REVIEW www.rsc.org/softmatter | Soft Matter
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Effect of polyelectrolyte/surfactant combinations on the stabilityof foam films
Nora Kristen and Regine von Klitzing*
Received 21st August 2009, Accepted 23rd November 2009
First published as an Advance Article on the web 7th January 2010
DOI: 10.1039/b917297a
What happens if polymers are added to a macroscopic foam or to foam films? This is an important
question for many technical applications, but it is also important for materials and life science. This
paper reviews the effect of the surface composition on the thickness and stability of aqueous foam films
containing surfactants and polymers. The surfactant concentration is below the critical micellisation
concentration (cmc) and the critical aggregation concentration (cac). The addition of polymers to foam
films leads to the formation of surface active polymer/surfactant complexes or to depletion of polymers
close to the interfaces, which has a strong effect on the film stability. The review mainly concentrates on
the dilute polymer regime (below the overlap concentration c*), and results at the semi-dilute regime
(above c*) are briefly reviewed. A principle concept for the relation between the surfactant/polymer
combination and the film thickness and stability is developed. It allows the prediction, of whether an
unstable Newton black film (NBF) or a stable common black film (CBF) will be formed.
1 Introduction
The stability of macroscopic colloidal dispersions like foams,
emulsions or suspensions is mainly governed by the stability of
the microscopic and mesoscopic thin liquid films between the
compartments (air bubbles, droplets, particles). The study of
aqueous free-standing films (foam films) is of great relevance
especially in two respects: 1) besides the plateau borders, these
single films can be considered to be the building blocks of a foam.
Stranski-Laboratorium f€ur Physikalische und Theoretische Chemie,Institut f€ur Chemie, Technische Universit€at Berlin, Strasse des 17. Juni124, D–10623 Berlin. E-mail: [email protected]; Fax: +49-30-31426602; Tel: +49-30-31423476
Nora Kristen
Nora Kristen holds a Masters
degree in chemistry from the
University of Basel. She did her
Masters thesis in the Physical
Chemistry 1 Department at
Lund University on DNA/RNA
adsorption at phospholipid
model membranes in 2006. Since
2006, she has been a PhD
student in the group of Prof. Dr
Regine v. Klitzing at the Stran-
ski Laboratorium of Physical
and Theoretical Chemistry at
the Technical University of
Berlin. Her research interests
are foam film behaviour and surface complexation of poly-
electrolyte/surfactant mixtures.
This journal is ª The Royal Society of Chemistry 2010
Clarifying the properties of each of these single films could lead
to a better understanding of the behavior of the whole macro-
scopic foam, such as its stability; 2) on the other hand, the free-
standing film corresponds to a slit pore geometry which enables
the study of the effect of confinement on the structuring of
colloidal particles, aggregates or macromolecules.
While a large number of reviews on foam films of pure
surfactants already exist1,2 much less has been published about
foam films of aqueous polyelectrolyte/surfactant mixtures. These
mixtures are of interest due to many applications in cleaning and
cosmetics and the films represent also model systems for bio-
logical membranes. Fig. 1 shows a schematic drawing of a single
foam film containing polyelectrolytes.
Regine von Kitzing
Regine v. Klitzing studied
physics at Technical University
of Braunschweig and University
of G€ottingen. Afterwards she
specialized in physical chemistry
at Institute of Physical Chem-
istry, Mainz, and finished her
PhD in 1996. From 1996 to 1997
she was a post-doc at Centre de
Recherche Paul Pascal
(Pessac/Bordeaux), then assis-
tant researcher and lecturer at
Stranski Laboratorium of
Physical and Theoretical
Chemistry, TU Berlin (1998–
2003). In 2004 she was group leader at Max-Planck-Institute for
Colloids and Interfaces, Potsdam, then held a professorship in
physical chemistry at Kiel University (2004–2006). She is now
Professor of applied physical chemistry at TU Berlin.
Soft Matter, 2010, 6, 849–861 | 849
Fig. 1 Schematic representation of a foam film containing oppositely
charged polyelectrolytes.
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This review addresses the effect of the chemical composition of
the film surface on the interactions, i.e. film thickness and
stability, of foam films formed from aqueous polyelectrolyte/
surfactant solutions. The structure formation of polyelectrolytes
within the film bulk is rather a minor aspect in the review and it is
only considered if the surface is directly involved like during the
stratification process itself. The liquid free-standing film is highly
relevant, since its thickness can easily be varied by changing the
outer pressure and the surface composition. Since the film
interfaces cannot be directly investigated, the disjoining pressure
isotherms are related to studies at a single air/water interface. It
cannot be excluded that the surface composition changes when
the opposing surface is approached during film thinning.
In general, electrical double layer forces dominate the disper-
sion forces. In contrast to this, the stability of non-aqueous films
is mainly determined by dispersion forces.3 Therefore studies are
reviewed where the charge and the hydrophobicity of the surfaces
of foam films is modified. In order to avoid structural forces, i.e.
stratification, the surfactant concentration is below the critical
micelle concentration (cmc), the concentration where micelles are
formed in the bulk, and the polyelectrolyte concentration below
the overlap concentration (c*), the point where polyelectrolytes
start to overlap and form a network. In addition, it is important
to choose the concentration ratio of both compounds such that
the critical aggregation concentration (cac) is not exceeded, since
otherwise aggregates are formed in the solution and no homo-
geneous films can be formed.
The question arises: what happens to a foam film if polymers
are added? Especially in the case of charged polymers the film
could swell due to increases in osmotic pressure. But the result is
not so simple and a prediction not so easy. Depending on the
polyelectrolyte/surfactant mixture, different foam films are
formed: either a thick common black film (CBF) or a thin
Newton black film (NBF). The results for the polyelectrolyte/
surfactant charge combination can be generalized for all inves-
tigated charge combinations, and they are summarized in
Table 1. This review gives a deeper insight into the reasons for
Table 1 Type of limiting free-standing film observed for specificcombinations of polyelectrolyte and surfactant charge. Comparison toresults obtained by other groups. Adapted from ref. 8
non-ionic surf. cationic surf. anionic surf.
polyanion CBF4–6,8 CBF4,5,7,9,10 CBF6
polycation NBF11,12,14,15 CBF11–13 —non-ionic polymer CBF14 — CBF16,17
850 | Soft Matter, 2010, 6, 849–861
such behavior and a principle concept is developed which allows
the prediction of whether a NBF or a CBF is formed.
