Study on the Performance of Lambda Cyhalothrin Microemulsion with

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Study on the Performance of Lambda Cyhalothrin Microemulsion with Biodiesel as an Alternative Solvent Chih-Ping Chin, Chi-Wei Lan, and Ho-Shing Wu* Department of Chemical Engineering and Materials Science, Yuan Ze University, 32003, Taiwan ABSTRACT: This study investigates a lambda cyhalothrin 2.5% (w/w) microemulsion formulation, with biodiesel as an alternative solvent, as a potential pesticide delivery system for oil-soluble pesticide active ingredients (AIs). This study presents a pseudoternary phase diagram of the investigated system, at room temperature by titration, and the surfactant/cosurfactant mass ratios (S/C), the oil-to-surfactant/cosurfactant mass ratios (O/SC) and the optimum formulation. This study also investigates the continuous structural inversion from water-in-oil to oil-in-water microemulsions after dilution with the water phase. The electrical conductivities of the selected system at constant S/C (5:1) and O/SC (1:1.2) ratios with biodiesel were also studied, and the percolation phenomenon was observed. The study examines the performance and stability of a formulation with biodiesel in comparison to commercial formulations. Residue and phytotoxicity tests were carried out on cabbage plants in a greenhouse. Experimental results indicate that biodiesel has good handling characteristics when it forms a microemulsion. The microemulsion formulation, with biodiesel as a solvent, had an acceptable transparent appearance, broader transparency temperature range, and better performance and stability than the commercial formulation. Using biodiesel in a pesticide formulation had no adverse effects on the AI activity and is comparatively safe for crops and plants.

1. INTRODUCTION Challenges in developing new pest control strategies include identifying novel active compounds and improving pesticide delivery at the biological level.1 In a pesticide delivery system, the oil-in-water (O/W) microemulsion is also called green pesticide for its use of water, instead of organic solvents. This approach saves a large amount of organic solvent, reducing the potential risk to the environment and improving safety during storage and transportation. A microemulsion (ME) is a mixture with a composition containing at least three types of components: active ingredients (AIs), amphiphiles, and water, at appropriate ratios. A microemulsion is thermodynamically stable, and usually a transparent liquid solution. 2 In pharmaceuticals, microemulsions are the objects of investigation in relation to drug delivery3 because of their advantages (thermodynamic stability, ease of preparation, transparency, low viscosity, considerable potential for solubilizing a variety of drugs). The most important characteristic of a microemulsion is its droplet size in the dispersed phase. This is usually very small, below 100 nm in diameter. Such a small droplet size improves the efficacy of pesticides penetrating the surface of target pests or plants. Some active ingredients, such as avermectins and endosulfan,4 azadirachtin,5,6 carbaryl, permethrin, and tetramethrin,7 chlorpyrifos and hexaflumuron,8 cypermethrin and deltamethrin,9 emamectin benzoate,10 ivermectin,11 milbemycins,12 rotenone,13 and pirimicarb14 have all been prepared as microemulsion in pesticide formulations. In microemulsion forming, certain mixtures of nonionic surfactants can enhance the solubility of water in water-in-oil microemulsions.15,16 Microemulsions with a broader transparent temperature range can also be produced using certain mixtures of ionic and nonionic surfactants.17 Microemulsions typically display rich phase behavior because of the variety of structures present (water-in-oil (W/O), oil-in-water (O/W), or © 2012 American Chemical Society

