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A Neutron Reflectivity Study of Drainage and Stratification of AOT Foam Films† T. Ederth* and R. K. Thomas Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, U.K. Received October 22, 2002. In Final Form: April 1, 2003 Structural properties of Aerosol-OT (AOT) foam films were studied by neutron reflectometry. The drainage of large (10 cm2), horizontally oriented AOT films under zero applied external pressure is slow and proceeds from thick colored or gray films to black films of approximately 200 Å thickness over many hours, resulting in quasi-static conditions over the time scales required for the acquisition of reflectivity profiles throughout the whole drainage process. After formation of the foam filmswhile still showing colored interference fringessand during the early stages of thinning, the appearance of Bragg diffraction peaks demonstrates that the film contains multilamellar structure, in agreement with studies at free air/water interfaces. Upon further reduction of the film thickness, below approximately 250 Å, the reflectivity profile is well fitted by a three-layer slab model with two surfactant layers and an aqueous core with a high surfactant content. The composition of the aqueous core and the manner in which the reflectivity during the latter stages of draining evolves from Bragg peaks to a Kiessig fringe structure indicate stratification within the thin foam film.
1. Introduction Foams are a key component of many systems; foams occur in commonplace products such as shampoos, beer, mattresses, bread, ice cream, or cork, and in industrial applications such as firefighting, mineral flotation, paper deinking, or decontamination of radioactive components. In many applications the effectiveness of the foam is dependent on its macroscopic stability, a property which in turn is closely related to the structural, elastic, or rheological properties of individual foam lamellae or its interfacial films. Over the past few years, some puzzling features of relevance to foam films and foam stability have been observed in polyelectrolyte/surfactant complexes in aqueous systems; thin-film balance and direct force measurements have demonstrated the presence of oscillatory force profiles, appearing as a result of internal structure, in thin aqueous films.1-3 In some cases these structural features have been correlated with organization at interfaces or in the bulk, via surface tension, neutron, or X-ray scattering data, and may result in steric stabilization of foam films. Furthermore, very small quantities of a polyelectrolyte may stabilize an otherwise unstable foam formed from an oppositely charged surfactant.4 Here the stabilizing effect is more likely a change in surface viscoelasticity, rather than a steric effect in the film. In these examples, lack of structural information (beyond layer thicknesses) from the interior of the film prevents firm conclusions about the origin of the observed effects * To whom correspondence should be addressed. Present address: Department of Physics and Measurement Technology, Linko¨ping University, SE-581 83 Linko¨ping, Sweden. E-mail:
[email protected]. † Part of the Langmuir special issue dedicated to neutron reflectometry. (1) Bergeron, V.; Langevin, D.; Asnacios, A. Langmuir 1996, 12, 1550. (2) Klitzing, R. v.; Espert, A.; Asnacios, A.; Hellweg, T.; Colin, A.; Langevin, D. Colloids Surf., A 1999, 149, 131. (3) Bergeron, V.; Claesson, P. M. Adv. Colloid Interface Sci. 2002, 96, 1. (4) Asnacios, A.; Klitzing, R. v.; Langevin, D. Colloids Surf., A 2000, 167, 189.
