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

Polydisperse ethoxylated fatty alcohol (EFA) surfactants can improve the pe rformance of crop protection agents. At the cuticular level they act as acc elerators of penetration by increasing the mobility of active ingredients i n the cuticle, the barrier properties of which are mainly caused by cuticul ar waxes. Polydisperse Genapol C-050 (GP C-050, average formula C12.5E5.8) was also found to increase mobility in wax-extracted polymer matrix membran es (MX) of bitter orange and pear, indicating that sorption of surfactants increased segmental mobility of polymethylene chains in cutin and wax. Sorp tion into MX of the active fraction of GP C-050 from 5g litre(-1) micellar solutions was in equilibrium in less than Ih after establishing contact. Th is is almost 100-fold faster than with cuticular membranes (CM). Temperatur e dependence of solute mobilities in CM was studied in order to measure act ivation 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 mod el compounds WL 110547 and bifenox in Citrus, Pymcs and Stephanotis CM by m ore than 100 kJ mol(-1), This corresponds to 50 to 275-fold increases of mo bilities at 15 degrees C. Our data suggest that the decrease in activation energies with the concomitant accelerating effect on mobility contributes c onsiderably to the effects of so-called activator surfactants. High tempera ture and accelerators act similarly on barrier properties of CIM. It is sho wn that effects of both monodisperse and polydisperse EFA surfactants on so lute mobility are reversible and that radiolabelled C12E8 penetrated pear C M rapidly. However, rates of penetration were lowered by excess amounts of WL 110547 and especially phenylurea. Partition coefficients of seven organic solutes between Capsicum fruit cuti cles and GP C-050 were very low and, with the exception of methylglucose, s maller than I. They decreased with lipophilicity and differed about 100-fol d. Especially for the lipophilic compounds they were orders of magnitude lo wer than octanol/water or cuticle/water partition coefficients, which indic ates the limited usefulness of these values for an understanding of penetra tion of active ingredients from formulation residues. (C) 1999 Society of C hemical Industry.