Micellization and catalytic properties of cationic surfactants with head groups functionalized with a hydroxyalkyl fragment

Article abstract of DOI:10.1021/je300753d

The catalytic activity of two homological series of cationic surfactants bearing a hydroxyalkyl fragment in the head groups R(CH₃)₂N⁺(CH₂CH₂OH)Br^-and R(CH₃)₂N⁺

Full text of DOI:10.1021/je300753d

Article
pubs.acs.org/jced
Micellization and Catalytic Properties of Cationic Surfactants with
Head Groups Functionalized with a Hydroxyalkyl Fragment
Alla B. Mirgorodskaya, Ekaterina I. Yackevich, Victor V. Syakaev, Lucia Ya. Zakharova,* Shamil K. Latypov,
and Alexander I. Konovalov
State Budgetary-Funded Institution of Science, A.E. Arbuzov Institute of Organic and Physical Chemistry of Kazan Scientific Center
of Russian Academy of Sciences, 8, ul. Akad. Arbuzov, Kazan, 420088, Russia
S
* Supporting Information
ABSTRACT: The catalytic activity of two homological series of cationic surfactants
bearing a hydroxyalkyl fragment in the head groups R(CH3)2N+(CH2CH2OH)Br−
and R(CH3)2N+(CH2CH2CH2OH)Br− toward the cleavage of the p-nitrophenyl
esters of carbonic acids of different hydrophobicity (acetate, caprilate, caprinate,
laurate, myristate) is shown to exceed that of typical cationic surfactants with the
trimethylammonium (TMA) headgroup. The catalytic effect increases with the alkyl
chain length of surfactants and nonmonotonous changes in the series: acetate <
caprilate < myristate < laurate < caprinate, reaching 750 times. NMR and IR
spectroscopy studies and the surface potential calculations revealed that the higher
catalytic effect of hydroxyalkylated surfactants is not due to their higher surface
potential and binding capacity toward substrates. This is in line with finding that
binding constants for TMA series are higher than for their hydroxyalkylated
analogues, which was demonstrated by the fitting of kinetic data in terms of the
pseudophase model. The microenvironment factor rather than concentrating effect is responsible for the advanced catalytic
properties of hydroxyalkylated surfactants in the micellar phase.
the dodecyl analogues with the successive introduction of
hydroxyalkyl substituents into the head groups.32 These studies
demonstrated an increase in the aggregative capacity with
hydroxyalkylation, as well as the advantages of such
functionalization from the viewpoint of nanotechnological
applications, that is, the design of nanocontainers, nanoreactors,
and so forth. However, the understanding of the role of
hydroxyalkyl groups in the physicochemical properties of
surfactants is unclear until now. In some cases their effects
are treated in terms of the increase in the volume of head
groups inducing the shielding effect around the charged atom,
while another viewpoint assumes the contribution of hydrogen
bonds to the interactions of head groups with each other and
solubilized compounds. In addition, upon their functionaliza-
tion, changes in the micropolarity of the surface layer occur,
which can strongly affect the surfactant interactions with
different guest molecules. To elucidate the mechanism of the
hydroxylalkyl groups' effect on both the supramolecular
structure and the functional activity of surfactants more
experimental data are necessary.
INTRODUCTION
■
The molecular structure of surfactants, namely, the nature of
the head groups, the length and structure of hydrophobic tails,
as well as solution pH, and the presence of electrolytes or
organic solutes, strongly control their aggregation behavior. In
its turn, the morphology of the surfactant assemblies
determines the thermodynamic stability and physicochemical
properties of the organized systems.1−5 Cationic surfactants are
widely involved in various nanotechnological strategies,6−12 and
therefore much attention is devoted to the correlation of their
chemical structure with their aggregation and functional
activity. The nature of head groups markedly affects the
aggregation, solubilization, and catalytic properties of cationic
surfactants,13−27 with the role of substituents in head groups
capable of H-bonding being of particular interest. These
investigations are stimulated by the wide application of cationic
surfactants as drug and gene carriers, micellar catalysts,
emulsifiers, inhibitors of corrosion, and so forth. The role of
hydroxyalkyl substituents in the DNA condensation mediated
by cationic agents, in the regulation of lipase activity toward the
hydrolysis of ester bonds, and in the formation of polymer−
colloid complexes has been elucidated.28−31 The structural
behavior of the typical cationic surfactant, that is, cetyltrime-
thylammonium bromide, is compared with that of cetyldime-
thylhydroxyethylammonium and cetyldimethylhydroxyethylam-
monium bromides, with the critical micelle concentrations
(cmc's) and thermodynamic parameters of the micellization
determined.17 Similar investigations have been carried out for
Our recent investigations are focused on the catalytic activity
of organized systems in the nucleophilic substitution in
phosphorus acid esters.33−37 Micellar surfactant solutions are
known to strongly influence the properties of solubilized
Received: July 6, 2012
Accepted: September 19, 2012
Published: October 2, 2012
© 2012 American Chemical Society
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compounds including their reactivity,38−41 thereby resulting in
considerable changes in the mechanism and rate of reactions.
