Effect of interface modification on forces in foam films and wetting films

Advances in Colloid and Interface Science 114–115 (2005) 253 – 266 www.elsevier.com/locate/cis

Effect of interface modification on forces in foam films and wetting films Regine v. Klitzing* Max-Planck-Institut fu¨r Kolloid- und Grenzfla¨chenforschung, D-14424 Potsdam, Germany Institut fu¨r Physikalische Chemie, Christian-Albrechts-Universita¨t Kiel, Ludewig-Meyn-Strasse 8, D-24118 Kiel, Germany Available online 21 March 2005

Abstract The paper reviews the effect of the surface composition on forces within aqueous foam and wetting films. In both types of films the charge of the air/water interface is varied by different surfactants. In wetting films the charge and the hydrophobicity of the solid substrate is changed by polymer coatings. The addition of polymers to foam films leads to the formation of surface active polymer/surfactant complexes or to depletion near the interfaces. The dissolution of polyelectrolytes within the film bulk can lead to structural forces. The selection of studies is made with respect to two questions: (1) What is the reason for charges at the air/water interface and (2) what is the mechanism of long-range hydrophobic interaction? D 2005 Elsevier B.V. All rights reserved. Keywords: Foam film; Wetting film; Disjoining pressure; Hydrophobic forces; DLVO forces; Structural forces; Polyelectrolyte multilayers

Contents 1. 2.

3.

4.

5.

Introduction . . . . . . . . . . . . . . . . . . Forces in thin liquid films . . . . . . . . . . 2.1. DLVO forces. . . . . . . . . . . . . . 2.2. Structural forces . . . . . . . . . . . . Methods. . . . . . . . . . . . . . . . . . . . 3.1. Thin-film pressure balance (TFPB) . . 3.2. Captured bubble technique. . . . . . . Foam films . . . . . . . . . . . . . . . . . . 4.1. Pure surfactant foam films . . . . . . . 4.2. Mixed polyelectrolyte/surfactant films . 4.2.1. Effect on structural forces. . . 4.2.2. Effect on total film thickness . 4.2.3. Effect on film stability . . . . Wetting films . . . . . . . . . . . . . . . . . 5.1. High energetic surfaces . . . . . . . . 5.1.1. Likely charged interfaces . . . 5.1.2. Oppositely charged interfaces . 5.2. Low energetic surfaces. . . . . . . . . 5.2.1. DLVO forces . . . . . . . . . 5.2.2. Water depletion layer . . . . . 5.2.3. Nanobubbles . . . . . . . . .

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T Tel.: +49 331 567 9232; fax: +49 331 567 9202. E-mail address: [email protected]. 0001-8686/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cis.2004.12.004

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6. Summary and conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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 paper deals with the effect of the interface composition on the forces within mesoscopic thin films. Thereby foam films and wetting films are in the focus of interest. Methods to study thin films between solid interfaces are reviewed elsewhere (e.g. [1]), and they are not considered within the paper. The study of aqueous free-standing films (foam films) is of great relevance especially in two respects. On one hand, these single films can be considered to be the building blocks of a foam. Clarifying the properties of each of these single films could lead to a better understanding of the behavior of the whole macroscopic foam, such as its stability. On the other hand, the free-standing film corresponds to a slit pore geometry which allows to study the effect of confinement on the structuring of colloidal particles, aggregates or macromolecules. The liquid free-standing film is highly relevant in this respect, since its thickness can easily be varied by changing the outer pressure. Since the film interfaces cannot be directly investigated, the disjoining pressure isotherms are related to studies at a single air/water interface. Wetting films play an important role in all processes of particle interaction. For a successful froth flotation it is essential that the thin film between the solid particle and the air bubble ruptures during the contact time. This problem can be simplified by studying the stability of a wetting film between an air bubble and a solid substrate. Two rupture mechanisms of thin films are possible: spinodal dewetting (capillary wave mechanism) or nucleation: during nucleation a hole is formed which expands above a critical radius r cr [2]. Beyond r cr the hole can disappear. No attractive forces are necessary. Density fluctuations inside the film close to hydrophobic spots or tiny gas bubbles can cause the rupture. Spinodal dewetting is related to growing fluctuation waves which in turn are caused by any kind of attractive forces. Attractive elecrostatic, van der Waals and so-called blong range hydrophobic forcesQ (e.g. [3–5]) amplifies the amplitude of the fluctuation, then. According to theoretical predictions [6,7] this instability leads to the rupture of the film especially during its drainage. The review focuses on aqueous films. In general, the dispersion forces are negligible in comparison with the electrical double layer forces. In contrast to this, the stability of non-aqueous films is mainly determined by dispersion forces [8].

While DLVO forces are well understood, the mechanisms of long-range hydrophobic forces are more or less unclear. They are mainly used to explain long-range attraction between hydrophobic surfaces or the rupture of films at thicknesses of several tens of nanometers, where the range of attractive dispersion forces is too short. An overview about forces beside DLVO forces is given by Ninham in [9]. In the present paper different forces which could contribute to long-range hydrophobic attraction are discussed. Furthermore, the reason for the electrostatic repulsion in foam films of pure nonionic surfactant solutions and the charge reversal during adsorption of cationic surfactants at the film surfaces is discussed. Therefore studies are reviewed where the charge and the hydophobicity of the surfaces of foam and wetting films are modified. The forces are mainly manipulated by the addition of polyelectrolytes. They are either dissolved in the film bulk or they are adsorbed at the film interfaces.

