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Effect of polymerЦsurfactant structure on its solution viscosity.

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Asia-Pac. J. Chem. Eng. 2011; 6: 78–84
Published online 29 May 2010 in Wiley Online Library
( DOI:10.1002/apj.461
Special Theme Research Article
Effect of polymer–surfactant structure on its solution
Sandeep Badoga, Sudip K. Pattanayek,* Anil Kumar and Lalit Mohan Pandey
Department of Chemical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India
Received 22 July 2009; Revised 27 March 2010; Accepted 23 April 2010
ABSTRACT: Polymer–surfactant interactions in solutions are important in wide range of applications. The interactions
lead to the formation of various polymer–surfactant structures. The solution viscosity is dependent on the structure.
Here we have studied the dependence of viscosity and structure on the composition of polymer–surfactant mixture.
We have used non-ionic polymer polyvinyl pyrrolidone (PVP) and negatively charged surfactant sodium dodecyl
sulfate (SDS). PVP behaves as a negatively charged polyelectrolyte in pure water at pH 7.4. It interacts with SDS
through hydrophobic interaction. This leads to the polymer saturation point (PSP). PVP behaves as a positively charged
polyelectrolyte at pH 2.4. The SDS neutralizes the opposite charge of polymer. After the neutralization point of PVP,
excess SDS interacts with the polymer through hydrophobic interaction. From the data of charge neutralization, charge
on the polymer is calculated. From the data of PSP on PVP at various concentrations, we calculated the number of
SDS molecules per PVP molecule. The structure of PVP–SDS complexes is observed by using transmission electron
microscope at room temperature. The viscosity data are explained from the observed structure of PVP–SDS mixture.
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
KEYWORDS: PVP–SDS mixture; polymer–surfactant interaction; hydrophobic interaction; water soluble polymer
The structure of polymer–surfactant agglomerates plays
an important role in various products, such as food products, pharmaceutical, cosmetics and paints,[1,2] and various technologies, such as enhanced oil recovery,[3] nano
particle synthesis,[4] metal extraction and dye removal
from waste water. The study of polymer–surfactant
system is also important in understanding of the phospholipids (a zwitterionic surfactant present in the cell
wall)–protein/DNA interactions, which are central to
many basic cellular processes.[5,6] The interactions of
various polymers with many different surfactants have
been studied by many researchers. At a fixed low polymer concentration and very low surfactant concentration, some surfactants are free in the bulk solution
and majority of the surfactants remain on the surface
of mixture exposed to air. With increase in surfactant
concentration, the free surfactant in the bulk starts interacting with polymer. This is called critical aggregation
concentration (CAC). With further increase in surfactant
concentration, micelles start building on the polymer; at
*Correspondence to: Sudip K. Pattanayek, Department of Chemical
Engineering, Indian Institute of Technology Delhi, Hauz Khas, New
Delhi 110016, India. E-mail:
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Curtin University is a trademark of Curtin University of Technology
some surfactant concentration, it completely saturates
the polymer. This point is known as polymer saturation
point (PSP). Between CAC and PSP, the surfactants
are present on the surface of mixture, in free spherical micelles, in bulk as individual molecule and on the
polymer. PSP is independent of molecular weight above
a certain molecular weight, but dependent on polymer
concentration in dilute regime[7,8] . The presence of surfactant in various forms gives rise to a variation in
viscosity with the change in surfactant concentration.
The type of interaction and proportion of various
agglomerates depends on the charge on polymer, charge
on surfactants and their chain lengths, hydrophobicity of polymer and surfactant molecules and solvent
temperature.[9 – 17] The oppositely charged polymer and
surfactant interact strongly and might give rise to charge
neutralization. The addition of charge or neutral surfactant in a polyelectrolyte solution has lesser effect on
its solution properties. The question arises what would
happen to polyelectrolyte–surfactant interactions, if
we change only the charge on the polyelectrolyte by
keeping the surfactant and hydrophobic moiety of the
polymer same?
