Influence of micelles on the basic degradation of carbofuran

Article abstract of DOI:10.1021/jf0505574

The effect of micellar aggregates upon the stability of carbofuran in basic media has been studied. The effect of the presence of micelles upon the basic hydrolysis of carbofuran is a function of the nature of the surfactant monomer. Important catalysis of basic hydrolysis of carbofuran in the presence of colloid aggregates with positive surface charge has been reported. On the other hand, the presence of anionic and nonionic surfactants implies a large inhibition of the basic hydrolysis of carbofuran. Both catalysis and inhibition are due to the association of carbofuran with the micellar core. The kinetic constants for the basic hydrolysis of carbofuran in these microheterogeneous media have been obtained on the basis of a micellar pseudophase model. No significant changes in the intrinsic reactivity of HO^- against carbofuran have been observed.

Full text of DOI:10.1021/jf0505574

7172 J. Agric. Food Chem. 2005, 53, 7172−7178
Influence of Micelles on the Basic Degradation of Carbofuran
†
‡
,§
MANUEL ARIAS, LU ´ı S GARC ´ı A-R ´ı O, JUAN C. MEJUTO,*
PEDRO RODR ´ı GUEZ-DAFONTE,§ AND JESUS SIMAL-G AÄ NDARA#
AÄ rea de Edafolog ´ı a y Qu ´ı mica Agr ´ı cola, Facultad de Ciencias de Ourense, Universidad de Vigo,
Ourense, Spain; Departamento de Qu ´ı mica f ´ı sica, Facultad de Qu ´ı mica, Universidad de Santiago de
Compostela, Santiago de Compostela, Spain; Departamento de Qu ´ı mica f ´ı sica, Facultad de Ciencias de
Ourense, Universidad de Vigo, Ourense, Spain; and AÄ rea de Nutrici o´ n y Bromatolog ´ı a, Facultad de
Ciencias de Ourense, Universidad de Vigo, Ourense, Spain
The effect of micellar aggregates upon the stability of carbofuran in basic media has been studied.
The effect of the presence of micelles upon the basic hydrolysis of carbofuran is a function of the
nature of the surfactant monomer. Important catalysis of basic hydrolysis of carbofuran in the presence
of colloid aggregates with positive surface charge has been reported. On the other hand, the presence
of anionic and nonionic surfactants implies a large inhibition of the basic hydrolysis of carbofuran.
Both catalysis and inhibition are due to the association of carbofuran with the micellar core. The
kinetic constants for the basic hydrolysis of carbofuran in these microheterogeneous media have
been obtained on the basis of a micellar pseudophase model. No significant changes in the intrinsic
reactivity of HO- against carbofuran have been observed.
KEYWORDS: Carbofuran; cationic micelles; anionic micelles; nonionic micelles; basic hydrolysis
INTRODUCTION
Scheme 1
In the environment, pesticides degrade into a variety of other
substances as a result of interactions with soil, water, sunlight,
and oxygen (1). The presence and potential environmental
effects of pesticide breakdown products are only now being
included in environmental fate studies by the agencies respon-
sible for monitoring concentrations of these compounds in the
environment. When these breakdown products are monitored,
they are commonly found in measurable quantities in water,
sediments, and plant and animal tissues (2, 3). Living organisms
also degrade pesticides. Soil microbes are responsible for
catalyzing the breakdown of many pesticides, and in mammals,
most pesticides are broken down in the liver to more soluble
compounds, which are then excreted. Generally, pesticide
breakdown products are less toxic than the pesticide itself. The
rate of breakdown of a particular pesticide in the environment
is a function of the pesticide, its location (whether it is adsorbed
to soil or sediment particles, on an exposed leaf surface, in water,
etc.), temperature, soil or water pH, and the moisture content
of the surrounding medium.
to control a broad spectrum of insects on corn, rice, alfalfa,
grapes, and other foodstuffs. It is sprayed directly onto soil and
plants just after emergence to control beetles, nematodes, and
rootworm. The mechanism of action for carbofuran activity
against insects is its rapid but reversible inhibition of acetyl-
cholinesterase.
