Polyelectrolytes in thin liquid films

Current Opinion in Colloid & Interface Science 15 (2010) 303–314

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Current Opinion in Colloid & Interface Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c o c i s

Polyelectrolytes in thin liquid films Cagri Üzüm 1, Nora Kristen 2, Regine von Klitzing ⁎ Stranski-Laboratorium für Physicalische und Theoritische Chemie, Technische Universität Berlin, Straße des 17, Juni 124, D-10623 Berlin, Germany

a r t i c l e

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Article history: Received 5 May 2010 Accepted 11 May 2010 Available online 16 May 2010 Keywords: Thin films Polyelectrolytes Polymeric surfactants Stratification Depletion force Polymer brushes Thin Film Pressure Balance Colloidal-probe technique

a b s t r a c t The review addresses the influence of polyelectrolytes on the statics and dynamics of thin liquid films. Both, changes of interfacial and bulk properties, contribute to the overall behaviour of thin films formed from aqueous polyelectrolyte solutions. Therefore, the chapter is separated into two parts: polyelectrolytes at film interfaces and polymers in film bulk. © 2010 Elsevier Ltd. All rights reserved.

1. Introduction The stability of aqueous colloidal dispersions like foams, emulsions and suspensions is mainly determined by the stability of thin liquid films between air bubbles, liquid droplets and solid particles, respectively. The addition of polymers can affect both the drainage (dynamics) and the static properties of the thin films. This is due to their excess or depletion in the interfacial region and their texture or structure formation in the film bulk of polymers. The chapter mainly focused on the effect of charged polymers, i.e. polyelectrolytes on the properties of thin liquid films and is separated into two parts. In the first section the polymer adsorption at film surfaces is described and the second section addresses the structure formation of polyelectrolytes in the film bulk above the overlap concentration C⁎ (semi-dilute regime). 2. Polyelectrolytes at film interfaces The following section focuses on the adsorption/depletion of polyelectrolytes at film surfaces. Since the film surfaces themselves are difficult to access, in most of the cited papers studies of polyelectrolyte adsorption at a free surface without any opposing surface are correlated with film studies like on film thickness, stability, and drainage. Most of the homopolyelectrolytes in water do not form stable foam films (free-standing films) due to missing amphiphilic ⁎ Corresponding author. Tel.: + 49 30 314 23476; fax: + 49 30 314 26602. E-mail addresses: [email protected] (C. Üzüm), [email protected] (N. Kristen), [email protected] (R. von Klitzing). 1 Tel.: + 49 30 314 24323; fax: + 49 30 314 26602. 2 Tel.: + 49 30 314 26774; fax: + 49 30 314 26602. 1359-0294/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.cocis.2010.05.009

character and surfactants have to be added to form stable films. In this case the interaction between surfactant and polyelectrolyte plays a decisive role on the drainage and stability of foam films and therefore for the macroscopic foam (subsection 2.1). In contrast to this homopolyelectrolytes block copolymers can have an amphiphilic character, which might lead to an excess at the film surfaces and to stable foam and emulsion films (subsection 2.2). Finally the adsorption at solid surfaces is considered (subsection 2.3). 2.1. Surfactant/polyelectrolyte complexes at film interfaces Adding polyelectrolytes to surfactant solutions changes surface properties and foaming behaviour dramatically. To get a deeper insight into the correlation between surface composition and foaming behaviour, the free air/water interface and the properties of the corresponding single foam film after the addition of polyelectrolyte were studied. Depending on the polyelectrolyte/surfactant mixture, either electrostatic or hydrophobic interactions dominate the system. The interactions and the corresponding changes of the surface properties are usually studied with tensiometry or elasticity measurements, neutron or X-ray reflectometry, or ellipsometry. In case of strong hydrophobic interactions between polymer and surfactant, a depletion of surfactant at the interface occurs. Therefore, the surface tension increases in comparison to the pure surfactant system, as for PSS/C16TAB [1]. Complexes that arise from the hydrophobic interactions between the molecules are charged and therefore hydrophilic, which explains the depletion of the surfactant from the interface. In systems containing likely charged surfactant and polyelectrolyte the strong electrostatic repulsion dominates the interactions between the compounds, and the surface tension shows no change after the

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addition of polyelectrolyte compared to the pure surfactant solution [2]. Mixing of oppositely charged compounds results in a surface tension curve with a very characteristic shape and is studied since several decades [3–5]. Typical concentration ranges for studying these systems are from 10–5 to 10−2 monoM for the polyelectrolyte and from 10− 6 to 10− 2 M for the surfactant. At low surfactant concentrations, the surface tension is lower compared to the pure surfactant due to the formation of complexes at the interface. The counterions of the polyelectrolytes are exchanged with dissociated surfactant molecules and released as this is energetically favourable for the surfactant due to the resulting decrease in electrostatic repulsion between the likely charged groups. Additionally, the release of the counterions increases the entropy of the system. The complexation of the polyelectrolyte and the surfactant can lead to very hydrophobic and hence, surface-active complexes. In this concentration regime, only loosely packed surface monolayers are formed at the surface [6]. With further increase in the surfactant concentration, the surface tension reaches a plateau which is characterised by the critical aggregation concentration, CAC. At this point, polyelectrolyte/surfactant aggregates are formed in the bulk that turns the solution turbid. The aggregates are described as surfactant half micelles that are formed on the polyelectrolyte chain. When foam films are formed in this concentration regime, the aggregates are trapped in the film or at the film surface and inhomogeneous films are formed. With further addition of surfactant, these aggregates might be solved again and the solution gets clear again. The second break point in the surface tension curve is the critical micelle concentration, CMC, at which micelles are formed in the bulk solution. From this point on, the surface tension stays constant upon the addition of surfactant, since all surfactant molecules are integrated into micelles. A detailed description of the surface properties of polyelectrolyte/surfactant mixtures can be found in the recent reviews of Taylor et al. [7] and Kristen et al. [8]. 2.1.1. Oppositely charged polyelectrolyte/surfactant mixtures In a study on foam films of PAMPS/C14TAB mixtures, the correlation between the complexes at the surface and the stability of the films was investigated [9]. In this work, the net charge of the system was tuned by varying the polyelectrolyte concentration between 10− 5 M and 10− 3 M, while the surfactant concentration was fixed at 10− 4 M. At very low polyelectrolyte concentrations, the net charge in the system is positive due to the dissociated cationic surfactants at the surface. With increasing polyelectrolyte concentration, the charge is reduced. The stability of the foam films follows this trend: At low charges, very stable films up to a disjoining pressure of several kPas are formed, while the stability is decreased when the charge is reduced. At the nominal isoelectric point (IEP) of the system, where the nominal charge of the system equals the nominal surfactant charge no stable films are formed at all. Above the IEP the film stability increases again. These findings were firstly interpreted by a charge reversal from positively over neutral to negatively charged film surfaces. A closer look of the film surface properties shows that the simple image of charge reversal at the film surfaces with increasing polyelectrolyte concentration is not correct, as summarized in the following. The investigation of the surface properties showed that the coverage is highly dependent on the polyelectrolyte/surfactant ratio. Surface tension measurements on the system revealed that at polyelectrolyte/surfactant ratios below 1, very hydrophobic complexes are formed at the surface, which strongly reduce the surface tension. When the ratio approaches 1, the surface complexes are suddenly released from the surface and only a thin surfactant layer covers the surface. This indicates a change in the nature of the complexes because they seem to get hydrophilic again due to the lower amount of surfactant on the polyelectrolyte chains and the resulting higher charge on the complexes. At higher polyelectrolyte concentrations (around C⁎), new complexes are adsorbed at the

