Experimental Evidences of Self-Assembly in Foam Films from

Jan 15, 2003 - Systematic foam film experiments are performed with amphiphile solutions. Specific film parameters are extracted which are related to t...
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Langmuir 2003, 19, 1215-1220

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Experimental Evidences of Self-Assembly in Foam Films from Amphiphilic Solutions Plamen Tchoukov, Elena Mileva,* and Dotchi Exerowa Institute of Physical Chemistry, Bulgarian Academy of Sciences-Sofia 1113, “Acad.G.Bonchev” Str., bl.11, Bulgaria Received August 13, 2002. In Final Form: October 23, 2002 Systematic foam film experiments are performed with amphiphile solutions. Specific film parameters are extracted which are related to the film lifetimes and reflect the coupling of film dynamics and the reorganization of surfactant assemblies present both in the film bulk and in the adsorption layers on its interfaces. A distinct qualitative sign of the above-stated coupling is the onset of peculiar unstable black patterns (black dots and “black spots”). Their occurrence is correlated with the plateau regions of the surface tension isotherms of the initial surfactant systems. The enhanced onset of the unstable black formations results in a sharp increase of the foam film lifetimes. This effect is interpreted as related to the disintegration of the amphiphilic structures in the process of film drainage. These experimental results are viewed as experimental evidences for the presence of amphiphilic structures in the primary surfactant solutions.

I. Introduction Throughout the past decade, an appreciable progress has been achieved in the experimental investigation and the theoretical modeling of micellar solutions.1-7 The most important outcome of these studies is the understanding that the onset and the reorganization of self-assembled structures are cooperative phenomena reflecting the delicate balance between intraaggregate and interaggregate interactions. The labile character of the surfactant assemblies brings about serious difficulties in the identification of these species. The prevailing number of experimental evidences of self-assemblies is related to overall bulk properties of the surfactant systems.5,6,8 It is necessary, however, to look for more varied methods that can address the characteristic properties of the amphiphilic structures and of the self-assembling phenomenon itself in a more diverse manner. The presence of a phase boundary in a solution with bulk surfactant structures influences the assembled entities, and the size distribution in close vicinity of the interface is different as compared to the homogeneous bulk.9 This difference potentially contains valuable structural information about the specific surfactant aggregation properties. There are numerous studies on amphiphilic assemblies at solid boundaries.10-14 Still, there is an * Address correspondence to this author. Tel: (+359 2) 979 3583. Fax: (+359 2) 971 2662. E-mail: [email protected]. (1) Curr. Opin. Colloid Interface Sci. 1996, 1, (3). (2) Curr. Opin. Colloid Interface Sci. 1997, 2, (4). (3) Rajagopalan, R. Curr. Opin. Coll. Interface Sci. 2001, 6, 357. (4) Gelbart, W.; Ben-Shaul, A. J. Phys. Chem. 1996, 100, 13169. (5) Structure and Dynamics of Strongly Interacting Colloids and Supramolecular Aggregates in Solution; Chen, S., Huang, J., Tartaglia, P., Eds.; NATO ASI Series, Ser. C: Mathematical and Physical Sciences, Vol. 369; Kluwer Academic: Dordrecht, The Netherlands, 1992. (6) Micelles, membranes, microemulsions and monolayers; Gelbart, W., Ben-Shaul, A., Roux, D., Eds.; Springer-Verlag: New York, 1994. (7) Russanov, A. Micellization in surfactant solutions; Chemistry: Sanct Petersburg, 1992 (in Russian). (8) Tanford, C. The hydrophobic effect: Formations of micelles and biological membranes; Wiley: New York, 1980. (9) Israelachvili, J. Langmuir 1994, 10, 3774. (10) Gu, T.; Zhu, B.; Rupprecht, H. Prog. Colloid Polymer Sci. 1992, 88, 74. (11) Niu, S.; Gopidas, K.; Turro, N.; Gabor, G. Langmuir 1992, 8, 1271. (12) Jonsson, B.; Wa¨ngnerud, P.; Jo¨nsson, B. Langmuir 1994, 10, 3542.