The micellar rate effect of surfactants is mainly determined by
noncovalent binding of reagents, which results in concentrating
the reactants in micelles and changes in their environment. The
functionalization of head groups allows us to invoke additional
binding mechanisms, that is, the H-bonding that may enhance
catalytic effect.
Herein, the influence of the structure of cationic surfactants
with hydroxyalkyl fragments in head groups on their
aggregation behavior and catalytic activity in the hydrolysis of
carbonic acid esters has been systematically studied. Two
homological series of ammonium surfactants differing in the
distance of their OH-groups from the cationic center, that is,
R(CH3)2N+(CH2CH2OH)Br− (1a−d) and R(CH3)2N+-
(CH2CH2CH2OH)Br− (2a−d), in which R = n-C12H25 (a),
n-C14H29 (b), n-C16H33 (c), and n-C18H37 (d) has been
investigated and compared with nonfunctionalized cationic
surfactants of trimethylammonium (TMA) series R-
(CH3)3N+Br− (3a−d). Apart from their aggregation behavior,
the kinetics of basic hydrolyses of p-nitrophenyl esters of
carbonic acids differing in hydrophobicity has been measured
(Scheme 1). It can be expected that the substrates were
displaced from the outside to the nonpolar core with an
increase of their alkyl chain length, which would affect their
reactivity.
of pKobs is the mean of three to five independent
determinations obtained at different pH values.
The reaction kinetics was monitored through changes in the
p-nitrophenolate anion absorption at 400 nm. The initial
substrate concentration of 5·10−5 mol·kg−1 was maintained
throughout the kinetic study. The observed rate constants
(kobs) were determined as follows: ln(A∞ − A) = −kobst +
const; here A and A∞ are the absorbances of the micellar
solutions at point t during and after completion of the reaction,
respectively. The kobs values were calculated using the weighed
least-squares computing methods (Figures S4 and S5 of the
Supporting Information, SI). The kobs values were triply or four-
fold measured, and the mean value was taken into
consideration, with the experimental error being within 4 %.
Kinetic theory and quantitative treatment of kinetic data are
given in the SI, Figures S1−S5. The reaction under study, that
is, the nucleophilic substitution in the carbonic acid esters, is a
bimolecular reaction, and generally it is second-order. A
commonly used approach for the kinetic study of such
processes is to carry out them under the excess of one of
reagents, thereby reducing processes to the first-order reactions.
This transformation is supported by the linearity in the ln(A∞
− A) versus t coordinates (Figures S4 and S5 of the SI). To
provide pseudofirst rate conditions, we carried out the reaction
in basic buffer solution, in which the concentration of OH-ions
is much higher as compared to that of substrate. In these
conditions, only the substrate concentration determines the
reaction rate.
Scheme 1. Schematic Representation of the Hydrolysis of p-
Micellar catalysis assumes the distribution of reagents
between two phases (Scheme S1 of the SI).1,45 Herein, kinetic
data were treated in terms of the pseudophase model, which
assumes that micellar solution consists of two pseudophases,
that is, volume phase and a micellar phase. The reagents
(substrate, S, and nucleophile, OH) are distributed between the
two phases in accordance with the partition coefficients
expressed as follows:
Nitrophenyl Esters of Carbonic Acids
P = [S]m /[S]0
PNu = [OH]m /[OH]0
EXPERIMENTAL SECTION
S
■
Chemicals. Commercial alkyltrimethylammonium bromides
3a−d (Sigma), p-nitrophenol, p-nitrophenyl acetate (PNPA),
p-nitrophenyl caprilate (PNPC), p-nitrophenyl caprinate
(PNPCn), p-nitrophenyl laurate (PNPL), and p-nitrophenyl
myristate (PNPM) (Fluka) of 99 % purity were used.
Hydroxyalkylated surfactants 1a−d and 2a−d were synthesized
through the reaction of dimethylaminoethanols with corre-
sponding alkyl bromides.17,42 The structure of surfactants
synthesized was proved by elemental analysis, IR, and H1 NMR
spectroscopy.
