The focus of this work is on the effects of multi-valent ions on the behavior of end-tethered polyelectrolyte brushes. The uses and applications of polyelectrolyte brushes are vast, appearing in both nature (e.g., surfaces of cartilage and mammalian lung interiors) and commercially (e.g., skin care products, shampoo, and surfaces of medical devices). This makes understanding the physical properties of these brushes important for many different applications. Using the Surface Forces Apparatus (SFA), we are able to study the forces of interaction between two polyelectrolyte brushes end-tethered to cylindrical surfaces. Depending upon the surrounding environment, which plays a critical role in brush behavior, the brushes we study can exhibit a wide range of physical properties.

In the presence of mono-valent counterions, polyelectrolyte brushes demonstrate purely repulsive forces when moved into contact. As the concentration of mono-valent ions in solution is decreased, electrostatic screening between the charged monomer segments of the polyelectrolyte chains is also decreased allowing the chains to extend further. These swollen chains at lower mono-valent salt concentrations also exhibit force versus distance profiles which reveal a softer brush. However, when multi-valent ions are added into solution, the force profiles portray a much more interesting story. The presence of multi-valent ions creates adhesive forces between the brushes after contact, which vary depending upon concentration. While the multi-valent counterions also display repulsive forces, it is the adhesion between brushes which allows for a much wider range of physical properties. The relevance of this work exists in the regularity at which multi-valent ions exist physiologically and in commercial applications.