A defect-free PAA hydrogel-network model with a cubic topology is used in all MD simulations. This topology has been used previously and allows us to calculate the ionic density around the chains aligned in the x, ŷ, and ẑ directions conveniently. The PAA polymer chains are cross-linked by bonding each chain end to a charge-neutral crosslinker bead to a continuous hydrogel structure with no free dangling ends (see ). The polymer chains are flexible, and the network nodes are free to move (not fixed at the lattice positions), which accounts for any fluctuations. The number of acrylic acid (AA) monomers between two crosslinkers are N = 10. The cubic network is composed of 240 AA monomer units and a total of 8 crosslinkers. For the purpose of this study, the crosslinker beads are modeled as charge-neutral dummy atoms that can bond to six chain ends with a molecular mass equivalent to that of a carbon atom. A single AA monomer consists of a single carboxylic group and can become negatively charged by releasing its H + counterion when the polymer is dissolved in water (see Figure ). In order to model counterions, 240 Na + ions are added in the simulation box at random positions to fulfill the electro-neutrality condition throughout the simulation box. The total number of atoms during the simulations is constant (i.e., canonical ensemble). Note that the dissociation degree of the PAA chain in water depends on the pK value of its monomers and on the solution properties such as pH and salinity since it is a weak polyelectrolyte . Since we are using only water as a solvent, we assume that the pH is 7 and PAA chains are fully ionized at pK ≈ 4.26 (i.e., pH -pK ≈ 3), at which PAA is fully ionized , and ignore the variations in pK values .