Article pubs.acs.org/JPCB
Sparingly Soluble Pesticide Dissolved in Ionic Liquid Aqueous Tengfei Fan, Xuemin Wu, and Qingrong Peng* College of Science, China Agricultural University, Beijing 100091, China ABSTRACT: Ionic liquids may be considered as “environment-friendly solvents” for sparingly soluble pesticides. In this study, a series of aqueous ionic liquids (ILs) with different cations and different anions was used as environment-friendly alternative to harmful organic solvents sparingly dissolved in soluble pesticides (metolachlor, acetochlor, clethodim, thiamethoxam, and prochloraz). The aggregation behavior of aqueous ILs was investigated through surface tension measurement. Minimum area per IL molecule (Amin) values from the surface tension measurement showed that alkyl chain length and the halide anions strongly affect the aggregation behavior of ILs and the solubilization of pesticides. The solubility of metolachlor, acetochlor, clethodim, thiamethoxam, nitenpyram, and prochloraz in aqueous ILs increased. More importantly, the solubility of prochloraz in [C10mim][I] became 5771-fold higher than that in pure water. The substantially enhanced solubility of the above pesticides proved that aqueous ILs are promising environment-friendly solvents for pesticides that are commercially processed in emulsifiable concentrate (EC) formulation.
1. INTRODUCTION
Prochloraz is currently used as an imidazole fungicide for gardening and agriculture in Europe, Australia, Asia, and South America. Prochloraz is used on wheat, barley, mushrooms, cherries, turf on golf courses, and flower production.3 To satisfy the requirement of using sparingly soluble substrates and to reduce harmful organic solvents, various chemicals could be solubilized with ionic liquids (ILs) and/or mixtures of ILs and/or IL-based solvent systems.4−7 ILs are accepted as new green chemical solvents, which are liquid at room temperature or at temperatures near room temperature. ILs comprise a distinct set of salts composed of organic cations and inorganic anions. The cations are usually made up of inorganic polyatomic anions and methylimidazolium, pyridine, methylpyridine, or pyrrolodinium, and so on. Moreover, ILs have many unique properties, such as lack of measurable vapor pressure, excellent thermal stability, and recyclability. These characteristics are more pronounced in ILs compared with water or common organic solvents. More importantly, the physicochemical properties of ILs can be tailored by the choice of cations and anions. Thus, ILs are used to create “designer liquids”. ILs have elicited much interest for applications in chemical synthesis,8,9 electrochemistry,10 biocatalytic transformations,11 and analytical and separation sciences.9,12 Among hundreds of ILs, ILs with 1-alkyl-3-methylimidazolium cation and viz [Cnmim]+ have received much attention and have been extensively studied.13−15 ILs based on imidazolium, pyridinum, or methylpyridinum cations and halide anions undergo aggregation beyond a critical concentration called critical aggregation concentration (CAC).16,17 The CAC
Large amounts of organic toxic solvents are added in liquid formulations because most pesticides are sparingly soluble in water. Regulatory authorities have answered the public’s call for “green chemistry” by replacing harmful active ingredients and formulating inerts. Metolachlor, acetochlor, clethodim (herbicides), thiamethoxam (insecticides), and prochloraz (fungicide) (Figure 1) are poorly water-soluble pesticides that are widely used in agriculture. All of them are poorly soluble in water, whereas nitenpyram (insecticides) is highly water-soluble. The molecular structures of the six pesticides under study are as follows: Metolachlor, acetochlor, and clethodim are widely used as herbicides. Metolachlor and acetochlor are members of the chloroacetanilide herbicides, and are used for broadleaf weeds and annual grasses in domestic soybean and corn crops. Main structural differences are seen in the nonchlorinated alkyl group attached to the anilide moiety and in the methyl or ethyl substituents on the benzene ring. Clethodim belongs to the acetyl-coenzyme A carboxylase (ACC)-inhibiting cyclohexanedione herbicide. Clethodim is used in peanut plants to control annual and perennial grasses.1 Thiamethoxam is the first commercial neonicotinoid insecticide from the thianicotinyl subclass that acts by binding to nicotinic acetylcholine receptors. Thiamethoxam exhibits exceptional systemic characteristics, and provides excellent control of a broad range of commercially important pests, such as aphids, jassids, whiteflies, thrips, rice hoppers, Colorado potato beetle, flea beetles, wireworms, and some lepidopteran species.2 Nitenpyram is also a neonicotinoid insecticide like thiamethoxam. However, unlike thiamethoxam, nitenpyram is a water-soluble pesticide. © 2014 American Chemical Society
Received: May 31, 2014 Revised: August 25, 2014 Published: September 15, 2014 11546
dx.doi.org/10.1021/jp505376w | J. Phys. Chem. B 2014, 118, 11546−11551
The Journal of Physical Chemistry B
Article
Figure 1. Structures of pesticides used in this study.
