Home The influence of herbicidal anions on chemical shifts in NMR, phytotoxicity and surface properties of pyrrolidinium surface-active ionic liquids
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The influence of herbicidal anions on chemical shifts in NMR, phytotoxicity and surface properties of pyrrolidinium surface-active ionic liquids

  • Marta Wojcieszak

    Marta Wojcieszak is a Ph.D. student in Department of Chemical Technology, Poznan University of Technology. She is involved in study of surface properties of new compounds.

         , Anna Syguda

    Anna Syguda is a Ph.D. in chemistry in Department of Chemical Technology, Poznan University of Technology. Her research interests are focused on organic chemical technology, in particular the synthesis of multifunctional compounds such as ionic liquids.

         and Katarzyna Materna

    Katarzyna Materna is a Professor in Department of Chemical Technology, Poznan University of Technology. Her research interests are focused on study of surface properties of new compounds.

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Published/Copyright: February 9, 2024
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Tenside Surfactants Detergents
From the journal Tenside Surfactants Detergents Volume 61 Issue 2

Abstract

Six surface-active ionic liquids (SAILs) with herbicidal anions were synthetized (with a yield of ≥ 88 %) and their structure was confirmed by NMR spectra. Subsequently, their surface properties, phytotoxicity and bulk aggregation behavior in aqueous solution investigated. The compounds studied have an amphiphilic structure and, due to the presence of two long alkyl chains in the cation, they displayed surface activity (CMC values, ranging from 0.13 mmol L−1 to 1.36 mmol L−1). Our results provide explanations for the role of counterions in the physical and chemical properties of SAILs. Indeed, the aromatic anion affects the hydrophilic charge of the surface-active compounds. The SAILs containing the [MCPA]− and [MCPP]− anions were the most phytotoxic to representatives of dicotyledonous plants compared to the other compounds. By analysis of the structure of SAILs, we demonstrate that counterions play an important role in self-assembly and adsorption processes in aqueous solutions, and therefore, in the potential biological application of these compounds.

Keywords: ionic liquids; chemical shifts in NMR; critical micelle concentration; AFM analysis; phytotoxicity

1 Introduction

Surface-active ionic liquids (SAILs), considered by the scientific community as a subclass of ionic liquids (ILs), are attracting increasing interest ( 1, 2, 3 ). This attention is due to the fact that SAILs not only possess the properties of traditional ILs, but also those of surfactants. Thus, in addition to properties such as low melting point, negligible vapour pressure and recyclability ( 4 ), they have the ability to influence surface and interfacial tension, wettability and promote foamability ( 1 , 5 ). In addition, SAILs have a tendency to self-assemble into various structures, including, e.g., wormlike, rod-like, vesicles, liquid crystalline phases, and tubules ( 1 ).

ILs are composed of different anions and cations, tailored for specific needs and purposes ( 4 ). From a structural and definitional point of view, ILs consist of organic or inorganic cations and similarly organic or inorganic anions. For SAILs the same principles of construction apply as for ILs. However, due to their affinity for amphiphilic compounds ( 3 ), SAILs typically have long alkyl substituents, often exceeding eight carbon atoms in the cation ( 1 , 6 ). Buettner et al. ( 1 ) suggest that the presence of these alkyl chains in the structure of SAILs is significant because long alkyl substituents have lower molecular charges compared to conventional surfactants. As reported in the literature ( 7 ), anions can also influence the surface activity of amphiphilic ILs, although to a lesser extent. Saien et al. ( 7 ) and others scientists ( 8, 9, 10, 11 ) have described the most typical inorganic and organic anions, such as bromide [Br]−, chloride [Cl]−, tetrafluoroborate [BF4]−, acetate [CH3COO]−, p-toluenesulfonate [Tos]−, tetrafluoroborate [AS]−, and trifluoroacetate [CF3COO]− anions. These authors highlighted that the size of the anions has a critical effect on the surface activity. Bulky counter anions show a lower susceptibility to hydration with higher polarisability, which can consequently lead to a limitation of the tendency of compounds to aggregate ( 12 , 13 ). In addition, as mentioned above when describing the effect of the cation on the surface properties of SAILs, the number of carbon atoms in the alkyl chain is also important for the anion structure under consideration. Sakthivel et al. ( 14 ) described that the relevant alkyl chain length of the anion can contribute to increase or decrease the adsorption capacity of SAILs. Another aspect relates to the so-called “structure modulation”, where the selection of the right or favourable ions can improve the interaction between cation and anion, thereby affecting the surface activity of compounds ( 1 , ).