All carbamates and related products enter the ecosystem with
potential adverse effectsm and recently their implications as
potential endocrine disrupters have been demonstrated (4).
Carbofuran is moderately persistent in soils with a half-life from
30 to 120 days depending on environmental conditions (such
as the organic and moisture contents and pH of the soil);
degradation occurs by exposure to sunlight to yield 2-hydroxy-
furadan and furadan phenol, by chemical hydrolysis and
bioactivity. Carbofuran is highly mobile in soils and appears in
the waters because of its high solubility (5, 6); it has a high
potential for groundwater contamination of aquifers by leaching
of treated fields, and it can also contaminate surface waters as
a result of runoff from treated fields. Direct photolysis and
photooxidation (via hydroxyl radical) is thought to be the major
pathway to degradation of carbofuran in water. When compared
Carbofuran, 2,3-dihydro-2,2-dimethylbenzofuran-7-yl meth-
ylcarbamate, is one of the carbamate pesticides (see Scheme
1
), a large family of pesticides derived from carbamic acid.
Carbofuran is used extensively in agriculture as an insecticide
*
Author to whom correspondence should be addressed (fax 34-988-
3
87031; e-mail xmejuto@uvigo.es).
†
AÄ rea de Edafolog ´ı a y Qu ´ı mica Agr ´ı cola, Universidad de Vigo.
‡
Universidad de Santiago de Compostela.
§
Departamento de Qu ´ı mica fisica, Universidad de Vigo.
#
AÄ rea de Nutrici o´ n y Bromatolog ´ı a, Universidad de Vigo.
1
0.1021/jf0505574 CCC: $30.25
© 2005 American Chemical Society
Published on Web 08/12/2005

Degradation of Carbofuran
J. Agric. Food Chem., Vol. 53, No. 18, 2005 7173
with other insecticides, carbofuran is less persistent than
organochlorine, but it is more persistent than organophosphorus
pesticides, the degradation of which has been studied in natural
waters.
Surfactants, or amphiphiles, are surface-active nonionic
molecules or organic salts that, either alone or in combination
with a wide variety of other ionic and nonionic solutes, aggregate
spontaneously and with a high degree of cooperativity in solution
to form a variety of assemblies (or association colloids) whose
structures depend both on solution composition and on the
structures of the components, primarily the surfactant (7-9).
Micelles are highly dynamic, often polydisperse aggregates
formed from single-chain surfactants (10) beyond the critical
micelle concentration (cmc). Micellization is primarily driven
by bulk hydrophobic interactions between the alkyl chains of
the surfactant monomers and usually results from a favorable
entropy change (11). The overall Gibbs energy of the aggregate
for ionic surfactants (12) is a compromise of a complex set of
interactions, with major contributions from headgroup repulsions
and counterion binding. It has long been known that aqueous
micelles can influence chemical reaction rates and equilibrium.
Early studies of micellar effects on reaction rates and equilibrium
are described in extensive monographs (7-9, 13-16). Much
of the impetus for the study of reactions in micelles is that they
model, to a limited extent, reactions in biological assemblies,
and the term “biomimetic chemistry” has been coined to describe
this general area of study.
Figure 1. Influence of NaOH concentration upon pseudo-first-order rate
-
5
constant of basic hydrolysis of carbofuran. [carbofuran]
M. T 25 C.
) 1. 94 × 10
)
°
The kinetic data always fitted the first-order integrated rate equation
satisfactorily (r > 0.999; eq 1); in what follows, k denotes the pseudo-
0
first-order constant:
ln(A - A ) ) ln(A - A ) - k t
(1)
t
∞
0
∞
0
where A , A , and A are the absorbance at t ) 0, at a given time, and
0 t ∞
at t ) ∞, respectively.
In the case that the half-time life of the reaction was <5 min, kinetics
were recorded at Applied Photophysics DX17 MV sequential stopped-
flow spectrophotometer thermostated at 25.0 ( 0.1 °C.