To answer this question, we need to choose a polymer
which can acquire positive, negative or neutral at different solution condition. Polyvinyl pyrrolidone (PVP) can
The samples at different pH were prepared by adding
HCl to the above-mentioned mixture until a desired pH
was achieved. All viscosities were measured after passing the solution through Whatmann grade-1 filter paper.
Figure 1. Resonating structures of PVP in water.
Vibration viscometer
be made positive, negative or neutral by adjusting pH
of its solution. PVP in deionized water exists in negatively charged[18] form due to its resonating structure,
as shown in Fig. 1. However, it can be made positively
charged polyelectrolyte at very low pH. This fact has
been discussed in the section on Results and Discussion.
The literatures available on PVP–surfactant have
focused on the interactions of the PVP in water with
negatively charged SDS using various techniques.[18 – 21]
The interaction has been studied as a non-ionic polymer and negatively charged surfactant. Here, we are
interested to see the effect of molecular weight of PVP
and concentration of PVP on the variation of the polymer–surfactant structure and corresponding viscosity
with change in sign of charge (from positive to negative) on the PVP. The structure of the PVP with
added surfactant is expected to depend on the charge on
the polymer and surfactant and solution condition. The
viscosity data are explained in terms of the nano- to
micro-structures observed under transmission electron
microscope (TEM).
Viscosities of solutions were measured using Sinewave Vibro Viscometer SV-10 model manufactured
by A&D Company. This viscometer consists of two
sensor plates that vibrate at a constant frequency of
30 Hz, which gives a constant shear rate. The Vibro
Viscometer measures the driving electric current to
vibrate the sensor plates with uniform frequency and
amplitude, and then viscosity is given by correlation
between driving electric current and viscosity.
We measured the viscosity of solution in water containing mixture of polymer, surfactant and any other
component such as salt or acid. This is called as solution viscosity denoted by ηsolution . We also measured
viscosity of solution containing surfactant only, which
we have denoted as ηsolvent . From the measured viscosities, we have determined reduced viscosity defined
by ηred = (ηsolution /ηsolvent − 1)/c, where c is the polymer concentration in mg/dl. The reduced viscosity can
be interpreted as the excess viscosity contribution of
polymer–surfactant agglomerates per unit polymer concentration. Units of ηsolution and ηred that we have used
are cp and (mg/dl)−1 , respectively.
Materials and Methods
Transmission electron microscope
We have chosen the polymer PVP of various molecular
weights 58, 360 and 1300 K and anionic surfactant
sodium dodecyl sulfate (SDS). PVP of molecular weight
58 and 1300 K was purchased from Sigma Aldrich
and 360 K from CDH. SDS was purchased from
LOBA Chemicals. HCl was used for adjusting pH of
a polymer–surfactant solution.
Philips-made model no. TEM CM-12 was used to
visualize the size and structure of polymer–surfactant
A drop of approximately 0.2 µl of solution was
poured into a TEM grid containing carbon coat on top
using a microliter syringe. This grid was inserted into
the microscope at room temperature.
Sample preparation
Samples with various SDS concentrations were prepared. These concentrations were expressed in terms
of critical micelle concentration (CMC) of SDS. The
CMC of SDS was 8 mM at 25 ◦ C. The concentrations
were typically x CMC, where x = 0.1, 0.2, 0.3, 0.5, 0.7,
1, 2, 3.5, 5, 7.5, 10, 15, 20, etc. We added 0.1152 g SDS
into 50 ml of deionized water to prepare x CMC solution. The solutions were stirred vigorously for 1 h. We
further added the required amount of PVP to the surfactant solution. The solutions were shaken vigorously
using a shaker for 2 h.
Zetasizer Nano Series MALVERN ZS90, model no.
ZEN3590, with 90◦ optics (scattering angle) was used to
measure the zeta-potential of polymer in solution. The
size of the polymer was measured using Delsa Nano
submicron particle size analyzer.