Although SAILs are a relatively new group compared to ILs or surfactants, they have applications in several fields such as medicine for cell signalling, nutrient or DNA transport, and as drug delivery systems or drug carriers in targeted therapy ( 18, 19, 20, 21 ). On a larger scale, SAILs are used in enhanced oil recovery (EOR) processes ( 2 , ). This is because amphiphilic ILs are able to reduce the interfacial tension (IFT) between oil and water under the environmental conditions characteristic of oil fields ( 22 , 25 ). An important application for SAILs is seen in nanotechnology, where they can simultaneously play a trifunctional role as a template, stabiliser and additive to facilitate the dissolution of reagents during the course of catalysis ( 26 , 27 ). In agronomy, SAILs have found use in weed control. The good wetting properties of amphiphilic compounds make it possible to increase their field of action on the leaf surface, thereby improving the efficiency of weed destruction ( 28 , 29 ). In another agronomic practice, SAILs are useful as solubilisers to create emulsifiable concentrate formulations containing pesticide (water insoluble) and amphiphilic ILs ( 30 ).

SAILs based on the pyrrolidinium cation, which possess good surface-active properties and the ability to self-organize, have already been previously discussed in the literature ( 31, 32, 33, 34 ). Despite the fact that the most commonly studied SAILs are cationic compounds, we want to focus on the role of anions and their impact on aggregation behavior of amphiphilic compounds in aqueous solutions. In addition, no attention has been paid to the construction of such a pyrrolidinium structure with potential herbicidal properties with two long alkyl substituents in the cation. Therefore, this aspect was also considered in these studies and not only the surface activity but also the herbicidal properties of the compounds were evaluated.

2 Experimental

2.1 Materials

Pyrrolidine 99 %, 1-bromodecane 98 %, (2,4-dichlorophenoxy)acetic acid (2,4-D) 98 %, active carbon and reagents for two-phase system titration (dimidium bromide 95 %, patent blue V sodium salt 97 %, sodium dodecylsulfate(VI) 98 %) were purchased from Sigma-Aldrich. (4-Chlorophenoxy)acetic acid (4-CPA) 98 % was purchased from Koch-Light Laboratories Ltd. (3,6-Dichloro-2-methoxy)benzoic acid (dicamba) 95 %, (4-chloro-2-methylphenoxy)acetic acid (MCPA) 97 %, (±)-2-(4-chloro-2-methylphenoxy)propionic acid (MCPP) 96 % and (3,6-dichloro)-2-picolinic acid 97 % (clopyralid) were purchased from Organika-Sarzyna (Poland). All solvents (acetonitrile, ethyl acetate, chloroform, toluene) and NaHCO3 were purchased from Avantor and used without further purification. The herbicides used in the synthesis (4-CPA; 2,4-D; MCPA; MCPP; dicamba and clopyralid) were first purified by dissolution in hot toluene, addition of activated carbon, filtration of the hot solution to remove colored impurities absorbed by activated carbon, and crystallization from cold toluene.

2.2 Methods

2.2.1 Preparation of 1-decylpyrrolidine

Pyrrolidine (0.55 mol) was dissolved in toluene (100 mL) and stirred with 1-bromodecane (0.25 mol). The reacting vessel was heated to 70 °C under a reflux condenser for 24 h. The produced pyrrolidinium hydrobromide was filtered and washed with toluene. Then, toluene and excess pyrrolidine were evaporated from the filtrate. Finally, the product was distilled under reduced pressure.

2.2.2 Preparation of 1,1-didecylpyrrolidinium bromide

1-Bromodecane (0.1 mol) was added to a round-bottomed flask that contained a vigorously stirred mixture of 20 mL of acetonitrile and 0.1 mol of 1-decylpyrrolidine. The reaction mixture was stirred at its boiling point under a reflux condenser for 24 h. Subsequently, the acetonitrile was removed under reduced pressure, and 50 mL of ethyl acetate was added to the raw post-reaction mixture at a low temperature. The precipitate was filtered off, washed with ethyl acetate and dried in a vacuum desiccator at room temperature.

2.2.3 Preparation of 1,1-didecylpyrrolidinium SAILs with herbicidal anions

The reagent mixture consisting of 0.01 mol of the selected herbicide in acid form (4-CPA, 2,4-D, MCPA, MCPP, dicamba or clopyralid), 20 mL of distilled water and 0.011 mol of a 10 % aqueous solution of NaHCO3 was mixed in a round-bottomed flask equipped with a magnetic stirring bar, a reflux condenser, and an addition funnel. The mixture was heated at 70 °C until the solution became clear. Then, 0.0095 mol of 1,1-didecylpyrrolidinium bromide dissolved in 30 mL of water was added and stirred for 30 min at room temperature. The products were extracted from the aqueous phase with 30 mL of chloroform and washed with distilled water until no bromide ions were detected using AgNO3. After removing the chloroform, the product was dried under reduced pressure at 60 °C for 24 h. The yields and surfactant content for each IL obtained are presented in Table 1. In order to verify their chemical structure, the synthesized SAILs and their precursor were subjected to 1H NMR and 13C NMR analyses using a BRUKER ASCEND™ 400 MHZ NANOBAY spectrometer. Tetramethylsilane was used as the internal standard, while CDCl3 was applied as a solvent. The instrument was operating at 400 MHz and 100 MHz for 1H NMR and 13C NMR analyses, respectively.