bicontinuous (BC)). The formation process and gradual changes in microemulsion microstructure can be monitored quantitatively by measuring the electrical conductivity of the system.18 Except surfactants, a variety of structures in microemulsion are also affected by temperature variation, pH, and electrolyte concentration. With the addition of cosurfactants and cosolvents such as short and medium chain alcohols,19,20 the phase behavior, formation, and properties of microemulsions may be altered by partitioning the aqueous and oleic phases and modifying the solvent properties of these phases.21−23 Different alcohols affect the formation and transmittance of microemulsions in varying manners.24 Solvent polarity also influences the phase behavior and stability of microemulsions.25,26 A difference in solvent polarity changes the solubilization positions in the micelle of microemulsions. To achieve a homogeneous oil phase, petroleum derived solvents such as petroleum, diesel, xylene, trimethylbenzene, ketones, and alcohols must be used to dissolve solid oil-soluble AIs. However, the toxicology and nonbiodegradation nature of these solvents make them hazardous to the environment and human health. Aromatic solvents may also produce collateral damage through drift and volatility, proving harmful to crops and plants. As a result, concerns about the environmental effects of incorporating such petroleum-based solvents have motivated the search for more environmentally friendly methods of crops protection. Vegetable oil was first proposed as an adjuvant in emulsifiable concentrate (EC) formulations.27 Fischer−Tropsch (FT) hydrocarbons were also proposed as solvents, or carriers, and FT-derived white oil was proposed as a Received: Revised: Accepted: Published: 4710

May 30, 2011 March 1, 2012 March 8, 2012 March 8, 2012 dx.doi.org/10.1021/ie201151p | Ind. Eng. Chem. Res. 2012, 51, 4710−4718

Industrial & Engineering Chemistry Research

Article

carrier for pesticide and fertilizer formulations.28 FT naphtha and FT diesel were also proposed for pesticide formulations,29 and alcohol alkoxylate compounds (fatty alcohol and/or fatty acid alkoxylated) were proposed as pesticide carriers.30 The herbicide quizalofop-P was prepared as 5% (w/w) EC formulation using biodiesel as solvent.31 These experimental results show that biodiesel increases the efficacy of weed control due to the increased rate of penetration. In slow-release formulations, natural lipids serve as vesicles that incorporate the pesticide and are adsorbed in clay. This also enhances the water solubility of the pesticides.32 Biodiesel is made from renewable biological resources such as vegetable oils and animal fats. It can be produced by techniques such as ultrasonic cavitation, hydrodynamic cavitation, microwave irradiation, response surface technology, and the two-step reaction process. Previous studies present detailed production processes.33−35 Biodiesel is environmentally beneficial because it is renewable, biodegradable, nontoxic, relatively stable, and has low emission profiles and a high flash point. The active ingredient used in this study is lambda cyhalothrin (CAS no. 91465-08-6), a synthetic pyrethroid insecticide and acaricide used to control a wide range of pests in a variety of applications. This ingredient was chosen because its microemulsion is widely used to control insects in field crops, home landscaping, and garden markets in Taiwan. It is similar to pyrethroid cyhalothrin. Figure 1 shows

