Novel eco-friendly ionic liquids to solubilize seven hydrophobic pesticides

Journal Pre-proof Novel eco-friendly ionic liquids to solubilize seven hydrophobic pesticides

Wenzhuo Wang, Kai Sheng, Fengmao Liu, Yuke Li, Qingrong Peng, Yangyang Guo PII:

S0167-7322(19)34550-7

DOI:

https://doi.org/10.1016/j.molliq.2019.112260

Reference:

MOLLIQ 112260

To appear in:

Journal of Molecular Liquids

Received date:

13 August 2019

Revised date:

2 December 2019

Accepted date:

4 December 2019

Please cite this article as: W. Wang, K. Sheng, F. Liu, et al., Novel eco-friendly ionic liquids to solubilize seven hydrophobic pesticides, Journal of Molecular Liquids(2018), https://doi.org/10.1016/j.molliq.2019.112260

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© 2018 Published by Elsevier.

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Novel eco-friendly ionic liquids to solubilize seven hydrophobic pesticides Wenzhuo Wang, Kai Sheng, Fengmao Liu, Yuke Li, Qingrong Peng*, Yangyang Guo College of Science, China Agricultural University, Beijing 100091, China

Abstract: In line with the requirements of green chemistry, the preparation of eco-friendly

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pesticide formulations has become an inevitable development trend. In this work, we

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explored the feasibility of four new ionic liquids (ILs, [Octyl trimethyl ammonium] anions,

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anions=proline, valine, leucine, tartaric acid) as environmentally-friendly solvents for

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pesticide emulsifiable concentrates, and also as alternatives to traditional imidazole-based ILs

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since they do not contain the imidazole ring and the anions are natural substances. The results showed that the solubility of pesticides (clethodim, metolachlor, acetochlor, prochloraz,

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thiamethoxam, glyphosate, acetamiprid) in water with 25%w/v ILs was significantly

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improved, especially for acetamiprid, where the solubility was increased 19 -fold, while it

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only increased about 4 to 5-fold after adding imidazole-based ILs in our previous work. In addition, the solubilization mechanism was explored by many methods and found that the solubilization effect does not completely depend on the critical micelle concentration (CMC) value but has a great relationship with the hydrophilicity of the ILs which may affect the size of micelles and release behavior to pesticides. Finally, 1H NMR was used to investigate the solubilization sites of ILs for glyphosate, and the interaction between pesticides and anions was explored by the quantum chemical method.

Keyword: Solubilizer, Insoluble pesticides, Ionic liquids, Solubilization mechanism

1

Journal Pre-proof 1. Introduction Pesticides are important materials for agricultural production and are widely used for preventing and controlling pests and diseases from earliest times to the present day. There are many available formulations for the pesticide, however, emulsifiable concentrate (EC) still accounts for the largest proportion in sales and is the major formulation in China.

f

EC is one of the traditional pesticide formulations, with high active ingredient content,

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good stability and convenient application [1]. In the preparation of EC, it is often necessary to

pr

dissolve the active ingredient into a large amount of organic solvents such as benzene, xylene,

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etc., which not only cause environmental pollution, but also endanger human health [2]. In

Pr

recent years, with the increasing awareness of environmental protection among the state and the public, the demand for developing new eco-friendly pesticide formulations has become

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more intense [3]. Reducing the use of organic solvents as well as exploring new green

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alternative solvents have become the main development trend for EC in the future [4-5]. In

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our early work, traditional imidazole-based ionic liquids (ILs) were chosen as the alternative solvent for organic solvents to study the dissolution of water-insoluble pesticides, and the results show that the solubility of pesticides in water was significantly improved due to the addition of ILs. These works confirmed the feasibility of ILs as alternative toxic solvents for EC and provide guidance for the follow-up work. [6-9]. However, the potential toxicity of traditional imidazole-based ILs has attracted widespread attention with the deepening of research in this field [10-11]. At present, many studies have confirmed that traditional ionic liquids have certain biological toxicity and cytotoxicity [12-13], which can affect the growth and development of algae [14], aquatic 2

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organisms [15] and soil organisms [16]. The toxicity of ionic liquids has always been related with its structure, in particular, the types of cations in ionic liquids [17], the length of alkyl chain linked by cations [18] and the types of anions [19-20]. At present, many studies have compared the toxicity of ILs with different cations to cells [21], microorganisms [22-23], and the results show that the imidazolium-based ILs general have lower EC50 or IC50 value than

f

ammonium-based ILs which indicated that the imidazolium-based ILs has higher toxicity.

