Polydisperse ethoxylated fatty alcohol surfactants as accelerators of cuticular penetration. 1. Effects of ethoxy chain length and the size of the penetrants

Quantifying the effect of non-ionic surfactant alkylphenol ethoxylates on the persistence of thiabendazole on fresh produce surface

BACKGROUND

Understanding the role of adjuvants in pesticide persistence is crucial to develop effective pesticide formulations and manage pesticide residues in fresh produce. This study investigated the impact of a commercial non-ionic surfactant product containing alkylphenol ethoxylates (APEOs) on the persistence of thiabendazole on apple and spinach surfaces against the 30 kg m-3 baking soda (sodium bicarbonate, NaHCO3 ) soaking, which was used to remove the active ingredient (AI) in the cuticular wax layer of fresh produce through alkaline hydrolysis. Surface-enhanced Raman scattering (SERS) mapping method was used to quantify the residue levels on fresh produce surfaces at different experimental scenarios. Four standard curves were established to quantify surface thiabendazole in the absence and presence of APEOs, on apple and spinach leaf surfaces, respectively.

RESULTS

Overall, the result showed that APEOs enhanced the persistence of thiabendazole over time. After 3 days of exposure, APEOs increased thiabendazole surface residue against NaHCO3 hydrolysis on apple and spinach surfaces by 5.39% and 10.47%, respectively.

CONCLUSION

The study suggests that APEOs led to more pesticide residues on fresh produce and greater difficulty in washing them off from the surfaces using baking soda, posing food safety concerns. © 2023 Society of Chemical Industry.

Measuring the photostability of agrochemicals on leaves: understanding the balance between loss processes and foliar uptake

BACKGROUND

Photostability is an important property in agrochemicals, impacting their biological efficacy, environmental fate and registrability. As such, it is a property that is routinely measured during the development of new active ingredients and their formulations. To make these measurements, compounds are typically exposed to simulated sunlight after application to a glass substrate. While useful, these measurements neglect key factors that influence photostability under true field conditions. Most importantly, they neglect the fact that compounds are applied to living plant tissue, and that uptake and movement within this tissue provides a mechanism to protect compounds from photodegradation.

RESULTS

In this work, we introduce a new photostability assay incorporating leaf tissue as a substrate, designed to run at medium throughput under standardized laboratory conditions. Using three test cases, we demonstrate that our leaf-disc-based assays provides quantitatively different photochemical loss profiles to an assay employing a glass substrate. And we also demonstrate that these different loss profiles are intimately linked to the physical properties of the compounds, the effect that those properties have on foliar uptake and, thereby, the availability of the active ingredient on the leaf surface.

CONCLUSIONS

The method presented provides a quick and simple measure of the interplay between abiotic loss processes and foliar uptake, supplying additional information to facilitate the interpretation of biological efficacy data. The comparison of loss between glass slides and leaves also provides a better understanding of when intrinsic photodegradation is likely to be a good model for a compound's behaviour under field conditions. © 2023 Society of Chemical Industry.

Mechanistic Aspects and Effects of Selected Tank-Mix Partners on Herbicidal Activity of a Novel Fatty Acid Ester

Only a limited number of contact herbicides exist in agricultural production. While systemic herbicides are more efficient also at suboptimum spray coverage with long-lasting weed control, contact herbicides provide several advantages. There is no translocation to fruits or roots of plantation and other crop, low risk for resistance development, and minor risk for spray-drift damage. Besides, synthetic products that often have toxicological or residues issues, natural fatty acids, particularly pelargonic acid (PA), have contact activity and are safer for home and garden use. We recently described a methyl capped polyethylene glycol ester of pelargonic acid (PA-MPEG) that acts independent of acid formation. Both, PA-MPEG and PA are applied at high rates per hectare to achieve excellent weed control. Here, we report about potential additives to increase PA-MPEG efficacy. The herbicidal active, 1-decanol, and the non-phytotoxic alkylated seed oil-based adjuvant, Hasten^TM, improved performance and outperformed a commercial PA herbicide. Both, PA-MPEG and PA appear to mainly act by the disintegration of bio-membranes besides having effects on transpiration. The main suggested effect is desiccation due to cutting the water continuum at the site of evaporation in the intercellular spaces. The synergistic action of the adjuvant Hasten^TM and its practical uses are also discussed.

