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Langmuir 2003, 19, 4880-4883
Fine-Tuning the Wetting Behavior of Polyelectrolyte Films with Sodium Dodecyl Sulfate Rita J. El-Khouri and Malkiat S. Johal* Division of Natural Sciences, New College of Florida, 5700 North Tamiami Trail, Sarasota, Florida 34243 Received January 31, 2003. In Final Form: April 6, 2003 Charge-alternating polyelectrolyte multilayers containing the anionic surfactant sodium dodecyl sulfate (SDS) were constructed by adsorption from aqueous solution. The polyelectrolyte films contained the polycation PEI (poly(ethylenimine)) and the polyanion PAZO (poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt]). SDS adsorption on the PEI/PAZO/PEI trilayer was studied using dynamic tensiometry and single wavelength ellipsometry. Advancing contact angle measurements of SDS adsorption onto PEI surfaces indicate that surfactant adsorption is complete within 1 min. The contact angle increases with surfactant concentration, from 30° (0 mM SDS) and plateaus to a constant value of 80° beyond 0.01 mM SDS. Constructing films from varying concentrations of PEI allowed the degree of PAZO interpenetration to be qualitatively measured. Contact angle and ellipsometric thickness studies indicate that PAZO interpenetrates 32 Å into the terminal PEI layer, above which the surface is composed entirely of PEI. Maximum SDS adsorption is observed above the interpenetrated region, consistent with maximum electrostatic interaction between the terminal PEI layer and SDS.
Introduction Varying the surface properties of polymer films is important in a wide range of applications.1-3 In particular, taking advantage of electrostatic adsorption of surfactants onto polyelectrolyte surfaces provides a convenient method of fine-tuning the wetting behavior of polymer films. Due to displacement of small counterions, the association between polyelectrolytes and ionic surfactants is entropically and electrostatically driven, with little contribution from hydrophobicity.4-8 The polycation PEI (poly(ethylenimine)) has the highest known charge density of all polyelectrolytes and a strong tendency to form complexes with anionic surfactants.9,10 In fact Van der Berg and Staverman first suggested complex formation between PEI and the common anionic surfactant sodium dodecyl sulfate (SDS) over 30 years ago.11 As the charge density of the polycation increases, greater amounts of anionic surfactant are adsorbed to the polymer. Degree of protonation often determines the charge density of cationic polyelectrolytes. Approximately 75% of the amino groups of PEI are protonated at pH 2 with the degree of protonation decreasing linearly to zero charge at ∼pH 11.12 This work investigates the wetting behavior of a PEI surface containing various amounts of adsorbed SDS. The * To whom correspondence should be addressed. E-mail: johal@ ncf.edu. Fax: (941) 359-4396. (1) Contact Angle, Wettability and Adhesion; Mittal, K. L., Ed.; V.S.P. Intl Science: Utrecht, The Netherlands, 2003. (2) Jones, R.; Richards, R. W. Polymers at Surfaces and Interfaces; Cambridge University Press: Cambridge, 1999. (3) Garbassi, F.; Morra, M.; Occhiello, E. Polymer Surfaces: From Physics to Technology; John Wiley & Son Ltd.: Chichester, 1998. (4) Satake, I.: Yang, J. T. Biopolymers 1976, 15, 226. (5) Bronich, T. K.; Cherry, T.; Vinogradov, S. V.; Eisenberg, A.; Kabanov, V. A.; Kabinov, A. V. Langmuir 1998, 14, 6101. (6) Thu¨nemann, A. F. Langmuir 2000, 16, 824. (7) Wallin, T.; Linse, P. J. Phys. Chem. 1996, 100, 17873. (8) Wallin, T.; Linse, P. Langmuir 1996, 12, 305. (9) Claesson, P. M.; Bergstro¨m, M.; Dedinaite, A.; Kjellin, M.; Legrand, J.-F.; Grillo, I. J. Phys. Chem. B 2000, 104, 11689. (10) Thu¨nemann, A. F.; Kubowicz, S.; Pietsch, U. Langmuir 2000, 16, 8562. (11) Van den Berg, J. W. A.; Staverman, A. J. Recl. Trav. Chim. Pays-Bas 1972, 91, 1151. (12) Reveda, T.; Petkanchin, I. J. Colloid Interface Sci. 1997, 196, 87.
polycation surface is generated by sequential adsorption of polyelectrolytes of opposite charge onto a glass substrate, with the outer layer as PEI.13-15 The surface wettability of charge-alternating polyelectrolyte layers is determined primarily by the outermost layer.16,17 Rubner has shown that wettability can be fine tuned by varying the pH of the adsorbing polyelectrolyte.16 In that work, changes in pH resulted in layers with different thicknesses and surface wettabilities. These changes were attributed to varying degrees of layer interpenetration. In fact, certain layer combinations have yielded water contact angles as low as 99%) was recrystallized twice (13) Decher, G. Science 1997, 277, 1232. (14) Decher, G.; Hong, J. D.; Schmit, J. Thin Solid Films 1992, 210/ 211, 831. (15) Handbook of Polyelectrolytes and Their Applications, Volume 1: Polyelectrolyte-Based Multilayers, Self-Assemblies and Nanostructures; Tripathy, S. K., Jayant, K., Singh, N. H., MacDiarmin, A. G., Ed.; American Scientific Publishers: Stevenson Ranch, CA, July 2002. (16) Yoo, D.; Shiratori, S. S.; Rubner, M. F. Macromolecules 1998, 31, 4309. (17) Chen, W.; McCarthy, T. J. Macromolecules 1997, 30, 78. (18) Casson, J. L.; Wang, H.-L.; Roberts, J. B.; Parikh, A. N.; Robinson, J. M.; Johal, M. S. J. Phys. Chem. B 2002, 106, 1697. (19) Casson, J. L.; Johal M. S.; Roberts, J. B.; Wang, H.-L.; Robinson, J. M. J. Phys. Chem. B 2000, 104, 11996.