ment. Measurements were taken daily before the tensiometer calibration. The measuring cell was kept at constant temperature of 293 ± 0.2 K. The surface tension of each solution was measured five times, and an average value was calculated. The reproducibility of the measurements was within ±0.1 mN/m. For each IL, the surface tensions of nine solutions of various concentrations were examined. 2.3. Solubility Studies. The solubility of pesticides was measured at 298 K in 30 wt % IL water solution prepared using a weighing balance (Sartorius, Germany) with an accuracy of ±0.0001 g. The 5 mL sample vials containing 3 mL of IL solutions were saturated with pesticide. The solutions were agitated by vortex mixing for 10 min, and then placed in an ultrasonic bath (Crest Ultrasonics, USA) for 15 min. The precipitated pesticides were removed by filtration through a 0.22 μm Millipore filter. The amount of pesticides in the resulting clear filtrate was determined using a UV spectrophotometer (Shimadzu UV-1800, Japan) at 230 nm for metolachlor, prochloraz, and acetochlor, 280 nm for clethodim, and 260 nm for thiamethoxam and nitenpyram.
is an important property of ILs because it enables the dissolution of many sparingly soluble substrates, which leads to the concept of tailor-made solvents for a specific application.16,17 The CAC of ILs can be determined through various methods, such as surface tension measurement, nuclear magnetic resonance (1H NMR),18,19 interfacial tension, fluorescence,5 apparent molar volumes,20 and electrical conductivity.21,22 The aim of this study was to extend the application of ILs in agriculture. The effects of hydrophobic chain length and hydrophilic head groups of ILs on solubilization of fungicide, insecticide, and herbicide were investigated. Results of this study may provide valuable information needed for the selection of ILs for the solubilization of pesticides, which may help in reducing the required amount of organic solvents.
2. EXPERIMENTAL SECTION 2.1. Materials. The ILs used were 1-butyl-3-methylimidazolium chloride ([C4mim][Cl]), 1-butyl-3-methylimidazolium bromide ([C4mim][Br]), 1-butyl-3-methylimidazolium iodide ([C 4 mim][I]), 1-hexyl-3-methylimidazolium chloride ([C 6 mim][Cl]) 1-hexyl-3-methylimidazolium bromide ([C 6 mim][Br]), 1-hexyl-3-methylimidazolium iodide ([C 6 mim][I]), 1-octyl-3-methylimidazolium chloride ([C 8 mim][Cl]), 1-octyl-3-methylimidazolium bromide ([C 8 mim][Br]), 1-octyl-3-methylimidazolium iodide ([C 8 mim][I]), 1-decyl-3-methylimidazolium chloride ([C 10 mim][Cl]), 1-decyl-3-methylimidazolium bromide ([C 10 mim][Br]), 1-decyl-3-methylimidazolium iodide ([C 10 mim][I]), 1-dodeyl-3-methylimidazolium chloride ([C 12 mim][Cl]), 1-dodeyl-3-methylimidazolium bromide ([C 12 mim][Br]), 1-dodeyl-3-methylimidazolium iodide ([C 12 mim] [I]), 1-butyl-3-methylpyridinium bromide ([C 4 mpy][Br]), 1-hexyl-3-methyl morpholinyl bromide ([C6mm][Br]), 1- hexyl-3-methylpiperidyl bromide ([C6mp][Br]), and 1-hexyl-3-methylpyrrolidyl bromide ([C6mpyr][Br]). They were purchased from Shanghai Cheng Jie Chemical Co., Ltd., and not subjected to further purification. The purity of the ILs were higher than 99%. Metolachlor, acetochlor, clethodim, thiamethoxam, nitenpyram, and prochloraz (purity >97%) were procured from China Agricultural University 2.2. Surface Tension Measurement. Surface tension was measured by a Wilhelmy plate using a filter paper, which was cleansed by annealing in alcohol flame before each measure-
