Dynamic Force Spectroscopy of Oppositely Charged Polyelectrolyte Brushes

Ion pairing is the main driving force in the formation of polyelectrolyte complexes, which find widespread use in micellar assemblies, drug carriers,and coatings. In this paper we examine the actual ion pairing, forces in a polyelectrolyte complex between two oppositely charged polyelectrolyte brushes. Using colloidal probe AFM, we study the effect of salt on the strength of the model complex. We find it strong dependence of attractive force on salt concentration up to a critical salt concentration beyond which no attraction call be measured. By changing the salt concentration, the time allowed for formation of the complex. and file rate at which the complex is disrupted, we are able to measure the kinetics of complex formation and to estimate the ion pairing forces in equilibrium polyelectrolyte complexes. We use a kinetic model for the dynamics of complex disruption as a function of disruption rate to find a typical force of ion pairing in the complex. The model shows that our model polyelectrolyte complex behaves as a series of bonded ion pairs loaded as a zipper.