Bridging contributions to polyelectrolyte brush collapse in multivalent salt solutions

Abstract

ABSTRACTPolyelectrolyte brushes are essential in many aspects of surface functionality, particularly for colloidal stabilization and lubrication in biological and materials science applications. It has been shown experimentally that the brushes undergo an abrupt shrinkage in the presence of multivalent counter‐ions. This transition is studied here using a phenomenological mean‐field approach with a model that specifically includes bridging of the polyelectrolyte chains by the multiple charges on the multivalent counter‐ions. Using an energy balance represented by the sum of electrostatic, polymeric and entropic mean‐field terms, additional parameterized phenomenological terms are introduced for counter‐ion condensation and for the attractive interaction between adjacent polyelectrolyte chains to account for the bridging effect. The free energy is minimized with respect to the counter‐ion populations and the brush height. In agreement with experimental observations, increasing the concentration of multivalent ions leads to a sharp collapse of the polyelectrolyte brush height. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 284–291