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Oscillation Phenomenon at Polymer Adsorption Sergiy Minko,* Andriy Voronov,† and Emile Pefferkorn Institut Charles Sadron, CNRS, 6 rue Boussingault, 67083 Strasbourg Cedex, France Received May 23, 2000. In Final Form: July 13, 2000
Adsorption of polymers is a fundamental phenomenon playing a very important role in techniques and nature: polyelectrolyte mono- and multilayers, colloidal stabilization/flocculation introduced by adsorbed polymers in numerous technologies, adsorption mechanism of adhesion, stabilization of soils and their filtration properties, aggregation and sedimentation of clay particles with adsorbed biopolymers in rivers and seas, adsorption of proteins on surface of implants, and adsorption of biopolymers on the surface of cell membranes. Despite the intensive investigation of polymer adsorption for the past several decades, there are still gaps in understanding the process. Particularly, changes occurring during long periods of time of equilibration in adsorbed layers were found to be very slow and have a complicated nonequilibrium character.1-3 The kinetics of polymer adsorption appears in the literature as a two-step process. The first step is the diffusion-limited adsorption, where the amount of the adsorbed polymer increases with the square root of time. Chains enter into contact with the surface and adsorb retaining their solution conformation. The second step is the slow reconformation of the adsorbed chains, when the chains become progressively flatter. When the surface is crowded, the adsorption slows down because of the interaction between the neighboring adsorbed macromolecules.4 Polymer coils occupy the adsorbent surface randomly until a layer state allows no free place between adsorbed coils for newcomer molecules. Adjacent molecules strongly repulse each other in the adsorbed layer. Further random occupation of the adsorption sites on the surface is possible if chains diffuse on the surface or subsequently adsorb onto and desorb from the surface. Repulsion between adsorbed chains acts in the direction opposite to flattening of the adsorbed coils. It was reported5 that at saturation only 20% of chain segments were in close contact with the substrate while at a low coverage this value was 50%. In the way of the contradiction between the opposite trends, the system approaches a free energy minimum of equilibrium state which might result in complicated kinetics.6,7 It was assumed that a fast adsorption-desorption exchange results in a monotonically increasing kinetic curve and smooth adsorption isotherm.8 Nevertheless, some non* Corresponding author. Present address: Institut fu¨r Polymerforschung, Hohe Strasse 6, 01069 Dresden, Germany. † Present address: Physikalische Chemie II, Universita ¨t Bayreuth, Universita¨tsstrasse 30, 95447 Bayreuth, Germany. (1) Pefferkorn, E.; Carroy, R.; Varoqui, R. J. Polym. Sci. Polym. Phys. Ed. 1985, 23, 1997. Pefferkorn, E.; Haouam, A.; Varoqui, R. Macromolecules 1989, 22, 2667. (2) Johnson, H. E.; Granick, S. Macromolecules 1990, 23, 3367. Frantz, P.; Granick, S. Phys. Rev. Lett. 1991, 66, 899. Johnson, H. E.; Granick, S. Science 1992, 255, 966. (3) Fu, Z.; Santore, M. Macromolecules 1999, 32, 1939. (4) Pefferkorn, E. Adv. Colloid Interface Sci. 1995, 56, 33. (5) Killman, E. Polymer 1976, 17, 864. (6) Schneider, H. M.; Frantz, P.; Granick, S. Langmuir 1996, 12, 994. (7) van Eijk, M. C. P.; Cohen Stuart, M. A. Langmuir 1997, 13, 5447.
