Effects of monodisperse alcohol ethoxylates on mobility of 2,4-D in isolated plant cuticles

Alcohol ethoxylates significantly synergize pesticides than alkylphenol ethoxylates considering bioactivity against three pests and joint toxicity to Daphnia magna

Seeking alternatives for alkylphenol ethoxylates (APEOs) have been a heavily researched topic in the surfactant industry and agricultural systems. In this study, the combined effects of different ethoxylates and pesticides on the bioactivity against three pests and toxicological risks to Daphnia magna were investigated. Results showed that alcohol ethoxylates (AEOs) had higher synergistic effects on the bioactivity of pesticides against Spodoptera exigua, Agrotis ipsilon and Aphis citricola than did APEOs. In terms of the joint toxicity of the ethoxylates and pesticides to D. magna, additive index method, toxicity unit method, V value method and isobologram method were used in the tests. All of these methods indicated that the joint effects of APEOs + acetamiprid and APEOs + indoxacarb upon D. magna turned from synergism to antagonism with the increasing EO (ethylene oxide) numbers. Those of AEOs exhibited similar trends. Overall, AEOs may be potential alternatives for APEOs in agriculture as they synergize pesticides against three pests significantly more than do APEOs. However, further research should investigate the compounds' environmental risks to aquatic organisms because the AEOs were highly toxic to D. magna.

Binary mixtures of alcohol ethoxylates, nonylphenol ethoxylates and pesticides exhibit comparative bioactivity against three pests and toxicological risks to aquatic organisms

Nonylphenol ethoxylates are widely used surfactants in the industry and agriculture. However, seeking for alternatives has been imperative considering their effects of the hormonal and other toxicological risks. In the current study, the synergistic effects of nonylphenol ethoxylates or alcohol ethoxylates on the bioactivity of indoxacarb and acetamiprid were compared. Results showed that synergistic ratios of nonylphenol ethoxylates (TX-7∼TX-30) and alcohol ethoxylates (MOA-5∼MOA-20) against Spodoptera exigua, Agrotis ipsilon and Aphis citricola decreased with the EO (ethylene oxide) numbers, although different magnitudes of decreases were observed. Single toxicities of all ethoxylates to Daphnia magna and Brachydanio rerio also dramatically decreased with the EO numbers. In terms of joint toxicity, the combined effects of all ethoxylates and pesticides upon D. magna turned from synergism to antagonism with the increasing EO numbers; the combined effects of nonylphenol ethoxylates and pesticides turned from synergism to antagonism with the increasing EO numbers of ethoxylates, whereas alcohol ethoxylates and pesticides always showed antagonistic effects whatever EO numbers. Overall, alcohol ethoxylates may be potential alternatives for nonylphenol ethoxylates as they exhibited nearly comparative bioactivity against tested pests and toxicities to D. magna and B. rerio.

Kinetic Resolution of the Interactions between Agrochemical Products and Adjuvant Systems upon Mixing

The addition of an adjuvant to a pesticide usually occurs in a mix-tank, before spray application to the crop. Their interaction is potentially crucial to overall efficacy but has received little attention from a physical-chemical perspective. Study was undertaken by laser diffraction, Raman spectroscopy, and small-angle X-ray scattering to resolve these physical processes. It was shown that migration of the pesticide into the adjuvant droplet occurred in all cases studied. The level of transfer was dependent upon adjuvant level, adjuvant solubility, and surfactant level. For suspension pesticides, dissolution of crystallites within the droplet occurred to a degree limited by solubility. The results directly demonstrate the transfer of the pesticide into the adjuvant carrier. This indicates that for emulsion-based pesticides, application to the target is likely as a homogeneously mixed droplet, whereas for suspension pesticides, solubility may limit transfer and dissolution, leading to heterogeneity in the applied particles.

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.

Effects of poly- and monodisperse surfactants on 14C-epoxiconazole diffusion in isolated cuticles of Prunus laurocerasus

BACKGROUND

Surfactants are known to enhance the foliar uptake of agrochemicals. It was the aim of this study to compare the enhancing effect of three polydisperse surfactants (Brij 30, Plurafac LF300 and Wettol LF700) and five monodisperse alcohol ethoxylates (C12 E3, C12 E4, C12 E5, C12 E6 and C12 E8) on (14)C-epoxiconazole diffusion in cuticles isolated from cherry laurel (Prunus laurocerasus L.).

