Hydrolysis of p-nitrophenyl perfluoroctanoate in mixed surfactant systems

Article abstract of DOI:10.1039/c4ra02888k

The kinetics of the hydrolysis reaction of p-nitrophenyl perfluoroctanoate were studied in the presence of different amphiphilic systems: two hydrocarbon surfactants (sodium dodecyl sulfate, SDS, and polyoxyethylene(23)lauryl ether, Brij-35), a perfluorinated detergent (perfluorononanoic acid, PFNA) and in mixtures of SDS-PFNA and Brij-35-PFNA. The study was performed at different compositions of the mixtures (characterized by the molar fraction of PFNA, α_PFNA), and for each α_PFNA the concentration was varied over a wide range. The kinetic probe p-nitrophenyl perfluoroctanoate showed a very strong interaction with the surfactants. In the presence of the mixed surfactants, the behaviour of the probe changed dramatically with the composition of the mixtures, which indicates that this substrate is very sensitive to the effect of the micellar media, being a very good molecule to study these types of surfactant systems. The use of the studied ester allowed us to confirm the presence of two different aggregates in the mixture Brij-35-PFNA as was previously observed with other techniques. The presence of hydrocarbon-rich micelles at low α_PFNA, as well as perfluorinated rich micelles at high α_PFNA was also confirmed. The kinetic behaviour of p-nitrophenyl perfluoroctanoate in SDS-PFNA mixtures fully agreed with our previous description of the aggregates formed in this system. The presence of a practically pure PFNA micelle at high α_PFNA could be also demonstrated from the kinetic results.

Full text of DOI:10.1039/c4ra02888k

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PAPER
Hydrolysis of p-nitrophenyl perfluoroctanoate in
mixed surfactant systems†
Cite this: RSC Adv., 2014, 4, 28606
*
Marıa Florencia Torres, Rita H. de Rossi and Mariana A. Fernandez
´
´
The kinetics of the hydrolysis reaction of p-nitrophenyl perfluoroctanoate were studied in the presence of
different amphiphilic systems: two hydrocarbon surfactants (sodium dodecyl sulfate, SDS, and
polyoxyethylene(23)lauryl ether, Brij-35), a perfluorinated detergent (perfluorononanoic acid, PFNA) and
in mixtures of SDS–PFNA and Brij-35–PFNA. The study was performed at different compositions of the
mixtures (characterized by the molar fraction of PFNA, aPFNA), and for each aPFNA the concentration was
varied over a wide range. The kinetic probe p-nitrophenyl perfluoroctanoate showed a very strong
interaction with the surfactants. In the presence of the mixed surfactants, the behaviour of the probe
changed dramatically with the composition of the mixtures, which indicates that this substrate is very
sensitive to the effect of the micellar media, being a very good molecule to study these types of
surfactant systems. The use of the studied ester allowed us to confirm the presence of two different
aggregates in the mixture Brij-35–PFNA as was previously observed with other techniques. The presence
of hydrocarbon-rich micelles at low aPFNA, as well as perfluorinated rich micelles at high aPFNA was also
confirmed. The kinetic behaviour of p-nitrophenyl perfluoroctanoate in SDS–PFNA mixtures fully agreed
with our previous description of the aggregates formed in this system. The presence of a practically pure
PFNA micelle at high aPFNA could be also demonstrated from the kinetic results.
Received 1st April 2014
Accepted 18th June 2014
DOI: 10.1039/c4ra02888k
www.rsc.org/advances
models.8,9 In some cases it has been found that the use of
micellar media results in different products from those
obtained in homogeneous media.10 Some reports also show
1. Introduction
The use of water as a solvent for reactions has numerous
advantages when compared with organic solvents. Specically,
its non-toxic character and ability to accelerate reactions with
polar transition states can bring environmental and kinetic
advantages over organic solvents. Yet, its major disadvantage
derives from the fact that most organic compounds are insol-
uble in water, thus co-solvents are generally needed, reducing
its benets. An alternative to co-solvents involves the use of
surfactants. They are used in much lower concentrations than
organic solvents, are non-volatile and in some cases can be
recycled, turning them into environmentally friendly
alternatives.1
The use of micellar systems as reaction media has been
broadly studied due to their ability to modify reaction rates and
selectivity, and due to their ability to solubilize organic
substrates.2–4 These systems are widely used in emulsion poly-
merization reactions,5 as replacement of volatile organic
solvents, as templates in the synthesis of new materials
(specially nanoparticles)6,7 and as enzyme and membrane
enantioselective reactions in the presence of surfactants.11
The ability of micelles to modify reaction rates depends on
several factors:12–14 (a) lower dielectric constant in the micelles
than in water, which causes a solvent effect; (b) stabilization/
destabilization of the transition state by the head groups of the
surfactants; and (c) concentration of the reactants through
interactions with the micelle surface or through insertion into
the micelle itself. These effects are known as Micellar Media
Effects (MME).