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Meanwhile, Kristina Radocheeviqi et al. [24] compared the effects of ionic liquids with

pr

different kinds of anions on the cytotoxicity of CCO, and found that the anion species also

e-

contributed to the differences in toxicity of ILs when the cations were kept constant. Now,

Pr

more and more studies have introduced amino acids as anions into ionic liquids, which can reduce the biological toxicity of ionic liquids. For example, Shuanggen Wu et al. [25]

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compared the toxicity of [C4mim] [Pro], [C4mim] [Val] and [C4mim] [Br] to wheat growth,

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and found that amino acid anions had lower toxicity. In summary, the toxicity of ionic liquids

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should be concerned when they are used，to address these earlier concerns of toxicity, we decided to improve the structure of traditional ionic liquids and explore more environmentally friendly solvents as alternative to toxic solvents for pesticide formulation. Currently, Conductor-like Screening Model for Real Solvents (COSMO-RS) has been successfully applied to calculate the thermodynamic properties and to explain the interactions between compounds. COSMO-RS is a quantum chemical model developed by Klamt and co-workers to predict thermodynamic properties [26]. In the COSMO-RS theory, each molecule is described by the shielding charge σ of its surface. In order to perform effective statistical thermodynamic calculations, the molecular surface always be divided into several 3

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small blocks, which are independent of each other, and the surface shielding charge is calculated by the COSMO method in quantum chemistry. The surface shielding charge density of all the small blocks constitutes the entire molecule and can be represented by a one-dimensional graph, which is called a σ-profile. After obtaining the surface shield charge distribution of each molecule, the interaction can be described by the shielding charge of each

f

small blocks on the surface of the molecule.

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In this work, we designed four new ionic liquids（ILs, [Octyl trimethyl ammonium]

pr

anions, anions=proline, valine, leucine, tartaric acid) as alternative to toxic solvents. These

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novel ILs are more eco-friendly than traditional ionic liquids, since they do not contain the

Pr

imidazole ring and the anions are natural substances. The structural and abbreviations of ILs are shown in Fig 1. To test these ILs, we attempted to solubilize seven pesticides commonly

al

used in the field as study targets, including clethodim, metolachlor, acetochlor, glyphosate,

rn

thiamethoxam, acetamiprid and prochloraz (Fig 2). The mechanism of solubilization of these

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pesticides were investigated by measuring the surface properties, the viscosity, the hydrodynamic diameter and 1H NMR of ILs. Additionally, in order to further explain the interaction between the anions of ionic liquids and pesticide molecules, the σ- files was calculated using the quantum chemical method COSMO-RS. Finally, the release behavior of pesticide in ionic liquid aqueous system was evaluated. The purpose of this work is to provide reference and scientific basis for the development of eco-friendly EC of pesticide.

4

rn

al

Pr

e-

pr

Figure 1. Structure and abbreviations of four ILs

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f

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Figure 2. Structure and molecular weight of seven pesticides

2. Experimental 2.1.

Materials

ILs used in this study: Octyl trimethyl ammonium proline ([OTMA] Proline), Octyl trimethyl ammonium valine ([OTMA] Valine), Octyl trimethyl ammonium leucine ([OTMA] Leucine), Octyl trimethyl ammonium tartaric acid ([OTMA] Tartaric acid) were entrusted to Shanghai Cheng Jie Chemical Co., Ltd. to prepared, and the purity of the ILs were higher than 99%. Clethodim, metolachlor, acetochlor, prochloraz, thiamethoxam, glyphosate and 5

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acetamiprid were procured from China Agricultural University, and the purity of the pesticides were above 97.5%. 2.2.

Solubility Studies

The solubility of the seven pesticides was tested according to the method reported by Maswal et al. [27] The specific procedure was as follows: An analytical balance (Sartorius,

oo

f

Germany) with an accuracy of ±0.0001g was used to accurately weigh a certain amount of IL into a 5 mL sample vial and 3 mL of 25 %w/v IL water solution with excessive pesticide

pr

were prepared. The mixed solution was then vortexed for 10 minutes, and placed in an

e-

ultrasonic bath (Crest Ultrasonics, USA) for 15 min at 298K±0.1 K. The precipitated

Pr

pesticide that remained insoluble was removed by filtration through a 0.45 μm Millipore Millex-LG filter made of polytetrafluoroethylene. After that, the amount of pesticides in the

al

transparent filtrate was measured using a UV spectrophotometer (Shimadzu UV-1800, Japan)

rn

at 230 nm for metolachlor, prochloraz, and acetochlor, 260 nm for thiamethoxam, 246 nm for

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acetamiprid, and 280 nm for clethodim. The solubility of each pesticide was measured three times. 2.3.

