorimetry (DSC); the data are presented in Table 2. Thermal stability was measured as both the temperature of onset to 5% sample decomposition (T5%) and 50% sample decomposition (T50%). Comparison of the thermal stabilities between all prepared compounds was

ammonium/phosphonium halides with sodium salts of the herbicides or acid−base reactions of corresponding ammonium/phosphonium hydroxides with the neutral herbicidal acids.13,14,19,21,22 These previously prepared compounds included quaternary protic and aprotic ammonium/phosphonium salts of herbicides, and we hypothesized that these cations would also be suitable for the combinations of ions selected in our work. To address the structural effect and size of anionic herbicides, we chose glyphosate (I, Scheme 1), dicamba (II, Scheme 1), and MCPA (III, Scheme 1) acidic herbicides, as herbicidal anion precursors. As cation precursors, long chain alkylated cations were chosen in order to impart hydrophobicity (i.e., to decrease the water solubility).27 In addition, the chosen cations possessed surfactant properties that would promote the absorption of the applied herbicides by the weeds.28 One phosphonium and five ammonium cations were used (Scheme 1), trihexyl(tetradecyl)phosphonium ([P 6 661 4 ] + , A), oleylmethylbis(2-hydroxyethyl)ammonium ([Etq-O]+, B), cetyltrimethylammonium ([CET]+, C), benzalkonium ([BA]+, D), cocotrimethylammonium ([Arq C/35]+, E), and didecyldimethylammonium ([DDA]+, F). All the HILs were prepared by two-step synthesis. In the first step, corresponding ammonium/phosphonium hydroxides were obtained from ammonium/phosphonium halides A−F utilizing a KOH ion-exchange reaction (carried out in a round6263


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ACS Sustainable Chemistry & Engineering Table 3. Solubility of DSHILs

a

#

DSHIL

Water

Methanol

DMSO

Acetonitrile

Acetone

Isopropanol

Ethyl acetate

Chloroform

Toluene

Hexane

1 2 3 4 5 6 7 8 9 10

A-I-II B-I-II C-I-II D-I-III E-I-III F-I-III B-I-III C−I−III B-I-III(H)c C-I-III(H)c

−a + + + − ± + + + +

+b + + + + + + + ± +

+ − − ± − + − − − −

±c − − − − ± − − − −

± − ± − − ± − ± − ±

± ± + ± ± − ± + − +

+ − − − − + − − − −

+ + + + + + + + + +

+ − − − − ± − − − −

± − − − − ± − − − −

Not soluble (−). bSoluble (+). cLimited solubility (±).

temperature range. (The presence of hydroxyethyl groups in the [Etq-O]+-based HILs (2, 7, and 9) presumably is the reason for the absence of any melting point or glass transition in the recorded temperature range. 31 ) Wax 5 ([Arq C/35][Gly]0.85[MCPA]0.15) did not show melting on the first heating cycle but, rather, featured a reversible, low-temperature glass transition at −31 °C, crystallization on heating from a supercooled phase at −21 °C, and a subsequent melting transition at −11 °C. The most interesting behavior was observed for solids both prepared using the [CET]+ ammonium cation, compounds 3 ([CET][Gly]0.85[Dic]0.15) and 8 ([CET][Gly]0.85[MCPA]0.15). For these compounds, there were no glass transition observed. Both have demonstrated melting, which was observed for each heating cycle (85 and 83 °C, respectively), crystallizing slowly on cooling into a poorly crystallizing solid (broad peak at 77 °C for 3 ([CET][Gly]0.85[Dic]0.15) and at 74 °C for 8 ([CET][Gly]0.85[MCPA]0.15). Interestingly, both liquids exhibited a solid−solid transition with disorder (low heat capacity), possibly because of the rearrangement of the long aliphatic chains in the molecules at 38 °C on heating for 3 and the similar temperature (36 °C) on cooling for 8. On further cooling, another such solid−solid transition into a more organized and ordered solid was observed with release of extra heat at 20 °C for 3 ([CET][Gly]0.85[Dic]0.15) and at 8 °C for for 8 ([CET][Gly]0.85[MCPA]0.15). Solid−solid transitions were also observed during heating at slightly higher temperature than during cooling: solid−solid transition from high degree of orderness to disorder at 27 and 15 °C for 3 and 8, respectively, and further transition of this disordered crystalline solids into poorly crystalline solids 49 and 40 °C for 3 and 8, respectively. These solids finally melted at 85 and 83 °C for 3 and 8, respectively. Corresponding monoanionic HIL, [CET][MCPA], possessed only a melting point at Tm= 57 °C, [CET][Dic] exhibited a melting point (Tm= 101 °C) and crystallization (Tcryst= 117 °C), while [CET][Gly] showed a melting point (Tm = 78 °C) and crystallization (Tcryst = 68 °C). Comparing results obtained for 3 and 8 with monoanionic salts, we can conclude that DSHILs 3 and 8 have no single salts as impurities due to no peaks observed corresponding to these salts. Solubility Studies. The solubility of the prepared DSHILs were investigated in 10 solvents chosen in descending order of the value of their Snyder polarity index: water 9.0, methanol 6.6, dimethyl sulfoxide (DMSO) 6.5, acetonitrile 6.2, acetone 5.1, ethyl acetate 4.3, isopropanol 4.3, chloroform 4.1, toluene 2.3, and hexane 0.0 (Table 3).32 A commonly employed solubility protocol was followed in this study,33 where typically

done through assessment of the onset of thermal decomposition for the first 5% weight loss (T5%) as providing a more accurate assessment of thermal stability than the 50% onset of thermal decomposition. The melting points and crystallization points are denoted as Tm, and Tcryst, respectively. The glass and solid−solid transitions are shown as Tg and Ts‑s, respectively. While glyphosate itself is a high-melting solid with Tm = 184.5 °C, among the glyphosate DSHILs we prepared, all synthesized compounds satisfied the definition of an “ionic liquid”, having a melting point below 100 °C. Here, DSHILs 1, 4, and 6 were liquids, compounds 3 and 8 were low melting noncrystalline solids (although they did crystallize in the DSC cell, see DSC description below), and the other five (2, 5, 7, 9, 10) were waxes where only 5 exhibited a glass transition, and the others showed none. Because all compounds 1−10 contain a glyphosate anion, we first compared their decomposition with glyphosate-free acid. The glyphosate-free acid has been reported to decompose in three steps, the first being the decomposition with melting at 230 °C.29 The commonly used sodium glyphosate salt Na[Gly] decomposes at 140 °C,30 while previously reported HILs on glyphosate19 had reported T5% of 155−210 °C. In the case of DSHILs, most exhibited a single-step decomposition, similar to previously reported HILs,19 higher than the glyphosate-free acid or its sodium salt; the lowest T5% being 155 °C for 5 and highest being 218 °C for 9. Among the ammonium salts, [EtqO]+ (B)-based DSHILs 2, 7, and 9 had the highest thermal stability (T5% of 215−218 °C). The DSHILs of [CET]+ (10) had higher thermal stability as compared to the one with equal ratio of cation to anion (8), although no such obvious effect was noticed for [Etq-O]+ cation-based HILs (7 and 9). The stability of glyphosate-based DSHILs containing either dicamba or MCPA was a function of cation; that is, the glyphosate− dicamba DSHILs seemed to be more stable thermally as compared to glyphosate−MCPA DSHILs with [CET]+ as the cation (3 and 8). The opposite trend was observed for [EtqO]+-based DSHILs (2 and 7). The DSC data (Table 2) provided some interesting behavior. Liquid 1 ([P66614][Gly]0.85[Dic]0.15), a phosphonium DSIL, showed no transitions in the investigated temperature range of −100 to 105 °C, while liquids 4 ([BA][Gly]0.85[MCPA]0.15) and 6 ([DDA][Gly]0.85[MCPA]0.15), both ammonium DSILs, demonstrated only glass transitions at −22 and −19 °C, respectively. Four waxy DSHILs, again all ammonium derivatives, 2 ([EtqO][Gly]0.85[Dic]0.15), 7 ([Etq-O][Gly]0.85[MCPA]0.15), 9 ([Etq-O][Gly][MCPA-H]0.18), and 10 ([CET][Gly][MCPAH]0.18), demonstrated no transitions in the investigated 6264