sulfonates and iso-butanol, was purchased from Huntsman (Utah, United States). Ethylan NS-500K, a nonionic surfactant, polyalkoxylated butyl ether, was purchased from Akzo Nobel (Amsterdam, Netherlands). The solvent Solvesso 150, a mixture of solvent naphtha and naphthalene, was purchased from Exxon Co. (Texas, United States). Solvesso 150 is commonly used in pesticide microemulsion formulations. Biodiesel, fatty acid methyl ester (FAME), was supplied by Greatec Green Energy Co., Ltd. (Changhua, Taiwan). Solvesso 150 and biodiesel are often used as solvents for commercial and biodiesel formulations. Water was purified by double distillation in a glass apparatus and then deionized using a Millipore MilliQ Water System (Millipore Corporation, Bedford, United States). Propylene glycol, an antifreezing agent, was purchased from Mitsui Chemicals, Inc. (Tokyo, Japan). All substances were used as received without further purification. 2.2.1. Microemulsion Preparation. Construction of Phase Diagram. The pseudoternary phase diagram was constructed by the titration of homogeneous liquid mixtures of oil, surfactant, and cosurfactant with water at room temperature.37−41 Termul 5030, the surfactant (S), and Ethylan NS-500K, the cosurfactant (C), were weighed at different surfactant/cosurfactant (S/C) mass ratios (3:1, 4:1, and 5:1), in screw-cap, dark-brown glass vials and mixed using a magnetic stirrer (IKA, model RTC B S1, Staufen, Germany). The speed of magnetic stirrer was maintained at 1000 rpm, and stirring took place at 60 °C for 1 h. The mixture was then stored overnight at room temperature. At a constant mass ratio of 1:2, mixtures of oil phase (O) and lambda cyhalothrin/solvent (biodiesel/Solvesso 150) were weighed in glass flasks with stoppers, mixed using a magnetic stirrer for 1 h, and then stored overnight at room temperature. This produced oil phase and surfactant/cosurfactant blends. The oil and surfactant/cosurfactant ratios varied from 9:1 to 1:9. Water phase was added to each oily mixture, drop-by-drop, during magnetic stirring. Samples were stirred during titration to allow equilibration. Following the addition of water, the mixture was visually examined for transparency. The changes in the sample’s visual appearance, from turbid to transparent and vice versa, were observed. Transparent, single-phase, low viscous mixtures were designated as microemulsions. After water titration, titration of water and surfactant/cosurfactant mixtures with oil were performed in the same manner to identify the microemulsion region borders. 2.2.2. Selection of Microemulsion Formulations for Lambda Cyhalothrin. For further studies of the pseudoternary phase diagrams constructed, the Termul 5030/Ethylan NS500K/lambda cyhalothrin/solvent mixture with optimal S/C ratio was selected first. Five potential microemulsions of various SC/O ratios (1.2, 1.4, 1.6, 1.8, and 2.0) were also selected and prepared as 2.5% (w/w) lambda cyhalothrin microemulsion. To prepare the microemulsion, appropriate quantities of Termul 5030, Ethylan NS-500K, lambda cyhalothrin, solvent (biodiesel/Solvesso 150), water, and propylene glycol (an antifreezing agent) were introduced into screw-cap glass vials. The mixtures were stirred with a magnetic stirrer of 100 rpm at room temperature to accelerate the formation of the transparent systems. Transparent and single-phase formulations formed 3 h later. Both pesticide-loaded microemulsions were prepared 48 h before analysis and stored at room temperature. The optimal SC/O ratio and storage stability of different formulations were then determined on the basis of the transmittance of microemulsions with a spectrophotometer

Figure 1. Structures of lambda cyhalothrin, a mixture of highly active isomers of cyhalothrin, comprises equal quantities of (S)-α-cyano-3phenoxybenzyl (Z)-(1R,3R)-3-(2-chloro-3,3,3- trifluoropropenyl)-2,2dimethylcyclopropanecarboxylate and (R)-α-cyano-3-phenoxybenzyl (Z)-(1S,3S)-3-(2-chloro-3,3,3-trifluoropropenyl)-2,2-dimethylcyclopropanecarboxylate.

the structure of this compound. The detailed descriptions of its chemical and physical properties can be found in presented literature.36 This study has three specific objectives. First, different types of lambda cyhalothrin 2.5% microemulsions with a minimum amount of surfactants mixture and different solvents (biodiesel and petroleum-based solvent) were formulated to obtain formulations for application. Second, this study investigates the gradual changes in the microstructure after the addition of a water phase into the mixture of oil and surfactants. Third, this study identifies the influence of different solvents on the stability, performance, conductivity, phytotoxicity, and physical and chemical properties of the selected microemulsion formulations.

2. MATERIALS AND METHODS 2.1. Materials. The 95% lambda cyhalothrin active was supplied by Chia-Tai enterprise Co. (Taoyuan, Taiwan). Termul 5030, a mixture of nonionic emulsifiers, alky aryl 4711

dx.doi.org/10.1021/ie201151p | Ind. Eng. Chem. Res. 2012, 51, 4710−4718


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Figure 2. Pseudoternary phase diagram for systems consisting of water (w), Termul 5030 (S), NS-500K (C), AI and biodiesel (O), at S/C ratios (w/ w) = (a) 3/1, (b) 4/1, (c) 5/1. Diagrams apply to room temperature measurements. AI, lambda cyhalothrin; 1, O/W microemulsion region; 2, W/O microemulsion region; 3, bicontinuous region; 4, unstable region.