Measurements of IL surface activity

2.3.1. Electrical conductivity The conductivity of four aqueous ILs was measured at 288, 293, 298, 303, 308 K using a CyberScan CON1500 benchtop conductivity meter. The desired temperature was maintained within ±0.1 K by a bath circulator, and samples were measured three times at each temperature. The conductivity was plotted against the concentration of ILs aqueous solution, 6

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wherein the concentration corresponding to the curve inflection point is the CMC value of the solution. 2.3.2. Steady-state fluorescence measurements The specific determination method of steady-state fluorescence refers to our previous research [9]. The fluorescence emission spectra of pyrene (fluorescent probe) in each IL

oo

f

aqueous solution were measured at 298 K using a PerkinElmer LS-55 fluorescence spectrophotometer (PE company, UK). The excitation wavelength was set to 335 nm, and the

pr

emission spectrum was recorded at 350 to 500 nm, in which the slit widths for emission and

e-

excitation were fixed to 2.5 and 10 nm, respectively. The pyrene shows a fine structure in the

Pr

370-400 nm region of the steady-state fluorescence emission spectrum, and the ratio of the fluorescence intensities of the first (373 nm) and third (384 nm) vibrating peaks i.e. I 3/I1,

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shows good sensitivity to changes in the polarity of the solvent, and thus pyrene can be used

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the aqueous solution.

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as a probe for changing the polarity of the solvent and determining the CMC value of IL in

2.3.3. Surface tension measurement.

The surface tension of the ILs aqueous solution was measured using a tensiometer with flat plate method at 298 K, so as to obtain a surface activity parameter and the critical micelle concentration (CMC) value of the solution. Measurements were made after the tensiometer calibration, and the surface tension of each solution was taken as the average of five measurements with a reproducibility of ±0.1 mN m -1 .

7

Journal Pre-proof 2.3.4. Spectrophotometric study Spectrophotometric measurements were performed in a UV 1601 Shimadzu (Japan) spectrophotometer using a quartz cuvette of 10 mm path length. A series of ILs aqueous solutions were prepared and the spectra were recorded over a wavelength range of 200-400 nm. The maximum absorption wavelength (λmax) is plotted against the ILs concentration, and

Measurements of viscosity

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2.4.

f

the concentration corresponding to the inflection point was denoted as the CMC.

pr

The viscosity was measured by viscosimeter (Brookfield DV1 viscometer), the four

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25%w/v IL aqueous solution were prepared in 50 mL centrifuge tube respectively. After

H NMR of four ILs was determined using a Bruker Avance DPX300 NMR spectrometer.

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1

Nuclear magnetic resonance spectroscopy

al

2.5.

Pr

ultrasound, the viscosity was measured at 288, 293, 298, 303 and 308 K in three replicates.

D2O as solvent. 2.6.

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(Bruker Corporation). About 15 mg of the sample was dissolved in 25% ILs solution with

Measurements of the average diameter

1% w/v and 25% w/v IL aqueous solutions were prepared for four ILs, respectively. The average diameter of IL aqueous solutions was measured by Dynamic Light Scattering (DLS) at 298K, and the samples were measured in triplicate. 2.7.

COSMO-RS methodology

Herein, COSMO-RS was used to calculate the σ-profiles of ionic liquids and pesticides. 8

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The first step is to calculate the target compound by quantum chemistry. The full geometry and energy of ionic liquid anion and pesticide molecules were calculated at the DFT BP TZVP level using the TURBOMOLE V7.3 software package which seamlessly connected with COSMOthermX (version 19.0), and the geometry was optimized to ensure that the energy was the lowest and the conformation of the compound was the most stable [28]. On a second step, the cosmo-files obtained by the above quantitative calculation was introduced

Release behavior study

pr

2.8.

oo

f

into COSMOthermX, and the σ-files of different compounds were compared.

e-

The release of pesticide from micelles was conducted according micellar dialysis

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method [29-30]. Acetamiprid was selected as the target pesticide, and 2mL of 25%w/v IL aqueous solution with pesticide was added into the dialysis tube, then immersed in 150mL of

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0.5% w/v sodium dodecyl sulfate (SDS). The use of SDS was conducive to the release of

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pesticide. The dialysis system was kept at 298K and rotates at 120 rpm in an air bath constant

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temperature oscillator. Periodically extract and add fresh release media of the same volume to dialysis medium. The extracted samples were filtered by 0.45 μm Millipore Millex-LG filter and determined by spectrophotometer.

3. 3.1.

Results and discussion Solubilization of pesticides

The solubility of pesticides in pure water and LogKow are shown in Table 1. Kow is the coefficient of distribution between n-octanol and water, as well as the indicator to evaluate the hydrophilic or hydrophobic properties of the compound. It can be seen that the solubility 9

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of the seven pesticides varies greatly, and the logKow values are small. The smaller logKow is, the higher hydrophilicity of the pesticide. Table 1. Physicochemical properties of seven pesticides pesticide

solubilitya

clethodim

metolachlor

acetochlor

glyphosate

thiamethoxam

prochloraz

acetamiprid

5.45

0.53

0.24

12.0

4.1

0.034

4.2

--

2.9

4.14

<-3.2

-0.13

3.53

0.80

359.9

283.8

269.8

169.1

376.7

222.7

Molecular

in pure water

e-

a

291.7

pr

mass

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logKow

f

(g·L-1)

Pr

Table 2 shows the solubility of the seven pesticides in 25 %w/v aqueous ILs at 298 K. Compared with the solubility in pure water, the presence of IL significantly improved the

