# Vapor pressure of immiscible liquids

I have read that vapour pressure of a mixture of two immiscible liquids is $$P=P^0_A + P^0_B$$ where A and B are immiscible liquids.

If they are immiscible, they would form separate layers. Thus since the more dense liquid would be below the less dense liquid, it would have no interface with the gas phase. Given this lack of an solid-gas interface, should the pressure due to the lower layer be zero?

• Will the equation be more plausible if you actually achieve a mixture by stirring the two layer system strong enough to achieve a dispersion, in which the former layers are no longer observable? – Klaus-Dieter Warzecha May 17 '16 at 19:21
• What if their densities are the same? What then is the driving force to form horizontal (rather than vertical) separation? Something else to ponder - why would a layer of B over A impact the equilibrium vapor pressure of A? (Even being 'immiscible' there is always some concentration of A in B - you can't beat the entropic contribution to the free energy). – Jon Custer May 17 '16 at 20:47
• They don't necessarily have to form separate layers, they could just be in separate containers such as the arrangement shown here: chemistry.stackexchange.com/questions/112854/… – Karsten Theis Sep 25 '19 at 19:35