Mechanism Of Interface Layer Mediated Amphiphilic Molecules Regulating The Flexibility Of Pesticide-loaded Microcapsules To Improve The Interaction Efficiency With Foliar Surfaces

In recent years,pesticide microencapsulation has shown great potential in improving utilization and declining environmental risk of pesticides.At present,the interfacial polymerization of pesticide-loaded microcapsules（MCs）preparation system is widely used in industrial pesticide formulation preparation,and the pesticide-loaded MCs performed slow-release property,which is prominent in control against soil-borne diseases and pests.Due to the differences in application scenarios,crop characteristics,pest occurrence features and other factors,the initial release of pesticide from MCs was insufficient,the foliar adhesion and rainfall-scouring resistance were poor,which cannot meet the control needs of leaf diseases and pests.This study is based on the analysis of the encapsulation principle of interface polymerization,inspired by"snail suction cup",the"pesticide-loaded MCs flexibility"strategy was proposed in this study,which improved MCs’interaction efficiencies,such as deformation,adhesion and release properties,with leaf surface are improved by controlling the structure and composition of shells for the better foliar performance of pesticide-loaded MCs.MDI（4,4-methylenediphenyl diisocyanate）was used as the oil phase monomer,water or PEG400 as the water phase monomer,pyraclostrobin was used as model pesticide,and nonionic surfactant octaphenyl polyoxyethylenes（OPs）and cosurfactant small alcohols with different usages and various types and characteristics were used to control encapsulation interface,affect encapsulation process,and achieve efficient regulations of the structures and properties of pesticide-loaded MCs.The prepared flexible pesticide-loaded MCs performed excellent foliar interaction efficiency and application effect.The results are as follows:1)The effects of OP-10 usage difference on the flexibility and application performance of pesticide-loaded MCs were studied.In interfacial polymerization encapsulation,the usage increasing of OP-10（0,0.5,1 and 2%）in aqueous phase could significantly increase the aggregation and participation of OP-10 in encapsulation interface and reaction to reduce the compactness of shells,which the compactness indexes of Shells-0%OP-10,Shells-0.5%OP-10,Shells-1%OP-10 and Shells-2%OP-were 1.10,0.48,0.36 and 0.07,respectively.The pyralcostrobin-loaded MCs’Young’s modulus and Reduced modulus were declined significantly,and the Adhesion were significantly improved,which led to the deformation and adhesion properties of MCs were improved.Among them,MCs-2%OP-10 has a strong scouring resistance on peanut foliar surface,and the control effect on peanut leaf spot in rainy season was significantly better than other treatments.And,the release property of pyraclostrobin-loaded MCs was significantly improved.In the rapid release solution,the release speeds of pyraclostrobin from MCs-0%OP-10,MCs-0.5%OP-10,MCs-1%OP-10and MCs-2%OP-10 were 0.022,0.077,0.097 and 0.52%/min,respectively,which resulting in an increase in their acute toxicity on zebrafish（Danio rerio）.However,compared with conventional formulation EC,microencapsulation performed a significant effect on toxicity declining of pyraclostrobin.The acute toxicity grade of EC to zebrafish is Extremely toxic(LC₅₀=0.043 a.i.mg/L),and that of MCs-2%OP-10,which performed the fastest release property,is Highly toxic(LC₅₀=0.329 a.i.mg/L).2)The effects of different type OPs on the flexibility and application performance of pyraclostrobin-loaded MCs were studied.For different types,the hydrophile-lipophile balance value（HLB value）of OP molecule is improved by increasing the degree of polymerization of polyoxyethylene ether chain.At 2%usage,OP-7,OP-10,OP-15 and OP-21 added in oil phase are driven by amphiphilicity,and redistributed in oil-water interface and oil-water phases at various proportions,which caused the different levels of regulatory effects on encapsulation reaction and MCs characteristics.With increase of their HLB value,more OP molecules enter into the aqueous phase and their aggregation at the encapsulation interface decreases,weakening their regulatory effect on encapsulation reaction.The interfacial polymerization of two-phase monomers processed smoothly,which increased the compactness of shell.Among them,the shells of MCs-2%OP-21 had the highest compactness,the strongest rigidity and slow-release property.On the contrary,OP molecules with low HLB value are more likely to remain at oil phase and enter the two-phases interface,which participate in and regulate interfacial polymerization to decline the compactness of shell,then the oil and water two-phases monomers fully contact and react to increase the thickness and the contents of ether bonds,ester bonds,and hydroxyl groups of shell.Among them,MCs-2%OP-7 had the best deposited deformation and foliar adhesion,which caused the highest control effect on peanut