Selection of organosilicone surfactants for tank-mixed pesticides considering the balance between synergistic effects on pests and environmental risks

In this study, the bioactivities of binary mixtures of organosilicone surfactants and indoxacarb against two Lepidopteran pests were investigated along with their environmental risks. All of the tested organosilicone surfactants had obvious synergistic effects on the contact toxicity of indoxacarb against Spodoptera exigua and Agrotis ipsilon. However, all of the organosilicone surfactants exhibited certain antagonism for indoxacarb against S. exigua in terms of stomach & contact toxicity; both Silwet-408 and Silwet-806 exhibited additivity against A. ipsilon, whereas Silwet-618 and Silwet-DRS-60 exhibited synergism and slight antagonism, respectively. All of the tested chemicals were highly toxic to Daphnia magna, among which Silwet-DRS-60 had the lowest acute toxicity (EC₅₀ of 94.91 μg/L). However, these chemicals were less toxic to Brachydanio rerio. Silwet-DRS-60 had a low toxicity to B. rerio, while Silwet-408, Silwet-806 and Silwet-618 were moderately toxic to B. rerio. For the joint toxicity evaluation of organosilicone surfactants and indoxacarb to D. magna and B. rerio, the additive index method, concentration addition method and toxicity unit method were robust in judging synergism or antagonism, whereas other methods were more conservative; the V-value method and equilibrium curve method exhibited high robustness and viability in evaluating the combined effects of binary mixtures. Overall, we should carefully select organosilicone surfactants for premixed or tank-mixed pesticides in agriculture to obtain a balance between synergistic effects on pests and environmental risks.

The Mode of Action of Adjuvants-Relevance of Physicochemical Properties for Effects on the Foliar Application, Cuticular Permeability, and Greenhouse Performance of Pinoxaden

We comprehensively studied the complexity of the mode of action of adjuvants by uncoupling the parameters contributing to the spray process during foliar application of agrochemicals. The ethoxylated sorbitan esters Tween 20 and Tween 80 improved the efficiency of pinoxaden (PXD) in controlling grass-weed species in greenhouse experiments by aiding retention, having humectant properties, maintaining the bioavailability, and increasing the cuticular penetration of PXD. The nonethoxylated sorbitan esters Span 20 and Span 80 showed minimal effects on retention, droplet hydration, or cuticular penetration, resulting in reduced PXD effects in the greenhouse. Tris(2-ethylhexyl)phosphate (TEHP) does not contribute much to retention and spreading but strongly enhances the diffusion of PXD across isolated P. laurocerasus cuticular membranes. As TEHP was most efficient in controlling the growth of grass-weed species, we propose that the direct effect of penetration aids on cuticular permeation plays a key role in the efficiency of foliar-applied agrochemicals.

Mixture toxicity assisting the design of eco-friendlier plant protection products: a case-study using a commercial herbicide combining nicosulfuron and terbuthylazine

The development of environmentally friendly plant protection products (PPPs), including pesticides, is a challenge nowadays. A commercial herbicide combining terbuthylazine and nicosulfuron as active substances (a.s.) was selected as a model PPP. The suitability of manipulating the ratio between a.s. towards alternative formulations with reduced impacts in a non-target indicator (Lemna minor) was assessed. The efficacy of such eco-friendlier a.s. ratios was then assessed using a target weed, Portulaca oleracea. Single and mixture toxicity testing with L. minor revealed an antagonistic joint action of the a.s., suggesting an environmentally protective effect of the combination compared to single dosing of a.s. The efficacy testing against the target weed of single and combined treatments of the a.s. showed that (i) the a.s. behave antagonistically throughout the whole P. oleracea response surface; (ii) there were no environmentally safe a.s. combinations ensuring target-efficacy; (iii) terbuthylazine alone was effective in controlling P. oleracea with no environmental hazardous potential, dosed at concentrations 10-fold lower than those involved in commercially recommended application doses. Overall, this case-study suggests that modelling tools widely used in the field of environmental risk assessment of PPPs may also have application in PPP design stages for a more efficient meeting of efficacy and environmental friendliness requirements.

Water Sorption Isotherms of Surfactants: A Tool To Evaluate Humectancy

Fundamental experimental data for moisture absorption of non-ionic polydisperse surfactants with differing ethylene oxide (EO) content and variable aliphatic portions were measured at relative humidities between 0 and 95% at 25 °C. Remarkable differences in moisture absorption were observed between surfactant classes but also within one series of surfactants differing in either EO content or the long-chain aliphatic fraction. Both the EO units as well as the entire molecular structure, including also the lipophilic domain, were discussed to account for the humectant activity of surfactants. Water sorption isotherms showed an exponential shape, which was argued to be associated with the formation of a "free" water domain. These humectant properties might be relevant to the behavior of a foliar-applied spray droplet of agrochemical formulation products because the uptake of active ingredients will be enhanced as a result of deferred crystal precipitation.

