During my time in the industry, I have noticed that the finer details of emulsion formation are often poorly understood, even by people who are otherwise very knowledgeable.
Given that it is a subject that is not generally taught in schools or universities, most people are not exposed to it in any detail prior to entering the industry, and explanatory texts that are geared towards industrial rather than academic chemists are few and far between, this is not entirely surprising.
As a result, I have found there are two particular mistaken beliefs, or canards, which are frequently cited as fact (even on this forum), despite being impossible to reconcile with orthodox science or actual evidence. With that in mind, I would like to take this opportunity to examine them in detail and set the record straight. Canard 1: O/W emulsions will invert to
W/O if the oil phase comprises over 50% of the formula, and vice versa.
(Whether this means 50% w/w, 50% w/v, 50% v/v
or 50 mole % is never clarified.)
The HLB of any emulsifier at a given temperature is
constant; it is an intrinsic property of the emulsifier, and it does not depend
on the relative proportions of the oil and water phases.
Therefore, at a constant temperature (hence, constant HLB) it is physically impossible for an emulsion to
invert once a given ratio of phases has been exceeded, because there is
no way for the emulsifiers to ‘know’ how much of each phase is present.
If the HLB did somehow depend on the ratio of the two phases, the
process of determining the HLB would be much more laborious than it is in
practise, and the requisite calculations would be considerably more complicated.
Plus, the system would be acting in a manner grossly inconsistent with the normal laws of physics.
Phase inversions are possible, but whether or not they occur
depends on how the emulsifiers’ HLB varies with temperature, and the range of
temperatures encountered during the emulsion’s formation. Canard 2: O/W emulsions are formed
if the oil phase is added to the water phase, and conversely, W/O emulsions are
formed if the water phase is added to the oil phase.
Before examining this statement in detail, it is important
to bear two fundamental physical principles in mind.
In any given system, wherever possible, nature will act to a)
maximise entropy (as per the second law of thermodynamics), and b) minimise the
amount of free energy available for thermodynamic work. (The latter is known as the .)
Entropy in the thermodynamic sense is defined as the degrees
of freedom available to a given system, in other words the range of movement
available to its constituent atoms and molecules.
A homogeneous mixture has more entropy than two separate
phases, because in the latter case, the molecules at the interface of the two
phases have restricted movement.
Because of this, nature will favour homogeneous mixtures unless
they are physically impossible to form.
For an emulsion which contains high HLB emulsifiers, it does
not matter which phase is added to which.
If high HLB emulsifiers are present, the most stable emulsion is the one
in which the water phase is continuous, i.e. O/W, regardless of which phase is initially continuous
This is due to the fact that water has much stronger intermolecular
bonds than oil, and so a continuous water phase has less energy available for
thermodynamic work than a continuous oil phase, making it more
It is also because water droplets have a greater tendency to
aggregate than oil, again due to the strong intermolecular bonds.
These statements also apply to mixtures of high and low HLB
emulsifiers. These could in principle
form either O/W or W/O, but in practise they are always O/W because a
continuous water phase is thermodynamically favoured.
If the emulsion only contains low HLB emulsifiers, it is not
physically possible for O/W emulsions to be formed, simply because water does
not mix with either oil or low HLB emulsifiers.
The only way to form a homogeneous mixture in such a system
is through formation of a W/O emulsion, and this must be done by adding the
water phase to the oil phase, so that the water phase remains dispersed and
In summary:For systems containing
high HLB emulsifiers, whether on their own or mixed with low HLB emulsifiers:
Oil into water forms O/W
Water into oil forms O/W
For systems containing
low HLB emulsifiers only:
Oil into water separates
Water into oil forms W/O
This can be summarised even further:
The nature of an emulsion is determined solely by the nature of its