Critical micellar concentration values of nonionic surfactants have
been obtained using surface tension measurements. The surface tension,
Micellar catalysis of organic reactions has been extensively
studied (17-23). This type of catalysis is critically determined
by the ability of micelles to take up all kinds of molecules. The
binding is generally driven by hydrophobic and electrostatic
interactions. The uptake of solutes from the aqueous medium
into the micelle is close to diffusion controlled, whereas the
residence time depends on the structure of the surfactant
-
1
σ/m Nm , was measured using a Kruss tensiometer (model K9) using
the procedure of the Wilhelmy plate (35). Data were the averages of
1
0 measurements and exhibited deviations of <0.4% in all instances.
All of the measurements of surface tension had been carried out at
2
5.0 ( 0.1 °C.
-
4
molecule and the solubilizate and is often of the order of 10
-
Nonlinear regressions were carried out using Grafit 3.0 supplied by
-
6
1
0
s (24). Solubilization is usually treated in terms of a
Erithacus Software Ltd.
pseudophase model in which the bulk aqueous phase is regarded
as one phase and the micellar pseudophase as another. The time-
averaged location of different solubilizates in or at a micelle
has been a topic of contention (25). Apart from saturated
hydrocarbons, there is usually a preference for binding in the
interfacial region, that is, at the surface of the micelle (26, 27).
Such binding locations offer possibilities for hydrophilic and
hydrophobic interactions and avoid unfavorable disturbances of
the interactions between the alkyl groups of the surfactant
molecules in the core of the aggregate. The preferential binding
of aromatic molecules at the micellar surface has been explained
at least in part by the ability of the π-system of the molecule to
form a weak hydrogen bond with water (28).
RESULTS AND DISCUSSION
Basic Hydrolysis of Carbofuran in Water. Although the
hydrolysis of aryl esters in aqueous media has been thoroughly
studied, we examined the alkaline hydrolysis of carbofuran (36)
to ensure good consistency in the evaluations of the experimental
results. The mechanism of the cleavage reaction is shown in
Scheme 1.
To determine the rate constant of basic hydrolysis of
carbofuran in water the concentration of NaOH was varied
-2
between 3.8 × 10 and 0.3 M. The concentration of carbofuran
-
5
was always 1.94 × 10 M in order to keep pseudo-first-order
conditions as we describe under Materials and Methods.
Experimental results are shown in Figure 1. The kinetic
equation for this reaction is V ) kw[NaOH] [carbofuran]. In the
pseudo-first-order conditions k0 ) kw[NaOH]. The value of kw
has been obtained from the slope of the liner regression of k0
versus [NaOH] (see Figure 1), and it was estimated in kw )
Our research group has carried out extensive studies about
the influence on reactivity of micellar media (29, 30) and other
colloidal aggregates (31-34). This paper presents a comparative
study of carbofuran cleavage in the presence of anionic, cationic,
and nonionic micellar aggregates.
-
1
-1
1
.16 ( 0.01 M s .
MATERIALS AND METHODS
Effects of Anionic Micelles. The effect of sodium decyl
All reagents were Merck or Sigma-Aldrich products of the maximum
commercially available purity, and none required further purification.
All aqueous solutions were prepared by weight using double-distilled
water.
sulfate (SdS), sodium undecyl sulfate (SUS), and sodium
dodecyl sulfate (SDS) on the basic hydrolysis of carbofuran
-
-
-3
was studied at [OH ] constant ([HO ]total ) 5.90 × 10 M,
-
-3
-
-3
[
HO ]total ) 6.40 × 10 M, and [HO ]total ) 6.70 × 10
M
Reaction kinetics was recorded at 25.0 ( 0.1 °C in a Kontron Uvicon
23 spectrophotometer equipped with thermostated cell carriers. The
for SdS, SUS, and SDS, respectively), and the concentrations
of the different surfactants were varied typically between 0 and
0.12 M. Figure 2 shows the pseudo-first-order rate constant,
k0, to be markedly decreased by all of the anionic surfactants.
