Pyrrolidinium herbicidal ionic liquids

Abstract

In this work, sixteen new pyrrolidinium herbicidal ionic liquids (HILs) with phenoxycarboxylate anions were synthesized and characterized. The effect of the alkyl or alkoxymethyl chain length and type of anion on the physicochemical properties, such as density, viscosity and refractive index of HILs, as well as on the thermal stability and surface activity, were determined. The oxygen presence in the structures of the prepared HILs significantly affects their thermal and surface properties. Furthermore, the herbicidal efficacy was tested in greenhouse experiments by using common lambsquarters (Chenopodium album L.) as a test plant. The novel pyrrolidinium ionic liquids were more effective than the commercial herbicides.

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Introduction

Ionic liquids (ILs) are the subjects of intense research in many scientific and industrial centers. Practically unlimited application possibilities of ILs result from the vast number of cation–anion combinations, estimated at 10¹⁸, and their unique properties.¹ By selecting a suitable cation and anion, we can design multifunctional salts.

The third generation of ILs is becoming more and more popular, demonstrating targeted biological properties with selected physical and chemical properties.^2,3 They are new pharmaceuticals^4–9 and phytopharmaceuticals.^10–13 In 2011, a new group of ILs called herbicidal ionic liquids (HILs) was described,¹⁴ that enrich phytopharmaceuticals. The HILs are composed of cation and anion, with a melting point below the boiling point of water at atmospheric pressure, and in which at least one of the ions manifesting herbicidal activity. They were designed with dual functionality where both cation and anion added to the beneficial properties of the salt. The use of HILs can reduce the dose of herbicide per hectare, while controlling its toxicity (toxic herbicides as phenoxy acids or esters may become nontoxic as HILs¹⁴) and having unique physicochemical properties like low volatility and thermal stability. The HILs as ionic compounds don't evaporate, making them safer for neighboring crops of dicotyledonous and staff engaged in spraying.¹⁵ Furthermore, some salts included in this group are biodegradable, as has been shown by Pernak¹⁶ and Ławniczak.¹⁷ Nowadays HILs are known with one herbicidal function contained one herbicidal ion: (4-chloro-2-methylphenoxy)acetate (MCPA),^14,18 (2,4-dichlorophenoxy)acetate (2,4-D),^18–20 2-(4-chloro-2-methylphenoxy)propionate (MCPP),²¹ (3,6-dichloro-2-methoxy)benzoate (Dicamba),¹⁵ 2-phosphonomethylaminoacetate (glyphosate),²² metsulfuron methyl (MS-M),²³ 5-[2-chloro-4-(trifluoromethyl)phenoxy]-N-(methylsulfonyl)-2-nitrobenzamidate (fomesafen),²⁴ 3,6-dichloropyridine-2-carboxylate (clopyralid),²⁵ 3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide (bentazone)²⁶ and 4-(4-chloro-2-methylphenoxy)butanoate (MCPB),²⁷ or oligomeric HILs with two anions: MCPA and Dicamba.²⁸ In these compounds the second actives (cations) were chosen from food- or cosmetic-grade antibacterial cations, antifungal cations, surfactants, etc. HILs with dual herbicidal function: 2-chloroethyltrimethylammonium cation as a growth regulator with MCPA²⁹ or 2,4-D³⁰ anion, have been also described. Moreover quaternary ammonium salts with 2,4-D or MCPA moieties in the cation as ester groups of the esterquats also are known.³¹ Due to their multifunctionality, HILs do not overload the environment, as opposed to other commercial products, which represent mixtures of many different compounds. In addition, herbicides in the form of HILs with long alkyl chains exhibit surface activity and lower liquid-contact angle of the surface of the leaf, which for foliar applied herbicides plays a key role in the penetration of the active substance in the plant.²⁰ Furthermore, several novel reports also show that ILs may be used in order to improve the immunity of plants to pathogenic factors, e.g. by inducing plant systemic acquired resistance towards viral diseases.^32,33

