Surfactant Effects on Environmental Behavior of Pesticides - Reviews of Environmental Contamination and Toxicology - page 87

mostly utilized to investigate a solubilized locus (Grieser and Drummond 1988;
Ta¸cio˘lu 1996). Because a pesticide is generally a complex polar molecule, its
interactions with surfactant would be difficult to interpret. Therefore, the solubiliza-
tion of simple organic molecules are reviewed first to determine the effect of each
functional group or chemical structure on solubilization, and then pesticide exam-
ples are discussed.
Simple Organic Molecules
In relation to their bioavailability and soil-water partitioning, solubilization of
PAHs by surfactants has been extensively studied, for example, by using naphtha-
lene, phenanthrene, and pyrene. The apparent water solubility of PAH increases
with the concentration of nonionic surfactants with their micelle partition coeffi-
cients (log K m ) proportional to octanol-water partition coefficient (log P ) (Edwards
et al. 1991; Grimberg et al. 1995). MSR values ranging from 0.04 to 0.3 with log
K m of 4.5-6.5 in the presence of nonionic surfactants showed the high ability of
micelles to solubilize PAH molecules. Ionic surfactants also solubilize a variety of
hydrophobic organic chemicals including PAH with similar correlation between log
K m and log P (Cho et al. 2002; Valsaraj and Thibodeaux 1989). In the case of
cationic pyridinium surfactants, decreasing carbon number in the hydrophobic
chain reduced the log K m value (Paria and Yuet 2006). Because the slope of log K m
versus the log P plot is less than unity, the micellar phase solubilizing organic
chemicals is considered to be less hydrophobic than octanol. The observed correla-
tion with log P implies that the most probable site of solubilization for hydrophobic
chemicals is a micellar core, but it depends on the size and structure of solubilizate
and surfactant molecules. Guha et al. (1998) demonstrated through solubilization
of PAH mixtures by Triton X-100 micelles that a less hydrophobic PAH such as
naphthalene is located at the micellar core-Stern layer interface, whereas the more
hydrophobic phenanthrene is mainly in the micellar core. This difference in the
solubilization site as well as competitive solubilization of PAHs having similar
hydrophobicity was also confirmed by changes in 1 H-NMR chemical shifts of
surfactant hydrogens (Bernardez and Ghoshal 2004).
Multiregression analysis on solubilization of hexachlorobenzene in various
micelles has shown that both MSR and K m are dependent on hydrophobicity of sur-
factant (Jafvert et al. 1994). More carbons in the alkyl or aralkyl chain and fewer
ethoxy units resulted in larger MSR and K m values. Liu et al. (2000) also reported
the relationship of log K m with log P and, furthermore, hydrophobicity of a sur-
factant molecule and the total molecular surface area of a solubilizate were found
to be good descriptors for log K m . Simple hydrocarbons such as hexane and
cyclohexane tend to be solubilized in a micellar core. Their higher MSR values of
0.5-4.0 in alcohol ethoxylate micelles, which decrease with an increase of HLB
indicate that the volume of a micellar core controls the solubilization (Diallo et al.
1994). In contrast, simple aromatic hydrocarbons and chlorinated alkenes were
considered to be located in either the core or the Stern layer of micelle via interaction