2.3.2. Conductivity Measurements. The solubility of the water phase in the selected oily mixture was monitored quantitatively by measuring the electrical conductivity. The water was added, drop-by-drop, to the mixture of oil and surfactant/cosurfactant. The conductivity (σ) of formulated mixture was measured by means of a conductivity meter (Suntex, model SC-170, New Taipei City, Taiwan) at 20 ± 2 °C. 2.3.3. Temperature Range Measurement of Transparent Microemulsion. The experiments in this study directly observed and measured the temperature range of the transparent microemulsion.46 A 10 mL sample was placed into a 25 mL flask and stirred. The flask was put into an ice bath and cooled. When the sample became turbid, or frozen, the measured temperature t1 obtained was the lower limit of the transparent temperature range. The sample was then heated at the rate of 2 °C/min and stirred gently. When the sample became turbid, the measured temperature t2 obtained was the upper limit of the transparent temperature range. The

(Thermo Spectronic Inc., model GENESYS 10, Cambridge, United Kingdom) at 520 nm. Further tests of different solvents were conducted to evaluate the performance of optimal formulations. 2.3.1. Microemulsion Characterization. HPLC Analysis of Lambda Cyhalothrin. A modified high-performance liquid chromatography (HPLC) method42−45 was used to determine the purity of the AI and the content of the micromulsions. A HPLC LC-10ATVP system (Shimadzu, Kyoto, Japan) was equipped with a vacuum degasser, a quaternary gradient pump, a heated column compartment, a UV detector, a 20 μL injection loop, and SISC-32 data process software (SISC, Taipei, Taiwan). This method used a reverse phase Luna 5u C18 column (250 × 4.6 mm I.D., 5 μm, Phenomenex Inc., USA). The mobile phase was prepared with methanol/water (95:5, v/v). The flow rate was maintained at 1.0 mL/min and the temperature of column was maintained at 40 °C. The injection volume of the sample was 20 μL, and the wavelength of the UV detector was 230 nm. 4712



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Figure 3. Pseudoternary phase diagram for systems consisting of water (w), Termul 5030 (S), NS-500K (C), AI and Solvesso 150 (O) at S/C ratios (w/w) = (a) 3/1, (b) 4/1, (c) 5/1. Diagrams apply to room temperature measurements. AI, lambda cyhalothrin; 1, O/W microemulsion region; 2, W/O microemulsion region; 3, bicontinuous region; 4, unstable region.

2.3.6. Field Trial for Residues. This study uses the Office of Prevention, Pesticide and Toxic Substance (OPPTS) test method to determine the magnitude of pesticide residue in, or on, raw agricultural commodities (RACs) and to determine pesticide use patterns that generate the most residue.49 The field experiments in this study used chemical analysis to quantify the terminal residue of AI in plant metabolism for two formulations. The test pesticide formulations were applied to RACs at the recommended concentration, and all formulations were tested with three replications. The pesticide was applied following the directions on the pesticide label. The RACs used in this study were cabbages planted in a greenhouse to avoid rain washing. The RACs were irrigated and sampled at regular intervals each day throughout the period from the last application of the pesticide to normal harvest. Each of the RACs was planted separately in different field trial sites for the different formulations, separated by an appropriate distance. In this study, daily samples of approximately 20 g were taken from the RACs to determine the AI of residue, using the mentioned analytical method. The curves for variation in residues, or minor metabolites of RAC, from different formulations were obtained by plotting the amount of residue with respect to the sampling day.

temperature range of the transparent microemulsion was from t1 to t2. 2.3.4. Droplet Size Measurements. The averaged droplet size (diameter) of the microemulsions was measured using dynamic light scattering (DLS) (Beckman Coulter Ltd., model N4 Submicrometer, California, United States). The microemulsions were diluted to 5 mM. After stability studies, further measurements of the average droplet size in microemulsions were carried out using DLS. A field emission scanning electron microscope was also used, and the results were compared. 2.3.5. Formulation Characterization. This study uses Collaborative International Pesticides Analytical Council (CIPAC) standard test methods to test the characteristics of formulations. Formulation characterization studies revealed the specific gravity and density measurement (MT3.2, CIPAC Handbook Volume F), stability at 0 °C (MT39.2, CIPAC Handbook Volume F), accelerated storage procedure (MT46, CIPAC Handbook Volume F), persistent foaming (MT47, CIPAC Handbook Volume F), pH value of formulation (MT75, CIPAC Handbook Volume F), spontaneity of dispersion (MT160, CIPAC Handbook Volume F), and viscosity measurement (MT192, CIPAC Handbook Volume L). All test methods follow the official CIPAC standard method outlined in the CIPAC Handbook Volume F47 and Volume L.48 4713