1-Methylpyrrolidine, also known as 1-methyltetrahydropyrrole is a cyclic tertiary amine that has a saturated 5-membered heterocyclic ring with methyl substituent. 1-Methylpyrrolidine is easy reacted with 1-bromoalkanes,^34,35 1-bromoalkoxyalkanes³⁶ and chloromethylalkyl ethers,³⁷ to give quaternary pyrrolidinium halides as precursors of pyrrolidinium ILs. This class of ILs provides alternative to popular and most common ammonium, pyridinium and imidazolium ILs. These ILs have found application in electrochemical development like rechargeable lithium-ion batteries and other electrochemical devices.^36,38–42 Furthermore, pyrrolidinium ILs were used for liquid–liquid extraction of Cu(II) ions⁴³ and extraction of olefins from paraffins, or cycloalkanes from cycloalkenes at infinite dilution, additionally for the separation of sulfur, or nitrogen compounds from aliphatic hydrocarbons, with high selectivity values observed.⁴⁴ Pyrrolidinium ILs have been found to be more environmentally friendly than imidazolium ILs. Biodegradability of pyrrolidinium, morpholinium, piperidinium, imidazolium and pyridinium ILs under aerobic conditions was examined. Most of the degradable compounds are among the quaternary pyrrolidinium cations. The biodegradability of 1-alkyl-1-methylpyrrolidinium ILs increased with lengthening side chain, changing from “not readily biodegradable” (ethyl substituent) to “inherently biodegradable” (butyl substituent) and “readily biodegradable” (octyl substituent).⁴⁵ The insertion of oxygenated chains in pyrrolidinium cation significantly reduces the toxicity and this kind of ILs can be classified as nontoxic for algae. The tests performed to check the effects on crustaceans confirmed that alkoxyl pyrrolidinium ILs had lower toxicity than alkyl counterparts.⁴⁶

The main aim of this work was the synthesis and characterization of new pyrrolidinium ionic liquids with phenoxycarboxylate anions, efficient in herbicidal action. 1-Methylpyrrolidine – cheap and reactive compound, giving safe and biodegradable quaternary pyrrolidinium salts was chosen as the source of cation.

Results and discussion

The precursors of ILs (1-alkyl- and 1-alkoxymethyl-1-methylpyrrolidinium halogenes) were obtained in Menschutkin reaction according to Scheme 1. Chlorides were prepared by treating 1-methylpyrrolidine with chloromethylalkyl ethers (chloromethyldecyl or chloromethyldodecyl ether) in hexane as a solvent and bromides were obtained in acetonitrile by treating 1-methylpyrrolidine with 1-bromoalkane (1-bromodecane or 1-bromododecane). All precursors of ILs were obtained as white solids. Chlorides were much more hygroscopic than bromides.

Scheme 1 Synthesis of 1-alkyl- and 1-alkoxymethyl-1-methylpyrrolidinium halogenes.

Pyrrolidinium ILs with phenoxycarboxylate anions were prepared in metathesis reaction in water, according to Scheme 2. Phenoxy acids (Fig. 1) used in synthesis (4-CPA, 2,4-D, MCPA and MCPP) were first purified by crystallization from toluene and using the active carbon to absorb colored impurities. All crystallized phenoxy acids were obtained as white solids. All of the obtained pyrrolidinium salts were liquids at room temperature, except for 1 and 2, which were waxes. They are new ILs. Purity of the obtained ILs and their precursors was determined by a direct two-phase titration technique (EN ISO 2871-1,2:2010) as surfactant content. The synthesized pyrrolidinium ILs with phenoxycarboxylate anions (1–16), their yields and surfactant content are presented in Table 1.

Scheme 2 Synthesis of pyrrolidinium salts with phenoxycarboxylate anions.

Fig. 1 Structures of phenoxy acids as herbicides.

Table 1 Synthesized pyrrolidinium salts

Salt	R^1	R^2	R^3	State^a	Yield (%)	Surfactant content (%)
a State is shown at 25 °C.
1	H	H	C10H21	Wax	85	98
2	H	H	C12H25	Wax	91	99
3	H	H	C10H21OCH2	Liquid	88	96
4	H	H	C12H25OCH2	Liquid	91	99
5	Cl	H	C10H21	Liquid	86	97
6	Cl	H	C12H25	Liquid	93	98
7	Cl	H	C10H21OCH2	Liquid	96	95
8	Cl	H	C12H25OCH2	Liquid	93	98
9	CH3	H	C10H21	Liquid	99	93
10	CH3	H	C12H25	Liquid	94	91
11	CH3	H	C10H21OCH2	Liquid	99	93
12	CH3	H	C12H25OCH2	Liquid	94	91
13	CH3	CH3	C10H21	Liquid	90	93
14	CH3	CH3	C12H25	Liquid	91	95
15	CH3	CH3	C10H21OCH2	Liquid	98	93
16	CH3	CH3	C12H25OCH2	Liquid	91	95

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For all room temperature ILs density, viscosity and refractive index were measured at 25 °C (Table S1 ESI†). Density of the pyrrolidinium ILs decreases with elongation of the alkyl or alkoxymethyl substituent (Fig. 2). Values of density for ILs with alkoxymethyl chain are always lower than for ILs with alkyl chain. Comparing the values density for all room temperature ILs the anions can be ordered according to decreasing values as follow: [2,4-D] > [4-CPA] > [MCPA] > [MCPP].

