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Interaction Forces and Zeta Potentials of Cationic Polyelectrolyte Coated Silica Surfaces in Water and in Ethanol: Effects of Chain Length and Concentration of Perfluorinated Anionic Surfactants on Their Binding to the Surface Cathy E. McNamee,*,† Mutsuo Matsumoto,† Patrick G. Hartley,‡ Paul Mulvaney,§ Yoshinobu Tsujii,† and Masaru Nakahara† Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan, CSIRO Molecular Science, Ian Wark Laboratory, Bag 10, Clayton South, Victoria 3169, Australia, and School of Chemistry, The University of Melbourne, Parkville, Victoria 3052, Australia Received February 22, 2001. In Final Form: July 2, 2001 Silica surfaces were premodified by the saturated adsorption of the cationic polyelectrolyte of poly(2vinyl-1-methyl-pyridinium bromide), P2VP, in water. The interaction forces between and the zeta potential of the silica surfaces were then measured in water and in ethanol solutions of perfluorinated anionic surfactants as a function of their chain length and concentration using the AFM surface force and electrophoresis methods. In water, the electrostatic repulsive forces between P2VP-modified silica surfaces in CF3CF2COONa, CF3(CF2)6COONa, and CF3(CF2)7SO3Li solutions of 0.1 mM were identical to the force curve in 0.1 mM NaNO3 but greatly decreased in 0.1 mM CF3(CF2)9COOLi (critical micelle concentration (cmc), 0.39 mM). The concentration increase of CF3(CF2)7SO3Li (cmc, 6.3 mM) from 0.1 to 1.0 mM caused the repulsive force curves to weaken and then to strengthen after passing through a zero repulsive force. The surface potentials obtained by the best curve fitting of the force curves agreed well with the zeta potentials, which indicated a surface charge reversal from positive to negative for a high concentration of CF3(CF2)7SO3Li. These observations were explained by the formation of a Stern layer due to specific counterion binding of the surfactant anions, which increased with the surfactant chain length and concentration. In ethanol, CF3(CF2)7SO3Li always showed strong repulsive force curves, when CF3(CF2)7SO3Li concentrations that were adjusted to give identical Debye lengths as those in water were used and the surface charge of P2VP-modified silica was the same as that in water. The surface potential obtained by the best curve fitting coincided with the zeta potential of positive sign, confirming no charge reversal. This suggested no obviously firm formation of a Stern layer by the surfactant ions. This was not always the case for shorter carbon-chain surfactants, since CF3(CF2)6COONa revealed a much weaker repulsive force curve than CF3(CF2)7SO3Li for concentrations of identical Debye lengths. This was explained in terms of an increased surfactant binding, due to the polarity difference between the solvent and surfactant molecules.
1. Introduction The adsorption of surfactants and polyelectrolytes is often used for the stable dispersion of multicomponent particles mixed in solution that is used to produce advanced materials. Surface force measurements provide direct information about key interactions on the nanometer scale acting between two substrate surfaces in solution. These interactions are directly connected to the stability of dispersed systems and include the doublelayer, van der Waals, steric, ion hydration, solvent structuring, and long-range hydrophobic forces.1 Most studies of the surface forces have been carried out on water systems incorporating inorganic electrolytes, surfactants, polyelectrolytes, polymers, and biomaterials. Ionic surfactants strongly bind to oppositely charged substrate surfaces due to electrostatic and entropy-driven forces, weakening and then strengthening the doublelayer forces with an increase in their adsorption density.2,3 * To whom correspondence should be sent. Tel: +81-774-38 3072. Fax: +81-774-38 3076. E-mail:
[email protected]. † Institute for Chemical Research, Kyoto University. ‡ CSIRO Molecular Science, Ian Wark Laboratory. § School of Chemistry, The University of Melbourne. (1) Israelachvili, J. Intermolecular & Surface Forces; Academic Press: London, 1994.
