[Alexey]

If you have to evaporate a water solution on a rotary evaporator, sometimes water starts foaming and "jumping". In this case one can add n-BuOH (5-50 vol % with respect to water - % depends on your case). This method works in really hopeless cases. Apparently, it reduces surface tense.

n-BuOH forms azeotrope with water.
b.p. azeotrope (45wt% H2O, 55wt%nBuOH) 93°C
(b.p. n-BuOH 117°C)

This azeotrope separates in your receiving flask on two phases :
-upper layer : nBuOH:H2O 80:20 wt%
-lower layer: nBuOH:H2O 8:92 wt%
(the upper layer probably can be reused)

If you add too little amount of n-BuOH it can start to foam again after all n-BuOH is gone. Also in some cases it's enough to add just a little bit of nBuOH at the beginning.

Excess of n-BuOH can be removed:
1)portion-wise with small portions of water. also sometimes it starts foaming again.
2)azeotropically with cyclohexane or toluene. It works even if your product is not soluble in these solvents (in this case sometimes sonification is required to get a rest of BuOH into these solvents).

n-BuOH has a specific smell, so you will know whether it's gone.

rotavapor evaporate water butanol foaming foam suppress

[Alexey.]

" I was struggling today; a published homotropanone prep calls for freezing + lyophilizing the entire reaction mix. I scaled that thing by a factor of four and since I did not want to lyophilize a half-liter of the reaction mix, I just put it on the rotavap and suddenly the reasons for the recommended lyophilization became painfully clear…

Desperate people would add n-octanol or even couple of drops of silicone oil to their mix but I did not want to introduce non-volatile impurities into the product. I was dreaming about silanizing the flask glass surface instead (a rinse with Me2SiCl2 and tributylamine in dichloroethane does it) but in the end I just poured 1mL of of hexamethyl disilazane (TMS)2NH straight into my aqueous mixture - and the foaming ceased like a miracle. It must have been the silicone film on the glass that produced this remarkable effect because when I later transferred the solution into another flask it started foaming crazy anew; and a little more (TMS)2NH and it was calm like a lamb again."

See also the comments in the original link below.

Reproduced with a permission from:
http://orgprepdaily.wordpress.com/2009/06/11/foaming/