2 Interactions in thin films
The disjoining pressure is the pressure which keeps the two
opposing surfaces apart from each other (P > 0) or brings them
together (P < 0, conjoining pressure). Thermodynamically, it can
be described by the negative derivative of the Gibbs energy by the
film thickness.
PðhÞ ¼ ��
vG
vh
�T ;P;A;n
(1)
It is an excess pressure within the film with respect to the
pressure of the outer bulk liquid. The disjoining pressure P
consists of several contributions, mainly DLVO and steric
interactions and structural (oscillatory) pressure.
2.1 DLVO interactions
DLVO forces consist of electrostatic and van der Waals inter-
actions.18,19 The electrostatic repulsion between two identically
charged interfaces with a distance h (which corresponds to the
film thickness h) is described by an exponential decay with the
Debye–H€uckel screening length 1/k.
P(h) ¼ P(0) exp(�kh) (2)
P(0) is connected to the surface potential J0 by the relation:20
PðhÞ ¼ 64kTrNg2 expð � khÞ¼�1:59� 108
�½cel �g2expð � khÞ ½Pa�
g ¼ tanhðzeJ0=4kTÞPð0Þ¼ 64kTrNg2
(3)
This corresponds to the solution of the linearized Poisson–
Boltzmann equation, which is only valid for low surface poten-
tials (<50 mV) or large distance, i.e. thick films. The surface
charge density can be calculated from the surface potential J0 by
using the Grahame equation:
s ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi8330RTc
psinh
FJ0
2RT(4)
The van der Waals contribution to the disjoining pressure
isotherm is described by:
PvdW ¼ �A
6p3(5)
where A is the Hamaker constant. In symmetric films like foam
films, A is always positive, resulting in an attractive van der
Waals force.
The thickness of the common black film (CBF) is determined
by DLVO forces, while the thinner Newton black film (NBF) is
stabilized by steric repulsion and does not contain any free
solvent molecules. A transition from a CBF to a NBF can be
induced by the addition of salt, which leads to a screening of the
surface potential, or by reducing the surface charge. This
confirms the electrostatic nature of the repulsive force stabilizing
the CBF. The transition from a CBF to a NBF corresponds to an
This journal is ª The Royal Society of Chemistry 2010
Fig. 2 Schematic representation of a disjoining pressure isotherm.
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oscillation of the disjoining pressure, which is depicted in Fig. 2.
This oscillation is derived from the attractive van der Waals
forces between the two opposing film interfaces. This attractive
part of the isotherm is mechanically unstable, and it cannot be
measured by a thin film pressure balance (TFPB), which is
further described in section 3. Instead, a step in film thickness
from the thicker CBF to a thinner NBF is detected.
Above a surface potential of 50 mV, simulations should be
done with the non-linearized Poisson–Boltzmann equation at
either a surface fixed potential or a fixed surface charge density.
Fig. 3 Center piece of a thin film pressure balance (TFPB). The inset
shows the application for foam films.
2.2 Structural forces
Beside the DLVO forces so-called structural forces can occur in
a film. This is manifested in an oscillatory disjoining pressure,
which is due to a layering of molecules between two film interfaces.
The layering is related to an oscillatory concentration profile with
a decay in amplitude from the interface towards the film bulk. The
relation between the oscillatory concentration profile and the
oscillatory pressure is still under discussion. An ostensive image of
the oscillatory pressure is the layer-by-layer expulsion of the
molecules which induces attractive depletion forces. With respect
to the class of molecules this kind of force is also called solvation or
hydration force. These structural forces have been observed for
spherical molecules entrapped between two mica plates in a surface
force apparatus (SFA) (e.g. ref. 20). The period of the oscillation is
connected to the diameter of a spherical molecule. Oscillatory
forces also occur in free-standing films containing liquid crystals,
colloidal particles (e.g. ref. 1 and references therein) or micelles.
The micellar systems have been investigated both in a SFA
between solid interfaces21 and in a TFPB between liquid inter-
faces.22,23 The step size scales with the surfactant concentration cs
as cs�1/3.24 This can be simply explained by a homogeneous distri-
bution of ‘‘hard spheres’’ in three dimensions where the distance
between two centers of mass is determined by the concentration of
spheres. In films of uncharged surfactants a stratification above the
cmc occurs as well, but in contrast to charged micelles, the step size
corresponds to the micelle diameter.25
Mixed polyelectrolyte/surfactant films are formed below the
cmc and the cac. Below the critical overlap concentration of the
polyelectrolytes c*, the film thins continuously with increasing
disjoining pressure. The isotherm shows a typical exponential
decay, indicating electrostatic repulsion. The slope is directly
related to the ionic strength. For polyelectrolyte concentrations
above c*, a step-wise thinning of the film occurs. It correlates
This journal is ª The Royal Society of Chemistry 2010
with an abrupt increase in slope of the isotherm branches from
a finite one below c*, related to the ionic strength, to a very steep
one above c*, the latter not being related to the ionic strength any
more.14,26 With increasing polyelectrolyte concentration the
number of steps increases and the steps become smaller (e.g.
ref. 26,27).
The oscillatory disjoining pressure is described by
PðhÞ ¼ Pð0Þexp
��h
l
�cos
2ph
d(6)
The three parameters which characterize the oscillation are the
amplitude P(0), the decay length l and the period d.
3 Thin-film pressure balance (TFPB)
The thin-film pressure balance (TFPB, Fig. 3) is mainly used to
study foam films, but recently it has been also extended to study
wetting films.28,29 With this method, disjoining pressure
isotherms (disjoining pressure P as a function of the film
thickness h, cf. Fig. 2) can be measured with the porous-plate
technique, developed by Mysels30 and Exerowa.31,32 The appa-
ratus allows the formation of a horizontal free-standing film, in
which the capillary pressure is balanced by the disjoining pres-
sure. The film is formed from an aqueous polyelectrolyte/
surfactant solution over a hole (diameter of about 1–2 mm)
drilled through a porous glass plate. The plate allows the liquid
to flow out of or into the film whenever the pressure is changed.
Additionally, the small pores (diameter: about 10 mm) of the
fritted glass plate make it possible to apply a pressure of 104 Pa.
