Mechanism of polyelectrolyte multilayer growth: Charge overcompensation and distribution

A mechanism for distributing excess polymer surface charge is used to model the growth of multilayers of strongly charged polyelectrolytes. Two parameters are required for the semiempirical analysis: the surface, or unrestricted, charge overcompensation level, phi, which is assumed to decrease exponentially from the film surface to bulk, and the characteristic length for this decay, l(cp), which is termed the charge penetration length. Modeling of the data reveals that only modest levels of polymer charge overcompensation are required to account for large increments in polymer thickness, realized at high salt concentration, since the excess charge is distributed over several layers. Experimentally, phi appears to be roughly independent of salt concentration. The thickness increment is primarily controlled by l(cp), which is about 2.5 nominal layers for the system studied. Whereas the growth conditions and polyelectrolyte type lead to the formation of intrinsically compensated multilayers in this work, conditions for obtaining extrinsic compensation are also discussed. Kinetic vs thermodynamic limitations for polymer addition during a deposition cycle are contrasted.