Surface tension measurements were performed with the du
Nouy ring detachment methods using tensiometer K6
(Kruss).43
Acid−base properties of p-nitrophenol (PNP) were inves-
tigated by spectrophotometry. A Specord UV−vis instrument
supplied by a temperature-controlled module with quartz
cuvettes of a 1 cm path length was used. Absorption spectra of
PNP were collected for the λ ranging from (250 to 600) nm,
with pH value varying. The observed pKa values (pKa,obs) of
PNP were determined using eq 144
in which the lower indices m and 0 denote the micellar and
bulk phases, respectively.
Depending on the ratio between these contributions, two
types of kinetic plots can be observed. In the case of the
moderate binding of the reagents a plateau is observed in
kinetic curves. In the case of the effective binding of reagents,
the contribution of the reaction in bulk phase is minor, and an
extremum type dependence occurs. The first case is usually
described by eq 2, which is analogous to Michaelis−Menten
equation, while the second type kinetics obeys eq 3. The
maximum in kinetic curves is due to the complete binding of
reagents at low surfactant concentrations, so that the dilution is
observed with further increase in surfactant concentrations
resulting in the decrease in the reaction rate.
kmKSC + k0
1 + KSC
kobs
=
(2)
here KS is the binding constant of substrate (S) associated with
the partition coefficient PS = [S]m/[S]0 as follows: KS = (PS −
1)V; indices 0 and m denote the water and micelles,
respectively; V is the molar volume of micellar pseudophase;
k0 and km (s−1) are the pseudofirst rate constants in water and
micelles, respectively; C is the surfactant concentration minus
cmc.
[PNP]
pKa,obs = pH + log
[PNP−]
(1)
where [PNP] and [PNP−] are the molar concentrations of the
neutral form and p-nitrophenolate ion, respectively. Each value
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k2,0 + kmKSKOH
(1 + KSC)(1 + KOHC)
Article
C
k′obs
=
(3)
in which k′obs is the second-order rate constant obtained by
division of the observed pseudofirst rate constant (kobs) by the
total nucleophile concentration; k2,0 is the second-order rate
constant of the reaction in water, km (= k2,m/V) characterizes
the reactivity in the micellar phase; C is the total surfactant
concentration minus cmc. To determine the true bimolecular
rate constant for the reaction in the micellar phase, k2,m, a value
for the volume of the micellar pseudophase, should be
estimated. The value V of 0.3 kg·mol−1 is taken for the
surfactants based on their molecular weight and density,46 with
no corrections introduced for the water in solvation shells. KS
and KOH are the binding constants of the substrate and
nucleophile.
To differentiate factors responsible for the micellar effects eq
3 is reduced to eq 4.45 Equations 3 and 4 are valid for the
systems with the extremum type of the kobs versus C curve. In
this case, a considerable acceleration of reactions occurs in
micellar systems as compared to molecular solution, that is, the
contribution of the reaction in the bulk phase, is neglected. The
maximum rate acceleration can be calculated as the ratio
between the pseudofirst rate constants in the micellar system
and water, respectively (eq 4):
Figure 1. Surface tension isotherms of hydroxyethylated cationic
surfactants; 25 °C. , 1a; , 1b; , 1c; , 1d.
□
■
○
●
discontinued at a definite concentration, which is generally
called a critical micelle concentration (cmc). Hydroxyalkylated
surfactants demonstrate lower cmc values as compared to
reference compounds 3a−d (Table 1). This trend agrees well
with refs 23 and 50 which indicates that micellization is
probably favored by the H-bonding between head groups and
their more effective solvation. Meanwhile, the main factor
contributing to the aggregation is the hydrophobicity of
surfactants controlled by the alkyl chain length. Indeed, for
all of the surfactant series the cmcs decrease by a factor of 10
with the transition from dodecyl to octadecyl tails (Table 1).
