Adsorbed polymer displacement

Effect of adsorbed polymer on the double-layer. Because of the presence of adsorbed train segments, the double layer is modified. The zeta-potential, , is displaced because the adsorbed polymer displaces the plane of shear. The parameters for describing adsorbed polymers are the fraction of the first layer covered by segments, 0, and the effective thickness, A, of the polymer layer, The insert gives the distribution of segments over trains and loops for polyvinyl alcohol adsorbed on silver iodide. Results obtained from double layer and electrophoresis measurements. 

Isolation of the molecules can be achieved, e.g., by adsorbing polymers in very low concentrations on suitable substrates. If clustering of the molecules can be avoided and the adhesion energies are sufficiently high, imaging of individual molecules by AFM methods, and here, in particular, intermittent contact or tapping mode AFM, is possible. Contact mode AFM is less advisable, since due to lateral forces the tip tends to displace and transfer molecules across the sample. The first example of a successful imaging of the conformation of a flexible polymer was reported by Kumaki et al., who studied PS-6-PMMA on mica (Fig. 3.8) [22]. 

Assuming that the W/O emulsion behaves as a near hard-sphere dispersion, it is possible to apply the Dougherty-Krieger equation [7, 8] to obtain the effective volume fraction, 4>. The assumption that the W/O emulsion behaves as a near hard sphere dispersion is reasonable as the water droplets are stabihsed with a block copolymer with relatively short PHS chains (of the order of lOnm and less). Any lateral displacement of the polymer will be opposed by the high Gibbs elasticity of the adsorbed polymer layer, and the droplets will maintain their spherical shape up to high volume fractions. 

It is also possible to study exchange kinetics of the adsorbed polymers using these techniques. In the case of IR spectroscopy, exchange between deuterated and hydrogenated polymer chains at the surface can be studied as a fimction of time. From the rate of change in the specific absorption bands one can estimate the time constant for exchanging a chain in the solution with one on the surface. In the same way it is possible to study the kinetics of one species displacing another adsorbed... 

An interesting feature of adsorbed polymers is that the chains do not desorb from the surface when exposed to the pure solvent. This apparent irreversibility of adsorption arises from the fact that the activation energy required to remove a chain from a surface is extremely high. However, it is possible to replace an adsorbed chain with a displacer molecule that has a higher affinity for the surface (34). 

Rachas, I. and Taylor, P., The displacement of adsorbed polymer from silica surfaces by the addition of a nonionic surfactant , Colloid and Surfaces, A Physiochem. and Eng. Asp., Vol. 161, No. 2 (December 1999)... 

Figure 6.1. Schematic illustration of temperature-induced elution. A—at elevated temperature adsorbed polymer is in a collapsed state and interacts with one ligand making the other available for binding of purified protein B—at low temperature adsorbed polymer is in an expanded state and interacts with a few neighboring ligands displacing bound proteins in solution.

Desorption of an adsorbed polymer by flushing with solvent proceeds particularly slowly and, in practice, it is impossible to desorb a significant amount in this way. However, we have seen that desorption of a polymer can be accomplished under the influence of an external shear force. In addition, desorption may be caused by competition with a polymeric or a monomeric displacer. In this section we will discuss exchange by a chemically identical polymer or by a polymer differing in size or chemical composition. 

So- far only non-adsorbing polymers have been considered, which flocculate by a depletion mechanism. Other molecules, which are slightly hydrophobic, may be able to be adsorbed at the oil-water interface. To prepare an emulsion a surfactant is also needed, but the surfactant is slowly displaced at the interface by the polysaccharide molecules. Adsorbing polymers are well known in colloid science, and their effect on the dispersion stability depends on the surface coverage of the polymer. At high concentrations, the drop surfaces are completely covered and... 

Adsorption and Desorption Adsorbents may be used to recover solutes from supercritical fluid extracts for example, activated carbon and polymeric sorbents may be used to recover caffeine from CO9. This approach may be used to improve the selectivity of a supercritical fluid extraction process. SCF extraction may be used to regenerate adsorbents such as activated carbon and to remove contaminants from soil. In many cases the chemisorption is sufficiently strong that regeneration with CO9 is limited, even if the pure solute is quite soluble in CO9. In some cases a cosolvent can be added to the SCF to displace the sorbate from the sorbent. Another approach is to use water at elevated or even supercritical temperatures to facilitate desorption. Many of the principles for desorption are also relevant to extraction of substances from other substrates such as natural products and polymers. 

The drawback of the described adsorbents is the leakage of the bonded phase that may occur after the change of eluent or temperature of operation when the equilibrium of the polymer adsorption is disturbed. In order to prepare a more stable support Dulout et al. [31] introduced the treatment of porous silica with PEO, poly-lV-vinylpyrrolidone or polyvinylalcohol solution followed by a second treatment with an aqueous solution of a protein whose molecular weight was lower than that of the proteins to be separated. Possibly, displacement of the weakly adsorbed coils by the stronger interacting proteins produce an additional shrouding of the polymer-coated supports. After the weakly adsorbed portion was replaced, the stability of the mixed adsorption layer was higher. 

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