Aggregation and catalytic activity of 2-hydroxybenzylated polyethyleneimines in water-organic solutions

Article abstract of DOI:10.1007/s11172-010-0243-8

The aggregation of 2-hydroxybenzylated derivatives of polyethyleneimine in a water-DMF medium in the absence and in the presence of cationic surfactants was studied. The catalytic activity of modified polyethyleneimines in hydrolysis of phosphorus acid esters depends on the pH of the medium and increases with the formation of polymer-colloidal complexes. The polymer-colloidal complexes based on surfactants with two hydroxyethyl substituents in the head group have the maximum catalytic activity (increase in the reaction rate by up to three orders of magnitude).

Full text of DOI:10.1007/s11172-010-0243-8

1
336
Russian Chemical Bulletin, International Edition, Vol. 59, No. 7, pp. 1336—1342, July, 2010
Aggregation and catalytic activity
of 2ꢀhydroxybenzylated polyethyleneimines in waterꢀorganic solutions
G. A. Gaynanova, A. V. Yurina, S. S. Lukashenko, E. P. Zhil´tsova, L. Ya. Zakharova,
†
L. A. Kudryavtseva, and A. I. Konovalov
A. E. Arbuzov Institute of Organic and Physical Chemistry,
Kazan Research Center of the Russian Academy of Sciences,
8
ul. Akad. Arbuzova, 420088 Kazan, Russian Federation.
Fax: +7 (843) 273 2253. Eꢀmail: gaynanova@iopc.knc.ru
The aggregation of 2ꢀhydroxybenzylated derivatives of polyethyleneimine in a water—DMF
medium in the absence and in the presence of cationic surfactants was studied. The catalytic
activity of modified polyethyleneimines in hydrolysis of phosphorus acid esters depends on the
pH of the medium and increases with the formation of polymerꢀcolloidal complexes. The
polymerꢀcolloidal complexes based on surfactants with two hydroxyethyl substituents in the
head group have the maximum catalytic activity (increase in the reaction rate by up to three
orders of magnitude).
Key words: 2ꢀhydroxybenzylated polyethyleneimines, surfactant, aggregation, catalysis,
hydrolysis.
The modern level of technical progress provides the
The works on studying the interactions in systems based
on polyethyleneimines (including those hydrophobically
modified) and cationic surfactants have appeared in the
development of new approaches to the solution of probꢀ
lems in the field of catalysis. One of the promising trends
is the use of supramolecular systems, whose biomimetic
character allows one to achieve high efficiency and selecꢀ
tivity of catalysts under mild conditions.1 A tendency to
the development of polycomponent compositions, whose
catalytic activity can be controlled by varying the nature
and ratio of reactant concentrations, is observed in the
1
2,18—21
recent time.
In aqueous solutions of such systems,
electrostatic interactions exert an unfavorable effect on
the formation of PCC, and combined structures are formed
—3
1
8,21
due to ionꢀdipole and hydrophobic interactions.
Modified polyethyleneimines are of certain interest.
For instance, additional hydrophobic fragments (benzyl,
alkyl) in a PEI molecule enhance their aggregation activiꢀ
ty and ability to complex formation toward molecules and
ions of lowꢀmolecularꢀweight organic compounds, as well
as the catalytic activity in hydrolysis of carboxylic and
4
,5
recent time. Nowadays the study of the properties of
polymerꢀcolloidal solutions is an intensely developing area
of supramolecular chemistry. Interest to the systems based
on surfactants and polymers is defined by their wide appliꢀ
cation in the production of detergents, oil regeneration,
2
2,23
phosphorus acid esters.
6
cosmetic industry, etc. From the viewpoint of biomiꢀ
In the present work, we investigated the selfꢀorganiꢀ
zation and catalytic activity of 2ꢀhydroxybenzylated
metics, surfactant—polymer systems are models of proꢀ
tein—lipid interactions. Among many polymers, we noꢀ
ticed polyethyleneimines (PEI), which are of practical inꢀ
terest because of their use as corrosion inhibitors, demulꢀ
polyethyleneimine derivatives 1—3 in an H O—DMF
2
(30 vol.% DMF) medium both as individual solutions and
as mixed compositions with cationic surfactants.
sifiers,7 biologically active substances, complexing
,8
7
9
10
agents, and carriers of drugs.