to be drawn and highlights the need for better methods for in situ structural characterization of foam films. Several structural studies using X-rays have been made in the past,5-8 with a particularly prominent contribution by Benattar et al.9-12 It appears that most experimental difficulties regarding measurements on foam films with X-rays have been overcome, making it the preferred method for structural characterization of Newton black films, but the level of detail provided by X-ray reflectivity experiments will be limited for multicomponent mixtures or complex systems, because the contrast (i.e., the electron density variation) between different components of the film is often insufficient. IR13-15 and Raman16,17 spectroscopic methods have also contributed valuable information about structure and organization in foam films, but these methods lack the specificity to distinguish between contributions from different components in complex multilayer structures. Considering the success with which neutron reflectivity has been applied to the study of surface and interfacial structure,18,19 it seems natural to renew efforts to apply this method also to foam films. Neutrons are particularly (5) Dasher, G. F.; Mabis, A. J. J. Phys. Chem. 1960, 64, 77. (6) Clunie, J. S.; Corkill, J. M.; Goodman, J. F. Discuss. Faraday Soc. 1966, 42, 34. (7) Platikanov, D.; Graf, H.; Weiss, A. Colloid Polym. Sci. 1990, 268, 760. (8) Platikanov, D.; Graf, H.; Weiss, A.; Clemens, D. Colloid Polym. Sci. 1993, 271, 106. (9) Be´lorgey, O.; Benattar, J. J. Phys. Rev. Lett. 1991, 66, 313. (10) Benattar, J. J.; Schalchli, A.; Be´lorgey, O. J. Phys. I 1992, 2, 955. (11) Guenoun, P.; Schalchli, A.; Sentenac, D.; Mays, J. W.; Benattar, J. J. Phys. Rev. Lett. 1995, 74, 3628. (12) Schalchli, A.; Sentenac, D.; Benattar, J. J.; Bergeron, V. J. Chem. Soc., Faraday Trans. 1996, 92, 553 and 2317. (13) Tian, Y. Langmuir 1992, 8, 1354. (14) Tano, T.; Umemura, J. Langmuir 1997, 13, 5718. (15) Zhang, Z. Q.; Liang, Y. Q. J. Colloid Interface Sci. 1995, 169, 220. (16) Lecourt, B.; Capelle, F.; Adamietz, F.; Malaplate, A.; Blaudez, D.; Kellay, H.; Turlet, J. M. J. Chem. Phys. 1998, 108, 1284. (17) Capelle, F.; Lhert, F.; Blaudez, D.; Kellay, H.; Turlet, J. M. Colloids Surf., A 2000, 171, 199. (18) Penfold, J. Curr. Opin. Colloid Interface Sci. 2002, 7, 139. (19) Lu, J. R.; Thomas, R. K.; Penfold, J. Adv. Colloid Interface Sci. 2000, 84, 143.
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well suited for structural studies of aqueous foam films; the differences in scattering length density (SLD) of protonated and deuterated material is significant, there is little diffuse scattering from bulk water, and the probing of two interfaces simultaneously enhances the sensitivity. In a pioneering study, Highfield20 used a vertical arrangement to study SDS/dodecanol black films and demonstrated that the aqueous core thickness can be determined directly from reflectivity data, but they were unable to maintain Newton black films for long enough times to acquire reflectivity profiles of a quality permitting details of the internal structure of such films to be studied. In this paper, we demonstrate how neutron reflectivity can be used to monitor structural characteristics of draining foam films, and we also address some methodological aspects of this type of measurement. The foam film is oriented horizontally to slow film drainage (and to minimize gravitational effects) and hence allows acquisition of reflectivity profiles of reasonable quality throughout the thinning process. Rather than starting with polyelectrolyte/surfactant mixtures, which are complex, we chose to perform an initial series of test experiments using a pure surfactant, Aerosol-OT, whose interfacial properties at the air/aqueous solution interface are well characterized and have been reported in previous publications.21-25 In brief, these results show that at bulk concentrations below the critical micellar concentration (cmc), AOT forms a surface layer whose thickness does not vary with concentration between cmc/300 and cmc, while the coverage varies between 132 and 78 Å2/molecule.23 Up to 10 × cmc, no features other than a surface monolayer are observed, while at ×20 cmc and higher concentrations, diffraction peaks appear in the reflectivity profiles, corresponding to multilayer structures with 175-180 Å layer spacing.21 At these concentrations, the lamellar and the homogeneous phases coexist, and the former apparently adsorbs to air/ aqueous solution interfaces, although the lamellar spacing at the surface is systematically shorter than for the bulk phase.24 Furthermore, the adsorbed lamellar phase gives strong off-specular scattering, characteristic of conformal roughness in the multilayer stacks, a consequence of large amplitude Helfrich undulations rather than air/water interfacial roughness.25 Direct application of these results to foam films must be made with caution, since the effects of the changing concentration in a foam film, from which water and surfactant might drain at different rates, the influence of the external pressure imposed by the second air/solution interface, or the kinetics of adsorption or formation of the lamellar phase at the interface cannot be properly evaluated. However, it appears sensible to use the results obtained for the structure near the air/solution interface as a starting point for modeling of the internal AOT foam film structure. 2. Experimental Section Sodium bis(2-ethylhexyl) sulfosuccinate (Aerosol-OT or AOT) (Sigma, >98%) was used as received to prepare 5 mM AOT solutions in D2O, from which the foam films were formed. (20) Highfield, R. R.; Humes, R. P.; Thomas, R. K.; Cummins, P. G.; Gregory, D. P.; Mingins, J.; Hayter, J. B.; Schaerpf, O. J. Colloid Interface Sci. 1984, 97, 367. (21) Li, Z. X.; Lu, J. R.; Thomas, R. K.; Penfold, J. Faraday Discuss. Chem. Soc. 1996, 104, 127. (22) Li, Z. X.; Lu, J. R.; Thomas, R. K. Langmuir 1997, 13, 3681. (23) Li, Z. X.; Lu, J. R.; Thomas, R. K.; Penfold, J. J. Phys. Chem. B 1997, 101, 1615. (24) Li, Z. X.; Weller, A.; Thomas, R. K.; Rennie, A. R.; Webster, J. R. P.; Penfold, J.; Heenan, R. K.; Cubitt, R. J. Phys. Chem. B 1999, 103, 10800. (25) Li, Z. X.; Lu, J. R.; Thomas, R. K.; Weller, A.; Penfold, J.; Webster, J. R. P.; Sivia, B.; Rennie, A. R. Langmuir 2001, 17, 5858.
Ederth and Thomas
Figure 1. The film-supporting frame, perspective and a crosssectional view. The ceramic fritsand the glass tube fused to itsacts as a reservoir, and is in capillary contact with the foam film, which forms from a sharp edge at the upper side of the steel frame.
Figure 2. Schematic view of the airtight Perspex cell containing the foam film and the surfactant solution. The foam films were suspended from knife edges on the upper side of a stainless steel frame in capillary contact with a porous frit, onto which a glass tube is attached to provide a reference pressure outside the cell (Figure 1). The frame and part of the frit were submerged in the AOT solution by lifting a Teflon trough containing the solution. When the trough was lowered again after allowing the frit to saturate in the solution for ca. 30 min, a foam film was produced in the metal frame. At this stage, the film thickness is inhomogeneous, as inferred from brightly colored and clearly visible interference patterns, but within about an hour, the colors have disappeared from the film, which then appears light gray. From this stage, the film was very stable and drainage slow. Under favorable conditions, thinning proceeds to thicknesses less than 200 Å over more than a day. A sealed Perspex cell with fully automated pressure adjustment was used to provide an equilibrated environment for the foam films; see Figure 2. The general principle of the cell is adapted from the thin-film balance described by Bergeron and Radke26 but modified to suit the particular demands of neutron reflectivity experiments; it has quartz entry and exit windows for the neutron beam and a film size with large enough area to enable reasonable time resolution in the measurements. For future experiments, the aim is to incorporate also an interferometer into the system, to provide an independent measure of the film thickness during the reflectivity experiments. The pressure in the cell is adjusted with a syringe pump constructed around a 300 mm linear actuator (SKF CARE33), driving a piston inside a Perspex tube. The piston is sealed to the interior of the tube by an O-ring and vacuum grease. The pressure is monitored with a 6 kPa differential pressure (26) Bergeron, V.; Radke, C. J. Langmuir 1992, 8, 3020.