surface due to the formation of a polyelectrolyte network in the film core. All these results are supported by surface elasticity measurements. However, comparing these findings to the stability of the corresponding foam films suggests that the surface coverage has no influence on the stability of the foam films. The most stable films are formed in a concentration regime where the surface tension has the highest values, which indicates a low coverage, while those films with the highest surface coverage are very unstable. Hence, the crucial point seems to be the net charge in the system. Similar trends have been also found for other polyelectrolyte/surfactants like PAMPS/ C12TAB and PSS/C12TAB [8,9]. In the example above the surfactant concentration was kept fixed below the CAC and the polyelectrolyte concentration was increased up to C⁎, but below the CAC in order to avoid the formation of heterogeneous foam films. In contrast to that, in most of the studies on oppositely charged surfactant/polyelectrolyte mixtures the polyelectrolyte concentration is kept constant and the surfactant concentration was varied, giving the opportunity to do measurements below C⁎ and through the CAC. In the last years mainly more complex polyelectrolytes, i.e. proteins were used in order to study oppositely charged surfactant/protein systems. In a recent work on lysozyme/ SDS and lysozyme/C10DMPO mixtures, foam films at different protein/ surfactant ratios have been studied [10]. Lysozyme is a protein with a positive net charge of 8e at neutral pH. At a protein concentration of 10− 5 M and very low surfactant concentrations, the air/water interface is covered by more or less pure lysozyme layer. Under these conditions, very unstable foams are formed. At high surfactant concentrations (N10− 3 M) a progressive displacement of the protein from the surface takes place. In this concentration regime, the surface tension of the mixed system coincides with the one of the pure system, which suggests a pure surfactant layer at the surface. From these solutions, very stable and homogeneous foam films can be formed, due to the strong repulsion of the two interfaces. While at low and high surfactant concentrations both surfactants have the same qualitative effect on surface tension, film and foam stability, the fundamental differences in the intermediate surfactant concentrations. The addition of SDS leads to the typical strong reduction of the surface tension followed by a plateau in surface tension around the CAC, indicating a strong interaction between lysozyme and SDS. At a SDS concentration of 7 × 10− 5 M, i.e. close to the nominal IEP, the surface activity of the complexes is very high, due to the fact that all charges are more or less compensated. The addition of slightly more surfactant to the system (2 × 10− 4 M) gives rise to additional hydrophobic interactions. This leads to the formation of aggregates and hence inhomogeneous but fairly stable foam films. In contrast to this, the tendency of C10DMPO and lysozyme to form complexes at the surface seems to be much more reduced and no CAC could be detected. Up to a C10DMPO concentration of 2 × 10− 4 M the surface is more or less covered by the protein and the films and foams are unstable. Above this concentration the surfactant covers mainly the surface and the films and foams become stable. One important parameter concerning the adsorption of polyelectrolytes to the interface is the degree of charge. Polyelectrolytes with a lower degree of charge form a thicker interface layer as they are only loosely connected to the surface and the uncharged parts dangle into the solution bulk. In contrast to that, highly charged polymers are flatly adsorbed to the surface and form therefore thin layers [7,8]. The charges at the polyelectrolytes can be either varied during synthesis or later by changing the pH. A comparison of adsorption behaviour of mixtures of cationic C16TAB and negatively charged pectin with different degrees of charge is shown in a study by Ropers et al. [11]. The degree of charge of the pectin has been varied by methylating the carboxyl groups of the polymer. For better comparison, the concentration of charged groups of the polyelectrolyte in the system has been kept constant at 0.24 mM. Surface tension measurements reveal that the polyelectrolyte with the highest degree of charge reduces the

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surface tension most effectively. The CMC of this system is one magnitude lower than the CMC of the pure surfactant and the surface tension curve shows the typical behaviour with a plateau close to the CAC. When the degree of charge is reduced, the reduction of surface tension is less and no CAC can be observed. The absence of CAC has been interpreted in terms of low cooperation between the alkyl chains of the surfactant (like in the C10DMPO/lysoyme system, described above). Surfactant molecules bound to the pectin chain are too far from each other to be assembled into stable micelles that are wrapped by polymers, so that no aggregates are formed. At surfactant concentrations higher than 2 × 10− 4 M all surface tension curves coincide on that of the pure surfactant, indicating, that the polymer/ surfactant complexes are released and the surface layer is mainly composed of C16TAB. Since pure pectin solutions do not foam, all foaming properties in this system arise from C16TAB or C16TAB/pectin complexes. The presence of pectin with the highest degree of charge (63%) results in slightly more stable films compared to those in the pure surfactant at the same concentration of 5 × 10− 5 M, due to the complexes at the surface. When the C16TAB concentration is increased to the CAC, inhomogeneous but very stable films are formed. This behaviour can be explained by the slower drainage of the film due to the aggregates that are trapped in the film. An increase in surfactant concentration to the point where all polyelectrolyte/surfactant complexes are released from the surface (≥10− 3 M) leads to a sharp decrease in the stability, down to the value of the pure surfactant. This indicates that the film behaviour is dominated by the pure surfactant layer at the interface. 2.1.2. Mixtures with a neutral component In order to complete the report on effect of charged compounds, the effect of both non-ionic surfactants and neutral polymers is described in the following. 2.1.2.1. Neutral surfactant. When a neutral component is involved in the polyelectrolyte/surfactant mixture and the interactions between the components are low, a different mechanism of adsorption occurs. In that case, competitive adsorption takes place, where the component with the higher surface pressure displaces the other compound. This can be demonstrated with Tween 20/casein mixtures; Tween 20 is a non-ionic commercially available surfactant, whereas casein is a milk protein that consists of several subunits, namely α–β- and κcasein. β-casein can be considered as diblock copolymer, since only the first 50 residues carry negative charges, which makes it amphiphilic and therefore, surface active [12,13]. Again the concentration of the protein is fixed at 5 × 10− 6 M, while the surfactant concentration was varied. First, mixtures of β-casein and Tween 20 are investigated. At low surfactant concentrations, the surface tension is dominated by the adsorption of the protein. This indicates a more or less pure polymer layer at the surface. When the surfactant concentration is increased over 2 × 10− 5 M, the behaviour of the surface tension changes abruptly. From this concentration on, the surfactant seems to control the adsorption process, since the surface tension isotherm coincides on that of the pure surfactant. The surfactant might therefore have forced the protein out of the surface layer. Solutions of pure β-casein form very stable foams, whereas Tween 20 is a poor foaming agent. When Tween 20 is added to the protein solution, foam film stability decreases continuously until it approaches that of the pure surfactant foam. The behaviour of the foam formed from the protein/surfactant mixtures reproduces perfectly the predicted mechanism of the protein displacement with increasing surfactant concentration [12]. In case of whole casein in the protein/surfactant mixture, the protein is not completely replaced in the surface layer. This is indicated by a lower CMC in the mixed system compared to the pure surfactant isotherm. This suggests that the surfactant interacts with some of the other protein subunits so that complexes are formed at

