Surface Adsorption of Oppositely Charged SDS:C12TAB Mixtures and

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Surface Adsorption of Oppositely Charged SDS:C TAB Mixtures and the Relation to Foam Film Formation and Stability Heiko Fauser, Martin Uhlig, Reinhard Miller, and Regine von Klitzing J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b06231 • Publication Date (Web): 14 Sep 2015 Downloaded from http://pubs.acs.org on September 21, 2015

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The Journal of Physical Chemistry

Surface Adsorption of Oppositely Charged SDS:C12TAB Mixtures and the Relation to Foam Film Formation and Stability Heiko Fauser,† Martin Uhlig,† Reinhard Miller,‡ and Regine von Klitzing∗,† Stranski-Laboratorium, Department of Chemistry, Technische Universitaet Berlin, Strasse des 17.Juni 124, 10623 Berlin, Germany, and Max Planck Institute of Colloids and Interfaces, Am Muehlenberg 1, 14424 Potsdam, Germany E-mail: [email protected]

Abstract The complexation, surface adsorption and foam film stabilistation of the oppositely charged surfactants, sodium dodecyl sulfate (SDS) and dodecyl trimethylammonium bromide (C12 TAB), is analyzed. The SDS:C12 TAB mixing ratio is systematically varied to investigate, whether the adsorption of equimolar or irregular catanionic surfactant complexes, and thus a variation in surface charge (i.e. surface excess of either SDS or C12 TAB), governs foam film properties. Surface tension measurements indicate that SDS and C12 TAB interact electrostatically in order to form stoichometric catanionic surfactant complexes and enhance surface adsorption. On the other hand it can be demonstrated that the SDS:C12 TAB mixing ratio and, thus a change in surface charge and composition plays a decisive role in foam film stabilisation. The present study ∗

To whom correspondence should be addressed Stranski-Laboratorium, Department of Chemistry, Technische Universitaet Berlin, Strasse des 17.Juni 124, 10623 Berlin, Germany ‡ Max Planck Institute of Colloids and Interfaces, Am Muehlenberg 1, 14424 Potsdam, Germany †

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demonstrates that varying the mixing ratio between SDS and C12 TAB offers a tool for tailoring surface composition and foam film properties, which are therefore not exclusively mediated by the presence of equimolar catanionic surfactant complexes. The SDS:C12 TAB net amount and mixing ratio determines the type, stability and thinning behavior of the corresponding foam film. These observations indicate the formation of a mixed surface layer, composed of the catanionic surfactant species surrounded by either free SDS or C12 TAB molecules in excess. Furthermore, a systematic variation in CBF-NBF transition kinetics is rationalized on the basis of a microscopic phase transition within the foam films. Fundamental knowlegde gained from this research gives insight into the surface adsorption and foam film formation of catanionic surfactant mixtures. The study helps to understand basic mechanisms of foam film stabilisation and to use resources more efficiently.

Introduction The performance of surfactant mixtures on foaming and foam stability is of interest for many industrial applications. Typical examples are enhanced oil recovery, fire fighting and advanced material synthesis 1 2 . In some processes foam formation is desired, whereas in others it should be avoided. This requirement highlights the importance to control foam stability. One way to fine tune the physical properties of foams is using oppositely charged surfactant mixtures as foam builders. Oppositely charged surfactant mixtures fascinate through their versatile bulk phase 3 4 5 and surface adsorption behavior 6 7 8 9 , which results into a diverse set of physico-chemical properties of the dispersion 10 . Due to electrostatic attraction between the oppositely charged surfactants, catanionic surfactant aggregates are formed 11 12 . Catanionic surfactant mixtures are promising systems for foam formation, since they reduce the amount of surfactant required and can assist in the formation of ultrastable foams 13 .


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In a dry foam with film thicknesses less than 100 nm, the film surfaces strongly interact and their stability is discussed in terms of surface forces 14 . Surface forces originate from adsorbed molecules at the air/water interface and create an excess pressure normal to the film interfaces called the disjoining pressure Π. According to the Derjaguin-Landau-VerweyOverbeek (DLVO) theory 15 , the disjoining pressure Π is the sum of long range repulsive electrostatic (Πelec ), short range attractive van der Waals (ΠvdW ) and repulsive steric (Πsteric ) forces 16 between the two foam film surfaces. Depending on the dominating surface force, two different types of foam films are formed: (A) If the film is stabilized by electrostatic forces, a Common Black Film (CBF) is formed, 10-100 nm thick. The structure of a CBF can be explained by a sandwich model of two adsorption layers of surfactant molecules with an aqueous core in between 17 . CBFs are stabilized by surface charges and generally observed for foam films formed by ionic surfactant solutions 14 18 19 20 . (B) Newton Black Films (NBF) on the other hand, are stabilized by steric forces and only 4-10 nm thick, depending on the thickness of the adsorbed surfactant layers. Stable NBFs are only formed, when the adsorption layer at the film surface is densely packed. A NBF consists of a bilayer of surfactant molecules with a high degree of order containing counter ions and hydration water 16 21 22 . Whether a CBF or NBF is formed depends on the interfacial charge of the film surfaces. This charge can be varied by the type of surfactant (nonionic, cationic or anionic), its concentration and the ionic strength (electrolyte concentration) in solution 17 23 . Another possibility to vary the interfacial charge is to introduce surfactant mixtures. In thin film studies, the interfacial charge is usually adjusted by mixing an ionic surfactant with an uncharged component such as nonionic surfactants 24 25 26 , polymers 27 28 or by adding charged polyelectrolytes 29 30 31 32 33 . Only recently Kristen-Hochrein et al. showed that the type and amount of anionic surfactant, added to a cationic surfactant solution, presents a tool for tailoring the thickness and stability of foam films 34 . Nontheless, systematic studies on the influence of oppositely charged surfactant mixtures on foam film stabilisation are scarcely existing.