2. Forces in thin liquid films The disjoining pressure is due to interaction forces between the two interfaces of the thin liquid film (e.g. [10]): P ¼ Pel þ PvdW þ Psteric þ . . . ðþ Pstructural Þ

ð1Þ

A profile of the resulting P(h) curve (disjoining pressure as a function of film thickness) is shown in Fig. 1. 2.1. DLVO forces The electrostatic and van-der-Waals forces form the DLVO forces (Derjaguin Landau [11] Veerwey Overbeek Π NBF

disjoining pressure

1. Introduction

265 265 265

~ exp(-κh) CBF ~ 5

-1/h3 10

120

h [nm]

Fig. 1. Sketch of a disjoining pressure isotherm: disjoining pressure P as a function of the film thickness h. The difference between the structure of a common black film (CBF) and a Newton black film (NBF) is indicated by simple cartoons. Taken form [84].

R.v. Klitzing / Advances in Colloid and Interface Science 114–115 (2005) 253–266

Pel ¼ P0 exp½  jðh  2h0 Þ

ð2Þ

with the Debye–Hu¨ckel length 1/j and the interface layer thickness h 0, which corresponds usually to the length of the surfactant molecules. P 0 is connected to the surface potential W 0 by the relation [13]: PðhÞ ¼ 64kT ql c2 expð jhÞ
 ¼ 1:59T108 ½cel c2 expð jhÞ; ½ Pa

ð3Þ

c ¼ tanhð zeW0 =4kT Þ

ð4Þ

P0 ¼ 64kT ql c2

ð5Þ

The surface potential W 0 corresponds to the f potential of the surface and [c el] is the electrolyte concentration. In the case of (symmetric) foam films the surface are likely charged and the electrostatic interactions are always repulsive and stabilizing. In (asymmetric) wetting films wetting films the interfaces can be oppositely charged which leads to attractive interactions. To take into account the different surface potentials of both interfaces c 2 is changed into c 1c 2. The film which is stabilized by electrostatic repulsion is called common black film (CBF) and its film thickness is sensitive to the ionic strength. It consists of two interfacial layers and a film core (solvent and additives). The van-der-Waals interactions are described by PvdW

A ¼ 6ph3

ð6Þ

The Hamaker constant for a film (medium 3) between two media 1 and 2 is given by A123

   3 e1  e2 e3  e2 ¼ kT þ A123 ðxÞ: 4 e1 þ e2 e3 þ e2

ð7Þ

A 123(x) is the frequency dependent part of the Hamaker constant. If the film is symmetric like in a foam film, the Hamaker constant is positive and P vdW becomes negative. If the film overcomes the electrostatic barrier, it either breaks due to the attractive van der Waals attraction or it is stabilized by repulsive steric forces between the adsorbed surfactant layers [14]. Such a film is called a Newton black film (NBF). This type of film does not contain any free solvent molecules anymore, and its thickness is about twice the length of the surfactant molecules adsorbed at the film interface. A transition from a CBF to a NBF is induced by the addition of salt, leading to a screening of the surface potential. The transition corresponds to an oscillation of the disjoining pressure because of the attractive van-der-Waals forces. This attractive part of the isotherm is mechanically unstable and cannot be measured with a thin-film pressure balance (TFPB, see chapter 3.1), but rather a step in film

thickness, from the thicker CBF to a thinner NBF, is detected. In wetting films the sign of the Hamaker constant depends on the value of dielectric constant of the intermediate medium 2 (film) with respect to the ones of the outer media. A can be positive, if e.g. e 1be 2be 3 or negative if e.g. e 1be 2Ne 3. Therefore P vdW can be either attractive of repulsive. The adsorption of thin layers like the adsorption of Silane at solid substrates (in wetting films) or the adsorption of a surfactant layer at the air/water interface changes the Hamaker constant marginal [15], since the Hamaker constant reflects rather the properties of the bulk than of the surface. 2.2. Structural forces The confinement of a fluid between two walls induces a layering of molecules or particles. Such a layering is related to an oscillatory decay of the particle or molecule concentration from the interface towards the film bulk, which itself induces a damped oscillatory disjoining pressure. However, while the decaying oscillatory concentration profile near the film interfaces is well understood and has been calculated in detail (e.g. [16,17]), the relationship between this profile and an oscillatory disjoining pressure is still under discussion [18–20]. To illustrate the oscillatory disjoining pressure, in Fig. 2 an exponentially decaying cosine function is used for a simulation of the experimental data: Pstructural ¼ Aexpð h=kÞcosð2pðh=nÞ þ /Þ;

ð8Þ

where A is the oscillation amplitude, k the decay length, n the oscillation period and / the phase shift. For colloids it can be several tens of nm depending on the colloidal diameter. The oscillatory or bstructuralQ forces have been discussed in detail in [13,21]. They cannot be described by the DLVO theory as they are due to the expulsion of molecules from the thin film.

2000

disjoining pressure / Pa

[12]). The electrostatic contribution to the disjoining pressure is described by:

255

∆h~25 nm

1000 0 -1000 -2000

mechanically stable mechanically unstable

-3000 0

25

50

75

100

film thickness / nm Fig. 2. Disjoining pressure P as a function of the film thickness h of a C12G2/PDADMAC film and its simulation by a damped oscillatory function. In a TFPB experiment only the mechanically stable parts are accessible while in AFM experiments, for suited systems, the whole oscillation can be measured. Adapted from [42].

