Article pubs.acs.org/JPCB

Surface Adsorption of Oppositely Charged C14TAB-PAMPS Mixtures at the Air/Water Interface and the Impact on Foam Film Stability Heiko Fauser,† Regine von Klitzing,*,† and Richard A. Campbell*,‡ †

Stranski-Laboratorium fuer Physikalische und Theoretische Chemie, Institut fuer Chemie, Technische Universitaet Berlin, Strasse des 17. Juni 124, D-10623 Berlin, Germany ‡ Institut Laue-Langevin, 6 rue Jules Horowitz, BP 156, 38042 Grenoble , Cedex 9, France S Supporting Information *

ABSTRACT: We have studied the oppositely charged polyelectrolyte/surfactant mixture of poly(acrylamidomethylpropanesulfonate) sodium salt (PAMPS) and tetradecyl trimethylammonium bromide (C14TAB) using a combination of neutron reflectivity and ellipsometry measurements. The interfacial composition was determined using three different analysis methods involving the two techniques for the first time. The bulk surfactant concentration was fixed at a modest value while the bulk polyelectrolyte concentration was varied over a wide range. We reveal complex changes in the surface adsorption behavior. Mixtures with low bulk PAMPS concentrations result in the components interacting synergistically in charge neutral layers at the air/water interface. At the bulk composition where PAMPS and C14TAB are mixed in an equimolar charge ratio in the bulk, we observe a dramatic drop in the surfactant surface excess to leave a large excess of polyelectrolyte at the interface, which we infer to have loops in its interfacial structure. Further increase of the bulk PAMPS concentration leads to a more pronounced depletion of material from the surface. Mixtures containing a large excess of PAMPS in the bulk showed enhanced adsorption, which is attributed to the large increase in total ionic strength of the system and screening of the surfactant headgroup charges. The data are compared to our former results on PAMPS/C14TAB mixtures [Kristen et al. J. Phys. Chem. B, 2009, 23, 7986]. A peak in the surface tension is rationalized in terms of the changing surface adsorption and, unlike in more concentrated systems, is unrelated to bulk precipitation. Also, a comparison between the determined interfacial composition with zeta potential and foam film stability data shows that the highest film stability occurs when there is enhanced synergistic adsorption of both components at the interface due to charge screening when the total ionic strength of the system is highest. The additional contribution to the foam stability of the negatively charged polyelectrolyte within the film bulk is also discussed.

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INTRODUCTION Aqueous oppositely charged polyelectrolyte/surfactant (P/S) mixtures play a major role in a wide range of industrial applications, from detergency to personal care products to drug delivery vehicles.1,2 It is important to understand the interactions of such species both in the bulk phase and at surfaces to predict their physicochemical behavior and in cases optimize perfomance. A strong impact on foam properties has been reported.3−6 Such mixtures may prevent unwanted foaming in chemical production or promote foam stabilty if desired. In general, combinations of oppositely charged polyelectrolytes and surfactants show a broad variety of surface adsorption behavior, leading to diverse impact on the resulting physical properties of the whole system.7−9 The stability of a foam depends essentially on the stability of the thin films that separate the dispersed phase. The properties of these thin foam films are governed by adsorbed molecules at the air−water interfaces. In particular, P/S mixtures can be used to tune the properties of the arising foam film.10−15 The influence of mixtures is due to their excess or depletion in the interfacial region and their texture or structure formation in the film bulk. Depending on the charge combination of the used © 2014 American Chemical Society

polyelectrolytes and surfactants, either a thick common black film (CBF), or a thin Newton black film (NBF) is formed as a final state before film rupture.16,17 The CBF is electrostatically stabilized due to the presence of surface charges, whereas the NBF is stabilized by steric repulsion of the adsorbed complexes. Earlier studies on mixtures of the anionic polyelectrolyte poly(acrylamidomethylpropanesulfonate) sodium salt (PAMPS) with the cationic surfactant tetradecyl trimethyl ammonium bromide (C14TAB) lead to a surprising conclusion:18,19 the surface coverage has no significant influence on the stability of the foam films. The most stable films were formed in a concentration regime where the surface tension exhibited high values, which indicates a low surface coverage. On the other hand films with the highest coverage were very unstable. It was deduced that an important reason for film stabilization was not the surface coverage but the total net charge in the system. Received: September 23, 2014 Revised: December 4, 2014 Published: December 4, 2014 348

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An important challenge is to rationalize foam film stability in terms of the surface properties of P/S mixtures and, in particular, the interfacial composition (i.e., surface excesses of both polyelectrolyte and surfactant). Surface tensiometry alone is known to be limited in its capacity to resolve the interfacial composition as a result of complexation in the bulk which changes the activities of the components in the system.20 The interfacial composition can be resolved using neutron reflectometry (NR) with isotopic contrast variation,21,22 even though the precision of the polyelectrolyte surface excess is rather poor when it is not available in deuterated form. An optical reflection technique to study P/S interactions at the air−water interface is ellipsometry.23,24 While it is not sensitive to the interfacial composition, it can be applied in combination with other techniques that have different sensitivities to the individual components at the interface.25 Angus-Smyth et al. showed recently that comodeling NR and ellipsometry data is a feasible way to obtain quantitatively the interfacial composition of P/S mixtures at the air−water interface while conserving precious neutron beam time and deuterated materials.26 Even so, the same authors went on to show that the approach can break down when bulk aggregates of P/S mixtures that exhibit bulk phase separation interact directly with the surface layer to modify its surface properties via dissociation and spreading.27 In the present work, we examine the relation between the interfacial composition and foam film stability for PAMPS/ C14TAB mixtures. A combination of NR and ellipsometry measurements was used to study the complexation at the air− water interface and zeta potential measurements were carried out to determine the charge on the bulk complexes. The measurements were carried out at a rather low bulk surfactant concentration (with changing bulk polyelectrolyte concentration) to keep the bulk compositions well outside the phase separation region, so that the data could be interpreted in terms of equilibrium adsorption. These results are used to rationalize a rise in surface tension which was found previously for the same mixture and provide insight into the resulting stability of foam films.18

