Surfactant Effects on Environmental Behavior of Pesticides - Reviews of Environmental Contamination and Toxicology - page 89

between a solubilizate and surfactant molecule is one of the most important factors
in determining its locus of solubilization in micelles together with the size and
shape of the solubilizate and micelles.

Pesticides

Basic investigation on solubilization of pesticides in micelles has been less often
conducted, probably because of its practical usage in formulation including various
components other than surfactant, lower water solubility, and its complex structure.
Solubilization parameters in micelles for the pesticides previously reported are
summarized in Table 3. Because DDT (1) is very hydrophobic, with its metabolites
persistent in soil, its solubilization by surfactants has been studied extensively. The
log K m values above cmc estimated by the solubility enhancement and surface ten-
sion measurement studies are larger than 5, similarly to PAHs. Some association of
(1) with surfactant monomers below cmc also enhanced its water solubility but with
much smaller log K m values (Kile and Chiou 1989; Chiou et al. 1991). In contrast,
less hydrophobic NAA (2) showed much smaller log K m values in nonionic
surfactants, and its interaction with micelles could be confirmed by a shift of UV
absorption to a longer wavelength (Heredia and Bukovac 1992).
The solubilization of organophosphorus pesticides was examined in relation to
their micellar-catalyzed hydrolysis and the micelle binding constant K s tends to
increase with the log P value of each pesticide. The usage of cosurfactant such as
n -pentanol enhanced the solubilization of chlorpyrifos (65) in micelles (Kaushik et al.
2005). Han et al. (2006) estimated that fenitrothion (5) is solubilized in HDTMA
Br micelles as its aryl moiety is located from the outer core to the Stern layer of the
micelles, with its methoxy groups residing near the head groups of the surfactant.
Similar orientation of organophosphorus pesticides has been estimated in lipid
bilayers of the several L - -phosphatidylcholines by fluorescence depolarization of

probe molecules (Katagi 1995). The MSR values of coumaphos (7), atrazine (13),
and trifluralin (15) in Triton X-100 micelles were found to be much smaller than
those of PAHs as a result of their larger molecular sizes with less hydrophobicity
(Mata-Sandoval et al. 2000). For hydrophobic pesticides, van der Waals force is
most likely to account for their interactions with the micellar core, and less hydro-
phobic ones such as organophosphorus pesticides are likely to be located near the
micellar head groups via dipole-dipole interactions and hydrogen bonding.
The more hydrophilic pesticides having amide, carbamate, and imide linkages
showed smaller K s values than (5), which were not so sensitive to the head group
of surfactant, and their solubilization in the Stern layer was most likely. The longer
the alkyl chain in the hydrophobic part of ionic surfactants having the same head
group, the larger K s value was obtained for carbofuran (9) (Arias et al. 2005) and
procymidone (10) (Villedieu et al. 1995). In the case of nonionic alcohol and alkyl-
phenoxy ethoxylates, more hydrophilic surfactants with a higher HLB value
solubilized well polar pesticides such as alachlor (53) (Xiarchos and Doulia 2006).
Sulfonylurea herbicides with a high water solubility are also partitioned to ODTMA

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