Strong Charge Inversion and
Layer-by-Layer Assembly of Flexible Polyelectrolytes from Self-Consistent Field
Calculations
Qiang Wang
Department of Chemical and Biological Engineering
We have applied a continuum self-consistent field (SCF)
theory to flexible polyelectrolytes on flat surfaces either uncharged or
carrying opposite charges to the polyelectrolytes. We examined in detail the
effects of various parameters on the polyelectrolyte adsorption and surface
charge compensation by the adsorbed polyelectrolytes. The ground-state
dominance approximation (GSDA) was used to explore the large parameter space
involved, including the charge distribution and degree of ionization of the
polyelectrolytes, surface charge density, short-range (non-Coulombic)
surface-polymer interactions, solvent quality, and bulk polymer and salt
concentrations. The numerical results under GSDA were also compared with full
SCF calculations to examine the effects of molecular weight of the
polyelectrolytes. Strong charge inversion is found for relatively long
polyelectrolytes on oppositely charged, attractive surfaces in poor solvent at
high salt concentrations. At the mean-field level, the adsorption behavior of polyelectrolytes
at high salt concentrations can be understood by that of neutral polymers in
good solvent.
Based on the above results, we have further modelled the process of layer-by-layer assembly of flexible polyelectrolytes on flat surfaces. The multilayer has a three-zone structure. An exponential growth is found for the first several layers, followed by a linear growth for subsequent layers evolving towards a steady state. While adjacent layers are highly interpenetrating, stratification can be seen for every four or more layers. We have also examined the effects of surface charge density, bulk salt concentration, and solvent quality on the thickness and internal structure of the multilayer. Our results agree with most experimental findings on polyelectrolyte layer-by-layer assembly.