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the interface. At lower surfactant concentrations, whole casein/Tween 20 mixtures form thinner foam films compared to films consisting of β-casein and Tween 20 which seems to be the reason for the lower stability of the corresponding foam. In contrast to that, foam films formed from mixtures with higher surfactant content are considerably thicker than those of β-casein, so that these foams are much more stable [13]. 2.1.2.2. Neutral polymers. Neutral polymers are also able to stabilise foams if they are hydrophobic enough to adsorb at the surface. In a study by Jean et al. [14] the behaviour of foam films stabilised by PNIPAM and by PNIPAM/SDS is investigated. PNIPAM is a neutral, thermo-sensitive polymer with a lower LCST of 32 °C. Stable foam films can be formed from both, pure polymer solutions and polymer/ SDS mixtures below the LCST at a polymer concentration of 0.1 mg/ml. Due to the hydrophobicity of the polymer, PNIPAM is able to adsorb at the surface. However, above 32 °C, PNIPAM starts to clot at the surface, leading to inhomogeneous films that rupture immediately. At low surfactant concentrations (below 10−4 M SDS), the foam films behave like pure PNIPAM films and they are very thick (120 nm). Since PNIPAM is a neutral polymer, the film stability cannot arise from electrostatic repulsion. These films are rather stabilised by long-range steric forces that are generated when the surfaces come closer and the polymer layers start to overlap. In this case, the thickness of the film is much larger than 2 Rg which indicates long polymer chains that dangle from the surface where they are connected, into the film core thereby forming a very thick PNIPAM layer. When the surfactant concentration is increased over the critical aggregation concentration CAC, the surfactant starts to form micelles on the polymer chains in a pearlnecklace like structure. Due to the negatively charged surfactant molecules, these complexes are hydrophilic. When the SDS concentration is further increased, the polymer/surfactant aggregates are continuously replaced, so that at very high surfactant concentrations, the surface layer mainly consists of SDS molecules. Furthermore, the film thickness decreases with increasing surfactant concentration, since the higher ionic strength induces charge screening. All films have been studied at a constant capillary pressure of 50 Pa, at which they are stable for hours. Gas permeability measurements of PNIPAM and PNIPAM/SDS films were carried out [15] with the Diminishing Bubble method, depicted in Fig. 1. For that, a bubble of radius R is blown onto the surface by using a fine capillary tube. The pressure difference between both sides of the film gives rise to gas diffusion through the film from the bubble interior to the outside air. The evolution of the bubble radius and the film radius is monitored as a function of time. Concerning the gas permeability, pure PNIPAM films show different behaviours than films from mixed polymer/surfactant solutions. In contrast to pure SDS systems, the gas permeability coefficient of pure polymer films is principally determined by the permeability of the adsorbed polymer layer, whereas the film thickness only plays a minor role. When SDS is added to the system, the gas permeability increases, even to higher values than that of the pure surfactant system. The polymer/surfactant complexes that are situated at the film interface disrupt the ordering of the surfactant molecules, so that the surface layer is more permeable to gas molecules. The addition of NaCl accelerates the process. 2.2. Polymeric surfactants in thin films Certain types of block copolymers can possess surfactant properties and they are referred as polymeric surfactants. Their applications find place in the stabilisation of oil-in-water or water-in-oil emulsions, solid–liquid dispersions as well as foam films [16•,17•–25]. Preparation techniques of colloidal suspensions stabilised by various polymeric surfactants are proposed in a series of studies by Rotureau

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Fig. 1. Schematic drawing of a bubble on the solution surface. The image shows the foam film as observed by a microscope. R and r are the radii of the bubble and the film respectively. Taken from Ref. [15] with permission.

et al. [26–29]. Depending on the composition and concentration of the polymer, and the temperature of the system, polymeric surfactants can have a destabilising effect, such as defoaming [16•]. Recently, to extend their previous work [30], Blomqvist and coworkers studied the effect of non-ionic PEOx–PPOy–PEOx triblock copolymers on the foam stability. They altered the hydrophobic PPO and the more hydrophilic PEO chain lengths by varying x and y [16•]. The authors used Ross-Miles, Foam Column (Foamscan), FRAP and Surface Dilatational Rheology methods to study the relationship between the foam stability, the surface rheology and the steric repulsions. It was observed that long-range steric repulsions are essential for stable foams. The chain lengths of the hydrophobic and hydrophilic blocks play important roles on the stability, foams of PEOx–PPOy–PEOx with short hydrophilic chains being highly unstable. Exerowa and co-workers studied the thin foam and emulsion films stabilised by hydrophobically modified graft polymeric surfactant (INUTEC SP1) in a series of studies [17•–19]. The foam films were found to be relatively unstable with a rupture thickness of 8–9 nm [17•]. The interactions between the foam film surfaces are reported to be highly dependent on the salt concentration (Cel). Up to a critical electrolyte concentration (Cel,cr) (0.02 M NaCl), the film thickness (at constant capillary pressure) decreases with increasing ionic strength. That behaviour was attributed to the compression of the electrical double layer, and was explained by DLVO theory. DLVO forces vanished when Cel N Cel,cr and a steric repulsion was observed, caused by the large hydrated “loops” on the INUTEC SP1 chain due to its unique structure. In the second study of the series [18], the authors focused on the interactions between two oil droplets stabilised, again by, INUTEC SP1 molecules in aqueous media. DLVO originated repulsive interactions between two droplet surfaces were overtaken by steric repulsion when Cel N Cel,cr, where Cel,cr = 5 × 10− 2 M. In contrast to the foam film system, a very stable Newton Black Film (NBF) (∼7 nm) was observed. The NBF was resistant to rupturing even at high salt concentrations (2.0 M NaCl) and high capillary pressures (45 kPa). This strong stabilisation was attributed to short-range steric interactions of the strongly hydrated loops and tails, as in the foam films (see Fig. 2B). Similar loop-to-loop repulsions were also observed for other types of graft copolymers [21]. The next study of the same authors showed no effect of type of salt on the thickness and stability of the NBF [19] but a strong effect on the value of Cel,cr and the thickness of double

