Polydisperse ethoxylated fatty alcohol surfactants as accelerators of cuticular penetration. 2: Separation of effects on driving force and mobility and reversibility of surfactant action

Abstract

Polydisperse ethoxylated fatty alcohol (EFA) surfactants can improve the performance of crop protection agents. At the cuticular level they act as accelerators of penetration by increasing the mobility of active ingredients in the cuticle, the barrier properties of which are mainly caused by cuticular waxes. Polydisperse Genapol C-050 (GP C-050, average formula C12.5E5.8) was also found to increase mobility in wax-extracted polymer matrix membranes (MX) of bitter orange and pear, indicating that sorption of surfactants increased segmental mobility of polymethylene chains in cutin and wax. Sorption into MX of the active fraction of GP C-050 from 5g litre À1 micellar solutions was in equilibrium in less than 1 h after establishing contact. This is almost 100-fold faster than with cuticular membranes (CM). Temperature dependence of solute mobilities in CM was studied in order to measure activation energies (E D ) of diffusion in the presence and absence of aqueous surfactant solutions. Monodisperse fatty alcohol ethoxylates C8E3, C8E4 and C12E6, and (non-surface-active) tributylphosphate decreased E D of the model compounds WL 110547 and bifenox in Citrus, Pyrus and Stephanotis CM by more than 100 kJ mol À1. This corresponds to 50 to 275-fold increases of mobilities at 15°C. Our data suggest that the decrease in activation energies with the concomitant accelerating effect on mobility contributes considerably to the effects of so-called activator surfactants. High temperature and accelerators act similarly on barrier properties of CM. It is shown that effects of both monodisperse and polydisperse EFA surfactants on solute mobility are reversible and that radiolabelled C12E8 penetrated pear CM rapidly. However, rates of penetration were lowered by excess amounts of WL 110547 and especially phenylurea.Partition coef®cients of seven organic solutes between Capsicum fruit cuticles and GP C-050 were very low and, with the exception of methylglucose, smaller than 1. They decreased with lipophilicity and differed about 100-fold. Especially for the lipophilic compounds they were orders of magnitude lower than octanol/water or cuticle/water partition coef®cients, which indicates the limited usefulness of these values for an understanding of penetration of active ingredients from formulation residues.

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

“…No superior spreading probably took place in the presence of the high already PA-MPEG concentration, which is also a wetting agent on its own [ 23 , 27 ]. The lack of any significant influence on PA-MPEG efficacy by strong penetration enhancer, Genapol C 050, was at first surprising [ 41 , 42 ]. However, in the presence of 7.5% PA-MPEG in the spray, and assuming a cuticle/water partition coefficient of Genapol C 050 close to 1, the explanation of these results was that Genapol C 050 probably did not simply enter through leaf cuticles [ 43 , 44 ].…”

Agricultural adjuvants may impair leaf transpiration and photosynthetic activity

“…Diffusion data of cuticles reflect the data of reconstituted waxes, and the waxy coating is subsequently seen as the rate limiting factor of diffusion (Kirsch et al 1997). The strong influence of the cuticular waxes on the mobility shows no dependence on the wax amount or thickness, but to the species-specific variation of wax compositions (Baur et al 1999). The chemical composition largely determines the architecture of the waxes, which itself establishes the pathway for diffusing solutes.…”

“…POEA acts as a penetration improving agent, as well as rapeseed methyl ester (RME). Both are employed in agrochemical formulations as in-can and tank mix adjuvants, also due to their capability to act as a solvents for lipophilic active ingredients (Baur et al 1999, Brausch and Smith, 2007, Charlemagne, 1999. Penetration accelerating adjuvants are known to alter the permeability of the cuticles, particularly of the cuticular waxes (Schreiber, 2006) and may thereby influence the barrier properties of the cuticle.…”

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

“…Therefore, adjuvants are used to optimize the physicochemical characteristics of the spray liquid to improve the biological performance of the AI. This can be achieved through spray liquid modifying properties − like pH adjustment, foaming activity, retention, wetting and spreading behavior, or humectancy. − Penetration aids impact the bioavailability of the AI in the spray deposit , or modify the cuticular transport properties. ,− In consideration of the various processes during spray application, it is evident that adjuvants can either impact individual or several steps of the application procedure. Thus, one adjuvant can have multifunctional properties, which make the elucidation of the mode of action of adjuvants very complex.…”