It is known that polyethyleneimines can form aggreꢀ
gates in individual solutions, and polymerꢀcolloidal comꢀ
1
1,12
plexes (PCC) are formed in the presence of surfactants.
In addition, it is shown that PEI and polymerꢀcolloidal
complexes based on them act as catalysts of hydrolysis of
phosphorus acid esters.1
2,13
Many works are devoted to
the study of systems consisting of PEI and anionic surfacꢀ
1
3—17
tants (for example, PEI—sodium dodecyl sulfate
).
†
Deceased.
m = 0.12 (2), 0.16 (3)
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1306—1312, July, 2010.
066ꢀ5285/10/5907ꢀ1336 © 2010 Springer Science+Business Media, Inc.
1

2
2
ꢀHydroxybenzylated polyethyleneimines
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 7, July, 2010
1337
Cetyltrimethylammonium bromide (CTAB) and its
ꢀhydroxyalkyl derivatives, viz., cetyl(2ꢀhydroxyethyl)ꢀ
The specific electroconductivity (χ) of polymer and polyꢀ
merꢀcolloidal systems was measured on a CDMꢀ2d conductoꢀ
meter (Denmark). The critical association concentrations (CAC)
were determined by the break in the plot of the electroconducꢀ
tivity vs concentration of a polymer or amphiphilic compound.
dimethylammonium bromide (CHAB), cetyldi(2ꢀhydrꢀ
oxyethyl)methylammonium bromide (CDHAB), and
cetyl(2ꢀhydroxypropyl)dimethylammonium bromide
The electroconductivity of the H O—DMF (30 vol.% DMF)
2
(
CPAB), were chosen as cationic surfactants.
solution was accepted to be zero.
The effective hydrodynamic radius (R ) of aggregates in soꢀ
H
Experimental
lution was determined on a Photocor Complex instrument of
dynamic and static light scattering. A He—Ne gas laser with
a power of 10 mW and a wavelength of 633 nm served as
a radiation source. For each system, the hydrodynamic radius
was calculated as an arithmetical average of three measurements.
The measurement inaccuracy was not higher than 10%.
2
ꢀHydroxybenzylated PEI were synthesized from branched
polyethyleneimine (molecular weight 10 000) and 2ꢀdimethylꢀ
aminomethylphenol or 2ꢀdimethylaminomethyl(4ꢀisononyl)ꢀ
phenol (transamination reaction) by refluxing the reactants for
5
—10 h in pꢀxylene with the removal of volatiles followed by
distillation of the solvent and evacuation at 150 °C to a constant
Results and Discussion
weight. The samples synthesized were characterized by IR specꢀ
1
troscopy and Н NMR spectroscopy. The starting unsubstituted
The electroconductivity of 2ꢀhydroxybenzylated PEI
PEI (Fluka) were used without preliminary purification. The
molecular weight of the monomeric unit of 2ꢀhydroxybenzylꢀ
ated PEI was determined by potentiometric titration on
a pHꢀ150MA instrument, the measurement error being 0.01 pH
units. The fraction of free amino groups of polyelectrolytes (α) at
different pH values was calculated by the Henderson—Hasselꢀ
bach equation.24
1
and 2 was measured in an H O—DMF medium in the
2
absence (Fig. 1) and in the presence (Fig. 2) of CHAB.
The conductometric plots have two breaks, which are comꢀ
monly attributed to the onset of aggregation in the soluꢀ
tion (CAC ) and structural rearrangements in the system
CAC ). In the absence of a surfactant, the run of the
conductometric plots reflects the change in the number of
charge carriers formed due to the acidꢀbase interactions of
the amino groups of PEI with water. The decrease in the
slope of the plots after the start of PEI aggregation and
structural rearrangements (see Fig. 1) associated with the
formation of intramolecular micelles or intermolecular
aggregates is due to a decrease in the accessibility of the
amino groups for similar interactions.