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ACS Sustainable Chemistry & Engineering 0.1 g sample of each DSHIL was added to a certain volume of solvent and the samples were vortexed at 25 °C. On the basis of the volume of solvent used, 3 types of behaviors were observed: “soluble” applies to those where 0.1 g of compound completely dissolved in 1 mL of solvent, “limited solubility” applies to those where 0.1 g of compound completely dissolved in 3 mL of solvent, and “not soluble” applies to the those where 0.1 g of compound did not completely dissolve in 3 mL of solvent. All DSHILs were soluble in methanol and chloroform. Except for the HILs 1 and 6, DSHILs 2−5 and 7−10 were insoluble in acetonitrile, ethyl acetate, toluene, and hexane. Compounds 1 and 5 were insoluble in water, though they were soluble in methanol. Salts of [Etq-O]+ bearing the hydroxyethyl moiety were expected to be soluble in polar protic solvents; however, the solubility in water of the DSHILs with hydrophobic cations (2, 3, 4, and 6) was unexpected. [CET]+-containing DSHILs (3, 8, and 10) were the only ILs to be soluble in isopropanol. No drastic influence on the solubility profile was observed depending on the cation−anion ratio (7−10), except that the [Etq-O]+ salts with nonstoichiometric anion compositions (7 and 9) exhibited decreased solubility in methanol and isopropanol. Greenhouse Tests. The efficacies of DSHILs 1−10 were evaluated in a greenhouse. Both monocots such as winter wheat (Triticum aestivum L.) and eudicots like common lambsquarters (Chenopodium album L.), white mustard (Sinapis alba L.), and common amaranth (Amaranthus retroflexus L.) were chosen as the test plant species in this study as they have different susceptibilities to glyphosate treatments. The herbicidal activities of the synthesized DSHILs were tested on the plants separated into three groups as follows: (1) test group: plants sprayed with aqueous solutions of DSHILs 1−10, (2) reference group: plants sprayed with commercially available glyphosate or mixtures of glyphosate with dicamba or MCPA, and (3) control group: plants have not been sprayed. Mixtures for spraying reference group plants were prepared taking into consideration molar ratio between herbicidal ions in DSHILs. That is, for example, for compound 1 described with formula [P66614][Gly]0.848[Dic]0.152, the same percent of active as found in DSHIL was followed in making corresponding mixtures (i.e, 42.4 mol % glyphosate active was mixed with 7.6 mol % dicamba active). In addition, the total amount of actives sprayed matched in the applied DSHIL and the corresponding reference herbicidal mixture. The efficacy data is expressed as percent fresh weight reduction compared to the control group (see Experimental Section for additional detail) and are shown in Figure 1. The glyphosate−dicamba DSHILs were evaluated separately from glyphosate−MCPA DSHILs. In Figure 1, the efficacy of DSHILs was compared to reference herbicides: I = glyphosate, I+II = glyphosate−dicamba mixtures, and I+III = glyphosate− MCPA mixtures. It should be noted that the herbicidal performance can vary notably depending on the species of the plant employed, and in a previous study, the efficacy of Roundup differed for cornflower and white mustard.19 Therefore, efficacy results on different plant species were color-coded and are shown as follows: black bars for common lambsquarters as tested plants, red bars for winter wheat as tested plants, green bars for common amaranth as tested plants, and yellow bars for white mustard as tested plants. In the case of common lambsquarters and winter wheat plants (black and red bars, respectively, in Figure 1A), a

Figure 1. Greenhouse efficacy of (A) glyphosate−dicamba-based DSHILs 1−3 compared to commercial formulations (I = glyphosate, I +II = glyphosate + dicamba) and (B) glyphosate−MCPA-based DSHILs 4−10 compared to commercial formulations (I = glyphosate, I+III is glyphosate + MCPA) on common lambsquarters (black bar), winter wheat (red bar), common amaranth (green bar), and white mustard (yellow bar). Please note that standard deviations (SD) are shown on the plots as error bars; however, to assess statistical significance, please consult Tables 4 and 5 that use the lettered designation (a, b, c, d). Those identified by the same letter indicate no statistical difference.

seemingly higher efficacy (measured as fresh weight reduction) was found for DSHILs, when compared to the reference. On common lambsquarters, glyphosate itself (I, Figure 1A) exhibited the poorest performance among all tested herbicidal treatments, although could afford up to 63% efficacy when applied in combination with dicamba (I+II, Figure 1A). This efficacy was substantially improved with DSHILs 1−3, and DSHIL 1 was the most effective (78% weight fresh reduction). The performance of DSHILs followed the order 1 > 3 > 2. On winter wheat, glyphosate either alone (I) or applied in combination with dicamba (I+II, Figure 1A) exhibited equal efficacy of ca. 80% which again significantly increased with DSHILs 1−3 (to ca. 90%), again with DSHIL 1 being the most effective. In this case, performance of DSHILs followed the order 1 > 3 ∼ 2. Contrarily, in the case of common amaranth and white mustard (green and yellow bars, respectively, in Figure 1A), DSHILs’ efficacies were comparable to those of the references. Similar observations were obtained when glyphosate−MCPA DSHILs were tested. Thus, on common lambsquarters, glyphosate itself (I) and glyphosate−MCPA reference (I+III) exhibited similar effectiveness but poor performance of as low as ca. 40% efficacy (Figure 1B). Use of DSHILs 4−10 improved efficacy substantially. This is seen especially in the case of DSHIL 4 that exhibited ca. 70% efficacy and DSHILs 9 and 10 that were only slightly worse (ca. 65%). We note here that [CET]+ and [Etq-O]+-based DSHILs having neutral MCPA in the composition (compounds 9 and 10) were highly effective against common lambsquarters. 6265


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ACS Sustainable Chemistry & Engineering On winter wheat, DSHILs 4−10 again seemed to possess improved performance, albeit the difference with reference herbicides was not as drastic as in the case of common labsquarters. Similarly to glyphosate−dicamba DSHILs 1−3, glyphosate−MCPA DSHILs 4−10 did not show significant differences with the references on common amaranth and white mustard, although demonstrated high herbicidal efficacy of 86− 91%, indicating species-specific selectivity. In order to comment on the real difference or superiority of the synthesized DSHILs in comparison to the commercial formulation or mixture of commercial formulations, statistical analysis for each of the greenhouse trials was conducted. Here, one-way analysis of variance (ANOVA) or the Kruskal−Wallis (K−W) rank sum test were used to determine significant differences in the mean effects between glyphosate−dicamba DSHILs 1−3 and glyphosate−MCPA 4−10 when compared to reference herbicides glyphosate (I), mixture of glyphosate with dicamba (I+II), and glyphosate with MCPA (I+III). The effect size was prominently expressed in displaying the p-value, and the results of the statistical tests are shown by marking the individual value of each treatment by an appropriate letter in Tables 4 and 5. The different letters indicate the significant differences in the statistical data, while the same letter shows no statistical difference between the treatments.