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Thereby, an important challenge is to understand and control the influence of aggregation and surface adsorption of oppositely charged surfactant mixtures on foam film properties. As model system a mixture of sodium dodecyl sulfate (SDS) and dodecyl trimethylammonium bromide (C12 TAB) is used. Both surfactants are symmetric in the hydrocarbon chain length and differ only in head group properties. In addition pure C12 TAB does not stabilize foam films at bulk surfactant concentrations below its cmc, which makes it easy to recognize the effect of the catanionic surfactant mixture at the concentrations investigated within this work. To study the complexation of SDS:C12 TAB mixtures at the water/air interface and the impact on foam film formation and stabilization, a combination of surface tensiometry, dilational surface elasticty and thin film pressure balance measurements were performed. The SDS:C12 TAB mixing ratio was systematically varied to investigate if surface properties are governed by the adsorption of whether equimolar or irregular catanionic complexes. The results are used to rationalize the influence of the mixing ratio on surface composition and thus the type, stability and thinning behavior of the corresponding foam film.

Experimental section Materials and Sample Preparation The cationic dodecyl trimethyl ammonium bromide (C12 TAB) was purchased from SigmaAldrich (Steinheim, Germany) and recrystallized three times from acetone with traces of ethanol. The purity of the surfactant was verified by surface tension measurements. The anionic sodium dodecyl sulfate (SDS) with a purity of 99.9 % was purchased from SigmaAldrich and was used without any further purification. All sample solutions were prepared from deionized water (Milli-Q; total organic content = 4 ppb; resistivity = 18 mΩ · cm). The mixed SDS:C12 TAB sample solutions were prepared by combining equal volumes of stock solutions with twice the desired concentration of the respective surfactant. All glassware (except the foam film holder) was cleaned with the basic detergent mixture Q9 (Ferak Berlin 4 ACS Paragon Plus Environment

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GmbH) and rinsed thouroughly with water before use. The porous glass disc was cleaned with ethanol and water for several times. Prior to each measurement the foam film holder was boiled for 48 h in water.

Thin Film Pressure Balance - Disjoining Pressure Isotherms Disjoining pressure isotherms were measured using a thin film pressure balance (TFPB) utilizing the porous plate technique 17 35 . With this method free-standing horizontal liquid foam films can be investigated. This is a unique technique to measure interaction forces, thickness, drainage and stability of thin foam fims. The setup is fully described elsewhere 36 . Before the measurement, the film holder was immersed into the sample solution for at least 2 h. All measurements were performed at 23◦ C. The foam film was formed inside a hole of 1 mm in diameter that is drilled into a porous glass disk. Disjoining pressure (Π(h)) isotherms were obtained by interferometrically measuring the foam film thickness h, while varying the pressure applied to the foam film. The equilibrium foam film thickness was measured after the intensity of the reflected light was constant for 20 minutes. All Π(h) isotherms shown within this study are an average of 3 to 7 independent measurements. The measured disjoining pressure isotherms (Π(h) curves) were compared with model calculations according to the DLVO theory in order to determine the surface charge density q0 . During the measurement, video microscopy was used to monitor the foam film in real-time (frame rate 5 frames/s). The kinetics of CBF-NBF transition was obtained by image analysis using the Ulead Video Studio (Version 7) and the open source software ImageJ.

Simulation of the Disjoining Pressure Isotherm The experimental disjoining pressure isotherms have been analyzed using a numerical approach based on the Poisson-Boltzmann equation in planar geometry. In this approach only the electrostatic component of the disjoining pressure isotherms (Πelec ) is considered. The electrostatic component of the disjoining pressure is modeled by solving the non-linear, one 5 ACS Paragon Plus Environment


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dimensional Poisson-Boltzmann equation assuming a constant surface potential 37 38 . The numerical Poisson-Boltzmann approach starts with two planar surfaces with constant surface potential Ψ which are separated in z direction (normal direction to the surface) by the distance D. The intervening medium is in equilibrium with a bulk electrolyte solution. By symmetrical considerations, only one half of the system needs to be considered (0≤z≤D/2). In order to obtain a unique solution of the PB equation two boundary conditions need to be fullfilled: dΨ(z) dz

= Ψ0

(1)

=0

(2)

z=0

and dΨ(z) dz

z=D/2

During the calculation process, the surface potential (Ψ0 ) and the ionic strentgh are adjusted until the calculated curve coincides with the experimental data. The corresponding surface charge density q0 can be calculated by using the Grahame equation 15 25 32 . p eΨ0 q0 = 80 c0 kB T sinh 2kB T

(3)

Surface Tension The surface tension was measured with a K11 tensiometer (Kruess, Germany) using the du Nouy ring. Each sample solution was prepared one day beforehand. Prior to each measumerent the sample solution was placed in a Teflon vessel (diameter 5 cm) and equilibrated for 15 minutes. The surface tension was measured at 25◦ C until a constant value was detected.