A long-range attraction that cannot be explained by the van der Waals attraction appears when the substrate surface charges are neutralized by the ionic surfactant binding4 or when the surfaces are made hydrophobic by the deposition of hydrophobic Langmuir-Blodgett (LB) films onto substrate surfaces.5 The origin of the long-range attraction is still an open issue, as reviewed by Craig and others.6 Adsorption of polyelectrolytes provides a surface modification for colloid particles and the possibility of adjusting isoelectric points, allowing destabilization, stabilization, and amphipathicity, depending on their adsorption density. The cationic polyelectrolyte of poly(2-vinylpyridine) is fully charged at pH below 3 and adsorbs on mica7 and silica8 surfaces at a monolayer or submonolayer coverage (2) Parker, J. L.; Yaminsky, V. V.; Claesson, P. M. J. Phys. Chem. 1993, 97, 7706. (3) Sharma, B. G.; Basu, S.; Sharma, M. M. Langmuir 1996, 12, 6506. (4) Craig, V. S. J.; Ninham, B. W.; Pashley, R. M. Langmuir 1998, 14, 3326. (5) Tsao, Y.-H.; Evans, D. F.; Wennerstro¨m, H. Science 1993, 262, 547. (6) Craig, V. S. J.; Ninham, B. W.; Pashley, R. M. Langmuir 1999, 15, 1562. (7) Marra, J.; Hair, M. L. J. Phys. Chem. 1988, 92, 6044. (8) Biggs, S.; Proud, A. D. Langmuir 1997, 13, 7202.
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with a polymer main chain conformation parallel to the surface. The dominant force between the modified surfaces at low pH is the electrical double-layer repulsion, which depends on the adsorption coverage. At high pH, the nonionized adsorption film of poly(2-vinylpyridine) is thickened and a steric force becomes dominant at a large separation.7 A similar submonolayer coverage on mica is achieved with a low concentration of chitosan, a cationic polyelectrolyte, and is stable over 24 h in chitosan-free solution in the bulk.9 The repulsive surface forces of such modified mica are pH dependent, and the sign of the charge of the modified mica surface at pH ) 3.8 is positive, indicating charge reversal. At pH ) 9.1, the force is repulsive, but the sign of the charge is negative, due to the nonionization of the adsorbed chitosan films. Poly((3-(methacrylamido)-propyl)trimethylammonium chloride neutralizes mica surfaces with a low concentration.10 The addition of salts thickens the adsorbed films on mica by conformation changes, and the resulting forces of the modified surfaces become repulsive by a steric effect of the adsorbed films. Quaternarized poly(2-vinylpyridine) with a substitution of 86% adsorbs such that the polymer main chain is parallel to a silica surface in a low ionic strength solution, forming a thin adsorption film.11 The isoelectric point of silica is shifted from pH ) 2-3, which is typical for bare silica surfaces, to the higher pH of 6, when partially modified by quaternarized poly(2-vinylpyridine).12 This indicates that the surface charges comprise both the charges of the modified and the nonmodified silica surface portions. Numerous force studies have been performed on cationic polyelectrolyte modified silica or mica systems in anionic surfactant solutions.13-19 Bremmell and others13 studied the interaction forces between silica and mica surfaces premodified with poly{[2-(propinoyloxy)ethyl]trimethylammonium chloride}, charge density of 30%, in the presence of sodium dodecylbenzene sulfonate. The electrostatic repulsive nature of the forces involving the charge reversal depends on the surfactant concentration. At zero surface charge, the force appears as a long-range attraction. Claesson and others14 showed that sodium dodecyl sulfate (SDS) adsorbs onto mica surfaces pretreated with a highly positively charged polyelectrolyte. At low SDS concentrations (e0.1 critical micelle concentration (cmc)), a repulsive force is observed. At higher concentrations, a periodically oscillatory force becomes dominant, due to the formation of small surfactant aggregates along the polymer chains that extend to the bulk. SDS-polyelectrolyte aggregates, however, do not adsorb onto bare mica surfaces, when they are present in the bulk, resulting in forces explainable by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory at a given ionic strength.19 (9) Claesson, P. M.; Ninham, B. W. Langmuir 1992, 8, 1406. (10) Dahlgren, M. A. G.; Waltermo, A° .; Blomberg, E.; Claesson, P. M.; Sjo¨stro¨m, L.; A° kesson, T.; Jo¨nsson, B. J. Phys. Chem. 1993, 97, 11769. (11) Nievandt, D. J.; Gee, M. L. Langmuir 1995, 11, 1291. (12) Hartley, P. G.; Scales, P. J. Langmuir 1998, 14, 6948. (13) Bremmell, K. E.; Jameson, G. J.; Biggs, S. Colloids Surf., A 1999, 155, 1. (14) Claesson, P. M.; Dedinaite, A.; Blomberg, E.; Sergeyev, V. G. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 1008. (15) Kjellin, U. R. M.; Claesson, P. M.; Audebert, R. J. Colloid Interface Sci. 1997, 190, 476. (16) Claesson, P. M.; Fielden, M. L.; Dedinaite, A.; Brown, W.; Fundin, J. J. Phys. Chem. B 1988, 102, 1270. (17) Fielden, M. L.; Claesson, P. M.; Schillen, K. Langmuir 1998, 14, 5366. (18) Dedinaite, A.; Claesson, P. M. Langmuir 2000, 16, 1951. (19) Dedinaite, A.; Claesson, P. M.; Bergstro¨m, M. Langmuir 2000, 16, 5257.
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These previous studies have shown that the surface force nature greatly depends on the charge density of the substrate, the solution pH, the species of surfactants, the charge site distribution of the polyelectrolyte, the formation of aggregates, and the pathways of the surface modification. Despite these numerous studies, we surprisingly find that there are no systematic force studies on the effect of the chain length and concentration of ionic surfactants to a bare or polyelectrolyte-modified surface, involving the different nature of the solvent. The only exception is that for short-chain tetraalkylammonium ions on bare mica in water, which form a strongly adsorbed Stern layer but retain their electrostatic repulsion that is dependent on their concentration.20 In this paper, silica surfaces were premodified with a high charge density quaternarized polyelectrolyte, poly(2-vinyl-1-methyl-pyridium bromide), P2VP, to give saturated coverage.21 The surface forces and zeta potential were measured by using an atomic force microscope (AFM) technique and electrophoresis, respectively. These were performed as a function of the carbon chain length and concentration of perfluorinated anionic surfactants in water and also in ethanol. These studies serve for discussion of the double-layer structure and counterion binding of the surfactants to the P2VP-modified surface. 2. Experimental Section 2.1. Materials. The following electrolytes and surfactants were used in this study: NaOH (Aldrich, 99.99% purity), HCl (Aldrich, 99.99% purity), NaNO3 (Aldrich, 99.995% purity), CF3CF2COONa (Aldrich, 97% purity), CF3(CF2)2COONa (PCR, 98% purity), CF3(CF2)6COONa (SCM Chemicals, 97% purity, cmc ) 36 mM22), CF3(CF2)7SO3Li (cmc ) 6.3mM23), and CF3(CF2)9COOLi (cmc ) 0.39 mM24). The last two surfactants were synthesized in our laboratory and recrystallized three times from dioxane.25 No impurities were detected, when checked by elementary analysis and NMR spectroscopy. A guaranteed grade of ethanol (99.5% purity) was obtained from Kantoh Chemical Co., Inc. The quaternarized polyelectrolyte, poly(2-vinyl-1-methyl pyridinium bromide) (P2VP, (C8 H10 Br N)n)250) from Polysciences, Inc., had a molecular weight of 50 000 and a degree of methyl pyridinium substitution of approximately 90%. The aqueous solutions were prepared using doubly distilled water passed through a reverse osmosis membrane and ionexchange resin. The polished silica substrates for the AFM force measurements were purchased from Nipponchikagaku, Ltd., Japan. The silica particles (Ube Nittoh Kagaku, Japan) for the zeta potential and AFM force measurements were prepared by a sol-gel method and had a radius of 0.15 and 5 µm, respectively. Both had a 0.6% size distribution, as determined by electron microscopy. 