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Foam Films Stabilized by Dodecyl Maltoside. 1. Film Thickness and Free Energy of Film Formation RM. Muruganathan, R. Krustev,* H.-J. Mu¨ller, and H. Mo¨hwald Max-Planck Institute of Colloids and Interfaces, Am Mu¨ hlenberg 1, Golm 14476, Germany
B. Kolaric and R. v. Klitzing Stranski Laboratory for Physical and Theoretical Chemistry, TU Berlin, Strasse des 17 Juni 112, 10623 Berlin, Germany Received March 4, 2004. In Final Form: May 9, 2004 Foam films stabilized by a sugar-based nonionic surfactant, β-dodecyl maltoside, are investigated. The film thickness and the film contact angle (which is formed at the transition between the film and the bulk solution) are measured as a function of NaCl concentration, surfactant concentration, and temperature. The film thickness measurements provide information about the balance of the surface forces in the film whereas the contact angle measurements provide information about the specific film interaction free energy. The use of the glass ring cell and the thin film pressure balance methods enables studies under a large variety of conditions. Thick foam films are formed at low electrolyte concentration. The film thickness decreases (respectively the absolute value of the interaction film free energy increases) with the increase of the electrolyte concentration according to the classical DLVO theory. This indicates the existence of a repulsive double layer electrostatic component of the disjoining pressure. An electrostatic double layer potential of 16 mV was calculated from the data. A decrease of the film thickness on increase of the surfactant concentration in the solution is observed. The results are interpreted on the basis of the assumption that the surface double layer potential originates in the adsorption of hydroxyl ions at the film surfaces. These ions are expelled from the surface at higher surfactant concentration.
Introduction Studies on thin foam films are one of the most suitable methods for obtaining information about the interactions between fluid interfaces1 because of their simple geometry and easy preparation. The practical interest in these films stems from the fact that several of their properties are determined by the same forces which also govern the stability of the colloids. Foam films are the basic building blocks of foams, and their properties are similar to those of emulsion films or films in the concentrated dispersions. Studies on foam films have been intensively performed since the middle of the last century. The main part of these studies has been summarized in books and reviews.2-6 Foam films consist of two adsorbed surfactant monolayers separated by an aqueous layer of the solution from which the film is formed. During the formation, first a thick film is established which becomes thinner by drainage of the solution due to the capillary pressure in the meniscus. Finally, equilibrium films are obtained with a uniform thickness depending on the concentration of the ions in the solution. If the films become thinner, they appear black in reflected light and are called black films. Two types of black films exist. The thicker common black film (CBF) appears at lower electrolyte concentrations. Its thickness and stability are controlled by the electrical * Corresponding author. Telephone: ++49 (0) 331 567 9255. Fax: ++49 (0) 331 567 9202. E-mail:
[email protected]. (1) Israelachvili, J. Intermolecular and Surface Forces; Academic Press: London, 1991. (2) Sheludko, A. Adv. Colloid Interface Sci. 1967, 1, 391. (3) Ivanov, IB., Ed. Thin Liquid Films, Surfactant Science Series; Marcel Dekker: New York, 1988. (4) Prud’homme, R.; Kahn, S. Foams; Marcel Dekker: New York, 1996. (5) Exerowa, D.; Kruglyakov, P. Foam and Foam FilmssTheory, Experiment, Application; Elsevier: Amsterdam, 1998. (6) Klitzing, R. V.; Mu¨ller, H.-J. Curr. Opin. Colloid Interface Sci. 2002, 7, 42.