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Surface Elasticity Surface elasticity of the SDS:C12 TAB mixtures was measured at room temperature using a PAT1 (Sinterface Technologies, Berlin, Germany). This device created a pendant drop of the surfactant solution at the tip of a capillary. The capillary was placed in a closed cuvette with a small reservoir of the sample solution at the bottom to prevent evaporation. The drop was equilibrated for 2 h prior to each measurement. To determine the dilatational surface elasticity harmonic oscillations of the drop surface were induced by a computercontrolled dosing system. Surface area A and surface tension γ were calculated as a function of time via drop shape analysis. The frequency of the drop oscillation was 0.2 Hz. The dilational deformation of the droplet (and therefore variation of surface area) causes a change of the surface concentration of the surfactants adsorbed: expansion leads to decrease in the surfactant surface concentration and hence, to an increase in the surface tension γ. Viceversa the contraction of the interface results in a decrease of γ. At a given area variation the change in surface tension is a measure of the dilational elasticity ε(ν) 39 .

ε (ν) =

∂γ = εr + iεi = εr + i2πνη ∂lnA

(4)

Therefore, the dilatational surface elasticity is a complex number of the elastic modulus εr (storage modulus) and the dilational surface viscosity εi (dissipation modulus). The surface elastic modulus is calculated from the amplitude ratio of the oscillating surface tension and surface area, whereas the phase shift between the two determines the dilational surface viscosity.



Results & Discussion Catanionic surfactant complex formation in SDS:C12 TAB mixtures and adsorption at the air-water interface Surface tension measurements are a good indicator of surfactant adsorption at the air/water interface. Figure 1 compares the surface tension isotherms of the pure surfactant solutions (anionic SDS and cationic C12 TAB) with the isotherm measured for an equimolar mixture of both surfactants (referred to as SDS:C12 TAB 5:5 mixture in the following). The high critical pure C12TAB pure SDS C12TAB : SDS (5:5)

80

Surface Tension [mN/m]

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70 60 50 40 30 20 1E-7

1E-6

1E-5

1E-4

1E-3

0,01

0,1

total surfactant concentration [mol/l] Figure 1: Equilibrium surface tension isotherms for pure SDS and C12 TAB solutions and the equimolar SDS:C12 TAB mixture. For the mixture the surface tension isotherm is shifted to much lower concentrations, indicating the formation of a highly surface active species. The arrows indicate the 3 concentrations investigated in more detail in Figure 2. The error bars of the surface tension measurements correspond to the size of the symbols. micellar concentrations (cmc) of the pure surfactants (1.5·10−2 mol/l for C12 TAB and 9·10−3 mol/l for SDS) reflect their good aqueous solubility and moderate surface activity 40 . In contrast, the 5:5 mixture levels off at much lower concentration (around 3 · 10−5 mol/l). This 8 ACS Paragon Plus Environment

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observed enhanced surface adsorption is similar to results from other studies of SDS:C12 TAB mixtures 41 42 . In case of the SDS:C12 TAB mixture the enhanced surface activity results from the formation of highly surface active stochiiometric catanionic surfactant complexes, also denoted as ion pair amphiphiles 11 . The reduced electrostatic repulsion between these catanionic complexes leads to a denser molecular packing at the interface and causes a higher surface concentration. That is confirmed by the lower plateau value of the surface tension isotherm in Figure 1 : 28 mN/m for the catanionic surfactant mixture; in comparison to 38 mN/m for the pure surfactants. In general, a strong association between SDS and C12 TAB is expected from Collins law of matched water affinities: the trimethylammonium and the sulfate headgroup are similar in head group charge density, but of reverse polarity 43 44 . In addition, the symmetric chain length of the C12 TAB and SDS hydrocarbon tail results in a strong hydrophobic interaction. SDS and C12 TAB molecules combine to form charge neutral catanionic complexes. In this respect it is noteworthy that the measured surface tension isotherm of the SDS:C12 TAB mixture agrees well with isotherms reported for the nonionic surfactants, tetra- and penta-ethylene glycol monododecyl ether. 45 To investigate the influence of the SDS:C12 TAB mixing ratio on surface adsorption, 3 particular concentrations have been investigated in detail. One concentration was in the decaying slope (10−5 mol/l) and the other two are in the plateau region (5 · 10−5 and 10−4 mol/l) of the surface tension isotherm in Figure 1. At all concentrations and mixing ratios a enhanced surface adsorption and, thus a reduced surface tension (Figure 2) could be observed. Note that pure surfactant solutions of SDS (10:0), respectively C12 TAB (0:10) showed no pronounced surface activity at the concentrations investigated and the surface tension is close to the one of pure water. In case of SDS:C12 TAB mixtures, a symmetric behavior was observed. The absolute values of the surface tension for mixtures with excess concentration of the anionic SDS (mixing ratio 9:1 to 6:4) were similar to mixtures with excess concentration of the cationic C12 TAB (mixing ratio 4:6 to 1:9). Surface tension values at different SDS:C12 TAB mixing ratios coincide with the sur-



