Langmuir 2006, 22, 5013-5017
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Interaction Forces in Thin Liquid Films Stabilized by Hydrophobically Modified Inulin Polymeric Surfactant. 1. Foam Films D. Exerowa,*,† T. Kolarov,† I. Pigov,† B. Levecke,‡ and Tharwat Tadros§ Institute of Physical Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria, and ORAFTI Bio Based Chemicals, Aandorenstraat 1, 3300 Tienen, Belgium, and 89, Nash GroVe Lane, Wokingham, Berkshire RG40 4HE, United Kingdom ReceiVed January 4, 2006. In Final Form: February 24, 2006 Using the interferometric method of Scheludko-Exerowa for investigation of foam films, we have obtained results using a hydrophobically modified inulin polymeric surfactant (INUTEC SP1). Measurements were carried out at constant INUTEC SP1 concentration of 2 × 10-5 mol‚dm-3 and at various NaCl concentrations (in the range 1 × 10-4 to 2 mol‚dm-3). At constant capillary pressure of 50 Pa, the film thickness decreased gradually with an increase in NaCl concentration up to 10-2 mol‚dm-3 NaCl above which the film thickness remains virtually constant at about 16 nm. This reduction in film thickness with an increase in NaCl concentration is due to the compression of the double layer and at the critical electrolyte concentration (Cel,cr ) 10-2 mol‚dm-3) the electrostatic component of the disjoining pressure is completely screened and the remaining pressure is due to the steric interaction between the adsorbed polymer layers. Disjoining pressure-thickness (Π-h) isotherms were obtained at Cel < Cel,cr (10-4 - 10-3 mol‚dm-3) and Cel > Cel,cr (0.5, 1, and 2 mol‚dm-3). In the first case, the disjoining pressure isotherms could be fitted using the classical DLVO theory, Π ) Πel + Πvw, and using the constant charge model. At Cel > Cel,cr, the main repulsion is due to the steric interaction between the polyfructose loops that exist at the air-water interface, i.e., Π ) Πst + Πvw. Under these conditions, there is a sharp transition from DLVO to non-DLVO forces. In the latter case, the interaction could be described using the de Gennes’ scaling theory. This gave an adsorbed layer thickness of 6.5 nm which is in reasonable agreement with the values obtained at the solid-solution interface. The Π-h isotherms showed that these foam films are not very stable and they tend to collapse above a critical capillary pressure (of about 1 × 103 Pa), and these results could be used to predict the foam stability.
Introduction The study of interaction forces between macroscopic phases across an aqueous core layer proved to be a suitable method to predict the stability of foams and emulsions. The earlier studies were carried out using simple ionic and nonionic surfactants of a different nature and structure. However, studies on thin liquid foam and emulsion films using polymeric surfactants are relatively scare. The most comprehensive investigations have been carried out using A-B-A block copolymers of the poly(ethylene oxide)-poly(propylene oxide) (PEO-PPO-PEO), referred to as pluronics, synperonic PE, or polaxamers.1,2 The results showed that at low electrolyte concentration (less than 3 × 10-3 mol‚dm-3 NaCl) an electrostatic contribution to the disjoining pressure exists and that could be described by the DLVO theory. However, at higher electrolyte concentration (greater than 3 × 10-3 mol‚dm-3 NaCl), this electrostatic contribution is suppressed and the main contribution to the disjoining pressure is the steric repulsion which increases sharply as the thickness of the layer decreases below a critical value depending on the length of the poly(ethylene oxide) chain. The steric interaction could be analyzed using the brush-to-brush model suggested by de Gennes.3 In this case, the steric interaction was very similar to that produced using nonionic surfactants based on PEO. The brush-to-brush contact is established only at higher capillary pressure, and then, * Corresponding author. † Bulgarian Academy of Sciences. ‡ ORAFTI Bio Based Chemicals. § 89, Nash Grove Lane. (1) Exerowa, D.; Sedev, R.; Ivanova, R.; Kolarov, T.; Tadros, Th. F. Colloids Surf. 1997, 123, 277. (2) Sedev, R.; Exerowa, D. AdV. Colloid Interface Sci. 1999, 83, 111. (3) de Gennes, P. G. Scaling Conceptes in Polymer Physics; Cornell University Press: Ithica, New York, 1979.