double layer repulsion,5 in agreement with the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory.7,8 The electrostatic double layer repulsion is suppressed at higher electrolyte concentrations, and the equilibrium state is a very thin Newton black film (NBF). Its thickness is independent of the electrolyte concentration and is given by the direct interaction between the surfactant adsorption layers by short-range forces.1,5 The properties of foam films that have been investigated include equilibrium thickness, contact angle film/meniscus, thinning rate, film rupture, and film elasticity. The interaction free energy per unit of the film area ∆gf can be obtained from the contact angle.9-11 β-Dodecyl maltoside (here after β-C12G2) is a sugar-based nonionic surfactant. The hydrophilic head group of the maltoside consists of two sugar rings connected via an ether bond. The sugar-based “natural” surfactants12 are gaining ever growing awareness. They are of low toxicity, are biodegradable, and are widely used in protein solubilization and membrane studies.13 It is expected that the use of sugar surfactants as foaming agents will increase. This makes it necessary to accumulate information about the properties of foam film stabilized with such surfactants. Studies on sugar-based surfactant-stabilized foam films were initiated by Waltermo et al.14 Then a series of works was published dealing mainly with glucoside (one sugar ring) as surfactant.15,16 Recently, Stubenrauch et (7) Derjaguin, B. Theory of Colloids and Thin Films; Consultants Bureau: New York, 1989. (8) Verwey, E. J. W.; Overbeek, J. Th. G. Theory of the Stability of Lyophobic Colloids; Dover Publications Inc.: Minneola, New York, 1948. (9) Toshev, B. V.; Platikanov, D. Adv. Colloid Interface Sci. 1992, 40, 157. (10) Ivanov, I. B.; Toshev, B. V. Colloid Polym. Sci. 1975, 253, 593. (11) De Feijter, J. A.; Vrij, A. J. Colloid Interface Sci. 1978, 64, 269. (12) Holmberg, K. Curr. Opin. Colloid Interface Sci. 2001, 6, 148. (13) Hill, K.; v Rybinski, W., Stoll, G. Alkyl polyglycosides; VCH: Weinheim, 1997.
10.1021/la0494268 CCC: $27.50 © 2004 American Chemical Society Published on Web 06/22/2004
Foam Films Stabilized by Dodecyl Maltoside
al.17 have reported precise disjoining pressure/thickness measurements using the thin film pressure balance (TFPB) technique on foam films stabilized by β-C12G2. The inherent limitation of the TFPB technique is the minimum accessible pressure, which makes it impossible to obtain information about the forces acting between the film surfaces when the repulsive component of the interactions is weak. This is overcome in the present paper by using Scheludko-Exerowa’s5 ring cell, where the capillary pressure necessary for film formation is low. The measurement of the film contact angle formed at the transition between the film and the bulk solution was complementarily performed. This provides information about the specific film interaction free energy under those thermodynamic conditions. In a recent paper,18 we reported first results on studies of the gas permeability of foam films stabilized with β-C12G2. The results have shown an irregular dependence of the gas permeability on the temperature. Further investigations have to be carried out in order to clarify the mechanism of the gas permeability. The gas permeability of foam films depends on the film thickness.19 It increases when the film thickness decreases until a certain thickness. Below that thickness, the permeability is governed only by the adsorption density of the surfactant molecules at the film interfaces.20 This density depends on the bulk surfactant and electrolyte concentrations and additionally on the specific film interaction free energy.21 Further quantification of these effects needs results from measurements on the thickness and the contact angles of foam films stabilized by β-C12G2 at different NaCl concentrations, surfactant concentrations, and temperatures. In this paper, the foam film thickness was measured either as a function of the NaCl concentration while fixing the β-C12G2 concentration or as a function of the β-C12G2 concentration at a fixed concentration of NaCl. These experiments were complemented with measurements of the contact angle under the same conditions. All experiments were performed at a constant temperature of 25 °C. The TFPB technique was used to obtain the dependence of the film thickness on the applied pressure (Π(h) isotherms) under certain conditions, preset by the concentration dependencies. In a third group of measurements, the film thickness and the contact angle were measured as a function of the temperature at constant surfactant and salt concentrations. Experimental Section Substances. The nonionic surfactant dodecyl β-D-maltoside was purchased from Glycon biochemicals (Luckenwalde, Germany) and used without further purification. Sodium chloride (NaCl) GR grade (Merck, Darmstadt, Germany) was roasted at 600 °C for 5 h to remove surface-active contaminations. An Elga Labwater (Germany) purification setup was used to purify the water for the preparation of the solutions. The specific resistance of the water used was 18.2 MΩ cm, the pH was 5.5, and the total organic carbon (TOC) value was 1° contact angles typical for NBF. In the process of approaching the equilibrium film, small black spots appear. The spots grow, coalesce, and finally cover the whole film area. At this moment the film expands (see Figure 2a). The contact angle is calculated from the experimental values of the film radius before the expansion r1 and the film radius after the expansion r2 and the radius of the capillary R ) 2 mm, using the relation