layer, as expected by DLVO theory. Nedyalkov et al. observed a similar structuring in the wetting films containing the same polymeric surfactant and the same types of salt as above [31]. However, these authors reported a slow decrease in the NBF thickness as the salt concentration was increased, even when Cel N Cel,cr. Exerowa and co-workers proposed that the most effective stabilisation of emulsions can be achieved using ABA block- or ABn graft copolymers [20]. To better understand the stabilisation mechanism in the thin film between opposing emulsion droplets, the conformation of the two types of copolymers should be compared (see Fig. 2). Based on the interfacial tension and disjoining pressure measurements, the same salt driven transition from a DLVO originated repulsion to a steric repulsion was observed as has been discussed above. The length of the brush chains (oriented perpendicular to the surfaces) played an important role in the equilibrium thickness of the thin film. A different disjoining pressure profile for the block (ABA) and grafted (ABn) copolymers was observed when Cel N Cel,cr. For the grafted copolymer, the jump to the sterically stabilised NBF occurred at much lower capillary pressures, giving also a higher film stability. The different stabilising effects were attributed to “brush-to-brush” and “loop-to-loop” interactions dominating the block and grafted systems respectively. In Ref. [23], the reader can find a review of the recent studies on thin films containing non-ionic polymeric surfactants. Another recent paper surveys polymeric surfactants in disperse systems [25]. 2.3. Polymers adsorbed/grafted on the interfaces Reversible or irreversible adsorption of polymer chains on the surfaces has been an area of interest for over five decades. Although there are several theoretical and experimental studies investigating adsorbed and grafted polymers, the majority of the effort is given to unconfined surfaces. However, it should be noted that the confined systems cannot be studied apart from the unconfined (bulk) systems. Several reviews about adsorbed/grafted polymers on either unconfined surfaces or in thin-film geometry are available in the literature. Charged or neutral chains physically adsorbed or chemically grafted on the surfaces are reviewed in Refs. [32] and [33••] in terms of force measurements. The detailed review about polyelectrolytes in solutions and at surfaces by Dobrynin and Rubinstein [34] is a fundamental theoretical source on the topic in addition to Netz and Andelman's review about the adsorption of neutral and charged polymers [35]. Following the very detailed work of Ruhe et al. [36], Ballauf and Borisov also focused only on polyelectrolyte brushes, which they define as “systems consist of long polyelectrolyte chains that are grafted densely to planar or curved surfaces” [37]. On the other

Fig. 2. Conformation of (A) triblock (ABA) and (B) graft (ABn) copolymeric surfactants at oil/water interface. h: separation distance and δ: adsorbed layer thickness. The diagram is taken from Ref. [20] with permission.

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hand, systems in which colloidal particles are coated with a dense layer of linear polyelectrolyte chains on their surface are called “spherical polyelectrolyte brush” systems and they are reviewed in Ref. [38]. The following discussions will only include recent studies on adsorbed/grafted polymer chains in thin-film geometry. If sufficiently long and sufficiently charged polymers (polyelectrolytes) are adsorbed/ grafted densely to a solid surface, a polyelectrolyte brush is formed. Due to the confinement of a major fraction of counterions within the brush layer, there is a high counterion based osmotic pressure difference between inside the brush and the outer solution. Stretching of polyelectrolyte chains is governed by this osmotic pressure while the stretching in neutral polymers is an outcome of the inter-chain repulsion [37,39]. Compared to either non-stretched or non-charged polymer chains, polyelectrolyte brushes have enhanced segment–segment repulsions and electrostatic interactions [36]. As two brush bearing surfaces, such as colloidal particles, come close enough in the solution, a thin film is formed between the opposing surfaces. The brushes possess unique properties in the thin film compared to the solution bulk. Their significant effect on the interactions between the surfaces bearing them leads to an inhibited flocculation in colloidal systems and lets controlling of the material surface properties such as adhesion, lubricity and biocaompatibility [40,41]. In Ref. [42], the reader will find a recent review on the interactions between solid surfaces with adsorbed polyelectrolytes and the effects of adsorbed polymers on the colloidal stability. Additionally, Monte Carlo simulations run by Linse focus on the interactions between colloids with grafted diblock polyampholytes and give informative results [43]. Structuring of polyelectrolyte brushes confined in a thin-film geometry defines the normal forces between the thin-film walls as well as the friction between them. The conformation of polyelectrolyte brushes can be studied both by simulation and force measurement techniques. The study of Kampf et al. is a recent example for an experimental work on the topic [44]. In addition to their former work which showed the supreme lubrication capability of grafted polyelectrolytes in case of symmetrically brush coated surfaces [45], they studied an asymmetrical system in which only one of the opposing mica surfaces is coated with the diblock copolymer PMMA-b-PSGMA [44]. The brush coated mica surface was prepared by hydrophobizing it with a stearic trimethylammonium iodide (STAI) layer. Then the hydrophobic block of the diblock copolymer was adsorbed on that layer while the hydrophilic part was forming the brush (see Fig. 3). Monitoring the normal and shear forces with a Surface Force Balance (SFB), the authors observed that for a film thickness between 250 nm and 75 nm, the counterion distribution in the thin film was similar to that in the bulk region. They proposed that at that separation, the polyelectrolyte chains are fully stretched, forming brushes. As the thin film gets thinner, a faster increase in the repulsion between the surfaces was observed. This was attributed to the influence of a higher counterion concentration in the thin film resulted by ions in the brush layer itself. Finally, at a film thickness smaller than 20 nm, steric forces resulted by the compressed polymer brush chains were observed. It should be noted that in this study, the fully stretched chain length is reported to be about 30 nm. When the film thickness was decreased as thin as 4 nm, an adhesive jump in the force profile was observed. This suggests that the majority of the polymer chains were excluded from the thin-film confinement leaving a small fraction which was aligned parallel to the surface with a thickness of 0.5 nm, still decreasing the friction between the surfaces significantly. The exclusion of the polymer brush chains from the thin film might seem impossible for a chemically grafted brush but as mentioned above, here the brushes are only physically attached to the hydrophobic STAI layer. Raviv et al. expanded this work by measuring the normal and frictional forces between mica sheets both coated with a hydrophobic layer of STAI layer and the diblock copolymer discussed above [46]. Polyelectrolyte brushes were reported to have a thickness of L = 13 nm each. The authors observed long-range electrostatic repulsive forces between the brushed surfaces at surface separa-

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Fig. 3. (A) Chemical structure of PMMA-b-PSGMA copolymer. (B) Schematic representation (not to scale) of a hydrophobically anchored PE on an STAIhydrophobized mica surface and their positioning during the force measurements. Taken from Ref. [44] with permission.

