Development of a predictive uptake model to rationalise selection of polyoxyethylene surfactant adjuvants for foliage‐applied agrochemicals

Abstract

Composition‐concentration relationships between a series of C13/C14 polyoxyethylene primary alcohol (AE) surfactants and the foliar uptake enhancement of five model neutral organic compounds were examined in factorially designed experiments on wheat (Triticum aestivum L.) and field bean (Vicia faba L.) plants grown under controlled environment conditions. Model compounds were applied to leaves as c.0.2‐μl droplets of 0.5 g litre−1 solutions in aqueous acetone in the absence or presence of surfactants at 0.2, 1 and 5g litre−1. Uptake of the highly water‐soluble compound, methylglucose (log octanol‐water partition coefficient (P) = ‐ 3.0) was best enhanced by surfactants with high E (ethylene oxide) contents (AE15, AE20), whereas those of the lipophilic compounds, WL110547 (log P = 3.5) and permethrin (log P = 6.5), were increased more by surfactants of lower E contents, especially AE6. However, there was little difference between AE6, AE11, AE15 and AE20 in their ability to promote uptake of the two model compounds of intermediate polarity, phenylurea (log P = 0.8) and cyanazine (log P = 2.1). Absolute amounts of compound uptake were also influenced strongly by both surfactant concentration and plant species. Greatest amounts of uptake enhancement were often observed at high surfactant concentration (5 g litre−1) and on the waxy wheat leaves compared with the less waxy field bean leaves. The latter needed higher surfactant thresholds to produce significant improvements in uptake. Data from our experiments were used to construct a simple response surface model relating uptake enhancement to the E content of the surfactant added and to the physicochemical properties of the compound to be taken up. Qualitative predictions from this model might be useful in rationalising the design of agrochemical formulations.

Influence of two fatty amine surfactants on foliar absorption, translocation, and efficacy of 2,4-D triethanolamine salt

“…These authors demonstrated that reducing of the EO content of ethoxylated alkylamines to 5 or 10 mol reduced the surfactant-induced uptake of the polar compound glyphosate by field bean and wheat. In view of this result and the proposed model (Stock et al, 1993) it is clear that uptake experiments with more polar compounds (log KO, < -3) would elucidate whether changing the surfactant lipophilicity by partial substitution of EO by PO has the same effect as changing the EO content.…”

“…In our study, partial substitution of EO by PO does not seem relevant to uptake enhancement; thus, surfactant lipophilicity does not seem to influence the extent of uptake enhancement of 2,4-D salts. This outcome and the measured log KO, of -0.7 for 2,4-D TR indicate that 2,4-D salts fall into the "critical log KO, range" mentioned in the description of an uptake model (Stock et al, 1993). The critical log KO, range has been defined as the range in which surfactant lipophilicity (based on EO content) does not strongly influence the uptake enhancement of compounds; its upper and lower limits may vary according to plant species (Stock et al, 1993).…”

“…This outcome and the measured log KO, of -0.7 for 2,4-D TR indicate that 2,4-D salts fall into the "critical log KO, range" mentioned in the description of an uptake model (Stock et al, 1993). The critical log KO, range has been defined as the range in which surfactant lipophilicity (based on EO content) does not strongly influence the uptake enhancement of compounds; its upper and lower limits may vary according to plant species (Stock et al, 1993). The contribution of the nitrogen atom to the hydrophilicity of ethoxylated alkylamines need not weaken greatly the influence of EO content as indicated in a previous study (Gaskin and Holloway, 1992).…”

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

“…the partition coefficient. The fact that acceleration did not increase with increase in application of GP C-200 from 50 to 100 kg is most probably due to dilution of the solute in the surfactant, or in terms of eqn (9), the decrease in on doubling the C don amount of surfactant counteracted the surfactant e †ect on mobility (k*). We have shown before that, in the presence of surfactants which do not penetrate cuticles (such as "TweenÏ 80), rates of uptake decrease with increasing amounts of surfactant.…”

“…Equation (9) suggests that, at constant driving force, rates of penetration should be higher for lipophilic than for polar solutes. Rate constants of penetration in the absence of surfactants are not very di †er-ent with values of 2É2(^0É7) ] 10~6 s~1 for cyanazine and 1É5(^0É75) ] 10~6 s~1 for WL110547 nor are mobilities (k*) with values of 1É75 ] 10~6 s~1 (cyanazine) and 1É26 ] 10~6 s~1 (WL110547).…”