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 contain water depending on relative humidity and hygroscopic compounds present. Rates of uptake depend on solute mobility in and driving force across cuticles, which are proportional to the cuticle/formulation residue partition coefficient. Partition coefficients cuticle/glycerol (KCGly) and cuticle/poly(ethylene glycol) 400 (PEG400) (KCPEG) for seven organic compounds differing 6 orders of magnitude in octanol/water (KOW) or cuticle/water (KCW) partition coefficients have been measured. KCGly can be estimated from KCW and KOW values (r2 = 0.95). A plot of log KCGly versus log KCW had a slope of 0.69, indicating that glycerol is a better solvent for lipophilic solutes than water. In contrast, PEG400 (PEG) was a good solvent for polar and nonpolar solutes. All KCPEG values were below 1, differing <10-fold without correlation with lipophilicity. PEG400 sorbs water from air, and log K of the lipophilic compound, bifenox, increased linearly toward the value of log KCW with decreasing mass fraction of PEG400. Rates of penetration of bifenox differed drastically if KCPEG was modified by different humidities in the ambient air.