The fritted glass plate is filled with the respective sample
solution, and is connected with the external reference pressure Pr
(atmospheric pressure) by a glass tube. This film holder is
enclosed in a metal cell, which allows one to pressurize the film
using a piston pump. During the film drainage the capillary
pressure causes a sucking of film liquid into the plateau borders
until the disjoining pressure begins to affect the dynamics. At
equilibrium, the capillary pressure Pc and the disjoining pressure
P compensate each other. They can be directly calculated from
the difference between the pressure in the cell, Pg, and that of the
liquid reservoir, Pl:
Pc ¼ P ¼ Pg � Pl ¼ Pg � Pr � Drghc + 2g/rc (7)
The hydrostatic pressure of the liquid column in the glass tube
is given by Drghc (Dr¼ difference in density between the solution
and the gas, g: gravitation constant, hc ¼ height of the liquid
column in the glass tube above the film). The capillary pressure in
the glass tube is determined by 2g/rc (g ¼ surface tension of the
Soft Matter, 2010, 6, 849–861 | 851
Fig. 4 Disjoining pressure isotherms of foam films of different C14TAB
concentrations.
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liquid, rc ¼ radius of the capillary tube). The difference in pres-
sure inside and outside the cell (Pg � Pr) is measured by
a differential pressure transducer. The accuracy of the disjoining
pressure is mostly limited by the differential pressure transducer,
which has usually a specificity of 0.3% of its full range (i.e. 3–30
Pa). Above a pressure of 500 Pa, Pg � Pr is the determining
contribution to the disjoining pressure.
The film is illuminated by cold-filtered white light via
a microscope through a quartz window on top of the cell and is
monitored with video microscopy. The film acts like an inter-
ferometer, since the light which is reflected at the upper and the
lower interface superposes. A maximum intensity occurs at
a thickness of around 100–150 nm for visible light due to the
phase shift of p/2 at one of the film surfaces. The intensity
decreases with decreasing film thickness which gives a change in
‘‘color’’ from white (at the maximum) to light grey or dark grey
and even black in the case of several nm thick films (e.g. NBF).
Parallel to video microscopy, the film thickness is determined
by the interferometrical method of Scheludko.33 Therefore the
reflected intensity at one fixed wavelength (l ¼ 550 nm or
630 nm) is measured by a photomultiplier. The following equa-
tion is used to calculate the film thickness:33
h ¼ l
2pnarcsin
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiD
1þ
4R
ð1� RÞ2,ð1� DÞ
!vuuuut (8)
D ¼ I � Imin
Imax � Imin
(9)
R ¼ ðn� 1Þ2
ðnþ 1Þ2(10)
with n as refractive index of the liquid. I is the actual intensity and
Imin and Imax correspond to the last minimum and maximum
intensity values. In the case of thin films (e.g. NBF, h� 4 nm) the
different refractive indices of the different film regions have to be
taken into account. In general, a sandwich structure of the two
surfactant layers which are adsorbed at the film surfaces (ntail,
nhead, htail, hhead) separated by the film core (nc, hc) is used as a film
model.34 However the determination of the refractive index and
the thickness is rather imprecise, since the molecular density at the
film interfaces is not directly accessible. The surface density at an
air/water interface of a solution can be determined for instance by
surface tension measurements, but these results probably cannot
be transferred to the film interfaces. There is evidence for a higher
surface density in a NBF than in the former CBF,35–37 which is
manifested in a lower gas permeability of the NBF.
A reservoir of the sample is included inside the cell to saturate
the atmosphere inside the cell and prevent evaporation of the film
liquid. It is assumed that the equilibrium film thickness is reached
when the reflected intensity stays constant over a period of
20 min. The measuring cell is temperature controlled and the
quartz window within the top of the cell is heated to avoid
condensation. Unless stated otherwise, the measurements are
carried out under equilibrium conditions at room temperature.
852 | Soft Matter, 2010, 6, 849–861
4 Pure surfactant systems
For pure surfactant foam films it is assumed that the surfactant
molecules adsorb at the film interfaces in a more or less tight
packing. Below the cmc, the film bulk of a CBF contains
surfactant monomers.
Hence, in the case of charged surfactant molecules, they
determine the charge of the film surface and in the film bulk they
act like a salt contributing to the electrostatic screening. Both
effects are counteracting. In most of the studies on charged
surfactants the film thickness decreases with increasing surfac-
tant concentration. This indicates a dominating effect of the
electrostatic screening due to an increase of the ionic strength as
shown in Fig. 4. The increasing surface charge is reflected by an
increase in film stability, i.e. maximum disjoining pressure. If the
surface charge is high enough the electrostatic repulsion between
the film surfaces avoids a transition from a CBF to a NBF. From
the disjoining pressure isotherms, one can extract the value of the
surface potential in symmetric films (like foam films) but not the
sign of the potential. However, the surface charge density can be
estimated by eqn (3) and eqn (4). For instance, the addition of
SDS leads to surface potentials up to 150 mV close to the
cmc.38,39 In the case of cationic surfactants it is about 110 mV for
C12TAB40 and 125 mV for C16TAB2,41 close to the cmc. In all
studies, the potential remains constant around the cmc, which
indicates a saturated interface.
CBFs can be formed even from aqueous solutions of nonionic
surfactants, which indicates the occurrence of electrostatic
interactions. Above a certain surfactant concentration a transi-
tion to a NBF takes place. Due to the fact that the nonionic
surfactant does not increase the ionic strength, the only reason
for the CBF–NBF transition can be a loss in surface potential
caused by the adsorption of nonionic surfactant molecules.42–44
This indicates a precharged air/water interface which is replaced
and eliminated by nonionic surfactant molecules. By increasing
the surfactant concentration the potential decreases from about
60 mV at about 10�5 M to about 30–40 mV at 10�4 M. These
values are similar for different nonionic surfactants.44–46 That
means one would expect a charge reversal either for cationic or
anionic surfactants, detectable as a stability minimum close to
the isoelectric point (IEP). To our knowledge, it has not been
possible to monitor that so far: below a certain surfactant
concentrations the films become unstable and remain unstable
This journal is ª The Royal Society of Chemistry 2010
Fig. 5 Dependence of surface tension on the surfactant concentration
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when the surfactant concentration is further reduced.41 That
means that the sign of film surface charge cannot be figured out
by TFPB measurements at symmetric films like foam films.
However, there are many other experiments including bubbles/
droplets in an electric field47 and studies of wetting films,29,48
which lead to the conclusion that the air/water interface is
negatively charged. Besides the charge density, the sign of the
charge49 is still controversially debated in the literature.50,51
Especially, simulations of the air/water interface hint at posi-
tively charged surfaces,50 but to our knowledge, there is no
experimental proof for positive charges at the air/water interface.
Another debate is the reason for the charges, which might be the
adsorption of OH� ions.52
of the pure surfactant solution (open symbols) and after the addition of
5 � 10�3 monoM PDADMAC (filled symbols) for two different systems:
PDADMAC/C16TAB (triangles) and PDADMAC/b-dodecylmaltoside
(C12G2) (squares). Data taken from ref. 8.