1
4
(
2
4
ꢀNitrophenylbis(chloromethyl)phosphinate (4) was syntheꢀ
2
5
sized according to a known procedure. The purity of the subꢀ
strate was checked by the elemental analysis data and by the
correspondence of the rate constants of alkaline hydrolysis and
melting points to the literature data.
Cetyltrimethylammonium bromide (Sigma) with the major
substance content 99% was used. Cationic surfactants (CHAB,
CDHAB, CPAB) were synthesized according to known proꢀ
cedures.2
6,27
The kinetics of hydrolysis of substrate 4 was studied spectꢀ
rophotometrically on a Specord UVꢀVIS instrument at difꢀ
ferent pH values and the initial substrate concentration
In the presence of surfactants, the slope of the conducꢀ
tometric plots after each break increases (see Fig. 2). Perꢀ
–
5
–4
–1
5
•10 —1•10 mol L . The latter was achieved by the introꢀ
duction of 2—10 μL of a phosphinate solution in ethanol
χ/μS cm–1
χ/μS cm–1
–
1
(
0.02 mol L ) into quartz cells (thickness 0.5 cm, 1.0 cm) conꢀ
taining a solution of PEI or a PEI—surfactant mixture. The reꢀ
action course was monitored by a change in the absorbance of
solutions at the wavelength 400 nm (formation of the 4ꢀnitroꢀ
phenolate anion). The apparent (observed) rate constants for the
4
0
160
–
1
firstꢀorder reaction (kapp/s ) were calculated using the regresꢀ
sion method by the equation: ln(D – D ) = –0.434kappt + const,
30
20
10
120
∞
t
where D and D are the absorbance values at the completion of
∞
t
1
the reaction and at the time moment t.
8
4
0
0
0
The kinetic dependences were analyzed in terms of Eq. (1)
2
8
2
of the pseudophase model of micellar catalysis :
,
(1)
where kapp/s–1 is the apparent rate constant for the firstꢀorder
–
1
–1
reaction; k /s and k /s are the rate constants for the
0
m
firstꢀorder reaction in the solvent bulk and in the micellar
0
0.004
0.008
0.012
СPEI/mol L–1
–
1
phase, respectively; K /L mol is the binding constant of
b
the substrate with the micelle; С /mol L–1 is the surfacꢀ
Fig. 1. Plots of the specific electroconductivity (χ) vs concentraꢀ
tion of PEI 1 (1) and 2 (2) (H O—DMF (hereinafter, 30 vol.%
DMF), 30 °C).
Surf
tant concentration; CMC/mol L–1 is the critical micelle conꢀ
centration.
2

1338
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 7, July, 2010
Gaynanova et al.
χ/μS cm–1
χ/μS cm–1
α
3
2
1
1
0
0
0
0
.0
.8
.6
.4
.2
0
600
500
400
300
200
100
1
1
1
1
1
700
600
500
400
300
2
1
5
6
7
8
9
pH
Fig. 3. Plots of the fraction of free amino groups (α) vs pH of the
medium for solutions of PEI 1 (1) and 3 (2) and a 1—CDHAB
–
3.5
–3.0 –2.5
–2.0 logCCHAB
–1
–1
system (3) (СPEI = 0.01 mol L , СCDHAB = 0.01 mol L
,
Fig. 2. Plots of the specific electroconductivity (χ) of solutions of
PEI 1 (1) and 2 (2) in the presence of CHAB vs surfactant
H2O—DMF, 25 °C).
concentration (СPEI = 0.01 mol L–1, H O—DMF, 30 °C).
2
based on less hydrophobic PEI 1 (see Fig. 2). The depenꢀ
dence on the PEI structure is also observed for the CAC
values. In the absence and in the presence of a surfactant,
an increase in the hydrophobicity of the polymer faciliꢀ
tates association in the system (Table 1).
haps, this is due to a decrease in the degree of binding of
surfactant counterions upon the formation of mixed surꢀ
factant—polymer ensembles.
It looks likely that at low polymer concentrations agꢀ
gregation occurs predominantly due to interactions inside
The dynamic light scattering method was used to obꢀ
tain the effective radii of particles of PEI 3 at different
one polymer chains (CAC ), whereas an increase in the
1
concentrations of the polymer in an H O—DMF medium
2
concentration increases the probability of bonding between
polymer molecules (formation of supramolecular strucꢀ
tures) due to hydrophobic interactions and hydrogen bonds
at 25 °C. This plot passes through a maximum value of
–1
1
25 nm at CPEI = 0.03 mol L .