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
We have determined the size and zeta-potential of PVP
in water at various pH. This shows the PVP’s state at
Asia-Pac. J. Chem. Eng. 2011; 6: 78–84
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
Table 1. Zeta-potential and size of PVP 360 K at various
Mean size (nm)
Zeta-potential (mV)
106 ± 5
101 ± 5
83 ± 5
82 ± 5
80 ± 5
85 ± 5
90 ± 5
90 ± 5
75 ± 5
75 ± 2
99 ± 5
95 ± 5
In the presence of salt 0.001 M NaCl.
Potentials are measured using MALVERN ZS90.
various pH. This is discussed in the next section. In the
subsequent sections, we have shown the effect of PVP
concentration, chain length of PVP, temperature and
pH on the variation of solution viscosity and reduced
viscosity with SDS concentration.
Size and charge of PVP in water and low pH
PVP in solution can exist in negative, uncharged and
positive forms depending on the pH of the solution.
This is confirmed from the measurement of zetapotential and size of PVP in its 0.25 wt% solution at
pH ranging from 7.4 to 2.4 (Table 1). The measured
zeta potentials of the PVP at various pHs are listed
in Table 1. The negative zeta-potential indicates the
existence of negative charge on a PVP in deionized
water. Although PVP is an uncharged molecule, because
of its existence of resonating structures (Fig. 1) in water,
it attains negative charge. The negative charge is due to
the presence of 0.1% salt present in the polymer. We
also tested a few PVP solutions by adding 0.001 M
NaCl. The hydrogen ion (H+ ) protonates the nitrogen
element of PVP on deduction of pH of the solution. This
makes the PVP a positively charged polyelectrolyte,
as indicated by the positive zeta-potential of PVP in
solution of pH less than 3.07. PVP behaves like a
negatively charged polyelectrolyte at pH 7.4 and a
positively charged polyelectrolyte at pH 2.4.
Variation of viscosity of solution at pH 7.4
Figure 2(a) shows the variation of viscosity of solution
containing 360 K PVP and SDS mixture at pH 7.4 with
SDS concentration. The two parameters of the studies were temperature and PVP concentration. There are
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Figure 2. Effect of PVP concentration, temperature on
variation of (a) reduced viscosity with SDS concentration
and (b) solution viscosity. Molecular weight of PVP 360 K.
three distinct regimes as shown in Fig. 2(a): regime 1
corresponds to the weak increase in the reduced viscosity; regime 2 corresponds to the stiff increase of the
reduced viscosity and regime 3 corresponds to the stiff
decrease in the reduced viscosity. The crossover point
from regime 1 to regime 2 is the CAC and from regime
2 to regime 3 is the PSP. The CAC is found to be independent on the PVP concentration, but PSP is dependent
on the PVP concentration. The interesting thing which
was observed here is the height of the curves at PSP.
Higher the PVP concentration, the higher is the PSP
and reduced viscosity at the PSP. The more amount
of surfactant needed to saturate the more amount of
polymer demands higher PSP. But with increasing polymer concentration, the solution contains higher number
of elongated polymer–surfactant complex structure at
PSP. This leads to a higher value of reduced viscosity
of higher concentrated polymer solution at PSP. The
CAC and PSP values for various solutions are listed in
Table 2. From these data, we can calculate the number
of surfactant per polymer molecule, NPSP , as follows.
The SDS is present (1) on the surface of mixture,
(2) in micelles, (3) in bulk as individual molecule and
Asia-Pac. J. Chem. Eng. 2011; 6: 78–84
DOI: 10.1002/apj
Table 2. CAC and PSP of 360 K PVP solution.
0.25% at pH 7.4
0.4% at pH 7.4
0.5% at pH 7.4
0.6% at pH 7.4
0.25% at pH 2.4
0.5% at pH 2.4
0.75% at pH 2.4
(4) on the SDS-saturated polymer at PSP. With change
in polymer concentration, contribution toward reduced
viscosity of component 4 changes, whereas the total
contribution of other three components remains same.