the disjoining pressure isotherm follows the de Gennes’ scaling prediction.1,2 In this paper, we will describe results on a novel graft copolymer of hydrophobically modified inulin (linear polyfructose). This polymeric surfactant has been recently synthesized,4 and its adsorption and conformation have been investigated at the solidliquid interface.5,6 One would expect the adsorption and conformation of the graft copolymer to be similar at the airliquid interface. The interaction between the polymer layers can be investigated using the microscopic technique described in detail earlier.7 In this way, one can investigate the effect of polymer concentration and addition of electrolyte on the interaction between the polymer layers, and this is the main objective of this paper. The results obtained could also be applied in predicting foam stability using this macromolecular surfactant. Experimental Section Materials. The polymeric surfactant is inulin on which several alkyl groups have been grafted (INUTEC SP1, ORAFTI, Belgium). Its average molecular mass is approximately 4500 g‚mol-1. The inulin backbone (linear polyfructose) has a degree of polymerization greater than 23. Grafting was carried out using alkyl isocyanates in a nonaqueous solvent N-methyl pyrrolidone. The polymer was purified either by solvent precipitation or by supercritical CO2 extraction. The chemical structure of the polymeric surfactant is presented in Figure 1. (4) Stevens, C. V.; Meriggi, A.; Peristeropoulo, M.; Christov, P.; Booten, K.; Levecke, B.; Vandamme, A.; Pittevils, N.; Tadros, Th. F. Biomacromolecules 2001, 2, 1256. (5) Esquena, J.; Dominguez, F. J.; Solans, C.; Levecke, B.; Tadros, Th. F. Langmuir 2003, 19, 10463. (6) Tadros, Th. F.; Vandamme, A.; Levecke, B.; Booten, K.; Stevens, C. V. AdV. Colloid Interface Sci. 2004, 108-109, 207.
10.1021/la0600301 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/20/2006
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Figure 4. Thin liquid films cells (partial drawings): ScheludkoExerowa tube cell (a); Exerowa-Scheludko porous plate cell (b). Figure 1. Structure of inulincarbamates.
Figure 5. Schematic arrangement of the microinteferometric apparatus. Figure 2. Effect of INUTEC SP1 concentration on the specific electrical conductivity in aqueous solutions.
Figure 3. Effect of NaCl concentration on the refraction index in aqueous solutions of 2 × 10-5 mol‚dm-3 INUTEC SP1 (O) and NaCl (0) only. Aqueous INUTEC SP1 solutions were prepared in the presence and absence of NaCl. The NaCl (Merck) was heated at 500 °C before use to remove any organic impurities. The water used for preparation of all solutions was doubly distilled, and it had a specific conductivity of about 1 µS‚cm-1. Physical Properties of INUTEC SP1 Solutions. The specific conductivity of aqueous INUTEC SP1 solutions was measured at 23 °C using a LP conductoscope (Czech Republic). Figure 2 shows the variation of the specific conductivity with INUTEC SP1 concentration. The specific conductance remains virtually constant (about 1.6 µS‚cm-1) up to about 1 × 10-5 mol‚dm-3 INUTEC SP1 after which it increases with further increase of polymeric surfactant concentration. All solutions used in this research had a concentration of 2 × 