The role of counter ions and ion dissociation in establishing the equilibrium swelling of balanced and unbalanced polyampholytic hydrogels has been investigated experimentally and theoretically. The swelling dependence on both the net charge offset and the external bath salt concentration has been examined using an acrylamide based polyampholytic hydrogels. By careful consideration of the swelling kinetics, we illustrate the effects of ion dissociation equilibria and counter ion shielding in polyampholytic hydrogels near their balance point where both polyelectrolyte and polyampholyte effects are present. The theory considers a Flory type swelling model where the Coulombic interactions between fixed ions in the hydrogel resemble those of an ionic solid with a Debye screening factor. Theoretical predictions from this model are in qualitative agreement with our experimental results.

Modelling of heterogeneous processes, such as electrochemical reactions, extraction, or ion-exchange, usually requires solving the transport problem associated with the process. Since the processes at the phase boundary are described by scalar quantities and transport quantities are vectors or tensors, the coupling of them can take place only via conservation of mass, charge, or momentum. In this book transport of ionic species is addressed in a versatile manner, emphasizing the mutual coupling of fluxes in particular. Treatment is based on the formalism of irreversible thermodynamics, i.e., on linear (ionic) phenomenological equations, from which the most frequently used Nernst-Planck equation is derived. Limitations and assumptions made are discussed in detail. The Nernst-Planck equation is applied to selected problems at the electrodes and in membranes. Mathematical derivations are presented so that the reader can learn the methodology of solving transport problems. Each chapter contains a large number of exercises.