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Evidence of Surface Charge at the Air/Water Interface from Thin-Film Studies on Polyelectrolyte-Coated Substrates Katarzyna Ciunel,*,† Marc Arme´lin,† Gerhard H. Findenegg,† and Regine von Klitzing‡,§ Stranski-Laboratorium fu¨ r Physikalische und Theoretische Chemie, Technical University Berlin, Strasse des 17. Juni 112, D-10623 Berlin, Germany, Max-Planck-Institute of Colloids and Interfaces, Am Mu¨ hlenberg 1, D-14424 Potsdam, Germany, and Institute for Physical Chemistry, Christian-Albrechts-University Kiel, Ludewig-Meyn-Strasse 8, D-24118 Kiel, Germany Received February 4, 2005. In Final Form: March 26, 2005 The stability of thin water films on silicon substrates coated with cationic and anionic polyelectrolytes was investigated by the thin film pressure balance technique. Depending on the surface charge of the substrate, the water films are either stable (on negatively charged wafers) or rupture rapidly (on positively charged wafers). It is supposed that this behavior is due to a negative surface charge of the free water surface. The underlying assumption that the films’ stability is due to electrostatic interactions is supported by measurements of the disjoining pressure on silicon wafers with a native oxide layer, which indicates a decrease of the film thickness, and thus decreasing repulsive interaction between the two film interfaces, with increasing ionic strength.
1. Introduction The rupture of a thin liquid film separating two compartments of a medium is a crucial step in many processes of scientific and industrial relevance, such as the breaking of foams, the coalescence of emulsion droplets, or the coagulation of colloidal dispersions. The stability of thin films depends on the forces acting between its two interfaces and is characterized by the disjoining pressure (Π) and its dependence on film thickness. Generally, the disjoining pressure in the liquid film is dominated by longrange repulsive electrostatic interaction, short-range attractive van der Waals contribution, both of which are accounted for in the classical DLVO theory, and shortrange repulsive confinement interaction.1,2 In experimental studies of film stability, different techniques have been developed, depending on the type of system studied. For the investigation of interfacial forces in thin liquid foam films the thin film pressure balance (TFPB) in various modifications3,4 has proved to be a suitable technique. With this method, the disjoining pressure isotherm (Π vs film thickness, h) of a mechanically stable film can be recorded. In this work, we have adopted a modified TFPB technique to probe the existence of surface charge at air/ water interfaces. Direct experimental verification of the existence of surface charges at the air/water interface has remained elusive despite great experimental efforts, mostly because of difficulties in the determination of the net charge sign.4 The hydrophobic properties of the free water surface are expected not to differ from those of the oil/water interface. The oil droplets in an oil-in-water emulsion have been proved to be negatively charged. The * Author to whom correspondence should be addressed. † Technical University Berlin. ‡ Max-Planck-Institute of Colloids and Interfaces. § Christian-Albrechts-University Kiel. (1) Israelachvili, J. Intermolecular and Surface Forces; Academic Press: New York, 1992. (2) Bergeron, V. J. Phys.: Condens. Matter 1999, 11, R215. (3) Scheludko, A. Adv. Colloid Interface Sci. 1967, 1, 391. (4) Stubenrauch, C.; Klitzing, R. J. Phys.: Condens. Matter 2003, 15, R1197.
