# Anionic Polyelectrolyte-Cationic Surfactant Interactions in Aqueous Solutions and Foam Films Stability

D. Langevin, A. Asnacios, J. F. Argillier, V. Bergeron

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D. Langevin, A. Asnacios, J. F. Argillier, V. Bergeron. Anionic Polyelectrolyte-Cationic Surfactant Interactions in Aqueous Solutions and Foam Films Stability. Revue de l'Institut Français du Pétrole, 1997, 52 (2), pp.139-144. 10.2516/ogst:1997012. hal-02079074

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ANIONIC POLYELECTROLYTE-CATIONIC
SURFACTANT INTERACTIONS IN
AQUEOUS SOLUTIONS AND FOAM
FILMS STABILITY

A. ASNACIOS

Institut Curie¹

V. BERGERON

Rhône-Poulenc²

D. LANGEVIN

Centre de Recherche Paul Pascal³

J.-F. ARGILLIER

Institut français du pétrole⁴

INTERACTIONS ENTRE POLYÉLECTROLYTES ANIONIQUES ET TENSIOACTIFS CATIONIQUES EN SOLUTIONS AQUEUSES ET STABILITÉ DES FILMS DE MOUSSES

L'objectif de cette étude est d'étudier les interactions polymère/
tensioactif en solution aqueuse et à l'interface eau/air. Ces inter-
actions interviennent dans de nombreux phénomènes physico-
chimiques tels que la stabilisation de suspensions colloïdales et la
mouillabilité qui sont d'une importance majeure dans les applica-
tions pétrolières comme, par exemple, les boues de forage.

Plus précisément, nous avons essayé de caractériser les interac-
tions entre un copolymère anionique n'ayant pas d'activité de sur-
face (acrylamide/acrylamide sulfoné) avec un tensioactif de charge
opposée cationique (C₁₂TAB). Nos résultats montrent une diminu-
tion synergique de la tension superficielle (coadsorption) à des
concentrations en tensioactifs très faibles (10⁻³ à 10⁻¹ CMC).
À plus fortes concentrations, en particulier supérieures à la con-
centration critique d'agrégation (CAC), des complexes
polymères/tensioactifs se forment en solution et les macro-
molécules précipitent. Les films minces réalisés à partir de ces
solutions sont stables, alors que les films C₁₂TAB sont instables.

Les mesures de pression de disjonction sur des films mixtes avec
des concentrations en tensioactifs de deux ordres de grandeur en
dessous de la CAC montrent l'existence de forces de répulsion à
longue portée et une transition discrète d'épaisseur du film. À la
valeur de la CAC, nous obtenons des films mixtes avec des
réseaux similaires à un gel qui sont fortement affectés par la
vitesse d'amincissement du film.

ANIONIC POLYELECTROLYTE-CATIONIC SURFACTANT
INTERACTIONS IN AQUEOUS SOLUTIONS AND FOAM
FILMS STABILITY

The objective of this work is to study polymer/surfactant interac-
tions in aqueous solution and at the air/water interface. These
interactions are involved in many physicochemical phenomena,
such as colloidal stabilization and wettability which are of major
importance in oil application as for exemple drilling muds.

More precisely, we have attempted to characterize interactions bet-
ween a non surface active anionic copolymer (acrylamide/acryla-
mide sulfonate) and an oppositely charged cationic surfactant
(C12TAB). Our results show a synergistic surface tension lowering
(coadsorption) at extremely low surfactant concentrations (10-3 to
10-1 CMC). At higher concentrations, namely above the so called
Critical Aggregation Concentration (CAC), polymer-surfactant com-

<footnote>(1) Laboratoire de Physico-chimie Curie,
Section de recherche, 11, rue Pierre et Marie Curie,
75231 Paris Cedex 05 - France</footnote>

<footnote>(2) Rhône-Poulenc Industrialisation,
85, avenue des Frères Perret,
69192 Saint-Fons - France</footnote>

<footnote>(3) Avenue Albert Schweitzer,
33600 Pessac - France</footnote>

<footnote>(4) 1 et 4, avenue de Bois-Préau,
92852 Rueil-Malmaison Cedex - France</footnote>
plexes form in the bulk and the macromolecules precipitate out of
the solution. Foam films made from these mixed solutions are
stable while C₁₂TAB films are unstable.