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solubility of most pesticides. For example, clethodim show 3-13-fold higher solubility in

rn

aqueous IL than in pure water (5.45 g L-1), and the solubility of acetamiprid has increased by

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more than 19 times in [OTMA] Proline. In general, these four ILs have better solubilization effect on pesticides when compared with 25% %w/v aqueous of [C4mim] [Br], [C10mim] [Br] or Tween-20 which reported in our previous work [7]. In addition, the solubility of different pesticides varies greatly. There are two main reasons, namely the physicochemical properties of pesticides, especially solubility and logKow, and the size of pesticide molecules. Our previous studies have shown that pesticides with large water solubility and small logKow have greater solubility in ILs aqueous solutions, this was also demonstrated in our present study. Molecular size is also an important factor affecting the solubilization effect. It was found that thiamethoxam and acetamiprid have similar solubility 10

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as well as logKow, but the solubilization effect was greatly differed in IL aqueous solution. This may be due to the size of the molecule, as the molecular weight of acetamiprid (222.7) is smaller than that of thiamethoxam (291.7), the former to enter the micelle more readily during the formation of micelles. Table 2. The solubility of seven pesticides in 25 %w/v aqueous ILs at 298 K

glyphosate

acetamiprid

clethodim

thiamethoxam

f

Solubility (g L-1) prochloraz

metolachlor

acetochlor

150

80

54

40

<1

<1

<1

9

52

75

18

<1

<1

43

52

49

34

<1

<1

<1

10

50

28

<1

17

<1

[OTMA]

pr

Proline [OTMA]

Pr

[OTMA] Valine

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acid

21

al

[OTMA] Tartaric

21

e-

Leucine

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ILs

In addition to the properties of pesticides, the types of ionic liquids also affect the

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dissolution effect. In this study, four ILs with the same cations but different anions were selected, and the solubility of seven pesticides in different IL aqueous solution was compared. The results reveal that [OTMA] Proline has the best effect on improving the solubility of pesticides, followed by [OTMA] Leucine and [OTMA] Valine, and [OTMA] Tartaric acid was the worst. Such variation might be attributed to many reasons, such as the surface properties, aggregation behavior of the ILs, the viscosity of ILs, and so on. Therefore, in the subsequent study, we discussed the factors influencing the solubility of pesticide.

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Surface properties of aqueous ILs

In this study, all the ILs are amphiphilic molecules with hydrophobic groups and hydrophilic groups, like surfactants. Studies have shown that surfactants used as solubilization agents is related to the formation of micelles [31]. In short, when the concentration of surfactants reaches the critical micelle concentration (CMC) in the solution,

f

the surfactant is no longer arranged in a monolayer, but forms micelles. At this time,

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water-insoluble compounds would enter the hydrophobic micelle nucleus, and the solubility

pr

could increase sharply. CMC is an important parameter of surfactant, and when the

e-

concentration of surfactant in solution reaches CMC, some physicochemical properties of the

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solution will change abruptly. As such, this shift in parameters can be used to determine the CMC of the surfactant. Herein, in order to study the effect of IL micelle aggregation on

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pesticide solubility, CMC of 25 %w/v aqueous ILs were measured at 298 K by surface

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tension method, conductivity method, UV method and steady-state fluorescence method, and

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the results are shown in Fig 3.

As shown in Table 3, although the CMC values obtained by the above four methods were different, these still showed the similar trend. i.e. [OTMA] Proline > [OTMA] Tartaric acid > [OTMA] Leucine > [OTMA] Valine. However, it was unexpectedly found that the CMC values of the four ILs were irrelevant to the solubilization effect in this study, which is not consistent with our conventional understanding, that is, the smaller the CMC value corresponds to better the solubilization effect. Therefore, we speculated that there are other factors may dominate the effective of solubilization. The nature of the surfactant is determined by several factors, wherein the hydrophilicity tends to affect its solubility in water, 12

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which in turn may affect the quantity and size of micelles [32]. In general, the hydrophilicity of surfactant can be measured by hydrophilic-lipophilic balance (HLB) value, and the larger HLB value means the higher hydrophilicity [33-34]. The HLB values of four ILs were calculated based on the Davies group method. By comparing with the solubility results, it was found that [OTMA] Proline had the highest HLB value (21.375), whereas [OTMA] Tartaric

f

acid had the lowest HLB value (18.8). It reveals that when the concentration of IL in solution

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exceeds the CMC value, the factors determining the solubilization effect are not only CMC

pr

value, but also closely related to the amount of IL in aqueous solution, that is to say, the

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hydrophilicity of IL.