Using this argument, we can say that excess amount of
surfactant needed is due to the addition of extra polymer
into solution. Hence, the NPSP can be written as
ws /Ms
wp /MP
where wp is the difference in weight of polymer at two
different concentrations, ws the weight differences
of total surfactant at PSP of the two different PVP
concentrations, and Ms and MP are the molecular
weights of SDS and PVP, respectively. The PSP of
solutions containing 0.25, 0.4, 0.5 and 0.6% PVP are
2.5, 3.5, 4.5 and 5.5 CMC, respectively. The amount
of SDS required to saturate the excess polymer is
calculated from the required SDS at PSP for the listed
amount of PVP (shown in Table 2). The average amount
of SDS required to saturate 1 g of PVP is 1.84 g, which
is equivalent to NPSP = 2540. The reported[22] amount
of required SDS per gram of PVP in 0.1 NaCl solution
is 2.3 g. The higher value of the reported case is due
to the screening of electrostatic repulsion among the
micelles in the presence of salt.
With increase in temperature, reduced viscosity
remains same, whereas viscosity of the solution decreases as the viscosity of the solvent decreases. The
polymer contribution toward rise in viscosity weakly
depends on temperature, implying that the structure of
PVP–SDS agglomerates does not change in this temperature range.
We plot the viscosity of solution vs SDS concentration shown in Fig. 2(b). The important characteristics of this figure is that viscosity decreases and then
increases with SDS concentration after PSP. Interestingly, this trend has not been not reported earlier. At
PSP, SDS saturates the PVP completely. After the PSP,
the increasing SDS concentration in solution affects the
solution properties in two ways. First, it increases the
number of free spherical SDS micelles in the mixture,
and second, increases the amount of soft material in
the solution. The decrease in the number fraction of
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Figure 3. Effect of molecular weight of PVP on the variation
of reduced viscosity with SDS concentration at 24 ◦ C.
elongated structure is observed due to the first effect.
The earlier works[23,24] on different systems suggest that
the viscosity of the solution containing spheres is lower
than that of the elongated structure at low shear rate.
Based on the above finding, we can explain the observed
decrease of viscosity of solution with increasing amount
of SDS after PSP. However, the increase in viscosity
is found due to the increase in amount of soft materials in the mixture. Thus, we see an increasing trend of
viscosity after the decreasing trend.
Variation of viscosity with molecular weight
of polymer
The effect of molecular weight of PVP on the variation
of reduced viscosity with SDS concentration is shown
in Fig. 3. The solutions contain 0.25% PVP at pH 7.4
and temperature 24 ◦ C. The molecular weights of PVP
are 58, 360 and 1300 K. PSP of PVP is not observed
with 58 K PVP, whereas it is observed for long chain
PVP 360 and 1300 K. The hydrophobic parts of 360
and 1300 K PVP interact with hydrophobic tails of
SDS. Once they are saturated with SDS, electrostatic
repulsion among the micelles formed on a polymer
makes it to elongate. The maximum elongation is
at PSP. From the figure, it is clear that the PSP
is independent of molecular weight above a certain
molecular weight. It depends only on the amount of
PVP present in solution. The polymer saturation is
absent on a low-molecular-weight polymer. This is due
to a specific size needed for joining few spherical
micelles leading to elongated structure. The length
would depend on the electrical repulsion between the
micelles on the PVP and interaction between the PVP
and SDS. A minimum length of a polymer is required
to run over a number of micelles. The required length
can be explained from the charge condensation on a
polyelectrolyte,[25] which says that the distance between
two consecutive charges on a polyelectrolyte should
Asia-Pac. J. Chem. Eng. 2011; 6: 78–84
DOI: 10.1002/apj
be such that the repulsion between two charges is
kT. Applying this concept, the required distance to
maintain kT repulsion energy between two micelles
with n0 bare electronic charges should be separated by
distance of n02 lB , where lB is the Bjerrum length, which
is 0.703 nm at room temperature. The Bjerrum length
shows that two unit charges should be at a separation
of 0.703 nm. This indicates that a long separation is
required to keep the interaction energy of the order of
kT between the two micelles, which have more than
one electronic charge. In fact, a small PVP chain such
as that with molecular weight of 58 K[26] which has
end-to-end distance of 39 nm cannot run over many
micelles. After this chain length, one can expect to have
more micelles to grow on a PVP chain. On addition of
micelles, the chain extends and can accommodate more
number of micelles maintaining the repulsion length.