tions, D N 2L due to the free counterions outside the brushes, exerting an osmotic pressure. For D b 2L, the repulsive forces are governed by the steric interaction between the brushes themselves. It was concluded that the friction between the polymer-bearing surfaces were very low due to weak interpenetration of the compressed brushes. It was reported that in contrast to neutral brushes, in which resistance to interpenetration is resulted by polymer configurational entropy, in polyelectrolyte brushes interdigitation is prevented by osmotic pressure of counterions trapped in the brush layers. Dunlop and his co-workers observed the same behaviour of long-range electrostatic and short-range steric intersurface repulsion in addition to low degree of brush interdigitation for polyelectrolyte brushes grafted directly on mica sheets [47]. In this case however no exclusion of the brush layer under a strong confinement was observed due to the fact that they are chemically bound to the mica surfaces. Addition of salt into the system caused a remarkable decrease in the long-range double layer repulsion, also resulting in a contraction of the brush. The authors reported a good theoretical matching of their results with the original work of Pincus [48]. Negligible interpenetration of brushes on the opposing surfaces, even when the inter-surface separation is as thin as half the length of a stretched chain, was also proposed by Sirchabesan and Giasson in their dissipative particle dynamics simulations for highly charged poly(tert-butyl methacrylate)-block-poly(sodium sulfonate glycidyl methacrylate) copolymer [41]. On the other hand, these authors noted that in the presence of salt (N10− 4 M), significant interpenetration of the two brush layers may occur under compression. The polyelectrolyte brush in salt solution and the neutral brush in a good solvent were found to have a similar dependence of interpenetration thickness on separation distance, grafting density and polymer size. However, less interpenetration is predicted for charged chains.

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Kegler et al. used the Optical Tweezer method to experimentally study the effects of compression, ionic strength and grafting density on the structuring of grafted DNA [49]. A transition from mushroom conformation (see Ref. [36]) to brush structure was observed as the grafting density was increased. For the DNA brushes, the brush thickness was found to be independent of the salt concentration for lower salt concentrations. As the salt concentration was increased, the brush thickness under the confinement of two colloidal particles was observed to be highly affected by ionic strength. The forces between two confining walls were found to be mainly steric. Liberelle and Giasson observed a similar ionic strength effect on the structuring of grafted polystyrene-poly(acrylic acid) diblock copolymers using the Surface Force Apparatus [50]. In contrast to the experiments of Kegler et al. [49] and the simulations of Wynveen and Likos [51] on grafted DNA (where the surface forces at increased confinement are dominated by the bending of the rigid chains), Dominguez-Espinosa et al. found out that for more flexible poly(acrylic acid) (PAA) grafted surfaces, the interactions in the thin film are mainly entropic due to the counterion distribution inside the brush [52]. They observed an ionic strength dependent scaling of the brush thickness. Increasing the pH resulted in a decrease in the salt concentration inside the brush and as a consequence resulted in more stretched (longer) chains. In contrast to that pH dependence, Elmahdy et al. observed a decrease in the brush thickness as the pH was increased for the polyelectrolyte poly(2-vinylpyridine) (P2VP) in their Optical Tweezer measurements [53]. For pH b4, the brush thickness was found to be independent of the pH. In addition to pH and salt concentration, the type of salt might play a role in the electrostatic/osmotic properties of a system. In their coarse-grained polyelectrolyte model simulations, Cao et al. investigated the effect of valence of counterions on the conformation and interpenetration of apposing polyelectrolyte brushes [54]. It was shown that for a lower grafting density, the brushes with multivalent counterions tend to collapse more. When the grafting density is large enough, the brush thickness becomes independent of the counterion valence and is only proportional to grafting density. In large separations of the opposing brush surfaces, the interpenetration of the brush layers was observed to be more in case of monovalent counterions compared to trivalent ones. As the film was compressed more, the difference in interpenetration behaviour of monovalent and multivalent counterion systems decreased and finally the interpenetrated polyelectrolyte brushes got indistinguishable from the neutral polymer systems. For the sake of simplicity, polyelectrolyte brushes can be considered to possess simple scaling arguments [55]. In the osmotic brush regime counterions are trapped inside the brush, and the brush height depends only on the balance between counterion osmotic pressure and chain elasticity. In their molecular dynamics simulations, Kumar and Seidel studied the scaling of disjoining pressure on the separation of two polymer brush bearing planes [55]. They reported that the scaling of disjoining pressure depends on the separation as well as the grafting density. For high grafting densities, a transition from good solvent behaviour to a melt regime was proposed. Unlike the studies above, remarkable interpenetration was observed in addition to compression/bending of the grafted chains. For large separations, brush height is found be independent of the thin-film thickness as expected but when the two brush layers come in contact and are compressed further, either counterion distribution or the packing properties determine the osmotic pressure and the brush thickness. If the grafting/adsorbing density of the chains is low, polyelectrolyte bridging can occur. Bridging is a phenomenon that occurs when sufficiently long polyelectrolytes are adsorbed to two or more surfaces of opposite charge. It is a thin-film property that can induce the attractive forces between the thin-film surfaces (macroions, colloidal particles, etc.). A recent review about bridging interactions can be

found in Ref. [56]. A colloidal-probe AFM study by Sparkel et al. on the dynamics of polymer bridge formation might also be of interest [57]. Another appearance of polymer on thin-film surfaces is the case of adsorbed dendrimers. This case won't be discussed here. In case of interest, the reader can refer to Refs. [56,58,59], recent studies about the structure of adsorbed dendrimers and their effects on the interactions between the confining surfaces in a thin film. 3. Polymers in film bulk Besides the composition and structure of the interfacial region also the structuring of polyelectrolytes in the film bulk (subsection 3.1) has an influence on the thin-film properties like stability in case of foam and emulsion films and film thickness and drainage (subsection 3.2). 3.1. Structuring of polymers Polymer systems possess not only unique bulk properties but also interesting inter-chain interactions in thin-film geometry. Understanding these interactions is the key to understand the macroscopic properties (e.g., rheology and stability) of the system. Studying the thin films of aqueous polymer solutions is also important for revealing the microscopic properties of the system such as ion distribution and chain conformation leading to structuring. Structuring in charged and neutral polymer solutions has been of great interest for the last decades and structuring of polymers in thin-film geometry is closely related to structuring theories in bulk. Some classic and recent studies discussing the structuring of polymers in the bulk can be found in the following references [34,35,60–67]. Despite the similarities of bulk and thin-film structuring, there is a significant difference between these regions in terms of conformational and translational entropies of the polymer. Due to a reduced entropy in the thin-film confinement, polymer chains in the gap prefer to be excluded to the bulk (layer-by-layer) as the confining surfaces get closer. The expulsion of one layer leads to a lower polymer concentration within the thin film than in the corresponding reservoir (e.g. meniscus). The temporarily lower concentration of polymers in the thin film causes an osmotic pressure which creates an attractive force between the confining surfaces. These attractive depletion forces occur when each single polymer layer is pressed out of the film and they lead to oscillatory forces (structural forces) during the film drainage. The structural parameters such as the period and the decay length of the oscillatory force curves give information about the inter-chain distance and the persistence of the ordering respectively. The peak height of the structural oscillations depends on the strength of inter-chain and surface–chain interactions [68•]. The variation of inter-chain distance with respect to polymer concentration in bulk allows to derivation of scaling laws. The inter-chain distance scales with the concentration as C− 1/3 in dilute regime and as C− 1/2 in semi-dilute regime, where the concentration is larger than the overlap concentration, C⁎ [60]. The transition between these two scaling laws in thin-film geometry was shown experimentally for the semi-flexible polyelectrolyte, potassium polyacrylate (KPAA) [69]. After their first observation in polymer systems by Milling in 1996 [70], oscillatory structural forces have been studied using Surface Force Apparatus (SFA), Colloidal-Probe Atomic Force Microscope (CP-AFM), Total Internal Reflection Microscopy (TIRM), Optical Tweezers and Thin Film Pressure Balance (TFPB). Recent reviews on the direct measurement of polymer induced forces with the first four methods above can be found in Ref. [33••]. Another recent review covers the CP-AFM studies in thin films [71•]. An overview of TFPB measurements is given in Ref. [72]. This part however, focuses on recent theoretical and experimental approaches on the structuring of polymers under confinement — i.e., in thin-film geometry. It should also be noted that as the adsorbed polymers were discussed in Section 2.3, the structuring of only mobile