9
disappearance of the absorbance at 280 nm due to carbofuran
consumption was followed. The initial concentration of carbofuran (1.94
-
5
×
10 M) was always much lower than that of the other reagents.

7174 J. Agric. Food Chem., Vol. 53, No. 18, 2005
Arias et al.
Figure 2. Influence of surfactant concentration upon pseudo-first-order
rate constant of basic hydrolysis of carbofuran:( ) SdS micelles; ( ) SUS
micelles; ( ) SDS micelles. Solid lines represent fit of eq 2 to the
exptl, and calculated rate constant,
0
Figure 3. Experimental rate constant, k
b
O
calcd
k
0
(obtained from eq 2), for the basic hydrolysis of carbofuran in the
) SdS micelles, ( ) SUS micelles and
4
presence of anionic micelles: (
b
O
-
5
experimental data. [carbofuran]
) 1.94 × 10 M. T ) 25 °C.
(
4
) SDS micelles. T 25 C.
)
°
Scheme 2
increasing the chain length of the surfactant. A higher value of
Ks (63, 87, and 100) implies a larger value of inhibition. In
fact, the ratio between inhibition and Ks value is almost constant,
9
.7, 11.2, and 10.5 for SdS, SUS, and SDS, respectively.
Effects of Cationic Micelles. The effect of hexadecyl
trimethylammonium bromide (CTABr), tetradecyl trimethylam-
monium bromide (TTABr), and dodecyl trimethylammonium
bromide (LTABr) on the basic hydrolysis of carbofuran was
This behavior can be rationalized in terms of the two-
pseudophase model (37-39) in which the reaction is assumed
to occur both in micellar pseudophase and in water pseudophase,
with an equilibrium distribution of carbofuran between the two
pseudophases. On the basis of electrostatic considerations (40),
the hydroxyl ions concentration at the micellar pseudophase
must be very low owing to the anionic character of the micellar
-
-3
studied at [OH ]total ) 1.0 × 10 M, and the concentrations
of the different surfactants were varied typically between 0 and
0
.15 M. The results (Figure 4) follow the typical biphasic
pattern. The reaction rates pass through a maximum as the
surfactant concentration increases. The existence of a maximum
can be explained in terms of two competing effects in the ion-
exchange model. Added surfactant increases the relative con-
-
headgroup. This virtually complete exclusion of OH means
that the reaction is taking place only in the aqueous pseudophase
-
centration of carbofuran and HO in the Stern layer, which
(see Scheme 2).
increases the reaction rate, and the ascending branch of the curve
is observed. As the concentration of surfactant increases, the
concentration of the reagents in the micellar pseudophase
decreases, and furthermore the excess of unreactive counterions,
This model yields the following equation:
-
k [OH ]
w
w
k0 )
(2)
-
-
1
+ K [D ]
X , competes with OH for available sites in the Stern layer,
so that the reaction rate decreases. The relative contribution of
these competing factors results in the experimental maximum.
This experimental behavior can be explained quantitatively again
on the basis of the pseudophase model. The overall reaction
rate was, therefore, equal to the sum of the rates at the micellar
and aqueous pseudophases (see Scheme 3).
s
n
-
In eq 2, [OH ]w is the NaOH concentration in the bulk water
that is approximately equal to the total NaOH concentration,
kw is the bimolecular rate constant in bulk water, Ks is the
binding constant of the substrate to the micellar pseudophase
{
Ks ) [carbofuran]m/([carbofuran]w[Dn])}, and [Dn] is the
concentration of micellized surfactant ([Dn] ) [surfactant]t -
cmc). The cmc values were obtained from the kinetic data and
calculated as the minimum concentration of surfactant necessary
to produce a change in the reaction rate. These values in the
Scheme 3 allows us to obtain the equation
-
-
k [OH ] + k K [OH ]
m
w
w
m
s
k0 )
(1)
-
1 + K [D ]
s n
presence of OH are satisfactorily correlated with the expected
behavior of cmc values when the length of the surfactant chain
increases, which is a linear decrease in the logarithm of cmc
with an increase in the number of carbons in the chain (41).