The exact chain length for this crossover needs the
idea of bare charge on the micelles and the interaction
potential between the polymer and the micelles.
Beyond the threshold molecular weight of PVP, the
higher the molecular weight of PVP, the more extended
would be the structure. The number of micelles per unit
chain length of PVP is roughly fixed neglecting the
end effect. If we compare polymer solutions containing
same amount of polymer of 360 and 1300 K molecular
weights, the total chain length of polymer is same in
solution. So the amount of SDS required to saturate the
different size PVP is equal.
We note the difference of the reduced viscosity height
at PSP. We found that the longer the chain, higher is the
height of reduced viscosity. This is due to the longer
elongated structure at PSP for higher molecular weight
polymer. In addition, we observe that reduced viscosity
of 58 K PVP at PSP is not substantial. This suggests
that the 58 K PVP is not at its elongated structure.
Asia-Pacific Journal of Chemical Engineering
Figure 4. Effect of pH on the variation of reduced viscosity
with SDS concentration. PVP used: 360 K and 0.25%.
Figure 5. Effect of PVP concentration at pH
2.4 on the variation reduced viscosity with SDS
concentration. PVP used: 360 K.
Variation of viscosity with different pH
The variation of reduced viscosity of 0.25% 360 K PVP
solution at 24 ◦ C with SDS concentration at various pH
is shown in Fig. 4. Depending on the pH, two different
types of viscosity variations are found. If the pH is
above 3.0 (corresponds to zero charge on PVP), the
trend of viscosity variation is similar to that described
in Fig. 2, whereas, for the pH below 3.0, the reduced
viscosity initially decreases, followed by an increase in
PSP point and finally decreases with an increase in SDS
At low SDS concentration, the positively charged
PVP interacts with oppositely charged SDS through
purely electrostatic interaction. The electrostatic interaction prevails until the complete neutralization of PVP.
The SDS concentration at which the neutralization takes
place is denoted as polymer charge neutralization point
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
(PNP). PVP is at the collapsed state at PNP. The collapsed state has the lowest size. As the polymer has the
compact size at PNP, the PVP solution at PNP shows
minima of viscosity/reduced viscosity vs SDS concentration curve.
PNP is expected to depend on the PVP concentration
at a fixed pH. As we can see from Fig. 5, the PNP
(i.e. the minima of the reduced viscosity vs SDS
concentration curve) at pH 2.4 for 0.25, 0.5 and 0.75%
PVP concentration are 0.3, 0.5 and 0.7 CMC SDS,
respectively. The net charge in solution due to the PVP
depends on its amount present in the solution. Due to
this, PNP increases with increasing amount of PVP.
The amount of SDS required at PNP for various weight
fraction of PVP is listed in Table 2. The difference in
SDS concentration at PNP for the different amounts of
Asia-Pac. J. Chem. Eng. 2011; 6: 78–84
DOI: 10.1002/apj
Figure 6. Structure of PVP–SDS agglomerates at 25 ◦ C in solution
containing 0.25% 360 K PVP and (a) pH 7.4 and 1 CMC SDS, (b) pH
7.4 and 2.5 CMC SDS, (c) pH 2.41 and SDS 1CMC, and (d) pH 2.41 and
3.5 CMC SDS.
PVP is used to calculate the charge on the PVP. The
difference in the amount of SDS, which is required to
neutralize the charge present on the added difference
amount of PVP is used to calculate its charge. We can
calculate the charge on a PVP molecule, NNPN , using
the Eqn (1). The meaning of the terms for this case is
as follows: ws and wp are the weight differences of
surfactant and polymer, respectively, in the solutions at
PNP, and the NPSP is replaced by NPNP . The calculated
value of NPNP is 230.
