Sodium alginate and agar vs Cross linking alginate with calcium chloride?

Question

I am a biology student looking for som chemistry advice.

I am making hydrogels for an ag experiment (testing yield and moisture retention)

I was originally going to cross link the alginate with calcium chloride and citric acid to form hydrogels but I instead opted for sodium alginate mixed with agar to form a less dense gel for more stable water release. I’m relying primarily on the agar to give it strength and viscosity. Will leaving out the calcium affect the the hydrogels water holding capacity/ effectiveness?

In: Chemistry

Answer 1

I’m a chemistry PhD candidate who doesn’t work in this field but did extensive alginate cross-linking literature review for a grad school project.

Generally, you can cross-link alginate into a gel several different ways. The common way that was discovered some time ago is ion-mediated cross-linking with divalent metal ions (Ca2+ was the original, but others are being used now, even other valencies too). Since sodium alginate is a salt in solution, the chains are negatively charged (Na+ is the counter-ion) and thus repel each other. Higher valency metal ions like Ca2+ replace Na+ to further shield the electrostatic repulsion between chains, so much so that the chains can bind either side of Ca2+ to form chain-chain cross-links (the so-called egg-box model of gelation structure). Sheets of cross-linked polymer then entangle to form a gel matrix which can swell to hold water or other things (like drug molecules). You can also do covalent cross-linking, but I’ll ignore that for now since your question is ionic vs. no ions.

If you don’t do ionic cross-linking, and you just add acid, you end up protonating the alginate chains, making them neutral. This eliminates the electrostatic chain-chain repulsion, allowing them to tangle up into a more viscous solution. Depending on molecular weight, amount of acid, temperature, the solution viscosity will be different. I think it will depend on your specific case whether you think this is a stable enough gel to be “effective,” but the general theme is that it should be a far weaker/less stable gel than if you ionically cross-linked, and that would absolutely impact your water holding capacity. To what degree I don’t remember off the top of my head. I also don’t have a lot of knowledge on using agar in the same solution, since that wasn’t what my project involved, but it’s likely to help somewhat, though I doubt you would ever get as strong as a gel if you just ionically cross-linked.