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2.3.7. Phytotoxicity Trial. Pesticides may be harmful or lethal to plants. Phytotoxicity is the degree to which a chemical is toxic to plants. Phytotoxicity may express itself in a variety of ways, ranging from minor leaf speckling to plant death.50 The most common symptoms of phytotoxicity are chlorosis, burning, and distortion. This study uses the same species of cabbage as the residual experiments because of its high sensitivity and easy planting and observation. The insecticide was applied at the maximum concentration and double and triple the maximum recommended concentration to increase the margin of safety. A control set of plants was sprayed with water only. No other pesticides were applied during the trial. After two weeks, the number of leaves, leaf area, and appearance of the plant were recorded to determine the safety of the pesticide. In general, the most severe phytotoxicity symptoms, such as chlorosis, burning, and distortion of leaves, become apparent during the test period. Figure 4. Transmittance curves for the lambda cyholothrin-in-water system, at a constant S/C mass ratio (5:1), for different solvents and SC/O mass ratios: biodiesel (●); Solvesso 150 (○).

3. RESULTS AND DISCUSSION 3.1. Phase Behavior. Figures 2 and 3 show the pseudoternary phase diagram for the system studied (water/ lambda cyhalothrin/Termul 5030/NS-500K with different solvents). The formation of microemulsion systems was observed at room temperature. Examining the phase behavior of the system was the most suitable method for determining of the water phase, oil phase, surfactant, and cosurfactant concentrations for which the transparent, one-phase, lowviscous systems form. The ratios of the area of different regions in each pseudoternary phase diagram were also measured and calculated using a simple and accurate weighing method (Table 1). During the addition of water to the selected oily mixtures,

greatest transmittances were obtained at SC/O = 1.2 for the biodiesel formulation (63%) and SC/O = 1.6 for commercial formulation (85%). It is interesting to note that when the amount of surfactant/cosurfactant increases, the system becomes turbid. When biodiesel was in microemulsion formulation, the transmittance decreases by a maximum of 53−63%, resulting in a poor appearance compared to the commercial formulation (80−85%). Using the stability tests mentioned above, this study also determines the stability of the selected system at S/C = 5:1 with a variety of SC/O mass ratios and different solvents. Results indicate that the precipitation occurred at a SC/O mass ratio of 1.2 in biodiesel and commercial formulations. As in previous studies, the optimum mass ratios were S/C = 5:1 and SC/O = 1.4, respectively, for the lambda cyhalothrin-in-water system. Therefore, the optimal formula for lambda cyhalothrin 2.5% microemulsion formulation in the aqueous solution is lambda cyhalothrin, 2.5%−3%; biodiesel or Solvesso 150, 5%− 6%; surfactants (Termul 5030 and NS-500K at S/C 5:1), 10.5%−12.6%; propylene glycol as an antifreezing agent, 3%− 5%, and the rest as water to 100%. 3.2. Conductivity Measurements. The structural changes caused by varying the amount of water (Φw) and using different solvents at constant S/C = 5:1 and SC/O = 1.4 were also investigated to ascertain the relationship between the type of solvent used and the electroconductive behavior. The electrical conductivity (σ) of the selected oily mixture is a function of water content (Figure 5). According to the conductivity data obtained, the two investigated formulations studied vary with Φw, and the biodiesel formulation has a higher σ. The electrical conductivity of the two different oily mixtures was almost zero when Φw was smaller than 10% (w/w). For a water titration up to 60% (w/w), σ increased markedly. For Φw > 60% (w/w), the system conductivity gradually declined with the addition of water. Previous research shows that there is a strong correlation between the specific structures of microemulsion systems composed of ionic surfactants and their electroconductive behavior. As the water volume fraction increases, the electrical conductivity of these systems also increases slightly until reaching the critical Φw, which produces a sudden increase in conductivity. This phenomenon is known as percolation, and the critical Φw at which it is occurs is called the percolation