Water-catalyzed processes are especially interesting to study
MME. The rate of hydrolysis of phenyl chloroformate in cationic
micelles was reported to diminish when the polar head size and
the affinity of the counterion for the micelle increase.15 These
head-group and counterion effects indicate that depletion of
water in the interfacial region complements the charge effect in
the control of the reactivity in micelles. One way to assess the
importance of the charge effect independently from other MME
is to study the same reaction in the presence of ionic and non-
ionic micelles.16
A systematic kinetic study in micellar systems offers not only
information on the amphiphilic system17 but also on the reac-
tivity and reaction mechanism in these media. It also provides
information on the solubilization and location of the substrate
in the micelle.18
´
´
´
Instituto de Investigaciones en Fısico-Quımica de Cordoba (INFIQC-CONICET),
´
´
´
Departamento de Quımica Organica, Facultad de Ciencias Quımicas, Universidad
Nacional de Cordoba, Haya de la Torre y Medina Allende, Ciudad Universitaria,
´
´
Cordoba, X5000HUA, Argentina. E-mail: marfer@fcq.unc.edu.ar
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra02888k
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Base-catalyzed ester hydrolysis has been widely used as a about the nature of the mixture at the molecular level. We found
model reaction to evaluate MME in chemical reactions due to its that the changes in the observed rate constants are in good
well-known, relatively simple mechanism.19,20 Its study is also agreement with the results previously published regarding the
particularly important since it is a key reaction in biological properties of the surfactant mixtures and the results are
systems.
In the hydrolysis reactions of aryl alkanoates when the
reported here.
leaving group is a substituted phenoxide, the stabilization of the
transition state given by the micelle remains nearly constant
and is independent of the nature of the surfactant used.21 This
fact indicates that the reaction center and the leaving group are
almost completely in an aqueous environment in the transition
state. Yet, catalysis plays a major role when the length of the
acyl group chain is increased, with the order p-nitro-
phenyldodecanoate > p-nitrophenylhexanoate [ p-nitrophenyl
acetate.12 When the chain length is increased, the substrate
associates more strongly with the micelle causing a bigger effect
on the reaction.
The hydrolysis of p-nitrophenyl peruoroctanoate (1) was
previously studied in our laboratory and it was shown that it
aggregates in water at very low concentrations.22 The aggrega-
tion and hydrolysis reaction in water occur on a similar time
scale. In that work we conducted a detailed study of the effect of
several pure surfactants: sodium dodecyl sulphate (SDS),
dodecyltrimethylammonium bromide and chloride, polyoxy-
ethylene(23)lauryl ether (Brij-35), and peruoroctanoic acid on
the reactivity of compound 1 in water at pH ¼ 6.00 and ionic
strength of 0.3 M.22 All surfactants were able to associate with 1
at a rate considerably faster than that of the self-association of
the substrate, and well above the Critical Micellar Concentra-
tion (CMC) the hydrolysis took place in the micellar pseudo-
phase at a faster rate than that in the absence of the micelles.
However, we found differences in reaction rates when the ionic
nature of the surfactant changed. The rates observed in the
2. Results and discussion
The effect of the mixtures of Brij-35–PFNA and SDS–PFNA was
evaluated on the kinetic behaviour of 1. The study was per-
formed by varying the composition of the mixture, character-
ized by the molar fraction of the uorinated surfactant (aPFNA).
The hydrolysis reaction of ester 1 in water and in the presence of
different surfactants or mixtures was followed by UV-Visible
absorption spectroscopy, measuring the appearance of p-nitro-
phenol at 319 nm or the disappearance of the substrate at
260 nm. As the total change in optical density measured at the
higher wavelength was larger, resulting in more accurate rate
constant values, we only report the data obtained at this wave-
length. However, it should be noted that at 260 nm the observed
rate constants showed similar trends as those measured at 319
nm and the analysis of the data obtained led to the same
general conclusions.
The reaction was evaluated in the absence of buffer or
additional electrolytes to avoid complications in the interpre-
tation of the results associated with the exchange of additional
ions at the interface. In previous work25 we have demonstrated
that the reaction of aryl triuoroacetates is independent of the
pH below pH ¼ 7, therefore it is reasonable to assume that the
kinetic of 1 is also pH independent in acidic solution.
2.1. Hydrolysis of 1 in water
presence of nonionic surfactant had values ranging between Although this substrate was previously studied in water at pH 6
those in cationic and in anionic surfactants, that is, kcationic
and controlled ionic strength, we re-analized the reaction of 1 in
>
knonionic > kanionic. The differences observed when the nature of water without any additives to have the same conditions that in
the surfactant changed, indicates that it may be a good sensor of the following studies in the presence of surfactants.
micellar effects. Recently, we characterized two mixed surfac-
At all substrate concentrations, using water as solvent, good
tant systems, Brij-35-peruorononanoic acid (PFNA)23 and SDS– isosbestic points are observed in the absorbance vs. wavelength
PFNA.24 We established that in the Brij-35–PFNA system two plot (Fig. 1A is representative). However, the absorbance vs. time
non-coexisting aggregates are formed (evidenced by the pres- data ts to a double exponential equation implying the presence
ence of two CMC).23 We attributed the rst CMC to a mixed of two kinetic processes (see Fig. 1B). Table 1 summarizes the
aggregate that changes its morphology when reaching the variation of the observed rate constant with the substrate
second CMC. In the SDS–PFNA system instead, mixed micelles concentration in water. It can be seen that there is a tendency
are formed at low aPFNA; yet, aggregates richer in the uorinated for the rate constant k2 to decrease when concentration
surfactant are formed at high aPFNA but only one CMC is increases. This behavior is usually found in compounds that
observed.24
self-aggregate in solution: when aggregation increases, reac-
tivity decreases, especially in uorinated reactants.26
Fluorocarbon surfactants are known to have a slow aggre-
gation rate.27 In substrate 1 the slow aggregation manifests itself
in the fact that aggregation and hydrolysis occur at similar time
scales, which means that aggregation competes with hydrolysis.