The micellar solutions of cationic surfactants provide a wide
range of possibilities for the control of the rate of ion-molecular
reactions, including the basic hydrolysis of esters. These
processes generally proceed in the Stern layer of micelles and
strongly depend on the sign and magnitude of the micellar
surface potential. In the case of cationic surfactants, the
concentration of anionic nucleophiles can occur in the reaction
zone, which results in the acceleration of the reaction. Figures 2
to 5 summarize kinetic data on the hydrolyses of esters in the
cationic surfactants studied. The catalytic effect is manifested
on reaching the cmc and increases with an increase in the alkyl
chain length of surfactants, which is typical for the reactivity in
micellar systems. For all the surfactants studied the micellar rate
effect increases by an order of magnitude on transition from
dodecyl to octadecyl derivatives (e.g., see Figures 2 and 3 for
PNPA and PNPC, respectively). Surfactants with hydroxyethy-
lated head groups are markedly more effective than their
nonfunctionalized analogues, while hydroxypropylated surfac-
tants are intermediate. The catalytic effect expressed as the ratio
⎛
⎜
⎝
⎞
⎟
⎠
k2,m
k2,0
kobs
k0
KSKOH
=
·
2
V( KS + KOH
)
(4)
max
The first multiplier on the right characterizes the influence of
the micellar microenvironment (Fm), and the second factor
determines the concentration of the reagents in micelles (Fc).
NMR experiments were performed with a Bruker AVANCE-
600 spectrometer (14.1 T) equipped with a 5 mm diameter
gradient inverse broad band probehead and a pulsed gradient
unit capable of producing magnetic field pulse gradients in the
z-direction of 0.535 T·m−1. Frequencies are 600.13 MHz in 1H
and 150.90 MHz in 13C experiments. Chemical shifts are
reported in the δ (ppm) scale relative to the residual signal of
solvents [HDO (4.7 ppm); CD3OD (3.31 and 49.0 ppm, for
1H and 13C, respectively)]. Experiments were carried out at (30
0.2) °C. To avoid temperature fluctuations owing to the
sample heating by the magnetic field pulse gradients high
airflow was used (0.149 s−1). The Fourier transform pulsed-
gradient spin−echo (FT-PGSE)47−49 experiments were per-
formed by the BPP-STE-LED sequence.47 Data were acquired
with a (50.0 or 120.0) ms diffusion delay, with bipolar gradient
pulse duration from (2.2 to 6.0) ms, 1.1 ms spoil gradient pulse
(30 %), and a 5.0 ms eddy current delay. The bipolar pulse
gradient strength was incremented from (0.01 to 0.32) T·m−1
in 32 steps.
The FT-PGSE experiments were performed in triplicate, and
only the data with the high (>0.999) correlation coefficients of
ln I/I0 versus b were included [I/I0, the normalized signal
attenuation; b = γ2δ2g2(Δ − δ/3); γ, the gyromagnetic ratio; g,
the pulsed gradient strength; Δ, the interval between the
pulsed-gradients; δ, the pulsed gradient duration]. All peaks
were analyzed, and the values were averaged.
k
obs/k0 is markedly higher for hydroxyalkylated surfactants as
compared to TMA analogues (Figure 6: exemplified by
hexadecyl derivatives) and changes as follows: 1c > 2c > 3c.
It is noteworthy that, due to a certain acidity of hydroxyalkyl
groups (pKa (12.4 to 12.6)), these surfactants can exist in the
form of zwitterions in strong alkali solutions, thus showing a
nucleophilic activity.51,52 However under the experimental
conditions maintained in this study (pH 9.0) hydroxyalkylated
surfactants behave as typical cationic compounds. The above-
mentioned advantages of their catalytic activity over non-
RESULTS AND DISCUSSION
■
Tensiometry data exemplified by the surface tension isotherms
for cationic surfactants 1a−d (Figure 1) show that they exhibit
a marked surface activity, with all σ vs C dependences being
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Table 1. Values of cmc, the Surface Potential, Binding Constants, and pKa,m of p-Nitrophenol for Micelles of Cationic
Surfactants 1a−d and 2a−d
surfactant
R
cmc/mol·kg−1
KA/kg·mol−1
KB/kg·mol−1
pKa,m
Ψ/mV
1a
1b
1c
1d
2a
2b
2c
2d
3c
C12H25
C14H29
C16H33
C18H37
C12H25
C14H29
C16H33
C18H37
C16H33
0.0071
0.0049
600
620
13700
26600
67600
5.79
5.52
5.42
107
123
129
a
0.00070
0.00012
0.0068
0.0034
1320
290
395
6150
22800
78600
17300
5.83
5.39
5.30
5.32
105
131
136
133
a
0.00065
0.000096
1100
0.00080
230
a
cmc values for 1c equal 0.0072 mol·kg−1 (fluorometry), 0.0078 mol·kg−1 (conductometry), 0.0083 mol·kg−1 (microcalorimetry);43 cmc for 2c
equals 0.008 mol·kg−1 (conductometry).23
Figure 2. Observed rate constants of basic hydrolyses of PNPA (a) and PNPC (b) as a function of the concentration of cationic surfactants; pH 9.0;
□
■
○
●
25 °C; , 1a; , 1b; , 1c; , 1d.
