Langmuir 2002, 18, 9401-9408
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Controlling the Adsorbed Conformation and Desorption of Polyelectrolyte with Added Surfactant via the Adsorption Mechanism: A Direct Force Measurement Study George Maurdev, Michelle L. Gee,* and Laurence Meagher School of Chemistry, University of Melbourne, Parkville, Victoria 3010, Australia Received June 10, 2002. In Final Form: September 2, 2002 When complexed PSS/CTAB is coadsorbed onto silica, a long-range, predominantly electrosteric repulsion is measured which decays exponentially with surface separation. Adsorption is driven by locally hydrophobic, surfactant-rich regions along the PSS chain. These anchor the PSS chain to the surface, resulting in a classical loop and tail adsorbed conformation. An increase in pH results in an increase in electrostatic repulsion between the silica surface and PSS, which drives an initially rapid desorption of some of the PSS/CTAB complex. Any remaining PSS/CTAB desorbs via a slow unravelling, as points of attachment to the surface are broken over time, allowing the PSS to extend further into solution. This continues until desorption is complete after 66 h. In contrast, adsorption of the PSS/CTAB complex onto a preadsorbed CTAB layer leads to a less extended, more compact surface layer since now adsorption is driven not only by the hydrophobic surfactant-rich regions along the PSS chain but also by a hydrophobic attraction between the PSS backbone and any adsorbed CTAB. An increase in pH results in rearrangement of the PSS/CTAB complex to a more extended conformation since now there is more segment-segment and surface-segment repulsion. This is evidenced by the increase in the range of the net repulsion and the fact that the repulsion now decays exponentially in a manner similar to that obtained when PSS/CTAB is able to loop and tail into solution. More points of attachment must be broken to achieve complete desorption of the PSS/CTAB complex, and hence complete desorption is kinetically hindered.
Introduction The industrial importance of surfactant/polyelectrolyte mixtures in cosmetics, paints, and drug industries is well recognized. The application of polyelectrolyte/surfactant complexation in colloidal stability depends on the adsorption of these species at an interface and the subsequent modification of surface forces due to their adsorption. A body of work investigating their behavior both in solution and at solid/liquid interfaces is in the process being established.1,2 Association between polyelectrolyte and surfactant molecules can be established through hydrophobic interactions between the hydrophobic surfactant tail and the polyelectrolyte’s backbone.3 Another driving force to polyelectrolyte/surfactant complexation exists between oppositely charged polyelectrolytes and surfactants which are able to associate through ionic attraction of the surfactant headgroup and the charged moieties along the polyelectrolyte backbone.1 In such systems, polyelectrolyte/ surfactant association is very strong, driven by the replacement of counterions on both the polyelectrolyte and surfactant.4 Furthermore, the interaction is highly cooperative and can result in the formation of micelle-like aggregates along the polyelectrolyte chain well below the cmc of the surfactant.5 The surfactant concentration at which this kind of aggregation occurs is referred to as the * To whom correspondence should be addressed. (1) Goddard, E. D. Colloids Surf. 1986, 19, 301. (2) Lindman, B.; Thalberg, K. Polymer-Surfactant InteractionsRecent Developments. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: London, 1993; p 203. (3) Moudgil, B. M.; Somasundaran, P. Colloids Surf. 1985, 13, 87. (4) Almgren, M.; Hansson, P.; Mukhtar, E.; van Stam, J. Langmuir 1992, 8, 2405. (5) Hayakawa, K.; Kwak, J. C. T. Interactions between Polymers and Cationic Surfactants. In Cationic Surfactants; Rubingh, D. N., Holland, P. M., Eds.; Marcel Dekker: New York, 1991; Vol. 37, p 189.
