Polymer adsorption at the solid-liquid interface

This section revisits adsorption as the main fundamental mechanism of polymer retention in porous media in most practical situations. That is, it is assumed that mechanical retention has been screened out and that hydro-dynamic retention is small. 

An enormous body of experiment and theory on polymer adsorption exists, and the treatment here will be very cursory (Lipatov and Sergeeva, 1974 Parfitt and Rochester, 1983). As noted above, the most important single quantity is the amount of polymer adsorption as measured either as T (mass polymer/mass of rock) or by the more basic quantity, the surface excess, (mass polymer/unit area of substrate). 

For a bulk static adsorption experiment, a given amount of adsorbent substrate of total surface area, A, is put into a volume V of polymer solution of initial concentration, (mass/unit volume of solution). When the adsorption has reached equilibrium, the bulk solution concentration, C2, is then measured and the difference is denoted by AC (= — C2). The surface 

We now briefly consider the time to attain equilibrium in the polymer adsorption process. In most situations, adsorption is delayed by the time of access to the surface, which is often controlled by diffusion, thus favouring faster access of low molecular weight fractions. However, even after the macromolecules reach the surface, the following further changes are expected 

This latter effect leads to exchanges of small ions (including H ) between the surface and the solution. All these phenomena usually contribute to the delay in the attainment of equilibrium, which may vary from minutes to as long as weeks for certain systems. 

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In the various sections of this chapter, I will briefly describe the major characteristics of FT-IR, and then relate the importance of these characteristics to physiochemical studies of colloids and interfaces. This book is divided into two major areas studies of "bulk" colloidal aggregates such as micelles, surfactant gels and bilayers and studies of interfacial phenomena such as surfactant and polymer adsorption at the solid-liquid interface. This review will follow the same organization. A separate overview chapter addresses the details of the study of interfaces via the attenuated total reflection (ATR) and grazing angle reflection techniques. 

IV. AGGREGATE FRAGMENTATION INDUCED BY POLYMER ADSORPTION AT THE SOLID/LIQUID INTERFACE... 

I. Caucheteux, H. Hervet, F. Rondelez, L. Auvray, and J. P. Connon, Polymer adsorption at the solid-liquid interface the interfacial concentration profile, in New Trends in Physics and Physical Chemistry of Polymers (L. H. Lee, ed.). Plenum Press, New York (1989). 

Of particular interest has been the study of the polymer configurations at the solid-liquid interface. Beginning with lattice theories, early models of polymer adsorption captured most of the features of adsorption such as the loop, train, and tail structures and the influence of the surface interaction parameter (see Refs. 57, 58, 62 for reviews of older theories). These lattice models have been expanded on in recent years using modem computational methods [63,64] and have allowed the calculation of equilibrium partitioning between a poly-... 

FIGURE 7.27 Schematic representation of an adsorbed polymer chain at the solid-liquid interface. (From Sato, T. and Ruch, R., in Steric Stabilization of Colloidal Dispersion by Polymer Adsorption, Marcel Dekker, New York, 1980. With permission.)... 

An alternative (and perhaps more efficient) polymeric surfactant is the am-phipathic graft copolymer consisting of a polymeric backbone B (polystyrene or poly(methyl methacrylate)) and several A chains ( teeth ) such as poly(ethylene oxide). The graft copolymer is referred to as a comb stabiliser - the polymer forms a brush at the solid/liquid interface. The copolymer is usually prepared by grafting a macromonomer such as methoxy poly(ethylene oxide) methacrylate with poly(methyl methacrylate). In most cases, some poly(methacrylic acid) is incorporated with the poly(methyl methacrylate) backbone - this leads to reduction of the glass transition of the backbone, making the chain more flexible for adsorption at the solid/liquid interface. Typical commercially available graft copolymers are Atlox 4913 and Hypermer CG-6 supplied by ICI. 

In this chapter, some of the basic ideas on polymer adsorption at a solid-liquid interface are briefly discussed. The different types of polymer retention mechanism within a porous medium as referred to above are then reviewed, together with discussion of how these may be measured in the laboratory both static and dynamic adsorption are discussed in this context. Retention of HPAM and xanthan are then considered and the levels observed and their sensitivities to polymer, solution and porous medium properties are discussed. The effect of polymer retention in reducing core permeability is also considered. Finally, some work on the effect of polymer adsorption on two-phase relative permeability, which is of some relevance in the polymer treatment of producer wells in order to control water production, is reviewed. 

Figure 5.4. Schematic view of polymer adsorption at a solid-liquid interface showing loops, trains and tails and the corresponding segment distribution function, p(z).

Small-angle neutron scattering has been available for some twenty years now. By contrast, neutron reflectometry has only been actively pursued in the last five years. Like SANS, NR has been rapidly applied to many different materials, notably surfactants, and has not been confined to polymers alone. Essentially, NR can be used to measure the density profile and thickness of a surface layer provided that sufficient contrast is available. Applications of NR to polymers have included surface segregation, Langmuir-Blodgett films, interdiffusion and adsorption at the solid-liquid interface, and these will be mentioned here. 

Fleer, G. J., and J. Lyklema (1983), "Adsorption of Polymers", in G. D. Parfitt and C. H. Rochester, Eds., Adsorption from Solution at the Solid/Liquid Interface, Chapter 4, Academic Press, London. 

Alternatively, several workers have shown that not only is the soluble, zero-charged hydrolysis product considerably more surface active than the free (aquo) ion but also a polymeric charged or uncharged hydrolysis product may be formed at the solid-liquid interface at conditions well below saturation or precipitation in solution. Hall (5) has considered the coagulation of kaolinite by aluminum (III) and concluded that surface precipitates related to hydrated aluminum hydroxide control the adsorption-coagulation behavior. Similarly Healy and Jellett (6) have postulated that the polymeric, soluble, uncharged Zn(OH)2 polymer can be nucleated catalytically at ZnO-H20 interfaces and will flocculate the colloidal ZnO via a bridging mechanism. 

