Effect of calcium ions on the interactions between surfaces end-grafted with weak polyelectrolytes.

We study the interactions between two planar surfaces end-tethered with poly(acrylic acid) polymers in electrolyte solutions containing calcium ions, using a molecular theory. We found that by adding divalent calcium ions to an aqueous solution of monovalent ions 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, i.e., complexes of one calcium ion and two carboxylic acid monomers, that reduce the effective charge of the polymer layers and, at sufficient calcium ion concentrations, can cause the polymer layers to collapse. For calcium ion concentrations above approximately 1 mM, the repulsions between the opposing end-grafted surfaces disappear and attractions occur. These attractions are correlated with the occurrence of interlayer divalent calcium bridges and do not occur for poly(acrylic acid) layers in contact with reservoir solutions containing only monovalent ions. This result indicates the suitability of divalent calcium ions to control and change the interaction range and strength, which is a useful property that is desirable in the design of stimuli-responsive nanomaterials.