The quality of the fit of the experimental data to eq 2 was
satisfactory. In all cases the deviation was lower than 10% (see
Figure 3). The kinetic coefficients for these systems are shown
in Table 1.
from which we can obtain the equation
-
k [OH ] + (k K - k )mOH[Dn]
w
total
m
s
w
k0 )
(3)
1
+ K [D ]
s n
The inhibitory efficiency of the different micelles increases
with increasing chain length (6.5, 7.7, and 9.5 times-fold for
SdSS, SUSS, and SDS, respectively). This inhibitory efficiency
has been estimated as the ratio between the rate constant in
water and the limit value of the kinetic constant in the presence
of surfactant obtained using the pseudophase model equation.
The observed trend is due to the increase of the Ks value on
where kw is the bimolecular rate constant in bulk water, km is
the rate constant in the micellar pseudophase, Ks {Ks )
[carbofuran]m/([carbofuran]w[Dn])} is the binding constant of
the substrate to the micellar pseudophase, [Dn] is the concentra-
tion of micellized surfactant ([surfactant] - cmc), and mOH
-
denotes the [OH ]m/[Dn] ratio, which satisfies the following
equation

Degradation of Carbofuran
J. Agric. Food Chem., Vol. 53, No. 18, 2005 7175
Table 1. Kinetic Coefficients for the Basic Hydrolysis of Carbofuran in the Presence of Anionic, Cationic, and Nonionic Micellar Aggregates,
Obtained for the Application of the Pseudophase Model
/M- s-
1
1a
K
k
/s-
1
k2
/M- s-
1
1
surfactant
(CH CH OSO
CH OSO
OSO
effect
k
w
S
m
m
SdS, CH
SUS, CH
SDS, CH
3
2
)
8
2
3
Na
Na
Na
inhibition
inhibition
inhibition
inhibition
inhibition
1.16
1.16
1.16
1.16
1.16
±
±
±
±
±
0.01
0.01
0.01
0.01
0.01
63
87
100
29
±
4
3
3
1
1
3
(CH
(CH
2
)
9
2
3
±
±
±
±
3
2
)
10CH
2
3
2 2
Brij-35, C12H25(OCH CH )10OH
Brij-58
36
C
16
H
33(OCH
2
CH
37(OCH
(CH
2
)
20OH
Brij-78, C18
H
3
2
CH
2
)
20OH
inhibition
catalysis
1.16
1.16
±
±
0.01
0.01
38
20
±
±
1
3
LTABr, CH
TTABr, CH
2
)
11N(CH
3
)
3
Na
Na
Na
10.5
11
±
±
±
0.6
1.0
2
1.47
±
0.08
0.1
0.3
3
(CH
2
)13N(CH
3
)
3
catalysis
catalysis
1.16
1.16
±
±
0.01
0.01
48
52
±
±
2
5
1.5
1.8
±
CTABr, CH
3
(CH
2
)
15N(CH
3
)
3
13
±
a
Value taken from the pure water experiments.
Figure 5. Experimental rate constant, k0exptl, and calculated rate constant,
Figure 4. Influence of surfactant concentration on pseudo-first-order rate
constant of basic hydrolysis of carbofuran: ( ) TTABr micelles; ( ) LTABr
micelles; ( ) CTABr micelles. [carbofuran]
C.
k0calcd (obtained from eqs 3 and 4), for the basic hydrolysis of carbofuran
b
)
O
-
5
4
1. 94
×
10 M. T
)
25
in the presence of cationic micelles: ) TTABr micelles; (
(
b
O) LTABr
°
micelles; ( ) CTABr micelles. T 25 °C.
4
)
Scheme 3
counterion. The subscripts m and w denote micellar and aqueous
pseudophases, respectively.
