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Langmuir 1998, 14, 4300-4307
Competitive Adsorption of Poly(ethylene oxide) Chains with and without Charged End Groups Zengli Fu and Maria M. Santore* Chemical Engineering Department, 111 Research Drive, Lehigh University, Bethlehem, Pennsylvania 18015 Received December 16, 1997 Competitive adsorption between species of the same backbone chemistry and chain length, but differing in their end groups, has been studied employing the model system of fluorescein dianion terminated PEO (F-PEO) and the corresponding native PEO [poly(ethylene oxide)] onto negatively charged silica glass. Here, one end of each F-PEO chain is electrostatically repelled from the surface. Kinetic and equilibrium measurements in gentle shearing flow were accomplished with a combination of total internal reflectance fluorescence and optical reflectivity. Pure samples of F-PEO and native PEO exhibited identical transportlimited adsorption kinetics and high-affinity isotherms, but remarkable differences became apparent during competitive adsorption from mixtures of the two. The surface selectivity, which favored adsorption of native PEO, was screened as the ionic strength was increased. The selectivity was also reduced for chains of high molecular weight, though the molecular weight effect was not as intense as the influence of ionic strength. Coadsorption kinetics over a variety of bulk solution concentrations were in excellent agreement with predictions from a binary mixture treatment that involved Langmuir isotherms for each species and allowed the relative rates of adsorption and diffusion from solution to be varied. Comparison of theory and experiment revealed that the entire competitive adsorption process including the initial coadsorption and late-stage exchange was completely controlled by the diffusion rate of the chains to the surface.
Introduction Competitive adsorption of polymers is an important concern in chromatographic separations1 and applications relying on the control of colloidal stability.2 In general, for a given surface and solvent pair, polymers of different adsorption energies will experience surface selectivity. The polymeric species may differ in backbone chemistry, in chain length, or even in chain end groups. In addition to differences in adsorption energy, frequently there are differences in mass-transfer rates stemming from different diffusivities and bulk concentrations. As a result, complex kinetic and equilibrium behavior generally occurs during competitive polymer adsorption. Chain exchange processes are expected when the less-preferred species temporarily exceeds its equilibrium surface coverage before an adequate amount of the preferred species reaches the surface. Consequently, overshooting kinetics for the less preferred species may be seen during competitive adsorption.3-5 Competitive adsorption of chemically different polymers was studied long ago6 and has been explored more extensively in the recent literature7-9 using surface sensitive techniques such as attenuated total reflection Fourier transform infrared (ATR-FTIR)7,8 and optical reflectivity.9 An early study by Thies6 demonstrated that poly(methyl methacrylate) (PMMA) adsorbed selectively (1) Tyengar, D. R.; McCarthy, T. J. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1989, 30, 154. (2) Napper, H. D. Polymeric Stabilization of Colloidal Dispersions; Academic Press: London, 1983. (3) Vroman, L.; Adams, A. L. J. Colloid Interface Sci. 1986, 111, 391. (4) Dijt, J. C.; Cohen Stuart M. A.; Fleer, G. J. Macromolecules 1994, 27, 3219. (5) Fu, Z.; Santore, M. M. Macromolecules 1997, 30, 8516. (6) Thies, C. J. Phys. Chem. 1966, 70, 3783. (7) Johnson, H. E.; Granick, S. Science 1992, 255, 966. (8) Enriquez, E. P.; Schneider, H. M.; Granick, S. J. Polym. Sci., Part B 1995, 33, 2429. (9) Dijt, J. C.; Cohen Stuart, M. A.; Fleer, G. J. Macromolecules 1994, 27, 3229.
on silica from mixtures of PMMA and polystyrene (PS) in dilute trichloroethylene solutions. There were, however, limitations on the experimental precision for kinetic measurements at that time. Johnson and Granick7 reported the slow replacement of preadsorbed PS on oxidized silicon by PMMA and argued that nonequilibrium surface states were long-lived. Dijt et al.9 found that the displacement of PS and poly(butyl methacrylate) (PBMA) on silica by poly(tetrahydrofuran) (PTHF) was rapid and independent of surface processes. The displacement of PS by PBMA was, however, much slower and, to a large extent, kinetically controlled at the surface. Dijt et al. argued that the segmental adsorption energy plays an important role in the kinetics but the chain dynamics could be another important factor. Competitive adsorption of homopolymers with different chain lengths has been another focus of polymer adsorption studies.4,5,10,11 Thermodynamically, longer chains are more competitive for the surface because the adsorption of short chains involves a greater loss of translational entropy. The displacement of short chains by longer ones can be kinetically controlled by factors such as interfacial chain dynamics, cooperative motions, and large numbers of segment-surface contacts (energy barriers). The importance of different chain-end groups in affecting the polymer adsorption behavior has been noted recently.12-16 Most of these studies involved chain-end groups with significant attraction to the surface, often leading to increased surface coverage (relative to homopolymer adsorption) and brushlike layers. Frantz, Leonhardt, and Granick12 reported the preferential adsorption of carboxylic acid-terminated PS versus hydrogen(10) Vander Linden C.; Van Leemput, R. J. Colloid Interface Sci. 1978, 67, 63. (11) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: London 1993. (12) Frantz, P.; Leonhardt, D. C.; Granick, S. Macromolecules 1991, 24, 1868.
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Adsorption of PEO Chains
terminated chains from cyclohexane onto silicon oxide. The differential adsorption enthalpy for the carboxylic acid end group was estimated to be about 6.4 kT. Kawaguchi et al.13,14 investigated the adsorption of normal polybutadiene (PBR) and PBR terminated with a highly polar and surface-attracted group from carbon tetrachloride onto silica. They found that the terminated PBR had an adsorbed amount and a surface site (silanols) binding amount twice that for normal PBR while the bound fraction of the main-chain segments was about the same. There have also been reports15,16 that adsorption can be induced for chains that would not otherwise adsorb, by attachment of end groups with high surface affinity. Situations such as this typically exhibit an independence of the surface coverage on the molecular weight.15 We know of no work examining homopolymers whose chain ends are electrostatically repelled from the substrate. Such a system is important because it is tunable through ionic strength, and the competitive adsorption of such functionalized homopolymers comprises a means of probing the issue of varied charge density in polyelectrolyte adsorption. In the present study, we exploit a fluorescent terminal group that functions as a model charged end group and a label to distinguish the functionalized chains from the native ones. This work reports competitive adsorption behavior in mixtures of fluorescein-tagged PEO and native PEO of the same molecular weight on negatively charged silica. The fluorescein dye bears two negative charges at pH 7 and above, making the labeled chain ends electrostatically repelled from the surface. We employ total internal reflectance fluorescence (TIRF) to monitor the evolution of adsorbed populations of labeled PEO in the mixture and near-Brewster optical reflectivity (which does not distinguish between labeled and unlabeled species) to measure the total adsorbed amount. Hence, various populations can be distinguished in competitive coadsorption kinetics. Kinetic and equilibrium aspects of competitive adsorption are presented and compared with a theoretical treatment for coadsorption kinetics. Experimental Section Adsorption Substrate and Flow Cell. Soda-lime glass microscope slides (FISHERfinest) were used as the adsorption substrate. The bulk glass composition was reported by the manufacturer (Erie Scientific) to be as follows: SiO2, 72.1%; Na2O, 14.0%; CaO, 7.3%; MgO 3.8%; Fe2O3/FeO, K2O, Al2O3