Table 1. Area Ratio in the Pseudoternary Phase Diagrams for Different Solvents and Various S/C Ratios area ratio of phase regions (%) solvent biodiesel

Solvesso 150

condition (S/C, w/ w)

O/W ME

W/O ME

BC

unstable

3:1 4:1 5:1 3:1 4:1 5:1

6.9 9.6 14.1 4.2 3.7 6.7

35.0 37.9 48.7 33.5 40.2 39.4

5.8 4.8 0.9 2.0 2.2 3.1

52.4 47.7 36.3 60.3 53.9 50.8

there was a continuous transition from oil-rich systems (right side of the phase diagram) to water-rich systems (left side of the phase diagram). These results show that the phase behaviors of microemulsions are influenced by variations in the S/C mass ratio and the types of solvent used. Experimental results show that formulation with biodiesel caused broader areas of single-phase microemulsions (O/W and W/O). Because biodiesel consists of fatty acid methyl ester, it may act as a cosurfactant in formulation. The areas of single-phase microemulsions were increased by altering the S/C mass ratio. Experimental results indicate that the maximum area was S/C = 5:1. Experimental results also reveal that using biodiesel as an alternative solvent helps form stable microemulsions. This study also measures the transmittance of microemulsions in the lambda cyhalothrin-in-water system at a selected S/C = 5:1 with a variety of SC/O mass ratios and different solvents to ascertain the optimal mass ratio of SC/O (Figure 4). According to the transmittance data obtained, the 4714



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Table 2. Performance Index, Physical, and Chemical Properties of Lambda Cyhalothrin 2.5% Microemulsion for Different Solvents specification

biodiesel

AI content (%, W/W) appearance

2.31−2.69%

2.54 clear solution

decomposition rate, after hot storage 14 days (%) stability at 0 °C transparent temperaturerange (°C) persistent foaming (mL, 1000×)

≤5%

0.6

2.57 light yellow clear solution 1.4

no precipitate

qualified −5 to 55

qualified −6 to 63

volume of foam ≤60 mL; 30 min ≥70%

33.5

36

100

100

30 6.87 1.016

60 5.27 1.020

spontaneity dispersion (%, 1000×) viscosity (mPȧs) pH value density (g/mL)

Figure 5. Electrical conductivity (σ), as a function of water phase volume fraction (Φw), in a lambda cyhalothrin-in-water microemulsion system with SC/O 1.4, S/C 5:1, and different solvents: biodiesel (●); Solvesso 150 (○).

Solvesso 150

*

The specification of test items is based on the regulations announced by the Taiwan government.

threshold (Φp). The increase in the electrical conductivity as a function of water volume fraction is the result of the increase in the fraction of ions from surfactants that are not enclosed in the core of the microemulsions. Therefore, the high values of electrical conductivity at higher water volume fractions are the result of the extra ions (surfactants) present in the water. The gradual decrease in conductivity created upon reaching the critical Φw is likely caused by the enclosing of the extra surfactants. A W/O microemulsion formed in the region of low water content (Φw < 10%, w/w). Beyond the percolation threshold (Φp ≈ 10%, w/w), electrical conductivity increases linearly and sharply up to Φw ≈ 60% (w/w). Beyond Φp, there may be a network of conductive channels that exist, corresponding to the formation of water cylinders, or channels, in the oil phase because of the attractive interactions between the droplets of the water phase in the W/O microemulsion. When Φw > 60% (w/w), the electrical conductivity increases nonlinearly to a maximum of Φw ≈ 70% (w/w). After the maximum, the conductivity of the system was decreased slightly upon further addition of the water phase. The conductivity results in this study can also be explained by the dilution of the O/W microemulsion with water, which decreases the concentration of the dispersed oil droplets. Electrical conductivity curves indicate the occurrence of the three structural regions in a microemulsion: W/O (Φw < 10% w/w), bicontinuous (10% < Φw < 60% w/w), and O/W (Φw > 60% w/w). 3.3. Performance of Formulations. Table 2 summarizes the specification and experimental results for the two lambda cyhalothrin 2.5% microemulsion formulations selected. The contents of lambda cyhalothrin in commercial and biodiesel formulations were 2.54% and 2.57%, both of which are within the acceptable range. The experimental results for the decomposition rate of the two formulations show that both were within the acceptable specified range after accelerated storage treatment. In the experiment to measure low temperature stability, no precipitate or separated materials were observed in either the commercial or biodiesel formulation. The droplet size of the dispersed phase for two freshly prepared microemulsion