RESULTS

Rate constants (k*) of (14) C-epoxiconazole diffusion were measured in the presence and in the absence of the surfactants. Polydisperse surfactants increased the rates of foliar penetration of (14) C-epoxiconazole by factors of between 8 and 16. With monodisperse surfactants, enhancing effects on cuticular penetration were 2-16-fold. Effects were highest with alcohol ethoxylates of intermediate size, whereas they were lower for the smaller, more lipophilic and the larger, more polar monomers. In addition, diffusion of four monodisperse alcohol ethoxylates (C12 E3, C12 E4, C12 E5 and C12 E6 ) across cuticles was measured. Rate constants of alcohol ethoxylates decreased with decreasing lipophility and increasing molecular weight.

CONCLUSION

The results indicate that enhancement of foliar penetration across cuticles by surfactants was most efficient when both (14)C-epoxiconazole and surfactants had similar mobilities in the transport-limiting barrier of the cuticles. This observation should be of interest in future strategies to optimise foliar uptake of agrochemicals using surfactants.

Factors influencing the association between active ingredient and adjuvant in the leaf deposit of adjuvant-containing suspoemulsion formulations

BACKGROUND

For an oil adjuvant to enhance uptake of a particulate active ingredient (AI), it is hypothesised that closer association between the two should result in higher uptake. Accordingly, factors important for the spray deposit size on grapevine leaves have been investigated for a series of model suspoemulsion formulations containing colloidal crystalline AI or fluorescent pigment particles and an emulsion of an oil adjuvant with different degrees of wetting and different spray volumes.

RESULTS

Low spray volumes (<100 L ha(-1)) produced small deposits with high particle-adjuvant association. Complementary uptake studies showed increased uptake with decreasing deposit size, in agreement with the above hypothesis. Higher spray volumes produced larger deposits that consisted of annuli formed by pinning of the contact line by particles. Low surfactant concentrations favoured particles in the annulus and adjuvant separated in the centre. Intermediate surfactant concentrations produced annuli containing both particles and adjuvant, while with high surfactant concentrations the deposits were large with few annuli.

CONCLUSIONS

Small deposits result in high AI-adjuvant association. With larger deposits, annulus structures allow for enhanced AI-adjuvant association (5-20 times greater). The formation of annuli appears to be important in enhancing the biodelivery of particulate AIs in adjuvant-containing suspoemulsion formulations at intermediate spray volumes.

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.

Effects of accelerators on mobility of 14C-2,4-dichlorophenoxy butyric acid in plant cuticles depends on type and concentration of accelerator

Effects of diethyl suberate (DESU), diethyl sebacate (DES), dibutyl suberate (DBSU), dibutyl sebacate (DBS), and tributyl phosphate (TBP) on diffusion of 14C-2,4-dichlorophenoxy butyric acid (2,4-DB) across cuticular membranes (CM) was studied. Astomatous CM were isolated enzymatically from Stephanotis floribunda Brongn. leaves, and diffusion was measured at 20 degrees C. The alkyl-substituted dicarboxylic acids constitute a homologous series with carbon numbers increasing from C12 to C18. Molecular weights increased only moderately from 230.0 (DESU) to 314.5 (DBS), while partition coefficients varied over orders of magnitude from 92 (DESU), to 1213 (DES), to 15,988 (DBSU), to 210,762 (DBS). All the above compounds turned out to be accelerators as they increased 2,4-DB mobility by up to 40-fold with accelerator concentrations in the CM ranging from only 9.2 to 105 g kg(-1). Efficacy (2,4-DB mobility in the presence/mobility in the absence of accelerators) increased with increasing concentrations of accelerators in CM or in reconstituted cuticular waxes. Plotting efficacy vs accelerator concentration in the CM resulted in straight lines, and their slopes increased in the order DBS (0.14), DBSU (0.31), DES (0.51), and DESU (0.85). Hence, DESU was the most powerful accelerator in this series as it increased 2,4-DB mobility in the CM about 6 times more than DBSU. Waxes constitute the major barrier in plant cuticles, and plots of efficacy vs accelerator concentration in Stephanotis wax were also linear, but compared to CM slopes were steeper by factors of 3.20 (DBS), 2.97 (DBSU), 2.70 (DES), and 1.62 (DESU). TBP was similarly effective as DESU, but plots of efficacy vs concentration were not linear, and curves approached a plateau at 60-80 g kg(-1). These data are discussed with regard to suitability of these accelerators for formulating systemic pesticides.