Pr

Table 3. The CMC of four ILs obtained through different methods CMC（mmol·L-1）

IL type steady-state

UV method

surface tension

conductivity

method

method

255

150

224

150

120

90

179

50

100

50

119

30

20

50

37

[OTMA] tartaric acid [OTMA] Leucine [OTMA] Valine

250

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Proline

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[OTMA]

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fluorescence method

13

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pr

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f

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Figure 3. (a) The surface tension of aqueous solutions with different content of IL, (b) the

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steady-state fluorescence of aqueous solutions with different content of IL, (c) the

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conductivity of aqueous solutions with different content, (d) the ultraviolet absorption intensity of aqueous solutions with different content. 3.3.

Thermodynamic parameters of ILs

The conductivity data of ionic liquids at different temperatures were measured. We can see from Figure 3(c), the slope of the conductivity before and after the mutation point (i.e. CMC) were different. Usually, the ratio of the slopes above and below the CMC is defined as the counter ion dissociation degree (k), and the degree of counterion binding to micelles (β) can be obtained as 1minus k. The thermodynamic data such as the standard Gibbs free energy (∆Gmθ), the variation in 14

Journal Pre-proof the standard enthalpy of micelle formation (∆Hmθ) and the standard entropy change (∆Smθ) of ionic liquids have been calculated, and the relevant formulas are as follows[35]. 𝜃 ∆𝐺𝑚 = (1 + 𝛽 )𝑅𝑇𝑙𝑛𝑋𝑐𝑚𝑐 𝜃 ∆𝐻𝑚 = −(1 + 𝛽 )𝑅𝑇 2 𝜃 ∆𝑆𝑚 =

(1)

𝑑𝑙𝑛𝑋𝑐𝑚𝑐

(2)

𝑑𝑇

𝜃 −∆𝐻 𝜃 ∆𝐺𝑚 𝑚

(3)

𝑇

f

Where R is gas constant, β is degree of counterion ionization, and Xcmc is CMC expressed

oo

in mole fraction.

8 29 3 [OTMA ] Proline

29 8 30 3 30 8 28 8 29 3

[OTMA ] Leucine

29 8 30 3 30 8

223.2 6 223.5 2 223.8 0 225.4 5 234.7 2

0.4 5

118.3 2 118.6 8 118.7

e-

Pr

28

0.5

5

0.4 5

0.5

5

0.4

5

0.5

5

0.4

2

0.5 8

0.5

0

0.5 0

0.6 3

0.3 7

0.6 5

0.3 5

0.6

9 118.8 5 120.0

4

9

3

lnXcm c (mmol L-1)

al

(K)

β

k

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T

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Ironic liquid

CMC (mmo l L-1)

pr

Table 4. Thermodynamic parameters of micellization.

0.3 6

0.6 5

0.3 5

0.6

0.3 7 15

∆Gmθ ∆Hmθ （KJ/mol） （KJ/mol）

T∆Smθ （KJ/mol）

-5.51

-13.20

-0.64

12.56

-5.51

-13.43

-0.66

12.77

-5.51

-13.66

-0.69

12.97

-5.50

-13.87

-0.72

13.14

-5.46

-13.99

-0.71

13.28

-6.15

-14.72

-0.66

14.06

-6.15

-14.97

-0.68

14.29

-6.14

-15.22

-0.70

14.52

-6.14

-15.48

-0.72

14.76

-6.13

-15.71

-0.75

14.95

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8 30 3 30 8 28 8 29 29 8 30 3 30

42.03 161.6 3 169.9 9 179.3 1 179.5 7 186.2 0

0.2

4

6 0.7

0.2

3

7 0.7

0.2

3

7 0.7

0.2

6

4 0.4

0.5

4

6 0.4

0.5

8

0.4

1

9 0.5

3 0.5 4

10.89

-7.31

-17.80

-7.03

10.76

-7.30

-18.09

-7.29

10.79

-7.28

-18.35

-7.56

10.79

-7.18

-18.40

-7.62

10.77

-5.84

-13.98

-7.29

6.68

-14.10

-7.39

6.70

-14.20

-7.46

6.74

-5.73

-14.44

-7.61

6.83

-5.70

-14.58

-7.81

6.77

-5.73

0.4 7

-6.75

-5.79

2 0.5

-17.64

0.4

6

al

8

38.04

0.7

-7.37

rn

It can be seen that the formation of micelles is a spontaneous and exothermic process for any ionic liquid, since both ∆Gmθ and ∆Hmθ are negative. It should be noted that ∆Hmθ and

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[OTMA ] Tartaric acid

3

37.39

5

f

29

5

oo

[OTMA ] Valine

3

37.20

0.2

pr

29

0.7

e-

8

34.98

Pr

28

∆Smθ show different trends as the temperature changed. There are structural water molecules in the hydration layer around the micelle. When temperature increased, the hydration between the alkyl chain and the surrounding water molecules was reduced, and the water molecules bound around the alkyl chain has been released, which eventually leads to the decrease of ∆Hmθ and the increase of the chaotic degree of the system. In addition, the contribution of ∆Smθ to the energy of the system was greater than ∆Hmθ, that means the formation process of the micelle is mainly driven by entropy.

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Journal Pre-proof 3.4.