СPEI/mol L–1
R /nm
(
CAC ). The contribution of hydrogen bonds is determined
Н
2
0
0
0
0
.01
.02
.03
.05
85
110
125
72
by the number of free amino groups of the polymer, whose
fraction depends on the pH of the medium. The fraction
of the free amino groups (α) of PEI 1 and 3 in an
H O—DMF medium (Fig. 3) was calculated using the
2
Henderson—Hasselbach equation by the potentiometric
titration curves. At high pH values, polyethyleneimines
are almost neutral polymers. An increase in hydrophobicꢀ
ity of the polymer and the addition of a surfactant exert
a weak effect on the α value, although an increase in the
fraction of free amino groups could be expected in the
presence of cationic surfactants. It is known29 that the рКа
values of the compounds decrease in cationic micelles.
Another feature of the conductometric plots of surfacꢀ
tant—PEI binary systems is the dependence of the degree
of increasing the absolute values of the specific electroꢀ
conductivity of the solution on the polymer structure. In
the absence of a surfactant the electroconductivity is highꢀ
er in a solution of more hydrophobic PEI 2 compared to
a solution of PEI 1 having the same concentration (see
Fig. 1). On the contrary, in binary systems a sharper inꢀ
crease in the electroconductivity is observed for the system
Such behavior of the dependence of the effective radius
on the concentration was observed for alkylated PEI in
30
chloroform. Interestingly, the sizes of particles of 2ꢀhydrꢀ
oxybenzylated PEI at a concentration of 0.02 mol L–
are much larger than those in the case of the starting
1
branched PEI with the molecular weight 10 000 (R = 110
Н
Table 1. Critical association concentrations for 2ꢀhydroxybenꢀ
zylated PEI in an H O—DMF (30 vol.% DMF) medium in the
2
absence and in the presence of CHAB (conductometric method)
3
–1
3
–1
PEI, system
CAC •10 /mol L
CAC •10 /mol L
1
2
1
2
1
2
1.8
0.8
2.0
1.0
11.0
9.0
6.3
3.9
—CHAB
—CHAB

2
ꢀHydroxybenzylated polyethyleneimines
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 7, July, 2010
1339
and 4 nm,31 respectively). Probably, this indicates the forꢀ
mation of aggregates by hydrophobic interactions.
The catalytic activity of modified polyethyleneimine—caꢀ
2
–1
kapp•10 /s
40
tionic surfactant systems in H O—DMF medium was
2
studied for the hydrolysis of 4ꢀnitrophenylbis(chloroꢀ
methyl)phosphinate (4) (Scheme 1).
3
2
1
0
0
0
0
3
2
Scheme 1
1
7
.0
7.5
8.0
8.5
9.0
pH
Fig. 5. Plots of the apparent rate constant (kapp/s–1) for the
hydrolysis of substrate 4 vs pH of the medium for PEI 2 (1) and
–
1
i. PEI, H O—DMF (30 vol.% DMF).
2—CHAB (2) and 2—CDHAB (3) systems (С = 0.01 mol L
,
2
PEI
СSurf = 0.01 mol L– , H O—DMF, 25 °C).
1
2
The plots of the apparent rate constants (kapp) for the
hydrolysis of substrate 4 in individual solutions of PEI 1
and 2 and in their mixtures with cationic surfactants are
shown in Figs 4 and 5. The hydrophobicity of PEI exerts
almost no effect on their catalytic activity. The presence
of cationic surfactants increases the apparent rate conꢀ
stants and shifts the dependences toward lower pH values,
which makes it possible to use these catalytic systems for
the acceleration of hydrolysis of pꢀnitrophenylphosphiꢀ
nates under mild conditions (pH 7—8).
of cationic surfactants enhances the catalytic activity: on
going from CHAB to CDHAB, the catalytic effect inꢀ
creases and the reaction in a 2—CDHAB system at pH 7 is
accelerated by approximately 170 times, which can be
a consequence of the formation of polymerꢀcolloidal comꢀ
plexes in the systems studied.
