A message has been floating around saying that it is better to wash your hands with warm water, as it helps the soap create more foam. Is it true? If it is true, why is it so? Is there a temperature dependant factor in the soap's cleansing mechanism?
Is there a temperature dependant factor in the soap's cleansing mechanism?
Yes, because hydrophilic-hydrophobic interactions are always driven by entropy. Entropy contributes to Gibbs Energy which is the final quantity determining whether a reaction takes place or not (or in which direction it takes place for that matter). But your question implies other questions that should be answered.
Does soap create more foam with warm water?
I don't know. In general, mixtures exhibit phase separation at a lower temperature bound of miscibility and a upper temperature bound of miscibility. Some mixtures do not have a temperature range where no phase separation occurs, some are miscible in all circumstances, you get evaporation/freezing before you get phase separation, think water and acetone.
Does more foam in my soap help clean my hands?
No. Foam/bubbles are small sheets of water with surfactant towards the gaseous phase on the in- and outside. Hence the oily look and texture. What you cleans your hands is the surfactant, because it also helps solubilize hydrophobic stuff like certain proteins. The cleaning is done by hydrophilic stuff being dissolved in the water, and hydrophobic stuff being covered inside micelles of the surfactant, which are soluble in the water.
Does more soap help against viruses?
If your hands are really dirty, maybe. Surfactants are known to denature proteins. You wish to denature the virus proteins so that they can no longer carry out their functions. This would also rob your skin of its protective fatty outside layer. You need not worry to denature your own proteins unless you are trying to drink soap water. Also, aggressive surfactants like SDS work better than your standard fatty acid soap, being even more damaging to your skin as well.
How can I optimally protect myself against viruses on my skin?
Use a desinfectant like ethanol or iso-propanol with at least 70% concentration. It denatures proteins quite effectively. Usage for at least 30 seconds is the official recommendation to kill most common viruses and bacteria. I personally don't wash off, but wait for it to evaporate, which is a good heuristic for the 30 seconds. Make sure to use some sort of lipid replenishing cream every now and then to recover your skin.