However, this is not the case with hydrocarbon-derived esters,
thus only one kinetic process (corresponding to the hydrolysis
of the aggregated substrate) is observed. It is important to note
that neither k1 nor k2 represent rate constants of elementary
reactions. They are a combination of several elementary
Taking into account the results mentioned above regarding
the effect of surfactants on the hydrolysis of 1, we carried out a
study of the kinetic of the reaction in the presence of mixed
surfactants since we considered that it might give information
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Fig. 1 (A) Hydrolysis of 1 in water. Time elapsed between a and b: 30 minutes. T ¼ (25.0 ꢂ 0.1) ꢃC, MeCN ¼ 3.8%, [1] ¼ 2.5 ꢀ 10ꢁ5 M. (B)
Absorbance at 319 nm vs. time data for the spectra in (A). Data were fitted to a double exponential equation.
Table 1 Observed rate constants for the hydrolysis of 1 in water as a function of substrate concentrationa
[1], 10ꢁ5
M
k2, 10ꢁ2 sꢁ1
k1, 10ꢁ2 sꢁ1
[1]b, 10ꢁ5
M
k2b, 10ꢁ2 sꢁ1
k1b, 10ꢁ2 sꢁ1
1.00
2.50
3.36
0.18 ꢂ 0.01
0.150 ꢂ 0.002
0.149 ꢂ 0.004
2.04 ꢂ 0.03
2.1 ꢂ 0.1
1.6 ꢂ 0.1
1.08
2.47
3.45
0.191
0.212
0.118
1.03
0.89
0.469
a
T ¼ (25.0 ꢂ 0.1) ꢃC; solvent contains MeCN ¼ 3.8%; l ¼ 319 nm. The values of the rate constants were obtained by tting the absorbance versus
time data to the following equation: A ¼ AN + a1eꢁk1t + a2eꢁk2t. Each value corresponds to the average of at least three determinations; the errors are
b
standard deviations from the t. Data taken from ref. 22, ionic strength 0.3 M; pH 6.00; [buffer] ¼ 0.096 M (Na2HPO4/NaH2PO4).
reactions. To nd the relationship between the values of the different to what is usually observed in other reactions. As it can
observed k1 and k2 and every step of the mechanism it would be be seen in Table 2 the observed rate constant rst decreases and
necessary to know it in detail, which is not possible with the then suddenly increases to reach an almost constant value. This
available data.
result might indicate that at low micellar concentration more
Comparing the values of k1 and k2 with those previously than one molecule of 1 (pre-associated) interacts with the
published in the presence of buffer (pH ¼ 6) and 0.3 M ionic
strength we can see (Table 1) that k2 have similar values whereas
Table 2 Effect of single hydrocarbon surfactants on the observed rate
k1 are signicantly different. It was suggested that k1 have high
contribution of the hydrolysis rate while k2 is mainly related to
the rate of self-aggregation of 1. This may indicate that the
change in ionic strength modify the water structure in the
proximity of the reacting ester.
of hydrolysis of 1a
Brij-35
SDS
(0.09 ꢀ 10ꢁ3 M)
(8.25 ꢀ 10ꢁ3 M)
[Surf], 10ꢁ3
M
k2, 10ꢁ2 sꢁ1
k1, 10ꢁ2 sꢁ1
k, 10ꢁ2 sꢁ1
0.20
0.50
0.75
2.00
3.00
5.00
7.00
10.00
15.00
20.00
35.00
50.00
4.7 ꢂ 0.3
2.2. Hydrolysis of 1 in the presence of pure surfactants
0.29 ꢂ 0.01
0.21 ꢂ 0.02
0.7 ꢂ 0.2
0.60 ꢂ 0.02
The kinetic behaviour shown in the presence of pure surfactants
varies according to the surfactant employed. Table 2 depict the
observed rate constants for the hydrolysis of 1 at concentration
2.5 ꢀ 10ꢁ5 M in the presence of variable concentrations of Brij-
35 and SDS.
At concentrations below the surfactant CMC, two kinetic
processes are observed in a similar way to what happens in the
absence of surfactants, although the rate constants are
different. This observation shows that, below their CMC, these
surfactants are not efficient enough to disaggregate the
substrate. Once the surfactant concentration is well above the
CMC, only one kinetic process is detected. This fact indicates
that 1 is incorporated into the micelle faster than it self-aggre-
gates. In the case of Brij-35 the maximum value of the rate
constant observed (45–48 ꢀ 10ꢁ2 sꢁ1) is higher than the value
obtained for SDS (z1 ꢀ 10ꢁ2 sꢁ1). The behaviour of SDS is quite
38 ꢂ 6
45 ꢂ 1
0.52 ꢂ 0.07
0.30 ꢂ 0.02
0.259 ꢂ 0.002
1.04 ꢂ 0.07
1.07 ꢂ 0.01
43 ꢂ 1
48 ꢂ 5
0.99 ꢂ 0.03
a
[1] ¼ 2.5 ꢀ 10ꢁ5 M; T ¼ (25.0 ꢂ 0.1) ꢃC; solvent contains MeCN ¼ 3.8%;
l ¼ 319 nm. If only one value of rate constant is informed, it means that
the data t to a single-exponential equation. When two values are
informed, they were obtained by tting the absorbance versus time
data to the following equation: A ¼ AN + a1eꢁk1t + a2eꢁk2t. Each value
corresponds to the average of at least three determinations; the errors
are standard deviations from the t. The values in brackets
correspond to the previously determined CMC of the pure surfactants
(see ESI, Table S1† and ref. 23 and 24).