Contrasting physiological responses of dwarf sea-lavender and marguerite to simulated sea aerosol deposition

Plants of two wild native species from littoral areas, marguerite [Argyranthemum coronopifolium (Willd.) C.J. Humphries] and dwarf sea-lavender [Limonium pectinatum (Aiton) O. Kuntze], grown in an unheated plastic greenhouse, were sprayed 2 to 3 min per day over a 7-d period with different aqueous solutions containing (i) an anionic surfactant (S1); (ii) a solution simulating the composition of sea aerosol (S2); (iii) a solution simulating sea aerosol with anionic surfactant (S3), and (iv) deionized water alone (control). The plant resistance to sea aerosol and the ability to recover from treatments were studied. By the end of the spraying period, marguerite showed a significant reduction in growth compared with control. However, most of the growth parameters were significantly unaffected in dwarf sea-lavender when plants were treated with sea aerosol containing surfactant. Measurements of water relations variables in marguerite showed a slight decrease in leaf turgor potential after spraying with sea aerosol containing surfactant. The surfactant enhanced the foliar absorption of salt in marguerite plants, inducing reductions in leaf stomatal conductance and causing such damage in the photosynthetic apparatus that the level of net photosynthesis decreased and had not recovered by the end of the experiment. The treatments had no effect on leaf stomatal conductance and photosynthesis rate in dwarf sea-lavender plants. The response of the species studied to sea aerosol was related to the degree of salinity tolerance. Although both species are wild native plants from littoral areas, marguerite is not salt tolerant and was the most sensitive to the sea aerosol treatments, while dwarf sea-lavender, a halophyte species, was more efficient at decreasing the toxic salt content of the tissues as its growth and ornamental characteristics were not affected.

A mechanistic analysis of penetration of glyphosate salts across astomatous cuticular membranes

Penetration of glyphosate salts across isolated poplar (Populus canescens (Aiton) Sm) cuticular membranes (CM) was studied using Na+, K+, NH4+, trimethylsulfonium+ (TMS) and isopropylamine+ (IPA) as cations. After droplet drying, humidity over the salt residues on the outer surfaces of the CM was kept constant, and cuticular penetration was monitored by sampling the receiver solution facing the inner surfaces of the CM. Glyphosate salts disappeared exponentially with time from the surfaces of the CM. This first-order process could be quantitatively described using rate constants (k) or half-times (time for 50% penetration; t1/2). Humidity strongly affected the velocity of penetration, as k increased by factors of 5.3 (K-glyphosate), 6.9 (TMS-glyphosate), 7.1 (NH4-glyphosate), 8.5 (Na-glyphosate) and 10.5 (IPA-glyphosate) when humidity was increased from 70 to 100%. Depending on the type of cation and humidity, t1/2 varied between 4 and 70h, but the humidity effect was statistically significant only at 100% humidity, when half-times were highest with IPA-glyphosate and lowest with TMS-glyphosate. Glyphosate acid penetration was measured only at 90% humidity and found to be extremely slow (t1/2 = 866 h). Adding 0.2 g litre-1 of a wetter (alkylpolyglucoside) to the donor increased IPA-glyphosate rate constants by about four times, but increasing concentration produced no further increase in k. When donors contained 0.2 g litre-1 wetter, further additions of 4 g litre-1 Ethomeen T25 did not change rate constants measured with IPA-glyphosate at 90% humidity, while Genapol C-100 and diethyl suberate increased k by only 35%. Concentration of IPA-glyphosate (1, 2 and 4 g litre-1) did not influence k at 90% humidity, and pH of donor solutions (4.0, 7.7, 9.5) had no effect on k of K-glyphosate at 90% humidity. Temperature (10 to 25 degrees C) had only a small influence on velocity of penetration of IPA-glyphosate and K-glyphosate, as energies of activation amounted to only 4.26 and 2.92 kJ mole-1, respectively. These results are interpreted as evidence for penetration of glyphosate salts in aqueous pores.

Evidence for surfactant solubilization of plant epicuticular wax

The solubilization of isolated, reconstituted tomato (Lycopersicon esculentum Mill.) fruit and broccoli (Brassica oleracaea var. botrytis L.) leaf epicuticular waxes (ECW) by nonionic octylphenoxypolyethoxy ethanol surfactant (Triton X-100) was demonstrated in a model system by TLC and fluorescence analysis using pyrene as a fluorescent probe. ECW was solubilized at or above the surfactant critical micelle concentration; solubilization increased with an increase in micelle concentration. As shown by the fluorescence quenching of pyrene, surfactant solubilization of the ECW increased rapidly for the first 12 h, then approached a plateau, increased linearly with an increase in temperature (22--32 degrees C), and decreased linearly with the log of the polyoxyethylene chain length (range 5--40 oxyethylenes). These data are discussed in relation to surfactant effects on phytotoxicity and performance of foliar spray application of agrochemicals.

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.