Disjoining pressure measurements on mixed films with surfactant concentration two orders of magnitude below the CAC show the existence of long range repulsive forces and a discrete film thickness transition. At the CAC, we obtain mixed films with gel-like networks that are strongly affected by the film thinning rate.

INTERACCIONES ENTRE POLIELECTROLITOS ANIÓ-
NICOS Y TENSIOACTIVOS CATÍONICOS EN SOLUCIO-
NES ACUOSAS Y ESTABILIDAD DE LAS PELÍCULAS DE
ESPUMA

El objetivo del presente estudio consiste en estudiar las interacciones polímero tensioactivo en la interfaz agua/aire. Estas interacciones intervienen en numerosos fenómenos físicoquímicos como por ejemplo, la estabilización le las suspensiones coloidales y la humectabilidad que presentan una gran importancia en las aplicaciones petroleras, como por ejemplo, los lodos de perforación.

Habida cuenta de que la adición de un copolímero aniónico no ejerce ninguna actividad de superficie (acrilamida/acrilamida sulfonado) con una solución de tensioactivo de C₁₂TAB (10⁻³ a 10⁻¹ CMC), da lugar a una adsorción sinérgica de dos especies de interfaz aire/agua. Las películas de bajo espesor realizadas a partir de estas soluciones son estables mientras que las películas C₁₂TAB son inestables.

Las mediciones de presión de disyunción efectuadas mediante películas mixtas con concentraciones de tensioactivo de dos órdenes de magnitud por debajo de la Concentración de Agregación Crítica (CAC) vienen a demostrar la inexistencia de fuerzas de repulsión de largo alcance y una transición discreta de espesor de la película. Con el valor de la CAC, obtenemos películas mixtas con redes similares de gel. Estas estructuras de gel son sensibles a la velocidad de formación de la película, pero no se encuentran afectadas por el adelgazamiento de la película.

1 MATERIALS

. The cationic surfactant, dodecyl trimethyl ammonium bromide (C₁₂TAB) from Aldrich (99%), was recrystallized (2g C₁₂TAB: 10 ml ethyl acetate: 1 ml ethyl alcohol) 3 times before use. No minimum in the surface tension versus surfactant concentration was observed after purification.

The polyelectrolyte used is a random-block copoly-
mer of acrylamide (AM) and acrylamido methyl
propane sulfonate (AMPS) synthetized by SNF
Floerger that we will named AAS. The samples were
composed of 75 mol% of neutral AM and 25 mol% of
charged AMPS monomers. The polymer's chemical
structure has been characterized by titration via a bro-
midation reaction for the amide function and potentio-
metric titration for the sulfonate group. The molecular
weight and polydispersity of the polymer were mea-
sured by size exclusion chromatography (SEC) cou-
pled with multiangle light scattering, which gave
Mw = 2.8 10⁶ and Mw/Mn = 1.5. In NaCl 1 g/l, the
average radius of the polymer chains is Rwy = 110 nm.
To eliminate any traces of surfactant molecules and/or
low molecular weight impurities, the polymer solutions
were passed through an ultrafiltration unit with a
20.000 cut-off membrane. Final concentrations were
determined using a total carbon analyser Shimatzu
TOC 5050. After this purification, the polymer dis-
plays no surface activity at concentrations below
2000 ppm. Detailed descriptions and references con-
cerning the experimental techniques and methods can
be found in [1] and [9].

2 RESULTS

## 2.1 Properties of bulk solutions

2.1.1 Surface Tension

We have studied the changes induced by the presence
of a fixed amount of AAS copolymer on the surface
tension isotherm of the C₁₂TAB. Figure 1 displays the
surface tension isotherms (surface tension versus sur-
factant concentration) with and without 750 ppm added
copolymer. The comparison of the two curves reveals
that the mixed solutions show a synergistic lowering of
surface tension at very low surfactant concentrations.
Note that the AAS copolymer alone is not surface
active [1]. The surface tension isotherm of the mixed
solutions exhibits two plateaus identified by two charac-
teristic break points in the isotherm. The first, known as
the Critical Aggregation Concentration (CAC), corre-
sponds to the beginning of a significant number of poly-
mer/surfactant complexes in the *bulk*, while the second
corresponds to the CMC of polymer-free C₁₂TAB. The
coincidence between the surface tension curves with
and without polymer for concentrations of C₁₂TAB
above 9 mM arises from the fact that all of the polymer
has precipitated out of the solution at this point, leaving
an essential pure surfactant solution.