It is known that in the presence of PEI the hydrolysis
of phosphorus acid esters follows the general basic mechaꢀ
3
2
20
The presence of CHAB affects the kapp values for PEI
nism. This mechanism was confirmed to retain in polyꢀ
merꢀcolloidal systems.
2
more significantly than those for PEI 1. The plots of the
–
1
increase in the hydrolysis rate (kapp•k0 ) vs pH for the
systems based on PEI 2 are shown in Fig. 6. An increase in
the number of 2ꢀhydroxyethyl fragments in the head groups
The plots of the apparent hydrolysis rate constant vs
concentration of the cationic surfactants with the variaꢀ
tion of different parameters are presented in Figs 7—9. All
kapp•k0–1
2
–1
kapp•10 /s
2
2
1
1
4
0
6
2
8
4
0
160
3
1
20
3
2
8
4
0
0
0
2
1
1
7
.0
7.5
8.0
8.5
9.0
pH
7
.0
7.5
8.0
8.5
9.0
pH
Fig. 4. Plots of the apparent rate constant (kapp/s–1) for the
Fig. 6. Plots of the increase in the rate (kapp•k0–1) of the hydrꢀ
hydrolysis of substrate 4 vs pH of the medium for PEI 1 (1) and
olysis of substrate 4 vs pH of the medium for PEI 2 (1) and
—CTAB (2) and 1—CDHAB (3) systems (С = 0.01 mol L–
1
,
2—CHAB (2) and 2—CDHAB (3) systems (С = 0.01 mol L ,
–1
1
PEI
PEI
СSurf = 0.01 mol L–1, H O—DMF, 25 °C).
СSurf = 0.01 mol L , H O—DMF, 25 °C).
–1
2
2

1340
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 7, July, 2010
Gaynanova et al.
2
–1
2
–1
kapp•10 /s
kapp•10 /s
1
1
0
8
6
4
2
0
3
2
3
6
4
2
2
1
0
0
0.005
0.010
0.015
CSurf/mol L–1
0
0.005
0.010
0.015
CSurf/mol L–1
Fig. 7. Plots of the apparent rate constant (kapp/s–1) for the
hydrolysis of substrate 4 vs surfactant concentration (CSurf) for
Fig. 8. Plots of the apparent rate constant (kapp/s–1) for the
hydrolysis of substrate 4 vs surfactant concentration (CSurf) in
PEI—CDHAB systems for PEI 1 (1), 2 (2), and 3 (3) (СPEI =
3
—CPAB (1), 3—CHAB (2), and 3—CDHAB (3) systems
–
1
–1
(
СPEI = 0.01 mol L , H O—DMF, 25 °C).
= 0.01 mol L , H2O—DMF, 25 °C).
2
kinetic dependences are similar reaching a plateau at some
value of concentration. This suggests that the hydrolysis
reaction itself is preceded by the primary binding of the
substrate by polymerꢀcolloidal aggregates analogously to
the formation of enzyme—substrate complexes. The kiꢀ
netic dependences were analyzed using Eq. (1). The calꢀ
culation results are listed in Table 2. A comparison of the
cationic surfactants with one hydroxyalkyl group shows
that CHAB (see Fig. 7) containing the hydroxyethyl fragꢀ
ment is more active than CPAB which has the hydrꢀ
oxypropyl group. Probably, this is due to the steric hinꢀ
drances to aggregation in the case of bulky hydroxypropyl
fragments and, hence, to weakening the aggregate ability
for micelle formation and solubilization. The CDHAB is
more catalytically active than CHAB, because the presꢀ
ence of two hydroxyalkyl groups creates more possibilities
for the formation of polymerꢀcolloidal complexes due to
the hydrogen bonding.
However, in this case the acceleration of the reaction is
low because of the low hydrolysis rate constant in the
polymerꢀcolloidal phase. A comparison of the activity of
PEI 1—3 with the same surfactant (CDHAB) shows that
the highest catalytic effect was obtained for PEI 1 conꢀ
taining no hydrophobic radical in the hydroxybenzyl subꢀ
stituent. The reaction rate in this system increases by more
than two orders of magnitude.