Table 5. Statistical Data To Assess the Influence of Glyphosate−MCPA-based DSHILs on the Fresh Weight of Tested Plantsa Species

Treatments

Winter wheat

Common amaranth

Common amaranth

White mustard

Treatment mean ± B gives lower and upper 95% confidence bounds (CI). The letters (a, b, etc.) here signify the statistical differences. The same letter in the column means no statistical differences. a

Species

Treatments

Winter wheat

General Analysis: Degrees of freedom (Df), the F value, and the significance value (p) Analysis ANOVA K−W K−W ANOVA Df 8; 27 8 8 8; 27 F/χ2 0.72 22.64 11.98 15.67 p-value 0.6760 0.0039 0.1523 0.0000 Mean Values for Treatments/Results of Post Hoc Tests (α = 0.05) General Mean 56 86 94 87 4 D-I-III 70 a 89 ab 91 a 74 c 5 E-I-III 56 a 86 bc 93 a 81 bc 6 F-I-III 45 a 89 ab 93 a 81 bc 7 B-I-III 48 a 89 ab 94 a 88 ab 8 C-I-III 61 a 89 ab 96 a 89 ab 9 B-I-III(H) 68 a 89 ab 96 a 88 a 10 C-I-III(H) 66 a 91 a 95 a 94 ab I Gly 44 a 79 cd 96 a 93 a I+III Gly+MCPA 45 a 68 d 93 a 93 a HSD for Treatments with Observed Statistical Differences ±B 26 3 HSD 8

Table 4. Statistical Data To Assess the Influence of Glyphosate−Dicamba-Based DSHILs on the Fresh Weight of Tested Plantsa Common lambsquarters

Common lambsquarters

White mustard

common lambsquarters or common amaranth. On white mustard, DSHILs 2 and 3 seemed to perform the same and DSHIL 1 performed worse than the reference with statistically different performance. For comparison of DSHILs with each other, post hoc comparisons using the Tukey HSD test were conducted. Here, we looked not at statistical significance but the results in terms of measures of magnitude−meaning, not only evaluation of treatment affecting plants but how much it affected them. Comparison of DSHILs with each other based on effectivity also depended on species used. First, the comparison was conducted between glyphosate−dicamba DSHILs 1−3 (Table 4) to determine their relative performance. The test indicated that the in case of common lambsquarters treated with 1, 2, 3, glyphosate I and glyphosate−dicamba mixture I+II the effectiveness of all treatments was statistically the same (1 ≈ 2 ≈ 3 ≈ I ≈ I+II); this might have been the effect of low performance of 2 which affected the mean value. The same picture was observed for common amaranth species. On winter wheat, effectiveness of treatment with 1 and 3 was significantly higher than that of either I or I+II. Although the performance of 2 was higher than in the case of I+II, the difference was not statistically significant, overall indicating the order of performance to be 1 ≈ 3 > 2 ≈ I ≈ I + II. For white mustard treated with 1, the performance was significantly lower than the effects of any other treatments, while treatments with 2, 3, glyphosate I, and glyphosate−dicamba mixture I+II were not significantly different, indicating the overall performance order of 1 < 2 ≈ 3 ≈ I ≈ I+II. Next, comparison was conducted for glyphosate−MCPA DSHILs 4−10 (Table 5). Similarly to glyphosate−dicamba DSHILs 1−3, the test indicated no statistical difference

General Analysis: Degrees of freedom (Df), the F value, and the significance value (p) Analysis ANOVA ANOVA K−W ANOVA Df 3; 16 3; 16 3 3; 16 F/χ2 0.66 9.83 6.63 9.41 p-value 0.6760 0.0004 0.1567 0.0005 Mean Values for Treatments/Results of Post Hoc Tests (α = 0.05) General Mean 62 86 95 91 1 A-I-II 78 a 93 a 96 a 84 b 2 B-I-II 59 a 89 ab 92 a 92 a 3 C-I-II 68 a 90 a 96 a 90 a I Gly 44 a 79 c 96 a 93 a I+II Gly+Dic 63 a 81 bc 96 a 94 a HSD for Treatments with Observed Statistical Differences ±B 32 4 3 HSD 8 6

Treatment mean ± B gives lower and upper 95% confidence bounds (CI). The letters (a, b, etc.) here signify the statistical differences. The same letter in the column means no statistical differences. a

Interestingly, there was no statistical difference between glyphosate (I) or its mixtures with dicamba (I+II), or MCPA (I +III). Regarding comparison of the references with DSHILs, statistically, for all greenhouse tests, the analyzed DSHILs demonstrated higher efficiency in plant reduction (Tables 4 and 5) on winter wheat. Here, statistical analysis indicated significant differences in the mean effects between glyphosate−dicamba DSHILs 1−3 and glyphosate−MCPA 4−10 when compared to reference herbicides. However, no statistical differences were seen between DSHILs and the references on 6266


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ACS Sustainable Chemistry & Engineering between the treatments in common lambsquarters and common amaranth species. The Dunn test used for the winter wheat trials indicated significant differences between 5 (the lowest performing DSHIL, not significantly different from the effect of references I and I+III) and 10 (highest performing DSHIL), while all other DSHILs performed statistically similar between each other, yet better than the reference I or I+III. For white mustard, the test did not indicate significant differences between 7, 8, 9, and 10, which performed better than statistically the same 4, 5, and 6, indicating the overall performance order of 4 ≈ 5 ≈ 6 < 7 ≈ 8 ≈ 9 ≈ 10. Overall, the efficacy of the synthesized DSHILs was either higher or similar as compared to the reference mixture of commercial formulations (same amount of active) depending on the species of the plant under study. The DSHILs prepared here were tailor-made to integrate the advantages of two different herbicides in one herbicidal IL to result in the enhancement of herbicidal efficacy and a decrease in resistivity by the weeds. The above results are interesting as the same performance can be achieved just by the right choice of cation and anion in the double salts. For instance, if any weed develops a resistance toward one herbicide, another herbicide can be used with identical efficacy and negligible resistance. Thus, the DSHILs demonstrated in this study exhibit a wide scope compared to the commercial formulation or previously known HILs which comprised a single herbicidal anion. Also, the efficacies of these DSHILs were observed to be higher than the single anion IL in recent reports employing MCPA, dicamba, and 2,4-D as anions, especially toward white mustard.13,14,19,21,34 Field Tests. Because in greenhouse tests comparison of DSHILs with the references demonstrated higher efficiency in plant reduction on winter wheat, the glyphosate−dicamba DSHILs 1, 2, and 3, and glyphosate−MCPA DSHILs 7, 9, and 10 (where DSHILs 9 and 10 contained excess of neutral MCPA) were investigated for their efficacy against winter wheat (Triticum aestivum L). In addition, field tests were done on poppy field (Papaver rhoeas L.) and cornflower (Centaurea cyanus L.) under field conditions. The efficacy was compared to the efficacy of glyphosate itself (I) under identical conditions. The efficacy against tested plants was evaluated 3 and 5 weeks after treatment (WAT), and the results are shown in Figure 2. Generally, it can be concluded that all the salts showed high herbicidal activity, especially 1, 9, and 10 (90− 100% for all weeds after 3 WAT, Figure 2A), with DSHILs performing worse on cornflower. DSHILs 1, 9, and 10 performed better than 2, 3, and 7, indicating that the efficacy of glyphosate-based DSHILs containing dicamba as secondary anion were less effective compared to the ones containing MCPA. Snapshots of the field trials on these weeds after 1, 2, and 4 weeks of application with different DSHILs and the control are provided in Figures 3 and 4, respectively. Biodegradability. From the efficacy data in the greenhouse test for the synthesized DSHILs, it was realized that [P66614]+ (A), [Etq-O]+ (B), and [CET]+ (C) cation-based HILs had maximum weed control efficacy. The seven DSHILs containing these cations (1, 2, 3, 7−10) were selected for assessment of biodegradability through ready biodegradability tests (RBTs). RBTs are laboratory-scale tests for evaluating the ready biodegradation of chemicals which were performed in accordance with guidelines of The Organisation for Economic Co-operation and Development (OECD) specifically designed for the testing of chemicals.35 OECD lists seven types of tests

Figure 2. Weed control (A) 3 weeks after treatment and (B) 5 weeks after treatment by glyphosate-based DSHILs compared to commercial glyphosate formulation on winter wheat (red bar), poppy field (blue bar), and cornflower (cyan bar).

Figure 3. Weed control (A) 1 week after application and (B) 2 weeks after application by glyphosate-based DSHILs.

