PARTITION-COEFFICIENTS OF ACTIVE INGREDIENTS BETWEEN PLANT CUTICLE AND ADJUVANTS AS RELATED TO RATES OF FOLIAR UPTAKE

After spraying, water evaporates and foliar penetration proceeds from a formulation residue of active ingredients and adjuvants which may co ntain water depending on relative humidity and hygroscopic compounds p resent. Rates of uptake depend on solute mobility in and driving force across cuticles, which are proportional to the cuticle/formulation re sidue partition coefficient. Partition coefficients cuticle/glycerol ( K-CGly) and cuticle/poly(ethylene glycol) 400 (PEG400) (K-CPEG) for se ven organic compounds differing 6 orders of magnitude in octanol/water (K-OW) or cuticle/water (K-CW) partition coefficients have been measu red. K-CGly can be estimated from K-CW and K-OW values (r(2) = 0.95). A plot of log K-CGly versus log K-CW had a slope of 0.69, indicating t hat glycerol is a better solvent for lipophilic solutes than water. In contrast, PEG400 (PEG) was a good solvent for polar and nonpolar solu tes. All K-CPEG values were below 1, differing <10-fold without correl ation with lipophilicity PEG400 sorbs water from air, and log K of the lipophilic compound, bifenox, increased linearly toward the value of log K-CW with decreasing mass fraction of PEG400. Rates of penetration of bifenox differed drastically if K-CPEG was modified by different h umidities in the ambient air.