The average diameter of ILs

The average diameter of ILs with different content in aqueous solutions were determined by DLS. The result is shown in Figure 6. When the content of the ionic liquid is 1%, the average diameters of [OTMA] Proline, [OTMA] Leucine, [OTMA] Valine, [OTMA] Tartaric acid are 402.2 nm, 216.7 nm, 246.0 nm, and 252.0 nm, respectively. When the content of the ILs

f

reaches 25%, the diameters of the above four ILs in the aqueous solution are 1693 nm, 1235

oo

nm, 1256 nm, and 779.6 nm, respectively. This great distinction reveals the formation of

pr

micelles. It is very interested to find that the solubilization effect of the ionic liquid may be

e-

related to the particle size of the micelle. Although [OTMA] Proline has a large CMC value,

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its average diameters are larger after micelle formation, which provides more solubilization sites for pesticides. Conversely, [OTMA] Tartaric acid has the smallest average diameters

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with the worst solubilization effect. The results also confirmed that the size of micelles was

rn

related to the hydrophilicity of ionic liquids, which would affect the solubilization effect of

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pesticides. In addition, as the ILs content increases, the particle dispersion index (PDI) of all solution increases. It shows that the uniformity of the solution decreases after the micelles are formed.

17

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rn

al

Pr

e-

pr

oo

f

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Figure 4. The particle size distribution plot for 1%w/v and 5%w/v IL aqueous solution at 25℃. (a) and (b) for [OTMA] Proline, (c) and (d) for [OTMA] Leucine, (e) and (f) for [OTMA] Valine, (g) and (h) for [OTMA] Tartaric acid. 3.5.

The viscosity of ILs

Viscosity is one of the important physicochemical properties, which has great influence on 18

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mass transfer in the process of dissolution. Generally speaking, low solubility facilitates to the diffusion of matter and increase the reaction rate. To investigate the effect of ILs solution viscosity on pesticide solubility, we determined the viscosity of 25 %w/v four aqueous ILs at 298 K and calculated Ea according to Eq (4). The results are shown in Table 4. lnη = lnη0 + Eη/RT

(4)

f

Viscosity is usually related to molar mass and intermolecular interaction forces, including

oo

van der Waals forces, hydrogen bonds, etc., generally, the viscosity of water at room

pr

temperature is 1 mPa·s, which belongs to the low viscosity range, and the viscosity of some

e-

ILs aqueous solutions can be as high as tens to hundreds of mPa·s. The high viscosity of ILs

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is considered to be one of its main shortcomings that limits application in practical production. Recently, there have been many studies devoted to the design of ionic liquids with low

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viscosity. In this study, our four novel ionic liquids have low viscosity, especially compared

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with ionic liquids we used before, such as tetradecyl trimethyl ammonium bromide, the

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viscosity of whose 30 %w/v aqueous IL solution is as high as 168.8 mPa·s at room temperature. It can be said that ILs with low viscosity are more conducive to mass transfer and increase the solubility of pesticides. The types of anions in ILs may affect the intermolecular forces and subsequently the viscosity of ILs [36]. By comparing the viscosities of ILs with different anions in Table 4, it can be found that the viscosity of [OTMA] Proline was the highest among the four ILs, although this difference is subtle, the viscosity of all four aqueous ILs were low. Therefore, the effect of anionic species on the viscosity of these four ILs appears to be minor in this study. Besides, temperature is an important factor affecting the viscosity of ILs. An increase 19

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of temperature, would decrease the intermolecular interaction force and the viscosity of ILs [37-38]. By calculating activation energy (Ea) of ionic liquids, it can be seen that with the decrease of the viscosity of ionic liquids, the corresponding Ea also decreases. Table 4. The viscosity and Ea of four ILs at five temperature IL type

Viscosity mPa·s 298K

303K

308K

（KJ·mol-1）

4.7

4.4

16.1

4.3

4.1

15.8

4.0

3.9

10.6

4.1

4.0

10.4

6.8

5.8

5.6

[OTMA] Leucine

6.3

5.5

5.1

[OTMA] Valine

5.2

4.9

4.5

[OTMA] Tartaric acid

5.3

5.2

4.9

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Pr

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[OTMA] Proline

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293K

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288K

Ea

Figure 4. Viscosity of four 25 %w/v aqueous ILs at five temperature points. 3.6.

Detection of IL-pesticide interactions using 1H NMR

There are three main types of solubilization sites for micelles: 1) solubilization in the micelle core for some non-polar compounds, as shown in Fig 5 (a) [39-40]. 2) Solubilization in the micelle barrier area for compounds containing polar groups, where hydrophobic alkyl chains can be inserted into the core of micelles and hydrophilic groups can be arranged between the polar regions of IL (Fig 5 b) [41], and this method allowing to give the best 20

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solubilization effect. 3) Absorption into the outer shell of micelles for some macromolecule substances as shown in Fig 5 (c), this solubilization method has the worst solubilization

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effect.

