Kinetics of Polymer Adsorption

Tadros, Th.F. (1985) in Polymer Colloids (eds R. Buscall, T. Corner, and J. Stageman), Elsevier Applied Sciences, London, p. 105. 

(1953) Principles of Polymer Chemistry, Cornell University Press, New York. 

Cohen-Stuart, M.A., Scheutjens, J.M.H.M., Cosgrove, T., and Vincent, B. (1993) Polymers at Interfaces, Chapman Hall, London. 

Abeles, F. (1964) in Ellipsometry in the Measurement of Suifaces and Thin Films, vol. 256 (eds E. Passaglia, R.R. Stromberg, and J. Kruger), National Bureau of Standards Miscellaneous Publications, p. 41. 

Einstein, A. (1906) Investigations on the Theory of the Brownian Movement, Dover Publications, New York. 

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Ringenbach, E., Chauveteau, G., and Pefferkom, E.. Effect of soluble aluminum ions on polyelectrolyte-alumina interaction. Kinetics of polymer adsorption and colloid stabilization. Colloids Surf. A, 99, 161, 1995. 

Dijt JC, Cohen Stuart MA, Fleer GJ. Kinetics of polymer adsorption and desorption in capillary flow. Macromolecules 1992 25 5416-5423. 

This volume consists of four parts. The first part is devoted to theoretical studies and computer simulations. These studies deal with the structure and dynamics of polymers adsorbed at interfaces, equations of state for particles in polymer solutions, interactions in diblock copolymer micelles, and partitioning of biocolloidal particles in biphasic polymer solutions. The second part discusses experimental studies of polymers adsorbed at colloidal surfaces. These studies serve to elucidate the kinetics of polymer adsorption, the hydrodynamic properties of polymer-covered particles, and the configuration of the adsorbed chains. The third part deals with flocculation and stabilization of particles in adsorbing and nonadsorbing polymer solutions. Particular focus is placed on polyelectrolytes in adsorbing solutions, and on nonionic polymers in nonadsorbing solutions. In the final section of the book, the interactions of macromolecules with complex colloidal particles such as micelles, liposomes, and proteins are considered. 

FIGURE 15.2 Typical example of kinetics of polymer adsorption. 

The kinetics of polymer adsorption is a highly complex process. Several distinct processes occur simultaneously each with a characteristic time scale. Generally, it is difficult to separate these processes except under certain limiting conditions ... 

Recently, several other groups have tried to model the kinetics of polymer adsorption (and exchange) by taking into account diffusion and some aspects of reconformation of the adsorbed polymer at the surface [22-24]. 

In principle, dynamic aspects of polymer adsorption can be determined with the same methods as one uses to characterize static properties of the adsorbed polymer layer. Fleer et al. [1] have presented an overview of experimental methods for the determination of adsorption isotherms, the adsorbed layer thickness, the bound fraction, and the volume fraction profile. However, in order to determine the dynamics of some property of the adsorbed polymer layer, the characteristic time of the experimental method should be shorter than that of the process investigated. Moreover, flie geometry of the experimental system is often of crucial importance. These factors severely limit the applicability of some experimental methods. In this section we will particularly review those methods which have been successfully applied for characterizing the kinetics of polymer adsorption. 

An important aspect of the kinetics of polymer adsorption is the transfer of the polymer from solution to flic interface by convection and/or diffusion. At a bare surface, and in the absence of a barrier, the rate of adsorption is limited by mass transfer (see Section II.B). In this kind of experiment hydrodynamic conditions must be well defined in order to allow quantitative conclusions with respect to the kinetics of adsorption. However, for slow processes, like rearrangements in the polymer layer or slow polymer exchange experiments, flic rate of mass transfer to the surface does not significantly contribute to flic overall rate. In this case the design of the experimental cell is not critical and it suffices that the solution is stirred. 

