[EvaH (OP)]

William asks:

When a bar of soap gets used a lot and gets smaller, it seems to struggle to form suds properly. Is something other than just a smaller surface area going on?

What do you think?

[alancalverd]

Good question!

My guess is that most soaps include volatile and easily soluble components that aid lathering. As the bar shrinks, its surface/volume ratio increases so these components are lost more rapidly.

Basic soap is a mixture of sodium and potassium oleate and/or palmitate. "Soft" soap, particularly shaving soap, has more of the potassium salt and very little in the way of perfume, so it retains its character down to the last flake, but hand soap  is based on the less soluble sodium salt and gradually loses its potassium and volatile content, ending up like oldfashioned laundry soap.

I like hard soaps like Wrights Coal Tar and Pears transparent bar soap (not advertising - I think they are unique survivors of Victorian "muscular christianity and cold morning baths") which seem to be as consistent as Palmolive shaving stick soap (another traditional formula), but The Boss buys a variety of modern brands with softer bar texture and less "masculine" perfumes, which definitely change character with use.

[Petrochemicals]

The acidic properties degrade. If you apply soap to your hands to wash somewhere else the soap is far less affective than applying direct. Oxidisation i think.

[alancalverd]

Regular soap is slightly alkaline. It removes grease by converting it to ..... soap!

[Petrochemicals]

Regular soap is slightly alkaline. It removes grease by converting it to ..... soap!

Of course, its made from acid.

[evan_au]

This question was addressed on the podcast:
https://www.thenakedscientists.com/podcasts/question-week/why-old-soap-useless