matched water (ACMW; 89 g D2O in 911 g H2O) were prepared. For the samples with contrast matched surfactant, hC14TAB was mixed with dC14TAB in a 95.6:4.4 ratio. All prepared samples showed no turbidity or precipitation. Zeta Potential. A Malvern Zetasizer NanoZ (Malvern Instruments, Germany) instrument was used to measure the zeta potential. The Zetasizer Nano series calculates the zeta potential by determining the electrophoretic mobility and then applying the Henry equation. The electrophoretic mobility is obtained by performing Laser Doppler Electrophoresis. Neutron Reflectometry (NR). NR measurements were performed on the neutron reflectometer FIGARO at the Institut Laue-Langevin (Grenoble, France).29 The time-of-flight instrument was used with a chopper pair giving neutron pulses with 4.0% dλ/λ in the wavelength range λ = 2−30 Å. Data acquisitions were carried out at incident angles of Θ = 0.62 and 3.8°. Samples were monitored while they reached steady state prior to each acquisition. Specular neutron reflectivity profiles comprise the intensity ratio of neutrons in the specular reflection to those in the incident beam with respect to the momentum transfer, Q, defined by

Q=

4π sin θ λ

(1)

where θ is the incident angle. The reflectivity profiles were fitted with the Motofit,30 based on the interaction of neutrons with a stratified layer model on the basis of the Abeles matrix method. Ellipsometry. Samples were also measured with an Optrel Multiskop Ellipsometer (Optrel, Berlin, Germany) using a HeNe Laser (λ = 632.8 nm). Solutions were prepared in a triangular glass trough. Measurements were performed at an angle of incidence of θ = 50°, close to the Brewster angle θB = arctan(nsol/nair) = 53°, where the measured optical signal is sensitive to material located at the air/liquid interface. The data acquisition rate was around 0.2 Hz, and the spot size of the laser beam on the interface was roughly 1 mm2. Measurements were performed until a constant signal was detected, indicating that the system had reached equilibrium. The absolute values were averaged over a period of at least 300 s. Ellipsometry measurements are based on the change in polarization of light reflected at an interface. The relative amplitude and phase of the p- and s-components change by different amounts upon reflection. The relative attenuation, Ψ, and the relative phase shift, Δ, depend on the optical properties of the interface and on the angle of incidence θ. The ellipsometric angles are related to the Fresnel reflectivity coefficients of the parallel and perpendicular components, rp and rs, respectively.31 rp = tan ΨeiΔ rs (2)

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EXPERIMENTAL SECTION Materials. Deionized water was passed through a purification system (Milli-Q; total organic content = 4 ppb; resistivity 18.2 mΩ cm). C14TAB (tetradecyl trimethylammonium bromide) was purchased from Sigma-Aldrich (Steinheim, Germany) and recrystallized three times from acetone with traces of ethanol. The high purity of the surfactant was verified with surface tension measurements. Deuterated C14TAB (dC14TAB) from CDN Isotopes (Quebec, Canada) was used as received. The anionic polyelectrolyte PAMPS (with a nominal charge degree of 100%) was a gift from Andre Laschewsky and was used as received.28 It has a molecular weight of about 100.000 g/mol. Sample Preparation. All samples were prepared with a fixed bulk C14TAB concentration of 10−4 mol/L and different bulk PAMPS concentrations. The stated polyelectrolyte concentrations in this work are related to the respective monomer concentrations. The mixed P/S samples were prepared by combining simultaneously equal volumes of a solution with twice the desired concentration of surfactant with a solution with twice the desired concentration of PAMPS. All prepared samples showed no turbidity or precipitation and remained stable for several weeks. For NR, samples with deuterated C14TAB (dC14TAB) in D2O and air contrast

A limitation of ellipsometry measurements at the air/liquid interface is that Ψ is rather insensitive to thin films. Therefore, we present measurements of Δ only in this work. We define the effect of the surface layer on the measured ellipsometric phase shift as Δsurf = Δ(P/S) − Δ0, where Δ(P/S) is the measured parameter for a P/S solution and Δ0 is the value for pure water. In the thin film limit, Δsurf is linearly proportional to the ellipsometric thickness, η,32 Δsurf = 349

g(θ ) η λ

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where λ is the wavelength, g(θ) is a function that depends only on bulk properties and the angle of incidence θ given by32 g(θ ) =

layer. The contrast PAMPS/dC14TAB/D2O is most sensitive to the amount of polymer adsorbed to the surfactant head groups, as the scattering length density of dC14TAB is close to that of D2O, resulting in an inversion in the scattering length density profile normal to the interface. For this reason we extended the model to a three-layer model comprising (1) surfactant chains and (2) head groups, each with a volume fraction of unity, and (3) adsorbed hydrated polymer underneath with a fitted volume fraction. We fitted the thickness and water content of the polymer layer. In a last step, we performed corefinement on the three different contrasts using the three layer model until consistent parameters were achieved. The surfactant and polymer surface excesses, ΓC14TAB and ΓPAMPS, were calculated using

4π εr ,amb εr − sub cos θ sin 2 θ [εr − sub − εr ,amb][(εr − sub + εr ,amb)cos2 θ − εr ,amb] (4)

and εr,amb is the relative permittivity of the ambient medium (air), εr,sub is the relative permittivity of the substrate (liquid), and εr = n2 (the square of the refractive index). For optically isotropic interfaces, Drude demonstrated that η can be written as a function of the relative permittivity profile across the interface εr(z),33 η=