insignificant number of experiments with fluid interfaces.15,16 The latter, however, induce “milder” experimental conditions in their vicinities and are expected to allow more accurate differentiation between the specific adsorption effects on one hand and the solvent-mediated self-assembling phenomena on the other hand. Foam films are thin liquid layers bounded by two air/ liquid interfaces. The specific kinetic and thermodynamic properties of these films, and the disjoining pressure on the first place, create additional options for the impact on existing micellar entities. Accordingly, in the course of formation and the time evolution of a film that originates from surfactant system containing self-assembled structures, the immediate neighborhood of each amphiphile molecule and of every aggregate are essentially changed. So, the initial size distributions are expected to be altered and structural reorganizations and disintegrations are anticipated. The impact of these events might routinely be registered as specific thinning characteristics of the foam films via the well-developed microinterferometric techniques.17-19 The major goal of the current paper is to present the results from a series of new systematic investigations aiming at the examination of the influence of amphiphilic structures on foam film drainage behavior. Specific foam film properties are reported, particularly referring to thinning kinetics, which are interpreted as related to the presence and the reorganization of self-assembled aggregates. II. Materials and Instrumentation The foam films are investigated via the microinterferometric method of Scheludko-Exerowa.17,18 The ex(13) Tiberg, F.; Jo¨nsson, B.; Langmuir 1994, 10, 3714. (14) Manne, S.; Cleveland, J.; Glaub, H.; Stucky, G.; Hansma, P. Langmuir 1994, 10, 4409. (15) Staples, E.; Tucker, I.; Penfold, J.; Warren, N.; Thomas, R.K. Langmuir 2002, 18, 5139. (16) Staples, E.; Tucker, I.; Penfold, J.; Warren, N.; Thomas, R. K.; Taylor, D. J. F. Langmuir 2002, 18, 5147. (17) Scheludko, A. Adv. Colloid Interface Sci. 1967, 1, 391. (18) Exerowa, D.; Kruglyakov, P. Foam and Foam Films; Elsevier: Amsterdam, 1998. (19) Exerowa, D.; Kolarov, T.; Khristov, Khr. Colloids Surf. 1987, 22, 171.

10.1021/la020713q CCC: $25.00 © 2003 American Chemical Society Published on Web 01/15/2003

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Figure 1. Microinterferometric experimental setup with the measuring cell of Scheludko-Exerowa: 1, measuring cell in thermostating device; 2, microscope; 3, CCD video camera; 4, photomultiplier; 5, PC with capture video card; 6, Y(t) recorder.

perimental setup is schematically shown on Figure 1. Microscopic films are formed in the middle of a biconcave drop, situated in a glass tube of a diameter 0.4 cm, by withdrawing the liquid from it. In the lowest surfactant concentrations and the blank probe, a modification with an additional solution reservoir next to the film meniscus is used.18 The classical experimental scheme is additionally modified with a video registration via CCD camera (SONY, DXC-107P). The digitized image is processed with powerful computer using a capture video card. The experiments are performed exclusively with sodium dodecyl sulfate. This ionic surfactant was specially synthesized for us by Henkel KGaA, Germany and does not display a minimum in the surface tension isotherm. Tridistilled water with electrical conductivity k ) 1.0/1.1 × 10-6 Ω-1 cm-1 is used. The added electrolyte is sodium chloride “Suprapur” (Merck), heated at 600 °C before the experiments. All experiments are done with solutions of 0.5 M NaCl and of various surfactant concentrations. The temperature is maintained strictly at 22 ( 0.1 °C. The experimental procedure is accomplished according to the following schedule: For each surfactant concentration, three to four series of film experiments are performed. Each separate set within a series consists of about 50 films. Their radii are kept within the range of about 10-2 cm. The exact radius (rf) of each film is determined from the last snapshot of the video recording. The lifetimes of the foam films are measured as the time between the onset of the film and its rupture. III. Results The key idea in the choice of the amphiphilic substance (sodium dodecyl sulfate) is to make use of a surfactant with amply balanced hydrophilic-hydrophobic characteristics and whose adsorption and bulk solution properties have been extensively studied and well-known. A specific advantage of the microinterferometric technique is that the size of the film allows the possibility to deal with very low surfactant concentrations of the initial amphiphilic solutions. This feature gives a possibility to trace back the conditions for the initial onset of micellar structures. With this in mind, the foam films studied here are obtained from surfactant solutions that are within the so-called low and intermediate concentration range. The latter interval is defined as being orders of magnitude lower than both the critical micellar concentration values (CMC) and the close-packing values (denoted with Γ∞), but the surfactant quantity exceeds the scope of Henry law. As