The contribution of spray formulation component variables to foliar uptake of agrichemicals

BACKGROUND

The objective of the present study was to determine the contribution of the active ingredient (AI) and surfactant, and their concentrations, to the foliar uptake of agrichemicals, and to examine the physical properties that would need to be included in a model for foliar uptake.

RESULTS

All spray formulation component variables significantly affected uptake, explaining 73% of the deviance. The deviance explained by each factor ranged from 43% (AI concentration nested within AI) to 5.6% (surfactant). The only significant interaction was between AI and surfactant, explaining 15.8% of the deviance. Overall, 90% of the deviance could be explained by the variables and their first-order interactions.

CONCLUSIONS

Uptake increased with increasing lipophilicity of the AI at concentrations below those causing precipitation on the leaf surface. AI concentration had a far greater (negative) effect on the uptake of the lipophilic molecule epoxiconazole. The uptake of 2-deoxy-D-glucose (DOG) and 2,4-dichlorophenoxyacetic acid (2,4-D) increased with increasing hydrophile-lipophile balance (HLB) of the surfactant, the effect of HLB being far greater on the hydrophilic molecule DOG. However, the differences observed in epoxiconazole uptake owing to the surfactant were strongly positively related to the spread area of the formulation on the leaf surface. For all AIs, uptake increased in a similar manner with increasing molar surfactant concentration.

Transport properties of the mung bean (Vigna radiata) non-aerial hypocotyl membrane: permselectivity to hydrophilic compounds

BACKGROUND

Aerial plant surfaces are covered by a lipophilic cuticular membrane (CM) that restricts the transport of water and small solutes. Non-aerial tissues do not exhibit such a barrier. Recent data have shown that large relative to CM hydrophilic agrochemicals were able to pass at high rates through the non-aerial coleoptile.

RESULTS

A moderately large hydrophilic solute like PEG 1000 with a mean molar volume of 782 cm(3) mol(-1) was rejected by the non-aerial hypocotyl. Uptake of smaller solutes like urea (46.5 cm(3) mol(-1) ) was fast and with 99% after 1 day. Cut-off size estimations suggest a pore size diameter below 1.5 nm.

CONCLUSION

Aerial and non-aerial CM differ largely in their absolute barrier properties. This difference is related to the absence of embedded cuticular waxes in the non-aerial hypocotyl membrane, which make the CM physically dense and cause low solubility of hydrophilic solutes. The free volume for diffusion at the interface of the non-aerial hypocotyl cuticle to the environment is much larger resulting in higher penetration rates. It is suggested that diffusion through the non-aerial hypocotyl does not proceed in a real channel system with continuous aqueous phase but is more like transport through a filter with restricted diffusion in the pore openings.

The application of ToF-SIMS to the analysis of herbicide formulation penetration into and through leaf cuticles

Understanding the movement of the active ingredient in relation to the other formulation components following application is crucial to an overall understanding of herbicide performance. We describe the novel use of time-of-flight secondary ion mass spectrometry (ToF-SIMS) as a tool for following the movement of herbicide formulation components into and across plant cuticles. This technique provides new insights since it provides both high (sub-micron) spatial resolution combined with the chemical specificity associated with organic mass spectrometry. The components studied include the oligomeric ethoxylate surfactants Synperonic A7 and A20 and active ingredient Sulfosate (trimesium glyphosate). The movement of these molecules, both separately and when combined in a simple formulation, into the surface of Prunus laurocerasus leaves and across the isolated plant cuticle was investigated and clear differences in penetration/diffusion behaviour were identified. ToF-SIMS was uniquely able to (simultaneously) spatially resolve all the species involved, including the anion and cation components of the active ingredient. Also, using spectral reconstructions from the imaging raw data streams, the behaviour of individual oligomers within the surfactant distributions, could be assessed. The observations are discussed with reference to the action of surfactants identified in parallel micro-structural studies and the current understanding of herbicide uptake.

