Surfactants are important components in pesticide formulations. They can significantly improve the solubility, dispersibility, and wettability of pesticides by synergizing with pesticides, thereby increasing the biological activity of pesticides, reducing usage, and enhancing stability. Based on the characteristics of surface activity, pesticide adjuvants can be divided into two categories: surfactants and non-surfactants. Surfactants, as a type of amphiphilic molecules, can significantly reduce the surface tension of solvents at extremely low concentrations and change the composition and structure of liquid interfaces.
The application of pesticide adjuvants can be divided into applications in formulation processing and in pesticide use (Figure 1). In formulation processing, surfactant adjuvants are called formulation adjuvants, which are used to improve the physical and chemical properties of pesticide formulations to meet processing requirements and performance. In the process of pesticide use, surfactant adjuvants are also called spray adjuvants, which improve the physical properties of droplets and improve the adhesion, spreading and penetration of pesticides on plant surfaces. Different pesticide formulations require the addition of specific surfactant adjuvants to improve the physical and chemical properties of the solution, improve the targeted deposition of the solution, promote the absorption and penetration of active ingredients, and improve the safety of non-target organisms. Therefore, surfactant pesticide adjuvants play a vital role in the application of pesticides, and they have an impact that cannot be ignored in achieving efficient delivery of pesticides.
Figure 1 The role of surfactant pesticide adjuvants in pesticide formulation processing and pesticide application
The role of surfactant pesticide adjuvants in pesticide formulation processing
Surfactants, with their unique structure and properties of hydrophilic at one end and lipophilic at the other, can significantly improve the physical and chemical properties of pesticides, thereby improving the biological activity and control effect of pesticides.
(1) Application in emulsifiable concentrates
During the emulsifiable concentrate processing of pesticide formulations, surfactants have solubilizing and emulsifying effects. Surfactants can wrap water-insoluble pesticide molecules in the hydrophobic environment inside the micelles through their hydrophilic-lipophilic balance (HLB) characteristics, thereby achieving solubilization of the active ingredients of the pesticide. Surfactants can also bind to pesticide molecules through intermolecular forces such as hydrogen bonds and van der Waals forces, changing the polarity of pesticide molecules and thus increasing their solubility. This solubilizing effect allows pesticides to be made into emulsifiable concentrates. Emulsification is another key function of surfactants, which can turn pesticide technical and solvent into extremely small droplets and evenly disperse them in water to form a relatively stable emulsion.
In emulsifiable concentrate preparations, the most commonly used surfactants are mixtures of anionic and nonionic surfactants, such as calcium dodecylbenzene sulfonate and triphenylphenol polyoxypropylene polyoxyethylene block polymers. Most of the nonionic surfactants used in these emulsifiable concentrates are polymer-type, with the characteristics of large molecular weight and long molecular chain, and some of them also have branches and present a comb-like structure. Different pesticides require different emulsifier combinations. For example, for most organophosphate pesticides, the combination of polyphenyl core nonionic monomers and calcium salts is considered ideal; while for synthetic pyrethroid pesticides, a mixture of phenylethylphenol polyoxyethylene ether and alkylphenol polyoxyethylene ether can show better results when used with calcium salts. This precise compatibility selection is crucial to ensure the stability and effectiveness of pesticide formulations.
(2) Application in water-in-water emulsions and microemulsions
In the field of pesticide formulations, water-in-water emulsions and microemulsions are two important pesticide formulations. Both use water as the dispersion medium. Compared with traditional emulsifiable concentrate formulations, they are more environmentally friendly. Water-in-water emulsions are milky white opaque emulsions with large droplet sizes, while microemulsions are pesticide formulations with transparent appearance, high stability and small droplet sizes. In water-in-water emulsions, surfactants act as emulsifiers to reduce the interfacial tension between oil and water, thereby reducing the surface energy between the oil phase and the water phase, and stably dispersing the water-insoluble pesticide active ingredients in water. Surfactants can also enhance the stability of pesticide water-in-water emulsions through steric hindrance effects, preventing stratification and precipitation.