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Figure 5. Modes of solubilization of ILs. (a) solubilization in the micelle core, (b)

H NMR is commonly used as a method to explore the potential interaction between

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1

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solubilization in the micellar barrier area, (c) solubilization in the micelle shell.

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solvents and solute molecules [42]. In this study, we compared the 1H NMR spectra of four ILs in the presence or absence of glyphosate, and the results are shown in Table 5. Due to the

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interaction (such as solvophobic solvation) between the alkyl chain of solvents and the solute,

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the chemical shift of protons on the alkyl chain of solvents moves to upfield, which has been

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reported in literatures and has same trend in this work [43-46]. By comparison, it can be found that the chemical shifts of protons on alkyl chain move to upfield in different degrees after adding glyphosate, and the chemical shifts follow the order of long chain methylene > terminal methyl > α-CH2 - >β-CH2-. This result revealed that the solubilization sites of glyphosate are mainly in the micelle barrier formed by ILs as shown in Fig 5 (b). In addition, we also noticed that for the anions of ILs moved to downfield, especially for [OTMA] Proline. We speculated that there might be exist hydrogen bond between glyphosate and anions of ILs. Table 5. 1H chemical shift changes (δ, ppm) of each group on ILs after adding glyphosate IL type

α-CH2 -

β-CH2-

long chain methylene 21

terminal methyl

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0.02-0.03

0.03-0.05

0.04

[OTMA] Leucine

0.06-0.1

0.07

0.07-0.08

0.04

[OTMA]Valine

0.03-0.04

0.03

0.02-0.04

0.02

[OTMA] Proline

0.04-0.07

0.01

0.05-0.08

0.06-0.09

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[OTMA] Tartaric acid

Figure 6. a) 1H NMR spectra of [OTMA] Proline solution without glyphosate added, b) 1H NMR spectra of [OTMA] Proline solution after adding glyphosate. 3.7.

σ-Profiles prediction by COSMO-RS

The interaction between four anions and two typical pesticides were predicted by calculating the σ-profiles in COSMO-RS. σ-profiles are important molecular descriptors, which provide information about the interactions between compounds in solution. σ-profiles can be

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qualitatively divided into three main regions as usual: hydrogen bond donation (HBD) region (σ< 0.0082 e/Å2), non-polar region (-0.0082<σ<0.0082 Å2) and hydrogen bond acceptor (HBA) region (0.0082 e/Å2<σ). As shown in Figure 7 (a), the σ curves of the four anions were mainly distributed in the non-polar and HBA regions due to the loss of a proton. For metolachlor, there was a large peak in the non-polar region because of the benzene ring in the molecule, and a small peak in the HBD region since it contains Cl-. It is worth noting that the

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σ curve of metolachlor (orange line) in the non-polar region well complements that of tartaric

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acid anion (red line) as compared to other anions. This indicates that tartaric acid anion

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renders the IL a better solubilizer for metolachlor than other anions when the cations were the

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same. This is consistent with the experimental results that metolachlor has the highest solubility in 25 %w/v aqueous [OTMA] Tartaric acid. Yan-Rong Liu [47] also studied the

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interaction of ionic liquid anions with cellulose by describing the σ-profiles, and the results

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demonstrated that anionic AC-, which was complementary to the cellulose model, had a better

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dissolution effect on cellulose. Unlike metolachlor, glyphosate has peaks in both HBD and HBA regions because of its hydroxyl group, carboxyl group and imino groups. The peaks of glyphosate (orange line) in HBD region were highly symmetrical with those of proline in HBA region (pink line), which also confirms that glyphosate and proline anion may interact via hydrogen bonds.

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The release behavior of pesticide

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3.8.

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Figure 7. σ-profiles of four anions (a) metolachlor and (b) glyphosate.

The experimental results of dialysis release study are shown in figure 8. During the first

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two hours, acetamiprid was busted out from the micelles, and then the release became smooth.

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[OTMA] Valine has the fastest release rate, and 92.7% of acetamiprid has been released in 2

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hours. [OTMA] Leucine and [OTMA] Proline have released more than 90% of acetamiprid in 4 and 6 hours, respectively. In contrast, although [OTMA] Tartaric acid has the smallest micelle particle size, it shown the slowest release rate. This is similar to the phenomenon reported by Pankaj singla [45-46]. We believe that in addition to the micelle particle size, the cause of this phenomenon may be related to the hydrophilicity of IL, more hydrophilic of the ionic liquid is, the easier of pesticides to be released.

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Figure 8. The release of acetamiprid from different IL system at 298K (n=3).