10-5 mol‚dm-3, and this eliminates the need to make corrections to the ionic strength. The refractive index n of 2 × 10-5 mol‚dm-3 INUTEC SP1 solutions was measured as a function of NaCl concentration using an Abbe´ refractometer (Zeiss, Germany). Figure 3 shows a linear dependence of n with increasing electrolyte concentration. The figure also shows the same increase for NaCl solutions in the absence of INUTEC SP1. Microinterferometric Method for Investigation of Foam Films. The foam film experiments were carried out using two cells described
in detail elsewhere.7 Figure 4 shows a schematic representation of the two cells. The cells are enclosed in an all-glass chamber (not shown) saturated with solution vapor. In the Scheludko-Exerowa tube cell8-10 (Figure 4a), a microscopic foam film (with radius r ) 50-100 µm) is formed in the middle of a biconcave drop hanging in a short vertical glass tube. A small portion of the liquid is sucked out of the drop through the capillary with a micrometrically driven pump. The two liquid/gas interfaces approach each other, and ultimately, a thin liquid film is created. The film thins out and at the end reaching one of the several possible states: equilibrium thickness, critical thickness, black spot, and black film formation. The Exerowa-Scheludko porous plate cell11,12 (Figure 4b) was used to monitor the disjoining pressure isotherm. The so-called “pressure balance technique” developed allows direct measurement of the interaction forces in a foam film. It is formed in a little hole drilled into a porous plate (Rasotherm sintered glass with a pore size of approximately 0.5 µm) soaked with the solution. The gas pressure inside the glass chamber is increased (up to 105 Pa) with a membrane pump (the capillary is left open). Pressure values lower than 103 Pa are measured with a water manometer ((5 Pa). Higher pressures are read from a standard membrane manometer ((0.6%). In both cases, the film thickness is monitored by measuring the reflection of monochromatic light (from an incandescent lamp equipped with an interference filter having a transmittance maximum at 546 nm) using the microinterferometric method. A schematic representation of the microinterferometric setup is shown in Figure 5. Assuming that the film is optically homogeneous and has a refractive index equal to that of the bulk solution, a film thickness hw, referred to as “equivalent film thickness”, is calculated.7,10 Using the above microinterferometric technique, one can obtain the thickness, the contact angle, lifetime, etc. of the film. Details of the measurements are described elsewhere7,10,12
Results and Discussion Influence of Electrolyte Concentration on Film Thickness (at Constant Capillary Pressure). Figure 6 shows the variation (7) Exerowa, D.; Kruglyakov, P. M. Foam and Foam Films, Elsevier: Amsterdam, 1998. (8) Scheludko, A. AdV. Colloid Interface Sci. 1967, 1, 391. (9) Exerowa, D.; Zacharieva, M.; Cohen, R.; Platikanov, D. Colloid Polym. Sci. 1979, 257, 1089. (10) Exerowa, D.; Kashchiev, D.; Platikanov, D. AdV. Colloid Interface Sci. 1992, 40, 201.
Foam Films
Figure 6. Effect of NaCl concentration, Cel, on the equivalent thickness, hw, of foam films from 2 × 10-5 mol‚dm-3 INUTEC SP1 aqueous solutions.