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This expulsion induces an attractive depletion force since the concentration within the film is lower than in the corresponding reservoir for a short time. Due to a constant period of the forces it is assumed that, each time, the same amount of material is squeezed out of the film. The period is somehow related to a characteristic length of the confined system such as the diameter in the case of spherical molecules [22,23], hard colloidal particles [24–29], liquid crystals [30,31] or micelles [19,32–36]. While the period is independent on the concentration in films of colloidal suspensions [29] the oscillation shows a strong concentration dependence in films containing soft colloids like micelles or polyelectrolytes. Soft particles like branched polyelectrolytes result in a period corresponding rather to the distance in the corresponding bulk solutions [37,38] and a scaling with the polyelectrolyte concentration as c 1/3. This is a typical scaling law for an isotropic distribution of spherical particles in three dimensions. The period in thin films containing micelles show the same scaling law [39] and for both types of colloids a layer-bylayer expulsion out of the film is suggested. Linear polyelectrolyte chains are assumed to form a transient network in the semi-dilute regime, and the network mesh size is of the same order as the oscillation period in the pressure curve of a thin film [40–42]. It scales with the polyelectrolyte concentration like c 1/2. In films formed from colloidal dispersions the decay length k is of several tens of nanometer. With regard to the class of molecules which form the supramolecular structure within the film, structural forces are also called solvation or hydration forces [43]. Structural forces with a much shorter range are due to the orientation of dipoles near interfaces. They can be either oriented perpendicular to the interface which leads to repulsive forces or parallel to the interface inducing attraction. The orientational structure decays with an exponential decay from the interface towards the bulk solution. The correlation between the dipoles is wellpronounced in polar solvents via hydrogen bonding. In the case of the layering of water molecules the decay length (or correlation length) k is about 1 nm [3]. If a second interface is approached to the first one these interfacial layers overlap and induce structural forces described by Eq. (8).

3. Methods 3.1. Thin-film pressure balance (TFPB) The disjoining pressure isotherms (disjoining pressure P as a function of the film thickness h) can be measured with the porous-plate technique in a thin-film pressure balance (TFPB; Fig. 3), developed by Mysels [44] and Exerova [45,46]. In this apparatus the capillary pressure is balanced by the disjoining pressure in a horizontal freestanding film. The film is formed from an aqueous

I

Ir

pg

Π Pump Fig. 3. The principle of a thin film pressure balance (TFPB) for the measurements of disjoining pressure isotherms in foam films and wetting films. Taken from [40].

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. On the other hand small pores (diameter: about 1 Am) of the fritted glass plate make it possible to apply a pressure of 104 Pa. The fritted glass plate is filled with the liquid of the film, and is connected to the external reference pressure P r (atmospheric pressure) by a glass tube. This film holder is enclosed in a metal cell, which allows to pressurize the film. 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 P c and the disjoining pressure P compensate each other. Pc ¼ P ¼ Pg  Pl ¼ Pg  Pr  Dqghc þ 2c=rc

ð9Þ

The hydrostatic pressure of the liquid column in the glass tube is given by Dqgh c (Dq=difference in density between the solution and the gas, g: gravitation constant, h c=height of the liquid column in the glass tube above the film). The capillary pressure in the glass tube is determined by 2c/r c (c=surface tension of the liquid, r c=radius of the capillary tube). The difference in pressure inside and outside the cell ( P gP r) is measured by a differential pressure transducer. The accuracy of the disjoining pressure is mostly limited by the difference pressure transducer, which has usually a specificity of 0.3% of its full range (i.e. 3–30 Pa). Above a pressure of 500 P a, P gP r 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 interferometer, 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 bcolorQ from white (at the maximum) to bright grey or dark grey and even black in the case of several nanometer thick films (e.g. NBF).

R.v. Klitzing / Advances in Colloid and Interface Science 114–115 (2005) 253–266

257

determined for instance by surface tension measurements, but probably these results cannot be directly transferred to the film interfaces. There are evidences for a higher surface density in a NBF than in the former CBF (e.g. [49]). Inside the cell a reservoir of the film liquid is included which allows to saturate the air inside the cell and that avoids 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 cell is thermostated 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. This apparatus can also be used to study wetting films [50]. Therefore a solid substrate (typically quartz or Silicon wafer) is put on one side. It is attracted by the solution filled porous plate via adhesion forces. Fig. 4. Photos of a step in film thickness. The darker spots correspond to the new thinner film thickness. The size of the photos: 300 250 Am. The time form top left to bottom right: about 1 min. Taken from [40].

Fig. 4 shows photos of a film draining in a discontinuous way with a step in its thickness. The thinner film thickness occurs as dark spots which extend to a new homogenous film thickness. The photo shows a typical bstepQ from a thicker to a thinner film. The origin for stratification can be a transition from a CBF to a NBF or structural forces, as it is mentioned in chapter 2. Parallel to video microcopy, the film thickness is determined by the interferometrical method of Scheludko [47]. Therefore the reflected intensity at one fixed wavelength (k=550 nm or 630 nm) is measured by a photomultiplier. The following equation is used to calculate the film thickness [47]: vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u k D u   h¼ arcsint ð10Þ 4R 2pn 1þ 2 d ð1  DÞ ð1RÞ

D¼

R¼

I  Imin Imax  Imin ðn  1Þ2 ðn þ 1Þ2

3.2. Captured bubble technique The more common method for the study of wetting films is the captured bubble technique as shown in Fig. 5. An air bubble is formed in front of a capillary tube of about 1 mm inside diameter in a solution, and it is pressed against a Quartz plate. Pressure changes are either applied by changing the pressure P i within the bubble, or the solution is sucked by micrometric piston through an outlet [51]. In equilibrium the capillary pressure compensates the disjoining pressure P i which is given by P=P iq flgz, with the height z of the solution within the cell and the solution density q fl. In [52] the forming film between the bubble and the Quartz plate is observed by an inverse microscope. The film thickness is measured by an interferometric method [47]: h¼

2m þ 1 p 2

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi # u d D u ! ð13Þ  Farcsinu pffiffiffiffiffiffiffiffiffiffiffiffiffi u 2 4 R R 12 23 t1 þ
pffiffiffiffiffiffiffiffiffiffiffiffiffi2 d ð1  DÞ 1  R12 R23

ð11Þ

ð12Þ

with n as refractive index of the liquid. I is the actual intensity and I min and I max correspond to the last minimum and maximum intensity value. 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 (n tail, n head, h tail, h head) separated by the film core (n c, h c) is used as a film model [48]. 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

k 2pn2

"

D¼

I  Imin Imax  Imin

R12 ¼

R23 ¼

ð n1  n2 Þ 2 ð n1 þ n2 Þ 2 ð n2  n3 Þ 2 ð n2 þ n3 Þ 2

ð14Þ

ð15Þ

ð16Þ

where m is the order of the interference, and d=0 for the refractive indices n 1bn 2bn 3.