linear effects were reported. The adsorption overshoot was documented.9 Nonmonotonic adsorption isotherms at polymer surface concentrations corresponding to transition between diluted and semidiluted regime of polymer surface concentration were theoretically predicted and obtained in experiments.10 Recently, we reported the oscillation in the average aggregate sizes of silica particles covered by protonated poly(2-vinylpyridine) (PVP) in water with time of adsorption.11 The same oscillatory behavior was observed for the contact angles of wetting liquids on layers of the adsorbed PVP on silica. However, it was not clear whether these oscillations resulted from changes in the adsorbed chain conformation and/or in the adsorption amount. In this paper we show that polymer adsorption has a cooperative character and runs by several subsequent adsorption-desorption steps. The adsorption experiment on the surface of spherical glass (Pyrex) beads of a diameter 0.5-10 µm was performed in such a manner as to enable precise measurements of polymer concentrations in water solution at pH 3.0 using the radiolabeled protonated poly(2-vinylpyridine). Details of the experimental method were published elsewhere.12 In the experiments, a solution of PVP in water was slowly pumped through the cell with a suspension of glass beads and the effluent was analyzed to determine the concentrations of radioisotopes. The surface area and the bulk volume were fixed whereas the polymer concentration in the injected fluid was adapted in order to allow the surface to bulk (and the inverse) transfer of PVP molecules to approach a quite equal number of adsorbed and solute macromolecules. These experimental conditions allowed (i) the changes of the adsorption amount to be measured with the highest precision from the change of the polymer concentration in solution, (ii) the rate of adsorption to be determined with accuracy, and (iii) the desorption to be not screened by adsorption because rates of both processes were expected to be of the same order of magnitude under these experimental conditions. A reactor of 20 mL containing 15 mL of water at pH 3.0 and 1.5 g of suspended glass beads was degassed first to create a true water/adsorbent interface. Then, after addition of the weighed dose of radioactive PVP solution under stirring, the cell was immediately completely filled with the solvent. This operation fixed the initial concentration of the radioactive PVP in solution C0 which decreases in time due to the adsorption on glass beads. A solution of radioactive polymer of concentration Cinj was then injected through the inlet aperture at the constant rate (0.0056 mL/min) with the automatic syringe. Usually Cinj was several times smaller than C0. The outlet aperture was fitted with a macroporous polyethylene filter in order to retain the glass beads within the cell and the effluent was collected in fractions of 0.247 ( 0.005 g. We obtained the same pH ) 3.0 values after the experiment was (8) Fleer, G. I.; Cohen Stuart, M. A.; Scheutjens, J. M. H.; Cosgrove, T.; Vincent, B. Polymer at Interfaces; Chapman & Hall: London, 1993. (9) Pefferkorn, E.; Elaissari, A. J. Colloid Interface Sci. 1990, 138, 187. Dorgan, J. R.; Stamm, M.; Toprakcioglu, C.; Je´roˆme, R.; Fetters, L. J. Macromolecules 1993, 26, 5321. (10) Minko, S.; Luzinov, I.; Evchuk, I.; Voronov, A. Adsorpt. Sci. Technol. 1995, 14, 251. Voronov, A.; Luzinov, I.; Minko, S.; Sidorenko, A.; Vakarin, E.; Holovko, M. Macromolecules 1997, 30, 6929. Voronov, A.; Pefferkorn, E.; Minko, S. Macromolecules 1998, 31, 6387. (11) Voronov, A.; Pefferkorn, E.; Minko, S. Macromol. Rapid. Commun. 1999, 20, 85. (12) Pefferkorn, E. J. Colloid Interface Sci. 1999, 216, 197.
10.1021/la000714n CCC: $19.00 © 2000 American Chemical Society Published on Web 09/08/2000
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Figure 1. Representation as a function of time of the specific radioactivity (CPM ) counts per minute) of the cell contents in absence of the adsorbent for the Cinj ) C0 ) 0.004 mg/g (squares) and Cinj ) 0.03 mg/g, C0 ) 0.15 mg/g (circles).
