When a pesticide product and an adjuvant are combined in a spray mix tank, the active ingredient can migrate from the product droplets or particles into the adjuvant oil droplets before the mixture reaches the crop. Webster et al. (2016) resolved this transfer kinetics for one emulsion-concentrate herbicide (clethodim) and one suspension-concentrate fungicide (tebuconazole) against an ethyl-oleate-based and a mineral-oil-based adjuvant: emulsion transfer was rapid and complete, suspension transfer was halted by the solubility limit of the active in the adjuvant oil, and high adjuvant loadings produced viscous systems that would not mix. The unresolved problem is to turn this two-product, two-adjuvant case study into a predictive, composition-based understanding: how surfactant content and oil composition separately control transfer rate and extent; whether the proposed compositional-ripening, log-P-directed mechanism generalizes across pesticide chemistries, formulation types, and complex multi-agrochemical tank mixes; and whether the transient suspoemulsion-like states that form when a suspension active cannot fully dissolve remain stable over the spray application window.

Background

Activator adjuvants are oil/surfactant additives mixed into the pesticide spray tank to improve foliar uptake: an emulsifiable oil phase (hydrocarbon or vegetable-oil derived) plus nonionic surfactants that disperse the oil in water. Pesticide products reach the tank either as emulsion concentrates (EC), in which the active ingredient is dissolved in an organic solvent that emulsifies on dilution, or as suspension concentrates (SC), in which solid crystalline active is suspended in water. The first physical event after the products meet in the tank is inter-droplet transfer: the active ingredient migrates between product droplets, adjuvant droplets, and suspended crystallites on the seconds-to-minutes timescale, so what is sprayed onto the crop may differ from what was poured into the tank. Until the work of Webster et al. (J. Agric. Food Chem. 2016, 64, 6139-6147), this stage had received little physical-chemical study; most adjuvant research concerned events on the leaf (wetting, cuticle penetration, rainfastness). Using laser diffraction (droplet-size-resolved intensity changes), confocal Raman spectroscopy of individual droplets (chemical identification of mixed droplets), and time-resolved synchrotron small-angle X-ray scattering, SAXS (which detects loss of crystalline diffraction from the solid active), they showed for the EC herbicide Status (clethodim) that transfer into both the ethyl-oleate-based adjuvant Hasten (EO) and the mineral-oil-based adjuvant Empower (MO) was rapid and essentially complete, proposed to occur by compositional ripening — the merger of composition between miscible emulsion droplets, surfactant-micelle mediated — with direction set by relative lipophilicity (octanol/water partition coefficient, log P: the more lipophilic phase acts as sink). For the SC fungicide Folicur (tebuconazole), transfer required dissolution of crystallites and stopped when the adjuvant droplet saturated: complete dissolution between 3:1 and 5:1 EO:product volume ratio, essentially none in MO at up to 10:1, consistent with measured tebuconazole solubilities of 6.37% versus 0.21% (w/w) in the two adjuvants. Where the oil wets but cannot dissolve the solid, the tank mixture becomes a transient suspoemulsion (an emulsion co-stabilizing suspended solid particles), argued — not demonstrated — to survive the sub-24-hour application window. At the highest adjuvant levels the MO/suspension system became highly viscous and would not mix, precluding higher loadings. The study was thus built from two commercial products and two commercial adjuvants, and its central quantities — transfer rate and extent — were measured case by case rather than predicted from composition.

Problem Statement

Establish a predictive, composition-based physical-chemical understanding of the first stage of adjuvant-pesticide interaction — transfer of the active ingredient from the pesticide product into the adjuvant phase upon mix-tank mixing — beyond the two-product, two-adjuvant case resolved kinetically by Webster et al. (2016). The single overarching objective is to relate adjuvant and pesticide composition to transfer rate and extent, and it comprises three jointly supporting parts: (1) decompose how the individual adjuvant components — surfactant content and oil composition — separately control the rate and the extent of active-ingredient transfer, rather than measuring commercial adjuvants only as wholes; (2) establish whether the proposed compositional-ripening mechanism and its log-P-directed transfer generalize across other pesticide chemistries and formulation classes, and extend to complex tank mixes containing multiple agrochemical products (e.g., fungicide + insecticide + foliar fertilizer) where competing surfactant demand may hinder transfer; and (3) for suspension products, where the solubility of the active ingredient in the adjuvant oil limits crystallite dissolution and transfer, establish whether the transient suspoemulsion-like states that form when crystallites persist are stable over the sub-24-hour crop-application window — a stability the source study argued, but did not demonstrate.