∫

[εr (z) − εr ,amb][εr (z) − εr ,sub] εr (z)

(5)

ΓC14TAB =

where z is the Cartesian coordinate perpendicular to the interface. The change in η from the different scattering by capillary waves from sample-to-sample is neglected. For a single uniform isotropic layer separated by sharp interfaces, eqs 3 and 5 may be reduced to Δsurf =

g (θ )(ϵr ,surf − ϵr ,amb)(ϵr ,surf − ϵr ,sub) ϵr ,surf λ

ϵr ,surf − ϵr ,sub dn dc

14 TAB

dC14TAB

bC14TABNA

(8)

and ΓPAMPS =

ρPAMPS dPAMPSνf ,PAMPS bPAMPSNA

(9)

where b, ρ, and d are scattering length, scattering length density, and the thickness of the respective layer. νf,PAMPS is the volume fraction of PAMPS. The used parameters for C14TAB and PAMPS are given in the Supporting Information. Compositional Analysis Method 2: NR Data from 2 Isotopic Contrasts at Low Q Only. The second method adopts a recent approach that was used by Á braham et al. to determine the interfacial composition of DNA/dodecyl trimethylammonium bromide (DNA/C12TAB)35 and poly(sodium styrenesulfonate)/C12TAB (PSS/C12TAB)36 mixtures. The approach is based on NR measurements at low Q only of two isotopic contrasts of the interfacial layer but with both recorded in ACMW involving different levels of deuteration of the surfactant. Samples for the determination of the interfacial composition of PAMPS/C14TAB mixtures were made in the isotopic contrasts PAMPS/cmC14TAB/ACMW and PAMPS/ dC14TAB/ACMW, where cmC14TAB is a mixture of 95.6% hC14TAB and 4.4% dC14TAB to give a scattering length density matched to air and ACMW. As such, the signal in the former contrast is sensitive only to adsorbed PAMPS (ΓPAMPS), and the signal in the latter contrast is sensitive to both adsorbed PAMPS and adsorbed C14TAB but weighted strongly on the latter. The scattering excesses measured for the two samples were used to solve uniquely the interfacial composition. The scattering length density (ρ) and thickness (d) of a single thin interfacial layer may be fitted at low Q with the product insensitive to details of a more complex model. The derived products of scattering length density and thickness were related to the interfacial composition through the solving of the following simultaneous equations:

×d (6)

where d is the layer thickness. The surface excess can then be calculated using de Feijters equation:34 Γ=

ρC

×d (7)

where dn/dc is the refractive index increment of the material in the surface layer. Data Evaluation. Traditionally, in the literature a method used to determine the interfacial composition of polyelectrolyte/surfactant mixtures is NR measurements over the full Q-range accessible while exploiting isotopic substitution of the surfactant and the subphase. Nevertheless, in the absence of deuterated polyelectrolyte, the precision in its surface excess is poor because its quantification is ambiguous due to a similar effect on the data from surface roughness which a priori is unknown. We carry out this method first, as well as two others which were used in recent studies to good effect. One is NR measurements at low Q-values only where the scattering excess in two different isotopic contrasts may be taken as a more direct quantification of the interfacial composition (at the expense of structural information which is not resolved).35,36 The other is the comodeling of one isotopic contrast from NR with ellipsometry data.26,27 The three methods in combination are applied in the present work, to our knowledge, for the time. Compositional Analysis Method 1: NR Data from 3 Isotopic-Contrasts over the Full Q Range. To obtain the surface composition, P/S mixtures of three different isotopic contrasts are comodeled to give consistent fits (PAMPS/ dC14TAB/ACMW, PAMPS/hC14TAB/D2O, and PAMPS/ dC14TAB/D2O). The contrast PAMPS/dC14TAB/ACMW is most sensitive to the amount of adsorbed surfactant. In a first approach, the interfacial structure is modeled as a single uniform layer consisting of surfactant chains only. From this, we obtained the surfactant layer thickness. In a next step, we extended the surfactant layer into a two-layer model consisting of surfactant chains and head groups. From this, we obtained the layer roughness and calculated the water content in the

(ρd)1 = NAbPAMPS ΓPAMPS

(10)

(ρd)2 = NA(bPAMPS ΓPAMPS + bdC14TAB ΓdC14TAB)

(11)

In this approach, the background was not subtracted from the reflectivity profiles before analysis. Instead, the background was refined to 2.3 × 10−5, which was the lowest level possible where a fit of a fictional layer on a clean air/ACMW measurement resulted in no surface excess. Following this calibration, any enhanced scattering from the measurements of 350

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the P/S mixtures could be attributed to the adsorption of polymer and/or surfactant. Compositional Analysis Method 3: NR Data from 1 Isotopic Contrast in ACMW at Low Q only Combined with Ellipsometry Data. The third method adopts a recent approach that was used by Angus-Smyth et al. to determine the interfacial composition of poly(ethylene oxide)/sodium dodecyl sulfate (PEO/SDS)26 and poly(ethylene imine)/SDS (PEI/SDS) mixtures27 at the dynamic air/liquid interface of an overflowing cylinder. The approach is based on the comodeling of an NR measurement at low Q in only one isotopic contrast of the interfacial layer involving deuterated surfactant in ACMW and an ellipsometry measurement. The approach assumes that appropriate experimental quantities, (ρ × d) in NR and Δsurf in ellipsometry, are additive functions of the surface excesses of PAMPS and C14TAB, ΓPAMPS and ΓC14TAB, respectively: (ρd) = NA(bPAMPS ΓPAMPS + bdC14TAB ΓdC14TAB)

Figure 1. Zeta potential for the surfactant/polyelectrolyte mixtures investigated. The bulk C14TAB concentration is fixed at 10−4 mol/L while the bulk PAMPS concentration is varied. The error bars correspond to the intrinsic experimental error of the measurement device.