Fig. 2 Values of density for pyrrolidinium ILs.

Pyrrolidinium ILs with alkoxymethyl chain are less viscous then ILs with alkyl chain (Fig. 3). Values of viscosity for pyrrolidinium ILs with alkyl chain are in range from 4.9590 to 8.7441 Pa s, while for the ILs with alkoxymethyl chain the viscosity valued from 0.4049 to 1.0006 Pa s. Considering the type of anion, values of viscosity for ionic liquids decrease in range of: [2,4-D] > [MCPP] > [4-CPA] > [MCPA].

Fig. 3 Values of viscosity for pyrrolidinium ILs.

Refractive index depends on the chain length (Fig. 4). For ILs with longer alkyl and alkoxymethyl substituent values of refractive index are lower. For alkyl-based ILs, the refractive index values are in range from 1.5004 to 1.5158 and in the case of alkoxymethyl-based ILs, values of refractive index are from 1.4933 to 1.5090. Type of anion also has influence for refractive index. Values refractive index for all ILs decrease in range of: [2,4-D] > [MCPA] >[MCPP] > [4-CPA].

Fig. 4 Values of refractive index for pyrrolidinium ILs.

All of the pyrrolidinium ILs are soluble in water, methanol, DMSO, acetonitrile, acetone, isopropanol, ethyl acetate, chloroform and toluene, but insoluble in hexane.

Thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) data for the ILs are presented in Table 2. For all prepared ILs glass transition were observed. In case of (4-chlorophenoxy)acetates (1–4) is in range from −59.2 to −38.5 °C for 3 and 2 respectively. In these group crystallization was observed for 1, 2 and 3 at −26.5, −13.5 and −36.6 °C. (4-Chlorophenoxy)acetates melt at 9.9 °C (1), 19.1 °C (2) and the lower melting point was observed for 1-dodecyloxymethyl-1-methylpyrrolidinium cation at −34.5 °C.

Table 2 The physicochemical properties of synthesized ILs^a

ILs	Tg (°C)	Tcryst. (°C)	Tm (°C)	Tonset5% (°C)	Tonset (°C)
a Tg – glass transition temperature, Tcryst. – temperature of crystallization, Tm – melting point, Tonset5% – decomposition of 5% sample, Tonset – decomposition of 50% sample.
1	−49.0	−26.5	9.9	209	251
2	−38.5	−13.5	19.1	209	242
3	−59.2	—	—	123	279
4	−46.4	−36.6	−34.5	126	286
5	−30.2	—	—	208	240
6	−37.2	—	—	208	260
7	−52.4	—	—	126	308
8	−46.0	−21.2	−17.0	133	316
9	−38.2	—	—	207	238
10	−37.8	—	—	210	249
11	−57.0	—	—	128	309
12	—	−49.6	−37.3	128	325
13	−28.5	—	—	208	238
14	−34.4	—	—	210	250
15	−58.1	—	—	125	301
16	−53.6	−48.0	−40.3	125	314

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For acetates with two chloride atom in anion (5–8) generally only glass transition was observed. These transition occurred at −30.2 °C and −37.2 °C for alkyl chain – respectively 5 and 6. For compounds with alkoxymethyl chain the glass transition occurred at lower temperatures −52.4 and −46.0 °C for 7 and 8. Only for 1-dodecyloxymethyl-1-methylpyrrolidinium (2,4-dichlorophenoxy)acetate crystallization was observed at −21.2 °C with melting point −34.5 °C.

1-Alkyl-1-methylpyrrolidinium (4-chloro-2-methylphenoxy)acetates glass transition was observed at −38.2 and −37.8 °C for 9 and 10 respectively. Replace of one carbon atom in alkyl chain with oxygen reduce glass transition to −57.0 °C for 11. For 12 crystallization occurred at −49.6 °C and melts at −37.3 °C.