sin θ )
δ22 - δ21 δ2
(2)
where δ1 ) r1/R and δ2 ) r2/R. This relation is valid under the assumption of constant pressure in the meniscus.24 The topographic method30 was used to measure the smaller contact angles typical for CBF. The method is based on the determination of the radii of the interference Newton rings (Figure 2b) when the film is observed in a reflected monochromatic light. Knowing the film thickness, the contact angle was calculated from
Figure 3. Dependencies of the (a) equivalent film thickness hw and (b) contact angle θ (2, topographic method; b, expansion method) on the concentration of NaCl. The β-C12G2 concentration is constant (1.0 × 10-3 M) and above the cmc of 0.17 × 10-3 M for the salt-free solution (see Figure 1). The temperature is constant (25 °C). All lines are only guides for the eye. rings; x2 is the distance between the first and the third Newton rings (Figure 2b); λ ) 456 nm is the wavelength; n is the refractive index of the solution. The process of film formation and obtainment of equilibrium was recorded with a video camera. Data acquisition was performed using a PC equipped with a frame grabber. The film size and the width of the Newton rings were evaluated by an image processing program. x1 and x2 were determined by averaging the data obtained from four measurements with successive 30° rotation around the center of each film. The β-C12G2 surfactant is a good foam stabilizer. Stable foam films are formed from its solution immediately after the filling of the cell and the formation of the biconcave drop in the glass ring. When the film was formed from such a nonequilibrated drop, it forms only very thin black films, irrespective of the surfactant and salt concentration. When the drop was left for equilibration, the thickness of the film was found to increase and the thinning process to slow. Consequently, all the measurements of the film thickness or the contact angle were performed after 2 h of equilibration time. Every experimental point is an average of at least 10 single measurements on different films. Sample standard deviations are shown as error bars in the figures. In all the experiments, the temperature was controlled with an accuracy of (0.1 °C. Additionally, the room temperature was adjusted to be equal to the temperature of the measurement and was kept constant with an accuracy of (0.5 °C.
Results tg2θ ) B2 - 4A(C - h/2)
(3)
where
A)
l 2x1 - x2 ; 2x1x2 x2 - x1
B)
2 l x2 - 2x1 ; 2x1x2 x2 - x1
l C) ; 2
l)
λ 4n
x1 is the distance between the first and the second Newton (27) Claesson, P. M.; Ederth, T.; Bergeron, V.; Rutland, M. W. Adv. Colloid Interface Sci. 1996, 67, 119. (28) den Engelsen, D.; Frens, G. J. Chem. Soc., Faraday Trans. 1973, 1, 237. (29) Benattar, J. J.; Schalchli, A.; Belorgey, O. J. Phys. I 1992, 2, 955.
The results for the dependence of the equivalent film thickness as a function of the NaCl concentration are presented in Figure 3a. The β-C12G2 concentration was 1.0 × 10-3 M, which is above the cmc of 0.17 × 10-3 M for the salt-free solution (see Figure 1). The calculated values for h1 and h2 are presented in Table 1 in the Supporting Information. The films were about 60 nm thick when no NaCl was added to the film-forming solution. The thickness decreases gradually with increasing salt concentration. CBFs with an equivalent film thickness of 15.7 nm were formed (30) Kolarov, T.; Scheludko, A.; Exerowa, D. Trans. Faraday Soc. 1968, 64, 2864.
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Figure 4. Disjoining pressure isotherms at two different NaCl concentrations: 9, no added salt; 0, 0.005 M. The β-C12G2 concentration is fixed at 1.0 × 10-3 M, and the temperature is 25 °C.
in the range of NaCl concentrations between 0.005 and 0.01 M. The further increase in the salt concentration above 0.05 M NaCl led to formation of films with an average equivalent film thickness of only 6.6 nm (respective film thickness 5.1 nm). We identify these films as NBFs. Their thicknesses are similar to those obtained by other authors.17 This shows that the film consists of two adsorbed surfactant layers with a thickness of around 2 nm each and an aqueous core of around 1 nm. This aqueous core is assumed to represent the hydration water of the surfactant head groups. The transition between CBFs and NBFs occurred slowly with formation of black spots in the already formed CBFs. The velocity of the formation and expansion of the black spots is slow at salt concentrations near the transition point and fast at higher concentrations. The contact angles θ were measured under the same conditions in the range of electrolyte concentrations (above 0.005 M) where black films were formed. Results are shown in Figure 3b. Two experimental methods were applied for these measurements. The topographic method was used to obtain the contact angle for concentrations of NaCl from 0.008 to 0.07 M where CBFs with small (
