Adhesion between oppositely charged polyelectrolytes in salt solution

The adhesion at the aqueous interface between a polyacid hydrogel and a grafted polycation layer (brush) is shown to be reversible on the addition of sodium chloride, which screens the charges on the gel and brush. Furthermore, it is shown that a double-network (DN) hydrogel both exhibits adhesive failure and stronger adhesion with the brush than a simple poly(methacrylic acid) (PMAA) hydrogel, which undergoes cohesive failure on detachment. The adhesive force at the hydrogel-brush interface significantly increases after adding dilute salt solution (of 0.001 M). Once the concentration of salt solution is raised above 0.05 M, both the adhesive force and work done in detaching both single-network and DN hydrogels from the brush surface are notably reduced. This result confirms that reversible adhesion between a hydrogel and polymer brush in salt solutions is possible, although a DN gel is required for adhesive failure.