Super Absorbent Chemistry 101
Acrylate polymers – in a dry state – resemble curled up curly-Qs or pigs tails.
When these molecules are placed in water, hydrogen bonding with the HOH surrounding them causes them to unfold and straighten out (like strands of spaghetti).
When the molecules straighten out, they increase the viscosity of the surrounding liquid. That is why several types of acrylates are used as thickeners.
Super absorbent chemistry requires two things: the addition of small cross-linking molecules between the polymer strands and the partial neutralization of the carboxyl acid groups (- COOH) along the polymer backbone (- COO- Na+).
Water molecules are drawn into the network across a diffusion gradient which is formed by the Sodium neutralization of the polymer backbone. The polymer chains want to straighten but are constrained due to the cross-linking. Thus, the particles expand as water moves into the network.
The water is tightly held in the network by Hydrogen-bonding. Many soluble metals will also ion-exchange with the Sodium along the polymer backbone and be bound.
Here’s what a super absorbent molecule looks like in greater detail:
And here’s what a super absorbent network looks like on a larger scale: