Chemistry - Why does hand sanitizer turn into a liquid after coming out of the bottle?

Solution 1:

A little research and perhaps a small experiment might help you figure something like this out on your own. First, have you tried wikipedia? A small mention of research can go a long way. Describing what product you are interested in never hurts either. Is the polyacrylic acid, a dispersant in this substance? That material can absorb water effectively, so here is what I would do:

Take a very small amount of water and smear it on the same surface that it does not "turn into a free flowing liquid" and see if it does with the addition of water. If it works, you might conclude that your alcohol-based rubs use a suspension material that when contacted with a little water, such as from your palms, will cause the gel to have just enough water absorbed inside to decrease its viscosity.

Perhaps you will like to share the results of your experiment.

EDIT: OP seems to be observing a gel demonstrating Thixotropy, this I have determined from a quick google search

Solution 2:

GermX thins when it hits your hands because of the sodium ions present in your hands. The polymer used in GermX is a polyelectrolyte - in this case it is poly(acyrlic acid) that has been neutralized with a base made with a large cation. The large cation is too large to effectively separate and quench the electrical repulsion between the neighboring negative charges on the acrylate monomers. When sodium ions are available, like when it hits your hands or when you add salt water, the smaller sodium ions may exchange with the large cations and collapse the polyelectrolyte. This collapse is thermodynamically favorable both in terms of enthalpy and entropy change. The answers using rheology to back up their claims are erroneous.

Source: PhD Polymer Science and Engineering, and my professor, who patented this technology.