Effect of calcium ions on the interactions of end-tethered weak polyelectrolytes

We use a molecular model, which combines theory with Monte Carlo and Molecular Dynamics simulations, to study the effect of divalent calcium ions on the interactions on the interactions between two planar surfaces end-tethered with poly(acrylic acid))(PAA). Polyelectrolyte-coated colloids and nanoparticles have nanotechnological and biomedical applications such as sensing and imaging. The addition of calcium ions to monovalent electrolyte solutions leads to a dramatic reduction in the size and range of effective interactions between the two polymer layers. This is caused by the formation of favorable calcium bridges that reduce the effective charge of the polymer layers and, at sufficiently high calcium ion concentrations, can cause the polymer layers to collapse. At physiological conditions and calcium ion concentrations above approximately 1mM, the repulsions between the opposing end-grafted surfaces disappear and attractions occur, accompanied by large structural changes of the opposing layers. These attractions are correlated with the occurrence of interlayer divalent calcium bridges and do not occur for PAA layers in contact with reservoir solutions containing only monovalent ions. Finally, the potential for microphase separation within the PAA layer is explored.