References

  1. Graham R. Webster, Nicole B. Bisset, David M. Cahill, Peter Jones, Andrew Killick, Adrian Hawley, Ben J. Boyd, "Kinetic Resolution of the Interactions between Agrochemical Products and Adjuvant Systems upon Mixing", Journal of Agricultural and Food Chemistry 2016, 64(31), 6139-6147, DOI 10.1021/acs.jafc.6b01673, https://doi.org/10.1021/acs.jafc.6b01673
  2. Nicole B. Bisset, Graham R. Webster, Yao-Da Dong, Ben J. Boyd, "Understanding the kinetic mixing between liquid crystalline nanoparticles and agrochemical actives", Colloids and Surfaces B: Biointerfaces 2019, 175, 324-332, DOI 10.1016/j.colsurfb.2018.11.063, https://doi.org/10.1016/j.colsurfb.2018.11.063
  3. Xiao-xu Li, Li-fei He, Xiu-yu Pang, Yang-yang Gao, Yang Liu, Peng Zhang, Guang Wei, Wei Mu, Bei-xing Li, Feng Liu, "Tank-mixing adjuvants enhanced the efficacy of fludioxonil on cucumber anthracnose by ameliorating the penetration ability of active ingredients on target interface", Colloids and Surfaces B: Biointerfaces 2021, 204, 111804, DOI 10.1016/j.colsurfb.2021.111804, https://doi.org/10.1016/j.colsurfb.2021.111804
  4. Graham R. Webster, Nicole B. Bisset, David M. Cahill, Peter Jones, Andrew Killick, Adrian Hawley, Ben J. Boyd, "Tristearin as a Model Cuticle for High-Throughput Screening of Agricultural Adjuvant Systems", ACS Omega 2018, 3(12), 16672-16680, DOI 10.1021/acsomega.8b02656, https://doi.org/10.1021/acsomega.8b02656
  5. Wanting Yang, Wei Zhong, Weidong Jia, Bin Wang, Yanzhao Yang, Hao Wan, Guorong Wu, "Atomization mechanisms and high-speed imaging of liquid sheet breakup in emulsion-based agricultural sprays: a review", Cogent Food & Agriculture 2025, 11(1), 2581332, DOI 10.1080/23311932.2025.2581332, https://doi.org/10.1080/23311932.2025.2581332
  6. Jing Ye, Zihao Wan, Chenyang Zhang, Luyao Zhang, Xuerui Yang, Jean-Marc Chovelon, Lei Zhou, Guangli Xiu, "Novel insights into the photochemical transformation of neonicotinoid insecticides: potential involvement of adjuvants", Journal of Environmental Chemical Engineering 2024, 12(5), 113994, DOI 10.1016/j.jece.2024.113994, https://doi.org/10.1016/j.jece.2024.113994
  7. Wanting Yang, Wei Zhong, Weidong Jia, Mingxiong Ou, Xiang Dong, Tie Zhang, Suming Ding, Pengpeng Yu, "The effect of oil-in-water emulsion pesticide on the evolution of liquid sheet rim disintegration and the spraying distribution", Crop Protection 2024, 177, 106547, DOI 10.1016/j.cropro.2023.106547, https://doi.org/10.1016/j.cropro.2023.106547
  8. Fengyan Wang, Zhen Hu, Carla Abarca, Michael Fefer, Jun Liu, Michael A. Brook, Robert Pelton, "Factors influencing agricultural spray deposit structures on hydrophobic surfaces", Colloids and Surfaces A: Physicochemical and Engineering Aspects 2018, 553, 288-294, DOI 10.1016/j.colsurfa.2018.05.074, https://doi.org/10.1016/j.colsurfa.2018.05.074