ON THE DIFFERENCE IN IONIZATION PROPERTIES BETWEEN PLANAR INTERFACES AND LINEAR POLYELECTROLYTES

Ionizable planar interfaces and linear polyelectrolytes show markedly different proton-binding behavior. Planar interfaces protonate in a si ngle broad step, whereas polyelectrolytes mostly undergo a two-step pr otonation. Such contrasting behavior is explained using a discrete-cha rge Ising model. This model is based on an approximation of the ioniza ble groups by point charges that are treated within a linearized Poiss on-Boltzmann approximation. The underlying reason as to why planar int erfaces exhibit mean-field-like behavior, whereas linear polyelectroly tes usually do not, is related to the range of the site-site interacti on potential. For a planar interface, this interaction potential is mu ch more long ranged if compared with that of the cylindrical geometry as appropriate to a linear polyelectrolyte. The model results are in s emi-quantitative agreement with experimental data for fatty-acid monol ayers, water-oxide interfaces, and various linear polyelectrolytes.