Table 1:

Reaction yield and purity of the synthesized pyrrolidinium SAILs with herbicidal anions.

Abbreviation of IL A− Surfactant content (purity) (%) Yield (%)
[Dec 2 Pyrr][4-CPA] [4-CPA]− 99.2 91
[Dec 2 Pyrr][2,4-D] [2,4-D]− 98.8 96
[Dec 2 Pyrr][MCPA] [MCPA]− 98.9 93
[Dec 2 Pyrr][MCPP] [MCPP]− 99.3 93
[Dec 2 Pyrr][Dicamba] [Dicamba]− 98.8 88
[Dec 2 Pyrr][Clopyralid] [Clopyralid]− 98.5 88

2.3 Surface activity studies

The surface tension (γ) and contact angle (CA) measurements were performed using a DSA 100 Drop Shape Analyzer (Krüss) at 25 °C. The Fisherbrand FBH604 thermostatic bath allowed one to maintain certain conditions with an accuracy of ±0.1 °C. The droplet shape method, in which an axially symmetrical droplet is produced at the tip of the syringe needle, was used to measure the surface tension. The image of drop was processed and digitalized, based on the drop profile analysis by the Laplace equation. Based on the results obtained from the method of surface tension measurement described above, the Gibbs energy (ΔG 0 ads), the surface excess concentrations (Γmax), the minimum surface occupied by a molecule at the interface (A min) and adsorption efficiency (pC20) were calculated by Eqs. (1–4) which were used in our previous works ( 3 , 28 , 29 ).

The adsorption efficiency (pC20) is given by:

(1) pC 20 = − log C 20

where: C20 is the concentration of the compound in the bulk phase required to reduce the surface tension of the water by 20 mN m−1, and the Gibbs energy (ΔG 0 ads) which characterizes the interactions between the bulk phase and the surface phase is given by:

(2) ∆ G ads 0 = − R T ln A sz

where: R is the gas constant, T the absolute temperature, and A sz the Szyszkowski equation constant.

Surface excess concentrations (Γmax) were calculated from the slope of the linear portion of the γ-log C plots (Figure 3) using the Gibbs isotherm:

(3) Γ max = − 1 R T · d γ d ( ln C )

where: R is the gas constant, T = absolute temperature, C = concentration of compounds.

From Γmax, the minimum surface area occupied by a molecule at the interface (A min) can be calculated from Eq. (4):

(4) Α min = 1 Γ max Ν A

where: N A = the Avogadro number.

In the current work, the contact angle (CA) is determined using an image of a liquid drop on a hydrophobic paraffin surface. The measurements were conducted with an accuracy of ± 0.1°. Once the actual shape of the deposited drop and the contact line have been determined, the data were adjusted to the mathematical model used to calculate the contact angle. The most precise method for determining this value was achieved using the Young-Laplace fit. Once the equation was successfully fitted, the contact angle was determined as the slope of the contour line at the point where the solid, liquid, and air phases meet.

2.4 Atomic force microscopy (AFM)

The samples of the compounds studied were dissolved in water, and then appropriate volumes of the solutions studied, at concentrations of SAILs before micellization process were deposited on freshly prepared mica substrates and dried. The microscope used for the study was an NX10 manufactured by Park Systems. The scan size was 512 px × 512 px, while the scan speed was between 0.3 Hz and 0.5 Hz. The images were processed using Gwyddion software.

2.5 Phytotoxicity test

The phytotoxicity of synthesized pyrrolidinium SAILs with herbicidal anions in relation to dicotyledonous plants was carried out on the basis of measurements of shoot and root growth inhibition of the plant according to the methodology described in our previous work ( 28 , 29 ). The garden cress (Lepidium sativum) was used as a model dicotyledonous plant. Commercial herbicides in acid form with an equimolar amount of NaHCO3 were used as reference samples. The tests were carried out using vertical plastic Phytotoxkit containers (Tigret company, Belgium). ZEW sand with a grain size of 0.1 mm to 1.0 mm was used as the substrate, the main component of which is quartz (SiO2), constituting over 98 %. The sand was previously screened and cleaned by washing it several times with tap water and deionized water and then dried in a dryer for 24 h at 105 °C. Each container was filled with (130 ± 0.1) g of sand. In a 100 mL volumetric flask, 0.25 mmol of the investigated compound was placed. Consequently, the initial solutions of the tested compounds were prepared at a concentration of 2.5 mmol L−1. Then a tenfold dilution was made by taking 10 mL of the initial solution and diluting it in a 100 mL volumetric flask. In the next stage, a tenfold dilution was made again. Finally, 0.025 mmol L−1 usable solutions of the compounds were obtained. Then 25 mL of the prepared solutions were taken and used to water the sand in a container, corresponding to 0.0048 mmol of the tested compound per kg of dry sand. One of the containers was prepared as a control, which contained only deionized water. Next, 10 seeds of tested plants were planted in each container and incubated for seven days at 25 °C. Seven days after sowing, the lengths of the shoots and roots were measured. The tests were carried out according to the PN–ISO 11269–1 (1998) standard.