negative zeta potential value depending on the pH was attributed to the specific location of hydroxide ions at the oil/water interface generating the negative charge on it.5,6 Near-edge X-ray absorption fine structure (NEXAFS) studies have provided evidence in support of the existence of two relevant orientations of water molecules at the air/ water interface. In the single donor configuration, water molecules are oriented with one H atom pointing to the gas phase while the other H atom acts as a donor for a hydrogen bond to a water molecule in the subphase. In the acceptor only configuration, both H atoms of water molecules are directed to the gas phase and the lone pair electrons of the oxygen atom are acting as donors for hydrogen bonds to water molecules in the subphase.7 The existence of these two water species is supported by sumfrequency generation (SFG) spectroscopy of the frequency range around 3700 cm-1, which selectively probes OH bonds oriented normal to the surface.8 Recent molecular dynamics (MD) simulations of the free surface of water also suggest that the molecules in the topmost layer are oriented mainly with the hydrogen atoms upward. This orientation causes a surface dipole moment and a surface potential of the order of +500 mV for the free liquid surface.9 The existence of such a surface potential offers an explanation for the positive adsorption of anions in the interface region depending on their polarizability as observed in molecular simulation studies by Jungwirth and Tobias.10,11 As a consequence, even solutions without any surface additives could show distinct charging effects. (5) Marinova, K. G.; Alagrova, R. G.; Denkov, N. D., Velev, O. D.; Petsev, D. N.; Ivanov, I. B.; Borwankar, R. P. Langmuir 1996, 12, 2045. (6) Beattie, J. K.; Djerdjev, A. M. Angew. Chem., Int. Ed. 2004, 43, 3568. (7) Wilson, K. R.; Cavalleri, M.; Rude, B. S.; Schnaller, R. D.; Nillson, A.; Petterson, L. G. M.; Catalano, T.; Bozek, J. D.; Saykally, R. J. J. Phys. Cond. Matter 2002, 14, L221. (8) Raymond, E. A.; Tarbuck, T. L.; Brown, M. G.; Richmond, G. L. J. Phys. Chem. B 2003, 107, 546. (9) Mamatkulov, S. I.; Khabibullaev, P. K.; Netz, R. Langmuir 2004, 20, 4756. (10) Jungwirth, P.; Tobias, D. J. J. Phys. Chem. B 2001, 105, 10468. (11) Jungwirth, P.; Tobias, D. J. J. Phys. Chem. B 2002, 106, 6361.
10.1021/la050328b CCC: $30.25 © 2005 American Chemical Society Published on Web 04/20/2005
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Figure 1. Thin film pressure balance adapted for the study of wetting films: Pressure cell containing the film holder with the solid substrate fixed to the bottom side of the porous glass plate by adhesion. Section across the porous glass plate with the thin liquid film on a specifically modified substrate (inset).
Force measurements on free-standing foam films also hint at the existence of surface charge at the film surfaces. As in the symmetric foam films, the two surfaces are equally charged, the electrostatic contributions to the disjoining pressure is always repulsive, and the sign of surface charge cannot be determined. Films which are stabilized by nonionic surfactants show a value of the surface potential of about 50 mV.12,13 Films stabilized by cationic surfactants exhibit a minimum in the surface potential at a certain surfactant concentration, which indicates a reversal from negative to positive sign of the surface potential with increasing surfactant concentration.14-16 The investigations of free-standing liquid films have led an intense discussion about the origin of the surface charge at the air/water interface.4 The present work is aimed to probe experimentally the presence and the sign of surface charges at the air/water interface. For this purpose, we study asymmetric films of the type air/water/solid (i.e., wetting films) which allow for changing the interaction between the interfaces of the film by modifying the solid substrate (see Figure 1). The surface charge of the substrate is controlled by adsorption of polyelectrolytes of the respective sign of charge. Many techniques for the investigation of the interfacial interactions in wetting films exist.17-19 We present results providing conclusive evidence of a negative sign of the air/water surface charge. The interactions within the film are dominated by electrostatic forces, as confirmed by changing the ionic strength. 2. Experimental Section 2.1. Materials and Surface Modification. Bi-distilled Millipore-Q water (pH 5.5, specific resistance 18 MΩ cm) was used for the preparation of the solutions. Sodium chloride was obtained from Merck (Germany) and roasted at 500 °C before use in order to remove organic impurities. Cationic polyelectrolytes used in the study were poly(ethylenimine) (PEI, 50 wt% solution in water), poly(allylamine (12) Bergeron, V.; Waltermo, A.; Claesson, P. M.; Langmuir 1996, 12, 1336. (13) Stubenrauch, C.; Schlarmann, J.; Strey, R. Phys. Chem. Chem. Phys. 2002, 4, 4504. (14) Balzer, D. Langmuir 1993, 9, 3375. (15) Aveyard, R.; Binks, B. P.; Esquena, J.; Fletcher, P. D. I.; Bault, P.; Villa, P. Langmuir 2002, 18, 3487. (16) Kolarov, T.; Yankov, R.; Espionova, N. E.; Exerova, D.; Zorin, Z. M. Colloid Polym. Sci. 1993, 271, 519. (17) Read, A. D.; Kitchener, J. A. J. Colloid Interface Sci. 1969, 30, 391. (18) Churaev, N. V. Adv. Colloid Interface Sci. 2003, 103, 197. (19) Diakova, B.; Filiatre, C.; Platikanov, D.; Foissy, A.; Kaisheva, M. Adv. Colloid Interface Sci. 2002, 96, 193.
