Ok so this occurred to me in the shower this morning; why does it seem like shampoo doesn’t foam up nearly as much when I skip my daily shampooing and wash it only every other or every third day?
How come?
More oil, less foam.
You noticed correctly. Wash it twice in a row and you will get lots of foam.
Hence, “lather, rinse, repeat.”
I could swear Cecil answered this, but I left my brain in my car when I came back from lunch.
A soap molecule has two ends, one is hydrophobic (water hating), while the other is hydrophyllic (water loving). Water is a polar molecule. Soap works by “grabbing” an oil molecule with the hydrophobic end, and being washed away by water by its hydrophylic end. The soap molecule, a lipid, looks a little bit like an octopus. When it “grabs” the oil with its tentacles, it can’t “grab” onto other soap molecules, so it doens’t lather as much.
As an aside, the “octopus” is between 400 and 800 nanometers, give or take. That’s why bubbles look like they have rainbows in them, because only the wavelengths of light that are the same size as the film (according to your line of sight) are allowed to pass through.