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: klitzing@chem.tu-berlin.de; 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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