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.
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.
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.