(12)

and Δsurf = A ΓC14TAB + BΓPAMPS

(13)

where A and B are scaling factors used to relate the sensitivity of the measured ellipsometric phase shift to the surface excess of C14TAB and PAMPS, respectively. A simple linear relationship between the quantities was assumed: the value A = 2.54 × 10−14 Å/mol was determined from measurements of Δsurf (ellipsometry) and (ρ × d) (NR) of a C14TAB monolayer at limiting surface coverage. The value B = 3.80 × 10−14 Å2/mol was determined by solving eqs 6 and 7 using the measured refractive index increment of the polymer, dn/dc = 0.13. Samples of pure C14TAB solution for the surface tension to surface excess calibration for the combined NR/ellipsometry modeling approach were measured in the isotopic contrast dC14TAB/ACMW only.

stability, and hence, aggregation and precipitation are hindered. Therefore, we can rule out precipitation as the cause of the surface tension rise for this (dilute) system. We note that in more concentrated mixtures, the effects of aggregate penetration at the surface, resulting in the modification of the interfacial properties, can be extremely important.39,40 Nevertheless, even though a very low coverage of kinetically trapped particles formed during mixing may be present at the interface in certain samples (an inevitable nonequilibrium feature of such systems), NR is insensitive to this effect, so we can treat the data on the basis of laterally homogeneous adsorption layers. Figure 1 exhibits a continuous increase in negative zeta potential with rising PAMPS concentration, indicating the formation of negatively charged complexes in the bulk solution. We return to this issue in the Discussion section. To monitor surface coverage and test if the rise in surface tension correlates with a depletion of adsorbed material at the surface, NR measurements were carried out. In a first approach, mixtures of PAMPS with deuterated C14TAB in ACMW were measured. In this case, the contribution of the solvent to the specular reflection is negligible and the signal arises mainly from scattering by the deuterated surfactant in the surface monolayer. Adsorbed polymer also contributes to the reflected signal, but because it is hydrogenous, it has a low scattering length density, so its contribution can be neglected in a first qualitative analysis. Figure 2 shows reflectivity profiles for dC14TAB at the cmc and dC14TAB/hPAMPS mixtures of varying polyelectrolyte concentrations in ACMW. For NR measurements in this isotopic contrast, a clear trend is observed. For mixtures with the lowest bulk PAMPS concentrations [10−5 and 3 × 10−5 (mono)mol/ L], the reflectivity profiles, and hence the scattering excess from adsorbed surfactant, are almost as high as that of pure C14TAB at its cmc (3.5 × 10−3 mol/L). If we recall that the bulk surfactant concentration for the mixtures was fixed at 10−4 mol/ L, this enhanced surfactant adsorption must be a result of a synergistic interaction with PAMPS at the interface. When the bulk PAMPS concentration increases and reaches the same concentration as that of the surfactant, a depletion of material

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RESULTS Our previous studies on freshly mixed PAMPS/C14TAB mixture recorded with respect to the bulk polyelectrolyte concentration at fixed surfactant concentration showed a striking small peak in the surface tension isotherm.13,18 Surface tension peaks have been described in different ways in the literature: either in terms of the loss of surface activity of complexes at chemical equilibrium37,38 or the comprehensive precipitation of aggregates in the bulk resulting in a depleted surfactant monolayer.35,36 The freshly mixed PAMPS/C14TAB samples in the present study were transparent to the naked eye, which indicates that precipitation is not the reason for the peak. To examine this issue, first we measured the charge on the bulk complexes using zeta potential measurements (Figure 1). These measurements would reveal if precipiation could occur at bulk compositions close to charge neutrality. In order to facilitate the comparison of our new data with our former findings, again we performed experiments at a fixed bulk surfactant concentration with respect to the bulk polyelectrolyte concentration. The surfactant concentration was fixed at 10−4 mol/L. The polyelectrolyte concentration was varied between 10−5 and 3 × 10−3 (mono)mol/L. No charge reversal from positive to negative could be observed. As the samples have sufficiently low bulk surfactant concentration to preclude charge inversion even at very high bulk polyelectrolyte concentrations, the charge on the complexes is always sufficient for them to retain colloidal 351

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Figure 2. Neutron reflectivity profiles for solutions prepared from dC14TAB and hPAMPS in ACMW. The bulk surfactant concentration for the mixtures was fixed at a concentration of 10−4 mol/L. In this contrast, the reflectivity is dominated by the surfactant. A reflectivity profile for dC14TAB at the cmc in ACMW is shown for comparison. The solid lines correspond to model fits.