The dilational rheological properties of imidazole gemini surfactants were used to prepare a series of water-in-oil emulsions, and the relationship between rheological properties and emulsion stability was revealed. The study pointed out that the larger the dilational modulus, the better the emulsion stability. This law was further explained by the change trend of the droplet radius over time: that is, the interfacial film with a higher dilational modulus will lead to a decrease in the Ostwald ripening rate, coalescence rate and oil separation rate, thereby improving the emulsion stability. These research results will further provide guidance for the selection of surfactant pesticide adjuvants and the preparation of stable emulsions. In addition, in water emulsions and microemulsions, commonly used emulsifiers include ethers, phenol ethers, block polyethers and phosphates. Surfactants can also be used as thickeners to prevent the aggregation of the oil phase caused by the large density difference between the oil phase and the water phase. Commonly used ones include sodium carboxymethyl cellulose, xanthan gum and gum arabic. In microemulsions, according to the mixed film theory and solubilization theory, the HLB value and critical micelle concentration of the surfactant are key factors. Nonionic surfactants are sensitive to temperature, and the temperature range of the prepared microemulsion is narrow, so they need to be used in combination with anionic surfactants; ionic surfactants are not sensitive to temperature and can be adjusted by adding auxiliary surfactants (such as medium- and long-chain polar organic matter) or salt; the amount of emulsifier is related to the type of pesticide, turbidity and preparation concentration, and is generally 2 to 5 times that of the oil phase. The use of these emulsifiers and thickeners helps to improve the stability and performance of pesticide preparations.
(3) Application in wettable powders
In the field of pesticide formulation, the performance of wettable powders mainly depends on their wettability, dispersibility and stability. Surfactants can be used as wetting agents to reduce the surface tension between pesticide particles and water, so that pesticide particles can be quickly wetted in water. Commonly used wetting agents include anionic surfactants, such as sodium dodecyl sulfate and sodium dodecylbenzene sulfonate, and nonionic surfactants, such as fatty alcohol polyoxyethylene ether and alkylphenol polyoxyethylene ether. As dispersants, ionic surfactants are ionized in water and adsorbed on the surface of the active ingredients of pesticides, making their surfaces carry the same charge, thereby generating electrostatic repulsion, preventing the active ingredients of pesticides from approaching and agglomerating each other, and ultimately maintaining a good dispersion state during the formulation and application process. Nonionic surfactants can form a steric hindrance layer on the surface of pesticide particles through their lipophilic groups. When the particles approach each other, the repulsive force generated by the steric hindrance layer prevents the particles from agglomerating. Commonly used dispersants include sodium (calcium) lignin sulfonate and sodium carboxylated lignin sulfonate. As a stabilizer, surfactants adsorb on the surface of pesticide particles to form a protective film, which prevents evenly dispersed pesticide particles from settling and agglomerating during storage and transportation, ensuring that the pesticide formulation can maintain good performance during use.
(4) Application in suspension concentrates
In pesticide suspension concentrates, surfactants promote the wetting and dispersion of pesticide particles by adsorbing on the surface of pesticide particles, while expelling air and acting as grinding aids, so that smaller particles can be formed during the grinding process. In addition, surfactants can also reduce the aggregation and precipitation of pesticide particles by reducing interfacial energy and forming a diffuse double layer, thereby enhancing the stability of suspension concentrates. Commonly used surfactants in suspension concentrates include anionic surfactants, such as fatty alcohol (alkylphenol) polyoxyethylene ether sulfonates and alkylphenol polyoxyethylene ether formaldehyde condensate sulfates; non-ionic surfactants, such as fatty alcohol polyoxyethylene ethers and polyoxyethylene polyoxypropylene ether block copolymers; and macromolecular surfactants, such as lignin sulfonates.
Oil suspension concentrates are a highly dispersed and stable suspension system formed by dispersing a solid pesticide raw material that is difficult to dissolve in oil in the form of fine particles in a non-aqueous medium (oil). Among them, methyl oleate competes with dispersants and emulsifiers for adsorption sites on the particle surface. Since methyl oleate has lower surface tension and stronger wetting power, its adsorption on particles is stronger than that of dispersants, making it difficult to further break the particles, thereby increasing the apparent viscosity of the system. The original drug has a certain solubility in methyl oleate, but the coating formed by traditional dispersants is not dense enough, making it easy for the exposed particle surface to dissolve and crystallize with methyl oleate.
(5) Application in water-dispersible granules
In pesticide water-dispersible granules, surfactants mainly play a wetting and dispersing role. The principle is to stabilize the dispersion state of pesticide particles by adsorbing on the surface of the original drug particles to reduce the interfacial free energy. Commonly used surfactants in water-dispersible granules include anionic surfactants, such as sodium dodecyl sulfate, sodium dodecylbenzene sulfonate and chloroalkyl sulfonates; non-ionic surfactants, such as fatty alcohol polyoxyethylene ether, polyoxyethylene/polyoxypropylene ether block copolymers and alkylphenol polyoxyethylene ether phosphates. The hydrophilic group characteristics of surfactants have a significant effect on the performance of pesticide formulations. For example, carboxyl groups have good dispersibility, but poor wettability and low stability; sulfate groups have good wettability, but poor stability in acidic media and are prone to produce a large amount of foam; while polyoxyethylene groups have good dispersibility and emulsification, low foaming and the ability to soften hard water. The hydrophilic group in the middle of the molecule has stronger wetting properties than the end. When selecting surfactants, it is necessary to consider the effect of their relative molecular weight on wettability and permeability. Surfactants with low molecular weight usually have better wettability and permeability, while surfactants with high molecular weight have a slower diffusion rate but have stronger adsorption capacity and the potential to provide long-term stability. For non-polar pesticides, it is recommended to use non-ionic or weakly polar surfactants, and vice versa, polar anionic polymer dispersants.