3 Results and discussion

3.1 Synthesis

A three-step synthesis was applied to obtain pyrrolidinium SAILs with herbicidal anions. In the first step, 1-decylpyrrolidine was synthesized according to the scheme shown in Figure 1. Initially, equimolar amounts of pyrrolidine, 1-bromodecane and sodium bicarbonate were used in the synthesis of 1-decylpryyolidine. A procedure analogous to the preparation of 4-alkylmorpholine described in our previous work ( 19 ) was applied. The yield was 45 %, indicating that sodium bicarbonate was not involved in the reaction, and that this function was taken over by pyrrolidine. Half of the molecules of the pyrrolidine used in the reaction were alkylated, while the other half reacted with the hydrogen bromide released. It was decided to modify the procedure by eliminating the use of sodium bicarbonate. For this purpose, pyrrolidine and 1-bromodecane were used in a molar ratio of 2.2:1. When heating the mixture under reflux at 70 °C, turbidity was observed, indicating the precipitation of pyrrolidinium hydrobromide, which precipitated as a polar compound from toluene. After filtering off the pyrrolidinium hydrobromide and evaporating the toluene, 1-decylpyrrolidine was distilled under reduced pressure ([137–138] °C, [10–11] mbar). Ultimately, a colorless liquid was obtained with a yield of 91 %.

Figure 1: Synthesis of pyrrolidinium SAILs with herbicidal anions.
Figure 1:

Synthesis of pyrrolidinium SAILs with herbicidal anions.

In the second step of the reaction (Figure 1), 1-decylpyrrolidine was quaternized with 1-bromodecane. To accelerate the formation of a polar product, the polar solvent acetonitrile, was used. To isolate the product from the post-reaction mixture, the acetonitrile had to be evaporated. Subsequently, a non-polar solvent, ethyl acetate, was added and the product was filtered. The resulting product obtained at room temperature was a white solid with a melting point of 37.5 °C–38.4 °C. The reaction yield was 93 %. The surfactant content (purity) was determined by two-phase titration (EN ISO 2871-1.2:2010) and was 98.1 %.

In the third step (Figure 1) an ion exchange reaction took place in the aqueous solution. The ion exchange reaction would not occur with herbicides in acid form because they are too weak acids to displace the bromide anion from 1,1-didecylpyrrolidinium bromide. Therefore, it was necessary to convert the herbicides into appropriate sodium salts. The solubility of these salts in water also facilitated the visually determination of the endpoint of the acid neutralization reaction, as the acid does not dissolve in water, whereas the sodium salt does. When sodium bicarbonate was added to herbicides in acid form, the evolution of carbon dioxide and the gradual homogenization of the solution were observed, indicating the formation of a sodium salt that was well soluble in water. Subsequently, the obtained sodium salts of herbicides were mixed with 1,1-didecylpyrrolidinium bromide. The observed turbidity of the solution indicated the formation of a product that was more hydrophobic than the starting substrates. The SAILs were extracted with chloroform and washed with water to remove the sodium bromide.

All compounds obtained after ion exchange were liquids under laboratory conditions (20 °C); and can therefore be classified as room temperature SAILs. The reaction yields of all the SAILs obtained with herbicidal anions were high, exceeding 88 % in all cases (Table 1). The purity of the obtained SAILs was determined by two-phase titration and was over 98.5 %. The residue (supplement to 100 %) was water, which was observed in 1H NMR spectra.

In the case of SAILs with phenoxycarboxylate anions ([Dec 2 Pyrr][4-CPA], [Dec 2 Pyrr][2,4-D], [Dec 2 Pyrr][MCPA], [Dec 2 Pyrr][MCPP]), a water signal of 3.7 ppm was observed. For SAILs with dicamba anion ([Dec 2 Pyrr][Dicamba]), the signal appeared at 3.50 ppm, for SAILs with clopyralid anion ([Dec 2 Pyrr][Clopyralid]) at 3.19 ppm, while for their precursor ([Dec 2 Pyrr][Br]) at 2.47 ppm. The different position of the signal of protons coming from water in 1H NMR spectra of compounds is caused by different interactions of water in these compounds. In the case of 1,1-didecylpyrrolidinium bromide, it is probably crystallization water, which is incorporated into the crystallization network of the compound. However, in other compounds, it is hydration water which forms hydrogen bonds with the carboxylate group of the anion. Due to the fact that these are different anions coming from different carboxylic acids, the shielding of the water protons is different in each case, therefore its position changes.