70

Surface Tension [mN/m]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

60

c(SDS+C12TAB) 10-5 mol/l 5·10-5 mol/l 10-4 mol/l

50

40

30

0,0 9:1 0,1 8:2 0,2 7:3 0,3 6:4 0,4 5:5 0,5 4:6 0,6 3:7 0,7 0,8 0,9 0:10 1,0 10:0 2:8 1:9

mixing ratio (SDS : C12TAB) Figure 2: Equilibrium surface tension for SDS:C12 TAB mixtures at three different bulk concentrations in variation of the SDS:C12 TAB mixing ratio. If a quantitative formation of catanionic complexes occurs, the dotted line shows the surface tension measured for equimolar SDS:C12 TAB mixtures at comparable concentration (details are given in the Supporting Information). The error bars of the surface tension measurements correspond to the size of the symbols. face tension measured for for sample solutions containing only the stoichometric catanionic SDS:C12 TAB complex at the corresponding concentration (doted line in Figure 2). For example, if catanionic complexes are formed quantitatively, a 5 · 10−5 mol/l 7:3 SDS:C12 TAB mixture forms 1.5 · 10−5 mol/l of catanionic complexes. In both cases, for a 5 · 10−5 mol/l 7:3 SDS:C12 TAB mixture and a 1.5 · 10−5 mol/l sample solution of only the catanionic complex, the measured surface tension was around 28 mN/m. A detailed comparison between the surface tension measured at different SDS:C12 TAB mixing ratios and the surface tension for equimolar SDS:C12 TAB mixtures is shown in the Supporting Information. These observations clearly indicate that surface adsorption is governed by the stoichometric formation of catanionic surfactant complexes and independent of the mixing ratio 41 42 46 .


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Only in the case of mixing ratios with a large excess concentration of SDS (9:1), respectively C12 TAB (1:9), the surface tension measured was lower as for the respective equimolar SDS:C12 TAB mixtures (doted line in Figure 2). This enhanced surface adsorption cannot be attributed to the stoichometric formation of catanionic surfactant complexes. In this case the enhanced surface adsorption propably results from a synergistic effect between the formed catanionic SDS:C12 TAB complexes and the free ionic surfactant molecules in excess concentration. This observation is an indicaton for the formation of a mixed surface layer composed of catanionic SDS:C12 TAB complexes together with excess of unbound SDS or C12 TAB molecules.

Influence of the SDS:C12 TAB mixing ratio on foam film properties Foam films and their characteristic disjoining pressure are strongly dependent on the composition of the surfactant monolayer at the film surface. Foam film investigations aim to find out whether surfactant adsorption at the surface is governed by the presence of equimolar catanionic surfactant complexes or if surface composition varies with the SDS:C12 TAB mixing ratio. Note, that a total surfactant concentration of at least 5 · 10−5 mol/l is required to obtain stable foam films. For SDS:C12 TAB mixtures of a total concentration of 10−5 mol/l, no stable foam films were observed. According to the SDS:C12 TAB phase diagram provided by Herrington et al. 47 at such low surfactant concentration the formation of micelles or vesicles in the bulk solution phase does not occur. Homogeneous foam films are formed at the low SDS:C12 TAB concentrations investigated. Foam films prepared from vesicular solutions exhibit microtubular networks (compare Figure 1 in the supporting information). According to the broad plateau in surface tension (Figure 2), one may expect that the surface is completely covered by catanionic surfactant complexes over a wide range of SDS:C12 TAB mixing ratios, which therefore dominate foam film stabilization. In this case the surface of the foam film would be covered by equimolar catanionic surfactant complexes and one may expect the formation of identical foam films at all mixing ratio. In order to 11 ACS Paragon Plus Environment


carry out a rigorous study of the SDS:C12 TAB surfactant mixture thin film pressure balance measurements were performed. This technique is predestinated to investigate foam film formation and to measure interaction forces in terms of the disjoining pressure. Figure 3 shows disjoining pressure isotherms for different SDS:C12 TAB mixtures and a total surfactant concentration of 5 · 10−5 and 10−4 mol/l obtained from thin film pressure balance measurements. In case of the equimolar 5:5 SDS:C12 TAB mixture a CBF is formed directly at the onset pressure, but transforms into a NBF already at slightly higher pressure. The thickness of the formed NBF was only around 6 nm and did not change upon increasing the pressure. The open black circles in Figure 3 show the disjoining pressure isotherm of

a)

b) pure SDS (10:0) 9:1 8:2 7:3 6:4 5:5

1000

100

NBF 0

20

Disjoining Pressure  [Pa]

Disjoining Pressure  [Pa]

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CBF 40

60

80

100

120

pure SDS (10:0) 9.5 : 0.5 9:1 8:2 7:3 6:4 5:5

1000

100

NBF 0

140

CBF 20

40

60

80

100

120

film thickness [nm]

film thickness [nm]

Figure 3: Disjoining pressure isotherms for SDS:C12 TAB mixtures of varying mixing ratio at a total surfactant concentration of a) 5 · 10−5 mol/l and b) 10−4 mol/l. The solid lines correspond to Poisson-Boltzmann simulations of the disjoining pressure isotherms assuming a constant surface potential Ψ. a foam film stabilized only by the anionic surfactant SDS. In this case a CBF is formed, which monotonically decreases in thickness with increasing applied pressure and no NBF formation was observed. For foam films stabilized from mixtures with an excess concentration of the anionic SDS (mixing ratio 6:4 to 9:1), a CBF was formed at onset pressure. The film thickness of the CBF decreased monotonically with increasing applied pressure and the film transformed into a NBF at higher pressure. The formation of a NBF requires sufficient surfactant surface coverage 24 34 48 . Only for SDS:C12 TAB mixtures showing minimum sur12 ACS Paragon Plus Environment