Glass transition for 2-(4-chloro-2-methylphenoxy)propionates (13–16) was observed. In case of compounds with alkyl chain at −28.5 °C for 13 and at −34.4 °C for 14. Incorporation of oxygen atom to alkyl chain reduced glass transition from −53.6 up to −58.1 °C for 16 and 15. In case 2-(4-chloro-2-methylphenoxy)propionates crystallization and melting point occurred only for 16 at −48.0 °C and −40.3 °C.

Obtained pyrrolidinium ILs exhibited multi step thermal decomposition. For compound with 4-CPA anion and 1-decyl-1-methylpyrrolidinium cation (1) decomposition (T_onset5%) begins at 209 °C. First, major step was ranged to 250 °C with 62% decomposition, the second is ranged from 250 to 278 °C and the last one 278–350 °C. Elongation of alkyl to twelve carbon atoms (2) do not influence on thermal stability (T_onset5%) 209 °C, the first step is ranged to 243 °C with around 40% degradation and second from 244 up to 301 °C with 57% decomposition. Incorporation oxygen atom to alkyl chain (3 and 4) dramatically reduced thermal stability, with T_onset5% respectively 123 and 126 °C. In case of 1-decyloxymethyl-1-methylpyrrolidinium cation the first step was ranged to 160 °C with only 19% decomposition, but the second from 160 up to 496 °C with 79% of decompose. For 4 the first step was ranged to 162 °C and 19% decomposition and the next exhibited 160–496 °C with 80% of decompose.

1-Decyl-1-methylpyrrolidinium (2,4-dichlorophenoxy)acetate (5) decompose at 208 °C, with first step up 292 °C and 82% of decompose. The second step is ranged 293–350 degree and 13% of decompose. For ILs with dodecyl substituent (6) only one step was observed, with degradation beginning at 208 °C. For 1-alkoxymethyl-1-methylpyrrolidinium ILs (7 and 8) thermal stability decreased respectively to 126 and 133 °C. In first case first step was ranged to 176 °C with 19% of decomposition and next step 176–489 °C (79%). For compound with dodecyloxymethyl substituent (8) first step ends at 185 °C (14%) and the second is ranged from 185 to 490 °C with 84% of decomposition.

1-Decyl-1-methylpyrrolidinium (4-chloro-2-methylphenoxy)-acetate (9) first step thermal degradation is ranged 282 °C with 82% of decomposition. The second step 282–323 degree and 17% of decompose. For IL 10 three steps were observed. First to 260 °C, the second 260–287 °C and third is ranged from 289 to 387 degree with degradation steps respectively 82, 10 and 2%. Incorporation of oxygen atom to alkyl chain in cation substituent (11 and 12) thermal stability was reduced to 128 °C. For 11 first step was ranged to 179 °C with 19% of decomposition and next step 179–493 °C with step around 81%. For compound which contained dodecyloxymethyl substituent (12) thermal decomposition steps were ranged 128–177 °C (17%) and from 177 up to 495 °C (81%).

In case of 1-decyl-1-methylpyrrolidinium 2-(4-chloro-2-methylphenoxy)propionate (13) two steps of thermal decomposition were observed. First, major to 285 °C with 85% of degradation and second was ranged from 285 to 389 °C. For compound with dodecyl substituent (14) first step is ranged to 260 °C with 83% of decomposition, next decomposition step occurred from 260 to 286 degree, and the third was in range 293–384 °C (2%). Replacement one carbon atom in alkyl chain with oxygen (15) reduced thermal stability to 125 °C. The first decompose step (20%) is ranged to 170 °C, an the next is in area from 170 up to around 500 °C with 79% step decomposition. In case 1-dodecyl-1-methylpyrrolidinium 2-(4-chloro-2-methylphenoxy)propionate (16) first step of thermal decomposition occurred between 125 and 167 °C, and second 167–488 °C with degradation steps 16 and 83%, respectively.