Modelling the effects of alcohol ethoxylates on diffusion of pesticides in the cuticular wax of Chenopodium album leaves

Cuticular waxes represent the first and, in most cases, the limiting barrier for foliar uptake of pesticides from solution. Sorption of pesticides in reconstituted cuticular wax (wax/water partition coefficients) of Chenopodium album L. and in isolated cuticular membranes (cuticle/water partition coefficients) of Prunus laurocerasus L. was determined. Diffusion coefficients of pesticides in reconstituted cuticular wax of C. album leaves were size-dependent, increasing with increasing molar volume. In the presence of alcohol ethoxylates, diffusion coefficients were enhanced by up to two orders of magnitude, and size selectivity was significantly decreased. The accelerating effect and the decrease in size selectivity were attributed to plasticisation of the cuticular wax by the alcohol ethoxylates increasing the fluidity in the wax. A free volume model adopted from polymer science was successfully applied to predict diffusion coefficients of pesticides on the basis of the transport properties of the wax (size selectivity and crystallinity), the molar volume of the diffusing compound and the accelerator concentration in the wax.

Comparison of sorption and diffusion by pyridate and its polar metabolite in isolated cuticular wax of Chenopodium album and Hordeum vulgare

Sorption and diffusion of the herbicide pyridate and its metabolite CL9673 were measured in reconstituted cuticular waxes isolated from Chenopodium album L. and Hordeum vulgare L. (cultivar Igri) leaves. The compounds have the same basic chemical structure, except that pyridate is characterized by a C8-alkyl chain bound via a thioester to the ionizable hydroxyl group of CL9673. Sorption of the weak acid CL9673 from aqueous solutions into cuticular waxes was pH-dependent, and the apparent wax/water partition coefficients decreased with increasing pH. Wax/water partition coefficients of pyridate were not dependent on pH, and they were about 4 orders of magnitude higher as compared to the nondissociated species of CL9673. Diffusion coefficients measured in reconstituted cuticular wax for CL9673 fitted established models predicting diffusion coefficients in relation to molar volumes. However, this was not the case with pyridate, which was characterized by a self-accelerating effect leading to diffusion coefficients, which were up to 2 orders of magnitude higher than predicted from the molar volume. This is a remarkable result since pyridate represents a compound combining the properties of an active ingredient and of a plasticizer.

Effects of temperature and concentration of the accelerators ethoxylated alcohols, diethyl suberate and tributyl phosphate on the mobility of [14C]2,4-dichlorophenoxy butyric acid in plant cuticles

Intrinsic activities of monodisperse ethoxylated dodecanols (MEDs), diethyl suberate (DESU) and tributyl phosphate (TBP) were investigated using Stephanotis floribunda leaf cuticular membranes (CMs) and [14C]2,4-dichlorophenoxy butyric acid (2,4-DB) as a model solute. When sorbed in cuticular membranes, MEDs, DESU and TBP increase solute mobility and are called accelerators for this reason. With MEDs, dose-effect curves (log mobility vs accelerator concentration) were linear but, with DESU and TBP, curves convex to the x axes were obtained that approached a maximum at 90 and 150 g kg-1, respectively. Accelerators increased the mobility of 2,4-DB in the CMs by 9- to 48-fold, and effects were larger at lower temperatures (range 15-30 degrees C). Activation energy for diffusion of 2,4-DB was 105 kJ mol-1, decreasing with increasing accelerator concentrations to 26 kJ mol-1 with DESU at 90 g kg-1 and 64 kJ mol-1 with TBP at 150 g kg-1. Thus, the intrinsic activity of DESU was much higher than that of TBP, which implies that, for a given effect, less DESU than TBP would be needed. MEDs were also very effective accelerators, lowering activation energies to 36 kJ mol-1. Data are discussed in relation to increasing rates of foliar penetration of active ingredients at low temperatures.

Surfactant effects on cuticular penetration of neutral polar compounds: dependence on humidity and temperature

Effects of poly(ethylene glycol) (PEG 400) and polydisperse fatty alcohol ethoxylates ("Genapols") on methylglucose penetration across cuticles of Pyrus communis were studied under different humidities and temperatures. All surfactants increased methylglucose penetration better than PEG 400. The efficacy was proportional to their own penetration. Genapol C-100 (C12.5E8.4; C, average number of carbon atoms in the alkyl chain; E, average number of ethoxy groups) effects were strongly concentration dependent (0.2-5 g/L). At 2 g/L distinct differences between surfactants were observed. Effects of Genapol C-050 (C12.5E5.8) did not depend on humidity (11-93% RH) and temperature (15-35 degrees C). They were maximal after application but decreased rapidly due to surfactant penetration. Genapol C-200 (C12.5E17) effects increased strongly with humidity and temperature both of which prevented its solidification. Genapol C-100 was superior under most conditions and never failed to increase penetration. There was no simple relationship between ethylene oxide content and surfactant effect.