10.1021/la034174g CCC: $25.00 © 2003 American Chemical Society Published on Web 05/10/2003
Letters
Langmuir, Vol. 19, No. 12, 2003 4881 substrates were immersed into water at a speed of 5 mm/min to a depth of 15 mm below the liquid level. Approximately 1000 contact angle measurements were taken for a single immersion, and the average advancing macroscopic contact angle is reported here. The precise dimensions of the plate were determined by optimizing the width to the precise surface tension determined for water and optimizing the thickness so that the contact angle was constant for all immersion depths. In practice, it was not necessary to adjust thickness, since a constant contact angle was always obtained for all immersion depths. Film thickness measurements were collected on a Rudolph Instruments Inc. (439L633P) single-wavelength manual photoelectric ellipsometer. Single-sided polished silicon wafers (1 in. round) were used as substrates. Data were collected at a beam incidence angle of 70° and a wavelength of 632.8 nm. A refractive index of 1.5 + 0i was used to manually calculate the ellipsometric film thicknesses from ∆ and Ψ parameters. The substrate measurements (the native oxide layer) were subtracted from the film measurements to determine total ellipsometric film thickness. The native oxide layer on silicon is identical to the surface of etched glass.1 Furthermore, others have reported a common substrate effect using both substrate types.20
Results and Discussion
Figure 1. Schematic of the PEI/PAZO/PEI trilayer film, containing adsorbed SDS. The thickness of the interpenetrated region was estimated from contact angle and ellipsometric thickness measurements (see text). Structures of both PEI and PAZO are also shown. from ethyl acetate before use. PEI and PAZO were used as received from Aldrich. Polydispersity data for these polyelectrolytes are not available. Ultrapure water (resistivity >18 MΩ cm) was used in all solution and substrate preparations. The concentration of aqueous PEI and PAZO was fixed at 1 mM, based on the molecular weight of the repeat unit. The pH values of the SDS, PAZO, and PEI solutions were approximately neutral. The sequence of adsorption on the substrate was first PEI (1 mM), followed by PAZO (1 mM), followed by PEI (1 mM), and finally SDS (variable concentration). The resulting film is denoted PEI/PAZO/PEI/SDS. In some experiments the concentration used to adsorb the second (outer) PEI layer was varied. The adsorption time for all species in all experiments was 5 min, except in kinetic measurements where SDS adsorption time was varied from 0 to 200 s. Dynamic advancing contact angles (θa) were obtained using the Wilhelmy plate method. In this method, the force (F) acting on the glass substrate of dimension lwt (length, width, thickness) submerged in water to a height h is given by F ) (Fglwt)g (FLhwt)g + 2(w + t)γ cos θa. Fg and FL are the densities of the glass substrate and water, respectively, and γ is the surface tension of pure water (72.8 mN/m). By zeroing the weight of the substrate ((Fglwt)g), F then depends only on surface tension, contact angle, substrate dimensions, and the upthrust term (FLhwt)g. Both dynamic advancing and receding contact angles were determined as a function of h using the Nima Technology dynamic surface tensiometer (DST9005). The surface tension of pure water was determined using the Du Nou¨y ring method, and this value was then used to determine the contact angle. One inch square glass (20) Chiarelli, P. A.; Johal, M. S.; Holmes, D. J.; Casson, J. L.; Robinson, J. M.; Wang, H.-L. Langmiur 2002, 18, 168. (21) Chiarelli, P. A.; Johal, M. S.; Casson, J. L.; Roberts, J. B.; Robinson, J. M.; Wang, H.-L. Adv. Mater. 2001, 13, No. 15, August 3, 1167.
The advancing water contact angle of a freshly cleaned (hydrophilic) glass side was determined to be 10° ( 1°. In the adsorbed polyelectrolyte film, the contact angle was found to systematically and reproducibly alternate from a value of 58° ( 2° to a value of 30 ( 2° as the terminal layer was changed from PAZO to PEI. SDS was adsorbed on the PEI/PAZO/PEI trilayer rather than on a single PEI layer (Figure 1). Advancing contact angles of SDS adsorbed on a single PEI layer show lack of reproducibility and suggest a substrate effect as reported by others.16,18-21 The terminal (outer) PEI layer in the PEI/PAZO/PEI trilayer is at least 40 Å from the glass substrate, and advancing contact angle measurements are reproducible. Figure 2a shows the change in the advancing contact angle of the PEI/PAZO/PEI/SDS system as the concentration used to adsorb SDS is increased. Very small concentrations (