2.2. Methods. 2.2.1. Surface Force Measurements. Preparation. The silica particles with 5 µm radius were dispersed in 15% H2O2 solution for 24 h and ultrafiltered more than 20 times with distilled water. They were further ultrafiltered with pure ethanol several times and kept in ethanol in a glass vessel. The silica particles in ethanol were spread on a cleaned glass slide plate. The tip end of the micropyramid of a v-shaped cantilever (Olympus Optical Co., Ltd.) was glued using 5-min-curing epoxy resin (Araldite, Ciba-Geigy Japan, Ltd.), and then a single silica particle from the glass plate was transferred onto the tip end. The spring constant (k) of the cantilever (k ) 0.16 or 0.02 N/m) (20) Claesson, P.; Horn, R. G.; Pashley, R. M. J. Colloid Interface Sci. 1984, 100, 250. (21) McNamee, C. E.; Matsumoto, M.; Hartley, P. G.; Nakahara, M. Colloids Surf., A, in press. (22) Kissa, E. Fluorinated Surfactants: Synthesis, Properties, Applications; Marcel Dekker: New York, 1994; p 222. (23) Shinoda, K.; Hato, M.; Hayashi, T. J. Phys. Chem. 1972, 76, 909. (24) Kunieda, H.; Shinoda, K. J. Phys. Chem. 1976, 80, 2468. (25) Bossev, D. P.; Matsumoto, M.; Nakahara, M. J. Phys. Chem. B 1999, 103 (39), 8251.
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was determined by the Cleveland method26 and was within (10% of the spring constant supplied by the manufacturer. The silica plates were cleaned prior to a force experiment by soaking them in a 1:1 solution of concentrated H2SO4 (Wako Pure Chemical Industries, guaranteed grade) and concentrated HNO3 (Nacalai Tesque, guaranteed grade) for 24 h. They were then transferred into a solution of 9 parts of 31% H2O2 (Santoku Chemical Industries, guaranteed grade) and 1 part of 28% NH3 (Nacalai Tesque, guaranteed grade) for another 24 h and then rinsed with copious amounts of distilled water. AFM images showed the absence of detectable contamination on the silica surfaces and a surface roughness of 0.35 nm over 25 µm2. The working surface of a liquid flow cell (Digital Instruments, Inc.) was cleaned with detergent (Extran MA 03, Merck) by using a cotton bud, rinsed with distilled water, cleaned with ethanol, and then rinsed in a stream of distilled water. The silicone O-ring and silicon tubing attached to the liquid flow cell were cleaned by sonication in a 1:1 mixture of water and ethanol for 30 min. They were then rinsed with distilled water. The substrate, silica probe, the working surface of the liquid cell, O-ring, and tubing were again cleaned at least two times after setting up and before commencing an AFM force experiment. This was achieved by inserting a solution of 9 parts of 31% H2O2 and 1 part of 28% NH3 into the liquid cell for 5 min and then by flushing the inside of the cell with a minimum of 100 mL of distilled water. Measurements. The surface forces between a silica substrate and silica particle in solution were measured at room temperature (∼25 °C) as a function of their distance using an atomic force microscope. An SPI3600 AFM (Seiko Instruments, Inc.) was modified with a different type of piezo actuator (Taiheiyo Cement Co., Japan, PMF-2020) for the distance control, regulated by a piezo driver (Taiheiyo Cement Co., PM-1100). This AFM was mainly used in this study. The peizo actuator had a high resistance (