Figure 3. Neutron reflectivity profiles for solutions prepared from cmC14TAB and hPAMPS in ACMW. This contrast is sensitive only to the polyelectrolyte at the interface. Data from a bare ACMW measurement (●) are shown where the background level was maximized so that a fit to a fictional interfacial layer resulted in zero surface excess. The solid lines correspond to model fits.

from the surface is indicated by the drop in reflectivity. If the bulk PAMPS concentration is further increased up to 3 × 10−3 mol/L, the adsorbed amount of surfactant increases again, but the scattering excess remains always lower than that of the pure C14TAB solution at its cmc. To obtain information directly about the adsorption of PAMPS at the air/water interface, NR measurements were carried out involving polymer with air-contrast-matched surfactant (cmC14TAB) in ACMW. This is a new approach35,36 which exploits the high flux at the natural low incidence angle of the FIGARO reflectometer to separate the signal from adsorbed hydrogenous polymer from the background.29 Figure 3 shows neutron reflectivity profiles for the investigated cmC14TAB/PAMPS solutions with different bulk polymer concentrations. These measurements represent a direct measure of the amount of PAMPS present in the interfacial layer as shown by enhanced scattering above the background level of a bare air/ACMW measurement. The black horizontal line represents a fit to the black data points and indicates zero specular reflection. It is clear that PAMPS is present at the surface for all the mixtures investigated. For bulk PAMPS concentrations between 10−5 and 10−4 (mono)mol/L, there are only minor changes in the polymer content at the interface, but the data are well above the zero specular reflection. At 3 × 10−4 (mono)mol/L, a drop of the scattering excess indicates a depletion of PAMPS from the surface. This indicates a surprising excess of PAMPS around a concentration of 10−4 mol/L, as C14TAB is already depleted around this concentration (Figure 2). If the PAMPS concentration is further increased, the polyelectrolyte enriches again at the surface. In contrast to the two approaches above using NR, where the former is sensitive primarily to the adsorbed C14TAB and the latter is sensitive only to the adsorbed PAMPS, ellipsometry measurements have roughly equal sensitivity to the two

components. As such, ellipsometry measurements can be taken as an approximate measure of the total surface excess. Figure 4 shows the phase shift of the investigated sample solutions with respect to a bare air/water interface (phase shift 0). The surface adsorption is enhanced for mixtures with low and high PAMPS concentration in comparison with surface depletion for intermediate PAMPS concentrations around 3 × 10−4 (mono)mol/L. To quantify the interfacial composition, we calculated the surface excesses of the two components using the three different evaluation approaches described in the Experimental

Figure 4. Measured phase shift from ellipsometry for the C14TAB/ PAMPS mixtures investigated with respect to the bare air/water interface (phase shift 0). The error bars shown correspond to average fluctuations in the signal over all the samples measured. 352

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Figure 5. Surface excess, Γ, for the C14TAB/PAMPS mixtures derived from the different analysis methods. The solid lines link the average values as a guide to the eye. For all mixtures investigated, the bulk concentration of C14TAB was fixed at 10−4 mol/L while the bulk concentration of PAMPS was varied. (a) Surface excess of C14TAB in the investigated C14TAB/PAMPS mixtures. The lower dotted black line corresponds to the surface excess of a pure C14TAB solution at a concentration of 10−4 mol/L. The upper solid black line corresponds to the surface excess of a pure C14TAB solution at the cmc (3.5 × 10−3 mol/L).41 (b) Surface excess of PAMPS in the investigated C14TAB/PAMPS mixtures. (c) Averaged values from the 3 different analysis methods.

As the bulk PAMPS concentration increases from 3 × 10−5 to 10−4 (mono)mol/L, Γ(C14TAB) drops dramatically to 1.1 × 10−6 mol/m2, which is close to the value found for the corresponding pure surfactant solution at 10−4 mol/L in the absence of added polyelectrolyte (dotted line in Figure 5a).41 At the same time, the surface excess of PAMPS remains, to within the precision of the measurements, equivalent to that for the samples with lower PAMPS concentrations. For this sample, the number of polyelectrolyte segments outnumber that of surfactant headgroups by about 3:1, which shows that the degree of binding at the interface has been reduced significantly. The presence of loops of polyelectrolyte on the interfacial structure may therefore be inferred. This change in structure is not supported directly by the NR measurements because the precision in the polyelectrolyte layer thickness for such thin layers (i.e., the NR measurements at high Q) are limited by the coupled effects on the data of layer density and interfacial roughness. The value of the thickness converges to slightly below 1 nm for all samples, but the effect on the chi2 value is minimal for thickesses up to 2 nm. An increase of the bulk PAMPS concentration to 3 × 10−4 (mono)mol/L results also in a depletion of the polymer from the interfacial layer. The remaining polyelectrolyte surface excess shows that there is still some PAMPS present at the surface. Further increase of the bulk polyelectrolyte concentration to 10−3 and finally 3 × 10−3 (mono)mol/L leads to an increase of both Γ(C14TAB) and Γ(PAMPS). Yet still PAMPS is in excess which shows that the polyelectrolyte binding to the surfactant remains weak and loops are present.

Section: (1) NR data from three isotopic contrasts over the full Q-range, (2) NR data from two isotopic contrasts in ACMW at low-Q only, and (3) NR data from one isotopic contrast of NR in ACMW at low-Q combined with ellipsometry data. Note that first, we calculated a surface excess of around 3.8 × 10−6 mol/m2 for the pure C14TAB surfactant at a concentration of 3.5 × 10−3 mol/L (cmc), which is in accordance with literature values calculated from NR41 and surface tensiometry.42 Figure 5 displays the values of the surface excess, Γ, of C14TAB and PAMPS for the investigated solutions calculated from the three different analysis methods. In the case of C14TAB, all approaches lead to similar values. The main reason for the very good agreement is that NR measurements of the isotopic contrast involving PAMPS and deuterated C14TAB in ACMW, which have good statistics and are therefore very well determined, are used in all three approaches. In contrast, and as expected, the deviations in the polymer surface excess between the different approaches are greater. Nevertheless, this is the first time that all three analytical methods have been applied to the same samples and, therefore, the precision in which the polyelectrolyte surface excess is measured has surely been optimized. The surface excess of PAMPS is almost identical to that of C14TAB for the samples with the two lowest bulk PAMPS concentrations measured. These data strongly support the model proposed by Asnacios et al., where the interfacial arrangement is neutral with full release of counterions into the bulk43 (i.e., equimolar binding of charged polyelectrolyte segments to the oppositely charged headgroups in the surfactant monolayer). At a bulk polymer concentration of 3 × 10−5 (mono)mol/L, the surface excess of surfactant almost converges with that of a pure C14TAB solution at its cmc (black line in Figure 5a). We recall that the bulk surfactant concentration in the mixtures was fixed at just 10−4 mol/L, and in the absence of polyelectrolyte the surface excess is rather low (dotted line in Figure 5a). This enhanced surfactant adsorption demonstrates the strong synergistic adsorption of the two components given that PAMPS is not surface active alone and that the bulk C14TAB concentration in the mixture is only around 3% of its cmc.