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R.v. Klitzing / Advances in Colloid and Interface Science 114–115 (2005) 253–266

Capillary tube

Air bubble

Wetting film Quartz

Bright for thin films Fig. 5. The principle of a captured bubble technique for the measurements of disjoining pressure isotherms of wetting films. Adapted from [52].

In order to induce surface waves in the wetting film, the trapped bubble is excited to radial oscillation by a broadband transducer. The meniscus vibrates radially and the circular wave runs towards the center of the film. The waves can be monitored with a stroboscopic light source [52].

4. Foam films As mentioned in chapter 2 the stability of a foam film depends on the interplay between stabilizing repulsive forces (electrostatic and steric interactions) and destabilizing attractive van der Waals forces. This chapter focuses on the effect of surface charge variation on the interactions in foam films. In contrast to solid interfaces the surface charge of a liquid interface can be easily changed by the charge of the surface active compound i.e. surfactants of low molecular weight (e.g. [53]) or amphiphilic diblock copolymers [53,54]. 4.1. Pure surfactant foam films In pure surfactant foam films the adsorption of surfactant molecules is detected by changes in film thickness, i.e. changes in interactions between the film surfaces. If the surface charge is high enough the electrostatic repulsion between the film surfaces avoids a transition from a CBF to a NBF. With a TFPB one can measure the value of the surface potential but not the sign of the potential. However an assumption of the surface charge can be done. In the case of ionic surfactants the film surfaces have the same sign of charge as the surfactant. The typical f potential is of the order of magnitude of 100 mVor even more [55]. The double layer potential increases with increasing surfactant adsorption in the range of small surfactant bulk concentrations [56]. At higher concentrations of the ionic surfactant, however, a saturation can take place due to the binding of counterions to the surface [57]. This is the opposite for films of nonionic surfactants where the repulsion between the surfaces is reduced with increasing surfactant concentrations. At low

surfactant concentrations the film shows a value of the surface potential |f| between 30 and 50 mV [58–60] which is high enough to stabilize a CBF. With increasing concentration the repulsion between the surfaces is reduced and a transition from a CBF to a NBF can be induced. The origin for surface charges in a foam film of nonionic surfactant has not been clarified, yet. But f potential measurements at the air/ liquid interface of the corresponding solutions result in negative surface charges [61,62]. Addition of anionic surfactants increases the negative f potential, whereas a cationic surfactant causes a charge reversal from negatively to positively charged interfaces [63,64]. For instance, the addition of C16TAB changes the surface potential from 14 mV to +123 mV [53]. The charge reversion takes place at a C16TAB concentration between 108 and 107 mol/l. In this concentration regime the film is unstable due to missing electrostatic repulsion. Discussing the origin of the charge in the presence of nonionic surfactants or the charge reversal in the case of ionic surfactants one has to take into account the fact that the water/air interface is assumed to be negatively charged even in the absence of any surfactant [62,65,66]. Beside the adsorption of impurities and other negatively charged ions, the most likely explanation for negative surface charges is the adsorption of OH ions at the air/water interface due to the dissociation of water [67]. The adsorption mechanism itself is still under discussion and only qualitative explanations e.g. steric aspects can be given [13,68]. On the other hand an excess of hydroxyl ions is difficult to image in a neutral pH regime. After dissolution of carbon dioxide from the atmosphere the pH of water is about 5.5, i.e. an OH concentration of less than 107 mol/l. An excess of one type of ion which leads to a significant surface potential can only be explained then by a strongly different dissociation equilibrium at the interface in comparison to the bulk equilibrium. Another explanation for a surface potential could be the orientation of water molecules at the air/water interface in the way that the hydrogen atoms predominately point to the air and the OH groups towards the water phase [69– 71]. The surface potential due to the orientation of the water molecules should be +500 mV [72]. The surface potential affects the ion distribution at the air/water interface [73,74]. The question arises if the orientation of water molecules can explain the repulsive forces measured in foam films containing nonionic surfactant. There, the surface potential is one order of magnitude lower, and the experiments with mixed polyelectrolte/ surfactant foam films give clear evidence for negative charged foam film surfaces (see below). A contradiction occurs in the literature concerning the orientation of the water molecules at the air/water interface. While [71–74] discuss a dipole orientation perpendicular to the interface as mentioned above, in [3] a lateral orientation of water dipoles is assumed. The former one seems to be reasonable due to the broken symmetry perpendicular to the interface.

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In mixed surfactant/polyelectrolyte films both the charge of the surfactant and of the polyelectrolyte can be varied. Two main questions arise. Firstly, how are the structural forces are affected by the choice of the surfactant, and secondly, how the thickness is influenced by the surfactant/ polyelectrolyte charge combination. In the studies presented in the following the surfactant concentration is below the critical aggregation concentration. 4.2.1. Effect on structural forces 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 on the choice of the surfactant [75,76]. This means that the interaction between the polyelectrolytes and surfactant molecules can be neglected with respect to their effect on structural forces. Fig. 6 shows the disjoining pressure isotherms of free-standing aqueous films 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 films [77] and between two solid interfaces in an AFM [78,79]. The period of the pressure oscillation remains constant.

disjoining pressure / Pa

4.2.2. Effect on total film thickness On the other hand the total film thickness is different for different surfactants which leads to the assumption that the interactions between polyelectrolyte and surfactant at the film interfaces are not negligible for the interactions within the film. Fig. 6 shows that for the case that both compounds are positively charged (PDADMAC/C16TAB) the film is

4000

C12G2 / PDADMAC

CBF

C16TAB / PDADMAC 3000

2000

Table 1 Type of the limiting free-standing film observed for specific combinations of polyelectrolyte and surfactant charge Polyanion Polycation Non-ionic polymer

Non-ionic surf.