Proton and carbon nuclear magnetic resonance spectra were obtained to determine the structures of the prepared compounds (Figures S1–S14 in the Supplementary Material). The values of chemical shifts and coupling constants for the 1H NMR spectra are listed in Table S1 (in the Supplementary Material). For all the compounds analyzed, differences in the chemical shift values of the protons located in close proximity to the positively charged nitrogen atom can be observed (Figure 2). The chemical shift values of the corresponding cation protons are the highest for the compound with the bromide anion and decrease for the remaining herbicidal anions. Therefore, it can be concluded that the replacement of the small bromide anion by a large organic anion causes the electron cloud of the ionic bond to shift towards the nitrogen atom. The result is an inductive effect affecting neighboring electron clouds, probably increasing the shielding of hydrogen atoms and changing the chemical shifts of the protons located at the quaternary nitrogen atom. The analysis of the 1H NMR spectra allowed us to rank the anions according to their increasing proton shielding capacities in the following order: [Br]− < [Clopyralid]− < [Dicamba]− < [4-CPA]− < [2,4-D]− < [MCPA]− < [MCPP]−.

Figure 2: Fragment of 1H NMR spectra for SAILs with the [Dec2Pyrr] cation showing differences in the chemical shifts of the corresponding protons.
Figure 2:

Fragment of 1H NMR spectra for SAILs with the [Dec2Pyrr] cation showing differences in the chemical shifts of the corresponding protons.

This phenomenon can be attributed to the electrostatic and steric interactions, which primarily affect the electron density near the quaternary nitrogen atom.

For 13C NMR spectra, the chemical shifts of the corresponding carbon atoms are listed in Table S2 (in the Supplementary Material). However, here the effect of shielding carbon atoms is less visible, depending on the type of anion.

3.2 Surface activity studies

Figure 3 shows the surface tension isotherms of SAILs. From the graph shown, it can be observed that the surface tension values decrease continuously and eventually become constant after reaching a plateau. The point of constancy on the surface tension-concentration relationship graph is identified as the CMC, and it is crucial for describing the micellization process. Micellization is a complex process influenced by various factors. According to Banjare et al. ( 35 ), it is based on electrostatic interactions between hydrophobic interactions and charged main groups between hydrocarbon tail groups. However, the role of the anion in micelle formation cannot be excluded. Before proceeding to a deeper analysis, it should be noted that the [Dec 2 Pyrr][Br] is a precursor of the synthesis, and its surface properties are also discussed.

Figure 3: Surface tension isotherms of SAILs in the aqueous solutions at 25 °C.
Figure 3:

Surface tension isotherms of SAILs in the aqueous solutions at 25 °C.

In order to discuss the surface activity of the compounds, specific parameters were determined, the values of which are summarized in Table 1. The critical micelle concentration (CMC), the value of the surface tension at CMC (γ CMC), the Gibbs energy (ΔG 0 ads), the surface excess concentrations (Γmax) and the minimum surface area occupied by a molecule at the interface (A min) can be assessed by surface tension measurements.

From the results presented in Table 2, it is clear that the synthetized SAILs have lower CMC values than conventional cationic surfactants such as DDAC, C 12 TAB, C 10 TAB and DomphB. This finding is also confirmed by the observations of Buettner et al. ( 1 ), who suggest that SAILs typically display a higher surface activity than conventional ammonium surfactants with similar alkyl chain lengths. Moreover, as highlighted by Saien et al. ( 7 ), ILs (which include SAILs) have larger micelle dimensions compared to conventional surfactants due to their bulkier head group. Another important aspect that should be addressed concerns the role of the anion of the synthesized compounds in determining the CMC value. As many discussions focus only on the amphiphilic part of SAILs, we decided to analyze the structure of the anions. Understanding the specific binding of counterions to micelles is a prerequisite for comprehending not only micellization but also any kind of aggregation in aqueous solution (see Figure 4).

Table 2:

Summary of surface properties of studied SAILs at 25 °C.