Figure 4. Weed control by glyphosate-based DSHILs 4 weeks after application: (A) untreated, (B) 1, (C) 2, (D) 3, and (E) I (control).

for determining the ready biodegradability of chemical compounds (301A-F and 310), and we used the OECD 301F protocol. The protocol allows classification of the DSHILs as either readily biodegradable (RB), inherently biodegradable (IB), or not readily biodegradable (NRB) based on specific criteria.36 According to this regulation, a DSHIL can be classified as readily biodegradable if 10−60% of it degrades in a time frame of 10 days or less. 6267


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reached at the end of the test. We would like to note that the amount of produced CO2 was compared with a control experiment which was not treated with any DSHILs. The comparison of CO2 production curves for samples containing the studied DSHILs and the control revealed that the activity of the microorganisms was not completely inhibited (i.e., active microorganisms have been present in the sludge at the end of the test, and results are not due to low microorganisms amount). This indicates, that, interestingly, none of the DSHILs under study show any sign of biodegradation even after 28 days (Table 6). These results suggest that none of the studied

Glyphosate has a tendency to bind to the soil, and soil-bound glyphosate is difficult to biodegrade. Free glyphosate is degraded by soil microbes into stable aminomethylphosphonic acid that binds to the soil irreversibly.37 Although, initially Monsanto claimed Roundup (a glyphosate formulation) to be biodegradable, it was soon realized that glyphosate has poor biodegradability.38 Dicamba and dicamba-based HILs were also reported to exhibit poor biodegradability.22 For these compounds, degradation starts with ether cleavage resulting in 3,6-dichlorosalicylic acid, which further eliminates chlorine and converts into 6-chlorosalicylic acid that does not undergo further biodegradation, contributing to the poor biodegradability of dicamba.39 MCPA is biodegradable, where ether cleavage results in the first metabolite 4-chloro-2-methylphenol, which further degrades via ortho-ring opening to participate in the citric acid cycle.40 Above all, one point of immense importance is that the known degradation pathways are for neutral molecules rather than the ions as in HILs or DSHILs. Nevertheless, the presence of carbon-rich cations could be critical in driving the biodegradation of not readily biodegradable anions or metabolites. The cations, [P66614]+, [CET]+, and [Etq-O]+, all possess at least one long alkyl chain which makes them an attractive carbon source. Also, [Etq-O]+ has a hydroxyethyl moiety in the structural framework making it more accessible to the microbes. These ammonium cations undergo dealkylation to form N-alkylamine and subsequently ammonia. The alkyl groups of these cations are mineralized through a stepwise oxidation for longer alkyl chains.41 To investigate the effect of DSHILs on microorganisms that carry out the biodegradation process, aerobic biodegradability was examined in a screening test for ready biodegradability (in accordance to the OECD 301F protocol).35 In these screening tests, conducted under aerobic conditions, DSHILs 1, 2, 3, and 7−10 were used as test samples (see Experimental Section) under excessive (per amount of sample) theoretical oxygen demand (ThOD, a calculated amount of oxygen required to oxidize a compound to its final oxidation products) of 100 mg O2, and biodegradation was measured by evaluating amount of CO2 produced. The amount of CO2 produced is shown in Figure 5, where the course of degradation is displayed graphically and the timeline is indicated where applicable. As shown, reported is the CO2 emission against time, and no plateau (indicating ready biodegradation, no more CO2 produced) has been

Table 6. DSHILs Biodegradation−Cumulative CO2 Production #

DSHIL

Biodegradation after 28 days (%)a

Classification

1 2 3 7 8 9 10

A-I-II B-I-II C-I-II B-I-III C-I-III B-I-III(H) C-I-III(H)

0 0 0 0 0 0 0

NRBb NRB NRB NRB NRB NRB NRB

a

Aerobic biodegradability was examined in a screening test for ready biodegradability (in accordance to OECD 301F protocol).36 bNot readily biodegradable.

DSHILs can be classified as readily biodegradable, a contradiction to the theory of hydrophobicity and hydrophilicity.42 This study suggests that the biodegradability of glyphosate cannot be improved through the double salt approach. Nonetheless, the higher efficacy of the DSHILs implies that a lower dosage of glyphosate can be applied, and thereby, the negative environmental impact reduced. Among the studied HILs, [CET]+-based DSHILs (3 and 10) had the highest impact on the microorganisms. Additionally, when the samples were treated with 10 μL of 60% aqueous solution of sodium lactate after 640 h (indicated by the dashed dark blue line in Figure 5), a sharp increase in the amount of CO2 production in the control was noted. The absence of any such sharp increase in the amount of CO2 production for the samples containing DSHILs might indicate inhibition of microbial activity of activated sludge by DSHILs. Even though the HILs synthesized in the present study have a wide scope of herbicidal activity, the low biodegradability may restrict their potential applications. The low biodegradability could result in bioaccumulation in the environment or in food chains upon long-term use. Nevertheless, it should be noted that this protocol is based on the use of activated sludge and not microbes from environment. Microorganisms isolated from the environment have been known to biodegrade the HILs much more efficiently as compared to sludge microbes.43 Certainly, suitable microorganisms that may biodegrade these DSHILs need to be identified.

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CONCLUSIONS New herbicidal forms are a major challenge for formulation scientists, and various techniques have been used to decrease soil−water solubility, including physical and chemical modifications, salt formation, solid dispersion, use of surfactants, complexation, etc., all based on changing the existing materials properties or delivery conditions of a given herbicide. We

Figure 5. Mean CO2 production by activated sludge in samples containing DSHILs 1−3 and 7−10 and the control. DSHILs concentrations according to theoretical oxygen demand (ThOD) were equal to 100 mg O2. 6268


