The rotovap allows us to distill
at low temperatures.
The low temperature is produced
because we modify the atmospheric pressure.
We remove air and create a vacuum.
And in this way we can promote evaporation at a very low temperature
Evaporation occurs at 50 degrees.
Then we pull the volatile aromas.
We produce a condensation and then the precipitation of the liquid
that lets us have this result and distilled water with all the aromas.
Let's use a new dimension of this product. In this case, the lemon peel.
So you just saw a remarkable demonstration
from Joan Roca using this very strange device called a rotavap.
So I now want to explain to you how a rotavap works.
The rotavap-- that's a short name of rotary evaporator--
operates by lowering the pressure that you're cooking something in.
So here again is the phase diagram of water.
And remember that we said that water can become a gas, liquid water can become
a gas by increasing the temperature from 23 degrees to 100 degrees Celsius,
if you're sitting at 1 atmosphere of pressure.
But you also can change the phase of the water by changing the pressure
and keeping the temperature fixed.
So if you look at the phase diagram of water,
and we drop the pressure sufficiently while maintaining the same temperature,
at that point the water will become a gas and it will actually vaporize.
So the equation from this week was that U interaction
as equal to C times k sub B T. And what that represented,
remember, is the physical balance between the fact that on one hand,
the molecules like to stick to each other-- that's the interaction energy--
and on the other hand, they like to jiggle around, which is the k sub B T.
And when these two are equal, that's when the molecules start
to evaporate off, liquid becomes a gas.
But now what I want you to imagine is that you
have a liquid which is contained of many different materials.
So just for the sake of argument, let's imagine that you have a cup
and it's filled with red molecules and blue molecules.
And imagine that the red molecules stick to the red molecules,
and they also stick to the blue molecules,
and the blue molecules stick to each other,
and they also stick to the red molecules.
But they stick with different strengths.
So maybe the red molecules are very weak.
That is, they bond to each other and to the blue molecules very weakly,
whereas the blue molecules bond more strongly.
So the question that I want you to imagine is,
how can we separate the red molecules from the blue molecules,
using only the principle of the equation that we've told you about,
So let's think about this.
So for the red molecules, given that they have some interaction energy,
there's some temperature at which the red molecules will boil,
if you have a substance of pure red molecules.
Similarly, if you take the blue molecules and you put them together,
there's some temperature at which the blue molecules will boil.
That's the interaction energy of the blue molecules divided by C k sub B.
So what you might imagine is that if you were sitting in a temperature
that somewhere between the red, the temperature for the red molecules
to boil off and the blue molecules to boil off, what you're going to get
is mainly red molecules boiling off.
So this is what I want you to now do.
So let's take the phase diagram that we had with these three different phases.
And I want to get rid of all of it, except for the phase
boundary that corresponds to the liquid-gas equilibrium boundary.
And so this plot's the pressure as a function of temperature.
So this curve, which basically represents
at every temperature, the pressure at which the gas and liquid are
in equilibrium is called the vapor pressure curve of the material.
We can put the curve down for water, and we also
add different substances on top of it.
We could add alcohol, for example, or we could add a flavor molecule.
And what you see is that these curves, which
happen at different positions for the different materials.
So for example, alcohol, which consists of ethanol,
which consist of molecules that are bound more weakly to each other
than water, has a different vapor pressure than that of water.
And what you see from our curves is there
is a critical pressure where the most volatile compounds will start
to come off much more than the others.
And so the gas that's above our substance
will be rich in the more volatile compounds,
only by changing the pressure, not by changing the temperature at all.
As you decrease the pressure further, more and more
of the different compounds will come off.
But if you do this carefully and you somehow
are able to collect the volatile compounds, the gas that you have
and reconvert it into a liquid, then that
will allow you to separate the two different compounds.