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(non-adsorbing) polymers is considered in the following discussions. However, sometimes the polymer chains on the surface and in the free thin film are in equilibrium and a sharp distinction between the two cases is not possible. In Leermakers and Butt's recent work [73] for example, the adsorbed and the mobile polymer chains are studied together. The authors expanded the classical restricted equilibrium approach of polymer adsorption by Scheutjens and Fleer [74,75] from dilute solutions to semi-dilute and concentrated polymer solutions as well as to polymer melts. In their generalised restricted equilibrium model, the polymer chains that are in contact with the confining surfaces (at least at one segment) were considered to be adsorbed. In addition to the calculations of adsorbed amount of polymers on the surfaces, theoretical force curves between two surfaces were derived. Neglecting the van der Waals and other types of interactions, their calculations for polymer melts, confined between two similar surfaces, gave oscillating interaction energies between the surfaces as they were brought closer. This oscillatory behaviour of surface forces is attributed to mobile polymer chains that are pushed out layer-by-layer from the thin film to the bulk (oscillatory force curves were observed experimentally as well and as will be discussed below and they have an overall decay length). As their system is a thin film of both absorbed and free polymers, Leermakers and Butt found two types of decaying behaviour which depend on the strength of the confinement, in other words the thickness of the thin film. When the confinement is less strong —i.e., in larger separations, there are still free polymer chains in the thin film and the decay length is proportional to the chain size. As the confinement gets stronger pushing the thin film thinner, the only chains that remain in the gap are the adsorbed ones, now being compressed. This compression results a steeper decay of the forces between the surfaces. Similarly, Turesson et al. simulated the confined system consisting of two similarly charged surfaces and a solution of oppositely charged polyelectrolyte in between [76]. The authors used a novel simulation technique allowing to study relatively longer chains. The structuring of 0.01 M flexible polyions with 60 monomers per chain was studied by monitoring the conformations of a single chain in the thin film. The results indicate that, as the confining surfaces come closer, the adsorption of chains gets more preferable. When the separation between the surfaces (h) is as large as 189 Å the chains are mobile in the thin-film forming layers (see Fig. 4). As the thin film gets thinner, “stratification” takes place as a result of surface charge reversal. The stratification causes an oscillatory free energy curve. When the separation is 125 Å, it was observed that a minority of the chains are attached to the surfaces, but still having long tails in the thin film. At the separations of 98 Å and 30 Å, the chains started to form bridges and coiled between the surfaces getting absorbed strongly. This shows the high dependency of polymer structuring on the thin-film thickness. Ditto, Jeon and Chun's Brownian dynamics simulations of the confinement of flexible and semi-flexible polyelectrolyte chains show the transformation of structuring and conformation with the thin-film thickness as well as on the ionic strength. In most of the theoretical studies, monodispersed polymers are considered for the sake of simplicity. In practice however, polymer solutions are more or less polydisperse systems. Recently, in addition to the density functional theory based work of Kim et al. [77], Yang et al. proposed a series of self-consistent-field theory (SCFT) based calculations to understand the effect of polydispersity on the transport of nonadsorbing polymers between the thin film and the bulk [78•]. The behaviour of polymer chains was discussed in terms of depletion forces between the thin-film surfaces. Their calculations propose that as the surface separation in the film decreases, the longer chains are pushed out of the thin film before the shorter ones, because the longer chains have more conformational entropy penalty for staying in the gap. They compared the amounts of polymer chains with lengths of nearly 34, 180 and 450 monomers giving an average length of 200 monomers. When the separation is around 8–9 times the radius of gyration (Rg) (of the chain with the average length), the majority of the chains in the thin film is moderately long chains (with

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180 and 450 monomers). As the thin film gets thinner, the longer chains are excluded to the bulk layer-by-layer and the ratio of shorter chains to the longer ones in the film increases. Finally, when the separation is 0.86 times Rg, the only chains that remain in the confined region are the shortest ones. It was noted that semi-dilute polymer solutions should possess a less effect of polydispersity on the structuring because in this regime short and long chains are entangled to each other and form mesh-like structures. Only the mesh-size, or correlation length, defines the structuring of these mesh-like networks. At this point, the controversy about the thin-film arrangement of polyelectrolyte chains in semi-dilute regime should be noted based on the work of Rapoport et al. [79•]. A considerable number of experimental studies propose a mesh-like structuring in thin films of semi-dilute polyelectroylte solutions [4,68•,80–89•–90] which are also supported by the theory [34,35,60,62,65,91–93]. However, Milling and Kendall [94] and Thedoly et al. [95] proposed a parallel alignment of polyelectrolyte chains forming lateral layers, which is in accord with the theoretical calculations by Jonsson et al. [96]. Rapoport and co-workers looked deeper into this issue by studying the system in molecular level for the first time [79••]. Surface tension measurements and a fluorescence spectroscopy coupled with a Thin Film Pressure Balance (TFPB) were used to investigate a pyrene-labeled poly(acrylic acid) solution. Pyrene is known to form excimers and it was shown that the excimer/monomer ratio increases during confinement. Different distributions of the Pyrene labels along the polyelectrolyte chains showed that the increase in excimer/monomer ratio is related to an increase in local concentration in the polyelectrolyte layers aligned parallel to the film surfaces. Supporting the network structuring in the film, Qu et al. observed that the inter-layer distance in thin-film geometry is identical to the average chain distance in bulk solution [68•]. The authors used colloidal-probe AFM and Small Angle X-Ray Scattering (SAXS) methods to study the confined and bulk structuring respectively. They showed that the target polyelectrolyte PAMPS has a scaling law of C− 1/2 in a wide concentration range, regardless of the charge fraction. The accordance of the inter-chain distance in the thin film and the bulk solution suggests that the confinement has no significant effect on the structuring and the isotropic transient network of polymers persists also in the thin film. In the same paper [68•], Qu et al. also proposed a new counterion condensation model in which the condensed counterions also have some mobility, affecting the conductivity and the structure of the transient network. Kleinschmidt and co-workers investigated the effect of polyelectrolyte chain stiffness on the thin-film structuring [97]. PAMPS, CM-Chitin, DNA and Xanthan having persistence lengths of 1, 5, 50 and 150 nm respectively, were studied using TFPB or CP-AFM. Stratification was observed in the thin films of flexible samples, PAMPS and CMChitin. For Xanthan, which is much stiffer, stratification could only be observed in low velocity compression of the thin film. No stratification or network formation was observed for the very rigid DNA, as expected. For Xanthan, which is much stiffer, stratification could only be observed in low velocity compression of the film. No stratification or network formation was observed for the very rigid DNA, as expected. It is assumed that the polymer stiffness/solution viscosity is the important parameter, and that network of stiff polyelectrolytes needs longer time to rearrange in comparison to more flexible ones. In case of aqueous solutions the viscosity is quite low and the drainage is too fast for rearranging the network of stiff polymers. To study the effect of polymer chain length on the structuring, Knoben et al. directly measured the depletion forces for a reversible supramolecular polymer solution [88,89•]. Supramolecules are special types of polymers in which the monomers are connected rather with weak forces such as hydrogen bonding or metal-ligand formation. The advantage of studying such a system is the ease of varying the chain length using “chain stopper” molecules. In the earlier work [88], the authors changed the mole fraction of the chain stoppers in a systematic way while monitoring the depletion length (which gives the polymer layer thickness). Two