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face tension, the surface is denseley covered with surfactant molecules and therefore NBF formation is possible. The pressure at which the CBF transforms into a NBF increases with increasing excess concentration of SDS. In case of 10−4 mol/l mixtures, the onset pressure for NBF formation increases from around 300 Pa for the 6:4 mixture to around 600 Pa for the 8:2 mixture. This observation is consistent with an increasing electrostatic repulsion (Πelec ) in the disjoining pressure isotherm. From simulation of the disjoining pressure isotherms (solid lines in Figure 3) the surface potential Ψ and therefore the surface charge density q0 15 could be determined . Figure 4 displays an increasing surface potential and surface charge density q0 with increasing excess concentration of SDS for the SDS:C12 TAB mixtures. In case of the 10−4 mol/l 5:5 the immediate formation of a stable NBF is observed. For this mixing ratio q0 is too low to stabilize a CBF, while the surface coverage is high enough to stabilize a NBF. Therefore a surface charge of q0 =0 mC/m2 is estimated With increasing excess concentration of SDS a CBF prior to NBF formation could be observed and the surface charge indreases to 1.64 mC/m2 for the 9.5:0.5 mixture (only CBF formation). In contrast to a NBF, CBFs are stabilized by surface charges and generally observed for foam films formed from ionic surfactant solutions 14 18 19 20 . The higher surface charge in case of mixtures with excess concentration of SDS may result from an excess of adsorbed SDS molecules at the surface. This variation in surface charge is a clear indication that surface composition changes with the SDS:C12 TAB mixing ratio and the surface is not solely covered with catanionic complexes. DLVO model fittings on the other hand underline that catanionic surfactant complexes are quantitatively formed at all mixing ratios. Figure 5 shows the good correlation between the Debye length (which corresponds to the ionic strength in solution) extracted from Poisson-Boltzmann simulations of the foam films in Figure 3, with the theoretical Debye length expected if a quantitative formation of catanionic surfactant complexes is assumed. Complete ion dissociation of a 10−4 mol/l SDS solution (into charged surfactants and counter ions) leads to a total concentration of 2 · 10−4 mol/l free ions in solution and, therefore exhibits a Debye length λ of 30.5 nm. Varying the mixing ratio of the SDS:C12 TAB mixture



2,0 2.0

100

c=5*10-5 mol/l (surface charge) c=10-4 mol/l (surface charge)

surface charge q0 [mC/m2]

1,8 1.8 1,6 1.6

c=5*10-5 mol/l (surface potential) c=10-4 mol/l (surface potential)

1,4 1.4

80

60

1,2 1.2 1,0 1.0

40

0,8 0.8 0,6 0.6

20

0,4 0.4

surface potential  [mV]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0,2 0.2 0

0,0 0.0 0,0 10:1

0,1 9:1

0,2 8:2

0,3 7:3

0,4 6:4

0,5 5:5

mixing ratio SDS : C12TAB Figure 4: Plot of the surface potential (open symbols) and respective surface charge (full symbols) extracted from Poisson Boltzmann simulations of the disjoining pressure isotherms in Figure 3. Since the NBF is immediately formed after foam film formation for the mixing ratio 5:5, the surface charge is estimated with 0 mC/m2 .


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45

complete ion dissociation quantitative formation catanionic complex PB fit

Debye length  [nm]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40

35

30

10:0

9:1

8:2

7:3

6:4

5:5

mixing ratio (SDS:C12TAB) Figure 5: Comparison of the expected Debye length λ if a quantitative formation of catanionic SDS:C12 TAB complexes in the 10−4 mol/l bulk solution is assumed, with the Debye lengths obtained from Poisson Boltzmann simulations of the disjoining pressure isotherms in Figure 3 b). The doted line correspond to the Debye length expected if a complete dissociation of all ions (surfactants and counterions) in solution occurs. from 9:1 to 5:5, increases the concentration of charge neutral catanionic complexes from 10−5 to 5 · 10−5 mol/l. Therefore, the concentration of free ions in solution decreases from 1.8·10−4 mol/l (9:1 mixture) to 10−4 mol/l (5:5 mixture). As a result the Debye length of the respective sample solution increased from 32.1 to 43.1 nm. The formation of charge neutral catanionic SDS:C12 TAB surfactant complexes leads to a reduced electrostatic repulsion in both, lateral and normal direction to the film surfaces. As a result the film surface is densely covered with catanionic complexes and very thin NBFs are stabilized at all SDS:C12 TAB mixing ratios. Counterintuively to what was observed for SDS:C12 TAB mixtures with excess SDS (mixing 9:1 to 6:4), a completely different behavior was observed for foam films stabilized from SDS:C12 TAB mixtures with excess concentration of C12 TAB (mixing ratio 4:6 to 1:9). In 15 ACS Paragon Plus Environment


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all cases, CBF-NBF transition occurred directly after film formation at the onset pressure and no electrostatic contribution to the disjoining pressure was detected. Furthermore, all observed foam films were unstable. On occassion, a stable NBF could be observed for several minutes, but the film gets ruptured during or after the NBF transition was completed. This observation was independant of the total surfactant concentration and 5 · 10−5 mol/l to 10−4 mol/l solutions showed similar stability. Note, in contrast to SDS, pure C12 TAB did not stabilize foam films in the investigated concentration regimes. In general, C12 TAB does not form stable foam films at concentrations lower than its cmc and the reduced foam film stability is normally explained with a lower surface elasticity 25 . Therefore, different surface elasticities may explain the different foam film stabilities observed for the SDS:C12 TAB mixtures.