5 Mixed surfactant/polyelectrolyte systems
In mixed surfactant/polyelectrolyte films both the charge of the
surfactant and that of the polyelectrolyte can be varied. Two
main questions arise: firstly, how is the thickness influenced by
the surfactant/polyelectrolyte charge combination and second,
how does the choice of the surfactant affect the structural forces.
The influence on structural forces is briefly reviewed at the end of
this paper (section 6). The main focus of the present paper will be
on the effect of the surfactant/polyelectrolyte charge combina-
tion on the film final thickness and stability at high pressures.
As mentioned in section 2.1 the interplay between the different
interactions between the surfaces decide whether a CBF–NBF
transition takes place or not. One predominant factor seems to
be the change in surface charge and therefore a change in the
electrostatic repulsion forces due to the adsorption of poly-
electrolytes at the surface. Since the composition of the film
interfaces is not directly accessible, in general the results of
surface tension53,54 and scattering measurements55,56 at the free
air/water interface are taken into account. Of course, it cannot be
excluded that the composition of the interface and the electro-
static screening length changes upon approach of a second
interface, since theoretical calculations predict such an effect.36
In the studies presented in the following, films containing both
like charged and oppositely charged polyelectrolyte/surfactant
mixtures are reviewed. In addition the effect of non-ionic
compounds (surfactant and/or polyelectrolytes) is discussed. In
all experiments the surfactant concentration is below the cac and
the cmc.
Fig. 6 Disjoining pressure isotherms of free-standing pure C16TAB film
(data taken from ref. 57) and mixed C16TAB/PDADMAC films (data
taken from ref. 8).
5.1 Like charged polyelectrolytes and surfactants
5.1.1 Surface characterisation. Fig. 5 shows the surface
tension of an aqueous solution of the positively charged surfac-
tant C16TAB before and after the addition of the polycation
poly(diallyldimethylammonium chloride) (PDADMAC) (trian-
gles).8 PDADMAC has no significant effect on the surface
tension of aqueous solutions containing cationic surfactant,8
which leads to the conclusion that no surface active complexes
occur. It is assumed that a surfactant layer is formed at the air/
water interface and that the polyelectrolytes are repelled from the
surface. This might even lead to a depletion of the poly-
electrolytes from the surface, visible at low surfactant concen-
trations up to 10�5 M. On the other hand the hydrophobic
backbone of the polyelectrolytes might enhance the adsorption,
This journal is ª The Royal Society of Chemistry 2010
which leads to a slight decrease in surface tension as observed at
higher surfactant concentrations above 10�5 M in Fig. 5.
5.1.2 Effect on film thickness. The disjoining pressure
isotherms of the foam films of pure surfactant and the mixed
system are compared in Fig. 6. The C16TAB concentration was
below the cmc and the PDAMAC concentration of 5 � 10�3
monoM (corresponding monomer concentration).8 The discon-
tinuous film thinning after adding polyelectrolytes is called
stratification and it is briefly discussed in section 6.
Both the pure and the mixed system show a CBF. The CBF is
very stable and no transition to a NBF has been observed up to
pressures as high as 5000 Pa and 20000 Pa, respectively. The
mixed polyelectrolyte/surfactant film is less stable (rupture at
5000 Pa) than the pure surfactant film (no rupture up to 20000
Pa), which makes it difficult to compare the final film thicknesses.
At low disjoining pressure, the film thicknesses are similar. On
one hand, the isotherm shows that the polyelectrolytes does not
act like a simple salt, which would decrease the film thickness due
to electrostatic screening. On the other hand, the addition of
polyelectrolytes does not increase the osmotic pressure so much
that the film thickness increases.
However, in spite of increasing electrostatic repulsions due to
the addition of polyelectrolytes the film ruptures at lower pres-
sure than the pure surfactant film. This indicates that the
Soft Matter, 2010, 6, 849–861 | 853
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mobility and fluctuation of polyelectrolyte chains might reduce
the film stability in comparison to a film containing pure
surfactant solution.
5.2 Oppositely charged polyelectrolyte/surfactant systems
5.2.1 Surface characterisation. Fig. 7 shows a characteristic
surface tension curve for oppositely charged polyelectrolyte/
surfactant systems. For these systems, surface tension is usually
depicted with fixed polyelectrolyte concentration and varied
surfactant concentration. Typical concentration ranges are 10�4
to 10�2 monoM for the polyelectrolyte and 10�6 to 10�2 M for the
surfactant. At low surfactant concentrations (below 10�4 M), the
addition of polyelectrolytes leads to the formation of surface
complexes that lower the surface tension compared to the pure
surfactant. This happens at the csac (critical surface aggregation
concentration).58,59 The driving force for the complexation
between oppositely charged polyelectrolytes and surfactants are
electrostatic and hydrophobic forces. The head groups of the
surfactant molecules are attracted by the charged polymer
segments. Additionally, the hydrophobic surfactant tails can
interact with the hydrophobic backbone of the polyelectrolyte.24
In this concentration regime, only loosely packed monolayers
are formed at the interface.54,56,60 The distance between the
surfactant molecules depends on the degree of charge of the
polymer and can be calculated from the Gibbs equation that is
applied to the surface tension isotherms. The polymer is linked to
the surfactant monolayer with the charged monomer units while
the uncharged parts dangle into the bulk solution. The lower the
degree of charge, the larger is the distance between the surfactant
molecules due to the longer distance between the charged
monomer units.54 In the case of C12TAB/PAMPS the area per
C12TAB molecule is 78 �A2 for a charge fraction of 25% and
100 �A2 for 10% charged monomer units which is much larger
compared to 48 �A2 of a surface densely covered with C12TAB
molecules.53,61 The degree of charge has an influence on the
thickness of the interfacial layer as well. Highly charged
polyelectrolytes such as PSS are flatly adsorbed to the interface,
polyelectrolytes with a lower degree of charge form thicker layers
due to the loops that are extended into the solution.55,62 In
a Langmuir–Blodgett film study by Lee et al.63 it was shown that
Fig. 7 Effect of PAMPS 25% on C12TAB surface tension: pure C12TAB
(open circles), with 0.3 mM (diamonds), with 1.5 mM (crosses), with
3.0 mM (filled circles). Reprinted with permission from ref. 53. Copyright
(1996) American Chemical Society.
