Best rechargeable cooler ice? Joules / cm^3

[wacki]

Two questions:

Q1) Reagrdless of price, what is the best rechargeable cooler ice material by volume? Thanks to hydrogen bonds, water has heat of fusion at 0 °C of approximately 334 joules (79.7 calories) per gram or cm^3. Water also has a higher specific heat capacity than any other material I know of. So it seems like it's the perfect material for keeping coolers cold.

EDIT: Looks like ammonia may be better but that's extremely dangerous

http://www.engineeringtoolbox.com/specific-heat-fluids-d_151.html

http://www.engineeringtoolbox.com/specific-heat-capacity-d_391.html

Q2) If the above is true, why don't you see cooler packs made for pure water? Insteaf people add the following:

hydroxyethyl cellulose (Cellusize)
sodium polyacrylate
silica gel

This I assume are just to make sure the packs are still "bendy". Alcohol is added to reduce bacterial growth. All of this reduces thermal performance. Am I wrong or is the ultimate cooler pack simply a nalgene bottle filled with ice?

 

[mindless1]

My (very old) cooler packs aren't bendy at all when frozen to (guessing here) ~ 0F in my deep freezer. I notice the sodium polyacrylate is a salt, might it lower the freezing point? On occasion I have put some regular table salt in ice packs to accomplish this, so they thaw before anything else goes much (if any) above 32F.

I don't know if any ingredients help with the frozen expansion rate. That's one issue with a DIY sealed bottle, that it's either underfilled to account for this or you'd leave the cap off while freezing it. Some of my older commercially made freezer packs seem to have a (deliberately) not-quite-air-tight plug in them, so the thickening agents are presumably to reduce liquid loss out that plug while it allows for pressure exchange.