Cationic surf.

Anionic surf.

CBF [75,122,41, 76,117,118] NBF [42,120,119,40] CBF [119]

CBF [121,75, 122,86,117] CBF [42,120,37] –

CBF [41,118] – CBF [123,124]

Comparison to results obtained by other groups. Adapted from [76].

about 30 nm thick which is an indication of a CBF. The CBF is very stable and no transition to a NBF has been observed up to a pressure as high as 4000 Pa. If the positively charged C16TAB is replaced by the nonionic C12G2 a CBFZ NBF transition is already induced at a pressure of about 800 Pa, and the film is only a few nanometers thick. The NBF is not stable and it breaks after a few minutes. 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. If PSS is combined with the positively charged C12TAB the film is also stable and no CBFZ NBF transition has been observed. The results for the polyelectrolyte/surfactant charge combination can be generalized for all investigated polyelectrolyte/surfactant charge combination, and they are summarized in Table 1. As mentioned in the introduction 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 polyelectrolytes at the surface. Since the composition of the film interfaces is not directly accessible, in general the results of surface tension [80,81] and scattering measurements [82,83] at the free air/water 70

surface tension / mN/m

4.2. Mixed polyelectrolyte/surfactant films

259

60

50

40

NBF 30

1000

10-7

10-6

10-5

10-4

10-3

10-2

surfactant concentration / mol/l 0 0

20

40

60

80

100

120

film thickness / nm Fig. 6. Disjoining pressure isotherms of free-standing PDADMAC films, stabilized with either C12G2 or C16TAB. Taken from [76].

Fig. 7. Surface tension in dependence of the surfactant concentration of the pure surfactant solution (open symbols) and after the addition of 5*103 monomol/l polyelectrolyte (filled symbols) for three different systems: PDADMAC/C12G2 (squares), PDADMAC/C16TAB (triangles) and PSS/ C12TAB (circles). Data taken from [76].

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surface tension / mN/m

80

70

60

50

40

0

20

40

60

80

100

polymer charge density /% Fig. 8. Surface tension in dependence of the polymer charge density of the random copolymer P(DADMAC-stat-NMVA). The polymer consists of different ratios of positively charged DADMAC and neutral NMVA [119]. The polymer concentration is 5*103 monomol/l and the SDS concentration is 105 mol/l. Taken from [84].

interface are taken into account. Of course, it cannot be excluded that the composition of the interface changes if a second interface is approached like in the free-standing film. The addition of PDADMAC has no significant effect on the surface tension of aqueous solutions containing nonionic or cationic surfactant [76], as shown in Fig. 7 (squares, triangles). Therefore it can be assumed that no surface active complexes are formed. In contrast to this a pronounced effect on the surface tension is observed after the addition of PSS to the oppositely charged C12TAB [81] (circles). The surface tension of the mixed system is reduced and shows the typical plateau at intermediate surfactant concentrations which is related to the critical aggregation

between polyelectrolyte and surfactant [76,81]. Results of X-ray and neutron reflectometry measurements confirm the assumptions with respect to the formation of surface active complexes [82,83]. The polymer charge density has a pronounced effect on the surface tension. As shown in Fig. 8, the surface tension of aqueous mixtures of the cationic P(DADMAC-statNMVA) with oppositely charged SDS exhibits a nonmonotonic dependence on the charge density of the polymer. A minimum in surface tension is detected at an intermediate degree of charge. Up to this charge density the surface tension decreases, which is attributed to the fact that the formation of surface active complexes becomes more pronounced with increasing charge density. Above this degree of polymer charge the surface tension increases with increasing charge. In this regime the surface tension is mainly governed by the conformation of the polyelectrolyte chains. With increasing charge density the polyelectrolyte chains stretch out due to intra-chain electrostatic repulsion, and thus the width of the interface layer and the adsorbed amount are assumed to decrease [84]. The results of the studies at the air/liquid interface are applied to the film interfaces and they lead to the conclusion that in films of ionic surfactants a pronounced repulsion between the surfaces takes place [76]. In the case of equally charged polyelectrolytes and surfactants both compounds repel each other, which enhances the overall electrostatic repulsion within the film. In films of oppositely charged surfactant and polyelectrolyte it is assumed that the polyelectrolyte chains adsorb at the interface and that polyelectrolyte/surfactant complexes are formed. A charge reversal at the interface occurs which again leads to an electrostatic repulsion between the interface and the polyelectrolyte within the film core. Both situations lead to the formation of a CBF which is stable up to high pressures.

a) Polyelectrolyte/surfactant: same charge

CBF

Π

CBF

b) Polyelectrolyte/surfactant: opposite charge

CBF h Fig. 9. Model of a film for which the film surface and the polyelectrolytes have same sign of charge. This can be adjusted (a) if polyelectrolyte and surfactant have the same sign of charge or (b) if they are oppositely charged and the surface charge is reversed by the adsorption of polyelectrolytes. Right hand side: schematic disjoining pressure curve of the CBF. Taken from [76].