Dilational surface elasticity in dependance of the SDS:C12 TAB mixing ratio Surface tension data indicates a high surface coverage at all SDS:C12 TAB mixing ratios and the concentration of catanionic complexes is always sufficient to induce NBF formation. On a contrary, a completely different foam film stability was observed for SDS:C12 TAB mixtures with excess concentration of either SDS or C12 TAB. Foam films stabilized from mixtures with excess concentration of SDS (9:1 to 5:5) were stable, whereas films stabilized from mixtures with excess concentration of C12 TAB (4:6 to 1:9) were unstable and ruptured. In this respect it was investigated to what extend dilational surface elasticity properties account for the different foam film stabilities observed. Figure 6 exhibits the measured dilational surface elasticity at a drop oscillation frequency of 0.2 Hz for SDS:C12 TAB mixtures at a total surfactant concentration of 5 · 10−5 mol/l in dependance of the overall mixing ratio. Similiar to what was found for the surface tension (Figure 2), a symmetric distribution of the dilational surface elasticity was observed. The dilational surface elasticity for SDS:C12 TAB mixtures with excess concentration of SDS (9:1 16 ACS Paragon Plus Environment

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to 7:3) is similar to the one for mixtures with excess concentration of C12 TAB (3:7 to 1:9). Maximum surface elasticity was observed for the equimolar 5:5 mixture, representing the mixing ratio with the highest concentration of formed catanionic complexes. Noteworthy is the observation that no dilational surface elasticity could be measured for the respective pure surfactant solutions at such low concentration. In case of 5 · 10−5 mol/l SDS solutions however, stable foam films could be observed. C12 TAB on the other hand did not form stable foam films. 100

dilational surface elasticity [mN/m]

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100

drop osillation frequency 0,2 Hz

90

90

80

80

70

70

60

60

50

50

40

40

30

30

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20

10

10

0 10:0 0,0

9:1

8:2 0,2

7:3

6:4 0,4

5:5

4:6 0,6

3:7

2:8 0,8

1:9

0 0:10 1,0

mixing ratio (SDS : C12TAB) Figure 6: Dilatational surface elasticity in variation of the SDS:C12 TAB mixing ratio at a total surfactant concentration of 5 · 10−5 mol/l. The error bars of the dilational surface elasticity correspond to the size of the symbols.

From the experimental data in Figure 6 it can be concluded that the dilational surface elasticity is governed by the formed catanionic complexes and, thus fails to explain different foam film stabilities found for SDS:C12 TAB mixtures with either excess concentration of SDS or C12 TAB. As a result, there must be another reason for the complete destablilisation of foam films stabilized from SDS:C12 TAB mixtures with excess concentration of C12 TAB. 17 ACS Paragon Plus Environment


CBF-NBF transition kinetics and its dependance on the SDS:C12 TAB mixing ratio A CBF-NBF transition starts with the nucleation of a small NBF spot within the CBF. The NBF spot spontaneously grows over time, until the transformation into a NBF is completed. The resulting CBF-NBF transition kinetics were analysed via video microscopy analysis: the NBF domain diameter d(t) always grew proportional to tn (Figure 7), but the exponent n systematically varied with the SDS:C12 TAB mixing ratio. This observation is intriguing and deserves particular attention. Table 1 shows that the exponent n ranges from n = 1/3 (very slow transition) for mixtures with excess concentration SDS (9:1 to 6:4), over n = 1/2 for the equimolar 5:5 mixture and reaches values of n = 2/3 (4:6 to 3:7) and, finally close to n = 1 (very fast transition) for mixtures with excess concentration of C12 TAB (2:8 and 1:9). Table 1 exhibits that this trend is independent of the total surfactant concentration.

a)

b)

1000

100

10

SDS : C12TAB (c = 5*10-5mol/l) 1:9 2:8 3:7 4:6 5:5 8:2 9:1 fit d(t) xn 0,1

1

10

100

1000

NBF diameter [m]

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100 SDS : C12TAB (c = 10-4 mol/l) 2:8 3:7 5:5 8:2 fit d(t) xn

10 0,1

time [s]

1

10

100

time [s]

Figure 7: Time dependance of the NBF domain diameter growth d(t) for CBF-NBF foam film transitions stabilized from different C12 TAB:SDS mixtures at a total surfactant concentration of a) 5 · 10−5 mol/l and b) 10−4 mol/l. The solid lines correspond to fits with d(t)∝ tn . The CBF-NBF transition velocity is mainly influenced by the electrostatic barrier 23 . The electrostatic barrier corresponds to the maximum disjoining pressure prior to NBF formation and depends on the respective surface charge. If the electrostatic barrier is high, 18 ACS Paragon Plus Environment

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Table 1: Fitted exponents n from CBF-NBF transition kinetics in Figure 7 according to d(t) ∝ tn . SDS:C12 TAB 5 · 10−5 mol/l 10−4 mol/l

9:1 0.35

8:2 0.33 0.31

6:4 0.30

5:5 0.45 0.45

4:6 0.57

3:7 2:8 1:9 0.68 0.75 0.82 0.65 0.95

the CBF-NBF transition is slow. If the electrostatic barrier is low, or surpressed by electrostatic screening at sufficient high electrolyte concentration, the transition is fast. In case of SDS:C12 TAB mixtures with excess concentration of SDS (mixing ratio 9:1 to 6:4), the electrostatic barrier increased with an increasing excess concentration of SDS (Figure 3). In all cases the NBF formed within an already equilibrated CBF and the increasing electrostatic barrier may explain the slow CBF-NBF transition observed at these mixing ratios. On the other hand the CBF-NBF transition was always proportional to n = 1/3, although the height of the electrostatic barrier varied between 250 and 560 Pa for the respective SDS:C12 TAB mixtures. On the contrary, the CBF-NBF transition for foam films stabilized by mixtures with excess concentration of C12 TAB (4:6 to 1:9) was very fast (n = 2/3 to 1). In these cases the foam films were unstable and NBF formation already occured during drainage of the film foam, what may explain the fast transition kinetics. Another explanation for the different CBF-NBF transition rates could be that a microscopic phase separation takes place while the CBF tranforms into a NBF. In general, very slow CBF-NBF transitions are reported for mixtures of oppositely charged surfactants 34 and nonionic surfactants 49 . A CBF-NBF transition is considered to be a phase transition of the whole foam film 50 . The structure of a CBF can be explained by a sandwich model of an aqueous core between to surfactant monolayers at the surface, whereas a NBF represents a well-defined solid like surfactant bilayer, containing only hydration water and counter ions. 16 For a mixture of two nonionic surfactants Stubenrauch et al. observed that the CBF-NBF transition can take up to several hours 49 . Based on the vacancy model by Exerowa 17 it was concluded that a microscopic phase separation into a surfactant-rich and a surfactant-poor