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Besides, zero-order kinetics, first-order kinetics, Higuchi kinetics, the Hixson-Crowell

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model and the KP model were used to evaluate the release of acetamiprid. The corresponding

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correlation coefficients and rate constants of the five models are calculated respectively, as shown in table 7. By comparing R2, we can see that the first-order dynamic model has the

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best fitting effect. According to the KP equation, the n of [OTMA] Proline and [OTMA]

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Leucine are less than 0.5, revealing that the release of acetamiprid follows the Fickian

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diffusion mechanism. For [OTMA] Valine and [OTMA] Tartaric acid, where n is slightly greater than 0.5 suggesting a coupled erosion–diffusion existed in this process. Table 6. Rate constants and regression correlations using different kinetic model for the release behavior of acetamiprid from different IL system. zero order kinetics

first

order

Higuchi kinetics

kinetics IL system

R2

K0 (h-1)

R2

Hixson-Crowell

KP model

model K (h-1)

R2

K (h-1)

R2

K (h-1)

R2

K

n -1

(h ) [OTMA] Proline

0.777

36.08

0.965

4.26

0.915

14.62

0.917

4.06

0.976

3.73

0.43

[OTMA] Valine

0.846

28.47

0.991

4.42

0.962

3.57

0.961

4.25

0.978

3.90

0.52

[OTMA]

0.781

37.88

0.983

4.46

0.912

11.52

0.932

4.13

0.961

4.10

0.46

Leucine 25

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21.11

0.974

4.47

0.965

-2.74

0.950

4.30

0.976

3.74

acid

4. Conclusions In this study, four new ILs were designed and their dissolution effects on seven common pesticides were tested. The results showed that the solubility of pesticides in water increased

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significantly after the addition of ILs. This may provide ideas and foundations for the

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development of eco-friendly pesticide formulations. In addition, the research explored the

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solubilization mechanism of ILs. The CMC values of the four IL were obtained by testing

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surface tension, electrical conductivity, UV spectra and steady-state fluorescence. However,

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unlike the previous research results, the solubilization capacity of ILs does not completely depend on the CMC value but has a great relationship with the hydrophilicity of the ILs in

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this study, and the hydrophilicity of the ILs would affect the size of micelles and release

H NMR spectra. It was found that the chemical shifts of hydrogen atoms in ILs alkyl chain

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1

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behavior to pesticides. Furthermore, the solubilization sites of four ILs were determined by

move to upfield obviously after adding glyphosate, and the peak shape also broadened compared to ILs alone especially seen for [OTMA] Proline. Therefore, the solubilization sites of IL to glyphosate are mainly in alkyl chains. In addition, we found that the chemical shift of hydrogen atoms on ILs anions shifted to low field in varying degrees after adding glyphosate. We speculate this too due to the interaction between IL anions and glyphosate via hydrogen bonds, and this is supported by the COSMOthermX calculation of the σ-profiles of glyphosate. Finally, the release behavior of micelles to pesticides was determined by dialysis release method, and the results showed that valine released the fastest and tartaric acid the 26

0.56

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slowest. The purpose of this study is to provide the basis and ideas for designing eco-friendly solvents to replace toxic solvents in pesticide EC. AUTHOR INFORMATION Corresponding Author E-mail: [email protected]

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Tel: +86 10 62736957

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Journal Pre-proof Figure captions Fig 1. Structure and molecular weight of seven pesticides. Fig 2. Structure and abbreviations of four ILs Fig 3. a) The surface tension of aqueous solutions with different content of IL, b) the steady-state fluorescence of aqueous solutions with different content of IL, c) the conductivity of aqueous solutions with different content, d) the ultraviolet absorption

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intensity of aqueous solutions with different content. Fig 4. Viscosity of four 25 wt% aqueous ILs at five temperature.

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Fig 5. Modes of solubilization of ILs. a) solubilization in the micelle core, b) solubilization in

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the micellar barrier area, c) solubilization in the micelle shell.

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Fig 6. a) 1H NMR spectra of [OTMA] Proline solution without glyphosate added, b) 1H NMR spectra of [OTMA] Proline solution after adding glyphosate.

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Fig 7. σ-profiles of four anions a) metolachlor and b) glyphosate.

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Figure 8. The release of acetamiprid from different IL system at 298K (n=3).

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Novel eco-friendly ionic liquids to solubilize seven hydrophobic pesticides Wenzhuo Wang, Kai Sheng, Fengmao Liu, Yuke Li, Qingrong Peng*, Yangyang Guo College of Science, China Agricultural University, Beijing 100091, China Author contributions: Wenzhuo Wang: Investigation, Writing - Original Draft

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Kai Sheng: Investigation, Data Curation

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Fengmao Liu: Conceptualization, Writing - Review & Editing

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Yuke Li: Visualization, Formal analysis, Software

Qingrong Peng: Supervision, Writing - Review & Editing

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Yangyang Guo: Software.

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Journal Pre-proof Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Graphical abstract

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Journal Pre-proof Highlights

New ILs were designed as alternative toxic solvents to pesticide formulations.

2.

Investigate the factors affecting the solubilization effect of ionic liquids.

3.

The solubilization sites of ILs for glyphosate were determined by 1H NMR.

4.

The interaction between pesticides and anions was explored by COSMO-RS.

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