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DISCUSSION In general, foam film properties are strongly dependent on the material adsorbed at the air/water interfaces. In this respect, first we discuss surface adsorption properties monitored for this with respect to the previously measured surface tension. In the second section, we refer to the correlation of surface adsorption with the previously measured foam film stability. Surface Adsorption. Our new results on the surface composition of PAMPS/C14TAB mixtures can now be placed in the context of the surface tension results reported previously by our group.18,13 From surface tension measurements, we 353

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know that for a pure C14TAB solution of 10−4 mol/L, the surface tension is close to that of pure water. The introduction of only 10−5 (mono)mol/L of PAMPS produces a large synergistic lowering of the surface tension (from 71 to 55 mN/ m) (Figure 6), evidencing the presence of PAMPS and C14TAB at the air/water interface. In this concentration regime, the bulk surfactant concentration exceeds the bulk PAMPS concentration (in monomer units) by 1 order of magnitude. The counterions of the polyelectrolyte are exchanged by the surfactant molecules, which is energetically favorable due to the resulting decrease in electrostatic repulsion between the surfactant headgroups at the surface. The accompanying release of counterions increases the entropy.44,45 This exchange leads to the formation of P/S complexes in the bulk, which are surface active. The data for the lowest two bulk PAMPS concentrations studied by NR shows that the surface composition is 1:1. This shows that the polyelectrolyte adopts a flattened conformation, and stoichiometric binding in the surface layer is achieved. In the case of strongly interacting P/S mixtures, it is assumed that the interactions are caused predominantly by electrostatic attractions between the charges on the polyelectrolyte and the surfactant head groups. The lack of correlation between the composition of the bulk complexes which are negatively charged (see Figure 1), and the interfacial composition which is neutral, shows that the adsorption process is mediated by a synergistic interaction at the surface itself rather than simply the adsorption of preformed complexes from the bulk. The interfacial composition of the mixture at a PAMPS concentration of 10−4 (mono)mol/L (boundary between region I and II) merits due discussion. At this point, the surfactant surface excess decreases dramatically, whereas an excess of PAMPS is present at the surface, but the surface tension remains low. In the case of charged polyelectrolytes, long-range intra- and intermolecular electrostatic repulsions become operative. Whereas higher bulk surfactant concentrations lead to higher shielding for PAMPS concentrations of 10−5 and 3 × 10−5 (mono)mol/L, intramolecular repulsions stretch the polymer backbone after the depletion of C14TAB at a PAMPS concentration of 10−4 (mono)mol/L. Such behavior has been reported for highly charged polyelectrolytes such as PSS.46 On the other hand, the reduction in the surfactant surface excess with a roughly constant polyelectrolyte surface excess indicates that the polyelectrolyte forms loops and the

interfacial structure becomes more substantial. Interfacial loop formation is reported for polyelectrolytes with a lower degree of charge density,47,48 although what happens in our case is not completely clear. None of the specular reflectivity profiles of our measurements exhibit a Kiessig fringe, indicating the presence of thick and dense interfacial layers. Indeed the thickness of the PAMPS layer still converges to below 1 nm with an upper limit of 2 nm, compatible with the deviation for the different analysis methods. In spite of the low amount of adsorbed surfactant at this bulk composition where the number of charges on the surfactant and polyelectrolyte in the bulk are the same, the surface tension has not yet increased above 55 mN/m. This may be explained by the weak amphiphilic character of PAMPS due to its weakly hydrophobic backbone and hydrophilic functional groups. Although the pure PAMPS solutions have a surface tension close to that of pure water in the studied concentration range, the surfactant mediated adsorption at the air−water interface might result in a reorientation of the PAMPS chains with the backbones directed toward the air. This physical picture would mean that parts of the PAMPS chains which are not compensated by surfactant molecules can contribute to the decrease in surface tension. As the bulk PAMPS concentration increases to 3 × 10−4 (mono)mol/L (boundary of region II), the surface tension also increases from 52 to 67 mN/m. Our bulk characterization of the charge on the complexes using zeta potential measurements showed that over the same range of bulk PAMPS concentrations, the charge on the bulk complexes become significantly more negative (i.e., the number of surfactant molecules per polymer molecule increases substantially). The rise in surface tension may therefore be attributed to interfacial depletion, resulting from a reduction in the surface excesses of both PAMPS and C14TAB due to a reduction in surface activity of the polyelectrolyte as a result of lower surfactant binding. A further increase of the bulk PAMPS concentration to above 3 × 10−4 (mono)mol/L (region III), again results in a lowering of the surface tension. This trend can be explained by the large increase in total ionic strength of the system at these bulk compositions. Self-screening of electrostatic interactions at higher ionic strengths renders the polyelectrolyte less charged.49,50 Additionally, binding of charged polyelectrolyte segments to surfactant headgroups in the surface monolayer is more favorable, resulting in greater synergistic adsorption at the interface and therefore a lower surface tension, as can be seen from the surface excesses in Figure 6. Former observations resulting from surface tension and surface elasticity measurements lead to the conclusion that after the rise in surface tension, PAMPS is completely depleted from the surface, leaving only a C14TAB layer at the surface. However, this interpretation is not consistent with the new data of the interfacial composition in the present work. Although we find a surface excess which is close to that for a 10−4 mol/L C14TAB solution,41 there is always an excess of polyelectrolyte present at the air/water interface. In summary, the surface tension maximum for freshly mixed, dilute, homogeneous PAMPS/C14TAB mixtures results from different regimes of interfacial behavior with respect to the bulk composition. First we may recall that the samples all have a rather low bulk surfactant concentration so the bulk complexes are always negatively charged. The three regimes may be described as follows: (1) at low bulk PAMPS concentrations, there is synergistic adsorption of polyelectrolyte and surfactant