of the equivalent film thickness at 23 °C with NaCl concentration (in the range 1 × 10-4 to 2 mol‚dm-3) at constant INUTEC SP1 concentration (2 × 10-5 mol‚dm-3) and at constant capillary pressure of about 50 Pa. These results were obtained using the cell described in Figure 4a, and the thickness values were an average of five independent measurements. The error bars in the thickness values were higher at lower electrolyte concentrations. This deviation from the mean value becomes insignificant at higher electrolyte concentrations (greater than 10-2 mol‚dm-3 NaCl). The results of Figure 6 clearly show a rapid reduction in film thickness with increase in NaCl concentration till 10-2 mol‚dm-3 NaCl above which the film thickness remains virtually constant at about 16 nm. This reduction in film thickness with increase in NaCl concentration is due to the compression of the double layer, and at a critical electrolyte concentration (Cel,cr ) 2 × 10-2 mol‚dm-3), the electrostatic component of the disjoining pressure is completely screened and the remaining thickness is due to non-DLVO forces most likely due to the steric interaction between the adsorbed polymer layers. Similar results have been obtained before for nonionic surfactants,7,10,13 nonionic phospholipids,7,14,15 as well as A-B-A block copolymers of PEOPPO-PEO.1,2,7 Disjoining Pressure Isotherms. Disjoining pressure isotherms were obtained using the cell described in Figure 4b. In these experiments, the INUTEC SP1 concentration was the same as described above, namely 2 × 10-5 mol‚dm-3. Isotherms were obtained at various electrolyte concentrations: 1 × 10-4 and 1 × 10-3 mol‚dm-3 (below Cel,cr) and 0.5, 1, and 2 mol‚dm-3 (above Cel,cr). All measurements were obtained at equilibrium, i.e., when the film thickness did not show any change with time. The agreement in film thickness between two sets of measurements were in general not very good, and this is due to the irregularities in the film thickness. However, the general trend in the variation of film thickness with disjoining pressure is as expected (see below). Figures 7 and 8 show the results at 1 × 10-4 and 1×10-3 mol‚dm-3 NaCl, respectively. Different symbols indicate two different experimental runs. In 1 × 10-4 mol‚dm-3 NaCl, the disjoining pressure started to increase rapidly when the film thickness decreased below 40 nm, whereas in 1 × 10-3 mol‚dm-3 NaCl, this rapid increase occurred at 30 nm. This difference is due to the difference in the double layer thickness which is higher (11) Exerowa, D.; Scheludko, A. Compt. Rend. Acad. Bulg. Sci. 1971, 24, 47. (12) Exerowa, D.; Kashchiev, D. Contemporary Phys. 1986, 27, 429. (13) Kolarov, T.; Cohen, R.; Exerowa, D. Colloids Surf. 1989, 42, 49. (14) Cohen, R.; Koynova, R.; Tenchov, B.; Exerowa, D. Eur. Biophys. J. 1991, 20, 203. (15) Cohen, R.; Yamanaka, T.; Exerowa, D. Comm. Dept. Chem. 1991, 24, 505.
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Figure 7. Measured disjoining pressure, Π, vs equivalent thickness, hw, for foam films from 2 × 10-5 mol‚dm-3 INUTEC SP1 and 10-4 mol‚dm-3 NaCl aqueous solutions: run 1 (O) and run 2 (0). The lines represent DLVO evaluation as a sum of double layer repulsion at constant potential (solid line) and constant charge (dashed line) modes and van der Waals attraction. The best fit to the experimental data corresponds to a diffuse double layer potential at infinity separation of -28 mV.
Figure 8. Same as in Figure 7 but for 10-3 mol‚dm-3 NaCl. The best fit to the experimental data corresponds to a diffuse double layer potential at infinity separation of -14 mV.
Figure 9. Measured disjoining pressure, Π, vs equivalent thickness, hw, for films from aqueous solutions of 2 × 10-5 mol‚dm-3 INUTEC SP1 and 0.5 (O), 1 (0), and 2 (4) mol‚dm-3 NaCl.
in 1 × 10-4 mol‚dm-3 NaCl when compared with that in 1 × 10-3 mol‚dm-3 NaCl. In both cases, the film remains stable up to a disjoining pressure of about 4000 Pa above which film collapse took place. Stable films in the range 15-80 nm could be obtained in both cases. The mechanism of film rupture at a disjoining pressure above 4000 Pa is difficult to explain at present. One may speculate that it is related to the magnitude of stabilizing forces. Similar rupture behavior has been reported for foam fims from both ionic and cationic surfactants.7 Figure 9 shows disjoining pressure isotherms at NaCl concentrations above Cel,cr namely 0.5, 1, and 2 mol‚dm-3. At such high electrolyte concentration, the variation of disjoining pressure with film thickness follows roughly the same trend, namely a
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Figure 10. Schematic presentation of three-layer film model: The adsorption layers of thickness h1 and refraction index n1 are composed of the hydrophobic dodecyl chains (DDC) and hydrophilic polyfructose (inulun) chains (PFC) of the INUTEC SP1 molecule. h2 and n2 are respectively the thickness and the refraction index of the aqueous core. d is the distance between the planes of onset of double layer repulsion and needs additional modeling.