Abbreviation CMC (mmol L−1) γ CMC (mN m−1) pC20 ΔG 0 ads (kJ mol−1) Γmax × 106 (mol m−2) A min × 1019 (m2) CA (°)
[Dec 2 Pyrr][4-CPA] 0.26 28.0 4.68 −30.20 4.49 3.70 33.98
[Dec 2 Pyrr][2,4-D] 0.13 27.8 4.82 −27.90 6.08 2.73 38.41
[Dec 2 Pyrr][MCPA] 0.18 26.6 4.75 −30.32 4.93 3.37 37.29
[Dec 2 Pyrr][MCPP] 0.20 27.7 4.84 −29.98 4.18 3.97 40.21
[Dec 2 Pyrr][Dicamba] 0.43 26.6 4.53 −30.60 3.90 4.26 37.36
[Dec 2 Pyrr][Clopyralid] 0.76 28.1 4.34 −30.78 3.31 5.02 36.39
[Dec 2 Pyrr][Br] 1.36 24.6 4.01 −24.61 4.82 3.46 24.88
DDAC ∼2.00a
C 12 TAB 15.1b 36.4b
C 10 TAB 67.0c 40.0c
DomphB 1.78d 36.5d

  1. DDAC, didecyldimethylammonium chloride; C 12 TAB, dodecyltrimethylammonium bromide; C 10 TAB, decyltrimethylammonium bromide; DomphB, domiphen bromide. aFrom ( 36 ), bFrom ( 37 ), cFrom ( 38 ), dFrom ( 39 ).

Figure 4: Comparison of CMC values depending on the anion for SAILs with the [Dec2Pyrr] cation.
Figure 4:

Comparison of CMC values depending on the anion for SAILs with the [Dec2Pyrr] cation.

Analyzing the results presented in Figure 4, it can be seen that the CMC value of the precursor [Dec 2 Pyrr][Br], (1.36 mmol L−1) is higher than that of the SAILs (ranging from 0.13 mmol L−1 to 0.76 mmol L−1). The precursor contains a bromide anion which tends to polarize, whereas SAILs such as [Dec 2 Pyrr][2,4-D], [Dec 2 Pyrr][4-CPA], [Dec 2 Pyrr][MCPA], [Dec 2 Pyrr][MCPP] based on phenoxyacetic acid contain aromatic anions, which give these compounds a significant hydrophobic character. This hydrophobic character is crucial for the micelles formation. The literature explains that more hydrophobic aromatic anions are located further inside the micelles and form a kind of surrounding that separates them from the bulk water ( 1 , 40 ). Additionally, the substitution of the aromatic ring can affect the micellization behavior. Substitution of one chlorine atom for the aromatic ring consequently leads to higher CMC results than the substition with two chlorine atoms. For [Dec 2 Pyrr][MCPA] and [Dec 2 Pyrr][MCPP], where they have the same substitutes connected to the aromatic ring, the CMC values are at the same level. The ability to micellize for [Dec 2 Pyrr][Clopyralid] and [Dec 2 Pyrr][Dicamba] is lower than for the rest of SAILs. In this case, the size of the anions probably impacts their charge screening efficiency. The γ CMC values of the SAILs range from 24.6 mN m−1 to 28.0 mN m−1, respectively. These results demonstrate that synthetized compounds have a higher level of surface activity than conventional ammonium surfactants.

On the other hand, the interpretation of another parameter describing the surface activity of the studied SAILs, the adsorption efficiency (pC20), shows that [Dec 2 Pyrr][MCPP] exhibits the highest tendency to adsorb at the air/water interface. This is due to the fact that for the indicated compound, the pC20 value is the highest. The explanation for the surface activity of different materials relies on the packing densities of surface-active compounds at the air/aqueous solution interface. For this reason, the values of Γmax and A min are interpreted. It can be seen that an increase in the A min values is accompanied by a decrease in the value of Γmax. Kumar et al. ( 41 ) have clearly shown the above relationship, illustrating that a higher Γmax value and a lower A min value indicate more packing of monomers at the interface, whereas a lower Γmax value and a higher A min value indicate less packing of monomers at the interface. Indeed, for [Dec 2 Pyrr][2,4-D] the highest values of Γmax are observed at the smallest A min. Moreover, all analyzed SAILs are prone to bend close to the same level. This is related to the same length of the alkyl chains present in the cation ( 42 ). Negative ΔG 0 ads values illustrate that the adsorption process of the studied SAILs is spontaneous ( 29 , 43 ).

Considering the application aspect of the synthesized compounds, it is worthwhile to focus on understanding their wettability properties. For this purpose, the wettability of the paraffin was investigated, which, as a model surface, allows a preliminary assessment of the potential application of SAILs. There is a division in the literature which classifies SAILs according to the value of the contact angle (CA). SAILs with a CA value between 0° and 90° are considered to have good wetting properties ( 28 , 44 , 45 ). In the case of synthesized SAILs, the CA value is in the range of 33.98°–40.21°, so we can classify these compounds as having good wetting properties. However, in the long perspective, the wettability of biological systems, such as weed leaves, should be studied, since ILs have herbicidal anions. The CA value for [Dec 2 Pyrr][Br] equals to 24.88°, but it has not been included in the above discussion due to its lack of potential use as a herbicide.

3.3 AFM analysis

Considering the tendency of surface-active compounds to aggregate, and following the definition of CMC, we decided to focus on observing the behaviour of the SAILs analyzed prior to the micellization process. The imagines of individual compounds are presented in Figure 5.

Figure 5: AFM results for SAILs (A–F), showing the difference in surface coverage. (Left column) Topography of selected areas of compounds. (Right column) 3D view of the test surfaces. A. [Dec 2 Pyrr][4-CPA], B. [Dec 2 Pyrr][2,4-D], C. [Dec 2 Pyrr][MCPA], D. [Dec 2 Pyrr][MCPP], E. [Dec 2 Pyrr][Dicamba], F. [Dec 2 Pyrr][Clopyralid]. The concentration of solutions is before the micellization process.
Figure 5:

AFM results for SAILs (A–F), showing the difference in surface coverage. (Left column) Topography of selected areas of compounds. (Right column) 3D view of the test surfaces. A. [Dec 2 Pyrr][4-CPA], B. [Dec 2 Pyrr][2,4-D], C. [Dec 2 Pyrr][MCPA], D. [Dec 2 Pyrr][MCPP], E. [Dec 2 Pyrr][Dicamba], F. [Dec 2 Pyrr][Clopyralid]. The concentration of solutions is before the micellization process.

Based on the results presented in Figure 5, it is generally observed that the SAILs formed objects with a symmetrical round shape ( 28 , 29 ). However, for [Dec 2 Pyrr][2,4-D] and [Dec 2 Pyrr][MCPA] the objects are less symmetrical. In addition, when analyzing the values of the average size of the agglomerates formed on the mica surface, trends can be observed: [Dec 2 Pyrr][Dicamba] < [Dec 2 Pyrr][MCPA] < [Dec 2 Pyrr][4-CPA] < [Dec 2 Pyrr][Clopyralid] < [Dec 2 Pyrr][2,4-D] < [Dec 2 Pyrr][MCPP]. These trends correspond to the following values (136.8 nm < 311 nm < 510 nm < 840 nm < 1167 nm < 1381 nm). Despite the fact that the objects formed in this case have a different average value, the influence of the anions on the difference in surface coverage cannot be clearly emphasized.

3.4 Phytotoxicity determination

The results of the phytotoxicity tests against garden cress are shown in Figure 6. All SAILs were found to be more phytotoxic (or comparably phytotoxic) than commercial herbicides in the form of acids with an equimolar addition of sodium bicarbonate. The addition of bicarbonate to the acids was necessary to convert them into water-soluble sodium salts. The SAILs containing the [MCPA]− and [MCPP]− anions were the most phytotoxic to representatives of dicotyledonous plants, followed by the ILs with the [2,4-D]− anion. Among the phenoxycarboxylates, the SAILs with the [4-CPA]− anion exhibited the least phytotoxicity. Notably, the IL with the [Dicamba]− anion was less phytotoxic than the IL with the [4-CPA]− anion. Conversely, the compound with the [Clopyralid]− anion was the least phytotoxic. A similar trend was observed in our previous work ( 29 ), using the same herbicide anions and the 1-alkyl-1-benzylmorpholinium cation. In addition, ILs from the group of esterquats containing dicamba in the ester substituent of the cation ( 46 ) and the [MCPA]− anion were found to be the most phytotoxic to garden cress, while those containing the [Clopyralid]− anion exhibited the least phytotoxicity, confirming the observations addressed in this study. Furthermore, in ( 47 ) the phytotoxicity of imidazolium ILs with the [4-CPA]− anion was investigated on cornflower. All imidazolium ILs were found to be more phototoxic than the reference herbicide, sodium 4-chlorophenoxyacetate. However, in this study, 1,1-didecylpyrrolidinium 4-chlorophenoxyacetate demonstrated comparable effects to the reference herbicide.

Figure 6: Average root and shoot length of garden cress (Lepidium sativum) seedlings grown in sand with added SAILs and commercial herbicides with an equimolar amount of NaHCO3.
Figure 6:

Average root and shoot length of garden cress (Lepidium sativum) seedlings grown in sand with added SAILs and commercial herbicides with an equimolar amount of NaHCO3.

It can be easily seen that the phytotoxicity results correlate with the CMC values. This means that the better the surface activity, the better the compound interacts with the plant.

4 Conclusions

The ability of newly synthetized SAILs (yield ≥ 88 %) to adsorb and micellize in aqueous solution, was studied. Due to the amphiphilic structure of the compounds, their surface properties were compared with those of conventional cationic surfactants, such as DDAC, C 12 TAB, C 10 TAB and DomphB. The literature data suggest that the new SAILs exhibited higher surface activity than the listed surfactants. The behavior of the compounds prior to the CMC was carried out using AFM analysis. Based on the studies mentioned, it was found that pyrrolidinium surfactant SAILs form aggregates. Moreover, according to the analyses performed, it is concluded that the type of the anions affect the micellization properties, and the most surface-active compound was identified as [Dec 2 Pyrr][2,4-D]. This observation is attributed to the presence of aromatic counterions and the functional groups attached to it. Interestingly, the surface properties of the compounds coincide with their phytotoxicity. Thus, the surface activity as one of the physicochemical properties, can be considered as an excellent tool for a better interpretation of the biological activity of the analyzed compounds. We hope that the research and considerations presented in this publication will contribute to a deeper understanding of the role of herbicidal anions in explaining the phenomena occurring at the interface and in the solution itself, and will stimulate the informed use of SAILs in the industry.