formulations was measured using DLS and found to be approximately 18 nm (d0.9) for both commercial and biodiesel formulations (Figure 6). After accelerated storage treatment,

Figure 6. Distribution of droplet size, for the dispersion phase of two freshly prepared lambda cyhalothrin 2.5% microemulsion formulations, with biodiesel (●) and Solvesso 150 (○).

the droplet size of the dispersed phase for the two formulations was approximately 18 nm (d0.9) for both commercial and biodiesel formulations (Figure 7). Experimental results indicate that, after accelerated storage treatment, the droplet size of the dispersed phase for the two formulations remained the same as that for the original formulations, with no reaggregation in droplet size. These results show that biodiesel and commercial solvent formulations both exhibit a stable distribution of droplet size for the dispersed phase. Therefore, the biodiesel formulation appears to be as stable as the commercial formulation in terms of its physical and chemical properties. The results of other performance tests, such as persistent foaming and spontaneity dispersion tests, show no difference between biodiesel and commercial formulations. These results indicate that the biodiesel formulation exhibits similar perform4715



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Figure 8. The variation in residual concentration of lambda cyhalothrin with time, for equal dosage (25 ppm) treatment on RACs, for two lambda cyhalothrin 2.5% microemulsion formulations, with biodiesel (●) and Solvesso 150 (○).

Figure 7. Distribution of droplet size, for the dispersion phase of two lambda cyhalothrin 2.5% microemulsion formulations, with biodiesel (●) and Solvesso 150 (○), after accelerated storage procedures.

ance in pesticide formulations. The biodiesel formulation has a higher transparency temperature range, viscosity, and density than the commercial formulation using petroleum-based solvents, but simultaneously has a lower pH value. The higher transparency temperature range improves the stability of the formulation in various environments. The lower pH value of biodiesel, because of its fatty acid profile, occurs so that its multiple-sources may cause the decomposition of pesticides that have a low tolerant to acid.51 Therefore, it is necessary to carefully select pesticides for formulation. 3.4. Residues and Phytotoxicity in Field Trials. The residue trials applied the commercial and biodiesel formulations of lambda cyhalothrin 2.5% microemulsion to the RACs at the recommended maximum application concentrations (25 mg/L, diluted ratio: 1000×) specified by the Bureau of Animal and Plant Health Inspection and Quarantine, Taiwan. In phytotoxicity trials, the commercial and biodiesel formulations were sprayed on the RACs three times at two concentrations (25 and 50 mg/L; diluted ratio, 1000× and 500×). These concentrations represent the maximum and twice the maximum recommended application ratio. The RACs used in this study were cabbages planted in pots with commercial potting soil with a pH value ranging from 5 to 7. Following good horticultural practices, the environmental conditions were maintained in greenhouses with a temperature of 25−32 °C, humidity of 60−80%, and sufficient light to ensure good plant growth. The pots were large enough to allow normal growth. All pots were irrigated with 500 mL of water at 8:00 a.m. daily during the experimental period. Approximately 10 g samples of the two different formulations, at a constant concentration, were taken from the RACs to determine the AI residue, using the analytical method for residues trials, as mentioned. The curves for variation in residues, or minor metabolites of RACs with different formulations, were obtained by plotting the amount of residues with respect to the sampling days. The RACs were also monitored for signs of visual phytotoxicity and mortality. Observations continued for at least for 7 days after the RACs had emerged. Figure 8 shows the declining lambda cyhalothrin curves for the two formulations. Experimental results show that the amount of biodiesel formulation is 58.1 ppm, which is similar to the commercial formulation (59.2 ppm). These results show