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4.2.3. Effect on film stability The addition of polyelectrolytes affect 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

4000

disjoining pressure [Pa]

Following this argumentation line, the high stability of the CBF containing nonionic surfactant and a polyanion is explained by a strong electrostatic repulsion between the polyanion and the negatively 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. The surfaces are assumed to be slightly negatively charged due to the adsorption of OH ions or the orientation of the water molecules with the OH groups towards the film bulk (see above). The polyelectrolyte is attracted by the interface, but no complexes are formed. This could reduce the surface potential and therefore the electrostatic repulsion, and a transition to a NBF occurs. This means that the osmotic pressure of the added polycation does not play any role. Otherwise the film thickness should increase after the addition of the polyelectrolyte. Schemes of the different film structures are shown in Figs. 9 and 10. The films containing the corresponding monomers DADMAC and Na–SS show qualitatively the same behavior concerning the thinnest accessible thickness as the respective polyelectrolyte film [76]. The addition on DADMAC to a C16TAB film leads to a CBF and the addition of DADMAC to a C12G2 film results in a NBF (see Fig. 11). If DADMAC is replaced by Na–SS a CBF is obtained. These results are remarkable, since usually the addition of low molecular weight salts like NaCl leads to the formation of a NBF (e.g. [60]). Therefore it is assumed that not the polymeric character is responsible for the final film thickness but rather the properties of the monomer unit and its adsorption or depletion at the film surface. This means that the monomers cannot be considered as simple salt. An open question is if the transition from the properties of simple salt to the ones of monomers is related to the size and/or the polarizability of the ions.

CBF

261

DADMAC / C12G2 DADMAC / C16TAB

3000

2000

1000

NBF 0

0

20

40

60

film thickness [nm] Fig. 11. Disjoining pressure isotherms of films containing DADMAC at the corresponding monomer concentration as in Fig. 6. The concentrations are DADMAC: 5 103 mol/l; C12G2: 105 mol/l; C16TAB: 104 mol/l. Adapted from [76].

minutes after its formation. This means that the steric repulsion of the nonionic surfactant layer at the film surface are rather weak. If polyelectrolyte and surfactant are identically charged the stability is increased due to the electrostatic repulsion. In the case of oppositely charged polyelectrolytes and surfactants the stability is increased due to the formation of surface active complexes. In addition the charge reversion of the surface leads to electrostatic repulsion which enhances the stability. While a pure C12TAB is unstable [85,86], the film is stable up to several thousands of Pa after the addition of PSS. In addition, the stabilizing effect of polyelectrolytes is due to oscillatory forces which might hamper the film drainage [87].

5. Wetting films This chapter describes the correlation between the composition of the solid interface and its interaction with the opposing air/liquid interface in a wetting film. By the variation of the surface coating the stability of the wetting

Π CBF

NBF

CBF

NBF h Fig. 10. Model of a film for which the film surface and the polyelectrolytes are oppositely charged. This can be only adjusted by mixing negatively charged polyelectrolytes and non-ionic surfactant. Right hand side: schematic disjoining pressure curve of the CBF–NBF transition. Taken from [76].

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film can be tuned. As already mentioned in the introduction a film can break either due to nucleation or due to spinodal decomposition. A precondition for spinodal dewetting is the existence of attractive forces between the solid surface and the gas bubble. In wetting films like air/water/Silica the van der Waals forces are repulsive. The only attractive DLVO force can be electrostatic attraction if the solid surface is oppositely charged to the air/water interface. Although there are no attractive interactions a water film ruptures on a hydrophobic interface. In general this lack of knowledge is filled with so-called b(ultra)long-range hydrophobic forcesQ (e.g. [3–5,9]) in a range of several nanometers up to hundreds of nanometers. Until now it is unclear what hydrophobic forces are. One part of the chapter deals with an overview on interactions which might be parts of these hydrophobic forces. In order to study the development of fluctuations within a film, surface waves have been induced mechanically [52]. The basic idea is to study the stability of disperse systems under the influence of external excited oscillations. Already Scheludko predicted that thermal perturbations on free liquid interfaces grow if the film is thin enough and attractive forces are involved [7]. If the attractive forces are not compensated by repulsive ones the film will break at a finite film thickness, referred as its critical film thickness h cr. 5.1. High energetic surfaces In the following charged hydrophilic surfaces are considered. 5.1.1. Likely charged interfaces A wetting film on a clean Silica is tremendously stable [88,89] up to an ionic strength of 0.1 mol/l [90]. Fig. 12 shows the disjoining pressure isotherm of a (free-standing) foam film and a wetting film on Silicon, both stabilized by a non-ionic sugar surfactant. The wetting film is thicker and more stable than the foam film [91] due to the higher surface potential of the Silicon surface (between 70 and 80 mV

disjoining pressure / Pa

no rupture

1000

Wetting film on Silicon Foam film

rupture

Wetting film: κ −1 = 30 nm Foam film: κ −1 = 29 nm

100

10 20 30 40 50 60 70 80 90 100

film thickness / nm Fig. 12. Disjoining pressure isotherm of a foam film and wetting film (against a silicon wafer) formed from an aqueous C12G2 solution.

