Electrostatic repulsion between two parallel plates covered with polyelectrolyte brush layers. Effects of interdigitation

A three-stage model is presented for the electrostatic repulsion between two parallel plates covered with polyelectrolyte brushes (i.e., plate-like soft particles) in an electrolyte solution. This is an extension of a previous theory (Ohshima, Colloid Polym Sci 277: 535, 1999), which assumes that when the two brushes come into contact, they are squeezed against each other but they do not interdigitate. The present theory covers the case where interdigitation between brushes occurs and proceeds until full interdigitation occurs. Then, compression of the polyelectrolyte brushes starts. That is, in the present model, the interaction process consists of three stages, i.e., interaction before contact of the polyelectrolyte brushes (stage 1), interdigitation (stage 2), and compression (stage 3). It is found that for highly charged polyelectrolyte brushes, the repulsive force between the two plates becomes almost constant independent of the plate separation during interdigitation.