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16.82 MΩ·cm at 25 °C. Benzalkonium chloride (purity 95%) was obtained from Sigma-Aldrich (St. Louis, MO, USA). Cetyltrimethylammonium chloride (50% aqueous solution) was purchased from Stockmeier Chemia (Poznan, Poland), and cocotrimethylammonium chloride (35% aqueous solution) was purchased from Julius Hoesch Düren (Düren-Hoven, Germany). Oleylmethylbis(2-hydroxyethyl)ammonium chloride (Ethoquad O/12, purity 75%) and didecyldimethylammonium chloride (Arquad 2.10−50, purity 50%) were purchased from Akzo Nobel (Amsterdam, The Netherlands). Trihexyl(tetradecyl)phosphonium chloride was kindly donated by Cytec (Niagara Falls, ON, Canada). 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 purification. Glyphosate (purity 95%) was kindly supplied by Monsanto. MCPA (purity 96%) and Dicamba (purity 99%) were purchased from PESTINOVA (Jaworzno, Poland). Commercial herbicides used as the reference treatments: Roundup 360 SL (360 g of glyphosate per 1 L, Monsanto Europe S.A./N.V., Belgium), Chwastox Extra 300 SL (300 g MCPA per 1 L, CIECH Sarzyna S.A., Poland), and Dicamba 480 (480 g dicamba per 1 L, SL Sharda International Ltd., India). Syntheses of HILs. All HILs were prepared by a two-step synthesis, where the first step was the synthesis of the corresponding hydroxides and the second step was an acid−base reaction with the herbicide (or mixture of herbicides). General Synthesis (Method A) of DSHILs 1−8. In a roundbottomed flask containing a Teflon-coated magnetic stirring bar, 0.05 mol phosphonium or ammonium chloride was dissolved in 25 mL of anhydrous methanol followed by the addition of 0.05 mol potassium hydroxide dissolved in 25 mL of anhydrous methanol. The mixture was then stirred for 1 h at room temperature. The inorganic byproduct (potassium chloride) precipitated as a white solid and was carefully separated from solution by filtration into a round-bottomed flask containing a magnetic stir bar. To this filtrate, a stoichiometric amount of a mixture of glyphosate and dicamba (or MCPA) in a molar ratio of 5.6:1 was poured to neutralize the phosphonium or ammonium hydroxide and allowed to stir for another 1 h at room temperature. Then, the solvent was evaporated using a rotary evaporator, and the obtained products were washed with isopropanol and dried under vacuum at 50 °C for 24 h. Trihexyl(tetradecyl)phosphonium 2-[(phosphonomethyl)amino]acetate-3,6-dichloro-2-methoxybenzoate (A-I-II, 1). Obtained as a yellow liquid in 95% yield and 99% purity. 1H NMR (DMSO-d6, 298 K, 300 MHz), δ (ppm) 0.84 (t, J = 6.8 Hz, 12H), 1.21 (s, 48H), 1.38 (m, 2H), 2.17 (s, 8H), 2.85 (d, J = 12.3 Hz, 2H), 3.31 (s, 1H), 3.43 (m, 2H), 3.78 (s, 0.5H), 6,96 (d, J = 8.6, 0.2H), 7,08 (d, J = 8.6, 0.2H); 13 C NMR (DMSO-d6, 298 K, 75 MHz), δ (ppm) 168.3, 165.8, 151.5, 140.7, 127.8, 126.8, 125.3, 125.2, 61.0, 56.3, 51.2, 46.4, 44.7, 31.6, 30.7, 29.4, 29.0, 22.4, 22.1, 20.9, 18.2, 17.8, 14.1, 14.0, 13.8. Oleylmethylbis(2-hydroxyethyl)ammonium 2[(phosphonomethyl)amino]acetate-3,6-dichloro-2-methoxybenzoate (B-I-II, 2). Obtained as a yellow wax in 97% yield and 98% purity. 1H NMR (CDCl3, 298 K, 300 MHz), δ (ppm) 0.88 (t, J = 6.6 Hz, 3H), 1.27 (m, 22H), 1.66 (s, 2H), 2.00 (m, 4H), 3.23 (s, 3H), 3.36 (s, 2H), 3.60 (m, 4H), 3.90 (s, 0.5H), 4.01 (s, 4H), 5.34 (m, 2H), 6.72 (m, 3H), 6.99 (d, J = 8.6, 0.2 H), 7.09 (d, J = 8.6, 0.2H); 13C NMR (CDCl3, 298 K, 75 MHz), δ (ppm) 170.7, 168.9, 151.9, 138.9, 130.4, 129.9, 129.5, 127.8, 126.2, 125.4, 64.2, 63.0, 61.7, 55.5, 50.2, 45.8, 44.0, 32.6, 31.8, 29.7, 29.3, 27.2, 26.5, 22.6, 14.0; 31P NMR (CDCl3, 298 K, 121 MHz), δ (ppm) 9.4 (s). Cetyltrimethylammonium 2-[(phosphonomethyl)amino]acetate3,6-dichloro-2-methoxybenzoate (C-I-II, 3). Obtained as a white solid in 97% yield and 98% purity. 1H NMR (CDCl3, 298 K, 300 MHz), δ (ppm) 0.81 (t, J = 6.5 Hz, 3H), 1.18 (m, 26H), 1.62 (m, 2H), 2.10 (s, 2H), 3.25 (m, 9H), 3.34 (s, 2H), 3.86 (s, 0.5H), 6.92 (d, J = 6.5 Hz, 0.2H), 7.03 (d, J = 6.5 Hz, 0.2H); 13C NMR (CDCl3, 298 K, 75 MHz), δ (ppm) 168.2, 151.6, 139.9, 127.7, 127.3, 126.0, 125.4, 66.6, 61.5, 53.0, 49.5, 45.3, 31.8, 29.6, 29.2, 26.1, 23.0, 22.5, 14.0. Benzalkonium 2-[(phosphonomethyl)amino]acetate-4-chloro-2methylphenoxyacetate (D-I-III, 4). Obtained as a yellow liquid in 95%

propose that the agricultural industry explore pure liquid rather than solid forms of herbicides, new materials that could give rise to new active formulations and to new delivery methodologies which might not be possible for solids. In this work, we have demonstrated the synthesis and application of glyphosate-based DSHILs incorporating dicamba or MCPA as secondary anions. Their physical properties (thermal behavior, solubility) and herbicidal efficacy, as well as biodegradability, were studied. Notable high thermal stabilities and controlled solubilities were realized for all the DSHILs compared to glyphosate-free acid itself or in mixture with other herbicides. The DSHILs demonstrated better efficacy under both greenhouse and field trials on some of the tested plant species, while they were similar to standard herbicides on other test species. According to statistical analysis, for all greenhouse tests, the analyzed DSHILs demonstrated higher efficiency in plant reduction on winter wheat. Here, effectiveness of treatment with containing both glyphosate and dicamba was significantly higher than that of either commercial formulation (glyphosate, dicamba, or mixture of these). However, no statistical differences were seen between DSHILs and “references” on common lambsquarters or common amaranth. For MCPAcontaining glyphosate salts, on white mustard, the test again indicated significant differences, but no statistical difference was found between the treatments in common lambsquarters and common amaranth species. The efficacies of these DSHILs were observed to be higher than the single anion IL in recent reports employing MCPA, dicamba, and 2,4-D as anions, especially toward white mustard. Overall, the efficacy of the synthesized DSHILs was either higher or similar as compared to the reference mixture of commercial formulations (same amount of active) depending on the species of the plant under study. In field tests, the salts showed high herbicidal activity on winter wheat (Triticum aestivum L) and on poppy field (Papaver rhoeas L.), although with DSHILs performing worse on cornflower. The efficacy of dicamba-containing salts was less effective compared to the MCPA-containing ones. The DSHILs based on [CET]+ and [Etq-O]+ had better herbicidal efficacy for all plant species. In general, [CET]+ salts exhibited an optimum herbicidal activity with the glyphosate− dicamba anion, whereas [Etq-O]+ salts performed better with glyphosate and neutral MCPA. Although the DSHILs were found to be not readily biodegradable, improved herbicidal activity, stability, and solubility are useful aspects. In addition, development of herbicides that result in combined activity of glyphosate and synthetic auxin inhibitors could be used for herbicide-resistant crop management, allowing a greater emphasis on long-term sustainable practices than other techniques provide (e.g., entirely new herbicides, spraying crops with glyphosate, followed by another herbicide soon after, or the mixing and rotating of herbicides). Finally, the concept of double salts will result in synthesis of novel HILs providing a wider scope for fine-tuning of the properties that may enhance the biodegradability and with the adequate choice of ions can result in multiple functionality contributing to overall lower environmental impact.

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EXPERIMENTAL SECTION

Materials. All materials were used as supplied unless otherwise noted. Deionized (DI) water was obtained from a commercial deionizer (Culligan, Northbrook, IL, USA) with specific resistivity of 6269


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Table 7. Amounts of DSILs and Commercial Formulations (or Mixture of Commercial Formulations) Sprayed #

Abbrev.

Formula, [Cat][Herb1]x[Herb2](1−x)a

MW,g/mol

DSIL, g/ha

Gly I, g/hab

Dic II, g/ha

MCPA III, g/ha

1 2 3 4 5 6 7 8 9 10

A-I-II B-I-II C-I-II D-I-III E-I-III F-I-III B-I-III C-I-III B-I-III(H) C-I-III(H)

[P66614][Gly]0.848[Dic]0.152 [Etq-O][Gly]0.848[Dic]0.152 [CET][Gly]0.848[Dic]0.152 [BA][Gly]0.848[MCPA]0.152 [Arq C/35][Gly]0.848[MCPA]0.152 [DDA][Gly]0.848[MCPA]0.152 [Etq-O][Gly]0.848[MCPA]0.152 [CET][Gly]0.848[MCPA]0.152 [Etq-O][Gly][MCPA-H]0.179 [CET][Gly][MCPA-H]0.179