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micelle but when the micelle concentration increases, is the of the substrate increases the reaction rate; on the other hand,
monomer the one that interacts with the micelle. the strong interaction of the substrate with the surfactant
On the other hand, 1 shows only one kinetic process at all makes the ester less available to react with water.
concentrations of PFNA even at concentrations below its CMC Brij-35 is a non-ionic surfactant so it would not affect the
(Table 3). This result indicates that PFNA interacts with 1 even transition state of the hydrolysis reaction through electrostatic
at very low concentration and prevents its self aggregation. The interactions, but it does affect the aggregation of 1. Hence,
rate constants determined for this process are somewhere when the substrate associates with Brij-35 micelles its hydro-
between the values of the two kinetic processes observed for the lysis rate is increased (Table 2) partly due to the fact that 1 is
substrate in pure water, and it decreases with the increase in disaggregated. The strong association of Brij-35 with per-
surfactant concentration reaching
a value approximately uorinated compounds was previously attributed to interac-
constant above the CMC of PFNA. These results are similar to tions between the –Cd+–Fdꢁ dipoles of the substrate and the
those previously reported for the kinetic of 1 in the presence of corresponding –Cd+–Odꢁ of the oxyethylene units of the surfac-
peruoroctanoic acid.22
tant.30,31 Also, it is known that ether groups of polyoxyethylene
The effect of the surfactants on the rate constants depends surfactants interact with interfacial water.32 El Eini et al.
on their nature, and may result from a combination of factors reported that polyoxyethylene surfactants have water molecules
such as localization of the substrate in the micelle and the trapped in the polyoxyethylene headgroup with 5.2 to 10.5 water
transition state of the reaction. It is well known that uorinated molecules per ethylene oxide unit as the hydrophilic chain
chains are highly hydrophobic, so it can be expected that the length increases.33 In the case of the microemulsions of an
substrate chain is associated with the micelle. However, the analogous surfactant, Brij-30 (shorter headgroup), enhance-
aromatic ring moiety is less lipophilic, so this part of the ment of the nucleophilic character of the interfacial water at low
molecule can be located in a more aqueous region. The inter- interface hydration was conrmed by NMR studies and corre-
facial region is less polar and “less aqueous” than water, so the lated with the catalysis shown for solvolysis of 4-nitrobenzoyl
reaction is expected to be inhibited when compared to pure chloride.32 A similar effect was proposed to explain the
water as was observed for several different systems. For increase in the solvolysis constants found for 4-nitrobenzoyl
instance, the spontaneous hydrolysis of phenyl chloroformate chloride in Brij-35 micelles.34 The authors suggest that Brij-35
in water–ethylene glycol was studied in presence of alkyl- micelles offer an environment with a concentration of water
trimethylammonium surfactants. The hydrolysis was retarded signicantly lower than that in the aqueous phase35 but that
in the presence of micelles. This was expected since deacyla- can be compared with a highly hydrated nonionic micro-
tions are inhibited by a decrease in solvent polarity and water emulsion. The increment seen in the hydrolysis rate constants
content, which occurs in micellar interfacial regions.28
for compound 1 in the presence of Brij-35 may result from two
With PFNA two other factors have to be considered to explain effects: the interaction of the ester with the micelles, that
the observed behaviour: (i) PFNA has a very low pKa12,29 so, when disaggregate it, plus the particular water environment present
dissolved in water, it can be considered an anionic surfactant. in Brij-35 micelles. However, the rate constants obtained at the
As mentioned above, the presence of anionic surfactants maximum Brij-35 concentration (almost 0.5 sꢁ1) is still 10
destabilizes the transition state thus inhibiting the reaction. (ii) times smaller than the estimated rate constant for the neutral
As both, surfactant and substrate, have peruorinated hydro- hydrolysis of monomeric ester 1, that is 5 sꢁ1 22 The results can
.
phobic chains, they associate strongly and the substrate is dis- be rationalized in terms of the partitioning of the reactant
aggregated even at low surfactant concentrations (only one between water and micelles whose interfacial regions are less
kinetic process is observed). This substrate–surfactant associa- polar than pure water and similar to mixed organic solvents as
tion has two opposite effects: on one hand, the disaggregation reaction media.14
Finally, in the presence of SDS, two kinetic processes are
observed below its CMC, with a decrease in the observed rate
constant (k). This is similar to the behaviour shown in PFNA,
Table 3 Effect of PFNA on the observed hydrolysis rate of 1a
although SDS below its CMC is not efficient enough to disag-
gregate the substrate, thus only the effect of the negative head
PFNA (2.11 ꢀ 10ꢁ3 M)
group is observed. When the CMC of SDS is reached, only one
kinetic process is observed, indicating that 1 associates with the
micelle and k increases up to 0.01 sꢁ1. This increment can be
attributed to substrate disaggregation. Despite this, the values
reached are lower than those in Brij-35 and pure water since
SDS is an anionic surfactant and, as already mentioned, ionic
strength and negative charge affect hydrolysis rate, in addition
to lesser accessibility to water when the substrate associates to a
micelle. The negatively charged interface in anionic surfactants
can also contribute to producing a decrease in the rate values
observed due to the destabilization of the transition state. This
behaviour was exhibited in several systems: the rate constant for
[Surf], 10ꢁ3
M
k, 10ꢁ2 sꢁ1
0.05
0.25
0.50
1.00
2.00
4.00
5.00
6.00
7.00
0.8 ꢂ 0.2
0.49 ꢂ 0.02
0.480 ꢂ 0.001
0.3183 ꢂ 0.0006
0.28 ꢂ 0.02
0.116 ꢂ 0.001
0.106 ꢂ 0.003
0.1017 ꢂ 0.0008
0.103 ꢂ 0.001
a
[1] ¼ 2.5 ꢀ 10ꢁ5 M; T ¼ (25.0 ꢂ 0.1) ꢃC; solvent contains MeCN ¼ 3.8%;
l ¼ 319 nm.
