Buletinul Ştiinţific al Universităţii “Politehnica” din Timisoara, ROMÂNIA
Seria CHIMIE ŞI INGINERIA MEDIULUI
Chem. Bull. "POLITEHNICA" Univ. (Timişoara)
Volume 53(67), 1-2, 2008
Complex Formation Between Cationic Polyelectrolytes
and Anionic Surfactans
M. Mihai*, G. Dabija**, C. Costache**
*
Chemical Engineering, University Politehnica of Bucharest, Faculty of Apply Chemistry and Material Science,
1-7 Polizu Street, sect 1, 011061 Bucharest, Romania
**
Technology of Inorganic Substances and Environmental Protection, University Politehnica of Bucharest,
Faculty of Apply Chemistry and Material Science, 1-7 Polizu Street, sect 1, 011061 Bucharest, Romania
Abstract: The polyelectrolyte – surfactant complexes formation has been studied. The critical micelle concentration for
every polyelectrolyte and surfactant which had been used has been measured on this purpose The phase behaviour of
mixtures of a cationic polyelectrolyte (Praestol 644) and an anionic surfactant (sodium lauryl sulphate – SLS) has been
studied. Three phase regions were identified for a given polyelectrolyte concentration, with increasing surfactant
concentration. The rheological consequences of interactions of polyelectrolytes with surfactants of the opposite charge are
experimentally studied. The results reveal that the Praestol 644 solution has the Bingham pseudoplastic, reopectic and
viscoelastic and the complex solutions lose the viscoelastic and rheopectic rheological properties.
Keywords: polyelectrolyte, surfactant, complex polyelectrolyte – surfactant, rheology
polyelectrolyte chain (Konop 1997). The polyelectrolyte
stabilizes the large charge on the micelle surface and no
counter ion condensation on the micelle surface is needed
(Goddard 1993). These effects stabilize the spherical
micelles, reflected in micelles forming at a much lower
concentration when oppositely charged polyelectrolytes are
present (Colby 2000).
1. Introduction
Polyelectrolytes are used in a number of technical
applications, such as water treatment. With polyelectrolytes
can change the surface properties of colloids, rheology of
solutions, wettability etc. In particular, cationic
polyelectrolytes are used in the water treatment because of
their ability to interact with negatively charged surfaces.
Using surfactants in combination with polyelectrolytes
increases the width of the applications even further, and
mixtures of polymers and surfactants in aqueous solution
have been used for colloidal stabilization or flocculation as
well as rheology control.
The interaction between polyelectrolytes and
oppositely charged surfactants can be understood
considering electrostatic and hydrophobic interactions
(Vautrin 2006). At low concentrations, the surfactant binds
individually to the polyelectrolyte through electrostatic
interactions. A cooperative association occurs at the critical
aggregation concentration, cac, as the concentration is
raised due to hydrophobic interactions between the
surfactant tails. This aggregation process leads in some
cases to a bead-and-necklace type structure, where
surfactant aggregates are located along the polyelectrolyte
chain (Bastardo 2005).
The addition of a simple salt lowers the viscosity of
polyelectrolyte solutions because salt screens the
electrostatic repulsion that expands the size of the
polyelectrolyte. The addition of an equivalent amount of
oppositely charged surfactant has a larger effect on
viscosity (Abuin and Scaiano 1986). This is because
flexible polyelectrolytes bind to the spherical surfactant
micelles by wrapping around the exterior of the micelle,
thereby shortening the effective length of the
2. Experimental
The cationic Praestol 644 is a commercial
denomination of a copolymer of polyacrylamide with
acrylic acid. The association between the cationic
polyelectrolyte Praestol 644 and the anionic surfactant
sodium lauryl sulphate (SLS) has been studied using the
surface tension measurements of the solutions. These are
simple measurements that monitor the polyelectrolyte and
surfactant aggregates formed in solution.
The rheological behaviour of the solutions are
experimentally studied with rheoviscosimetre Rheotest RV
3. Results and discussion
In order to study the influence of association degree in
micelles of the used substances on the complex formation
the micellar critical concentration of each substance was
experimentally determined. The results are presented in
Figure 1.
The consequences of interactions of polyelectrolytes
with surfactants of the opposite charge are presented in
figures 2 – 5
The values of the critical aggregation concentrations
and of the critical saturation concentration for each solution
are presented in the table 1.
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Chem. Bull. "POLITEHNICA" Univ. (Timişoara)
Volume 53(67), 1-2, 2008
TABLE 1. The dependence of the concentrations cca and ccs from
the Praestol 644 concentration
ccs, 10 -3
80
1.0 10 -4
3.0
5.5
70
-4
3.0
6.0
60
4.4 10 -3
3.2
6.0
σ, mN/m
7.5 10
2
cca, 10 -4
c, %
-4
C o m p le x P ra e s to l 6 4 4 s o lu tio n , c = 7 .5 1 0 %
a n d S L S s o lu tio n
90
50
40
From the diagram analysis of those figures results that
for cSLS > 310 -4 % the surfactant molecules are associated
in micelles but the macromolecules of polymer are not
associated and the complex forms by engraftment of
surfactant micelles on the macromolecules polymer chain.