hydrochloride) (PAH, MW ) 65 000 g/mol) and poly(diallyldimethylammonium chloride) (PDADMAC, MW ) 100 000 g/mol). Poly(styrenesulfonate) sodium salt (PSS, MW ) 70 000 g/mol) was used as anionic polyelectrolyte. PDADMAC was received from Werner Jaeger (Fraunhofer Institut, Potsdam, Germany). All other polyelectrolytes were purchased from Aldrich (Steinheim, Germany) and used without further purification. Silicon wafers were used as solid substrate. The wafers were cleaned by treatment with a 1:1 H2O2/H2SO4 mixture (30 min) followed by extensive rinsing with water and Millipore water. The wafers were coated by adsorption of polyelectrolytes using the layer-by-layer method introduced by Decher.20 Water contact angle measurements were made using the instrument OCA 20 by Dataphysics (Filderstadt, Germany). The silicon wafers were mounted in a closed glass cell containing water to ensure a water-saturated atmosphere adequate to the force measurement condition. The contact angle of sessile drops was determined by an image analysis program every 10 min until an constant value was reached. 2.2. Film Pressure Apparatus. The film pressure of water films on solid substrate was measured by a modified TFPB method, using the porous-plate technique to determine the disjoining pressure, Π, as a function of film thickness, h.21-23 A single thin liquid film is formed in a hole (1 mm diameter) drilled through the porous glass plate connected to the atmospheric pressure by a capillary tube. The film holder is enclosed in a hermetically sealed stainless steel cell. Before the film is formed, the porous plate is wetted by the solution. The TFPB apparatus is adapted for studies of wetting films by attaching the solid substrate (silicon wafer) to the bottom side of the porous glass plate by adhesion (Figure 1). A thin liquid film is formed on the top side of wafer by adjusting the gas pressure in the cell. The pressure applied by a piston pump sets the disjoining pressure while the film becomes thinner and the interfaces approach each other. At equilibrium, the disjoining pressure is equal to the capillary pressure within the film.24 The film thickness is measured interferometrically3,23 by white light reflected from the top and bottom surface of the film. The light beam passes through an objective and an interference filter (550 nm) and is detected by a photomultiplier. Simultaneously, the thinning of the film is recorded by video microscopy.
3. Results and Discussion Figure 2 shows CCD images illustrating the drainage of the water film on silicon wafers coated with different polyelectrolyte layers. The wafer shown in Figure 2a was coated with one layer of polycation PEI. In this case, it was generally found that (20) Decher, G. Science 1997, 210/211, 831. (21) Mysels, K.; Jones, M. N. Discuss. Faraday Soc. 1966, 42, 42. (22) Exerova, D.; Scheludko, A. Chim. Phys. 1971, 24, 47. (23) Scheludko, A.; Platikanov, D. Kolloidn. Zh. 1961, 175, 150. (24) Toshev, B. V.; Ivanov, I. B. Colloid Polym. Sci. 1975, 253, 558.
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Figure 2. CCD images of water films on differently coated silicon substrates (a) dewetting water film on Si/PEI; (b) stable water film on Si/PEI/PSS at Π ) 430 Pa; (c) the same film as in (b) at a later stage of the thinning process at Π ) 1700 Pa.
the water film ruptures immediately after pressure increase, forming small islands of accumulated liquid on the dewetted substrate. Film rupture immediately after a pressure increase was also found for water films on wafers coated with the polycations PAH or PDADMAC, indicating that thin films of water are not stable on coatings with positive excess charge. A different behavior was found for the water films on a negatively charged surfaces, either silicon wafers with a native oxide layer or coated with the polyanion PSS as the outermost layer. Figure 2b shows a water film formed on a silicon wafer with a PEI/PSS coating: In this case, the water film is thinning continuously as the pressure in the cell is gradually increased and the film persists stable up to high values of the disjoining pressure (Π ) 1700 Pa; Figure 2c). We suppose that the stability of the water films on negatively charged substrate is due to electrostatic repulsion between the air/water interface and the substrate. This conclusion is based on the fact that the Hamaker constant for the system silicon/water/air is positive,25 i.e., corresponding to attractive van der Waals interactions,1 and thus the stability of the films cannot be due to van der Waals interactions. This argument relies on the assumption that the ultrathin polyelectrolyte film (