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transducer with ∼0.1% resolution (Omega PX635). A custombuilt controller keeps the pressure at a preset value by averaging the pressure over a certain time interval and adjusting the position of the piston in the syringe if required. A stability of (2 Pa is easily achieved over the entire pressure range of the sensor. Due to the long response time of the system, this arrangement gives better stability than a conventional PI controller with continuous adjustment of the pressure. Higher precision within a narrower pressure range can be achieved by connecting a large dead volume between the syringe and the cell. The experiments described here were performed at zero applied pressure, and the control system was only used to keep the pressure inside the hermetically sealed system equal to the reference pressure outside the cell. Applying a pressure to the film markedly increases the drainage rate even for large films (for an example, see further down), though since our interest was mainly in monitoring the structural evolution in the film during drainage, we tried to keep drainage as slow as possible. The specular neutron reflectivity, R, is measured as a function of the wave vector transfer Q
Q)
4π sin θ λ
(1)
directly from a surfactant solution. The advantages of this procedure are ease of preparation and rapid drainage, though in our case a horizontal film is essential, since one of the objectives is to enable control of the film thickness via the applied pressure. However, the potential influences of gravity and thermal fluctuations on the film must be assessed, since the size of the film enhances these effects. Our soap film is a rectangular membrane, for which the two-dimensional wave equation provides an appropriate description under the following assumptions: the deflections are small compared to the dimensions of the film, the weight of the film is small compared to the film tension, and the film tension remains constant upon deflection. These criteria are met for the thin foam films under investigation, and the only external force acting on the film is gravity. The film is rigidly attached to the supporting frame and thus subject to homogeneous Dirichlet boundary conditions. If the soap film extends in the xyplane on the region 0 < x < a, 0 < y < b (see Figure 1) and deflections occur along the z-direction, the boundary value problem for the displacement function z ) z(x,y,t) is
where θ is the glancing angle of incidence and λ the neutron wavelength. R(Q) is related to the neutron scattering length density profile perpendicular to the surface, F(z), via its Fourier transform, Fˆ (Q)
R(Q) )
2
16π |Fˆ (Q)|2 Q2
(2)
However, rather than fitting models of F(z) using this relation, the reflectivity can be calculated directly using optical matrix methods for layered structures27 and the fit to the data optimized with respect to the thickness and scattering length density of each layer and the roughness of their interfaces; this method is particularly suitable for models of the moderate complexity required in this study. The reflectivity experiments were performed on the SURF28 and CRISP29 time-of-flight reflectometers, facing the 25 K hydrogen moderator at the ISIS pulsed neutron source (Didcot, U.K.). Measurements were made at a 1.5° angle of incidence and at approximately 6% resolution (smaller angles of incidence were not used due to difficulties in aligning the beam against a sample with such low reflectivity as the foam films) and over the Q range 0.05-0.5 Å-1 (corresponding to λ between 0.6 and 6.5 Å). Data beyond 0.25 Å-1 are not plotted, because the signal approaches the background noise level of the experiment. Reflectivity profiles were collected during periods of 10 or 15 min. Acquired data was normalized against the reflectivity of a liquid D2O sample, and a flat background of 5 × 10-7 was subtracted from all reflectivity profiles before proceeding with further analysis of the data. The assumption of a flat background is invalid whenever there is strong off-specular scattering (which might very well be the situation, judging from the results at the air/aqueous AOT solution interface, as outlined in the Introduction), though the 2D data required for proper assessment of the consequent errors are not yet available for the AOT foam films. All experiments were performed at 20 °C.
3. Properties of Macroscopic Soap Films Of the several experiments that have been conducted on macroscopic soap films with X-ray, neutron, or infrared methods in the past, most have used vertical films drawn (27) Heavens, O. S. Optical properties of thin solid films; Butterworths: London, 1955. (28) Penfold, J.; Richardson, R. M.; Zarbakhsh, A.; Webster, J. R. P.; Bucknall, D. G.; Rennie, A. R.; Jones, R. A. L.; Cosgrove, T.; Thomas, R. K.; Higgins, J. S.; Fletcher, P. D. I.; Dickinson, E.; Roser, S. J.; McLure, I. A.; Hillman, A. R.; Richards, R. W.; Staples, E. J.; Burgess, A. N.; Simister, E. A.; White, J. W. J. Chem. Soc., Faraday Trans. 1997, 93, 3899. (29) Penfold, J.; Ward, R. C.; Williams, W. G. J. Phys. E 1987, 20, 1411.
∇2z -
1 ∂2z g ) c2 ∂t2 c2
0 < x < a,
(3)
0