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the hydrolysis of 4-nitrophenyl chloroformate decreases in the
presence of anionic and non-ionic micelles, and it is more
sensitive to the structure of the head group than to the length of
the surfactant hydrophobic tail. In this case the effects are not
ascribed either to sizeable differences in the micelle–ester
association constants nor to entirely different average solubili-
zation/reaction sites in the micellar pseudophases. They arise
mainly from the medium effect, the interaction of the reactant
state with the anionic surfactant and the destabilization of the
transition state by the charged interface.36
In summary, there are different effects that can inuence the
reaction rate of a compound in the presence of micellar media.
It is not easy to determine their contribution to the effect
observed, or which one is more important to predict substrate
behaviour.
Fig. 2 Observed rate constant for the hydrolysis of 1 as a function of
PFNA molar fraction at [S]total ¼ 5 ꢀ 10ꢁ3 M for the mixture Brij-35–
PFNA. [1] ¼ 2.5 ꢀ 10ꢁ5 M; T ¼ (25.0 ꢂ 0.1) ꢃC; the solvent contains
MeCN ¼ 3.8%; l ¼ 319 nm.
2.3. Hydrolysis of 1 in the mixed system Brij-35–PFNA
The results of the hydrolysis of 1 in mixtures of Brij-35–PFNA are
summarized in Table 4. Under all conditions the absorbance
values vs. time t to a simple exponential equation, indicating
that only one kinetic process takes place.
Fig. 2 shows that the largest decrease occurs before aPFNA
¼
0.5 is reached. From aPFNA ¼ 0.5 to aPFNA ¼ 1, the rate decrease
is less signicant, probably indicating that the composition
change in the micelle is less important. Compound 1, strongly
associated with the peruorinated surfactant, may be sensing
an intramicellar demixing, i.e., a region in the micelle enriched
with PFNA, due to the enrichment of the aggregate in the per-
uorinated surfactant. That could be the reason for the lesser
dependence of the observed rate constant with aPFNA. A similar
behaviour is evidenced at other constant concentrations of the
mixture analysed.
As it was reported before,23 mixtures of PFNA with Brij-35
have two critical micellar concentrations (ESI, Table S1†) which
have been attributed to a change in morphology of the micelle
when the concentration increases.
From the analysis of the data in Table 4 it can be seen that
for a constant total surfactant concentration higher than 1 ꢀ
10ꢁ4 M, the observed rate constant decreases with the increase
in molar fraction of PFNA. For instance, at a total surfactant
concentration of 5 ꢀ 10ꢁ3 M, the observed rate constants vary
from 25.9 ꢀ 10ꢁ2 sꢁ1 to 0.45 ꢀ 10ꢁ2 sꢁ1 for aPFNA ¼ 0.1 and 0.9
respectively (Fig. 2). When the molar fraction of PFNA
increases, its content in the mixture also rises, and the
observed rate constant at aPFNA ¼ 0.9 decreases to a similar
value to that obtained in the presence of a 10 times lower
concentration of pure PFNA (Table 3, [PFNA] ¼ 0.5 ꢀ 10ꢁ3 M,
k ¼ 0.48 ꢀ 10ꢁ2 sꢁ1).
Table 4 Observed rate constants (10ꢁ2 sꢁ1) for the hydrolysis of 1 in Brij-35–PFNA mixturesa
aPFNA
[Surf], 10ꢁ3
M
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.9
0.01
0.05
0.1
0.2
0.5
0.7
1
2
3.5
5
2.13 ꢂ 0.01
4 ꢂ 2
3.6 ꢂ 0.9
2.9 ꢂ 0.5
3.4 ꢂ 0.3
2.1 ꢂ 0.1
3.5 ꢂ 0.2
3.51 ꢂ 0.09
6 ꢂ 2
2.48 ꢂ 0.07
2.38 ꢂ 0.07
3.5 ꢂ 0.4
2.6 ꢂ 0.1
2.9 ꢂ 0.1
2.18 ꢂ 0.08
2.4 ꢂ 0.2
1.64 ꢂ 0.05
1.22 ꢂ 0.04
2.3 ꢂ 0.5
2.4 ꢂ 0.3
2.0 ꢂ 0.2
3.1 ꢂ 0.2
2.35 ꢂ 0.01
3.4 ꢂ 0.1
7.7 ꢂ 0.3
2.9 ꢂ 0.2
1.58 ꢂ 0.03
1.3 ꢂ 0.1
4.0 ꢂ 0.2
11.9 ꢂ 0.8
12.5 ꢂ 0.8
1.26 ꢂ 0.08
2.9 ꢂ 0.4
8.3 ꢂ 0.7
8.9 ꢂ 0.6
5.18 ꢂ 0.09
7.8 ꢂ 0.2
2.8 ꢂ 0.1
16.1 ꢂ 0.6
21.8 ꢂ 0.4
16.9 ꢂ 0.6
4.55 ꢂ 0.03
5.9 ꢂ 0.2
14 ꢂ 1
22 ꢂ 1
16.5 ꢂ 0.4
19.7 ꢂ 0.2
9.9 ꢂ 0.4
7.9 ꢂ 0.2
5.1 ꢂ 0.6
1.7 ꢂ 0.1
2.5 ꢂ 0.1
19.9 ꢂ 0.8
24.8 ꢂ 0.9
8.3 ꢂ 0.3
3.6 ꢂ 0.2
6.8 ꢂ 0.6
1.89 ꢂ 0.05
1.9 ꢂ 0.1
1.6 ꢂ 0.1
25.9 ꢂ 0.6
22.6 ꢂ 0.4
16.8 ꢂ 0.2
5.01 ꢂ 0.07
4.21 ꢂ 0.08
3.73 ꢂ 0.03
2.8 ꢂ 0.1
2.7 ꢂ 0.3
2.5 ꢂ 0.2
0.447 ꢂ 0.008
7
1.61 ꢂ 0.01
10
15
20
30
50
18.3 ꢂ 0.3
13.8 ꢂ 0.2
9.3 ꢂ 0.3
6.17 ꢂ 0.08
5.48 ꢂ 0.09
28.9 ꢂ 0.5
15.8 ꢂ 0.4
16.6 ꢂ 0.7
27.2 ꢂ 0.9
25.6 ꢂ 0.4
a
[1] ¼ 2.5 ꢀ 10ꢁ5 M; T ¼ (25.0 ꢂ 0.1) ꢃC; the solvent contains MeCN ¼ 3.8%; l ¼ 319 nm. The data t to single-exponential equation. Each value
corresponds to the average of at least six determinations and the errors are standard deviations of the t. The CMCs previously obtained for different
compositions of this mixed system are summarized in ESI, Table S1† and ref. 23.
