[e12e]

> I desperately hope this is the case, but a quick google couldn't find a link. Do you happen to have a citation that you'd recommend?

Cdc agrees:

https://www.cdc.gov/handwashing/faqs.html

> Is it better to use warm water or cold water?

> Use your preferred water temperature – cold or warm – to wash your hands. Warm and cold water remove the same number of germs from your hands. The water helps create soap lather that removes germs from your skin when you wash your hands. Water itself does not usually kill germs; to kill germs, water would need to be hot enough to scald your hands.

Soap helps break down/encapsulate/dissolve oils/fat that aren't readily soluble in just water.

[lazide]

Which, that breakdown and encapsulation happens faster at higher temperatures - it’s a pretty predictable chemical process. At scalding hot or higher temps, the surfactants can sometimes break down or form weird side products though.

[e12e]

It does happen faster at higher temperatures, but apparently not so much that it makes a difference in terms of cleaning your hands. I'm a bit surprised by that, but that's what studies seem to imply.

[lazide]

Do you have a link to the studies? I definitely notice a huge difference in washing with soap in cold water (camping), which is hard to not leave a film, and warm water which does a pretty effective job quite quickly with the same exact soap.

[e12e]

I didn't encounter any good primary sources, but a few references that seemed to indicate no difference in bacteria reduction between warm/cold water for washing hands. It may be that cold water is simply good enough.

[lazide]

The tricky part here is how the study is constructed. If you take a dry swab with a known bacterial culture, wash in water for x minutes at temp y, and graph it, that might make sense. It just doesn’t reflect real world situations, which almost always involve some kind of biofilm, oil/grease, dirt or similar mass in a complex interaction.

Especially once lipids get involved (almost all hydrophobic), especially in combination with other types of substances like oil soaked dirt or dead skin (like what you get on a doorknob at the micro level), it just doesn’t work the same way on the same timescales with colder temps - it fundamentally can’t, chemistry doesn’t work that way. [https://www.chemguide.co.uk/physical/basicrates/temperature....]

If you want the same speed of reaction - aka same amount of oil converted to a not oil - with lower temps (and hence lower molecular velocities and less Brownian motion) you need more reactive chemicals and more aggressive physical action to get compound A in contact with compound B enough to have the reaction you need. It’s as true for soap as it is for anything else.

And this all of course has a giant * on it, since above a certain point the energy in the system causes parts of it to disassociate or form unwanted side reactions, so there is a limit where it ‘blows apart’, ‘crashes’, or ‘can’t stick’ to extend the analogy. I can’t think of any soap that would have that problem at temps within human ‘not going to immediately get 3rd degree burns’ ranges.

[blub]

Is it really the foam that makes soap effective? I understood that it's not the case.

[e12e]

Note, they say "soap lather" I take that to mean "a mix of soap and water" - not necessarily thick foam.