Figure 1 also indicates that at low surfactant concen-
trations (10⁻⁵ M C₁₂TAB), the surface activity of the
surfactant is negligible, yet there is a substantial lower-
ing of the surface tension when the (non surface active)
polymer is present. This effect implies coadsorption of
AAS and C₁₂TAB molecules at the air/solution inter-
face and formation of a highly surface active
polymer/surfactant complex.

2.1.2 Bulk Viscosity

Additional information concerning the C₁₂TAB-AAS interactions can also be obtained from bulk viscosity measurements. Thus, in Figure 2, we plot the bulk viscosity (normalised by its value for a 750 ppm AAS solution) for 750 ppm AAS-C₁₂TAB and 750 ppm AAS-KBr solutions versus C₁₂TAB or KBr concentration. As can be seen in Figure 2, for C₁₂TAB concentrations below the CAC, the decrease in viscosity with added surfactant is identical to that of added KBr. However, above the CAC, C₁₂TAB has a much stronger effect than KBr. In fact, slightly above the CAC, the AAS-C₁₂TAB mixed solutions become turbid, we observe a white precipitate and the measured viscosities correspond to that of pure water (we actually find the same value for pure C₁₂TAB solutions).

These observations indicate that, below the CAC,
C₁₂TAB is completely dissociated and behaves as a
simple salt. That is, the increased ionic strength upon
addition of surfactant or simple electrolyte screens the
electrostatic repulsions between charged groups on the
polymer backbone, leading to smaller hydrodynamic
volumes and lower relative viscosities. However, at the
CAC, the surfactant concentration is high enough to
form (in the bulk!) tightly packed polymer-surfactant
complexes that eventually precipitate out of the solu-
tion to form a separate phase. This precipitation of the
macromolecules is consistent with the coincidence
between the surface tension curves with and without
polymer for C₁₂TAB concentrations above 9 mM.

<figure><img src="image_2.png" /><figcaption>Figure 1
Effect of AAS copolymer on C₁₂TAB surface tension: (○) C₁₂TAB; (●) C₁₂TAB + 750 ppm of AAS.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Figure 2
Comparison of the effect of C₁₂TAB and salt on the relative viscosity of a 750 ppm solution of AAS: (●) AAS/C₁₂TAB; (◊) AAS/KBr.</figcaption></figure>

In summary, below the CAC, while there is no com-
plexation between the polymer and the surfactant in the
bulk, the two species coadsob at the solution/air inter-
face to form a highly surface active complex. Above
the CAC, complexation occurs in bulk, the polymer
molecules precipitate out and the mixtures finally
recover the surface tension and viscosity of the poly-
mer-free surfactant solutions.
### 2.1.3 Ellipsometry

Ellipsometric measurements allow us to determine the thickness of the adsorbed mixed layer. We find that the layer properties are also very dependent on the surfactant concentration. Again, the CAC appears as the limit between two different regimes. Below the CAC, one finds homogeneous thin layers with thicknesses between 1 and 6 nm that suggest a flat polymer conformation. Above the CAC, the adsorbed layer becomes heterogeneous and thicker. The surface layer is then formed of domains of different thicknesses ranging from 40 to 80 nm. More information and details concerning these results (including a thermodynamic analysis) can be found in [1] and [2].

## 2.2 Properties of foam films made from mixed solutions. Disjoining pressure isotherms

We have studied foam films at two characteristic $C_{12}$TAB concentrations: 0.05 and 1 mM, which are respectively below and slightly above the CAC. We note that foam films made from $C_{12}$TAB alone at these concentrations are completely unstable and we are unable to determine the disjoining pressure isotherms for the pure surfactant solutions.