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The analysis of the data at each molar fraction (Table 4 and and CMC2) hydrocarbon-rich mixed micelles are formed and
Fig. 3) has three regions: (i) a region under the rst CMC of the the presence of some uorinated compound in the aggregates is
mixtures, where the observed rate constants do not change or responsible for the decrease in rate compared with pure Brij-35
decrease slightly; (ii) a region between both CMC, where the micelles. On the other hand the micelles formed aer CMC2 are
observed rate constants increase; (iii) a third region, at high uorocarbon-rich micelles, therefore the hydrolysis of 1, espe-
[S]total, where the rate constants diminish sharply. This behav- cially at high aPFNA, has a rate constant similar but not equal to
iour is common to all the solutions with PFNA molar fraction that found in the pure uorocarbon surfactant. We suggest that
higher than 0.1. For the solution with aPFNA ¼ 0.1 the decrease the presence of some Brij-35 in the micelle is responsible for the
in rate aer the second CMC is less marked.
small acceleration of the rate compared with the value in pure
It is worth noting that, at a surfactant concentration lower peruorinated surfactant. The picture of the system from the
than the rst CMC, only one kinetic process is observed, with a kinetic data is in good agreement with our previous results on
rate constant value similar to that of the fastest process for the the characterization of this mixture using other techniques.23
reaction in water (k1 in Table 1). As mentioned earlier, the
fastest kinetic process is mainly associated with the hydrolysis
reaction. This behaviour is similar to that found in pure PFNA.
The second region seen in the plots of Fig. 3 occurs at
surfactant concentrations above the rst CMC value. The rate
2.4. Hydrolysis of 1 in the mixed system SDS–PFNA
Contrary to the observed behaviour of the mixtures Brij-35–
PFNA, SDS–PFNA mixtures show only one CMC.24 However
nuclear magnetic resonance results indicate that there are
increase detected aer the rst CMC is similar to that found in
intramicellar demixing at high aPFNA
.
pure Brij-35; however, the maximum rate constant value
obtained was 1.5 to 2 times lower than the value observed at the
same concentration of pure Brij-35, even at low aPFNA. Besides,
the maximum is considerably higher than the value obtained at
the corresponding concentration of pure PFNA. This result may
indicate that within this concentration range (between CMC1
Table 5 summarizes the results of the hydrolysis of 1 in SDS–
PFNA mixtures. It can be seen that the behaviour of 1 depends,
again, on the composition of the mixture. At low surfactant
concentrations two kinetic processes are observed, but for
aPFNA ¼ 0.2 already at 7 ꢀ 10ꢁ4 M of total surfactant concen-
tration, only one kinetic process is observed indicating that the
Fig. 3 Observed rate constants for the hydrolysis of 1 as a function of total surfactant concentration for Brij-35–PFNA mixture ([S]total). aPFNA
¼
0.2, CMC1 ¼ 1.1 ꢀ 10ꢁ4 M, CMC2 ¼ 5.7 ꢀ 10ꢁ4 M; aPFNA ¼ 0.5, CMC1 ¼ 1.6 ꢀ 10ꢁ4 M, CMC2 ¼ 7.4 ꢀ 10ꢁ4 M; aPFNA ¼ 0.7, CMC1 ¼ 2.4 ꢀ 10ꢁ4 M,
CMC2 ¼ 1.0 ꢀ 10ꢁ3 M. [1] ¼ 2.5 ꢀ 10ꢁ5 M; T ¼ (25.0 ꢂ 0.1) ꢃC; the solvent contains MeCN ¼ 3.8%; l ¼ 319 nm.