The surface-active properties of the ILs are summarized in Table 3. The surface activity of ionic liquids can be evaluated by γ_CMC, which is the effectiveness of surface tension reduction. For aqueous solutions of the ILs, the surface tension decreased from the value of water value to a minimum between 27.4 and 35.6 mN m⁻¹. This demonstrated that synthesized pyrrolidinium ILs exhibited intermolecular hydrophobic interactions, making it easy to form aggregates in water, especially when the length of the substituent increased. Comparing ILs with alkyl substituent to those with alkoxymethyl substituent it observed, that always the lower values of γ_CMC were recorded for ILs with alkoxymethyl substituent. The CMC value decreased as expected from the increased hydrophobicity owing to the elongation of the hydrocarbon chain. The adsorption efficiency, pC₂₀, is defined as the negative logarithm of the surface-active compound concentration in the bulk phase required to reduce the surface tension of the water by 20 mN m⁻¹. Thus, the greater the pC₂₀ value means the higher the adsorption efficiency of the surface-active compound. The efficiency of adsorption of a surface-active compound at the surface, as measured by the pC₂₀ value, increased with an increase in the number of carbon atoms in the hydrophobic chain. The other parameter, Π_CMC is the surface pressure at the CMC, being defined by: Π_CMC = γ_o − γ_CMC, where γ_o is the surface tension of pure solvent and γ_CMC is the surface tension of the solution at the CMC. This parameter indicates the maximum reduction of surface tension caused by the dissolution of surfactant molecules and, therefore, becomes a measure for the effectiveness of the surfactant to lower the surface tension of the solvent. It can be seen that the highest values of Π_CMC were obtained for compounds with alkoxymethyl substituent, which indicates that this substituent is superior to the alkyl substituent in the effectiveness of surface tension reduction (Π_CMC).

Table 3 Surface activity parameters of synthesized pyrrolidinium ILs

IL	CMC (mmol L^−1)	γCMC (mN m^−1)	ΠCMC	pC20	CA (°)
1	9.77	35.6	37.2	2.4	70.1
2	2.45	34.3	38.5	3.2	66.4
3	5.01	29.5	43.3	2.9	46.2
4	1.23	27.5	45.3	3.6	40.6
5	7.94	33.0	39.8	2.8	57.3
6	2.51	33.2	39.6	3.3	58.1
7	2.57	29.0	43.8	3.4	43.1
8	1.29	28.4	44.4	4.0	42.8
9	4.90	33.5	39.3	2.8	59.8
10	2.45	33.5	39.3	3.4	59.6
11	7.85	29.1	43.7	3.1	45.6
12	1.24	27.4	45.4	4.0	40.6
13	9.66	33.7	39.1	2.8	61.4
14	2.57	34.4	38.4	3.7	67.9
15	8.91	28.1	44.7	2.9	41.6
16	1.26	28.0	44.8	3.9	40.1

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For wetting, the reduction of the contact angle of the drop from 40.1 to 70.1 was observed, the minimum value was determined for 16 and the maximum – for 1 (Table 3). Better wettability of the paraffin surface, which is similar to the surface of plants, was observed for salts with alkoxymethyl substituent and may be useful in future applications of these ILs as herbicides.

For all pyrrolidinium ILs, the weight reduction of fresh common lambsquarters (Chenopodium album L.) with comparison to control was measured. The efficacy of tested ILs was compared with commercial products containing MCPA-salt and 2,4-D-salt. The results are shown in Table S2 (ESI†) and Fig. 5. Data indicate that the level of weed control depended on the structure of tested compounds. In case of ILs with 4-CPA (1–4) and MCPP (13–16) anions, shorter alkyl or alkoxymethyl substituent denotes a slightly lower activity as compared to those with longer alkyl chains. The inverse relationship was observed for ILs with 2,4-D (5, 6, 8) and MCPA (9–12). The exception to this rule is 7. The most active compound was 11, which exhibited the efficacy of 67%, but 2, 9 and 16 also gave good results. The correlation between length of alkyl or alkoxymethyl chain and herbicidal activity (fresh weight reduction of common lambsquarters) was not observed.

Fig. 5 Efficacy for pyrrolidinium ILs with phenoxycarboxylate anions against common lambsquarters.

The biological activity of ILs was better compared with commercial herbicide and therefore the tested pyrrolidinium salts can be classified as new HILs.

Conclusions

In conclusion, new ionic liquids with pyrrolidinium cations were synthesized and characterized. The physicochemical properties of obtaining salts (viscosity, density, refractive index values, and solubility) were influenced by the length of the alkyl or alkoxymethyl substituent. Additionally, the values of density, viscosity and refractive index were affected by the oxygen presence in the alkyl chain or type of anion. TGA and DSC analyzes confirmed that all obtained salts were thermally stable, this also noted the relationship between stability and the presence of oxygen in the alkyl chain. A similar important role of oxygen was confirmed during the study of surface activity. Furthermore, our pyrrolidinium ionic liquids showed better herbicidal action than commercial herbicides (towards common lambsquarters), therefore can be classified as HILs.