### 2.2.1 Below the CAC

Figure 3 displays the disjoining pressure isotherm for a 0.05 mM $C_{12}$TAB solution with 750 ppm added AAS copolymer. This isotherm contains two repulsive branches which we accentuate further by sketching in lines between the data points. The first branch is encountered at rather thick films, $h \approx 125$ nm, and is stable until pressures near 500 Pa are reached. Imposing a higher pressure results in a discrete transition to a new metastable equilibrium thickness of about 75 nm. The transition process is pictured in Figure 4, where the lightly colored region corresponds to a film thickness of 125 nm and the darker (grey) expanding portion is the thinner 75 nm region which eventually covers the entire film. In thin-liquid films this type of transition is termed "film stratification" [3]. Both of the repulsive branches of the isotherm are reproducible, and hysteresis along a branch upon lowering the imposed pressure is not observed. Finally, once the pressure exceeds 1000 Pa the film ruptures.

The film stratification observed in Figure 3 is quite striking because it is the first observation of this

phenomenon in foam films made from solutions well below the CMC, and also below the CAC! All previous observations have been made in highly concentrated surfactant solutions or in solutions containing microscopic particles [3, 4, 5, 6, 7]. In these cases stratification is due to supramolecular structuring of the surfactant aggregates (i.e. micelles or bilayers) or of the microscopic particules in the films. However, our mixed solutions are very dilute in both surfactant and polymer, and it is unlikely that the present stratification arises from the structuring of micelles, lamella or mixed aggregates. Thus, we have first suggested that this stratification could be due to bridging between the two adsorbed layers [8]. Another possibility is that the distance between branches might be related to the mesh size $\xi$ of the temporary netowrk formed by the macromolecules in a semi-dilute regime [9]. In this regime, the macromolecules can be seen as ideal chains of rods of size $\xi$, and rods are uncorrelated in orientation [11]. An estimation of the overlap concentration $c*$ clearly show that our systeme is in the semi-dilute regime [2].

Recent observations show that the number of branches increases and the inter-branche distance decreases with increasing polymer concentration [10]. This behaviour is in agreement with the correlation length $\xi$ dependence on polyelectrolyte concentration.

<figure><img src="image_4.png" /><figcaption>Figure 3
Disjoining pressure isotherm for foam films formed from a mixed solution containing 0.05 mM $C_{12}$TAB and 750 ppm AAS.</figcaption></figure>
<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>Figure 4
Stratification event for a foam film formed from a mixed solution containing 0.05 mM C₁₂TAB and 750 ppm AAS.</figcaption></figure>

The important characteristic of semi-dilute polyelec-
trolyte solutions is that the correlation length ξ, the
inter-rod distance d and the Debye length λ are of the
same order [11]. There is a strong electrostatic repul-
sion at distances smaller than ξ, and the structure factor
shows a peak at a wavevector q* ≈ 1/ξ. Thus, the dis-
tance between branches is probably on the order of ξ.

One can casily imagine that the macromolecules
(behaving as a succession of sub-units of length ξ
uncorrelated in their orientation) can adjust their con-
formations to stay in a film of a thickness of only a few
times ξ. Moreover, it is worth noting that the viscosity
behaviour of our solutions implies that the polyelec-
trolyte molecules, even in the semi-dilute regime, are
not entangled enough to show a reptation dynamic [2].
This implies that molecules can be easiliy squeezed out
of the film at cach transition, allowing the remaining
ones to flatten themselves to satisfy the constraint of a
film thickness ξ smaller. The whole process is then
equivalent to having a film formed of a close packing
of spheres of size ξ, a layer of such spheres being
squeezed out at each thickness transition.