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Fig. 4. Top left and right figures: h = 98 Å. The chain (darker one) lies in a flat configuration at one of the charged surfaces (front and side views respectively). Bottom left picture: h = 125 Å. One tail of the chain has desorbed from the surface. Bottom right picture: h = 189 Å. “Stratification” of the system has occurred. The labeled chain now resides in the layer created in the middle of the slit. Figures taken from Ref. [76] with permission.

different regions were observed. For higher chain stopper fractions, in other words for shorter chains, the depletion layer thickness was found to depend on the chain length as in dilute solutions. For lower amount of chain stoppers, the chains are long enough to form mesh-like structures and the depletion layer thickness (mesh-size) was found to be independent of the chain length. In the latter study [89•], depletion forces were measured in semi-dilute regime, where again the thickness of the depletion layer gives the correlation length in the solution. A weaker scaling of the layer thickness with respect to concentration was observed compared to the theory. This difference was attributed to nonequilibrium effects on the measurements such as hydrodynamic drag or entrapment of chains in the gap. Bymaster and co-workers also showed two different concentration regimes with different types of layers [98]. They used a density functional theory (DFT) based model called interfacial statistical associating fluid theory (iSAFT) and observed a depletion layer on two different length scales. In semi-dilute regime, the depletion layer was defined by the mesh-size and in dilute regime the layers were formed by polymer coils, having a thickness comparable to the radius of gyration, Rg. An effect of polymer chain length on the structuring was proposed as well. As a summary, it can be concluded that there is no universal scaling law and structuring behaviour for polymer systems. In addition to polymer architecture, chain length, stiffness, charge fraction and concentration, the ionic strength of the solution and the strength of confinement determine the thin-film structuring. The surfactant in the system and the type of the confining surfaces has no significant effect on the thin-film structuring.

3.2. Foam dynamics The dynamics of foams have been widely discussed in literature in the last years. In the following, we will concentrate on two major topics: the dynamic process of stratification and the behaviour of foams containing polyelectrolytes. 3.2.1. Stratification dynamics In the previous section, the structuring of polyelectrolytes in confinement was discussed. When the two interfaces of the film approach each other, layers of the polyelectrolyte network are pushed out of the film. For foam film, a stepwise thinning of the film occurs. In this section, this stepwise thinning phenomenon is discussed in terms of stratification. The stratification process starts with small discontinuities, visible as small black dots at the film interface. These dark domains spread over the whole film and the new equilibrium thickness corresponds to a different branch of the oscillating disjoining pressure isotherm. This process is driven by a difference in film tension between the inner and the outer parts of the domain (see Fig. 5). The film tension in the thinner domain is smaller and energetically more favourable, thus it leads to the growth of the domain. The excess material from the thinner regions is transported to the bulk reservoir in a small rim that surrounds the growing domain. This rim is visible as a bright ring on the film as it's thicker than the other regions of the foam film due to the excess liquid from the stratification. The height of the rim depends on the particular system that is investigated and is therefore a material constant [99].

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The stratification velocity is a dynamic process that depends on many parameters, for example, the properties of the polyelectrolyte, the type of the surfactant, the film thickness, or the position of the domain in the film. Especially the polyelectrolyte/surfactant combination of the system is essential. If both compounds are oppositely charged, hydrophobic complexes are formed due to the electrostatic interactions (see Section 2.1). These complexes are situated at the surface and alter thereby the properties of the film. In contrast to that, in systems where surfactants and polyelectrolytes are likely charged or neutral surfactants are used, the addition of the polyelectrolyte basically just changes the ionic strength in the film, but has no influence on the properties of the interface. In a study by Beltran et al., foam films from negatively charged carboxy-methycellullose stabilised by differently charged surfactants have been investigated. In these systems with oppositely and likely charged components, it has been shown that domains in foam films with surface-active complexes grow slower than those in films, where only surfactant molecules cover the surface. It is assumed that this is due to the polyelectrolyte chains that are connected to the surface and dangle into the film core, thereby slowing down the stratification process [100••,101]. When likely charged components are used, the velocity of domain growth is dependent on the rim that is formed during domain expansion. This rim becomes unstable at a critical radius and breaks into droplets that surround the domain. This phenomenon is called Rayleigh instability. The typical behaviour of the domain expansion shows a scaling behaviour of the domain radius r ∼ t1/2 before the Rayleigh instability occurs and an increase with t after the rim is broken [101]. In case of oppositely charged surfactants and polyelectrolytes, the regime of constant growing velocity was never observed. The position of the domain in the film has an additional effect on the domain growth velocity. If it is situated near the film border, the domains grow faster, especially when they are already large enough. Yet to our knowledge, the reasons for that behaviour remain unclear [99]. In case of several domains coexisting in the film, the growth velocity remains roughly the same for the individual domains, but of course, the total area change increases due to the additive property of the area. The results presented above apply to the films in the lower thickness region (≤40 nm). For thicker films it's difficult to investigate the domain growth velocity, because the domains are formed very close to each other and rapidly coalesce. However, in a qualitative sense one can state that the stratification process is slower for thick films than for thinner ones [99]. Another important parameter that affects the stratification process is the velocity of the approaching interfaces. During the expulsion of one polyelectrolyte layer from the film, the remaining network needs to readjust. For example, the colloidal-probe AFM is an excellent tool to control the speed of the approaching interface. When the polyelectrolytes are flexible, such as PSS or PAMPS, the network in the film core can easily rearrange and the films are rather insensitive to the thinning velocity. In contrast to that, stiffer polyelectrolytes like DNA or Xanthan need more time to adjust during stratification. Hence, no oscillatory force can be observed for these polymers when the interfaces approach too fast. In foam films, this problem can be overcome by increasing the viscosity of the solution and thereby reducing the velocity of the domain growth. This gives the polymer network time to adjust and a stepwise thinning of the film can be observed [97]. 3.2.2. Foaming The investigation of the foam film properties usually aims for a better understanding of the foaming behaviour of the corresponding solution. However, it is not trivial to make a correlation between a foam film and the respective foam for various reasons. i) A foam is a threedimensional system, composed of foam films and plateau borders. If one of the films ruptures, the system has to rearrange, so that a single