[29]) in comparison to the air/water interface covered by a non-ionic surfactant (between 30 and 50 mV, see chapter 4). The same slope indicates the same screening length which corresponds to an ionic strength of about 104 mol/l as used in the experiment. With increasing ionic strength the wetting film becomes less stable, which indicates that the film is mainly stabilized by electrostatic repulsion. The film thickness of the water film on a hydrophilic substrate like metallic or mineral surfaces is much thicker than predicted by DLVO theory [92,93]. It is assumed that an excess of hydrogen bonds occur at the air/water interface [94]. It decreases with decreasing film thickness which enhances the DLVO repulsion within the films [95]. The effect results from so-called long range surface forces on the structure of the first water layers. A further hint for the importance of non-DLVO forces is given by washing experiments. While a water film on a H2SO4 treated SiC is stable, film rupture occurs on a HF treated surface [51]. Neither the surface potential nor the Hamaker constant is varied by the cleaning process. This leads the authors to the conclusion that structural forces are important. P structural is supposed to be positive at hydrophilic surfaces which increases the stability and negative at hydrophobic surfaces which destabilizes the film [13]. Qualitatively the same results are obtained for pure water films and water films containing non-ionic surfactant. The results give a clear evidence for the existence of negative charges at the air/water interface as discussed in chapter 4. The question is again if the dipole forces of water molecules oriented with the hydroxyl group towards the film bulk are sufficient for explaining the strong repulsion and stability of wetting water films at low ionic strength. The drainage behavior has been manipulated by the adsorption of polyanions at the Silicon surface. For instance, polyelectrolyte multilayer has been prepared by alternating adsorption of polycations and polyanions [96,97] at the Silicon substrate. In the case of polyanion terminated multilayer a water film is again stable due to the reasons discussed above, but the drainage velocity decreases in comparison to the one near the bare silicon surface. This can be explained by an increase in surface roughness due to the tails and loops of the polyelectrolyte chains directed towards the wetting film. This effect is enhanced in the case of polyelectrolyte multilayers which contains strongly swelling polymers like polyacrylic acid [50]. Beside homopolymers also amphiphilic blockcopolymers have been used for coating the solid substrate. The wetting film was studied both on quartz [53,98] and on SiC [99]. At low ionic strength electrostatic repulsion is responsible for the film stability. From a certain ionic strength on the film thickness remains constant, and it is of the same order of magnitude as the sum of the thicknesses of the adsorbed polymer layers at the both opposing interfaces. Soft steric repulsion is assumed to govern the equilibrium film thickness in wetting films containing block-copolymers at high ionic strength.

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5.1.2. Oppositely charged interfaces In contrast to foam films, wetting films can be prepared with oppositely charged interfaces. In this case the electrostatic interaction between the negatively charged air/water interface and the positively charged surface becomes attractive. Depending on the ionic strength and the surface potential the repulsive van der Waals contribution can be overcome or not. Coming from pure water films on Silicon or glass the sign of the air/water interface and/or of the solid interface can be reversed. The charge of the air/water interface has been changed, by the addition of a cationic surfactant. For instance, a wetting film containing Cn TAB against a negatively charged solid surface (silicon, polyanion coated silicon) is unstable and breaks more or less immediately [50]. Studies on C16TAB wetting films against Quartz indicates a change in the potential of the Quartz surface from 100 to +150 mV [100]. The charge reversal takes place at a surfactant concentration between 105 and 104 mol/l. Due to the fact that the charge reversal of the air/solution interface occurs between 108 and 107 mol/l as mentioned in chapter 4, the opposing interfaces are oppositely charged between these two concentrations of charge reversal, which leads to a film rupture in between 107 mol/ l and 105 mol/l C16TAB [53]. The sign of the surface charge of the solid surface (Silicon or quartz) has been reversed by the adsorption of small cations (e.g. Al3+ [101]) or polycations [50]. In [101] the wetting films are stable for blong timeQ, although many holes are formed. The distance between the holes is quite constant which could be a hint for the existence of fluctuation waves enhanced by electrostatic attraction. The theoretically predicted wavelength is of the same order of magnitude as the distance of the holes. An immediate film rupture has been observed for water films against a polycation coated silicon wafer [50]. It can be concluded that films with oppositely charged surfaces breaks due to spinodal decomposition. 5.2. Low energetic surfaces While a water film on Silica is stable (see above), it breaks immediately on a methylated surface [89,90,102]. To take domain and defect formation and roughness of a brealQ interface into account the correlation between morphology of transferred LB-layers and the rupture of wetting films has been studied [103]. The results on the methylated surface are more reproducible with a smaller error than the results on the LB films. While the wetting film on the methylated surface breaks at a thickness between 50 and 70 nm the critical film thickness of a wetting film on a LB film even reaches 250 nm. Surface heterogeneities of a respective height have never been observed and can be excluded. Theoretical connection between surface structure and hydrophobic attraction is unknown. The most probable explanation for the rupture of thick films seems to be air bubbles sticking to surface irregularities. Hydrophobic spots might act as nuclei for bubbles, but no direct hints exist, that the film starts to break near the bubble. However the

263

formation of bubbles sticking to the heterogenous surface explain the rupture of thick films rather than ultra-longrange attraction. Perhaps, also bubble bubble coalescence could be possible. It is even more difficult to imagine film rupture at smooth interfaces. At homogeneous methylated surface no bubble formation has been observed. With increasing contact angle, i.e. increasing hydrophobicity, the critical thickness of rupture increases and the life time of the film decreases [101]. Attractive structural force between hydrophobic interfaces has been already mentioned above [13]. The typical decay length is of the order of nm up to 10 nm [104–106]. The attraction between an uncharged hydrocarbon and fluorocarbon was measurable even up to a distance of 80 nm. The reason for such a long range has been not clarified, yet. Probably, these are similar forces which let break a thick aqueous wetting films on silanized silicon. Obviously a certain number of surface properties play a role which leads to the conclusion that several different types of hydrophobic forces exist [103]. In the following possible types of forces are discussed. 5.2.1. DLVO forces According to Laskowski and Kitchner [90] the negative f-potential of the silica surface is more or less unchanged after the methylation reaction, since only a small amount of the superficial hydroxyl groups are eliminated. However, the sign of charge is not changed by the hydrophobization and there should be at least a small electrostatic repulsion between the hydrophobized Silica and the negatively charged air/water interface. The dispersion forces are repulsive which leads to a stable wetting film (see chapter 2). Even for systems where they are attractive, their range of 10 nm could not explain a critical film thickness of 100 nm. 5.2.2. Water depletion layer The instability of aqueous films against hydrophobic interfaces could be explained by the reduction in water density near a planar, low energetic surface [71,107,108]. For instance neutron reflectometry measurements at a polystyrene/water interface result in a 2.5 nm thick interfacial layer whose water density is 90% of the respective bulk value [107]. The respective vacuum layer thickness is about 2–3 2. On hydrophobic self-assembled monolayers the vacuum layer is about 5 2 and on parafilm substrates the vacuum layer thickness is about 1 2 [71]. Molecular dynamics (MD) simulations confirm these experimental results [72]. The thickness of the depletion layer decreases even at a small amount of roughness and it increases at high temperature. The depletion layer has a rather low compressibility. After degassing of water the depletion layer is still detectable. However, since this layer thickness is between 1 and 2 orders of magnitude below the critical film thickness of film rupture and cannot explain the dewetting of films thicker than 10 nm. The structure of the depletion layer is still under discussion. Eventually, this depletion layer is related to the structuring of water