Figure 6. Surface tension of C14TAB/PAMPS solutions with a fixed bulk surfactant and variable bulk PAMPS concentrations. The dashed horizontal line corresponds to the surface tension of pure C14TAB at a concentration of 10−4 mol/L. Three regions separated by vertical dashed lines are identified for discussion in the text. The graph is taken from ref 18. 354

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to form a neutral adsorption layer which lowers the surface tension. (2) At intermediate bulk PAMPS concentrations, the decreasing number of surfactant molecules per polymer chain results in a loss of surface activity first of the surfactant and then of the polyelectrolyte as well. With the interface depleted, the surface tension increases. (3) At high bulk PAMPS concentrations, the total ionic strength of the system has increased significantly. The increase in ionic strength due to increase in PAMPS concentration has 2 counteracting effects. First, the attraction between the surfactant and polymer in the bulk would be screened which, in addition to a decrease in surfactant/polymer ratio, would lead to more hydrophilic surfactant/polymer complexes that have increasing stability in solution. This effect seems to be rather minor, since an increase in the adsorbed amount of polymer is observed at the interface. Second, simultaneously, the charges of the surfactant head groups are screened, which leads to the observed increase in the adsorbed amount of surfactant at the interface. Due to the increase in positive charges at the interface, the more negatively charged PAMPS molecules can adsorb as well. Clearly this effect dominates the screening of the attraction between opposite charges. This screening of interfacial interactions results in an increase in the polyelectrolyte and surfactant surface excesses which result in a lowering of the surface tension. As a result, the presented experimental data on the interfacial composition of PAMPS/C14TAB mixtures has allowed us to propose an explanation for the surface tension peak reported previously. It is interesting to note that its physical basis is fundamentally different in nature to that in higher concentrated, strongly aggregating mixtures, which results from slow comprehensive precipitation.35,51 Relation to Foam Film Stability. One predominant factor in the stabilization of foam films is the change in surface charge and therefore a change in the electrostatic repulsion forces due to the adsorption of polyelectrolyte at the surface. The formation of an electrostatic stabilized CBF for oppositely charged P/S mixtures is commonly reported in the literature.16,18,19,45,52 Consequently, there has to be a rationalization of the charges present in the system in all cases. Since the composition of the film interfaces is not directly accessible, in general, the results of surface tension and NR measurements at the air/water interface are taken into account. Of course, it cannot be excluded that the composition of the interface changes if a second interface approaches, like in a freestanding film. In the case of C14TAB/PAMPS mixtures, the formed foam films were quite thick (40−100 nm), homogeneous, and no discontinuous thinning (stratification) was observed.13 Therefore, we assume that formed complexes remain in the film bulk and may contribute to the film stability. At low PAMPS concentrations (region I), Γ(C14TAB) is close to the value found at the cmc; additionally, Γ(PAMPS) is commensurately high as well. From the amount of surface active material at the interface, one may intuitively expect very stable foam films, but the observed stability is low.18 To emphasize this point, Figure 7 is reproduced from ref 18, where the maximum disjoining pressure before film rupture may be taken as a measure of the film stability. In fact the stability is even lower than for a film formed from a pure C14TAB solution at a concentration of 10−4 mol/L in the absence of added polyelectrolyte. It was discussed previously that screening of charges by the coadsorption of surfactant and polymer may be the underlying reason for this lack of stability.18 The present

Figure 7. Stability of C14TAB/PAMPS foam films: maximum disjoining pressure Πmax before film rupture versus PAMPS concentration. The horizontal dashed line corresponds to the stability of the pure surfactant film at 10−4 mol/L. The same three regions that are shown in Figure 6 are also marked for reference in the discussion in the text. The graph is taken from ref 18.

data on the interfacial composition confirm this point: the low stability coincides with a neutral adsorption layer, where the polyelectrolyte segments binding in equimolar stoichiometry to the surfactant headgroups in the surface monolayer. It should be added that other factors concerning the interfacial rheology may also be important. For example, it has been discussed that the formed P/S adsorption layer becomes rigid and is therefore not very flexible, making it fragile to rupture.8 Complete destabilization of the foam films is exhibited at bulk PAMPS concentrations slightly below 10−4 (mono)mol/ L18 (beginning of region II). It seems reasonable that changes in the interfacial composition, structure, and/or topology are responsible for the complete destabilization. Directly at a bulk PAMPS concentration of 10−4 (mono)mol/L, we have demonstrated an unexpected large excess of polyelectrolyte at the surface (Figure 5), which leads to a surplus of anionic charges at the surface. High anionic excess charge would lead to high surface potential and very stable foam films. One would expect the most stable films at this concentration, but only weakly stabilized CBF are observed. Obviously, here the polymer conformation must play an important role in the foam film stability. The possible change in polyelectrolyte conformation discussed above for this sample merits further investigation. Increase of the bulk PAMPS concentration to 3 × 10−4 (mono)mol/L and above (region III) results in depletion of surfactant and polyelectrolyte from the surface. This is consistent with former observations by surface tension and surface elasticity. On the contrary, an enhanced foam film stability was observed only at the highest bulk PAMPS concentration measured. From the present work, we have learned that for these samples, there is (1) the most negatively charged complexes in the bulk and (2) enhanced synergistic adsorption of both components at the interface, which we attribute to charge screening due to the elevated total ionic strength in the system. In summary, it is absolutely clear that this is a highly complex system, and there is no simple relation between either the surface tension or the surface composition and foam film stability. Obviously, the surface potential determined from foam film measurements18 does not correlate with the surface composition. For example, at a bulk PAMPS concentration of 10−4 (mono)mol/L, there is a high surface excess of PAMPS, but the surface potential (and therefore surface charges) is low. 355