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phase took place. The surfactant-poor phase forms a CBF, while the surfactant rich phase forms a NBF and the two films coexist. The respective phase transition is very slow and accounts for the slow CBF-NBF transition kinetics. A similar microscopic phase separation could explain the different CBF-NBF transition kinetics observed for the SDS:C12 TAB surfactant mixtures. During the CBF-NBF transition, the surface layer may separate into two different phases. An inner phase consisting of catanionic complexes and a surrounding phase of mainly the ionic surfactant (SDS or C12 TAB) in excess. The catanionic phase is densely packed and forms a NBF. For mixtures with excess concentration of SDS (9:1 to 6:4) the NBF is surrounded by an electrostatically stabilized CBF. The resulting electrostatic barrier may explain the slow CBF-NBF transtion. The transition behavior for mixtures with excess concentration of C12 TAB (4:6 to 1:9) is contrary. C12 TAB does not form stable CBFs and the transformation into a NBF is not hindered by an electrostatic barrier. This results in a faster transition kinetic and lower film stability. In addition to the systematic shift in CBF-NBF transition kinetics for different SDS:C12 TAB mixing ratios, the transition kinetics show a distinct dependence on the existence of Rayleigh instabilities. Rayleigh instabilities appear if excess liquid is hindered from dissipation and accumulates in the rim between the CBF and the NBF 17 . As a consequence droplets are formed which are thicker than the surrounding film and consequently reflect more light (Figure 8 a)). In case of mixtures with an excess concentration of SDS (9:1 and 8:2 - slow transtition) the NBF domain always expanded with a homogeneous liquid rim, containing no Rayleigh instabilities. For SDS:C12 TAB solutions with an excess concentration of C12 TAB (3:7, 2:8 and 1:9 - fast transition) on the other hand, Rayleigh instabilities were observed. For intermediate SDS:C12 TAB mixing ratios (5:5 and 6:4), Rayleigh instabilities occured directly after NBF nucleation, but disappeared soon after (Figure 8 a) and b)). A closer analysis of the CBF-NBF transition (Figure 9) revealed that the disappearance of Rayleigh instabilities was connected to a small change in the CBF-NBF transition kinetics (d(t) ∝ tn ). For the 5:5 SDS:C12 TAB mixture the exponent n droped slightly from 0.55 (domain growth


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a)

b) CBF Rayleigh instability NBF

Figure 8: Images of a horizontal foam film during CBF-NBF transition. The pictures are examplary for a 5:5 SDS:C12 TAB mixture at a total surfactant concentration of 5 · 10−5 mol/l. a) View of the transition domain with Rayleigh instabilities (bright spots in the rim) 0.6 s after NBF onset. b) View of the same film without Rayleigh instabilities 3.6 s after NBF onset. with Rayleigh instabilities) to 0.45 (domain growth without Rayleigh instabilities) (Figure 9 a)). This shift was independent of the total surfactant concentration and observed for 5 · 10−5 mol/l and 10−4 mol/l SDS:C12 TAB mixtures (compare also Figure 1 in the Supporting Information). A more pronounced shift was detected at the 6:4 mixing ratio (Figure 9 b)). Directly after the NBF nucleation, Rayleigh instabilities were present and the exponent n was 0.55. After approximately 4 s the Rayleigh instabilities vanished and n droped to 0.3. Rayleigh instabilities only occur if the NBF domain expansion d(t) is proportional to tn with n > 0.5. If n drops below 0.5, Rayleigh instabilities vanish. In this case the NBF grows too slow and, thus the excess liquid is not hindered from drainage. As a result the liquid rim separating the NBF from the CBF expands homogeneous. This observation is different to what is generally reported in literature 51 52 : Beltran et al. reported a change from a diffusion-like (d(t) ∝ t0.5 , without Rayleigh instabilities) to a linear growth (d(t) ∝ t, with Rayleigh instabilities). One reason for the different behavior could be the different



a)

b)

1000

100

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SDS : C12TAB (5:5) with Rayleigh instabilities without Rayleigh instabilities fit d(t)  xn : n = 0,55 fit d(t)  xn : n = 0,45 10 0,1

NBF diameter [m]

1000

NBF diameter [m]

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1

SDS : C12TAB (6:4) with Rayleigh instabilities without Rayleigh instabilities fit d(t)  xn : n = 0,55 fit d(t)  xn : n = 0,30 10 0,1

10

time [s]

1

10

time [s]

Figure 9: Detailed analysis of the CBF-NBF transition for a a) SDS:C12 TAB (5:5) mixture and b) (6:4) mixture in relation to the occurrence of Rayleigh instabilites. The bulk total surfactant concentration was fixed at 5 · 10−5 mol/l. origin of the foam film transition in both cases. For SDS:C12 TAB mixtures a CBF-NBF occurs, whereas in case of 51 52 it is a stepwise film thickness transition originating from the expulsion of surfactant-polyelectrolyte complexes from the film bulk. On the other hand, in both cases no Rayleigh instabilities are observed if the film transition is too slow. In general, a foam film transition (in particular a CBF-NBF transition) is a complex phenomena and is strongly affected by hydrodynamics, the film thickness and viscosity. 17 The influence of not only the film thickness on the transition kinetics but also the hydrodynamics within in the film bulk on the transition kinetic is intriguing and merits further investigation.

