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Polyelectrolyte adsorption on metals: Effects of an applied surface potential
Abstract
Electrostatic interactions may direct the behavior of polyelectrolytes at solution/solid interfaces, with these comparatively long-ranged interactions influencing both the adsorbed chain conformation and the amount of polymer adsorbed. The microscopic structure of adsorbed polyelectrolyte layers is difficult to measure, however, and present knowledge on the subject derives from a relatively small number of experiments. To provide further insight into the role of electrostatic interactions, this thesis examines, using in situ ellipsometry, the effect of a variable applied surface potential on the structure of an adsorbed polyelectrolyte layer at the solution/metal interface. Previous investigators reported that significant changes in layer thickness accompanied variations in surface potential, a phenomenon presumably traced to the attraction or repulsion of segments from the surface. However, oxidation/reduction of the surface also accompanies variations in this potential, and these chemical rearrangements were ignored when the ellipsometric data were analyzed, resulting in an inaccurate determination of thicknesses and adsorbances. Our study is thus the first to establish a method by which ellipsometry can be combined with voltammetry to correctly determine the effects of an applied surface potential on the structure of an adsorbed polyelectrolyte layer. As part of this method, we have established a protocol by which ellipsometric results can be corrected for surface oxidation/reduction. Two amphoteric polyelectrolyte systems are examined: $\gamma$-globulin, which possesses a rigid globular structure, and gelatin, which exhibits a flexible coil conformation in solution. Both are adsorbed onto platinum from phosphate buffers at pH values above their isoelectric points, creating a net negative polymer charge. Locally, however, both positive and negative charges exist simultaneously on the polymer chains. Due to the extremely rigid conformation of $\gamma$-globulin, provided by 16 disulfide bond linkages, no potential-induced changes in adsorbed layer thickness or plateau adsorbance are observed after ellipsometric data is corrected for surface oxidation/reduction. In contrast, gelatin's flexible nature would appear more conducive to adsorbed layer alterations with surface potential. Again, however, upon investigation, no changes in adsorbed layer thickness or amount adsorbed are detected, irrespective of ionic strength. The results suggest that lateral segment-segment interactions within a flexible polyelectrolyle layer are more important to layer structure than long-range segment-surface interactions.
Subject Area
Polymers|Chemistry
Recommended Citation
Yarmey, Marianne, "Polyelectrolyte adsorption on metals: Effects of an applied surface potential" (1996). Doctoral Dissertations Available from Proquest. AAI9639054.
https://scholarworks.umass.edu/dissertations/AAI9639054