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molecules near the hydrophobic interface as mentioned in chapter 2. The typical decay length for structural forces in water is about 1 nm which is of the same range as the thickness of the measured depletion layer. 5.2.3. Nanobubbles The long-range (10–100 nm) attraction measured between macroscopic hydrophobic surfaces (see [109] and references therein) has stimulated wide-ranging debate on the origins and implications of the phenomenon. One idea is that the attraction is due to preexisting bubbles that bridge the two approaching hydrophobic surfaces [109,110]. Force measurements at a hydrophobic interface [107,111,112] indicate the formation of nanobubbles at hydrophobic surfaces against the water phase. The diameter of nanobubbles varies in the literature from 20 nm up to 100 nm [112–114] which is in the same order of magnitude as the critical thickness of wetting film rupture. As a consequence a very thin free-standing film is formed between the nanobubble and the water/air interface of the wetting film (see Fig. 13). This thin air/water/air film has a positive Hamaker constant and leads to attractive van der Waals interactions, i.e. nucleation can occur [101]. If the electrostatic barrier is overcome, any further thinning of the wetting film will lead to rupture of the foam film. Thereby a hole with a three-phase contact line at the position of the former nanobubble appears, which can spread and cause dewetting of the solid surface. The hole needs a minimum radius r crit for growing. If the radius is smaller than r crit the hole disappears. With increasing hydrophobicity r crit decreases. In general, r crit is larger than h crit and the factor

is between 1 for strongly hydrophobic interfaces and 10 for less hydrophobic interfaces [101]. The formation of a foam film is supported by an increase in wetting film stability due to the addition of a small amount of negatively charged surfactant molecules [115]. They increase the electrostatic repulsion between the negatively charged air/water interfaces (see chapter 4). Nucleation of air bubbles might be the reason for the very high rupture thickness. In this image the stability of a thick wetting film (about 100 nm) depends on the stability of a thin foam film. Therefore Stfckelhuber et al. mention that bAn introduction of long-range hydrophobic interaction forces is neither necessary nor appropriate.Q However, the formation of nanobubbles is still under discussion. The pressure difference between the inside of a nanobubble of a radius of 10 nm and the surrounding water phase is about 107 N/m2 (100 bar). Generally, the disjoining pressure in wetting films of the presented studies is several orders of magnitude lower (103–104 N/m2). It is not clear whether the bubbles are in thermodynamical equilibrium or not. It arises the question why reflectometry studies and simulations result in a thin depletion layer (1–2 nm) as mentioned above and why force measurements show a clear evidence for the existence of nanobubbles. The answer might be given by AFM measurements where the tip has been approached several times [116]. If the tip is approached for the first time to the interface there is no indication for a nanobubble in the force curve. But after several approaches a long-range interaction has been

~ 20-100 nm

foam film

hydrophobic interface

hcr

rcr

Fig. 13. Sketch of a nucleation of a wetting film due to the formation of nanobubbles.

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detected. This leads to the conclusion that the nanobubbles are induced by the interaction of the probe with the surface and that the result depends on the history of the sample. These experiments argue against the formation of nanobubbles at the solid interface in a wetting film, since the opposed air/water surface is rather flat and there is no heterogenous (point-shaped) force field as around an AFM tip.

6. Summary and conclusion The present paper reviews the different forces in foam films and wetting films and focuses on the influence of the modification of the surfaces. There some disagreements between the studies of a single air/water interface and experiments at thin aqueous films like foam films and wetting films. The experimental and theoretical studies of a single air/water interface indicate an orientation of the water molecules with the hydrogen atom pointed to the air. This leads to a surface potential of +500 mV. Force measurements of foam and wetting films result in a surface potential of the air/film interface which is one order of magnitude lower. The addition of polyelectrolytes and the modification of the film surfaces, respectively, leads to the conclusion that the air/water interface is negatively charged. Under the assumption that the orientation of water molecules is the same at the free air/solution interface as at the film surface this paradox might be solved by the fact that the forces are due to the repulsion between the partially negatively charged oxygen which are pointed towards the film bulk. On the hand the dipole forces might be not strong enough to be detectable at film surface distances of several tens of nanometers. Another question which has not been clarified yet, is the physical nature of so-called blong-range attractive hydrophobic forcesQ which lead to a film rupture at thicknesses of the order of 100 nm. Different forces are considered. DLVO forces are repulsive for most of the considered system, and even if the dispersion forces would be attractive their range of about 10 nm would not explain the rupture of much thicker films. At hydrophobic surfaces a reduction in water density has been detected by neutron reflectometry or ellipsometry. This layer is a few nanometers thick and cannot explain the rupture either. While non-invasive methods like optical or scattering methods do not detect any gas cavities AFM measurements show a clear evidence for the existence of nanobubbles of a diameter between 20 and 100 nm at the hydrophobic interface. It has been proven that the bubbles are induced during the measurement. The presented studies on foam and wetting films include force measurements, too, but the force field is laterally homogenous in contrast to the force filed around a tip. The diameter of nanobubbles is of the same order of magnitude as the force range which is needed to explain the rupture of such thick films, but their formation is still unclear.

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Acknowledgement The author thanks Roland Steitz, Roland Netz and Pavel Jungwirth for fruitful discussions. The Fonds der Chemischen Industrie is acknowledged for financial support.

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