Surfactant Effects on Environmental Behavior of Pesticides - Reviews of Environmental Contamination and Toxicology - page 77

similar to that of ethanol. The fraction of bound ions is known to be 0.6 - 0.9 per
ionic head group, and the remaining counterions make a diffuse electrical double
layer by their thermal motions known as a Gouy-Chapman layer. Polar substrates
such as pesticides are then considered to be solubilized in the neighborhood of the
Stern layer. In the case of nonionic surfactants, the role of an ionic head group is
replaced by the polyethoxy moiety, whose structure may resemble that of a crown
ether (Mishra et al. 1992). Raman spectroscopy of octylphenoxy ethoxylates has
shown that the helix/coil conformation is predominant for the polyethoxy chain in
the presence of excess water via hydration of two water molecules per ether linkage
(Bartlett and Cooney 1986). Additionally, the mixing of surfactant with oil such as
hexane and long-chain alcohol (cosurfactant) in water gives a spherical microemul-
sion whose radius is much larger than that of a micelle (Paul and Moulik 2001).
Various spectroscopic investigations have shown that the interior of a microemul-
sion is more hydrophilic than micelles and is considered to produce a different
reaction environment from micelles (Almgren et al. 1980; Gregoritch and Thomas
1980; Mackay 1981).
B BiologicalEffects
Surfactants, especially nonionic ones, are widely utilized to increase the efficacy of
pesticides by modifying spray profiles, the penetration through cuticle structures of
weeds, crops, and insects to target sites, and translocation, but unfavorable phyto-
toxicity to crops is also caused by usage depending on surfactant structure and
concentration (McWhorter 1985). Some nonionic surfactants are known to inhibit
plant growth by affecting root elongation and disrupting the normal membrane per-
meability and photosynthetic activity. By means of bioassays on germination and
growth either in soil and hydroponic media for 17 higher agricultural plants,
Günther and Pestemer (1992) reported that most plants are more sensitive to linear
alkylbenzene sulfonate (LAS) and 4-nonylphenol, metabolites of nonionic
surfactants, than distearyldimethylammonium chloride. Phytotoxicity tests using
14 different cell suspension cultures have shown that tolerance to 4-nonylphenol is
correlated with the formation of unextractable residues via metabolic degradation
(Bokern and Harms 1997). Either the root uptake of 4-nonylphenol to shoots or the
extent of its metabolic degradation was dependent on the plant species (Bokern
et al. 1998), indicating that metabolism is one of the important factors to control
the phytotoxicity of 4-nonylphenol.
Because surfactants in pesticide formulation as well as their degradation
products are considered to move finally to water, their biological effects should
be examined not only for crops but also for aquatic species. Generally, cationic
surfactants are more toxic to aquatic species including fish, the water flea, and
algae than anionic surfactants (Ying 2006). Acute EC 50 (50% effective concentra-
tion) or LC 50 (50% lethal concentration) and chronic NOEC (no-observed-effect
concentration) values for cationic surfactants range from 0.1 to 10 ppm, whereas