The SDS concentration above PNP interacts with
polymer through its hydrophobic part with the hydrophobic part of PVP. The reduced viscosity of the solutions increases with increase in SDS concentration due
to this type of aggregation. The viscosities are continued to increase until the PSP of the PVP is reached. The
PSP of PVP at its concentration of 0.25, 0.5 and 0.75%
are 3.5, 5 and 6.5, respectively. We calculate NPSP to
be 1728. The value is smaller than that calculated for
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
PVP at pH 7.4. This is due to the collapsed structure of
PVP at low pH after neutralization.
We note that all the reduced viscosity curves meet
after PNP. Also, the height of the reduced viscosity
at PSP at pH 7.4 (Fig. 2) is higher than that at pH
2.4 (Fig. 5). The viscosity of solution containing lesser
elongated PVP at pH 2.4 and the spherical micelles
changes slightly with an increase in SDS concentration.
Due to less elongated PVP SDS structure, the reduced
viscosity at low pH decreases slowly after PSP.
TEM images
The TEM images obtained for solution containing
0.25% PVP at pH 7.4 and SDS concentration of 1 and
2.5 CMC are shown in Fig. 6(a) and (b), respectively.
Figure 6(a) corresponds to a point in between PSP
and CAC. We can observe the micelles and some
agglomerated structures. The second image corresponds
to the PSP. Here, we see elongated SDS-saturated PVP
Asia-Pac. J. Chem. Eng. 2011; 6: 78–84
DOI: 10.1002/apj
molecules. Figure 6(c) and (d), respectively, represents
the 0.25% PVP solution at pH 2.41 with concentration
of 1 and 3.5 CMC. Phase-separated collapse structure
is observed for 1 CMC SDS solution, whereas swelled
structure is observed at 3.5 CMC SDS concentration.
Clearly, we see distinctly different structures depending
on the condition of the solutions under TEM. Detection
of polymer–surfactant structure using TEM has two
challenges: (1) generating contrast between different
morphological features due to amorphous nature of
the structures and (2) the structures are susceptible to
significant structural change by a high-energy radiation
imposed by exposure to electrons. These two problems
are also there in the cryo TEM, which is generally used
for this type of solution mixture. The verification of
the structures should be done by recently developed
scanning TEM which uses spatially resolved electron
energy-loss spectroscopy.[27]
PVP acquires negative charge in pure water and positive charge at pH below 3.1. The PSP of negatively
charged PVP depends on the concentration of polymer
only, but independent on polymer chain length (molecular weight) above certain PVP chain length. In the
temperature range of 20–25 ◦ C, the solution viscosity
changes considerably at fixed surfactant and PVP concentration, but reduced viscosity weakly depends on
the temperature in this range, presumably due to the
unchanged PVP–SDS agglomerates within the temperature range. The solution viscosity above PSP depends
on the concentration of spherical micelles.
The positively charged PVP interacts with SDS electrostatically to form charge neutralized complex at low
SDS concentration. This continues up to charge neutralization point. At this point, phase-separated structures
are observed using TEM. With further increasing SDS
concentration, it interacts with the neutralized PVP via
hydrophobic interactions.
Asia-Pacific Journal of Chemical Engineering
TEM, Zeta-meter and viscometer, respectively, in their
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Authors thank Prof. G. B. Reddy, Prof. A. K. Ganguli
and Dr Rajesh Khanna of IIT Delhi for allowing use of
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2011; 6: 78–84
DOI: 10.1002/apj
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