Conclusions The present study demonstrates that SDS and C12 TAB interact electrostatically to form charge neutral catanionic complexes (Figure 10 a)) over a broad range of different SDS:C12 TAB mixing ratios. The resulting reduced electrostatic repulsion leads to a dense molecular packing at the interface. As a result surface tension is strongly reduced and sterically stabilized 22 ACS Paragon Plus Environment

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NBFs are formed at all SDS:C12 TAB mixing ratios. Although the surface adsorption properties are dominated by the resulting 1:1 complexes over a wide range of SDS:C12 TAB mixing ratios, it was demonstrated that foam film properties are strongly affected by the surfactant in excess concentration. The corresponding surface layer is composed of the catanionic surfactant complexes surrounded by an surplus of either SDS or C12 TAB molecules (Figure 10 b)). Different foam film stabilities are observed and attributed to a complex change in the topology of the surface layer occuring during the CBF-NBF transition, also reflected by a systematic variation of the CBF-NBF transition kinetics. It is most likely that a microscopic phase separation of the surface layer into a phase consisting of catanionic complexes (stabilizing the NBF) and a phase of the ionic surfactant takes place (Figure 10 c)). In case of excess concentration of SDS, a CBF is stabilized and a resulting higher surface charge density and, thus higher electrostatic barrier explains the slow CBF-NBF transtion. In case of excess concentration of C12 TAB (mixing ratio 4:6 to 1:9), the CBF-NBF transition is fast and the formed foam films are unstable and rupture. The surface concentration of C12 TAB is not sufficient to stabilize the foam films.

b)

9:1 8:2 7:3 6:4 5:5 4:6 3:7 2:8 1:9

-

catanionic complex

adsorption at air-water - - +- - - - + interface

foam film formation

+- - +- - - +

- - + - - - -+

C12TAB

c)

+- +- + - +

+- - +- - -+ +- +- + - + -+ -+ -+ -

no stable foam film

- - +- - - - +

SDS

SDS:C12TAB mixing ratio

Common black Newton black film (CBF) film (NBF)

- -+ - -+ -+

- -+ - -

a)

-+ -+

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Phase separation during CBF-NBF transition

Figure 10: Schematic representation of the a) Catanionic complex formation between oppositely charged SDS and C12 TAB molecules. b) Surface adsorption and foam film stabisation as a result from different surface monolayer composition. Monolayer composition is strongly dependent on the SDS:C12 TAB mixing ratio. c) Microscopic phase transition during the CBF-NBF transition: a phase consisting of the catanionic complexes (stabilizing the NBF) and a surrounding phase mainly consisting of the ionic surfactant in excess.

In summary, the present study verifies that surface adsorption is mainly dominated by 23 ACS Paragon Plus Environment


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the formation of stoichometric catanionic complexes, but the surfactant in excess has a distinct influence on foam film properties. By adjusting the bulk SDS:C12 TAB mixing ratio it is possible to tune the type, stability and thinning behavior of foam films. Already small amounts of excessive ionic surfactant are sufficient to completely change the resulting foam film properties. This leads to the conclusion that the surface is covered by a mixed surface layer composed of the catanionic surfactant species with either free SDS or C12 TAB molecules in excess, depending on the overall mixing ratio. Fundamental knowledge gained from this study may help to understand basic mechanisms of foam film stabilisation and inspire future research on the versatile influence of oppositely charged surfactant mixtures on foam film properties.

Acknowledgement We thank Adrian Carl for his help with the analysis of the CBF-NBF transition kinetics and Sabine Siegmund from MPI in Golm for her support on the oscillating drop measurements. The author is grateful to Dr. Bhuvnesh Bharti for his help to improve the manuscript. Financial support from COST CM1101 and MP1106 is acknowledged.

References (1) Rio, E.; Drenckhan, W.; Salonen, A.; Langevin, D. Unusually Stable Liquid Foams. Advances in Colloid and Interface Science 2014, 205, 74–86. (2) van der Net, A.; Gryson, A.; Ranft, M.; Elias, F.; Stubenrauch, C.; Drenckhan, W. Highly Structured Porous Solids from Liquid Foam Templates. Colloids and Surfaces A 2009, 346, 5–10. (3) Dubois, M.; Zemb, T.; Demé, B. Self-Assembly of Flat Nanodiscs in Salt-Free Catanionic Surfactant Solutions. Science 1999, 283, 816–819.


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(52) Beltrán, C.; Langevin, D. Stratification Kinetics of Polyelectrolyte Solutions Confined in Thin Films. Physical Review Letters 2005, 94, 217803.


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SDS:C12TAB -

9:1 8:2 7:3 6:4 5:5 4:6 3:7 2:8 1:9 Adsorption at air-water surface - - +- - - - +

foam +- - +- - - + film formation

- - + - - - -+

Page 31 of 31

Common black film

+- +- + - +

+- +- + - + -+ -+ -+ -

no stable foam films

Newton black film

Figure 11: Table of contents graphic.


Surface Adsorption of Oppositely Charged SDS:C12TAB Mixtures and

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