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Figure 2. Surface tension isotherm of sodium dodecyl sulfate solutions at electrolyte concentrations (NaCl) Cel ) 0.5 mol/l and temperature t ) 22 °C.

was previously shown for a series of low molecular amphiphilic substances, the surface tension measurements in aqueous solutions within the above-mentioned concentration range are often characterized with certain peculiarities:20-23 the surface tension isotherms usually contain kinks and plateau portions (Figure 2). The data were obtained by the high-precision spherotensiometric technique (accuracy (0.005/(0.01 mN m-1).24 It is well-established that there is a close relationship between thin liquid film formation and the properties of the surfactant adsorption layers.18,19,25 A manifestation of this interrelation is the onset of various black patterns in the process of film thinning. Systematic investigations of microscopic foam films from amphiphilic systems within the concentration range, which is much lower than the starting value of the usual black-spot and black-film formation, have not been performed. The only observation that was reported in this connection was the onset of the so-called “black dots”.18,26 The latter are miniature nonspreading spots with radii of about 5 µm. They appear when the thickness of the thin liquid layer diminishes to about 35 nm and the films become unstable. Here are reported the results from extensive studies of the evolution of foam films originating from surfactant solutions of the above-mentioned concentration range. The basic qualitative outcome is the following: in the course of the foam film drainage two types of unstable black pattern formations may be distinguished, namely, black dots and “black spots”. If the surfactant concentration is low, black dots do persistently appear.27 At higher amphiphile quantities covering the true horizontal part of the surface tension curve (Figure 2), larger unstable black formations are observed. We label them as “black spots”. One should beware, however, that these are not the well-studied black spots known as precursors of the usual black films and which have been extensively studied in the past.18,25,28 These “spots” are obtained at high surfactant concentrations when close packing (Γ∞) of the (20) Exerowa, D.; Scheludko, A. Bull. Inst. Phys. Chem. 1963, 3, 79. (21) Exerowa, D.; Nikolov, A. In Surfactants in Solution; Mittal, K. L., Ed.; Plenum Press: New York, 1984; Vol. 4, p 1313. (22) Nikolov, A.; Martynov, G.; Exerowa, D.; Kaishev W. Colloid J. (in Russian) 1980, 62, 672. (23) Nikolov, A.; Martynov, G.; Exerowa, D. J. Coll. Interface Sci. 1981, 81, 116. (24) Scheludko, A.; Nikolov, A. Colloid Polym. Sci. 1975, 253, 404. (25) Exerowa, D.; Kashchiev, D.; Platikanov, D.; Toshev, B. Adv. Colloid Interface Sci. 1994, 49, 303. (26) Exerowa, D.; Nikolov, A.; Zacharieva, M. J. Coll. Interface Sci. 1981, 81, 419. (27) Mileva, E.; Exerowa, D.; Tchoukov, P. Colloids Surf., A 2001, 186, 83. (28) Exerowa, D.; Kashchiev, D.; Platikanov, D. Adv. Colloid Interface Sci. 1992, 40, 201.

