Page 263 - Physical chemistry understanding our chemical world

230 PHASE EQUILIBRIA

undergraduate laboratory. Since plants contain such a small amount of this precious
oil, a ton of plant may be needed to produce a single fluid ounce. Some flowers, such
as jasmine or tuberose, contain very small amounts of essential oil, and the petals
are very temperature sensitive, so heating them would destroy the blossoms before
releasing the essential oils.
To add to the cost further, many of these compounds are rather sensitive to tem-
perature and would decompose before vaporizing. For example, oil of cloves (from
Eugenia caryophyllata) is rich in the phenol eugenol (V), which has a boiling point
◦
of 250 C). We cannot extract the oils via a conventional distillation apparatus.
H
C CH 2
H 2 C

OCH 3
OH
(V)

The most common method of extracting essential oils is steam
Heat-sensitive or distillation. The plant is first crushed mechanically, to ensure a
water-immiscible high surface area, and placed in a closed still. High-pressure steam
compounds are purified is forced through the still, with the plant pulp becoming hot as the
by steam distillation steam yields its heat of vaporization (see p. 79). The steam forces
at temperatures
the microscopic pockets holding the essential oils to open and to
considerably lower release their contents. Tiny droplets of essential oil evaporate and
than their usual boiling
mix in the gas-phase mixture with the steam. The mixture is then
temperatures.
swept through the still before condensing in a similar manner to a
conventional distillation.
Such ‘steam heating’ is even, and avoids the risk of overheating
Solvent extraction of and decomposition that can occur in hot spots when external heat-
essential oils tends to
ing is used. The steam condenses back into water and the droplets
generate material that coagulate to form liquid oil. Esters and essential oils do not mix
is contaminated with
with water, so phase separation occurs on cooling, and we see
solvent (and cannot be
sold); and mechanical a layer of oil forming above a layer of condensed water. The
pressing of a plant usu- oil is decanted or skimmed off the surface of the water, dried,
ally generates too poor and packaged.
a yield to be economi- The only practical problem encountered when collecting organic
cally viable. compounds by steam distillation is that liquids of low volatility
will usually distil slowly, since the proportion of compound in the
vapour is proportional to the vapour pressure, according to

p (oil) n (oil)
= (5.23)
p (water) n (water)