Although the macroscopic curvature of the surface can have a small effect on the equilibrium properties of the adsorbed polymer layer, it will have a strong effeet if the kinetics of polymer adsorption are dominated by diffusion. Examples of frequently applied well-defined geometries were presented in flie previous section. Wang et al. [34] have studied exchange kinetics in a spherical geometry. It was shown that the influence of bulk diffusion is reduced with decreasing radius of the adsorbent particle. For slow processes the macroscopic curvature will not be very important, and powders with poorly defined macroscopic curvatures ean still be used. 

The kinetics of polymer adsorption on porous substrates is much more difficult to tackle. Besides adsorption, desorption, and exchange, size exelusion has to be taken into account. Also, most in situ methods are not applicable to porous substrates. A major difficulty is that with all available methods smeared-out properties are measured while it is likely that strong gradients in the axial direetion of the cylindrical pore are present. The process of axial equilibration is poorly understood and in many cases extremely slow. Most studies were performed with porous substrates with broad pore size and shape distributions. Controlled-pore glasses, zeolites, or porous membranes could be used as model systems with pores of molecular size. Application of glass capillaries is interesting for controlling the hydrodynamics in a curved system. 

Total internal reflection fluorescence (TIRF) has also been used for characterization of the kinetics of polymer adsorption [46-55]. Atypical set-up [56] is shown in Fig. 6. A light beam is totally reflected at the solid side (2) of a solid-solution interface resulting, on the solution side (1), in an evanescent field with the wavelengfli of the ineident light similar to that of FTIR-ATR. The solution contains fluorescent moleeules which can adsorb at the solid-solution interface. Owing to the limited... 

The kinetics of polymer adsorption has also been studied in capillary flow by measiuing the electrokinetic streaming potential [ 0], In this method one measures the Z-potential which can be related to the position of the surface of shear, fi om which the thickness of the adsorbed polymer layer may be inferred. The method is particularly sensitive for detecting tails protruding into the solution. 

IV. EXPERIMENTAL STUDIES OF THE KINETICS OF POLYMER ADSORPTION ON OXIDES... 

In the following seetions some systematie investigations of the kinetics of homo- and co-polymer adsorption are discussed. For the adsorption of neutral polymers the transport from the bulk to flie surface usually dominates the process. Exchange experiments with neufral polymers will help to unravel the mechanisms of attachment, detachment, and rearrangement. The kinetics of polyelectrolyte adsorption, and the kinetics of polymer adsorption on porous substrates are discussed in separate sections. 

Dijt et al. [41] have studied the kinetics of polymer adsorption on a flat silica surface in a stagnation-point flow cell with a fixed-angle reflectometer. The effect of solution concentration, flow rate, and molar mass on the rate of adsorption of... 

It is not easy to perform measurements of kinetics of polymer adsorption or exchange in porous systems. Pore geometries, even in model systems like con-trolled-pore glass or Stober silicas, are usually poorly denned. In situ measurements are difficult to perform. Usually, indirect measurements are performed in which one measures batchwise the time dependence of the concentration of the polymer in intensively stirred bulk solutions. As discussed above, depending on the size of the polymer and the pore radius, adsorption can be extremely slow. Of course, one must realize that even in model porous systems nonideality of the pore geometry, such as the presence of tortuous pore channels with junctions and branches, a pore size distribution, and a non-uniform pore diameter, may thwart the interpretation of experimental results. 

Frantz, P., Granick, S. Kinetics of polymer adsorption and desorption. Phys. Rev. Lett. 66, 899 (1991)... 

Figure 1.2 gives another example of the kinetics of polymer adsorption - this time the adsorption of PEI on pulp fibers [17]. The curve for the lowest PEI concentration is fitted to Equation 1.1, with 7to = 3.5, kads = 0.2 min and kdes = 0. The curves for higher PEI concentrations were modeled by assuming that a fraction of the poly disperse PEI is small enough to penetrate the pores or the lumen. At higher PEI concentrations, more low-molecular weight PEI is available for pore penetration [18]. 

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