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This might indicate an important role of the polyelectrolyte conformation. Entanglement could lead to intrinsic charge compensation and foam film destabilization. Nevertheless, we found that the foam films are most stable when the complexes in the bulk have the highest overall charge (PAMPS concentration above 10−4 [mono]mol/L). We state that the bulk net charge contributes to the electrostatic stabilization of foam films. The interplay between these factors in determining the stability of foam films also merits further work. It may be noted that foam film stability and especially foaming may be strongly affected by dynamic absorption processes and that long adsorption times (up to 2 h) are indicated by ellipsometry for the investigated systems at low bulk PAMPS concentrations. Additionally, surface tension measurements in the literature confirm that it is often difficult to obtain the surface tension of dilute P/S mixtures on practically accessible timescales. As a result, there is uncertainty associated with the measured values in this region.21 On the one hand, this was taken into account by long equilibration times in former foam film studies. On the other hand, the main changes in the surface tension are rapid and it is the smaller changes in the final approach to equilibrium that seem to be very slow. Investigations at the expanding liquid surface of an overflowing cylinder53 give information on the coverage and composition of the adsorbed layers formed on a 0.1−1 s timescale, and it would be very interesting to compare the findings with those for well-equilibrated interfaces. The pattern of adsorption at short timescales was found to be quite different from that of well-equilibrated static surfaces.26,54 A continuation of our studies on such a device could shed light on the dynamic foam properties.

PAMPS molecules can adsorb as well. Clearly, this effect dominates the screening of the attraction between opposite charges. This results in an enhanced adsorption of both C14TAB and PAMPS at the surface and disproves the former assumption that after the rise in surface tension there is only a bare surfactant layer present at the surface. The origin of the surface tension maximum has turned out to be very different to that of strongly aggregating systems where the experimental data have been rationalized in terms of comprehensive precipitation. The present study verifies that there is not a clear relation between the interfacial composition and foam film stabilization for the polyelectrolyte/surfactant mixture investigated. For samples with low bulk PAMPS concentrations, we observe a high surface excess of polymer and surfactant and the surface structure is essentially completely neutral. The foam films formed with these neutral adsorption layers are even less stable than films stabilized by pure C14TAB. Complete destabilization of foam films (and hence a surface potential of almost zero) at intermediate bulk PAMPS concentraions coincides with substantial depletion of surfactant from the interface and a high excess of polyelectrolyte. On the other hand, enhanced film stability occurs at high bulk PAMPS concentrations, even though the total surface coverage is fairly low. The most stable films therefore have a surface structure with a modest excess of polyelectrolyte at the interface. The roles of not only the total net charge of the bulk complexes but also the ionic strength on the stability of foam films is intriguing and clearly merits further work.

CONCLUSIONS Our results demonstrate the correlation between the surface tension and the interfacial composition, as determined from NR and ellipsometry measurements at the air/water interface of PAMPS/C14TAB mixtures. For the low bulk surfactant concentration (10−4 mol/L) used in this work, the polyelectrolyte is always in excess in bulk complexes, yet at low bulk PAMPS concentrations, PAMPS and C14TAB interact synergistically at the air/water interface to form charge neutral adsorption layers. At the bulk composition where PAMPS and C14TAB are mixed in an equimolar ratio, we find a dramatic depletion of surfactant at the interface with an unexpected excess of bound polyelectrolyte. We attribute this to a complex change in the topology of the adsorbed layer, resulting from the change in stoichiometry of the bulk complexes. A rise in surface tension then occurs with further increasing bulk polyelectrolyte concentration, and this has been related to a reduction in both the C14TAB and PAMPS surface excesses. A further increase of the bulk PAMPS concentration increases significantly the ionic strength. This increase in ionic strength due to an increase in the PAMPS concentration has 2 counteracting effects. First, the attraction between the surfactant and polymer in the bulk is screened which, in addition to a decrease in surfactant/polymer ratio, would lead to more hydrophilic surfactant/polymer complexes that have increasing stability in solution. This effect seems to be rather minor, since an increase in the adsorbed amount of polymer is observed at the interface. Second, simultaneously, the charges of the surfactant head groups are screened, which leads to the observed increase in the adsorbed amount of surfactant at the interface. Due to the increase in positive charges at the interface, the more negatively charged

Scattering lengths and scattering length densities and neutron reflectivity profiles. This material is available free of charge via the Internet at http://pubs.acs.org.

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ASSOCIATED CONTENT

S Supporting Information *

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Tel: +33-476-207-097. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Nora Kristen-Hochrein and Martin Uhlig for their assistance with the NR measurements, Maximilian Zerbal for his support with the ellipsometry measurements, Andre Laschewsky for the kind donation of the PAMPS polyelectrolyte, Imre Varga for helpful discussions, and the ILL for allocations of neutron beam time on FIGARO.

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REFERENCES

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