854 | Soft Matter, 2010, 6, 849–861
PSS/C14TAB forms very homogeneous layers at the interface at
a surfactant/polyelectrolyte segment ratio of 1. This behaviour is
dependent on the surfactant chain length: shorter chain lengths
form layers with holes. The rigidity of the polyelectrolyte is
another important parameter concerning the layer thickness.
PSS and PAMPS are rather flexible polyelectrolytes, whereas
xanthan and DNA are more rigid due to their ability to form
double helices. Ellipsometry measurements show that they form
thicker and denser layers55 which can be interpreted as the helices
adsorbing flat to the surfactant monolayer. In general, the
adsorption process is very slow, especially in the low concen-
tration regime58,64 so that it is important to give the system time
to equilibrate.
When the surfactant concentration is further increased, the
surface tension reaches a plateau. This plateau is characterised by
the cac at which aggregates are formed in the bulk. These
aggregates turn the solution turbid when they reach a certain
dimension65 and are described as polyelectrolyte chains that are
decorated with surfactant micelles. The formation of surfactant
micelles on the polymer chains starts much below the cmc of the
pure surfactant system.
The second break point in the surface tension graph in Fig. 7 is
the cmc. Above this point the surface is densely covered and the
surface tension does not change any more. It is assumed that
most of the polymer is redissolved within the bulk in this regime
of the surfactant concentration.
The cac depends on many parameters like surfactant
chain length, polyelectrolyte concentration or degree of charge. It
decreases with increasing surfactant chain length due to
the stronger hydrophobic interactions between the two
compounds.66,67 Polyelectrolytes with a high degree of charge
interact stronger with the surfactant than those with only a few
charged monomer units. Therefore the cac decreases with
increasing polymer charge density.68 The chain length of the poly-
mer has no influence on the cac but in the case of PSS a length of at
least 20 monomer units is needed to show polymer behaviour.69
C12TAB/PAMPS(25%) in Fig. 7 is an example for a rather
hydrophilic system due to the short aliphatic chain of the
surfactant and the quite hydrophilic polyelectrolyte. The inter-
action between both compounds is rather weak due to the lack of
hydrophobic interactions and weak electrostatic interactions due
to the low degree of polymer charge.
In case of C16TAB/PSS (see Fig. 8) the hydrophobic interac-
tion is so strong due to the long aliphatic chain and the
Fig. 8 Surface tension isotherms of pure C16TAB (open squares) and
with 2.4 mM PSS (filled squares). Reprinted with permission from ref. 69.
Copyright (2004) American Chemical Society
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hydrophobic polymer backbone that the charges of both
compounds are directed towards the solution and that the
complexes become hydrophilic and water soluble. That leads to
a shift in surface tension to higher values than those of the pure
surfactant. For combinations of PSS with either C12TAB or
C14TAB the surface tension is always lower than for the
respective pure surfactant system.69
In contrast to PSS, mixtures of C16TAB with PAMPS (25%)
reduce the surface tension with respect to the pure surfactant
system54 like C12TAB/PAMPS (25%) does in Fig. 7. PAMPS
shows much weaker hydrophobic interactions with C16TAB than
PSS, which makes the complexes much less hydrophilic.
The concentration of the polymer has a more complex impact
on the point of aggregation: in general the surface tension at
a fixed polyelectrolyte concentration decreases monotonously or
remains constant with increasing surfactant concentration.
However, in the dilute concentration regime of the poly-
electrolyte (far below 10�3M) another phenomenon occurs: when
the surfactant concentration has reached the described plateau
(close to cac) the surface tension starts to increase again and
finally collapses on the curve of the pure surfactant as shown in
Fig. 9. This is more pronounced for surfactants with a long
hydrophobic tail. For instance, fully charged PAMPS show this
non-monotonous behaviour in the low concentration regime in
combination with C16TAB, whereas the maximum becomes
smaller with C14TAB and almost vanishes with C12TAB.70 So
far, this has only been observed for highly charged poly-
electrolytes, while 25% charged PAMPS in combination with
C16TAB does not show this non-monotonous behaviour. The
starting point of this rise in surface tension is shifted to lower
surfactant concentrations when the amount of polymer is
reduced. Taylor et al.71 propose a complex ordering of the
surface complexes including the formation of multilayers in this
concentration regime. These structures are described as several
layers with different polyelectrolyte/surfactant compositions,
usually a surfactant monolayer with up to 8 mixed layers
underneath.56,71,72 These results are supported by neutron
reflectivity experiments. Surface tension measurements indicate
a long equilibration time (more than one hour) of the surface in
this concentration regime, which might be a hint for the forma-
tion of a multilayer at the surface.70
Above the cac it is not possible to form homogeneous foam
films due to the aggregates that are trapped in the film.
Fig. 9 Surface tension measurements of PSS/C16TAB solutions with
different PSS concentrations: 0 mM (filled circles), 0.1 mM (open dia-
monds), 0.25 mM (filled squares). Reprinted with permission from ref.
71. Copyright (2003) American Chemical Society.
This journal is ª The Royal Society of Chemistry 2010
The surfactant/polymer ratio has an important influence on
the surface tension as well. At ratios close to 1, Monteux et al.73
observed very hydrophobic complexes in their surface tension
measurements for the C12TAB/PSS system. These findings are
supported by surface rheology studies74–77 where the surface
shows high elasticity which indicates a large amount of material
at the interface. It was even proposed that surface tension is just
a question of polymer/surfactant ratio69 due to the fact that the
surface tension was constant when plotted versus the ratio.
Fig. 10 shows the surface tension for C12TAB/PSS for different
PSS concentrations. With decreasing polymer concentration the
cac decreases. These findings are in contrast to the results for
C12TAB/PAMPS, where all surface tension measurements
collapse on one curve in a polyelectrolyte concentration regime
of 0.3 to 3 mM53 which means that the surface tension is not
dependent on the polyelectrolyte concentration in that case
(see Fig. 7). It is not fully clarified if the difference in charge
density or the hydrophobicity of the polymer backbone is
responsible for that difference.
The characterisation of the surface complexes has been the
subject of many studies in the past years. Concerning foaming
and foam film stability, it is hard or even impossible to make
predictions only from the surface coverage.78
5.2.2 Influence of the surface charge on foam film stabilities.
For oppositely charged polyelectrolyte/surfactant mixtures so far
only CBFs have been observed8 (see Fig. 11).
Since CBFs are formed due to the electrostatic repulsion of
the two opposing interfaces, either the surfactant or the
Fig. 10 Surface tension measurements of PSS/C12TAB solutions with
different PSS concentrations.
Fig. 11 CBF formed by PAMPS 100% and 10�4 M C14TAB.