functionalized analogues can be due to the differences in the
surface potential, which is responsible for the concentration of
hydroxide-ions in the reaction zone. The surface potential was
determined from the pKa,obs values of p-nitrophenol (PNP) by
the spectrophotometry method.23,53 Neutral form of PNP
demonstrates a lower affinity toward a micelle as compared to
the anionic form. The latter can be effectively bound by cationic
micelles by electrostatic interactions, which results in a decrease
in the observed pKa value in cationic micelles (Figure 7). The
observed constant of the acid−base dissociation Kobs depends
on the surfactant concentration as follows:45
minus cmc. The quantitative analysis of the (Ka,0 − Kobs)/C vs
obs dependence by the least-squares method gives the values of
binding constants. With an increase of the surfactant
concentration pKa,obs approximates to pKa,m; here pKa,m is the
pKa in the micelle. Table 1 summarizes the cmc values and
binding constants of neutral and anionic forms of PNP
calculated by eq 5.
K
Equation 6 gives the relation between the pKa,m value and the
surface potential:
Fψ
2.303RT
pKa,m = pKa,i
−
(6)
1 + KBC
1 + KAC
Kobs
=
·Ka,0
where pKa,i is the intrinsic pKa of the probe in the absence of an
electrostatic field, and F is the Faraday constant. The pKa,i value
can be equated to nonionic surfactants or modeled by addition
of electrolytes to the surfactant solutions. Herein the pKa value
of 7.6 for PNP in the Triton-X-100 solution is used.
(5)
where Ka,0 is the constant of the acid−base dissociation in
water; KA and KB are the binding constants of the acid and base
forms of the compound; C is the surfactant concentration
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Figure 3. Observed rate constants of basic hydrolyses of PNPA (a) and PNPC (b) as a function of the concentration of cationic surfactants; pH 9.0;
□
■
◆
◇
●
25 °C; , 2b; , 2c; , 2d; , 3b; , 3c.
binding constants of both forms increase with the alkyl chain
length of surfactants, which is probably due to the increase in
their solubilization capacity. Surface potentials somewhat
increase with an increase in the alkyl chain length,
demonstrating only a slight dependence on the presence of
hydroxyalkyl groups. These data somehow disagree with the
assumption29 that the decrease in the cmcs of hydroxyalkylated
surfactants is due to the shielding effect of head groups toward
the charged nitrogen. To compare the surface potentials and
observed rate constants of the hydrolysis of PNPA and PNPC
for three surfactants with the same alkyl chain length (e.g., for
1c, 2c, and 3c, see Table 1 and Figures 2 and 3) it is clear that
the Ψ value is slightly lower in the case of hydroxyethylated
surfactant versus the conventional TMA-analogue, while the
observed rate constants for the latter are ca. 10-fold lower. Thus
the surface potential is not the main factor determining the
catalytic activity of hydroxyalkylated surfactants. We assume
that the electrophilicity of the carbon atom in the carbonyl
group and hence the reactivity of substrates can increase due to
their H-bonding with the hydroxyalkyl fragment of surfactants.
To gain an insight into the substrate−surfactant interactions,
hexadecyl derivatives of hydroxyalkylated surfactants 1c and 2c
and conventional TMA analogue 3c were investigated by NMR
self-diffusion coefficient (Ds) measurements in both the
presence and the absence of substrates.
Figure 4. Observed rate constants of basic hydrolyses of PNPA as a
□
function of solution pH without any cationic surfactants ( ) and in
−1
the presence of 0.0025 mol·kg−1 ( ) and 0.005 mol·kg ( ) of 1c;
25 °C. Inset shows the effect of pH on the catalytic effect of 1c toward
the basic hydrolyses of PNPA.
○
●
For T = 298 K eq 6 can be transformed to:
The effective method for characterization of association of
amphiphiles is NMR self-diffusion.54,55 The association process
can be monitored through the difference in the self-diffusion
coefficients (DS) for the amphiphile monomers and aggregates.
The translational mobility of an amphiphilic species decreases
by 1−2 orders of magnitude, when it is bound by the micelle.
Due to a fast exchange in the NMR time scale between
Ψ = 0.0591(pKa,i − pKa,m
)
(7)
The binding constants of the anionic form of PNP by
micelles of the surfactants studied are ca. (20 to 70)-fold higher
as compared to the neutral form (Table 1). This fact is
responsible for the lower pKa values of PNP in the micellar
phase versus water by the (1.4 to 1.9) logarithmic unity. The
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Figure 5. Observed rate constants of basic hydrolyses of carbonic acid esters as a function of the concentration of cationic surfactants 1c (a); 2c (b);
■
□
◆
●
○
3c (c); pH 9.0; 25 °C; , PNPA; , PNPC; , PNPCn; , PNPL; , PNPM.
amphiphiles in the micelles and the bulk phase two-site model
can be used
diffusion coefficient of the “free” surfactant in the aqueous
phase.