The complex solution superficial tension is lower then the
one of the polymer solution. These properties recommend
the use of this solution in interfacial liquid – solid
processes.
c cs
c ca
20
1 E -8
1 E -7
1 E -6
1 E -5
1 E -4
1 E -3
0 .0 1
0 .1
cLLS, %
Figure 3. Superficial tension dependence for the aqueous concentration
solutions of the Praestol 644 and SLS complex for c = 7.5 10-4 % SLS
concentration solutions at 20 °C.
90
S LS solution
P raestol 644 solution
90
c o m p le x s o lu tio n
S L S s o lu tio n
30
C o m p le x P ra e s to l 6 4 4 s o lu tio n , c = 4 .5 1 0
a n d S L S s o lu tio n
-3
%
80
80
70
σ, mN/m
σ, mN/m
2
2
70
60
50
60
50
c o m p le x s o lu tio n
S L S s o lu tio n
40
40
30
30
1E -8
1E -7
1E -6
1E -5
c ca
c cm
c cm
20
1E -4
1E -3
0.01
ccs
20
1 E -8
0.1
1 E -7
1 E -6
1 E -5
1 E -4
1 E -3
1
Figure 4. Superficial tension dependence for the aqueous concentration
solutions of the Praestol 644 and SLS complex for c = 4.5 10-3 % SLS
concentration solutions at 20 °C.
Figure 1. Superficial tension dependence for the Praestol 644,
respective SLS aqueous solution
C o m p le x P ra e s to l 6 4 4 s o lu tio n , c = 1 1 0
a n d S L S s o lu tio n
-4
0 .1
c LLS, %
c S LS , % , c P , %
90
0 .0 1
90
%
C o m p le x P ra e sto l 6 4 4 a n d S L S s o lu tio n
80
80
70
2
60
50
σ, mN/m
σ, mN/m
2
70
c o m p le x s o lu tio n
S L S s o lu tio n
40
1 E -7
1 E -6
1 E -5
-4
1 E -4
1 E -3
0 .0 1
c = 1 .0 1 0 %
-4
c = 7 .5 1 0 %
-3
c = 4 .4 1 0 %
30
c cs
c ca
1 E -8
50
40
30
20
60
20
0 .1
1 E -5
1 E -4
1 E -3
0 .0 1
c LLS , %
c S LS, %
Figure 5. Superficial tension dependence for the aqueous concentration
solutions of the Praestol 644 and SLS complex for different SLS
concentration solutions at 20 °C
Figure 2. Superficial tension dependence for the aqueous concentration
solutions of the Praestol 644 and SLS complex for c = 1 10-4 % SLS
concentration solutions at 20 °C
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Chem. Bull. "POLITEHNICA" Univ. (Timişoara)
Volume 53(67), 1-2, 2008
The same results are obtained at higher concentration
of the polymer solution, while this concentration is lower
then the micellar critical concentration. At concentrations
c > 10 -2 % of the polymer solution the formed complex
precipitates entering in the heterogeneous zone of complex
formation. In the analysed situations the concentrations for
aggregation and critical concentration of saturation have
the same values for every experiment, cca = 3 · 10-4 %, and
ccs = 6 · 10-3 %. It results that between the Praestol 644
solution and SLS there are complexes with good defined
structure and composition forming.
The concentrations values of cca and ccs remain
constant independent from the quantities ratio of polymer
and SLS from the solution. The complexes solutions have
the superficial tension lower than the one of the polymer
solution, but higher than the one of the surfactant solution.
The superficial tension of the solution decreases only after
aggregation. In order to highlight the influence of the
admixture of surfactant on the polymer solution, the
rheogram of the Praestol 644 solution was experimentally
determined (figure 6). Figure 6 shows that the Praestol 644
solution has the Bingham pseudoplastic behaviour and the
τ, N/m
2
10000
1000
c
c
c
c
=
=
=
=
1.2
1.7
1.6
2.3
-3
10
-4
10
-3
10
-3
10
%
%
%
%
100
1
10
100
γ, s
1000
-1
a
C o m p le x P ra e sto l 6 4 4 a n d S L S s o lu tio n s
1 00 0
η, Pa s
rheological model is τ
C om ple x P ra esto l 6 44 a nd S LS solu tio ns
= 7.27 + 399 γ& 0.39 ; has reopectic
behaviour at γ < 22s −1 and viscoelastic behaviour with the
relaxation time tr = 1/30 s = 0.033 s for going diagram and
tr = 1/22 = 0.045 s for return diagram.
