c for counterions inside brush. The fourth term refers to the entropy of the counterions which are freely moving particles. And finally, the sixth term which takes the electrostatic interactions throughout the brush. This term accounts for all electrostatic interactions which occur inside brush. Here, ϕ 1 (z) is the dimensionless electrostatic potential because it has been divided by (k B T /e), where, e is the elementary charge. The Eq. ( ), is coupled to the Poisson-Boltzmann differential equation as follows,