First, the 0.005 mol·kg−1 surfactant solutions 1c, 2c, and 3c
were investigated (Table 2). Then to these samples, PNPA was
added, and similar NMR measurements were also carried out
(Table 2). Thus the sizes of aggregates were estimated based on
self-diffusion coefficients both prior to and after the substrate
D
= DmicPmic + Dfree(1 − Pmic)
(8)
sur
in which Dsur is the time-averaged diffusion coefficient of the
surfactant, Pmic is the fraction of micellized surfactant molecules,
Dmic is the diffusion coefficient of the micelle, and Dfree is the
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addition. In single surfactant solutions, the hydrodynamic
radius of micelles lies within the interval of (2.2 to 2.4) nm,
which corresponds to the aggregation numbers of (31 to 39) in
spherical approximation. The addition of the substrate PNPA
results in the increase of the size of aggregates by (5 to 15) %.
The DS value of the substrate upon its solubilization by
surfactant micelles should decrease due to the contribution of
self-diffusion coefficients of the bound substrate (eq 7).
Therefore the comparison of the DS value of free substrate
(i.e., in the absence of surfactants or below the cmc) with that
in micelles makes it possible to estimate the fraction of the
bound substrate, Pb (Table 2). As can be seen, Pb reaches 24 %
at the surfactant concentration of 0.005 mol·kg−1. This value
exceeds the cmc (Table 1) but lies below the plateau kobs region
in Figures 2, 3, and 5. Therefore Pb can considerably increase
with an increase in the surfactant concentration in the case of
PNPA and especially for the more hydrophobic esters, for
which the complete binding can occur. At the same time, the
self-diffusion coefficients of the substrate and hence its
solubilization by micelles practically do not depend on the
structure of head groups. The lack of any specific interaction
between the substrate and surfactant head groups is also
supported by the 13C chemical shifts. Namely, the electro-
philicity of the carbon atom is mostly determined by the
polarization of carbonyl group, which can be monitored in its
13C chemical shift.56 Essentially similar chemical shifts observed
for carbonyl group of acetate in D2O (172.4 ppm) and
surfactant (171.9 to 172.4) ppm solutions indicate that there is
the lack of change in charge density at carbon atom upon
substrate solubilization.
Figure 6. Micellar rate effect of cationic surfactants 1c, 2c, and 3c
toward the basic hydrolyses of p-nitrophenyl esters of carbonic acids.
Above investigations were carried out in aqueous solution.
Meanwhile, in the presence of micelles, the reaction in the
micellar pseudophase, that is, in the nonpolar microenviron-
ment, mainly contributes to the observed rate constant.
Therefore the above conclusion on the lack of the head-
group/substrate interactions may be invalid in nonpolar
microenvironment. Besides, hydrogen bonds between carbonyl
oxygen and OH groups of the substrate in water will be
suppressed by competitive interactions of these species with
water. Therefore the 13C NMR- and IR-spectroscopy studies
were carried out for 0.001 mol·kg−1 PNPA single solution
(CDCl3) and for (0.005 and 0.01) mol·kg−1 solutions of 1c and
3c (CDCl3). It turned out that there are almost no differences
in the 13C spectra of single substrate and micellar (1c and 3c)
substrate solutions. Similarly, in the IR spectrum of the
substrate the carbonyl stretching absorption is observed at 1720
cm−1 regardless of the presence and type of surfactants added.
Therefore, this experiment contradicts the assumption that the
activation of substrates occurred by hydroxyalkylated surfac-
Figure 7. Dependence of pKa of p-nitrophenol on the hydroxypropy-
lated surfactant concentration , 2b; , 2c; , 2d. Inset shows
□
■
●
○
dependence of pKa of p-nitrophenol on the concentration of
Triton-X-100 and , 1b; , 1c; , 1d.
