[adi]

Hi,

I have been doing some reading into Hydrogels, and I am a little bit confused. When the Sodium Polyacrylate is put into water, the Sodium ions dissociate, leaving behind negatively charged Carboxylate ions. What I'm confused about is 2 things. First, do all the sodium ions come off? As per some sources, not all the Sodium ions come off the polymer (equal balance between polymer network and surrounding water), but according to others, all of it does come off the chain. Which is correct?


My second question is, how does the water attract to the hydrogel? Is it just the negative oxygen in Carboxylate that forms a Hydrogen bond with the partial positive hydrogen in the water molecule? Or, do the sodium ions also attract the water molecules?

From this link (3rd paragraph in the conclusion section), it claims that all the Sodium dissociates from the polymer chain, and both the free Sodium cation and the carboxylate attract the water and form bonds (https://www.carolina.com/teacher-resources/Interactive/water-absorbing-polymers/tr10869.tr)

From this link (1st paragraph), it claims that only some of the Sodium dissociates from the polymer, and that the places where the sodium leaves is where the water molecules fill in the gap and form a hydrogen bond (https://www.cmu.edu/gelfand/lgc-educational-media/polymers/polymer-and-absorption/super-absorb-powder.html#:~:text=The%20cross%2Dlinks%20that%20connect,calcium%20and%20other%20mineral%20salts.).

I am getting all this contradicting info, and that has left me confused. So, any answer would be appreciated!

[Enthalpy]

Hi Adi,

I can't tell if all the sodium polyacrylate dissociates. It depends on the amount of water.

A hydrogen bond happens between neutral species, for instance between two water molecules. Here the carboxylate and the sodium ions are charged, this makes a stronger bond with the polar water molecules. It resembles closely the dissolution of NaCl in water, where Na⁺ attracts the negatively polarized side of H₂O and Cl^- the positively polarized side. If water is in small amount, it resembles the dessication by CaCl₂. The difference by sodium polyacrylate is that the negative ion remains a long polymer chain.

Both ions attract water. But since all + ions can't separate from all - ions due to the huge attraction, only the sum of both effects is observed.