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Figure 3. Evolution of unstable black pattern formations. Black dot (a-c) and the results of image analysis (d).

Figure 4. Evolution of unstable black pattern formations. “Black spot” (a-c) and the results of image analysis (d).

adsorption layers is ensured and the films containing them evolve into stable black films (Newton Black Films (NBF) for the conditions of the present experiment). The unstable black spots, in contrast, appear at much lower surfactant concentrations. These “spots” are related by genesis to the black dots but they increase in dimensions. The respective foam films are also unstable and rupture very soon thereafter. Both of the observed black patterns continue living just for seconds while they move (sometimes vigorously) within the background films. On Figures 3 and 4 are presented typical photos of the evolution of foam films and the results of the respective image analysis. The consecutive snapshots show the

moment of onset of the respective black formation (Figures 3a, 4a), an intermediate position (Figures 3b, 4b), and the last moment before film rupture (Figures 3c, 4c). As is to be seen on the photos (Figure 3a-c), the dots live for a relatively long time, 3-4 s, and do not grow in size. The juxtaposition of the surfactant concentration intervals of surface tension isotherm peculiarities (Figure 2) and of black-dot registration gave us grounds to consider the onset of the latter as an indicator for the presence of amphiphilic structures in the initial surfactant solutions from which the respective foam films are formed.27 So far as the onset and the evolution of the “black spots” are concerned, their lifetime is much shorter as compared to

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Figure 5. Raw lifetime results of foam films as function of film radii for various concentrations of initial sodium dodecyl sulfate solutions.

those of the dots (compare time scale on Figures 3a-c and 4a-c) and they grow considerably in size within just 1 s. The morphological difference between the two black formations is also quite well-distinguished. The black dots (Figure 3d) are shallow local thinnings with irregular thickness. The “spots” are thinner black patterns and may often be viewed as plane parallel portions (microfilms) within the background foam film (Figure 4d). So far as the investigated foam films originate from

surfactant solutions of concentrations that are lower than both the close-packing value (Γ∞) and the CMC, they all drain quickly and rupture in about a minute. Video recording and image analysis, however, permit closer examination of the drainage behavior. The raw results for the overall film lifetime against the film radius for a variety of concentrations are shown in Figure 5. Generally, the experiments run as expected,18,29 with a gradual growth of the lifetimes with increase of film radii for each

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Figure 7. Mean lifetimes of foam films against the surfactant concentration of initial sodium dodecyl sulfate solutions. The frame on the abscissa marks the concentration values of the plateau region from the surface tension isotherm.

IV. Discussion

Figure 6. (a) Slope of the linear fit as a function of surfactant concentration. (b) Relative standard deviation as a function of surfactant concentration. The frame on the abscissa marks the concentration values of the plateau region from the surface tension isotherm.

surfactant value. The lines represent the best linear fit: τ ) A + tgRrf, where A, tgR are the parameters of linear regression analysis. Evident change of the slope of the fit with the raise of the concentration in the scope of the plateau region of the respective surface tension isotherm (Figure 6a) is observed. The relative scatter of the results is presented in Figure 6b. It is characterized with the standard deviation from the linear fit, namely, SD )

x∑(τi-A-tgRri)2/(n-2). As can be seen, the run of the

relative standard deviation (SD/τm) against the surfactant concentration is a clear-cut sign for the reproducibility of the results of the foam film thinning process within the chosen surfactant concentration range. In Figure 7 are shown the cumulative results for the dependence of the mean lifetime (τm) taken from the linear fits against the concentration of the initial surfactant solutions for film radii of 100 µm. For the lower surfactant content, the curve has a distinct plane portion (τm ∼ 15 s). The latter is followed by a sharp increase for higher surfactant quantities. This concentration dependence is completely correlated with the respective plateau region in the surface tension isotherm, as it is shown in Figure 2. The major result of the film evolution studies is that within the surfactant concentration range, which outlines the plateau region of the surface tension isotherms, the foam film lifetimes exhibit a marked transition from lowlifetime values to high-lifetime values. In very low concentrations in the initial solutions (Henry region), the lifetimes are about τm ∼ 3-5 s and none of the above-described unstable black patterns is observed there.18 (29) Exerowa, D.; Kolarov, T. Ann. Univ. Sofia 1964/65, 59, 207.

