RELATIVE SOLUBILITIES OF BIFENOX AND 1-NAPHTHYLACETIC ACID (NAA) IN PLANT CUTICLES AND IN SELECTED PURE OR AQUEOUS GLYCOL ADDITIVES

The driving force for foliar penetration is the product of the partiti on coefficient (K) between the cuticle and the formulation residue, an d the concentration of the active ingredient in the spray residue. Eth ylene glycols and polyethylene glycols (PEGs) are often contained in c ommercial formulations, because they are good solvents and not toxic. Since they are humectants, the water content of the formulation residu e varies with humidity of the air. At 25 degrees C and 65% relative hu midity PEG 400 contains about 50% water. The partition coefficients fo r the lipophilic herbicide, bifenox, increased with increasing water c ontent of the ethylene glycols and PEGs, such that log K was a linear function of the mass fraction of water or PEG. The K value was about 0 .5 for the system cuticle/pure PEG and 27 500 for the system cuticle/w ater. When PEG contained 50% water, K was only 110. Partition coeffici ents of bifenox between cuticle and 'Tween' 80 depended also on humidi ty, having values of 0.75 in cuticle/pure 'Tween' 80 and 29.1 when 'Tw een' 80 contained 50% water. With 1-naphthylacetic acid (NAA), which i s a weak acid, partition coefficients also depended on water content o f PEG 400, but a maximum curve was obtained. The maximum partition coe fficient (211) was observed with a mixture of 30% PEG 400 and 70% wate r, which corresponded to a humidity of 88%. We suggest that this is du e to an effect of PEG 400 on the ionisation of NAA. Our data demonstra te that partition coefficients are affected by humidity, because solub ilities of lipophilic compounds and weak acids in PEG/water mixtures v ary greatly with the water content. This in turn affects driving force s, mainly by the effect of humidity on partition coefficient. With non -ionisable lipophilic compounds, partition coefficients, driving force s and rates of foliar uptake therefore increase with increasing humidi ty. (C) 1998 SCI.