Soft Matter, 2010, 6, 849–861 | 855
Fig. 13 Surface characterisation of three different polyelectrolyte
systems: a) surface tension and b) surface elasticity measurements. The
dashed lines corresponds to the surface tension and the elasticity,
respectively, of the pure surfactant.
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polyelectrolyte charge should dominate and determine the sign of
the surface charge. The working hypothesis was for a long time
that close to the isoelectric point (IEP) a charge reversal should
take place, which leads to a CBF at polyelectrolyte concentra-
tions below and above the IEP. At the IEP where repulsion
should be reduced two scenarios are possible: the formation of
a NBF or the destabilisation of the film. The surface charge can
be tuned by the variation of the added polyelectrolyte concen-
tration when one assumes that the surface tension is positive for
cationic surfactant films79 and decreases with increasing poly-
electrolyte concentration. To get more information about the
surface coverage during this process, surface tension and elas-
ticity measurements were carried out and compared to the foam
film stability. To test the influence of the surfactant chain length
and the hydrophobicity of the polyelectrolyte, two different
surfactants and polyelectrolytes were used, namely C12TAB and
C14TAB, both cationic surfactants, and PAMPS and PSS, both
linear polyelectrolytes with a degree of charge of nearly 100%.
Fig. 12 shows the stability of the foam films, which is displayed
by the maximum disjoining pressure before film rupture versus
the polyelectrolyte concentration. The nominal IEP in these
measurements is 10�4 M due to the surfactant concentration used
in the experiments. The graph shows that stable films are formed
below the IEP but when the polyelectrolyte concentration is
increased a minimum in stability is found. This minimum at
7.5 � 10�5 M is close to the nominal IEP of 10�4 M. That the
point of destabilisation is not exactly at the nominal IEP could be
due to the fact that the surfactant is not completely dissociated or
that surfactant/polyelectrolyte ratio at the surface is different
from that in the bulk. Once the IEP is crossed the film stability
increases again and foam films at a concentration of 10�3 M are
much more stable than the pure surfactant films (z900 Pa
for 10�4 M C14TAB and 0 Pa for 10�4 M C12TAB). All stable
films were CBF and no NBF was formed. From the stability
measurements one could conclude that the foam films are
stabilized by the cationic charge of the surfactant below and by
the additional negative charges of the polyelectrolyte above the
IEP. The study of the surface by surface tension and rheology
shows that this image is much too simple.10
Surface tension measurements (cf. Fig. 13a) show that the
addition of small amounts of polyelectrolyte leads to highly
surface active complexes that lower the surface tension strongly
compared to the pure surfactant. In this concentration regime all
Fig. 12 Foam film stabilities for different polyelectrolyte/surfactant
systems; maximum disjoining pressure Pmax before film rupture versus
polyelectrolyte concentration.
856 | Soft Matter, 2010, 6, 849–861
counterions of a polyelectrolyte chain are exchanged by
a surfactant molecule which makes the complex very hydro-
phobic.54 Above the IEP, where the number of polyelectrolyte
segments exceeds the number of surfactant molecules, less
surfactant molecules decorate one polymer chain so that it again
becomes hydrophilic.66,71 This results in a release of the complex
from the air/water interface indicated by an increase in surface
tension close to the value of the pure surfactant, leaving a more
or less pure surfactant layer at the film interface. These findings
are supported by elasticity measurements (cf. Fig. 13b). Samples
with low polyelectrolyte concentrations show a high surface
elasticity60,80 indicating a high amount of material at the surface,
whereas the release of the complexes appears in a drop in elas-
ticity directly above the IEP. Further increase of the poly-
electrolyte concentration leads to a rise in elasticity and
a decrease of surface tension. The reasons for this behaviour are
rather speculative and might originate in the network of poly-
electrolytes that is formed above c*. Due to this oscillation of the
polymer concentration within the film, it is suggested that the
surfactant is unequally distributed over the polymer chains so
that some of them become surface active again.81
Comparing the foam film stabilities to the results of the surface
tension and elasticity measurements, one can clearly see that the
stability of the foam films is independent of the surface coverage.
Concentration regimes with high surface coverage but low net
charge show low stabilities, whereas in the regime with high
surface tension and therefore low surface coverage, one can
observe very stable films. This indicates that the net charge of the
system plays an important role in foam film stabilisation. The
simulation of the surface potential of the C14TAB/PAMPS
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system confirms this picture. The pure surfactant film has
a potential of 70 mV which is decreased to 53 mV when 10�5 M
PAMPS is added so that the surface potential at low poly-
electrolyte concentrations seems to be dominated by C14TAB.
However, at a higher concentration of 10�3 M PAMPS a surface
potential of 83 mV is found which is higher than that of the pure
surfactant film even though no surface complexes are detected.
This seems to be an effective surface potential since it does not
only reflect the potential at the surface but includes all charges in
the system.
To investigate the influence of the surfactant chain length,
measurements of C14TAB and C12TAB with PAMPS are
compared. Pure C14TAB films are stable but with the addition of
the polyelectrolyte the stability decreases towards the IEP and
a complete destabilisation at that point can be observed. When
the polyelectrolyte concentration is further increased foam film
stabilities up to 1600 Pa are found.
Due to the shorter alkyl chain and the resulting lower Gibbs
elasticity57 pure C12TAB does not form stable films. Only after
the addition of polyelectrolytes can disjoining pressure isotherms
be observed.60 Fig. 12 shows that a concentration of 3 � 10�5 M
PAMPS is needed to stabilise the foam film. Further poly-
electrolyte addition leads to an increase the foam film stability
except for a stability minimum close to the nominal IEP. Two
major differences between the two stability graphs can be
observed: in the case of C12TAB/PAMPS no complete destabi-
lisation at the IEP is measured and foam films at high poly-
electrolyte concentrations are much more stable than with the
surfactant with a longer aliphatic chain. In the concentration
regime below the nominal IEP, surface tension measurements of
C12TAB/PAMPS give higher and elasticity measurements give
lower values indicating less hydrophobic complexes at the
interface. Together with the later decrease in surface tension and
increase in elasticity, respectively, at higher polyelectrolyte
concentrations this suggests a lower interactions between the
surfactant molecules and the polymer chains, leaving more free
surfactant molecules at the film surface to help to stabilise the
film.
