Brushes made of charged polymers (polyelectrolytes) display a variety of interesting features. This is so because with the corresponding mobile counter-ions there exists an addditional component carrying entropy. The counter-ions, driven away by their osmotic pressure, are drawn back into the brush due to electrostatic interaction, generating a delicate charge-balance that affects the properties of the polymer brush. Naturally, polyelectrolyte brushes are of particular interest in biophysics and bioengineering. The addition of salt modifies the range of electrostatic interactions through the Debye-screening. In recent studies we have analyzed how the brush height is changing with the concentration of salt inside the (good) solvent. The molecular dymamics simulations, featuring explicit salt ions (red squares), were in close agreement with an approximate simulation with Debye screening potential (black dots) and self-consistent field calculations using Biesheuvel's approach (green triangles), but in qualitative disagreement with previously published mean-field approaches (green dashed and magenta dotted curves). Considering the interplay of charge-balance, excluded volume and entropic spring interactions we have developed a Flory-type mean field approach which is capable of fitting the simulation data accurately (blue curve).

Highlights:

1. Su-Zhen He, Holger Merlitz, Long Chen, Chen-Xu Wu, and Jens-Uwe Sommer
   Polyelectrolyte brushes: MD simulation and SCF theory
   Macromolecules 43, 7845 (2010)
2. Long Chen, Holger Merlitz, Su-zhen He, Chen-Xu Wu, and Jens-Uwe Sommer
   Polyelectrolyte Brushes: Debye Approximation and Mean-Field Theory
   Macromolecules 44, 3109 (2011)