gradual increase in disjoining pressure with reduction in film thickness. These results are different from those obtained at Cel < Cel,cr which showed a much more rapid increase in disjoining pressure with reduction in film thickness. This difference must be related to the different behavior at low and high electrolyte concentrations. At Cel < Cel,cr, the dominant force responsible to disjoining pressure is probably of the DLVO-type, whereas at Cel > Cel,cr, the non-DLVO type of interaction predominates. In the latter case, the disjoining pressure isotherms are roughly the same for the three electrolyte concentrations studied. Theoretical Analysis of Disjoining Pressure. The disjoining pressure isotherms for polymer stabilized liquid films can be fitted with theory by considering the total interaction forces, namely van der Waals attraction (Πvw), double layer repulsion (Πel), and steric interaction (Πst). At Cel < Cel,cr, Πel predominates over Πst at least at high film thickness, and in this case, the disjoining pressure isotherms can be fitted using the classical DLVO theory, i.e., Π ) Πel + Πvw. In contrast at Cel > Cel,cr, Πst predominates over Πel, and in this case, Π ) Πvw + Πst. In the first case, the theoretical analysis of the disjoining pressure isotherms is based on the DLVO theory16,17 and a threelayer film model.18 When only DLVO forces are operative, Πel was evaluated by solving the complete Poisson-Boltzmann equation using the numerical procedure given by Chan et al.19 The solution allows either constant potential or constant charge boundary conditions to be considered as limiting cases of Πel. Πvw was calculated using the empirical equation of Donners et al.20 based on the exact Lifshitz theory.21 Figure 10 shows a schematic representation of the film threelayer model used. It consists of two adsorbed layers of INUTEC SP1, each of thickness h1 and refraction index n1, and an aqueous core layer of thickness h2 and refraction index n2. The total film thickness h ) h2 + 2h1 is different from the equivalent film thickness hw that is experimentally measured. This difference is accounted for by the correction factor hw,corr ) 2h1(n12 - 1)(n22 - 1) with h2 ) hw - hw,corr.18 The evaluation of hw,corr requires appropriate values for n1 and h1. In our case, the adsorbed layer is composed (see Figure 10) of the hydrophobic dodecyl chains (DDC) and the hydrophilic polyfructose (inulin) chains (PFC) (16) Derjagiun, B. V.; Landau, L. D. Acta Physicochim. URSS 1941, 14, 633. (17) Vervwey, E. J. V.; Overbeek, T. G. The Theory of the Stability of Liophobic Colloids; Elsevier: Amsterdam, 1948. (18) Duyvis, E. M. Thesis Utrecht, 1962. (19) Chan, D. Y.; Pashley, R. M.; White, L. R. J. Colloid Interface Sci. 1980, 77, 283. (20) Donners, W. A. B.; Rijnbout, J. B.; Vrij, A. J. Colloid Interface Sci. 1977, 60, 540. (21) Dzjaloshinski, I. E.; Lifshitz, E. M.; Pitaevskii, L. P. AdV. Phys. 1961, 10, 165.
