Environmental Engineering Reference
In-Depth Information
that in region 1 the relative concentrations of a solubilizate and X such as OH
−
increase in the Stern layer, but the further increase of a surfactant concentration
causes competition of available sites in the Stern layer between X and inert counte-
rion N. However, if the concentration of a counterion is kept constant, the lack of
further competition between reactive and inert ions results in the continuous
increase of the apparent hydrolysis rate (Fig. 7, dotted line). A similar effect of sur-
factant on hydrolysis in micelles has been reported for oil-in-water microemulsions
(Mackay and Hermansky 1981; Mirgorodskaya and Kudryavtseva 2002; Varughese
and Broge 1991), and significant change of hydrolysis rate was reported at the
phase transition (Hao 2000).
It is known that the PPIE model fails to describe some bimolecular reactions
such as nucleophilic aromatic substitution and amide hydrolysis (Broxton 1981;
Broxton and Sango 1983). When the reaction center of a solubilizate is located
more in the outer side of the Stern layer, reaction with a reactive ion across the
interfacial boundary proceeds more readily. Deviation from the PPIE model has
been reported for acidic hydrolysis of acetal and aldoxime esters where the appar-
ent rate constants increase in proportion to the acid concentration, which shows the
involvement of the reaction with H
+
across the interfacial boundary (Bunton et al.
1979; Gonsalves et al. 1985). A similar deviation in HDTMA Br micelles was also
found for alkaline hydrolysis of DDT (1) at high alkaline concentrations (Nome
et al. 1982; Stadler et al. 1984).
B HydrolysisofPesticides
Simple Organic Chemicals
The structure of the pesticide molecule is so complex in the clear understanding of
the effect of micelles on hydrolysis that micellar-catalyzed hydrolysis of simple
organic chemicals is discussed first. Kinetic parameters for typical chemicals are
summarized in Table
5. The hydrolysis of an organophosphorus ester in micelles has
been reviewed extensively by Fendler and Fendler (1975). Its alkaline hydrolysis is
affected only by cationic micelles, with the typical profiles in Fig. 7 being observed.
Even for the hydrophilic mononitrophenyl esters, the micelle partition constant to
HDTMA Br is large (10
4
-10
5
), and enhanced hydrolysis was found to originate from
reduction of the activation energy (Bunton et al. 1968). At the higher pH, the apparent
micellar catalysis is more evident for alkaline hydrolysis of
O
-
p
-nitrophenyl
O
,
O
-
diphenyl phosphate in HDTMA Br micelles (Bunton et al. 1969). Either anionic SDS
or nonionic Igepal micelles were found to markedly inhibit this alkaline hydrolysis,
possibly because of the stronger association with micelles and electrostatic repulsion
of OH
−
from the micellar surface (Bunton and Robinson 1969).
Carboxylic esters are another important chemical class in considering the micel-
lar catalysis of pesticide. Beme et al. (1965) reported the fivefold micellar catalysis
in alkaline hydrolysis of phenyl hexanoate in HDTMA Br micelles. Similar to
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