Exchanging the hydrophilic polymer PAMPS with the more
hydrophobic PSS, only minor changes occur. The stability graph
of C12TAB/PSS has the same shape as that of C12TAB/PAMPS
but is shifted to a higher stability. The surface tension below the
nominal IEP is slightly lower and the elasticity higher, indicating
that the hydrophobic parts of the polyelectrolyte are surface
active as well and contribute to the film stability. These
measurements show that the effect of the surfactant chain length
is more important than the hydrophobicity of the polyelectrolyte.
The surfactant influences the strength of the interactions between
the two components and hence the shape of the stability curve.
The difference in hydrophobicity of the polyelectrolyte has only
a slight effect on the stabilisation of the films.
Fig. 14 Disjoining pressure isotherms of free-standing pure C12G2 film
(data taken from ref. 44) and mixed C12G2/PDADMAC films. Data
taken from ref. 8.
5.3 Nonionic surfactant and polyelectrolytes
5.3.1 Surface characterisation. The effect of the addition of
polyelectrolytes (PDADMAC) on the surface tension of an
aqueous solution of nonionic surfactant C12G2 is rather minor.
This shows that no polyelectrolyte/surfactant complexes are
formed at the surface. The slight decrease in surface tension in
This journal is ª The Royal Society of Chemistry 2010
Fig. 5 (squares) indicates an adsorption of the polyelectrolytes
due to their hydrophobic backbones. As mentioned above, the
neat air water/interface is assumed to be negatively charged due
to the adsorption of OH� ions.29,47,48 The adsorption of nonionic
surfactants leads to a reduction of the surface charge, but the
surface remains negatively charged over a large concentration
range. The polyelectrolyte might be attracted by the interface,
but no surface complexes are formed.
5.3.2 Effect on film thickness. The films stabilized with
nonionic C12G2 (well below cmc) form a CBF and are shown in
Fig. 14. They are less stable than the films stabilized with
cationic C16TAB, since the pure C12G2 films already rupture at
6000 Pa.44 The electrostatic stabilisation is explained by the
negative charges at the air/water interface as mentioned above.
After the addition of 5 � 10�3 monoM PDADMAC a CBF–
NBF transition is already induced at a pressure of about
800 Pa, and the film is only a few nm thick. The NBF is not
stable and it breaks after a few minutes at 800 Pa. A NBF is
also observed for the pure C12G2 film after increasing the C12G2
concentration, due to the replacement of the negative charges at
the air/water interface by the non-ionc surfactant. Simple salt
also induces a CBF–NBF due to electrostatic screening. Those
types of NBF are much more stable, which is explained by
a high ordering of the surfactant molecules at the film surface,
giving a crystalline ordering of the whole NBF.82 The CBF–
NBF transition after addition of PDADMAC can only be
explained by electrostatic screening of the negative surface
charges, since the polyelectrolyte does not adsorb at the surface,
as indicated by surface tension measurements. Nevertheless, the
structure of the surface of the NBF seems to be less ordered and
less crystalline than for pure systems. This might be due to
a lower packing density of surfactants and/or the fluctuation/
mobility of polymer chains, which leads to a lower stability of
the NBF.
On the other hand, if the cationic PDADMAC is replaced by
the anionic PSS in the C12G2 film, a CBF of about 30 nm is
obtained and no NBF occurs up to a pressure of about 6000 Pa
(see Fig. 15). The mixed film shows a thicker final film thickness
than the pure surfactant film which indicates a stronger electro-
static repulsion within the film due to the increase of negative
charges after the addition of polyanion PSS.
Soft Matter, 2010, 6, 849–861 | 857
Fig. 15 Disjoining pressure isotherms of free-standing pure C12G2 film
(data taken from ref. 44) and mixed C12G2/PSS films. Data taken from
ref. 8.
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5.4 Surfactants and neutral polymers
Films containing neutral polymers like poly(N-methyl-
vinylacetamide) (PVNMA),14 poly(vinyl alcohol) (PVA),16 and
poly(vinylpyrrolidone) (PVP)17 show CBF.
PNIPAM is a neutral water-soluble polymer as well, but
additionally, it is thermoresponsive with a LCST of 32 �C. It
forms rather thick films (110 nm) at room temperature in the
presence of SDS that get even thicker when the temperature is
increased.83 This can be explained by an increase in hydropho-
bicity of the PNIPAM molecules, which forces them to adsorb at
the film surface. Above the LCST the polymer collapses and the
dense polymer aggregates at the film surface84 provoke the film to
rupture. At low SDS concentrations the film thickness is not
affected by the surfactant. Only at concentrations above 2� 10�4
M SDS does the thickness show a clear dependence on surfactant
concentration and decrease with further addition of surfactant.
This is explained by a higher electrostatic screening due to an
increase in counterion concentration. At low surfactant
concentrations the film thickness is dominated by polymer tails
that dangle into the film bulk, therefore it increases when poly-
mers with higher molecular weight are used (cf. Fig. 16). In the
presence of higher SDS concentrations, PNIPAM is progres-
sively displaced from the interface due to the interactions
between the polymer and SDS above the cac and can not influ-
ence the film thickness any more. The displacement from the
interface has also been observed for other neutral polymers like
PEO.85,86 For the determination of the cac of PNIPAM/SDS
Fig. 16 Schematic picture of the cross section of a PNIPAM/SDS film.
Reprinted with permission from ref. 83. Copyright (2009) American
Chemical Society.
858 | Soft Matter, 2010, 6, 849–861
systems, surface tension measurements have been found to be
unsuitable due to the low surface tensions for pure PNIPAM
solutions. Instead, isothermal titration calorimetry has been used
to investigate the cac.87
Foam films can also be stabilised by pure PNIPAM without
any surfactant.88 These films are strongly dependent on temper-
ature and can form inhomogeneities due to lateral temperature
fluctuations parallel to the film surface.
6 Stratification phenomena
One of the main topics in the investigation of foam films containing
polyelectrolytes is the discontinuous thinning of a film, the so-called
stratification process. This stepwise thinning of the foam film is
observed below the cac in the semi-dilute concentration regime of
the polyelectrolyte.89 Stratification occurs due to an oscillation of
the disjoining pressure in the film and is assumed to originate in
a polyelectrolyte network that is formed in the film core above
c*.9,12,26,27 When the applied pressure is increased, layers of the
network are pressed out of the film. This is an irreversible process,
since it is not possible to go back to another branch of the oscillation
when the pressure is reduced. The steps in thickness follow a power
law that scales with Dh f c�1/2 for linear polyelectrolytes and
Dh f c�1/3 for branched polymers like PEI.90 In the case of a linear
polymer, this corresponds to x, the mesh size of the polymer
network that is formed in the bulk.6,59,91 A good method to inves-
tigate these oscillations is the colloidal probe AFM.91 With this
apparatus one can measure on the same length scales, e.g. con-
cerning film thickness, but one can image the whole force oscillation
and not only the mechanical stable part like with a TFPB.