,
□
■
●
Table 2. NMR Self-Diffusion Data (Self-Diffusion Coefficients DS, Hydrodynamic Radius Rh, Fraction of the Bound Substrate
Pb, and Aggregation Number Nagg for Components of the Systems Based on Cationic Surfactants in the Presence and Absence
of PNPA; 30 °C
a
surfactant
CS/mol·kg−1
DS/(1010 m2·s−1) (surfactant)
DS /(1010 m2·s−1) (substrate)
Rh/A (micelle)
Pb
Nagg
1c
0
1.18
1.14
1.26
1.06
1.27
1.15
23.5
24.4
22.0
26.2
21.9
24.0
39
43
32
54
31
42
0.001
0
7.32
6.95
7.36
0.19
0.24
0.18
2c
3c
0.001
0
0.001
a
DS of free (nonmicellized) PNPA equals 8.76·10−10 m2·s−1.
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Table 3. Quantitative Treatment of Kinetic Data of the Reaction of the Esters in Surfactant Solutions of 1c, 2c, and 3c (Figure
5) in Terms of Equation 2 (pH 9.0, 25 °C)
a
surfactant
substrate
km/s−1
KS/kg·mol−1
cmc/(104 mol·kg−1
)
km/k0
1c
PNPA
PNPC
PNPCn
PNPL
PNPM
PNPA
PNPC
PNPCn
PNPL
PNPM
PNPA
PNPC
PNPCn
PNPL
PNPM
0.0263
0.0185
0.0375
0.0116
0.0102
0.0082
0.0092
0.0093
0.00655
0.00201
0.00168
0.0017
0.00208
0.00103
0.00074
260
1050
1900
990
2.07
3.75
3.4
80
185
750
290
255
25
4.86
1.58
5.7
600
2c
3c
240
720
1.2
92
2350
2000
1450
960
0.34
1.97
3.11
4.67
1.80
2.81
0.69
0.82
186
161
50
5.1
17
4500
18400
19100
5100
50
26
19
a
km/k0 is the acceleration of the reaction as compared to basic hydrolyses of the substrates under a solution pH of 9.0.
Table 4. Results of the Quantitative Treatment of Kinetic Data for Hydrolyses of PNPCn and PNPL in Micellar Solutions of
Cationic Surfactants 1c, 2c, and 3c (Figure 5) in Terms of eq 3; pH 9.0; 25 °C
substrate
surfactant
k2,m/kg·mol−1·s−1
KOH/kg·mol−1
KS/kg·mol−1
(kobs/k0)max
Fm
Fc
Fm·Fc
PNPCn
PNPL
1c
1c
2c
2c
3c
3c
10
100
45
6100
4200
550
2.0
266
124
116
123
346
247
531
230
170
118
36
7.4
232.5
172
1.25
1.45
0.96
0.10
0.01
PNPCn
PNPL
5.8
40
7100
3.9
44
5650
145
PNPCn
PNPL
0.52
0.39
130
140
10300
10100
38.4
22.8
24
tants through H-bonding. Another explanation of the influence
of hydroxyalkyl moieties on the reactivity of alkanoates may be
based on accompanying changes in the micropolarity of the
reaction site. However, as the monitoring of the steady-state
fluorescence emission spectra revealed, the I1/I3 ratio as a
measure of the micropolarity only slightly changes by
introducing one-by-one hydroxyalkyl moieties into the head
groups of 3c.28
It can be concluded that none of the above factors, that is,
changes in the surface potential or micropolarity and the
activation of the substrate by the OH-group of the head groups,
are responsible for the rate enhancement induced by the
functionalization of the surfactant head groups. Probably, more
detailed investigations involving modeling the transition state of
the reaction are required.
may occur toward the micellar interior, thus affecting the
reaction rate. As a whole, an increase in the surfactant
hydrophobicity results in the increase in the observed rate
constants and the catalytic effect (Figure 2, 3, and 5). The
increase in the substrate hydrophobicity exerts a non-
monotonous effect, so that the decrease in the reactivity occurs
in the case of the most hydrophobic esters, which can be due to
the micellization of high homologues.57 In the presence of
cationic surfactants, mixed micelles are formed instead of
individual substrate associates, thus influencing the reactivity of
amphiphilic esters.23,40 The micellar rate effect on the
hydrolysis of substrates differing in their hydrophobicity is
exemplified by hexadecyl derivatives of surfactants 1−3 (Figure
5). For all of the substrates their reactivity changes in the
following order 1c > 2c > 3c; that is, the highest catalytic effect
is observed in the case of the hydroxyethylated surfactant 1c.