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film's rupture influences the whole foam. ii) A foam film is investigated under well defined conditions and usually studied in equilibrium. On the other hand, foam formation occurs under dynamic conditions, where the foam films are thicker and the adsorption equilibrium has not been reached. Additionally, the capillary pressure is not constant but a function of foam height [102,103]. Nevertheless, in order to compare the surface properties and the thin-film measurement to the foaming behaviour, different parameters can be investigated; First of all, the foamability, which describes the ability of a polyelectrolyte/surfactant solution to produce foam. It depends on the adsorbed amount on the air/water interface and the rate of transport to the surface. 2) The foam stability or lifetime that is limited by the drainage of the liquid, bubble coalescence and Ostwald ripening of the dispersed air bubbles [104]. The drainage of the liquid in the films plays an important role in foam stability. When the foam dries, its structure becomes more fragile because the films become thinner and break more easily. The drainage can for example be followed by measuring the liquid stability, which is the time that is needed to decrease the initial liquid content in the foam film by 50%. This is a characteristic value, provided that the starting volume of the foams is identical [11]. For example, for C16TAB/pectin foams, the stability of the foam is drainage controlled. Both pectin and C16TAB are not good foaming agents, because the foam decays very fast. When pectin at a concentration of 0.1 mg/ml is added to the foaming solution, the stability increases, especially when the surfactant concentration is increased over the CAC. In this case, the drainage of the foam is strongly reduced due to the aggregates that are trapped in the film. When the surfactant concentration is further increased and all the aggregates are released from the film, the foam decays very fast again, due to the increased drainage velocity in the foam. Similar observations have been made by Saint-Jalmes et al. [105]. The authors compared the mechanisms of foam stability of pure SDS and casein/SDS mixtures and found remarkable differences. In case of pure surfactant foams, a homogeneously distributed surface coverage and charge was needed to produce stable foams, which were mainly stabilised by electrostatic repulsion of the interfaces. Stable foams could be produced by casein/SDS mixtures as well, although the surface coverage was very poor. It was proposed that the foam is stabilised by percolation process induced by aggregates that makes the film more rigid. This thesis is supported by investigations of hydrophobically modified cellulose and sodium dodecyl polyoxyethyl sulfate (AES) mixtures, where the most stable foams appeared in the concentration regime of aggregation formation, which seems to be a general trend [106]. Apart from the drainage process, the final film thickness in the foam seems to play a crucial role. In a comparison of foams made of whole

Fig. 5. Sketch of a growing domain of the radius R. The film thickness equals h0 inside the domain and h∞ 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. Figure reprinted with permission from Ref. [99] (Heinig P, Beltran C, Langevin D, Domain growth dynamics and local viscosity in stratifying foam films. Phys Rev E 2006;73:051607). Copyright (2010) by the American Physical Society.

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casein/Tween 20 and β-casein/Tween 20, the respective foam with the higher equilibrium thickness was observed to be more stable [13]. As mentioned above, thicker films seem to be less susceptible to film rupture. In general, the addition of polyelectrolytes to foam systems leads to an increase in the foam stability, as long as the concentration is kept in range where aggregates are trapped in the film.

SFA SFB TFPB TIRM

Surface Force Apparatus Surface Force Balance Thin Film Pressure Balance Total Internal Reflection Microscopy

References and recommended readings•,•• 4. Conclusions Polyelectrolytes in any type of thin liquid film can be positioned either on the surfaces (adsorbed/grafted) or in the thin-film bulk. The present review demonstrates that the transition between these two states mostly depends on the charge and concentration of the polymer and the thickness of the film. Chain length also has an effect on the polyelectrolyte position and conformation, leading to a selective existence of short and long chains in the film bulk. In most of the systems, ionic strength changes the chain–chain and surface–chain interactions dramatically. Apart from the thin films confined between two solid surfaces, the dynamics of foams can be understood by monitoring the stratification process, which leads to oscillatory disjoining pressure isotherms. Polyelectrolytes can possess stabilising or destabilising effects on the foam films depending on their ratio to the surfactant in the system. The charges on the polyelectrolyte and the surfactant might lead to different surface coverage mechanisms. On the other hand, specific type of polymers called polymeric surfactants can stabilise liquid-in-liquid dispersions as well as foam films in the absence of any additional surfactants. 5. Abbrevations Chemical compounds AES Sodium dodecyl polyoxyethyl sulfate CM-Chitin Carboxymethyl-chitin DNA Deoxyribonucleic acid KPAA Potassium polyacrylate PAA Poly(acrylic acid) PAMPS Poly(2-acrylamido-2-methylpropanesulfonic acid) PEO −CH$_{2}$−CH$_{2}$−O− PMMA-b-PSGMA Poly(methyl methacrylate)-block-poly(sodium sulfonated glycidyl methacrylate) PNIPAM Poly(N-isopropyl acrylamide) PPO −CH$_{2}$−CH(CH$_{3}$)−O− PSS Poly(styrenesulfonate) SDS Sodium dodecyl sulfate STAI Stearic trimethylammonium iodide Experimental methods and theory AFM Atomic force microscope C Polymer or polyelectrolyte concentration C⁎ Polyelectrolyte overlap concentration Cel Salt concentration Cel,cr Critical salt concentration CP-AFM Colloidal-Probe Atomic Force Microscope CAC Critical aggregation concentration CMC Critical micelle concentration DFT Density functional theory DLVO Derjaguin, Landau, Verwey, Overbeek FRAP Fluorescence recovery after photobleaching iSAFT Interfacial statistical associating fluid theory LCST Lower critical solution temperature monoM Concentration of the respective monomer units in mol/l NBF Newton Black Film Rg Radius of gyration SAXS Small Angle X-Ray Scattering SCFT Self-consistent-field theory

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