that the two formulations have the same hydrophobic/ lipophilic character as the RACs. In comparing the two formulations, this study assumes that the degradation of lambda cyhalothrin obeys the first order rate equation in both formulations. The biological half-life of lambda cyhalothrin in biodiesel formulation is 1.67 days. The decay equation is Ct = 38.088 e−0.4147t, R = 0.8989 in which Ct is the concentration of residue, t is time, and R denotes a regression factor. The biological half-life of lambda cyhalothrin in a commercial formulation is 1.63 days, and the degradation equation is Ct = 34.0388 e−0.4259t, R = 0.8977. These calculations show that the biological half-lives of the two formulations are similar. The biodiesel does not cause any further degradation in lambda cyhalothrin, indicating that it is just as stable as the commercial formulation. Phytotoxicity trials show some adverse effects for the commercial formulation, including chlorosis and burning on the leaves of RACs, as shown in Figure 9. These signs appeared on the 14th day after treatment for the commercial formulation at a concentration of 50 mg/L (2 folds of the maximum recommended application ratio). In contrast, no adverse effects appeared on RACs treated with the biodiesel formulation in different concentrations during the observation period. These observations indicate that the adverse effects that occurred with the commercial formulation may have been caused by the commercial solvent, Solvesso 150. Biodiesel is likely more compatible with plants because it is derived from plants. Thus, the lambda cyhalothrin 2.5% microemulsion formulation with biodiesel is safer to use in farming because it does not damage crops and plants.

4. CONCLUSIONS This study proposes the use of biodiesel as an alternative solvent for lambda cyhalothrin in microemulsion formulation. Results demonstrate that a good pesticide microemulsion formulation can be prepared with biodiesel. Using biodiesel as a solvent for solid, oil-soluble AIs in pesticide microemulsion formulations can also effectively increase the area of the singlephase microemulsion region, facilitating the formation of microemulsions. The performance test results for the selected formulations showed no significant differences between the 4716



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ME = microemulsion O = oil Q = mass of AI in the 25 mL diluted suspension at the bottom S = surfactant t1 = lower limit of transparent temperature range t2 = upper limit of transparent temperature range w = mass of formulation actually added to the cylinder (g) W = water Greek letters

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REFERENCES

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Figure 9. Symptoms of phytotoxicity for the commercial formulation at a concentration of 50 mg/L; chlorosis and burning show on the leaves of RAC (cabbage).

biodiesel and commercial formulations, expect for a lower transparency in appearance. Because of its broader transparent temperature range, biodiesel increases the stability of the formulation for storage and transportation in a variety of environments. The results of residue experiments and phytotoxicity tests in this study show that biodiesel has no adverse effects on the activity of AIs, and is safer than the commercial formulation for crops. These findings indicate that using biodiesel as a carrier does not harm plants, and also reduces environmental risks because of its biodegradable nature and high plant compatibility. Therefore, biodiesel appears to be an attractive substitute solvent for oil-soluble AIs in microemulsion formulations. In summary, biodiesel gives more added values, such as environmentally friendly and biodegradable, and it also allows a reduction in the use of petroleum-based solvents. All of these factors give biodiesel significant advantages as a substitute solvent for delivering pesticides and improving their performance.

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Φw = volume fraction of water Φp = percolation threshold σ = electrical conductivity

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +8863-4638800 ext 2564. Fax: +886-3-4631181. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Chia-Tai Enterprise Co., Ltd., Taoyuan, Taiwan, for kindly supplying the materials used in this study.

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NOTATIONS a = content of AI in the formulation AI = active ingredients c = mass of AI in the whole cylinder C = cosurfactant 4717



Article

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Study on the Performance of Lambda Cyhalothrin Microemulsion with

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