The calculations were performed by means of a nonlinear
fitting program based on Marquardt’s algorithm. The fit was
carried out using cmc values calculated from the kinetic data;
kw was the experimental value in bulk water, and Ks was
obtained from the fitting of kinetic data for the acid hydrolysis
OH-]
OH
-
[
+ KX [X ]
OH
2
total
total
to find the values of km and KX that best reproduce the kinetic
mOH + mOH
- â -
OH
X
[
]
(K
- 1)[Dn]
data. The solid line in Figure 4 represents the best fit of eqs 3
OH
-
and 4 to the experimental data, using the value of KX
â[OH ]
total
)
]
0 (4)
obtained from the influence of salts upon the basic hydrolysis
of carbofuran in the presence of cationic surfactants (30). This
allows us to obtain again values of km in good agreement with
OH
X
[
(K
- 1)[D ]
n
those found in the study of the influence of salts. The values of
OH
OH
-
-
-
-
where KX {KX ) [OH ]w[X ]/([OH ]m[X ]w)} is the ion-
exchange constant, obtained from the influence of salts upon
k0 (see below). The cmc values were obtained from the kinetic
data and calculated as the minimum concentration of surfactant
necessary to induce a change in the reaction rate (see inset in
Figure 1) and are listed in Table 1. â is the fraction of surfactant
ions neutralized by counterions, which the ion-exchange
pseudophase model assumes to be constant and independent of
the concentration of surfactant (14, 16). The usual value of â is
in the range of 0.6-0.9. We used â ) 0.8 (16) as it seems to
be the most frequently used value. Changes in â (in the range
of 0.6-0.9) do not affect the fitting results. The good fit to the
experimental data shows that the assumption of constant â is
satisfactory and, in our case, independent of the nature of the
OH
X
K
are in accordance with the values reported in the literature
(30, 42-45). The fitting results for different surfactants are
shown in Table 1. The quality of the fit of the experimental
data to pseudophase equations was satisfactory. In all cases the
deviation was <10% (see Figure 5).
For comparing the reactivities in the micellar pseudophase
with the corresponding reactivities in bulk water, km (defined
-1
in terms of moles per mole concentrations and expressed in s )
2
are converted into k m expressed in M. Using the well-known
-
1
2
molar volume of the Stern layer (∼0.14 mol ) (30), the k m
values are compatible with the corresponding one in pure water.
This fact implies that the catalytic effects observed are due to
the increase in the local reagent concentration at the Stern layer.
The kinetic coefficients for these systems are shown in Table
1.

7176 J. Agric. Food Chem., Vol. 53, No. 18, 2005
Arias et al.
Figure 6. Linear relationship between the log/log plot of K
for cationic surfactants.
s
and kmax/k
w
exptl, and calculated rate constant,
0
Figure 8. Experimental rate constant, k
calcd
k
0
(obtained from eq 5), for the basic hydrolysis of carbofuran in the
) Brij-35; ( ) Brig-38; ( ) Brij-78. T )
presence of nonionic micelles: (
b
O
4
25 °
C.
character of the micellar headgroup. In this case, the noncharged
-
headgroups open the possibility that HO can be present in the
micellar pseudophase, and hence, as in the case of cationic
micelles, a second pathway of reaction can be expected (see
Scheme 3).
Nevertheless, the experimental results show that the reaction
in the micellar medium seems to be negligible against the
reaction in the bulk water. For this reason, the behavior can be
modeled using Scheme 2, which yields eq 2.
The quality of the fit of the experimental data to eq 2 was
satisfactory. In all cases the deviation was <10% (see Figure
). The kinetic coefficients for these systems are shown in Table
Figure 7. Influence of surfactant concentration on pseudo-first-order rate
constant of basic hydrolysis of carbofuran: ( ) Brij-35; ( ) Brig-38; (
25 C. Solid lines represent
b
O
4)
8
1
-
5
Brij-78. [carbofuran]
)
1.94
×
10 M. T
)
°
.
fit of eq 5 to experimental data.