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of the INUTEC SP1 molecules. Hence, it is not unreasonable to approximate n1 by a value between 1.42 (corresponding to bulk dodecane) and 1.38 (corresponding to supersaturated aqueous solutions of inulin22), i.e., one may adopt n1 ) 1.40 ( 0.02. As far as h1 is concerned data are avalable from atomic force microscopy measurements which showed an adsorbed layer thickness at the hydrophobic glass surface-water interface of about 7 nm.23 Measurement using dynamic light scattering for polystyrene latex particles with an adsorbed INUTEC SP1 layer gave a thickness of about 4 nm.5 It seems therefore that the adsorbed layer thickness depends on the nature of the interface, and in the present calculation, we have assumed a layer thickness of 5 nm, which seems to be a reasonable estimate. With this value, we obtained hw,corr ) 12.5 ( 0.7 nm; that is, the assumptions made for n1 resulted in an error of less than 1 nm. The origin of electrostatic charge at the air-water interface containing adsorbed INUTEC SP1 surfactants is most likely due to the adsorption of OH- ions at the interface.7,24 Under these conditions, the distance d between the planes of the onset of Πel needs additional modeling: the diffuse double layer boundary is supposed to be located in the middle of the adsorbed layers. Hence, d ) h - h1. This is a rather rough approximation, but it has been proven to be satisfactory in practice.2,7,13 The results of the DLVO fits to the experimental results are shown in Figures 7 and 8. The cases of constant potential and constant charge boundary conditions are plotted by solid and dashed lines, respectively. The diffuse double layer (DDL) potential at infinity was used as a fitting parameter. Keeping in mind the reproducibility of the experimental data and approximations made, the fits seem to be satisfactory. In addition, it appears that the best fit is obtained when using the constant charge model. The corresponding DDL potentials at infinity are -28 and -14 mV for 1 × 10-4 and 1 × 10-3 mol‚dm-3 NaCl, respectively. As it can be seen, all of the experimental data for these relatively thick films are close to the theoretical calculations based on the constant charge model. Both the constant charge DLVO regime and the fitted values of the DDL potential at infinity are in good agreement with those reported for nonionic surfactants with relatively large hydrophilic heads.2,7,13 Thus, one may conclude that at Cel < Cel,cr only DLVO forces are responsible for film stability. At Cel > Cel,cr, the double layer repulsion is practically suppressed and this is manifested by the fact that the film thickness does not change any more for electrolyte concentrations above Cel,cr (see Figure 6). Under these conditions, Πel ) 0, and the experimental results cannot be explained in the frameworks of DLVO theory only. If one assumes that the only contribution to Π is Πvw, one obtains the solid line represented in Figure 11, which clearly shows a very large deviation from the experimental data (also shown on the same figure). This directly implies that there is an additional repulsive contribution to the disjoining pressure, namely Πst. It has been suggested by Tadros et al. that an adsorbed INUTEC SP1 molecule produces large “loops” of polyfructose between two adjacent dodecyl chains and strong repulsion occurs when the adsorbed layers begin to overlap.6 This additional repulsion usually referred to as steric repulsion, Πst,25 can be estimated from the data plotted in Figure 11 by subtracting the values of (22) Bot, A.; Erle, U.; Vreeker, R.; Agterof, W. G. M. Food Hydrocolloids 2004, 18, 547. (23) Nestor, J.; Esquena, J.; Solans, C.; Luckham, P.; Musoke, M.; Levecke, B.; Booten, K.; Tadros, Th. F. Langmuir (submitted). (24) Exerowa, D. Kolloid-Z. 1969, 232, 703. (25) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: New York, 1983.
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Figure 11. Same as in Figure 9 and the DLVO evaluation (solid line) under the conditions of suppressed double layer repulsion.
