The adsorption of weak polyelectrolytes and polyampholytes : an experimental study

The objective of this study was to collect systematic data on the adsorption behaviour of weak polyelectrolytes and polyampholytes. The measurements were performed on well-defined systems in order to be able to compare the results with the recently developed theories of Evers et al. and Bohmer et al. for the adsorption of weak polyelectrolytes. The adsorption of polyampholytes was studied in order to help bridge the gap between the theoretically well understood behaviour of polyelectrolytes at interfaces, and the adsorption characteristics of proteins, which are, so far. mainly experimentally documented. In order to compare experiment with theory it is essential to be able to vary the surface charge and the polymer charge independently. In Chapter 1 we describe the synthesis of a monodisperse, positively charged polystyrene latex with fixed surface charge. The positive charge results from quarternary ammonium groups and proved it to be independent of the pH in the range 4-10, as shown by streaming potential measurements on plugs. In Chapter 2 the adsorption data of poly(acrylic acid) onto this positively charged latex are reported and compared with theory. The agreement between theory and experiment is remarkably good. The theoretically predicted maximum in the adsorbed amount as a function of pH is fully confirmed experimentally. The maximum occurs because of two opposing trends. With increasing pH, the increased negative charge on the polyacid leads to a stronger attraction between surface and polyelectrolyte groups and, thus, to a higher adsorption. On the other hand, the higher repulsion between the segments opposes accumulation of polymer near the interface. At relatively low pH the latter effect is small and the electrostatic contribution to the adsorption energy gives high adsorbed amounts. At higher pH, the intersegmental repulsion leads to a decreased adsorption. It was found experimentally that the adsorption is almost independent of the salt concentration. This is also predicted by theory. The computations showed that the segments in contact with the surface are dissociated to such a degree that the surface charge is just balanced. The segments in loops and tails are dissociated to a much lower degree. The effective charge of the colloidal particle plus the adsorbed polymer layer is therefore small, so that salt will hardly affect the adsorbed amount. The adsorbed amount increases slightly with increasing molecular weight of the polyelectrolyte and with increasing surface charge. These trends also correspond to theoretical predictions; the agreement is again semi- quantitative. A next step was the adsorption of simple polyampholytes. Because well-defined homodisperse polyampholytes are not commercially available. we decided to synthesize such a macromolecule ourselves. The synthesis of a model polyampholyte is described in Chapter 3. The starting point for the polymerization was the tripeptide L-lysyl-L-glutamyl-glycine, with fully protected aminoand carboxylic functions. Coupling of two tripeptide molecules to a hexapeptide, two hexapeptide molecules to a dodecapeptide, etc. had to be attained by activation of the caboxylic terminal amino acid (c- terminus) with dicyclohexyl carbodiimide. It is known that this activator causes racemisation of the c-terminus. Therefore glycine was chosen as the c-terminus. The controlled coupling had to be given up at the dodecapeptide level due to the extremely low solubility of the fully protected peptide derivative. In Chapter 4 adsorption studies are described with this dodecapeptide (L-lysyl-L-glutamyl-glycine) 4 and with a commercially available random copolymer of lysine and glutamic acid, both on positively and on negatively charged polystyrene latices. It was found that the dependence of the adsorption on pH was the same for adsorption on the positively and on the negatively charged latex: high adsorbed amounts at low pH and virtually no adsorption at high pH. This behaviour is easily understood for the negatively charged latex: at low pH the positively charged macromolecule adsorbs easily on the negative surface, whereas at high pH the molecule and surface repel each other. However, for the positively charged latex one would expect just the reverse so that the observation is difficult to understand. A possible explanation is that at low pH the "chemical" affinity of the adsorbate for the adsorbent is larger than the electrostatic repulsion. while at high pH the very good solubility of the polyampholyte counteracts the electrostatic attraction with the surface to such an extent that the adsorption is only small. In conclusion, the adsorption behaviour of weak polyelectrolytes with only one type of charged group is now understood reasonably well, and the agreement between theory and experiment is excellent. For polyampholytes with anionic and cationic groups in the molecule there Is no symmetry in the behaviour with respect to positive and negative surfaces. Since, for purely non-specific electrostatic Interactions. such symmetry must exist. other factors such as solubility or formation of internal ionic bonds within the polyampholyte must be held responsible for the observations. The latter conclusion may be important for the understanding of proteins at interfaces.