659.9 546.6 460.5 463.4 485.5 499.5 543.5 457.4 574.4 488.3

1658 1373 1157 1164 1220 1255 1365 1149 1219 1037

360 360 360 360 360 360 360 360 360 360

84 84 84 − − − − − − −

− − 76 76 76 76 76 76 76

a

In greenhouse tests, the applications of DSHILs were applied as 2.5 mol/ha aqueous solutions of compounds under study using a spray chamber with a TeeJet 1102 flat-fan nozzle delivering 200 L/ha of spray solution at 0.2 MPa operating pressure. In field tests, all treatments were applied as aqueous solutions of compounds under study using a knapsack sprayer with AXR 100/03 flat-fan nozzles delivering 200 L/ha of spray solution at 0.3 MPa of operating pressure. yield and 99% purity. 1H NMR (CDCl3, 298 K, 400 MHz), δ (ppm) 0.88 (t, J = 6.9 Hz, 3H), 1.26 (s, 18H), 1.45 (s, 0.4H), 1.65 (s, 2H), 2.21 (s, 0.5H), 2.81 (d, J = 11.5 Hz, 2H), 3.11 (m, 2H), 3.16 (s, 6H), 3.57 (s, 2H), 4.74 (s, 0.4H), 5.99 (s, 3H), 6.75 (d, J = 8.8, 0.2 H), 6.92 (dd, J1,2 = 8.8, J1,3 = 2.7, 0.2H), 6.98 (d, J = 2.0, 0.2H), 7.36 (m, 3H), 7.54 (2H); 13C NMR (CDCl3, 298 K, 75 MHz), δ (ppm) 172.8, 170.0, 156.1, 132.9, 130.0, 129.6, 128.8, 128.1, 127.8, 125.8, 123.7, 112.6, 68.7, 67.0, 62.2, 59.7, 50.0, 49.3, 45.3, 40.3, 31.7, 29.4, 29.3, 29.1, 29.0, 27.5, 27.3, 26.1, 22.4, 16.2, 13.9; 31P NMR (CDCl3, 298 K, 121 MHz), δ (ppm) 8.2 (s). Cocotrimethylammonium 2-[(phosphonomethyl)amino]acetate4-chloro-2-methylphenoxyacetate (E-I-III, 5). Obtained as a yellow wax in 96% yield and 99% purity. 1H NMR (CDCl3, 298 K, 400 MHz), δ (ppm) 0.88 (t, J = 6.5 Hz, 3H), 1.26 (s, 20H), 1.66 (s, 2H), 2.22 (s, 0.5H), 3.14 (s, 2H), 3.25 (m, 9H), 3.32 (s, 2H), 3.62 (s, 2H), 4.40 (s, 0.4H), 6.72 (m, 0.2H), 7.03 (m, 0.4H); 13C NMR (CDCl3, 298 K, 75 MHz), δ (ppm) 172.6, 169.8, 156.0, 129.8, 128.2, 125.9, 124.0, 112.6, 68.3, 66.3, 63.5, 63.3, 52.9, 31.7, 29.4, 29.1, 26.1, 25.7, 22.6, 22.5, 16.2, 13.9; 31P NMR (CDCl3, 298 K, 121 MHz), δ (ppm) 8.7 (s). Didecyldimethylammonium 2-[(phosphonomethyl)amino]acetate-4-chloro-2-methylphenoxyacetate (F-I-III, 6). Obtained as a colorless liquid in 97% yield and 98% purity. 1H NMR (CDCl3, 298 K, 400 MHz), δ (ppm) 0.88 (t, J = 6.5 Hz, 6H), 1.26 (s, 28H), 1.63 (s, 4H), 2.24 (s, 0.5H), 3.10 (d, J = 6.5 Hz, 2H), 3.24 (m, 9H), 3.37 (s, 2H), 3.57 (s, 2H), 4.42 (s, 0.4H), 6.75 (m, 0.2H), 7.04 (m, 0.4H); 13C NMR (CDCl3, 298 K, 75 MHz), δ (ppm) 173.0, 169.6, 156.1, 129.7, 128.3, 125.9, 123.9, 112.7, 68.5, 62.7, 51.6, 49.5, 31.7, 29.3, 29.1, 29.0, 26.1, 25.2, 22.5, 16.3, 13.9; 31P NMR (CDCl3, 298 K, 121 MHz), δ (ppm) 8.5 (s). Oleylmethylbis(2-hydroxyethyl)ammonium 2[(phosphonomethyl)amino]acetate-4-chloro-2-methylphenoxyacetate (B-I-III, 7). Obtained as a yellow wax in 96% yield and 99% purity. 1 H NMR (CDCl3, 298 K, 400 MHz), δ (ppm) 0.88 (t, J = 6.7 Hz, 3H), 1.27 (m, 22H), 1.66 (s, 2H), 2.00 (m, 4H), 2.23 (s, 0.5H), 3.22 (s, 3H), 3.38 (s, 2H), 3.55 (s, 2H), 3.63 (m, 4H), 4.00 (s, 4H), 4.38 (s, 0.4H), 5.34 (m, 2H), 6.70 (d, J = 8.7, 0.2H), 6.76 (m, 3H), 7.01 (d, J = 2.4, 0.2H), 7.05 (m, 0.2H); 13C NMR (CDCl3, 298 K, 75 MHz), δ (ppm) 173.8, 170.6, 155.9, 130.5, 130.4, 130.0, 129.9, 129.5, 128.6, 126.0, 124.3, 112.6, 68.3, 64.2, 63.0, 55.5, 50.2, 49.9, 32.6, 31.8, 29.7, 29.5, 29.4, 27.2, 26.4, 22.6, 16.3, 14.0; 31P NMR (CDCl3, 298 K, 121 MHz), δ (ppm) 9.1 (s). Cetyltrimethylammonium 2-[(phosphonomethyl)amino]acetate4-chloro-2-methylphenoxyacetate (C-I-III, 8). Obtained as a white solid in 97% yield and 98% purity. 1H NMR (CDCl3, 298 K, 300 MHz), δ (ppm) 0.88 (t, J = 6.5 Hz, 3H), 1.26 (m, 26H), 1.65 (m, 2H), 2.22 (s, 0.5H), 2.96 (m, 2H), 3.24 (s, 9H), 3.39 (s, 2H), 3.57 (s, 2H), 4.38 (s, J = 6.5 Hz, 0.4H), 6.74 (d, J = 6.5 Hz, 0.2H), 7.03 (d, J = 6.5 Hz, 0.4H); 13C NMR (CDCl3, 298 K, 75 MHz), δ (ppm) 172.8, 170.9, 156.2, 129.9, 128.3, 126.0, 124.0, 112.7, 68.7, 66.4, 53.0, 49.3, 45.4,