The stratifications are affected by the salt content of the
solution and the degree of charge of the polyelectrolyte. The
addition of salt suppresses the occurrence of the steps whereas in
films with polyelectrolytes with a low degree of charge all steps
take place simultaneously with very low disjoining pres-
sures.9,14,92
The formation of the network of polymer chains has been
widely discussed in the literature. The question has arisen if it is
really a network that is formed in confinement or if there rather is
a layering of the polymers with a pronounced alignment parallel
to the surface.6,93,94 Fluorescence measurements81 of pyrene-
labeled PAA95 have shown that there indeed is a layer wise
arrangement but with a distance between the layers that corre-
sponds to a random polymer network.
Backbone rigidity also plays an important role in the stratifi-
cation process. For flexible polyelectrolytes the force oscillation
can be observed as long as the velocity of the two approaching
surfaces is not too fast.94 More rigid polymers show stratification
only when viscosity of the solution is large enough so that the
network has time to adjust, otherwise no stepwise thinning can
be observed.
The choice of the surfactant has no detectable influence on the
structuring of the polyelectrolyte chains within the film core
which leads to a fixed period of force oscillation at a certain
polyelectrolyte concentration independent of the surfactant
type.4,8 This means that the interaction between the poly-
electrolytes and surfactant molecules can be neglected with
respect to their effect on structural forces. Fig. 14 and 6 show the
disjoining pressure isotherms of free-standing aqueous films
This journal is ª The Royal Society of Chemistry 2010
Fig. 17 Sketch of a growing domain of the radius R. The film thickness
equals h0 inside the domain and hN at infinity. The film tension difference
between the inside and the outside results in a rim with height h1. Material
transport is marked by black arrows. Reprinted with permission from ref.
100. Copyright (2006) American Chemical Society.
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containing the polycation PDADMAC in combination with
either nonionic C12G2 or positively charged C16TAB. The
present results confirm that the surfactant has no influence on the
step size, and therefore on the structuring of the polyelectrolytes
within the film. Beside the charge also the elasticity of the
interfaces has no effect on the structural forces. They were
measured in films of polyelectrolyte solutions not only in foam
films but also in wetting films96 and between two solid interfaces
in an AFM.93,97 The period of the pressure oscillation remains
constant.
In contrast to that, the interactions between polyelectrolyte
and surfactant do affect the velocity of the stratification. The
stratification process starts with small discontinuities in the film
thickness, visible as small dark dots spreading over the whole
film. The velocity of the growth of these domains depends on the
boundary conditions of the surface. When the polyelectrolyte is
linked to the surface, the domain growth is much slower than in
the case of a depleted interface.98,99 The reasons for this could be
that the polyelectrolyte chains dangling from the surface slow the
domain growth process. The driving force of this domain growth
is the difference in film tension Ds between the two film parts.
The film tension in the inner part of the domain is smaller than
that of the thicker film,100 and as the system favours the lower
energy state of the inner part the domains are enlarged. Ds is
assumed to result in an increase of the film thickness of the rim
surrounding the domain so that h1 is a material constant
(cf. Fig. 17).
7 Conclusion
The results of the studies at the air/liquid interface are applied to
the film interfaces in order to understand the effect of the inter-
actions between surfactant and polyelectrolyte at the film surface
on the film stability.
In case of equally charged polyelectrolytes and surfactants
both compounds repel each other. No pronounced adsorption of
the polyelectrolyte at the surfactant covered surface could be
detected and the surface tension is not affected. The addition of
like charged polyelectrolyte enhances the overall electrostatic
repulsion within the film.
In films of oppositely charged surfactant and polyelectrolyte it
is assumed that polyelectrolyte/surfactant complexes are formed.
They stabilize the foam film by decreasing the surface elasticity.
The polyelectrolytes screen the surfactant charge laterally, so
This journal is ª The Royal Society of Chemistry 2010
that they can pack more densely in the surface. For instance,
C12TAB does not form stable films, but in combination with an
oppositely charged polyelectrolyte stable CBF are formed. The
adsorbed amount varies non-monotonically with both the
surfactant and the polyelectrolyte concentration.
At an excess of either polyelectrolyte segments or surfactants
a stable CBF occurs. At an excess of surfactant it is assumed that
polyelectrolyte/surfactant complexes adsorb at the film surface,
while at an excess of polyelectrolyte the surface layer consists
rather of pure surfactant. Close to the IEP the film becomes
unstable, but no NBF occurs.
Following this argument line, the high stability of the CBF
containing nonionic surfactant and a polyanion is explained by
an electrostatic repulsion between the polyanion and the nega-
tively charged film surface. The only combination where the film
interfaces and the polymers are oppositely charged is in films of
nonionic surfactant and positively charged polyelectrolytes. This
could reduce the surface potential and therefore the electrostatic
repulsion, and a transition to a NBF occurs. In case of non-ionic
surfactants no surface-active complexes are formed.
The addition of polyelectrolytes affects the stability of the
foam films, and it is highly related to the type of final film. While
the CBF is stable, the NBF breaks within the first minutes after
its formation. This means that the steric repulsion of the nonionic
surfactant layer at the film surface are rather weak. The reason
could be that the surfactant molecules are less densely packed
and/or that there is still polymer inside which reduces the packing
density or which leads to fluctuations and to film rupture. In
contrast to this, for stable NBF of pure surfactant a crystalline
packing of the surfactant molecules is assumed.82
Above the overlap concentration c* the stabilizing effect of
polyelectrolytes is due to oscillatory forces which might hamper
the film drainage.101 The velocity of each stratification step
depends on the composition of the surface. The dangling ends of
polyelectrolyte/surfactant complexes at the film surface reduce
the velocity in comparison to film surfaces consisting of pure
surfactant.
The results show that the osmotic pressure of the added
polycation does not play a dominate role. Otherwise the film
thickness should increase after the addition of the poly-
electrolyte.
Acknowledgements
The authors thank the German Research Council (DFG) and the
EU for financial support. The part of the reviewed studies
coming from the Klitzing lab were carried out within the frame of
the DFG priority program ‘‘Kolloidverfahrenstechnik’’ (KL
1165/10-1 and 2), the CRG 448/TP B10 (DFG) and the EU
Marie Curie network SOCON. Reinhard Miller is acknowledged
for the oscillating drop measurements.
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