The inhibitory effect of the different micelles is similar. This
The catalytic efficiency of the different micelles (viz., kmax/
kw) increases with increasing chain length (5.4, 24.9, and 39.8
for LTABr, TTABr, and CTABr, respectively). This trend 1:4.6:
can be explained by taking into account that the Ks values of
carbofuran for these three surfactants are almost the same (29
( 1, 36 ( 1, and 38 ( 1 for Brij-35, Brij-58, and Brij-78,
respectively).
7
.3 can be justified by taking into account the value of Ks (20,
4
8, and 52 for LTABr, TTABr, and CTABr, respectively). The
To sum up, the effect of the presence of micelles upon the
basic hydrolysis of carbofuran is a function of the nature of the
surfactant monomer. Both anionic and nonionic surfactants
inhibit the decomposition of carbofuran. However, the presence
of cationic surfactants catalyzes the decomposition of carbofu-
ran. Both catalysis and inhibition are due to the association of
carbofuran to the micellar core. No significant changes in the
relationship between the log/log plot of Ks versus kmax/kw is
shown in Figure 6.
The pseudophase ion-exchange model is highly successful
in explaining the results obtained for the alkaline hydrolysis of
carbofuran in cationic micelles. It can also be addressed that
hydrophobic forces drive the association of substrate with the
micellar pseudophase and that the catalysis observed in these
systems is not due to an intrinsically larger kinetic constant in
the micellar pseudophase but to a local concentration effect.
The relationship between the log/log plot of Ks versus kmax/kw
is a good evidence of this fact.
-
intrinsic reactivity of HO against carbofuran have been
observed. Changes in the apparent reactivity in the presence of
micellar aggregates are attributed to the concentration of reagents
in the micellar core (cationic surfactants) or the compartmen-
-
talization of reagents (electrostatic exclusion of HO of Stern
Nonionic Micellar Effects on the Basic Hydrolysis of
Carbofuran. The effect of Brij-35, Brij-58, and Brij-78 on the
layer and inclusion of carbofuran to the micellar core).
Considering that the presence and concentration of the anionic
surfactants in polluted waters is much larger than the other kind
of surfactants (especially the cationic surfactants), the inhibitory
effect observed is important for the persistence of the carbofuran
pesticide in the presence of the anionic surfactants hindering
its degradation.
-
basic hydrolysis of carbofuran was studied at [OH ] constant
-
-3
(
[OH ]total ) 6.20 × 10 M), and the concentrations of the
different surfactants were varied typically between 0 and 0.06
M (Figure 7), showing the pseudo-first-order rate constant, k0,
to be markedly decreased by all of the nonionic surfactants.
This behavior can be rationalized in terms of the two-
pseudophase model (37-39) in which the reaction is assumed
to occur both in micellar pseudophase and in water pseudophase,
with an equilibrium distribution of carbofuran between them.
In the case of anionic surfactant, electrostatic considerations
allow us to neglect the possibility of reaction in the micellar
pseudophase because the hydroxyl ion concentration at the
micellar pseudophase must be very low owing to the anionic
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Degradation of Carbofuran
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of free cyclodextrin in equilibrium with micellized surfactant.
Received for review March 12, 2005. Revised manuscript received June
Langmuir 1999, 15, 8368-8375.
26, 2005. Accepted June 27, 2005. We gratefully acknowledge financial
(
45) Dorrego, B.; Garc ´ı a-R ´ı o, L.; Herv e´ s, P.; Leis, J. R.; Mejuto, J.
C.; P e´ rez-Juste, J. Changes in the fraction of uncomplexed
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of balance between micellization and cyclodextrin-surfactant
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Chem. B 2001, 105, 4912-4920.
support from the Xunta de Galicia (PGIDT03-PXIC20905PN, PGID-
IT0TMT209003PR, and PGIDIT03RAG38301PR) and the Ministerio
de Ciencia y Tecnolog ´ı a (Projects BQU2002-01184 and AGL2003-
02244). M.A. thanks the Ministerio de Educaci o´ n y Ciencia of the
Government of Spain for a Ramon y Cajal Research Grant.
JF0505574