critical association concentration (cac). These micelle-like structures typically have aggregations numbers much lower than those for free micelles.4 Polyelectrolyte/surfactant coadsorption at the solid/ liquid interface has been investigated using techniques such as sum-frequency spectroscopy,6 attenuated total internal reflectance spectroscopy,7 and electron spin resonance spectroscopy.8 Only recently, the modification of colloidal forces by polyelectrolyte/surfactant coadsorption has been investigated by means of the surface forces apparatus (SFA) and the atomic force microscope (AFM). Claesson et al.9 have measured the interaction between two mica surfaces with coadsorbed poly((2-(propionyloxy)ethyl)trimethylammonium chloride) (PCMA) and sodium dodecyl sulfate (SDS). In this work, a net electrostatic potential was exhibited, and an oscillatory steric repulsion was seen due to structuring of the polyelectrolyte/ surfactant layer. Work has also been carried out investigating the interaction forces between mica surfaces immersed in a mixture containing the cationic Polymer JR-400 and the anionic surfactant sodium dodecyl sulfate, where the coadsorption of the polyelectrolyte/surfactant complex led to a significant bridging attraction.10 Shubin11 found that the complexation of SDS with a preadsorbed cationic hydrophobically modified hydroxyethylcellulose layer resulted in molecular conformational changes at the interface and a consequential change in surface forces. In another study,10 the cationic Polymer JR 400 was initially allowed to adsorb onto mica and SDS was subsequently (6) Duffy, D. C.; Davies, P. B. Langmuir 1995, 11, 2931. (7) Neivandt, D. J.; Gee, M. L.; Tripp, C. P.; Hair, M. L. Langmuir 1997, 13, 2519. (8) Esumi, K.; Masuda, A.; Otsuka, H. Langmuir 1993, 9, 284. (9) Claesson, P. M.; Dedinaite, A.; Blomberg, E.; Sergeyev, V. G. 1996, 100, 1008. (10) Ananthapadmanabhan, K. P.; Mao, G.-Z.; Goddard, E. D.; Tirrell, M. Colloids Surf. 1991, 61, 167. (11) Shubin, V. Langmuir 1994, 10, 1093.
10.1021/la0205349 CCC: $22.00 © 2002 American Chemical Society Published on Web 11/01/2002
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introduced. Again, it was found that the preadsorbed polyelectrolyte layer underwent rearrangement on addition of the surfactant. The resulting polyelectrolyte/ surfactant layer adopted a more extended conformation at the interface. An adhesion was measured on separation of the two surfaces, which was attributed to a combination of both hydrophobic and bridging interactions. The surface forces studies mentioned above have involved systems in which the polyelectrolyte has a negative adsorption free energy and added surfactant can modify the preadsorbed polyelectrolyte layer. In contrast, in the present work, we measure the surface forces in a system where the polyelectrolyte/surfactant complex has a net negative charge and the surface is near neutral. Hence, the polyelectrolyte does not adsorb unless surfactant is added. Specifically, we have used the atomic force microscope (AFM) to measure the surface forces between two silica surfaces with coadsorbed poly(styrenesulfonate) (PSS) and cetyltrimethylammonium bromide (CTAB), where the solution is initially adjusted to a pH of 1.9. This polyelectrolyte/surfactant system has been the subject of a previous study by us7 where it was found that addition of CTAB promotes PSS adsorption through the formation of a PSS/CTAB complex. The surface conformation of the PSS/CTAB complex at the interface depends on the history of the system, i.e., the order in which the two components were added to solution for adsorption. The present work builds on this earlier investigation by looking at how the surface interactions are affected by the adsorption history of the system. Two adsorption protocols were adopted: (1) coaddition: coadsorption of the PSS/CTAB complex preformed in bulk solution; (2) sequential addition: adsorption of PSS onto a preadsorbed CTAB layer. Experimental Section Atomic Force Microscopy. The interaction forces between two silica surfaces bearing coadsorbed polyelectrolyte/surfactant were measured using a Nanoscope III atomic force microscope (AFM) (Digital Instruments, Inc.) and the colloid probe method developed by Ducker et al.12,13 In this method, a spherical glass particle is attached to the microfabricated cantilever spring used in the AFM, providing a surface of known geometry. To scale the force measurements correctly, the spring constant must be known accurately. This was achieved using the resonance method proposed by Cleveland et al.14 This method gives the spring constant with an error of approximately 10%. An average value calculated from a sample of 10 cantilevers was used to scale the raw data obtained from the interaction force experiments. Cleaning Methods and Materials. All operations were carried out in a laminar flow clean cabinet to minimize any particulate contamination. The fittings and tubing used for injecting solutions into the AFM fluid cell were constructed from either Teflon or KelF polymers to facilitate rigorous cleaning. This involved soaking individual parts in a one percent surfactant solution (RBS 35, Pierce) overnight. Next, they were rinsed thoroughly with Milli-Q water and soaked in AR grade ethanol overnight. The ethanol was subsequently replaced followed by another overnight soak. These components were finally rinsed with distilled AR grade ethanol and blown dry using a highvelocity stream of filtered nitrogen. The spherical glass particles were obtained from Polysciences Inc. (Warrington, PA) and had a diameter of 6-7 µm. The flat silica surfaces used in this study were pure fused quartz (Suprasil, H.A Groiss Ltd.) and were polished by the manufacturer to give a root-mean-square roughness of