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Table 5 Observed rate constants (10ꢁ2 sꢁ1) for the hydrolysis of 1 in SDS–PFNA mixturesa
aPFNA (CMC)
0.2 (8.26 mM)
k
0.9 (3.10 mM)
0.5 (5.53 mM)
0.7 (4.03 mM)
k2
[Surf], 10ꢁ3
M
k2
k1
k1
0.77 ꢂ 0.02
k
0.70
0.75
1.00
2.00
2.50
3.00
4.00
5.00
6.00
7.00
7.50
10.00
15.00
20.00
30.00
35.00
0.64 ꢂ 0.05
0.0227 ꢂ 0.0006
0.039 ꢂ 0.001
0.028 ꢂ 0.005
0.25 ꢂ 0.03
0.37 ꢂ 0.06
0.102 ꢂ 0.002
0.170 ꢂ 0.004
0.24 ꢂ 0.02
0.365 ꢂ 0.007
0.092 ꢂ 0.001
0.105 ꢂ 0.009
0.21 ꢂ 0.01
0.134 ꢂ 0.002
0.1314 ꢂ 0.0003
0.1445 ꢂ 0.0005
0.112 ꢂ 0.001
0.136 ꢂ 0.002
0.105 ꢂ 0.003
0.103 ꢂ 0.002
0.130 ꢂ 0.002
0.099 ꢂ 0.002
0.134 ꢂ 0.001
0.35 ꢂ 0.02
0.117 ꢂ 0.001
0.110 ꢂ 0.001
0.1037 ꢂ 0.0008
0.102 ꢂ 0.001
0.120 ꢂ 0.001
0.122 ꢂ 0.002
0.0989 ꢂ 0.0009
2.1 ꢂ 0.1
4.6 ꢂ 0.1
5.1 ꢂ 0.4
4.5 ꢂ 0.1
[1] ¼ 2.5 ꢀ 10ꢁ5 M; T ¼ (25.0 ꢂ 0.1) ꢃC; the solvent contains MeCN ¼ 3.8%; l ¼ 319 nm. If only one value of rate constant is informed, it means that
a
the data t to a single-exponential equation. When two values are informed, they were obtained by tting the absorbance vs. time data to A ¼ AN
+
a1eꢁk1t + a2eꢁk2t. Each value corresponds to the average of at least two determinations and the errors are standard deviations of the t. The values in
brackets correspond to the CMC of the mixture at the corresponding aPFNA (ESI, Table S1† and ref. 24).
association of the substrate with the present peruorinated can modify the offered environment to the reaction when it is
surfactant inhibits the self-association of 1. At this molar frac- mixtured with hydrocarbon surfactants. This is reasonable
tion the maximum value of the observed rate constant obtained considering that peruorinated chain have more affinity among
is higher than in pure SDS and in pure PFNA. This result might themselves than towards hydrocarbon chains.
indicate that the interphase in this mixed micelle has the water
Although we cannot determine quantitatively the association
less structured and more available for the reaction. At higher constants of ester 1 with the mixed surfactant systems, we can
molar fraction the rate constants measured aer the CMC have make some qualitative estimates of the interaction. Due to the
about the same value as that in pure peruorinated surfactant presence of PFNA in mixtures with Brij-35, only one kinetic
which may indicate same intramolecular demixing and associ- event occurs even at lower concentrations than the rst CMC of
ation of the probe with the uorinated reach region of the the system. Only one kinetic process was found in all compo-
micelle. This is consistent with the fact that at low aPFNA the sitions of the mixture, indicating that the interaction of 1 with
micelles are mixed while at high aPFNA the aggregates become these aggregates is stronger than with pure Brij-35 micelles.
almost of pure PFNA, as reported elsewhere.24
In mixtures of PFNA with SDS instead, at concentrations
lower than the CMC, two processes are still present. The mixture
is not able to completely disaggregate the substrate when
micelles are not present. This could indicate a weaker interac-
tion of substrate 1 with this mixed system.
2.5. Comparison between the different systems under study
The behaviour of 1 in the presence of surfactants can be rep-
resented by Scheme 1, where R is the substrate, S is the
monomeric surfactant, M represents the micelles and P repre-
sents the products of the reaction. Each step has its rate and/or
equilibrium constants, which are not shown in the scheme for
clarity reasons. A detailed kinetic analysis is not possible due to
the many reaction paths involved. Depending on the surfactant
or mixtures of surfactant employed, different regions in Scheme
1 gain relevance. When the reaction takes place in SDS or Brij-
35, we observe two kinetic processes at concentrations below
the respective CMCs indicating that these monomeric surfac-
tants are not able to completely disaggregate the substrate;
thus, the reaction still occurs, partly, from the aggregated
substrate. On the other hand, PFNA can disaggregate the ester 1
Scheme 1 Representation of the hydrolysis of 1 in the presence of
even at concentrations lower than its CMC. In addition, PFNA surfactants.
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The effect of the different systems on the hydrolysis rate of 1
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As 1 has a long uorinated chain, it can be incorporated into
was mainly attributed to (i) destabilization of the transition state the micelles. Its behavior is strongly dependent on the charac-
in anionic systems; (ii) different content of water or different teristics of the aggregates and it can sense slight differences. It
polarity of the micellar interfaces in the mixed systems. We can clearly distinguish variations in the ionic nature of the polar
determined the polarity of the aggregate interfaces at different head as well as in the chain. It is a substrate which can be used
compositions of each system using pyrene as micropolarity effectively as a kinetic molecular probe in these systems.
probe37 (ESI, Table S2†). The ratio between the vibronic bands of
The differences noted in substrate 1 when the micellar
the uorescence spectra of pyrene in SDS–PFNA, (I1/I3), changes system changes show the importance of the nature of the
from 0.82 at aPFNA ¼ 0.1 to 1 at aPFNA ¼ 0.5. The value of the same surfactant polar head when dealing with reactions carried out in
ratio, (I1/I3), for pure PFNA is also 1, indicating that aer aPFNA
¼
colloidal systems.
0.5 the probe senses a similar polarity to that of an aggregate
formed only by PFNA, reinforcing the concept that the micelles at
high aPFNA in this mixture are mainly formed by PFNA as we
previously established.24 On the other hand, the same analysis for
Brij-35–PFNA mixtures shows changes in polarity from 1.08 to
1.31 for aPFNA 0.1 to 0.9 respectively, i.e., an increase in the ratio
with the increase in aPFNA. Considering that the value of the (I1/I3)
ratio is 1.59 in water,37 the polarity of the Brij-35–PFNA interface
is higher than that corresponding to SDS–PFNA. This fact is also
in good agreement with the higher hydrolysis rate observed in the
rst mixtures. The difference in the polarity of the micellar
interface could partly explain the difference evidenced in the rate
constants, as another factor inuencing micellar effects on
organic reactions.