2.2.2 Above the CAC

Mixed solutions at 750 ppm of AAS copolymer and 1 mM C₁₂TAB produces films with a quite different behavior than that seen at the lower C₁₂TAB concentration. In this case, the film thickness becomes very heterogeneous. This phenomenon is pictured in the series of photos shown in Figure 5. These images are taken from a VCR recording of the film and correspond to observations made with a color CCD camera mounted on the microscope. All four images are from the same film but correspond to different imposed pressures. The series begin with Figure 5a at an imposed pressure of approximately 50 Pa followed by progressively higher pressures (Figure 5d, P > 5000 Pa). As can be easily seen, the film pictured in Figure 5 is thick enough to produce iridescent colors from thin-film interference under white light illumination. Each color corresponds to a different film thickness and it is evident that the thickness is highly heterogeneous across the film. In fact, there are two types of heterogeneities: microscopic, on the order of 20 mm diameter, which appear as spots scattered throughout the film, and much larger irregularly shaped macroscopic domains. The microscopic spots corresponds to regions of about 1 mm thickness, while macroscopic domains have thicknesses of 100-200 nm. From the photos in Figure 5, it can also be observed that, as the pressure is increased, each macroscopic domain becomes thinner (as evident by the changes in color), yet the shape integrity of the domains remains intact. Finally, pronounced hysteresis of the film thickness behaviour is observed when the pressure is lowered.
The fact that the spots located in the film and the macroscopic domain shapes are unaffected by the overall thinning of the film under pressure increases suggests the existence of a gel-like network within the foam films made from mixed solutions with surfactant concentration above the CAC. It is interesting to note that the macroscopic domain thicknesses correspond quite well to twice the adsorbed layer thickness measured by ellipsometry at the air/solution interface. Moreover, since polymer-surfactant complex formation in the bulk begins at the CAC, one can imagine that micro-gel domains appear in the bulk solutions at this concentration and that such micro-gels can be adsorbed at the air/water interface. The presence of such a gel-like network is not surprising as we have observed white rubber-like aggregates in the bulk solution after the CAC. It is in fact worth noting that the gel-like films have a very long relaxation times (the imposed pressure on the film can be held constant for days without observing any significant change in the heterogeneous thickness profile). More details concerning these gel like films can be found in [2] and [8].

CONCLUSIONS

Oppositely charged AAS polyelectrolyte and $C_{12}$TAB surfactant interact strongly in aqueous solution. They form complexes in the bulk solution as well as at the air/water interface, and $C_{12}$TAB thin films can be stabilized by the addition of AAS. At fixed polymer concentration, both bulk, interfacial and film properties change dramatically with the surfactant concentration. One can essentially distinguish two different regimes:

- Well below the critical aggregation concentration (CAC < CMC), while there is no complexation in

the bulk, the polymer and the surfactant coadsorb at the air/water interface producing a synergistic surface tension lowering. The adsorbed layer is then homogeneous and thin (<6 nm) indicating a flat polymer conformation at the interface. Mixed films at these extremely low surfactant and polymer concentrations show discrete thickness transitions probably due to polyelectrolyte structuration in the films.

- Slightly above the CAC, polymer-surfactant complexation occurs in bulk and the macromolecules precipitate out (forming white rubber-like aggregates). The adsorbed mixed layer then has a thick and heterogeneous profile (>40 nm). Finally, mixed films exhibit a gel-like structure with heterogeneous thicknesses.

REFERENCES

1 Asnacios A., D. Langevin and J.F. Argillier (1996), *Macromolecules*, **29**, pp. 7412-7417.

2 Asnacios A. (1997), *Thesis*, Paris.

3 Bergeron V. and C.J. Radke (1992), *Langmuir*, **8**, p. 3020.

4 Exerowa D. and Z. Lalchev (1986), *Langmuir*, **2**, p. 668.

5 Richetti P. and P. Kékicheff (1992), *Phys. Rev. Letters*, **68**, 12, pp. 1951-1954.

6 Nikolov A. and D.T. Wasan (1992), *Langmuir*, **8**, p. 2985.

7 Bergeron V. and C.J. Radke (1995), *Colloid. Polym. Sci.*, **273**, p. 165.

8 Bergeron V., D. Langevin and A. Asnacios (1996), *Langmuir*, **12**, 6, pp. 1550-1556.

9 de Gennes P.G., P. Pincus, R.M. Velasco, and F. Brochard (1976), *Journal de Physique*, **37**, pp. 1461-1473.

10 Asnacios A., A. Espert, A. Colin and D. Langevin, submitted for publication in *PRL*.

11 Barrat J.L. and J.F. Joanny (1996), *Adv. Chem. Phys.*, **94**, p. 1.

Manuscript received in January 1997
<figure><img src="image_7.png" /><figcaption>Figure 5</figcaption></figure>

Sequence of photos taken from a single foam film formed from a mixed solution containing 1 mM C₁₂TAB and 750 ppm AAS copolymer. Each frame corresponds to a different imposed pressure on the film: (a) 50 Pa; (b) 250 Pa; (c) 1000 Pa; (d) > 5000 Pa.