An experimental and theoretical study of oligo- and polyelectrolyte adsorption

The purpose of this study is the description of the behaviour of a model polyelectrolyte near a model interface.Chapter 1 gives a general background and the outline of the investigations.The chapters 2 and 3 comprise the theoretical parts. Chapter 2 considers the applicability of equilibrium thermodynamics to polymer adsorption, an important prerequisite. We focus our attention on the applicability of Gibbs' law to polymer adsorption. In many cases results from surface tension measurements do not obey the classically formulated Gibbs' law, which is based on equilibrium thermodynamics. Thus, the use of equilibrium thermodynamics was questioned. We show that it is important to take the heterodispersity of the polymers used in adsorption studies into account when Gibbs' law is applied. Although not all problems could be solved completely, this result encourages the use of equilibrium thermodynamics in the polyelectrolyte adsorption theory.Chapter 3 describes the polyelectrolyte adsorption theory. Expressions are derived for the potential distribution in and the free energy of a double layer containing polyelectrolyte charge. In the case of the Debye-Hückel approximation of the Poisson-Boltzmann equation analytical expressions are obtained. When the full Poisson-Boltzmann equation is used, we have to rely on numerical procedures. The electrostatic interactions are incorporated in the phase equilibria theory of Flory and the Roe theory and the Scheutjens-Fleer theory of polymer adsorption. Electrostatic interactions strongly influence the conformation of the adsorbed polyelectrolyte. Because of the mutual repulsion of the charged segments the formation of loops and tails is strongly suppressed. Thus very flat adsorbed layers are predicted. For uncharged polymers the increase of the adsorption with chain length and polymer concentration mainly takes place in the loops and tails. As for polyelectrolytes, these are all but absent, their adsorption is nearly chain length-independent and the plateau value of the adsorption isotherms is much more constant than with uncharged polymers. Polyelectrolyte solutions are resilient against phase separation in much poorer solvents than uncharged polymers, depending on chain charge density and indifferent electrolyte concentration.The chapters 4 and 5 make up the experimental part. As the polyelectrolyte adsorption theory does not only apply to long polymer chains, but also to the adsorption of oligomers, it is important to test the theory for short chains, too.Chapter 4 describes the synthesis of these oligomers. Repeated coupling of chains of equal length yields a series of two, four, eight, sixteen and thirty-two segments long. For this kind of coupling reactions peptides are suitable compounds, since standard methods of synthesis have been developed for the benefit of protein research. Using only one type of amino acid, L-lysine, we obtained poly-L-lysine type oligomers.Chapter 5 presents characteristics for the adsorption of the model-polyelectrolyte poly-L-lysine on the model-colloid silver iodide. Several colloid chemical techniques can be applied to in vestigate this system both in the absence and in the presence of poly-L-lysine. We investigated adsorption isotherms, charge potential curves, coagulation kinetics and electrophoretic mobil ities. From the results the following picture of polyelectrolyte adsorption emerges:(i) At low ionic strength only a thin layer of adsorbed poly-electrolyte is formed.(ii) Under these conditions the adsorption is chain length in dependent, provided the chains are not too short.(iii) Because of the shielding of indifferent electrolyte some loops and tails are formed at high salt concentrations.(iv) Oligomer adsorption increases with chain length.This picture is in agreement with the theoretical predictions of chapter 3.Chapter 6 gives a quantitative comparison of the experimental and theoretical data. The quantitative agreement between theory and experiment is also satisfactory. This chapter also gives some, suggestions for further improvement of the theory and for additional experimental tests.