Surfactant Effects on Environmental Behavior of Pesticides - Reviews of Environmental Contamination and Toxicology - page 105

More detailed analysis on the hydrolysis mechanism of N -methyl- p -toluanilides in
HDTMA Br micelles has been conducted from the standpoint of Hammett correlation
(Broxton and Duddy 1979). The curve of the logk 2w versus s − plot indicates the
change of the reaction mechanism with substitution at the N -phenyl moiety, but the
constantly higher r value of 2.4 in the logk 2m versus s − plot indicates that the solvent-
assisted C-N bond breaking of the intermediate tetrahedral complex is a rate-
determining step in micelles. Similar analysis in HDTMA Br solution has shown that
alkaline hydrolysis of N -methyl- N -phenylacetamides obeys the B AC 2 mechanism
(Broxton and Duddy 1980), and bond forming to a tetrahedral intermediate becomes
a rate-determining step for N , N -diphenylbenzamides (Broxton et al. 1981). The ori-
entation effect in micellar hydrolysis was clearly demonstrated in alkaline hydrolysis
of pentyl N -(4-nitrophenyl)carbamate by using differently hydroxylated HDTMA Br
surfactants (Broxton and Chung 1986). The replacement of a methyl group by a
hydroxyethyl in HDTMA Br showed an insignificant difference in the alkaline
hydrolysis rate, but the introduction of OH at the 2-position of the hexadecyl moiety
increased the rate by a factor of 1.3-1.7. This change most likely indicates that the
carbonyl moiety of the carbamate is located in the Stern layer as the 2-OH oxygen of
the surfactant can easily attack. In the hydrolysis of Schiff bases, the entrapment of
the protonated bases by the polyethoxylate moiety of nonionic surfactant was consid-
ered to control its orientation in micelles (Mishra et al. 1992). Much less investigation
of the micellar effect on acid hydrolysis has been conducted. Mollett and O'Connor
(1976) investigated this for N -phenyl ureas, but neither nonionic nor ionic surfactants
significantly affected the hydrolysis rate.

Pesticides

The alkaline hydrolysis of pesticides has been examined mainly in the cationic
micelle system (Table 6). The k 2m values are generally less than the corresponding k 2w ,
indicating that the apparently enhanced micellar catalysis stems mainly from the
concentration effect. In the case of organophosphorus pesticides, the nucleophilic
attack of OH − at the central phosphorus via the S N 2 (P) mechanism is affected by the
solubilization site of the pesticide. The more hydrophobic parathion (3) is considered
to be favorably located toward the micellar core than its oxon (4), which resulted in
the apparent hydrolysis rate of (3) being less dependent on the concentration of
HDTMA Br than (4) (Purnanand and Danikhek 1985). The micelle-catalyzed hydrol-
ysis of (4) was inhibited by addition of inorganic salts in the order of NO 3 − , Br − , and
Cl − , which was consistent with the efficiency of counterions in competitive binding
with the micellar head groups. Balakrishnan et al. (2004, 2005) and Han et al. (2006)
investigated the hydrolysis of fenitrothion (5) in the HDTMA + micelles by using
various a -nucleophilic oximates. The accelerated hydrolysis was considered to
originate from both solubilization of (5) in the Stern layer and attraction of oximate
anions to the micelle surface. The hydrophobic oximate such as Ph(CH 3 )C NOH is

considered to be solubilized in micelles as the hydrophobic Ph(CH 3 )C moiety is