4. Experimental
4.1. Materials and methods
Aqueous solutions were made up from water puried in a Mil-
lipore apparatus. Acetonitrile HPLC grade (Mallinckrodt) was
used as received.
Ester 1 was prepared by the reaction of peruoroctanoic acid
chloride with p-nitrophenol following literature methods.38 The
product was obtained aer distillation of the remaining acid
chloride and phenol, and it was characterized by IR and UV-
Visible spectroscopy.
PFNA (Fluka, GC grade), SDS (Sigma-Aldrich, 99%) and Brij-
¨
35 (Riedel de Haen, 98%) were used as received. All surfactant
By using mixtures of surfactants we aim to nd synergism, it
means, better properties in the mixture compared with pure
surfactants. In this case, although the micellar system formed
by Brij-35–PFNA is more efficient than that of SDS–PFNA, since
the disaggregation of the substrate is performed at lower
surfactant concentrations with higher hydrolysis rate constants,
pure Brij-35 is still much better as the rate constants are higher
than those in the Brij-35–PFNA system.
The study of this hydrolysis reaction in these mixed surfac-
tant systems was, however, valuable to gain further knowledge
of these complex mixtures. These results are in very good
agreement with those obtained with other techniques dealing
specically with the characterization of the mixtures at a
molecular level.
solutions in water were prepared at least 24 hours before use to
ensure that the aggregation equilibrium was reached. Pyrene
(Aldrich, 98%) was used without purication.
4.2. Reaction kinetics
The reactions were followed measuring the change in absor-
bance with time for the appearance of p-nitrophenol at 319 nm
or the disappearance of 1 at 260 nm. The reactions of 1 in the
presence of Brij-35 or in the Brij-35–PFNA mixtures were carried
out in a stopped ow apparatus with unequal mixing. The
substrate was dissolved in dry acetonitrile and placed in the
small syringe (0.1 mL). The larger syringe (2.5 mL) was lled
with a water solution containing the surfactants. The reactions
of the substrate in water, with pure PFNA or SDS or in the SDS–
PFNA mixtures were measured in a conventional UV-visible
spectrophotometer. For these kinetic runs, 0.12 mL of a stock
solution of 1 in acetonitrile were injected into a 1 cm optical
pass length quartz cuvette containing 3 mL of water or an
aqueous solution of surfactant. In all cases the total acetonitrile
concentration was 3.8%, and the reactions were carried out at
3. Conclusion
The behavior of 1 was markedly different in both systems
studied, showing that this substrate is very sensitive to the
environment and to the effect of micellar media.
The differences in the kinetic behavior of 1 in the mixed Brij-
35–PFNA system when the total surfactant concentration
changed, allowed us to conrm the presence of two different
aggregates in this mixture as was previously observed with other
techniques.23 The presence of hydrocarbon-rich micelles at low
aPFNA, as well as peruorinated rich micelles at high aPFNA was
also conrmed.
ꢃ
(25.0 ꢂ 0.1) C.
As the reactions in PFNA have a pH around 3, we carried out
some of the reactions in Brij-35 using HCl 1 ꢀ 10ꢁ3 M as solvent
to check that the pH did not affect the rate constants. The values
of k in the presence and absence of HCl were the same within
the experimental error so we continued the experiments using
water as solvent.
The SDS–PFNA system was also evaluated. The sensitivity of
the substrate is quite amazing since the kinetic behavior fully
agrees with our previous description of the system obtained via
4.3. Micropolarity determinations
the application of different techniques to characterize the aggre- Studies were carried out by adding a very small volume of a
gates formed. The presence of a practically pure PFNA micelle at stock solution of pyrene in acetonitrile to a volumetric ask. The
high aPFNA could also be demonstrated from the kinetic results.24 corresponding surfactant or mixture of surfactants was then
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added. The dilutions prepared in this way contained 0.08% 14 C. A. Bunton, Adv. Colloid Interface Sci., 2006, 123–126, 333–
acetonitrile in water. The concentration of pyrene in the solu-
343.
tions was 1 ꢀ 10ꢁ6 M. The ratio of the bands (I1/I3) was deter- 15 L. Brinchi, P. Di Proo, F. Micheli, R. Germani, G. Savelli and
mined aer measuring the uorescence intensity of pyrene at
C. A. Bunton, Eur. J. Org. Chem., 2001, 2001, 1115–1120.
˜
´
´
374.5 and 384.0 nm. The excitation wavelength was 334 nm.
16 M. Munoz, A. Rodrıguez, M. d. M. Graciani and M. L. Moya,
Int. J. Chem. Kinet., 2002, 34, 445–451.
17 L. Onel and N. J. Buurma, J. Phys. Chem. B, 2011, 115, 13199–
13211.
Acknowledgements
This research was supported in part by Consejo Nacional de
18 R. Lombardo, C. Sbriziolo, M. L. T. Liveri and M. L. Turco
Liveri, Colloids Surf., A, 2006, 273, 1–9.
´
´
Investigaciones Cientıcas y Tecnicas (CONICET), Agencia
´
Nacional de Ciencia y Tecnologıa (FONCyT), Ministerio de
˜
19 C. Bobica, D. F. Anghel and A. Voicu, Colloids Surf., A, 1995,
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Ciencia y Tecnologıa de la Provincia de Cordoba (MINCyT) and
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21 O. S. Tee, Adv. Phys. Org. Chem., 1994, 29, 1–85.
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recipient of a fellowship from CONICET.
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22 M. A. Fernandez and R. H. de Rossi, Langmuir, 2006, 22,
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