3. RESULTS AND DISCUSSION 3.1. Surface Activity of ILs with Different Carbon Chain Lengths. The surface activity of ILs was measured using surface tension. The relationships between surface tension (γ) and concentration (C) are shown in Figure 2 and Figure 3. The abscissa and ordinate of the intersection point of the two tangent lines of the γ-C curve (Figure 2 and Figure 3) are the CAC and the lowest surface tension. The surface tension values increased at concentrations higher than CAC for C8-, C10-, and C12-based ILs (more pronounced in C8- and C12based ILs). We deduced that the increase of surface tension values can be attributed to the presence of impurities23 or increased surface excess of water.25 The imidazolium ring is preferentially hydrated by ring hydrogen and aromatic protons in hydrogen bonding with water. Increased surface tension values may also be caused by the transition of aggregates.24 The maximum surface excess concentration Γmax (μmol·m−2) at the air−aqueous interface was calculated using the Gibbs adsorption isotherm equation25 Γmax = − 11547
dγ 1 × nRT d[ln c] dx.doi.org/10.1021/jp505376w | J. Phys. Chem. B 2014, 118, 11546−11551
The Journal of Physical Chemistry B
Article
where γ is the surface tension in mN·m−1, Γmax is the maximum surface excess concentration in μmol·m−2, R is the gas constant (8.314 J·mol−1·K−1), T is the absolute temperature, and c is the IL concentration. The slope of the surface tension isotherm near the CAC is dγ/d[ln c]. The value of n is 2.26 The prefactor n is theoretically dependent on the surfactant type and structure, as well as the presence of electrolytes in the aqueous phase. With a 1:1 ratio of ionic surfactants in the absence of extra electrolyte, the thermodynamic treatment requires n = 2, implying an equimolar ratio between the surfactant anion and countercation in the interface.23 The minimum area Amin occupied by each molecule at the air/aqueous interface should reflect the packing density of surfactant molecules at the interface. Amin can be obtained using the following equation:25 Figure 2. γ−c curves of [Cnmim][X] (n = 4, 6, 8, 10, 12; X = Cl, Br, I) in aqueous solution at 298 K.
A min =
1 NA Γmax
Here NA is Avogadro’s number and Amin is in nm2. The calculated parameters for different ILs are summarized in Table 1. The CAC values of [Cnmim][Cl], [Cnmim][Br], and [Cnmim][I] decreased and the length of carbon increased (Table 1). Therefore, the adsorption behavior of ILs at air/ water interface was similar to that of conventional cationic surfactants. Amin (the area per IL molecule) systematically and drastically increased in the order [C12mim][X] < [C10mim][X] < [C8mim][X] < [C6mim][X] < [C4mim][X]. The results indicated that simple monolayers with alkyl chain oriented toward the water side were formed at the interface in C4 alkyl chain-based ILs, whereas the C6, C8, C10, and C12 alkyl chains interacted with one another to form closely packed layers.27 Comparison of surface active parameters for the four ILs with a common C6 chain but different cationic head groups revealed that the methylimidazolium moiety is more effective than methylpiperidine, morpholine and methylpyrrolidine at air/water interface. The CAC values increased in the order [C6mim][Br] 590 0.03432
3.1328 3.2728 4.2029 −0.1330 −0.6631 4.1033
KO/W is the n-octanol−water partition coefficient of chemicals.
Table 3 shows the solubility of the six pesticides in 30 wt % aqueous ILs. In aqueous [C10mim][I], the solubility of metolachlor, prochloraz, and acetochlor were 143.3, 196.2, and 173.4 g·L−1, respectively. In pure water, the solubility of metolachlor, prochloraz, and acetochlor were 0.53, 0.034, and 0.24 g·L−1, respectively (Table 2). On one hand, the solubility Table 3. Solubility of Pesticides in Different Ionic Liquids
solubility (g·L−1) ILs
metolachlor
acetochlor
clethodim
prochloraz
thiamethoxam
nitenpyram
[C4mim][Cl] [C4mim][Br] [C4mim][I] [C6mim][Cl] [C6mim][Br] [C6mim][I] [C8mim][Cl] [C8mim][Br] [C8mim][I] [C10mim][Cl] [C10mim][Br] [C10mim][I] [C12mim][Cl] [C12mim][Br] [C12mim][I] [C4mpy][Br] [C6mm][Br] [C6mp][Br] [C6mpyr][Br]
9.5 11.7 60.0 11.4 37.3 67.0 78.0 91.1 108.6 33.1 99.2 143.2 144.9 85.3 26.3 2.9 5.7 8.3 8.7
2.4 3.2 3.6 7.00 11.5 21.3 45.2 16.3 44.1 36.6 40.4 173.4 35.9 50.1 33.9 4.3 5.4 4.8 6.0
8.0 23.4 53 10.0 41.8 159.9 169 137.7 179.3 134.1 264.9 128.9 253.1 >1000 168.5 20.3 20.4 25.0 39.5
6.0 2.0 20.0