(6) Application in nanopesticides
As an emerging type of pesticide formulation, nanopesticides have significantly improved the effectiveness and safety of pesticides through their unique nanoscale, thereby promoting the sustainable development of agriculture. The ideal nanopesticide particle size should be less than 100 nm, but considering the feasibility of actual preparation and application, a particle size of less than 500 nm is also considered acceptable. Nanopesticides mainly include nanoemulsions and nanodispersions, among which nanodispersions are nanocrystals or amorphous particles composed of active ingredients. For long-term storage stability, surfactants or polymer stabilizers must be used. Lu et al. stored pesticides in soluble concentrates and diluted them with water to form nanodispersions when used. As shown in Figure 2, the lipophilic groups of the surfactant are adsorbed on the surface of the formed material, reducing its interfacial energy and stabilizing the particles through electrostatic repulsion and elastic-steric repulsion to form a nanodispersion.
Schematic diagram of the preparation of soluble concentrates and nanodispersions
The role of surfactant pesticide adjuvants in pesticide application
The pesticide use process is a complex and delicate process, which mainly includes the secondary dispersion process from pesticide preparation to liquid, the atomization process from liquid to droplets, the deposition process of droplets contacting plant leaves, and the absorption and conduction process of plant leaves to pesticides. In the process of pesticide use, surfactant pesticide adjuvants change the physical and chemical properties of pesticides, thereby enhancing the efficacy of pesticides and improving the control effect on pests. They play an indispensable role in the entire pesticide use process.
(1) Role in the secondary dispersion and atomization process of pesticides
In order to ensure that pesticides have sufficient lethality to pests and diseases, avoid pesticide damage to crops, and promote uniform distribution of pesticides on the surface of crops, high-concentration pesticide preparations must be dispersed in proportion with water or other solvents to prepare a suitable solution. During the secondary dispersion process, surfactant molecules will be oriented at the interface between the pesticide solution and the aqueous phase, with the hydrophilic group extending toward the aqueous phase and the lipophilic group extending toward the air or other non-aqueous phase. This oriented arrangement changes the properties of the solution interface, reduces the interfacial tension between the pesticide solution and the aqueous phase, improves the solubility, dispersibility and charge properties of the preparation, ensures that the active ingredients of the pesticide are evenly dispersed in the solution, and lays the foundation for subsequent use.
Pesticide droplets are mainly deposited in the middle and upper areas of the crop canopy. The droplet size has a greater impact on the canopy deposition distribution. Droplets with a size of 100 to 180 μm are more ideally distributed in the canopy. The large specific surface area of droplets can better cover the surface of crops, increase the probability of contact between pesticides and pests, and thus enhance the control effect. In addition, droplets of appropriate size can reduce the drift and loss of pesticides in non-target areas and reduce environmental pollution. During the atomization process of pesticide formulations, surfactants can adjust the size, density, surface tension and other properties of droplets, which helps to better form and stabilize the droplets, thereby improving the uniformity of pesticide spraying. Polymer additives can increase the shear viscosity and tensile viscosity of the liquid medicine, thereby increasing the median diameter of droplets and reducing droplet evaporation and drift. Oily additives can increase the median diameter of droplets, reduce the proportion of droplets less than 100μm, slow down the evaporation rate, and prolong the drying time. The median diameter of droplets is positively correlated with the dynamic surface tension of the liquid medicine, which affects the breakup and deposition of droplets.
(2) Role in the deposition and absorption and conduction of pesticides
During the deposition of pesticides, surfactants can change the wettability of droplets, reduce the bouncing phenomenon, make them spread better on the leaf surface, and increase the amount of pesticides deposited on the leaf surface. Surfactant molecules reduce the interfacial tension between the pesticide solution and the target surface such as the plant or pest body surface, making it easier for the pesticide solution to spread and deposit on the target surface, thereby increasing the contact area with the target organism and improving the utilization rate of pesticides, which is of great significance for reducing the amount of pesticides used and reducing costs. For example, silicone surfactants, due to their extremely low surface tension, can promote the rapid wetting of pesticide emulsions and penetrate into every tiny part of the leaves, stems and stalks of plants, so that the pesticide can play its maximum effect. In addition, surfactants can interact with substances on the target surface, change the hydrophilicity or hydrophobicity of the target surface, make its properties closer to the properties of the liquid, and promote the wetting and adsorption of the pesticide solution on the target surface. For example, natural glycyrrhizic acid can be assembled into one-dimensional nanofibers fixed on a rough hydrophobic surface, and effectively slow down the retraction rate of droplets. The bioamphiphilic emulsifier sodium deoxycholate forms a Janus emulsion through salinity-driven interfacial self-assembly. Due to its topological effect, the emulsion can adhere to the rice micropapillary, reducing the loss of pesticides caused by factors such as rain erosion, and improving the persistence of pesticides. Surfactants can also regulate the deposition morphology of pesticide active ingredients, so that the prepared cleavable fipronil amphiphile can wet the micro-nano wax structure of rice leaves, achieve a good leaf surface deposition morphology, effectively avoid the coffee ring effect, and thus achieve a more uniform coverage of the leaves.
