Ion Correlation-Driven Hysteretic Adhesion and Repulsion\nbetween Opposing Polyelectrolyte Brushes

Polyelectrolyte\n(PE) brushes are widely used in biomaterials and\nnanotechnology to regulate the surface properties and interactions.\nHere, we apply the electrostatic correlation augmented self-consistent\nfield theory to investigate the interactions between opposing PE brushes\nin a mixture of 1:1 and 3:1 salt solutions. Our theory predicts a\nhysteretic feature of the normal stress induced by strong ion correlations.\nIn the presence of trivalent ions, the force profile is discontinuous:\nrepulsive in the compression branch and adhesive in the separation\nbranch. The molecular origin of the hysteretic force is the coexistence\nof two collapsed modes: two separated condensed layers on each surface\nin the compression and a single bundled condensed layer in the separation.\nWith the systematic inclusion of ion correlations, our theory captures\nwell the hysteretic force, adhesive separation, “jump-in”\nand “jump-out” features, and the “specific ion\neffect”, all in good agreement with the reported experimental\nresults.