We studied the kinetics of hydrolysis of phosphinate 4
in the 3—CHAB system at different temperatures (see
Fig. 9) and calculated the effective activation parameters
(activation enthalpy ΔHeff and activation entropy ΔSeff≠
≠
)
as functions of the surfactant concentration (Fig. 10). As
can be seen from Fig. 9, the typical behavior of the temꢀ
perature plots of the rate constant is found in the region
below the CMC (region A) and when the reaction is enꢀ
2
–1
kapp•10 /s
As can be seen from the data in Table 2, in the series of
the 2ꢀhydroxybenzylated PEI considered, the highest bindꢀ
ing constant value was obtained for 2—CDHAB systems.
3
2
6
5
4
3
2
1
C
Table 2. Parameters of the hydrolysis of 4ꢀnitrophenylbisꢀ
(
(
chloromethyl)phosphinate in the polymerꢀcolloidal systems*
рН 8, 25 °C)
1
B
–
1
CMC•103
/mol L
km•k0–1
System
k /s
Kb
L mol–1
m
–1
/
A
1
2
3
3
3
—CDHAB
—CDHAB
—CDHAB
—CHAB
0.459
0.089
0.138
0.037
18
160
82
130
84
4.7
5.4
3.8
3.7
4.9
920
180
275
75
0
CSurf/mol L–1
0
0.005 0.010 0.015 0.020
Fig. 9. Plots of the apparent rate constant (kapp/s–1) for the
hydrolysis of substrate 4 in a PEI 3—CHAB system vs surfactant
—CHAB
0.0275
55
*
2
k0 is the rate constant of the nonꢀcatalytic process at pH 8,
5 °C.
concentration (CSurf) at 25 (1), 40 (2), and 50 °C (3) (СPEI
=
= 0.01 mol L– , H O—DMF).
1
2

2
ꢀHydroxybenzylated polyethyleneimines
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 7, July, 2010
1341
ΔHeff≠/kJ mol–1
–ΔSeff≠/J mol–1 K–1
possess the highest catalytic activity (an increase in the
reaction rate by more than two orders of magnitude).
4
3
2
1
0
0
0
0
0
2
2
80
40
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 09ꢀ03ꢀ00572ꢀa).
2
1
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2
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1
60
0
0.005 0.010 0.015 0.020
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5
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valov, V. P. Barabanov, Izv. Akad. Nauk, Ser. Khim., 2005,
According to Fig. 10, the compensation character of
≠
≠
the change in the parameters ΔHeff and ΔSeff is observed
with an increase in the surfactant concentration, and reꢀ
gion В coincides with the minimum values of both activaꢀ
tion parameters. As a whole, the transfer of the reaction
from the aqueous to micellar phase is accompanied by the
favorable change in the activation enthalpy from 38
6
30 [Russ. Chem. Bull., Int. Ed., 2005, 54, 641].
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000, 16, 4495.
1
1
2
–
1
(
in water) to 15 kJ mol (in micelles) and a decrease in
5. R. Meszaros, L. Thompson, M. Bos, I. Varga, T. Gilanyi,
Langmuir, 2003, 19, 609.
–
1
–1
the activation entropy from –167 to –221 J mol
K . It
can be concluded that the increase in the reaction rate is
controlled by a change in the effective values of the activaꢀ
tion enthalpy.
Thus, 2ꢀhydroxybenzylated PEI and cationic surfacꢀ
tants can form mixed aggregates exerting a catalytic effect on
16. H. Wang, Y. Wang, H. Yan, Langmuir, 2006, 22, 1526.
17. S. M. Bystryak, M. A. Winnik, Langmuir, 1999, 15, 3748.
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1
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the hydrolysis of phosphorus acid esters in an H O—DMF
2
(
30 vol.% DMF) medium under mild conditions. The catꢀ
alytic effect of these systems depends on the structure and
concentration of the cationic surfactant, pH of the soluꢀ
tion, and temperature. In the series of surfactants considꢀ
ered, the polymerꢀcolloidal systems based on CDHAB
(
Engl. Transl.), 2003, 44, 547].
2
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Received May 5, 2009;
in revised form March 24, 2010