leaf spot in rainy season.Compared to the MCs preparation process of OP added in aqueous phase,the concentration of OP added in oil phase is higher in the initial phase,and its regulatory effect on the encapsulation reaction is enhanced.While improving the flexibility of the pesticide-loaded MCs,the release property is significantly improved.Except for MCs-2%OP-21 was Highly toxic to zebrafish(LC₅₀=0.619 a.i.mg/L),the acute toxicity grades of other pyroclostrobin-loaded MCs were consistent with that of pyroclostrobin EW,which were Extremely toxic.3)The principle of OP in regulating the flexibility of MCs was clarified.OP molecules aggregate and arrange in oil-water interface,and affect the polymerization reaction of two-phase monomers by"steric hindrance".The mono-hydroxyl of OP molecule can form urethane with the isocyano of MDI,which can prevent the chain extension of the polymerized product and reduce crosslinking degree by"chain end sealing".The OP molecules that participate in and regulates the formation of shell by physical chimerism and covalent bonding not only reduce the compactness of shell to improve the deposited deformation of pesticide-loaded MCs,but also increase the content of hydrogen bonding groups such as ether bonds and ester bonds in shells,thus improving adhesion of MCs with leaf surface.4)The effects of small alcohol usage or type differences on the flexibility and application performance of pyraclostrobin-loaded MCs were studied.Increasing usage of small alcohols（n-butanol-0,0.05,0.1 and 0.2 mol/kg）or increasing the number of their carbon chain atoms（isopropanol,n-propanol,isopentanol,and n-pentanol）at the same usage（0.1 mol/kg）could increase the distribution amounts of small alcohols in oil water interface and oil phase,which increased the regulation on encapsulation reaction,reduced shell’s thickness and compactness to improve the deposited deformation of pesticide-loaded MCs.However,there are differences in the features of core between two methods.At usage of 0.1 mol/kg,pentanol was more capable to enter oil phase than butanol and propanol to"compete for MDI"to form hydrophobic compounds to improve core density.Both MCs-N-butanol 0.1 mol/kg and MCs-N-pentanol 0.1 mol/kg performed good foliar deformation and adhesion,and the former has the higher core fluidity,which is easy to release,and its sustained release performance was not as good as the latter,which performed a stronger sustained control effect against peanut leaf spot.5)The principle of small alcohols in regulating the flexibility of MCs was clarified.By changing the carbon chain molecular weight or structure of small molecular alcohols,their steric hindrance and oil-water partition coefficient can be regulated,thereby regulating their distribution amount between the oil-water interface and the oil-water two phases.In the interfacial polymerization system with MDI and PEG400 as two-phase monomers,after exogenous addition of different amounts or types of small alcohols at the initial stage of curing,they are redistributed at different levels between oil-water interface and oil phase,driven by concentration diffusion and amphiphilicity.As the usage of small alcohols increases and their molecular weight of carbon chains increases,their aggregation amount at oil-water interface and distribution amount in the oil phase significantly increase.The small alcohols in oil-water interface prevent the chain extension of the two-phase monomers polymerized product by"hydrophobicity modification",improve hydrophobicity of product and reduce shell compactness,and those entered oil phase form hydrophobic compounds by"competing for MDI"to increase core density,reduce core fluidity,and reduce the MDI participated in the encapsulation reaction to reduce shell thickness,thereby improving the deposited deformation and foliar adhesion of pesticide-loaded MCs.6)The effects of pesticide-loaded MCs release kinetics on their application performances were studied.By evaluating the release kinetics of pyraclostrobin-loaded MCs with various structures in different environments,it was found that at solid/gas interface,except for shell structure,the core state also had a regulator effect on pyraclostrobin release,while in liquid environments,the shell structure played a decisive role in releasing.The releasing performances of pesticide-loaded MCs at the solid/gas interface have a significant impact on the control effect against foliar diseases in greenhouse,and the releasing in the liquid environment was negatively correlated with their safety to fish.The flexible strategy of pesticide-loaded MCs proposed in this study provides a solution for improving their application performances on the leaf surfaces.The established amphiphilic molecular differentiation interface layer regulation method provided a strategy for efficient regulating on interfacial polymerization encapsulation systems.The correlation between structure and property of MCs provided a theoretical basis for improving pesticide utilization and eco-safety.