Corresponding author: Katarzyna Materna, Department of Chemical Technology, Poznan University of Technology, ul. Berdychowo 4, Poznan 60-965, Poland, E-mail:

Funding source: Ministry of Education and Science in Poland

Award Identifier / Grant number: 0912/SBAD/2308

About the authors

Marta Wojcieszak

Marta Wojcieszak is a Ph.D. student in Department of Chemical Technology, Poznan University of Technology. She is involved in study of surface properties of new compounds.

Anna Syguda

Anna Syguda is a Ph.D. in chemistry in Department of Chemical Technology, Poznan University of Technology. Her research interests are focused on organic chemical technology, in particular the synthesis of multifunctional compounds such as ionic liquids.

Katarzyna Materna

Katarzyna Materna is a Professor in Department of Chemical Technology, Poznan University of Technology. Her research interests are focused on study of surface properties of new compounds.

  1. Research ethics: Not applicable.

  2. Author contributions: Marta Wojcieszak: Methodology; writing: original draft; investigation; writing: review and editing; validation; data curation. Anna Syguda: Conceptualization; validdation; methodology; investigation; data curation; writing: original draft; writing: review; editing. Katarzyna Materna: Supervision; resources; project administration, funding acquisition.

  3. Competing interests: The authors states no conflict of interest.

  4. Research funding: This study was funded by the Ministry of Education and Science in Poland (0912/SBAD/2308).

  5. Data availability: Not applicable.

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Supplementary Material

This article contains supplementary material (https://doi.org/10.1515/tsd-2023-2571).

Received: 2023-11-14
Accepted: 2023-12-12
Published Online: 2024-02-09
Published in Print: 2024-03-25

© 2023 Walter de Gruyter GmbH, Berlin/Boston

Articles in the same Issue

  1. Frontmatter
  2. Biosurfactants
  3. Study on the properties of branched-chain alkyl glycoside nonionic surfactant and anionic surfactant in a mixed system
  4. Aggregation behavior and thermodynamic parameters of biosurfactants (NaC/NaDC) in aqueous medium of Emtricitabine and Lamivudine (anti-HIV drugs)
  5. Drug Release Systems
  6. Lamellar liquid crystals formed from sucrose ester and Brij97 solutions for curcumin delivery
  7. Surface Active Ionic Liquids
  8. The influence of herbicidal anions on chemical shifts in NMR, phytotoxicity and surface properties of pyrrolidinium surface-active ionic liquids
  9. Applications
  10. Study on an all-in-one foaming agent with corrosion inhibition for air foam flooding
  11. Study on the scale inhibition performance of organic chelating agent aided by surfactants on CaCO3 at high salinity condition
  12. Study on the influence of emulsification ability of oil displacement system on its chemical flooding recovery
  13. Effect of contaminated water (handwashing detergent) on seed germination traits in wheat, mung bean, and chickpea
  14. Synthesis
  15. Synthesis and application feasibility study of cetyltrimethylammonium p-toluenesulfonate
https://doi.org/10.1515/tsd-2023-2571
Keywords for this article
ionic liquids; chemical shifts in NMR; critical micelle concentration; AFM analysis; phytotoxicity

Articles in the same Issue

  1. Frontmatter
  2. Biosurfactants
  3. Study on the properties of branched-chain alkyl glycoside nonionic surfactant and anionic surfactant in a mixed system
  4. Aggregation behavior and thermodynamic parameters of biosurfactants (NaC/NaDC) in aqueous medium of Emtricitabine and Lamivudine (anti-HIV drugs)
  5. Drug Release Systems
  6. Lamellar liquid crystals formed from sucrose ester and Brij97 solutions for curcumin delivery
  7. Surface Active Ionic Liquids
  8. The influence of herbicidal anions on chemical shifts in NMR, phytotoxicity and surface properties of pyrrolidinium surface-active ionic liquids
  9. Applications
  10. Study on an all-in-one foaming agent with corrosion inhibition for air foam flooding
  11. Study on the scale inhibition performance of organic chelating agent aided by surfactants on CaCO3 at high salinity condition
  12. Study on the influence of emulsification ability of oil displacement system on its chemical flooding recovery
  13. Effect of contaminated water (handwashing detergent) on seed germination traits in wheat, mung bean, and chickpea
  14. Synthesis
  15. Synthesis and application feasibility study of cetyltrimethylammonium p-toluenesulfonate
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