Why do some solvents bubble more than others when boiling?

Question

This question about boiling milk reminded me of my everyday lab work at the rotavap: Some solvents and solutes are feared for the intense bubbling they create while the solution is boiling — be it at ambient pressure like the milk or at reduced pressure in the rotavap. For most of the infamous solvents, the bubbles will even create a somewhat stable foam (that usually ends up on the wrong side of the rotavap).

Well known in my lab are dissolved triphenylphosphane and the solvent toluene while isohexane (‘mixture of hexane isomers including methylcyclopentane’) is known to hardly ever give bubbles.

Why is this so? Why are bubbles often so much more stable even for just a solvent (toluene)?

Answer 1

Boiling depends on certain properties of a solvent:

1. Tendency to form an emulsion of bubbles due to low surface tension and separation of solvent phases, as in soap bubbles. Try boiling some dish-washing detergent in water.

2. Tendency to superheat, i.e. to go well above the boiling point and then to suddenly form bubbles, which is more pronounced at lower pressure. This can cause violent bumping, in addition to foaming. The cleaner the apparatus, the more likely, so boiling stones (chips) are added to nucleate more gentle boiling. A glass mug of water heated in a microwave can bump violently enough to blow open the door (and blow the fuse).