Surfactant Effects on Environmental Behavior of Pesticides - Reviews of Environmental Contamination and Toxicology - page 144

benomyl (61) in its soil application with Tween 20 for cotton may imply enhanced
root uptake (Rawlins and Booth 1968).
Application of SDBS with about a 10-fold amount of atrazine (13) in an outdoor
lysimeter study has shown that root uptake of 14 C and (13) in maize after the first
season increases by factors of 2 and 3, respectively (Scheunert and Korte 1985).
However, in field residue trials of (13), not only preemergence application of the
flowable and wettable powder formulations but also postemergence application of
(13) mixed with three different adjuvants gave insignificant differences in residues
of the metabolites in oats (Khan et al. 1981). Butachlor (74) applied to soil was
absorbed and translocated by rice plants, but its dissipation rate was independent of
formulation type (Kulshrestha 1987). These limited studies may show possible
enhancement or inhibition of the root uptake of chemicals including pesticides by
surfactant for some simple model systems, but under field conditions the effects are
likely to be diminished by various factors such as dilution, adsorption to soil, and
bacterial degradation of surfactant.
B FoliarUptake
Adjuvants in formulations, especially surfactants, are known to increase the foliar
penetration of pesticide (Bentson 1990; Bukovac et al. 2003). The surfactant having
a higher HLB value tends to increase hydration of the cuticle and as a result to
enhance the permeance of a hydrophilic chemical (log P < 3), whereas that having a
lower HLB is considered to cause the enhanced permeance of a hydrophobic chemi-
cal (log P > 3) via an increase in the fluidity of cuticular waxes (Baur 1998; Hess and
Foy 2000; Wang and Liu 2007). Penetration of pesticide and surfactant through the
wax region and the underlying cutin layer has been studied extensively by either
adsorption and desorption on a wax-coated disk (Schreiber and Schönherr 1993) or
mass transfer through an enzymatically isolated cuticular membrane in a diffusion
cell (Bauer and Schönherr 1992; Schönherr and Riederer 1989). It is considered
through diffusion experiments that the surfactant above cmc depresses the partition of
pesticide to a cuticle membrane by its solubilization to micelles and that the perme-
ance of the pesticide in cuticle increases when the surfactant is concomitantly
dissolved into the cuticle (Schönherr et al. 1991). The copermeation of pesticide and
surfactant molecules has been shown by Schreiber (1994) through the desorption of
pentachlorophenol from reconstituted barley waxes in the presence of dodecyl octae-
thoxylate surfactant. Burghardt et al. (1998, 2006) found that the extent of enhanced
diffusion of 15 monodisperse alcohol ethoxylates is proportional to their concentra-
tion in the isolated cuticle with a concomitant increase of pesticide diffusion. The
presence of surfactant was found to increase the free volume available for a pesticide
diffusion, showing the plasticizing effect of surfactant.
Stevens et al. (1988) examined the effect of nonylphenol octaethoxylate on the
foliar absorption and translocation of 14 C-glucose and 14 pesticides for four plant
species and shown that the log (% uptake) and log (% translocation) have a