The innate assumptions which constitute the basis of the conventional film drainage theory17,18,30-33 are the following: (1) The evolution of foam films is determined by the coupling of film hydrodynamics and the mass transfer of the surfactant; (2) The overall lifetime depends also on the time evolution of a film with critical thickness of about 35 nm. In foam films, these models presuppose the tangential mobility of the interfaces, as well as the notion that any surfactant self-assembly is excluded from the interpretation of the results. Thus, the general trend depending exclusively on the geometrical anisodiametry of the film flow is the following. In the drainage process, the radial outflow in the film carries the surfactant toward its periphery. The result is the onset of a surface tension gradient which evokes a tangential force opposite to the direction of the initial interface mobility of the film. To compensate this extra force, new quantities of the surfactant are to move from the bulk to the interfaces. In plane parallel foam films and if the surfactant nonassociation is true, this bulk flux comes from the meniscus regions. The raise of the surfactant concentration is expected to ease the compensation mechanism and to lead to a decrease of the film lifetime with the increase of the surfactant concentration. In our experiments, however, just the opposite tendency is registered (see Figure 7). These observations suggest the notion that, although completely general, the conventional film drainage picture seems insufficient for the explanation of the reported results. The clue in the resolution of this insufficiency lies, in our opinion, in the careful juxtaposition of the foam film experiments and the surface tension measurements within the same scope of the surfactant concentration. This concurrence evokes the idea that the concept of nonassociative state of the amphiphilic molecules should be reconsidered. The starting point of the interpretation of the experimental observations is the understanding that at specific experimental circumstances smaller bulk self-assembled structures (premicelles11,34,35) may appear at surfactant concentrations lower than the CMC-values.21-23,36,37 So, (30) Radoev, B.; Scheludko, A.; Manev, E. J. Coll. Interface Sci. 1983, 95, 254. (31) Dimitrov, D. Prog. Surf. Sci. 1983, 14, 295. (32) Mileva, E.; Radoev, B. Colloids Surf., A 1993, 74, 259. (33) Mileva, E.; Nikolov, L. Colloids Surf., A 1993, 74, 267. (34) Vold, M. Langmuir 1992, 8, 1082. (35) Brinchi, L.; Di Profio, P.; Germani, R.; Giacomini, V.; Savelli, G.; Bunton, C. Langmuir 2000, 16, 222. (36) Mileva, E.; Exerowa, D. Colloids Surf., A 1999, 149, 207.

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Figure 8. Experimental results for the critical thickness of foam films against the surfactant concentration of initial sodium dodecyl sulfate solutions. The frame on the abscissa marks the concentration values of the plateau region from the surface tension isotherm.

the foam films are considered as obtained from initial surfactant solutions that already contain self-assembled entities. The film itself is viewed as a system composed of a bulk phase and two 2D surface phases (air/liquid interfaces).36-38 Each of these phases may contain amphiphile structures which are characterized by bulk and interface size distributions, respectively. The drainage of foam films is essentially determined by the disjoining pressure (Π(h) * 0).17,18 In the present case, the electrolyte concentration is high and the van der Waals component (ΠVW) dominates the disjoining pressure. As was already shown in previous papers,27,36,37 the account for this specific “surface force” in the model scheme of the self-assembly predicts a rearrangement of the existing surfactant structures, both in the bulk of the film and at the interfaces. In thinner films, the destruction of the self-assembled entities is enhanced. Thus, the number of the free monomers increases upon film drainage. It is reasonable to expect that the newly released monomers would serve as additional reservoir to the air/liquid interface thus ensuring better coverage on the interfaces. When the film thins to the so-called critical thickness (of about 30-40 nm), it becomes unstable and thermal fluctuations bring about domains of irregular depths. The critical film thickness may be estimated according to Scheludko hypothesis:17 4