10 0
c
c
c
c
10
Praestol 644 c = 0.1% solution
1
=
=
=
=
1 .2
1 .7
1 .6
2 .3
-3
10
-4
10
-3
10
-3
10
%
%
%
%
10
10 0
γ, s
10 00
-1
τ, N/m
2
b
Figure 7. Rheogram of complex solution made with Praestol 644
c = 0.1 % for different SLS concentration solutions at 20 °C
in coordinate τ = f(γ) (a) and η = f(γ) (b).
100
going
return
The rheograms presented in figure 7 were obtained
for the complex solution formed by adding surfactant
solution of different concentrations in the polymer solution
with constant concentration. The Rheograms were
correlated trough nonlinear regression with Ostwald de
Waele τ = τ 0 + η a γ& n model and the model parameters for
each studied case are presented in table. The results
presented in figure 7 point out that by forming the complex
the polymer solution lose the viscoelastic and rheopectic
properties.
10
10
100
γ, s
1000
-1
a
η, mPa s
Praestol 644 c = 0.1% solution
100
TABLE 2. The dependence of the model parameters from the
concentration of the SLS solution
cSLS, %
0
1.710 -4
1.210 -3
1.610 -3
2.310 -3
going
return
10
10
100
γ, s
1000
-1
τ0
7.27
599.46
1312.40
1545.21
1882.77
ηa
399.00
141.59
56.82
36.09
7.46
n
0.39
0.49
0.59
0.65
0.92
The analysis of the values presented in the table point
out that if the surfactant concentration rises then the value
of the flow threshold of the Bingham solution rises
b
Figure 6. Rheogram of Praestol 644 c = 0.1 %.solution at 20 °C
in τ = f(γ)coordinate (a) respectively η = f(γ)(b)
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Chem. Bull. "POLITEHNICA" Univ. (Timişoara)
Volume 53(67), 1-2, 2008
(constant τ0), the value of the apparent viscosity of the
solution decreases (constant ηa) and the pseudoplastic
character of the solution decreases (constant n). The
rheological properties of the polymer solution are
significantly modified.
4. Conclusions
The interaction between polyelectrolytes and
oppositely charged surfactants are experimentally studied.
A cooperative association occurs at the critical aggregation
concentration, cac, when the surfactant aggregates are
located along the polyelectrolyte chain.
The rheology of polyelectrolyte solutions are
profoundly altered by the presence of surfactants. The
addition of an oppositely charged surfactant lowers the
viscosity of polyelectrolyte solutions. This is because
flexible polyelectrolytes bind to the spherical surfactant
micelles by wrapping around the exterior of the micelle.
3200
2800
τ, N/m
2
2400
2000
1600
c
c
c
c
1200
800
0
40
80
120
160
200
γ, s
240
=
=
=
=
1 .2
1 .7
1 .6
2 .3
-3
10
-4
10
-3
10
-3
10
280
%
%
%
%
320
REFERENCES
360
-1
Figure 8. Rheogram of complex solution made with Praestol 644
c = 0.1 % for different SLS concentration solutions at 20 0C
in coordinate τ = f(γ) at low share rate values
1. Abuin, E. B., Scaiano J. C., J. Amer. Chem. Soc., 1984, 106, pp.
6274-6683.
2. Konop, A. J., Polyelectrolyte Collapse Induced by Aggregation of
Oppositely Charged Surfactant, Masters Thesis, Pennsylvania State
University, 1997.
3. Konop A.J, Colby, R.H., Langmuir, 1999, 15, pp. 58-65.
4. Goddard, E.D., Ananthapadmanabhan, K.P., Interaction of
Surfactants with Polymers and Proteins, CRC Press, London, 1993.
5. Colby, R., XIIIth International Congress on Rheology, Cambridge,
UK, 2000, pp. 414-416.
6. Bastardo, L.A., Self assembly of Surfactants and Polyelectrolytes in
Solutions at Interfaces, Doctoral Thesis at the Royal Institute of
Technology Stockholm, 2005.
7. Vautrin, C., Stabilité et Structure d’Agrégats Cationiques, Thèse de
Doctorat de l’Université Versailles Saint-Quentin-en-Yvelines, 2006.
8. Monteux, C., Thèse de doctorat de l’Université Paris VI, 2004.
In the rheograms presented in figure 8 it is to be found
that at low share rate values the variation law τ = f(γ) is
different from the variation law for higher values of the
share rate. The reorganisation of the complex structure
formed under the action of the share stress may be the
explanation of this fact. It results that a hard-set complex
forms, because the resulted rheograms are similar with the
polymer solution rheogram. This behaviour is the result of
the cationic character of the Praestol 644 solution.
68