Q: Why is it that when you have a bubble bath, all the bubbles disappear when you use ordinary soap? Is it possible to stop this from happening?
A: Both bubble bath and soap are surface-active agents (surfactants) but they are normally of opposite and incompatible types, which is why the bubble bath collapses.
Surfactant molecules tend to be shaped like tadpoles with elongated tails. The head is water-attracting and may carry an electrical charge which can be either positive (cationic) or negative (anionic), while the tail is grease-attracting, water-repelling and carries no charge. The foaming and washing properties of these surfactants result from this characteristic structure.
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The bath problem arises because cationics are usually chosen for bubble-bath mixtures because of their high foaming characteristics, whereas soaps which have good washing characteristics are anionics.
When the two come together, the oppositely electrically charged heads are strongly attracted to each other, and the resulting entity is a 1:1 complex of the two surfactant molecules. Such structures have neither good foaming nor good washing characteristics, and the foam collapses. Fortunately for the bather, the soap tends to be used in excess and so its effect is not totally lost.
This different surfactant effect is also present in hair conditioners and shampoos, which is why two-in-one formulations have only recently become available. Any direct attempt to mix them would result in a sludge devoid of shampooing or conditioning properties.
In the new two-in-one shampoo and conditioner formulations a number of patented tricks are used, essentially to keep the conditioner separate from the shampoo until the dilution effect of rinsing out the shampoo releases it.