31.8, 29.6, 29.2, 26.2, 25.2, 22.6, 16.3, 14.0; 31P NMR (CDCl3, 298 K, 121 MHz), δ (ppm) 8.7 (s). General synthesis (Method B) of DSHILs 9 and 10. Typically, in a round-bottomed flask containing a Teflon-coated magnetic stirring bar, 0.05 mol ammonium chloride was dissolved in 25 mL of anhydrous methanol followed by the addition of 0.05 mol potassium hydroxide dissolved in 25 mL of anhydrous methanol. The mixture was then stirred for 1 h at room temperature. The inorganic byproduct (potassium chloride) precipitated as a white solid and was carefully separated from solution by filtration into a round-bottomed flask containing a magnetic stir bar. To this filtrate, a stoichiometric amount of glyphosate was poured to neutralize the ammonium hydroxide and allowed to stir for another 1 h at room temperature. Next, 0.0089 mol of MCPA was added into the flask (molar ratio glyphosate:MCPA = 5.6:1). Then, the solvent was evaporated using a rotary evaporator, and the obtained products were washed with isopropanol and dried under vacuum at 50 °C for 24 h. Oleylmethylbis(2-hydroxyethyl)ammonium 2[(phosphonomethyl)amino]acetate-4-chloro-2-methylphenoxyacetic acid (B-I-III(H), 9). Obtained as a yellow wax in 95% yield and 98% purity. 1H NMR (CDCl3, 298 K, 300 MHz), δ (ppm) 0.88 (t, J = 6.5 Hz, 3H), 1.27 (m, 22H), 1.66 (s, 2H), 2.00 (m, 4H), 2.23 (s, 0.5H), 3.22 (s, 3H), 3.39 (s, 2H), 3.59 (s, 4H), 3.80 (m, 2H), 4.00 (s, 4H), 4.43 (s, 0.4 H), 5.34 (m, 2H), 6.71 (d, J = 9.4, 0.2H), 7.06 (m, 0.4 H), 7.83 (s, 3H); 13C NMR (CDCl3, 298 K, 75 MHz), δ (ppm) 173.3, 170.6, 155.7, 130.5, 130.4, 130.3, 130.1, 130.0, 129.9, 129.5, 128.7, 126.1, 124.7, 112.7, 67.7, 64.2, 63.0, 55.5, 50.2, 45.7, 43.9, 32.6, 31.8, 29.7, 29.5, 29.2, 27.2, 26.5, 22.6, 16.3, 14.0. Cetyltrimethylammonium 2-[(phosphonomethyl)amino]acetate4-chloro-2-methylphenoxyacetic acid (C-I-III(H), 10). Obtained as a white wax in 98% yield and 99% purity. 1H NMR (CDCl3, 298 K, 300 MHz), δ (ppm) 0.88 (t, J = 6.5 Hz, 3H), 1.26 (m, 26H), 1.64 (m, 2H), 2.22 (s, 0.5H), 3.05 (m, 2H), 3.22 (s, 9H), 3.36 (s, 2H), 3.62 (s, 2H), 4.38 (s, J = 6.5 Hz, 0.4H), 6.72 (d, J = 6.5 Hz, 0.2H), 7.03 (d, J = 6.5 Hz, 0.4H); 13C NMR (CDCl3, 298 K, 75 MHz), δ (ppm) 172.9, 170.8, 156.2, 129.9, 128.3, 126.0, 124.0, 112.7, 68.7, 66.5, 53.0, 49.7, 45.4, 31.8, 29.7, 29.3, 29.2, 26.3, 23.0, 22.6, 16.3, 14.0. Methods. Nuclear Magnetic Resonance (NMR) Spectroscopy. All spectra were recorded using CDCl3 as the lock solvent and TMS as an internal standard. 1H NMRs were recorded using a Mercury Gemini 300 spectrometer operating at 300 MHz or a Mercury Gemini 400 spectrometer operating at 400 MHz. 13C NMR and 31P NMR spectra were also obtained using a Mercury Gemini 300 spectrometer at 75 and 121 MHz, respectively. Thermal Gravimetric analysis (TGA). TGA was performed using a Mettler Toledo Stare TGA/DSC1 unit (Leicester, UK) 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. Differential Scanning Calorimetry (DSC). Thermal transition temperatures were determined by DSC using a Mettler Toledo Stare DSC1 (Leicester, UK) unit under nitrogen. Samples between 5 and 15 6270


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ACS Sustainable Chemistry & Engineering mg were placed in aluminum pans and heated from 25 to 105 °C at a heating rate of 10 °C/min and cooled with an intracooler at a cooling rate of 10 °C/min to −100 °C and then heated again to 105 °C. Greenhouse Tests. Four plant species, namely, winter wheat (Triticum aestivum L.) and eudicots like common lambsquarters (Chenopodium album L.), white mustard (Sinapis alba L.), and common amaranth (Amaranthus retroflexus L.), were chosen to test the herbicidal efficacy under greenhouse conditions. The plants were grown in 0.5 L plastic pots containing commercial peat-based potting material. The plants were thinned to four per pot within 14 days after emergence and watered as needed. Greenhouse temperature was 20 ± 2 °C, humidity was 60%, and a photoperiod of 16/8 day/night hours was used. The applications of HILs were made using a spray chamber with a TeeJet 1102 flat-fan nozzle delivering 200 L/ha of spray solution at 0.2 MPa operating pressure when plants were at six-leaf growth stage. The sprayer was moving above the plants at a constant speed of 3.7 m/s. The distance between the nozzle and target plants was 40 cm. The plants were treated once with aqueous solutions of the compounds under study. The test plants were categorized into three groups: (a) plants treated with an aqueous solution of DSHILs 1−10, (b) plants treated with commercial formulations or mixture of commercial formulations (reference), and (c) nontreated plants (control). The plants were treated under identical conditions of spraying speed or nozzle distance from the top of plant. The concentration of commercial formulation (or mixture of commercial formulations) were equalized to correspond molar ratio between herbicidal ions in DSHILs. That is, for example, for compound 1 described with formula [P66614][Gly]0.848[Dic]0.152, the same percent of active as found in DSHIL was followed in making corresponding mixtures (i.e., 42.4% glyphosate active was mixed with 7.6% dicamba active). In addition, the total amount of actives sprayed matched in applied DSHIL and the corresponding reference herbicidal mixture. Table 7 presents the amounts of commercial formulation (or mixture of commercial formulations) sprayed. Fresh weight of plants was determined separately from each replication three weeks after treatment (WAT). The efficacy data is expressed as percent fresh weight reduction compared with control (nontreated plants). The experiments were arranged in a completely randomized setup with four replications. The herbicidal activity of the synthesized HILs (1−10) were compared to the commercial formulation of glyphosate (Roundup 360 SL) or the mixture of herbicides, glyphosate + dicamba (Roundup 360 SL + Dicamba 480 SL) or glyphosate + MCPA (Roundup 360 SL + Chwastox Extra 300 SL). Doses of active ingredients were glyphosate − 360 g/ha, MCPA − 73.9 g/ha, and dicamba − 83.7 g/ha. Field Tests. The field trial was conducted in 2016 in Winna Gora (western part of Poland, 52.04° N, 19.39° E). Three plant species were chosen to express the herbicidal efficacy of tested HILs: winter wheat (Triticum aestivum L), poppy field (Papaver rhoeas L.), and cornflower (Centaurea cyanus L.). The individual plot size was equal to 10 m2. All treatments were applied as aqueous solutions of compounds under study using a knapsack sprayer with AXR 100/03 flat-fan nozzles (TeeJet Technologies, Wheaton, IL, USA) delivering 200 L ha−1 of spray solution at 0.3 MPa of operating pressure; the concentrations of active while preparing commercial formulations or mixture of commercial formulations matched the molar ratio between herbicidal ions in DSHILs, similar to greenhouse tests. During application, the plants were at the following growth stage: winter wheat − BBCH 32 (stem elongation with two visibly extended internodes), poppy field − BBCH 32, and cornflower − BBCH 33 (stem elongation with three visibly extended internodes). The control treatments were not sprayed plants. The efficacy of all treatments was evaluated visually using a scale of 0 (no effect) to 100% (complete weed destruction). The herbicidal activity of the DSHILs were compared to the commercial formulation of glyphosate used at the dose of 360 g/ha (Roundup 360 SL). The experimental design was based on a randomized block with four replications. Each error margin range represents standard errors of the mean (SEM). The SEM values were calculated according to following equation:

SEM =

s n0.5

where SEM is the standard error of the mean, s is the sample standard deviation, and n is the number of samples. Statistics. In order to study the efficacy of the applied herbicides against weed species, statistical analysis for each of the greenhouse trials was carried out. Each experiment was performed in completely randomized design with r = 4 replications. In the first experiment v = 5 (1, 2, 3, I, and I+II) treatments were used, while the second experiment was of v = 9 (4, 5, 6, 7, 8, 9, 10, I, and I+III). First, in each case, the Pearson chi-square test was used to verify the null hypothesis that the population is normally distributed, and the Fligner−Killeen test of homogeneity of variances was used to test the null hypothesis that the variances for each treatment in one trial are the same.44,45 When the p-value obtained in both tests was higher than chosen significance level (α = 0.01), a one-way analysis of variance (ANOVA) was applied to study differences between studied treatments; in other case nonparametric Kruskal−Wallis (K−W) rank sum test was used (Tables 4 and 5). If the null hypothesis of ANOVA or K−W test was rejected, then, to compare the studied treatments, a multiple Tukey’s post hoc test or nonparametric pairwise multiple comparisons procedure based on Dunn rank sums test was applied. The results of the statistical tests are shown by marking the individual value of each treatment by an appropriate letter. The program package R (3.2.4) was used for the calculations (R Development Core Team, 2013).46 Biodegradability Tests. The tests were performed according to the OECD guideline for the 301F test. The oxygen uptake and CO2 production were determined every 5 h for 28 days using the MicroOxymax Respirometer (Columbus Instruments, Columbus, USA). The activated sludge was collected from a local municipal wastewater treatment plant (Czajka, Warsaw, Poland). Prior to use, the activated sludge was aerated for 7 days in a mineral medium which was also used for subsequent tests. The mineral medium consisted of KH2PO4 − 85 mg/L, K2HPO4 − 220 mg/L, Na2HPO4·2H2O − 220 mg/L, NH4Cl− 17 mg/L, CaCl2·2H2O − 37 mg/L, MgSO4·2H2O − 23 mg/L, and FeCl3− 0.25 mg/L. The measured pH was at 7.2. The test was performed in brown glass bottles (100 mL Simax) containing mineral medium, innoculum (cell density at ca. 106 cells/mL determined with plastic Paddle Tester for aerobic bacteria, Hach, Loveland, USA), and tested DSHILs at a concentration of ca. 10−30 mg/L, which was equal to 100 mg/L of theoretical oxygen demand (ThOD, calculated based on equation CcHhNnOoPp). 1 5 16⎡⎣2c + 2 (h − 3n) + 2 p − o⎤⎦ ⎡ mgO ⎤ 2 ⎥ ⎢ ThOD = M ⎣ mg ⎦