hcr ) x3Kλ2/64σb where λ ∼ 0.1rf is the length of the wave that has appeared because of the instability of the thin liquid film, rf is the radius of the foam film, and K is the van der WaalsHamaker constant. In Figure 8 are shown the experimental results for the critical thickness within the investigated concentration range. It remains virtually constant, and distinct peculiarities are not observed. So, the onset of the thermal fluctuations alone cannot cause the sharp lifetime increase as shown in Figure 7. These fluctuations, however, are closely related to the overall reorganization and restructuring of the self-assembled entities in the thinning foam film. The mechanism is the following: In the thinner films, the existing aggregates are continuously destroyed setting more free amphiphiles. (37) Mileva, E.; Exerowa, D. In Emulsions, Foams and Thin Films; Mittal, K. L., Kumar, P., Eds.; Marcel Dekker: New York, 1999; Chapter 15. (38) de Feijter, J. In Thin Liquid Films; Ivanov, I. B., Ed.; Marcel Dekker: New York, 1988; Chapter 1.

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Thus, conditions are ensured for the outbreak of still narrower quasi-equilibrium portions within the film. On the interfaces of these thinner places, the monomer molecules may form highly packed adsorption layers. These regions are registered via the microinterferometric techniques as unstable black patterns (dots and spots) (Figures 3-4). As already discussed,27 black dots appear when the electrolyte quantity is high enough so that an effective suppression of the electrostatic component of the disjoining pressure is achieved. Thus, at least in principle, it becomes possible for black pattern formations to appear.18 The total amphiphile quantity, however, remains largely insufficient and cannot ensure the dense coverage of the whole film interfaces. Therefore, at lowest concentration of the initial surfactant solutions only black dots are registered. Such films are exceedingly unstable and break up in a very short time. At higher concentrations, the dots may grow and embrace larger regions of the draining film. So far as the surfactant concentration is still much lower than the value corresponding to a uniform dense adsorption layer, these “spots” are also unstable and the films that contain them rupture soon thereafter. The “black spots” may be viewed as microfilms within the background foam film. Because of the substantial local slowdown of the outflow in the places where they outburst, a sensible delay of the overall drainage process is observed. Locally, within these microfilms, the mechanism of this retardation is the same as in the conventional plane-parallel films.17,18,30-33 It is related to the specific coupling of the hydrodynamics and the mass transfer of the amphiphile in the region where the spot appears. The presence of surfactant structures in the regions, adjacent to the spot, provides the source of monomers coming from the disintegration of the aggregates. During the film thinning, these monomers feed up the interfaces of the neighboring spot. This surfactant flow guarantees the tangential surface tension gradient that retards the further local thinning inside the spot region. For increased surfactant concentrations, the number of such spots is raised. The larger the number of these spots, the higher is the lifetime of the respective foam film. This mechanism ensures a sharp raise of the lifetime within a narrow concentration range. The latter is somewhat restricted because the specific feeding mechanism of the spots interfaces can be warranted just for a particular coupling of hydrodynamics and mass transfer of the surfactant coming from the disintegration of the amphiphilic structures. The major results of the present investigation may be resumed as follows: Specific film parameters are extracted from the systematic foam film experiments with initial solutions of low and intermediate surfactant concentration. They are related to the film lifetimes and reflect the coupling of film dynamics and the reorganization of existing surfactant assemblies both in the film and in the adsorption layers on its interfaces in the process of film drainage. The particular qualitative sign of above-stated coupling is the onset of various unstable black patterns (dots and spots). Their occurrence is correlated with the plateau regions in the surface tension isotherms of the initial surfactant solutions. All these results may be considered as experimental evidences for the presence of amphiphilic structures in the initial surfactant solutions. LA020713Q