Modeling Explicit-Ion Effects in Weak Polyelectrolyte Complexes

Polyelectrolytes may be classified into two primary categories (strong and weak) depending on how their charge state responds to the local environment. Both of these find use in many applications, including drug delivery, gene therapy, layer-by-layer films, and fabrication of ion filtration membranes. The mechanism of polyelectrolyte complexation is, however, still not completely understood. Here we detail computer simulations aimed at providing a comprehensive picture of the titration and complexation behaviour of these weak polylectrolytes and perform thermodynamic investigation through coarse-grained molecular simulations permitting us to calculate the free energy of complex formation and its dissection into energetic and entropic contributions. Our investigations indicate that entropic contributions indeed dominate the free energy of complex formation for strong polyelectrolytes, which is in agreement with experiments, but are less important than energetic contributions when weak electrostatic coupling or weak polyelectrolytes are present.