In the presence of water, the sodium atoms will disconnect from the sodium polyacrylate polymer and disolve into the solution as sodium ions (Na+), leaving behind a negatively charged polymer chain. This process is similar to dissolving table salt (sodium chloride — NaCl) in water, which results in the sodium and chlorine separating into Na+ and Cl- ions in solution.

 

When table salt dissolves in water, the sodium and chlorine ions quickly diffuse throughout the entire solution. However, when sodium polyacrylate dissolves, the sodium ions remain trapped within the large cross-linked polymer, which acts like a cage. Neutral water molecules can diffuse in and out of the cage, but the positively charged sodium ions remain somewhat attracted to the negative cage, and cannot easily escape.

 

At this point, the water inside the polymer cage contains many dissolved sodium ions, while the water outside remains relatively pure. Osmosis then drives more water molecules into the polymer cage in order to dilute the salty water inside. (In osmosis, when two different solutions are separated by a membrane, the solvent will tend to diffuse across the membrane in order to equalize the concentrations.) As the polymer soaks up more water molecules, it inflates and stiffens until it can expand no more, held in place by the many cross links.