where c, h, n, o, and p represent the number of atoms of a given element in the chemical formula of the tested DSHIL, and M is the molar mass of the tested DSHIL [mmol/mg]. Allylthiourea (1.16 mg/L) was added to inhibit nitrification. All samples were analyzed in triplicate together with controls (sodium benzoate without inoculum, tested substances without inoculum) and blanks (medium and inoculum without tested substances). Gas tight flasks were connected with a respirometric system and incubated in the dark at 22 °C for 28 days. The CO2 sensor range spanned the volume of 0−15%. Sensitivity of the respirometer was 0.2 μL/h. The biodegradation efficiency was calculated based on the oxygen uptake in each bottle (measured automatically by the Micro-Oxymax) and corrected for the oxygen demand of the blank with respect to the theoretical oxygen demand and the amount of DSHILs tested.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01224. 1 H, 13C, and 31P spectra of all compounds. (PDF) 6271


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(11) Pernak, J.; Czerniak, K.; Niemczak, M.; Chrzanowski, Ł.; Ławniczak, Ł.; Fochtman, P.; Marcinkowska, K.; Praczyk, T. Herbicidal ionic liquids based on esterquats. New J. Chem. 2015, 39 (7), 5715− 5724. (12) Pernak, J.; Niemczak, M.; Materna, K.; Zelechowski, K.; Marcinkowska, K.; Praczyk, T. Synthesis, properties and evaluation of biological activity of herbicidal ionic liquids with 4-(4-chloro-2methylphenoxy)butanoate anion. RSC Adv. 2016, 6 (9), 7330−7338. (13) Cojocaru, O. A.; Shamshina, J. L.; Gurau, G.; Syguda, A.; Praczyk, T.; Pernak, J.; Rogers, R. D. Ionic liquid forms of the herbicide dicamba with increased efficacy and reduced volatility. Green Chem. 2013, 15 (8), 2110−2120. (14) Pernak, J.; Giszter, R.; Biedziak, A.; Niemczak, M.; Olszewski, R.; Marcinkowska, K.; Praczyk, T. Alkyl(C16, C18, C22)trimethylammonium-based herbicidal ionic liquids. J. Agric. Food Chem. 2017, 65 (2), 260−269. (15) Ding, G.; Liu, Y.; Wang, B.; Punyapitak, D.; Guo, M.; Duan, Y.; Li, J.; Cao, Y. Preparation and characterization of fomesafen ionic liquids for reducing the risk to the aquatic environment. New J. Chem. 2014, 38 (11), 5590−5596. (16) Wang, B.; Ding, G.; Zhu, J.; Zhang, W.; Guo, M.; Geng, Q.; Guo, D.; Cao, Y. Development of novel ionic liquids based on bentazone. Tetrahedron 2015, 71 (41), 7860−7864. (17) Zhu, J.; Ding, G.; Liu, Y.; Wang, B.; Zhang, W.; Guo, M.; Geng, Q.; Cao, Y. Ionic liquid forms of clopyralid with increased efficacy against weeds and reduced leaching from soils. Chem. Eng. J. 2015, 279, 472−477. (18) Pernak, J.; Niemczak, M.; Shamshina, J. L.; Gurau, G.; Głowacki, G.; Praczyk, T.; Marcinkowska, K.; Rogers, R. D. Metsulfuron-methylbased herbicidal ionic liquids. J. Agric. Food Chem. 2015, 63 (13), 3357−3366. (19) Pernak, J.; Niemczak, M.; Giszter, R.; Shamshina, J. L.; Gurau, G.; Cojocaru, O. A.; Praczyk, T.; Marcinkowska, K.; Rogers, R. D. Glyphosate-based herbicidal ionic liquids with increased efficacy. ACS Sustainable Chem. Eng. 2014, 2 (12), 2845−2851. (20) Bica, K.; Rogers, R. D. Confused ionic liquid ions-a “liquification” and dosage strategy for pharmaceutically active salts. Chem. Commun. 2010, 46 (8), 1215−1217. (21) Pernak, J.; Niemczak, M.; Chrzanowski, Ł.; Ławniczak, Ł.; Fochtman, P.; Marcinkowska, K.; Praczyk, T. Betaine and carnitine derivative as herbicidal ionic liquids. Chem. - Eur. J. 2016, 22 (34), 12012−12021. (22) Ławniczak, Ł.; Syguda, A.; Borkowski, A.; Cyplik, P.; Marcinkowska, K.; Wolko, Ł.; Praczyk, T.; Chrzanowski, Ł.; Pernak, J. Influence of oligomeric herbicidal ionic liquids with MCPA and dicamba anions on the community structure of autochthonic bacteria present in agricultural soil. Sci. Total Environ. 2016, 563−564, 247− 255. (23) Chatel, G.; Pereira, J. F. B.; Debbeti, V.; Wang, H.; Rogers, R. D. Mixing ionic liquids − “simple mixtures” or “double salts”? Green Chem. 2014, 16 (4), 2051−2083. (24) Atilhan, M.; Anaya, B.; Ullah, R.; Costa, L. T.; Aparicio, S. Double salt ionic liquids based on ammonium cations and their application for CO2 capture. J. Phys. Chem. C 2016, 120, 17829− 17844. (25) Lagator, M.; Vogwill, T.; Mead, A.; Colegrave, N.; Neve, P. Herbicide mixtures at high doses slow the evolution of resistance in experimentally evolving populations of Chlamydomonas reinhardtii. New Phytol. 2013, 198 (3), 938−945. (26) Kelley, S. P.; Narita, A.; Holbrey, J. D.; Green, K. D.; Reichert, W. M.; Rogers, R. D. Understanding the effects of ionicity in salts, solvates, co-crystals, ionic co-crystals, and ionic liquids, rather than nomenclature, is critical to understanding their behavior,. Cryst. Growth Des. 2013, 13, 965−97. (27) Singer, E. J. Biological Evaluation. In Cationic Surfactants: Analytical and Biological Evaluation; Cross, J., Singer, E. J., Eds.; CRC Press: New York, 1994; pp 29−136.

AUTHOR INFORMATION

Corresponding Authors

*Juliusz Pernak. E-mail: [email protected]. *Robin D. Rogers. E-mail: [email protected]. ORCID

Michał Niemczak: 0000-0002-4364-8267 Robin D. Rogers: 0000-0001-9843-7494 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare the following competing financial interest(s): R.D.R. has partial ownership of 525 Solutions. J.L.S. is an employee of 525 Solutions. J.P., R.D.R., O.A.C., and J.L.S. are named inventors on related patent applications. McGill University maintains an approved Conflict of Interest Management Plan. R.D.R. has a grant through McGill University with a licensee of the related patents. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed. No writing assistance was utilized in the production of this manuscript.

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ACKNOWLEDGMENTS This research was undertaken, in part, thanks to funding from the Canada Excellence Research Chairs Program. This work was also supported by 03/32/DSPB/0708, Poznan University of Technology, Department of Chemical Technology.

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REFERENCES

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Two Herbicides in a Single Compound: Double Salt Herbicidal Ionic

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