Figure 12. Difference between the experimental and DLVO data from Figure 11, Πexp - Πvw, (O) plotted as a function of total film thickness, h. A fit to these data of de Gennes’ equation for steric repulsion between polymer “brushes” is shown also (solid line).
the theoretical Πvw from the experimental disjoining pressures Πexp, i.e., Πst ) Πexp - Πvw. Moreover, it has been recognized that hydrophilic headgroups longer than about 1 nm may be treated as “quasibrushes”.26 If this is also applicable to the “loops”, one may expect to use the theory for interacting polymer “brushes” to describe “loop-to-loop” steric interactions between adsorbed layers of INUTEC SP1. The de Gennes’ scaling theory gives the following expression for the repulsion between two brush layers:
Πst =
[( )
kT h D3 2h1
-9/4
-
( )] h 2h1
3/4
where D is the mean distance between the brushes and k and T are Boltzmann constant and absolute temperature, respectively.3,27 Figure 12 shows the best fit of our data for the difference Πexp - Πvw vs h (i.e., Πst vs h) with the de Gennes’ equation. The resulting value for h1 for this fit is 6.5 nm, which is only slightly higher than the value used in the three-layer model (5 nm). Moreover, it can be shown that the disagreement becomes less significant if an adsorbed layer of 6 nm is assumed in the threelayer model. In this case, the fit (not shown) yields 6.1 nm for h1. The above calculations clearly indicate that Πst is the most likely contribution to the disjoining pressure that is responsible for the stability of the films above Cel,cr. These results are in good agreement with our earlier conclusions using A-B-A block (26) Israelachvili, J. N.; Wennerstro¨m, H. J. Phys. Chem. 1992, 96, 520. (27) de Gennes, P. G. AdV. Colloid Interface Sci. 1987, 27, 18.
Figure 13. Effect of INUTEC SP1 concentration (0, 2 × 10-3 and O, 2 × 10-5 mol‚dm-3) on the disjoining pressure isotherms measured at 1 mol‚dm-3.
copolymers in foam films.1,2 Similar results are obtained at higher INUTEC SP1 concentration namely 2 × 10-3 mol‚dm-3 in the presence of 1 mol‚dm-3 NaCl as shown in Figure 13. The same figure also contains the data obtained at lower INUTEC SP1 concentration (2 × 10-5 mol‚dm-3). As can be seen, the Π(hw) isotherms for the two INUTEC SP1 concentrations are very close to each other. At the higher INUTEC SP1 concentration, a slightly higher capillary pressure (about 1.1 × 103 Pa) could be reached. Under this condition, one can conclude that the concentration of INUTEC SP1 does not influence the disjoining pressure isotherms within the range studied in this paper. This would allow for a future comparison of foam films with real foams that are usually obtained at higher polymer surfactant concentrations.
Conclusions The study of microscopic foam films produced from aqueous solutions of polymer surfactants based on inulin (INUTEC SP1) allows one to obtain the film thickness as a function of electrolyte concentration Cel as well as the dependence of disjoining pressure with film thickness (the disjoining pressure isotherms). This allows one to identify the contributions of the various interaction forces to the disjoining pressure in these foam films. At low electrolyte concentrations (10-4-10-3 mol‚dm-3) the main contributions to the disjoining pressure are the electrostatic repulsion, Πel, and the van der Waals attraction, Πvw, as described by DLVO theory. This applies to thicker films, 15-80 nm. At high electrolyte concentrations (0.5, 1.0, and 2.0 mol‚dm-3 NaCl), the main contributions to the disjoining pressure are the steric repulsion Πst and van der Waals attraction, Πvw. This is the case for thinner films 9-17 nm. In the first case (Cel < Cel,cr), the results could be fitted to the DLVO theory in particular when using the constant charge model. In the second case (Cel > Cel,cr), the steric repulsion arrises from the interaction between the polyfructose loops that exist at the air-water interface. In this case, the de Gennes’ scaling theory could be used to fit the data giving a layer thickness of 6.5 nm. It is worth noting that the foam films studied are relatively unstable, and the smallest thickness that can be obtained before film rupture was in the region of 8-9 nm. In such high electrolyte concentrations, a capillary pressure of about 1 × 103 Pa could be reached. Above this pressure, film rupture occurred indicating the instability of the foam film. These disjoining pressure isotherms can be used to predict the foam stability obtained using INUTEC SP1. LA0600301
