“The most significant discovery that resulted from our study is the extraordinary shift in the acid-base equilibria that is observed for weak polyelectrolytes upon incorporation into multilayered films in comparison to their dilute solution behavior.”

This paper describes a study of the acid-base equilibria of weak polyelectrolytes that have been assembled into multilayered films. The films are prepared using the layer-by-layer assembly technique, which involves the repetitive deposition of oppositely charged polymers onto a charged substrate from dilute solution. This fundamental topic appeals to a multidisciplinary audience ranging from biologists and biochemists interested in the behavior of natural weak polyelectrolytes to polymer chemists and materials scientists who are finding new and innovative ways of using polyelectrolytes in a variety of applications. In particular, the field of multilayered polyelectrolyte films prepared using the layer-by-layer assembly technique has received considerable attention in recent years from a highly diverse field of researchers. To a large extent, the behavior of weak polyelectrolytes is controlled by their acid-base dissociation. Therefore, knowing about the acid-base behavior of weak polyelectrolytes is essential for understanding their resulting physical properties.

The study was one of the first of its kind to examine the acid-base equilibria of polymer films prepared using the layer-by-layer assembly of oppositely charged polyelectrolytes. Previous studies had shown that multilayered films of weak polyelectrolytes exhibited a wide range of anomalous physico-chemical properties that are highly dependant on the pH used to assemble the films. Our paper addresses the root cause of these observations. In addition, we were one of the first to use the well known technique of microelectrophoresis to determine the dissociation constants of the polyelectrolytes in the films. The most significant discovery that resulted from our study is the extraordinary shift in the acid-base equilibria that is observed for weak polyelectrolytes upon incorporation into multilayered films in comparison to their dilute solution behavior.

The properties of weak polyelectrolytes are uniquely dependent on the pH of their local environment over the entire pH range because of their ability to gain and lose charge along their chains with a change in pH. This, in turn, strongly influences the physico-chemical properties of these polymers (i.e. solubility, chain conformation, etc.). The overall acid-base dissociation is governed by the dissociation of the individual repeat units that carry the charged moieties. The dissociation behavior of the individual repeat units is influenced by the local pH environment and the acid-base equilibria of the nearest neighboring repeat units. In the case of weak polyelectrolytes incorporated into multilayered films, the dissociation of each repeat unit and, in turn, the overall chain, is also affected by the acid-base equilibria of the repeat units of the polyelectrolytes in the neighboring layers. The acid-base equilibria in multilayer films is made more complex by the presence of both positively and negatively charged groups within the same film. The organization of the chains within multilayered films of weak polyelectrolytes depends on their degree of dissociation during the assembly process. Thus, by knowing how the organization of the chains influences the overall acid-base equilibria of the polyelectrolytes within the films, it is possible to begin to understand the resulting physico-chemical properties of the assemblies.

The research outlined in the paper stemmed from earlier work that Professor Chris Barrett had carried out as a post-doctoral fellow. Between 1998 and 1999, he had the opportunity to work with Professor Anne Mayes and Professor Michael Rubner of the Department of Materials Science and Engineering at MIT. At that time, little was known about multilayered films of weak polyelectrolytes. Professor Rubner and his group were extensively involved in much of the early work on preparing and studying such films. Their earliest studies revealed that multilayered films of weak polyelectrolytes exhibited a rich suite of anomalous physical properties, which strongly depended on the pH used to assemble the films. The ability to manipulate the film properties to such a large extent with pH prompted many other researchers to examine the potential of weak polyelectrolyte multilayered assemblies. In 2000, Professor Chris Barrett joined the faculty in the Department of Chemistry at McGill University where he continued to pursue the study of weak polyelectrolyte multilayered films. In 2001, we began investigating their fundamental acid-base equilibria behavior because we believe that it is the key to understanding how to effectively manipulate and control multilayered films of weak polyelectrolytes.