FIGURE 17.8 Illustration of adsorption of a polymer at the solid-liquid interface, inhibiting particle agglomeration via a steric barrier, (a) Adsorbed polymer on the surface of two particles (b) interpenetration of the adsorbed layers as the particle surfaces approach is energetically unfavorable owing to osmotic and entropic phenomena. (Reprinted from Meyers, D. (19ffiljrfaces, Interfaces, and CollojctefCH Publishers, Inc.,... 

Interestingly, protein adsorption is also a field of biological interfacial chemistry which parallels that of synthetic materials at the solid - liquid interface. A number of spectroscopic advances have been made which allow FT-IR to be used in kinetic monitoring of protein adsorption on metals and "biocompatible" polymers. In addition to providing in - situ measurements of total adsorbed protein, FT-IR can also yield information about perturbation of protein secondary structure in adsorbed layers. 

Polymers are also essential for the stabilisation of nonaqueous dispersions, since in this case electrostatic stabilisation is not possible (due to the low dielectric constant of the medium). In order to understand the role of nonionic surfactants and polymers in dispersion stability, it is essential to consider the adsorption and conformation of the surfactant and macromolecule at the solid/liquid interface (this point was discussed in detail in Chapters 5 and 6). With nonionic surfactants of the alcohol ethoxylate-type (which may be represented as A-B stmctures), the hydrophobic chain B (the alkyl group) becomes adsorbed onto the hydrophobic particle or droplet surface so as to leave the strongly hydrated poly(ethylene oxide) (PEO) chain A dangling in solution The latter provides not only the steric repulsion but also a hydrodynamic thickness 5 that is determined by the number of ethylene oxide (EO) units present. The polymeric surfactants used for steric stabilisation are mostly of the A-B-A type, with the hydrophobic B chain [e.g., poly (propylene oxide)] forming the anchor as a result of its being strongly adsorbed onto the hydrophobic particle or oil droplet The A chains consist of hydrophilic components (e.g., EO groups), and these provide the effective steric repulsion. 

Flocculation of alumina suspensions obtained by the sequential addition of polystyrene sulfonate (M j, = 4600) and cationic polyacrylamide (M, = 4,000,000) at pH 4.5 is compared in Figure 7.33 with that obtained using single polymers. While the anionic polystyrene sulfonate had only a minor effect, cationic polyacrylamide did not produce any flocculation. However, when used together, both polymers adsorb completely. This coadsorption is attributed to the interaction of complexes between cationic polyacrylamide and the polystyrene sulfonate at the solid-liquid interface. The mechanism of the superior flocculation obtained with the dual polymer system is illustrated schematically in Figure 7.34. The anionic polystyrene sulfonate adsorbs on alumina surface and acts as an anionic anchor for the adsorption of the long-chain cationic polymer, which ultimately provides interparticle bridging and excellent flocculation. 

These forces and hence the stability of the dispersions can be altered/controlled by the adsorption of ions, surfactants, or polymers at the solid-liquid interface. Adsorption of surfactants and polymers at the solid-liquid interface depends on the nature of the surfactant or polymer, the solvent, and the substrate. Ionic surfactants adsorbing on oppositely charged surfaces exhibit a typical four-region isotherm. Such adsorption can alter the dispersion stability mainly by changing the double layer interaction, which depends on the extent of adsorption. Thus, it is seen that alumina suspensions are destabilized by the adsorption of SDS when the zeta potential is reduced to zero. At higher concentrations, bilayered surfactant adsorption can occur with changes in wettability and flocculation of the particles by altering the hydrophobic interactions. 

Surface and Interfacial Aspects of Biomedical Polymers, vol. 2, Plenum Press New York, 1985 (b) Landau, M.A. Molecular Mechanism of Action of Physiologically Active Compounds, Nauka Moscow, 1981 (c) Protein at Interfaces Physicochemical and Biochemical Studies, Brash, JL. Horbett, T.A., Eds., ASC Symp. Ser., vol. 342, Amer. Chem. Soc. Washington, DC, 1987 (d) Parfitt, G.D. Rochester, C.H., Eds. Adsorption from Solution at the Solid/Liquid Interface, Academic Press London, 1983 (e) Sato, T. Ruch,... 

Effect of Pol3rmer. In recent years, as described previously, much attention, both experimental and theoretical, has been focused on surfactant-polymer interaction in solution. Less experimental work, however, is done on the interaction between polyelectrolyte and surfactant of similar charge at the solid-liquid interface. Static adsorption experiments from the chemical literature indicate that the polymer does not affect the adsorption of the surfactant onto solid material as long as the surfactant concentration is above the CMC, apparently owing to the availability of sufficient surface sites for adsorption of the surfactant molecules [40, 41],... 

The adsorption of polymers at the liquid/liquid interface is somewhat different from that at the solid/liquid interface as the polymer can penetrate both phases, x determines the adsorption behavior of polymers at liquid/liquid interfaces. The presence of the polymer at the interface between the two immiscible liquids lowers the surface tension. Determination of the adsorption isotherm (see Section IX.B) is more straightforward compared to particulate dispersions as surface tension measurements, interpreted using the Gibbs equation, can be used to give accurate adsorbed amounts. 

The broad aspects of adsorption-flocculation reactions of macromolecules at the solid-liquid interface have been reviewed by LaMer and Healy (220). For each polymer-colloid system, maximum flocculation occurs over a narrow concentration range of flocculant. 

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