Experimental

Materials

1-Bromodecane 98%, 1-bromododecane 98%, 1-methylpyrrolidine 97%, (2,4-dichlorophenoxy)acetic acid (2,4-D) 98% and active carbon were purchased from Sigma-Aldrich. (4-Chlorophenoxy)-acetic acid (4-CPA) 98% was purchased from Koch-Light Laboratories Ltd. (4-Chloro-2-methylphenoxy)acetic acid (MCPA) 97% and 2-(4-chloro-2-methylphenoxy)propionic acid (MCPP) 96% were purchased from Organika-Sarzyna (Poland). Chloromethyldecyl ether 99% and chloromethyldodecyl ether 99% were obtained in Department of Chemical Technology, Poznan University of Technology according to the method described previously.³⁷ All solvents, methanol, DMSO, acetonitrile, acetone, isopropanol, ethyl acetate, chloroform, toluene, hexane, and NaOH were purchased from Sigma-Aldrich (European market, Poznan, Poland) and used without further purification.

Synthesis

1-Alkyl-1-methylpyrrolidinium bromides. 1-Bromoalkane (1-bromodecane or 1-bromododecane) (0.12 mol) was added into a round-bottomed flask, which contained a vigorously stirred mixture of 20 mL acetonitrile and stoichiometric amount of 1-methylpyrrolidine. The reaction mixture was stirred at room temperature for 4 hours. Next, 20 mL of ethyl acetate was added and the mixture was cooled in the freezer for 24 hours. The precipitate was filtered, washed with ethyl acetate and dried at 50 °C under reduced pressure for 6 h. Yield of 1-decyl-1-methylpyrrolidinium bromide preparation was 90%, surfactant content 98% and the yield of 1-dodecyl-1-methylpyrrolidinium bromide preparation was 87%, surfactant content: 98%.

1-Alkoxymethyl-1-methylpyrrolidinium chlorides. 1-Methylpyrrolidine (0.12 mol) was placed in anhydrous hexane (30 mL) and the chloromethylalkyl ether (chloromethyldecyl ether or chloromethyldodecyl ether) (0.125 mol) was added. A reacting vessel was cooled with ice. The reaction mixture was vigorously stirred. Product immediately precipitated from the reaction mixture. The precipitate was crystallized from hexane and acetone. Finally, the product was separated, washed with hexane and dried in vacuum desiccator for 6 hours. Yield of 1-decyloxymethyl-1-methylpyrrolidinium chloride preparation was 82%, surfactant content 97% and the yield of 1-decyloxymethyl-1-methylpyrrolidinium chloride preparation was 86%, surfactant content: 96%.

Pyrrolidinium ionic liquids with phenoxycarboxylate anions. The appropriate phenoxy acid – 4-CPA; 2,4-D; MCPA; MCPP (0.01 mol), distilled water (20 mL), and a 10% aqueous solution of NaOH (0.011 mol) were 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 it became a clear solution. After that the quaternary pyrrolidinium chloride or bromide (0.01 mol) dissolved in water (30 mL) was added and stirred for 30 min at room temperature. Then product was extracted from the aqueous phase with chloroform (50 mL). After separation of the phases, the aqueous layer was removed, then chloroform from the organic phase was removed under reduced pressure until constant weight. Anhydrous acetone (20 mL) was added to dissolve the product. The precipitate (NaCl or NaBr) was filtered of and the filtrate concentrated in a vacuum evaporator. Finally the products were dried under reduced pressure at 60 °C for 24 hours.

Analysis

¹H NMR spectra were recorded on a Varian VNMR-S spectrometer operating at 300 MHz with tetramethylsilane as the internal standard. ¹³C NMR spectra were obtained with the same instrument at 75 MHz. CHN elemental analyses were performed at Adam Mickiewicz University, Poznan (Poland).

Density was determined by using an Automatic Density Meter DDM2911 with the mechanical oscillator method. The density of the samples (about 2.0 mL) was measured at 25 °C, controlled with a Peltier heater. The apparatus was calibrated by using deionized water as the reference. After each series of measurements, the densimeter was washed with methanol and acetone and dried.