finished and pH was measured in the solution extracted from the cell. The radioactivity counted in terms of counts per minute (CPM), corresponding to the polymer content of the sample, was reported as a function of time. The experiments were carried out for 70-150 h. Figure 1 presents two reference experiments performed in the absence of the adsorbent. These experiments show the constancy of the measured data for a long period of time and the expected monotonic decrease of the PVP concentration resulting from the injection into the reactor of a more diluted solution. The experiment in the presence of the adsorbent shows very pronounced oscillations in the PVP bulk concentration (Figure 2). This oscillatory phenomenon may result from the cooperative character of the adsorption and desorption steps. The period of 21 h was observed for different values of C0. The oscillation amplitude usually decreases with time but it is instantaneously stimulated by fast injection of a larger portion of the polymer. These pronounced oscillations of similar periodicity were found for several polymers of small polydispersity index and weight average molecular weight (Mw) ranging from Mw ) 78 000 g/mol to Mw ) 540 000 g/mol. Conversely, data obtained for polymers of larger molecular weight polydispersity which were prepared by mixing of two or more polymers of small polydispersity index display different shapes (Figure 3). In the latter case the oscillations are less pronounced and of shorter periodicity. We determined the oscillation with a periodicity of 12 h in the case of adsorption of uncharged PVP from ethanol on the glass beads (Figure 4). The phenomenon documented evidences a complex cooperative behavior at polymer adsorption. We may address two major questions concerning the mechanism and the periodicity of the process. The cycle consists of three steps: (1) a fast diffusionlimited adsorption, (2) a relatively long period where chains are more or less staying on the surface, and (3) a fast diffusion-limited desorption. The second stage corresponds to the reconformation and it is responsible for the oscillation period. We may assume the following scenario. The chains rapidly adsorb on the surface and occupy surface sites randomly. At some given moment there is no available free space on the surface and adsorption is stopped. It turns out that surface sites are occupied ineffectively because of random distribution of chains on the surface. Chains change their conformation to fit better to the interaction with the substrate. This
Figure 2. Adsorption kinetics of PVP (Mw ) 102 000 g/mol, Mw/Mn ) 1.26) from water solution for the Cinj ) 0.03 mg/g, C0 ) 0.15 mg: (a) representation as a function of time of the specific radioactivity of the cell contents during adsorption; (b) representation in terms of adsorbed amount per weight of the glass beads.
Figure 3. Adsorption kinetics of mixture (1:1) of two PVP samples with Mw ) 102 000 g/mol, Mw ) 278 000 (Mw/Mn ) 1.20) for the Cinj ) 0.03 mg/g, C0 ) 0.15 mg/g.
process runs until it is stopped by a strong repulsion between neighboring chains. At this point, all chains interact with each other in large areas on the surface. The free energy change due to segment-substrate interaction is compensated by elastic energy of coils and volume excluded effect. The layer is unstable at this point (a metastable state). Unoccupied sites on the surface are attacked by residual chains coming from the bulk (or belonging to other colloids and thus the process lead to colloid aggregation as reported in ref 11). All these factors
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Figure 4. Adsorption kinetics of PVP (Mw ) 102 000 g/mol, Mw/Mn ) 1.26) from ethanol solution for the Cinj ) 0.03 mg/mL, C0 ) 0.15 mg/mL.
result in cooperative desorption of chains from the large areas. The following adsorption step results in more optimal chain packing on the surface and so on. As the packing density progressively increases, the amplitude of the oscillation decreases and peaks split in smaller peaks (Figure 2) as long as effective (close to equilibrium) packing of all small areas is not complete. The similar scenario was analyzed using phenomenological models where the
Notes
condition for oscillation was found at very large time of reconformation (relaxation).13,14 For the case of adsorption of polymer samples of large polydispersity index, an optimal packing may be approached faster and consequently interactions between densely packed neighboring chains results in smaller periodicity and smaller overshoot. The phenomenon which is documented potentially may serve to determine the polydispersity of polymer samples, especially when other traditional methods are not or less effective (e.g., for very polar and natural macromolecules) and spectroscopic methods may be employed for effluent analysis. The oscillations may dramatically affect the surface characteristics and interfacial properties,11 and knowledge of conditions leading to their occurrence may be useful for the understanding of adsorption phenomena in artificial and natural systems as well as in multilayer processing of polyelectrolyte complexes. Acknowledgment. Financial support was provided by the Centre National de la Recherche Scientifique and NATO Linkage Program. We thank Mrs. J. Widmaier for technical assistance and V. Gafijchuk for fruitful discussions. LA000714N (13) Oshshima, H.; Fujita, N.; Kondo, T. Colloid Polym. Sci. 1992, 270, 707. (14) Filippov, L.; Filippova, N. J. Colloid Interface Sci. 1996, 178, 571.