The key indicators of target deposition efficiency (such as interface expansion modulus, interface properties and adhesion, etc.) can be regulated by surfactant pesticide adjuvants. The increase in the expansion viscosity modulus of the droplet gas-liquid interface means the enhancement of adhesion, the decrease in the bounce height on the target surface, and the enhancement of the deposition and wetting ability of the liquid. The molecular structure of the adjuvant can affect the molecular migration rate, the dynamic surface tension of the drug solution and the droplet spectrum; the increase in the migration rate is conducive to the formation of a suitable assembly, thereby increasing the deposition and wetting of the drug solution. The smaller the receding contact angle of the drug solution on the target surface, the more it can inhibit the drug solution from bouncing and enhance the deposition ability, while the lower contact angle and higher adhesion tension can also effectively inhibit the droplet bouncing. Li et al. found that didecyl dimethyl ammonium bromide (DDAB), a double-chain cationic surfactant, can not only inhibit the retraction and rebound of droplets, but also promote their full spreading on the surface of paraffin and Chenopodium quinoa leaves, even at ultra-low concentrations (0.05%), compared with the well-known sodium dodecyl sulfate (SDS) and sodium bis (2-ethylhexyl) sulfosuccinate (AOT). This phenomenon is attributed to the rapid adsorption kinetics of DDAB from the bulk to the newly formed interface (only 100 ms), which significantly reduces the surface tension, thus giving it excellent drug solution improvement ability. Li et al. further proposed a new method to control droplet splashing and diffusion through double-chain cationic surfactants of different lengths. Due to the differences in the diffusion rate of surfactants from the bulk phase to the interface during the impact process, the adsorption efficiency at the liquid-solid interface, and the relaxation rate of aggregates within a short period of impact, C10DDAB has the greatest inhibitory effect on droplet splashing. At the same time, due to the different adsorption effects of the liquid-solid interface and the diffusion driving force during the long spreading time, the C10DDAB droplets spread fastest on the lotus leaves and eventually achieve complete wetting. These findings reveal that surfactants have a suitable hydrophilic-hydrophobic ratio, a fast diffusion rate, and an unstable assembly, which can significantly inhibit droplet splashing, promote the spreading of droplets, and thus promote the deposition of droplets on superhydrophobic surfaces.
The role of surfactants in the use of pesticides is multifaceted. It can not only improve the penetration of pesticide active ingredients in plant epidermis, but also promote the penetration, absorption and conduction of pesticide active ingredients by leaves through the synergistic effect of dissolving wax layers and opening stomata, further enhancing the efficacy. In addition, after the application of pesticides, their stability on the target is also crucial. Functional adjuvants can enhance the ability of pesticides to withstand rainwater erosion and ensure that pesticides can maintain good efficacy after being affected by natural environmental factors. The surface characteristics and growth environment differences of different crops determine that their needs for pesticides and surfactants are also different. For example, crops with a waxy layer on the leaf surface require surfactants with stronger wettability; while crops with developed root systems may pay more attention to the role of surfactants in root absorption and conduction of pesticides. Bao et al. studied the impact, wetting, adhesion and leaf retention behavior of pyraclostrobin (PYR) droplets containing the surfactant Triton X (TX) series on the hydrophobic surface of onion leaves. The results showed that moderate wetting ability and high adhesion allowed the PYR solution sprayed on the onion leaves to retain the maximum amount of TX-102-added PYR on the leaves. In addition, TX-102 improved the penetration and absorption of PYR on onion leaves through the synergistic effect of opening stomata and dissolving the waxy layer. When screening surfactant pesticide adjuvants, we can study the interaction between adjuvants and active ingredients and target enzymes based on the rate of adjuvants penetrating cell membranes, morphological changes and their effects, and build a predictive model for the absorption and conduction process through machine learning. This will help develop efficient green pesticide formulations and adjuvants, and ultimately achieve reduced pesticide use and increased efficiency.