Further information can be obtained from the quantitative
treatment of kinetic data (Figures 2, 3, and 5) in terms of the
pseudophase model (eq 2). The results of analysis (Table 3)
reveal that the so-called kinetic cmc's extracted from the fitting
procedure are lower than those obtained by tensiometry. This
discrepancy can be due to (i) the contribution of the
premicellar assemblies to the micellar rate effect and (ii) the
promotion of the micellization by the additives of organic
substrates. Indeed, the more marked decrease in the kinetic
cmc is observed for the more hydrophobic esters. It is
noteworthy that the binding constants of substrates with
hydroxyalkylated surfactants 1c and 2c are similar, while those
with TMA analogues are (3 to 4)-fold higher. This is another
argument in favor of the conclusion that the higher catalytic
The high catalytic activity of hydroxyethylated surfactants 1c
and 1d let us to carry out kinetic studies in mild alkali
conditions (about pH 9.0), thereby eliminating side reactions
contributed by the hydroxyl-groups of surfactants. According to
these data (Figure 4) the observed rate constants of hydrolysis
of PNPA in alkali solutions increase with the addition of
hydroxyethylated surfactant 1c, with the catalytic effect (kobs
/
k0) depending on the solution pH. A rather sharp increase in
the catalytic activity with an increase in pH can be due to the
contribution of the nucleophiliciy of hydroxyethyl groups of the
surfactant at strong alkali conditions.
Apart from nonassociated substrates PNPA and PNPC the
kinetic studies of PNPCn, PNPL, and PNPM prone to
aggregation were also carried out in micellar solutions of
surfactant series 1−3 at pH 9.0. It can be assumed that with an
increase in the hydrophobicity of substrates their dislocation
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activity of hydroxyalkylated surfactants is not related to their
higher binding capacity toward the substrates.
potential and binding capacity toward substrates. This is in line
with the simulation of kinetic data in framework of
pseudophase model, which demonstrated the higher binding
constants for the TMA series as compared to hydroxyalkylated
analogs. Fitting parameters showed that microenvironment
factor rather than concentrating effect is responsible for the
advanced catalytic properties of hydroxyalkylated surfactants.
Equation 2 describes the reaction kinetics for the first-order
rate constants demonstrating the plateau-type concentration
dependence. In this case only the substrate distribution
between two phases is taken into account, and therefore the
reaction kinetics is treated in terms of the substrate binding
constant and the first-order rate constant in the micellar phase.
However, basic hydrolysis is related to the bimolecular reaction,
although the pseudofirst conditions were maintained. Indeed,
many concentration dependences in Figures 2, 3, and 5 exhibit
an extremum-type shape, which is typical for ion-molecular
reactions in cationic micelles in the case of the strong binding
of the substrate.45 These dependences were also treated in
terms of eq 2, with part of the curves before maximum being
considered. To obtain a deeper insight into the catalytic
mechanism, eq 3 was used for the quantitative analysis of
selected dependences. The calculated values of binding
constants for both reagents and second order rate constants
for reactions of hydrophobic substrates PNPCn and PNPL are
summarized in Table 4. The effective binding of substrates is
supported by the high values of KS ranging from (4000 to
10000) kg·mol−1, while the binding of hydrophilic nucleophile
is much weaker. The above-mentioned reverse correlation
between the binding constants and catalytic activity obtained by
fitting eq 2 to the kinetic data has been confirmed by
calculations with eq 3; that is, higher binding constants are
obtained for nonfunctionalyzed surfactants of the TMA type,
while their catalytic effect is lower as compared to their
hydroxyalkylated analogues.
In aqueous micellar solutions, the concentration factor Fc (eq
4) is the main contributor to the micellar rate effect, while the
micellar microenvironment exerts a negative effect upon the
nucleophilic substitution, with the Fm decreasing with the
substrate hydrophibicity.45 Such behavior is actually observed in
the case of the nonfunctionalized surfactant (Table 4); namely,
the concentration factor exceeds by 2 orders of magnitude.
However due to the decrease in the second-order rate constant
in the micellar pseudophase as compared to water the resulting
effect ranges from (20 to 45) times depending on the
substrates. For the case of the hydroxyalkylated surfactant
similar values of the concentration factor are also observed
(Table 4), while unlike the TMA series, Fm > 0, which results in
a marked increase in catalytic effects (550-fold acceleration).
This clearly indicates that the advantage in catalytic activity of
hydroxyalkylated surfactants is due to the activation of the
substrates. Probably, hydroxyalkyl fragments provide more
polarity in the reaction site as compared to the TMA series.
ASSOCIATED CONTENT
■
S
* Supporting Information
Kinetic theory, calculations of the rate constants, and the
binding constants of reagents in the micellar pseudophase. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: lucia@iopc.ru; fax: +7-843-2732253.
Notes
The authors declare no competing financial interest.
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