Viscosity was determined by using a rheometer (Rheotec RC30-CPS) with cone-shaped geometry (C50-2). The viscosity of the samples (about 1.5 mL) was measured at 25 °C. The uncertainty of the viscosity measurement was estimated to be less than 10⁻⁴ Pa s.

Refractive index was determined by using an Automatic Refractometer J357 with electronic temperature control.

The solubility of the salts in 10 representative solvents was determined according to the protocols described in Vogel's Textbook of Practical Organic Chemistry.⁴⁷ A sample of each salt (0.1 g) was added to a certain volume of the solvent and the samples were kept at 25 °C in a water bath (WNB 7, MEMMERT). Based on the volume of solvent used, two types of behaviors were recorded: ‘soluble’ applies to compounds that dissolved in 1 mL of the solvent and ‘not soluble’ applies to compounds that did not dissolve in 3 mL of the solvent.

Thermal analysis

Thermal transitions of the prepared salts were determined by DSC, with a Mettler Toledo Star^e TGA/DSC1 (Leicester, UK) unit, under nitrogen. Samples between 5 and 15 mg were placed in aluminum pans and heated from 25 to 120 °C at a heating rate of 10 °C min⁻¹ and cooled with an intracooler at a cooling rate of 10 °C min⁻¹ to −100 °C. Thermogravimetric analysis was performed using a Mettler Toledo Star^e 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⁻¹.

Surface activity

Surface tension measurements were carried out by the use of a DSA 100 analyzer (Krüss, Germany, accuracy ±0.01 mN m⁻¹), at 25 °C. The surface tension was determined using the shape drop method. Basically, the principle of this method is to form an axisymmetric drop at the tip of a needle of a syringe. The image of the drop (3 mL) is taken from a CCD camera and digitized. The surface tension (γ in mN m⁻¹) is calculated by analyzing the profile of the drop according to the Laplace equation. Temperature was controlled using a Fisherbrand FBH604 thermostatic bath (Fisher, Germany, accuracy ±0.1 °C). The values of the critical micelle concentration (CMC) and the surface tension at the CMC (γ_CMC) were determined from the intersection of the two straight lines drawn in low and high concentration regions in surface tension curves (γ_CMC vs. log C curves) using a linear regression analysis method.

The basis for the determination of the contact angle is the image of the drop on the examined surface (paraffin). After determination of actual drop shape and the contact line, the drop shape is adapted to fit a mathematical model used to calculate the contact angle. The most exact method to calculate this value is Young–Laplace fitting (sessile drop fitting). Complete drop contour is evaluated. After successful fitting of the Young–Laplace equation, the contact angle is determined as the slope of the contour line at the 3-phase contact point (solid–liquid and liquid–air). The measurements were carried out by the use of DSA 100 analyzer, Krüss (Germany).

Greenhouse experiment

Herbicidal activity of the pyrrolidinium ionic liquids was examined on common lambsquarters (Chenopodium album L.). The seeds were sown into soil-filled pots to a depth of 1 cm. After emergence the plants were thinned to five plants in each pot. The treatments were applied at four leaf stage using a moving sprayer occupied with Tee Jet 110/02 flat-fan nozzles delivering 200 L of spray solution per 1 ha at 0.2 MPa pressure. The distance from the nozzles to the tips of the plant was 40 cm, and the sprayer was moving above the plants, at a constant speed of 3.1 m s⁻¹. ILs and commercial herbicides were dissolved in water. All treatments were applied at a rate of 400 g of phenoxy acid anion in calculation for 1 ha. The standard herbicides were commercial products: Chwastox Extra 300 SL® and Aminopielik Standard 600 SL. Chwastox Extra 300 SL® contains sodium and potassium salts of MCPA – 300 g MCPA per 1 L and Aminopielik Standard 600 SL contains dimethylammonium (2,4-dichlorophenoxy)acetate – 600 g 2,4-D per 1 L. The plants were placed in a greenhouse at the temperature of 20 ± 2 °C, humidity of 60%, and photoperiod (day/night hours) 16/8. After 2 weeks, the plants were cut to soil level and weighed with an accuracy of 0.01 g. The efficacy of the herbicides was calculated on the basis of the reduction of plant fresh weight as compared to control (no sprayed plants). The study was carried out in four replications in a completely randomized setup.

Acknowledgements

This work was supported by grant PBS2/A1/9/2013, The National Centre for Research and Development, Warszawa, Poland.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra12157h

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