Seltzer Soda Top

Background:
Recently I bought myself a seltzer bottle, and am quite happy with it. I enjoy sparkling water, and instead of having to buy bottles of it at the store I have just bought a few dozen cases of CO2 cartridges, and add tap water whenever needed. This, along with a recent idea, inspired this simple product.

Problem:
Large bottles of soda generally go flat before they are emptied.

Solution:
Simple device that has threads to attach a soda bottle to on one side, and a sealable compartment for a CO2 cartridge on the other side. Between these two sections is a valve that automatically closes when unscrewed from bottle.

Use:
Attach this device to your partially used bottle of soda to keep it fizzy indefinitely.

Option:
Instead of unscrewing a cap, why not just go all the way and have seltzer-like access? Include a lever and spout, and any soda bottle can be a seltzer bottle.