Emulsion technology
SCS Summer school July 13,
2013
Steve Boothroyd
Thinking about how you would develop
any sort of product what are the key
things you should consider during
development?
My thoughts in no particular order of
importance
Does your Consumer love it?
Does it meet the Claims you want to
make for it?
Is it safe?
Is it stable?
Can you make it on a larger scale
consistently?
Can you afford it?
Does you consumer love it?
Why would you choose to use an
emulsion instead of another
technology?
Take a few minutes in your tables to
come up with 5 possible answers
Does you consumer love it?
Possible answers…
• They feel good!
• You can pack different actives in different
phases
• You have the option to change lots of things
• They can take a range of formats
• They are cost effective
What is an emulsion?
A dispersion of one or more immiscible
liquid phases in another, the distribution
being in the form of tiny droplets.
What is an emulsion?
A dispersion of one or more
immiscible liquid phases in another,
the distribution being in the form of tiny
droplets.
Simple emulsion types
Oil-in-water
Water
(continuous phase)
Oil droplet
(dispersed phase)
Water-in-oil
Oil
(continuous phase)
Water droplet
(dispersed phase)
Emulsion orientation
• The phase that is added tends to become the
internal phase
• The predominant solubility of the emulsifier
tends to determine the external phase
(Bancroft’s rule)
• Generally, the phase of the greatest volume
tends to become the external phase
• The phase in which the stirrer is placed tends to
become the external phase
Droplet size measurement
Laser method
Audio method
Optical
method
Laser Particle Analyser
Use of sound waves
(Malvern)
Microscopy
• Uses
• Droplet size and size distribution
• Quality of manufacturing process e.g.
•
•
•
•
undispersed thickener
Detecting unwanted crystallisation
Early indications of instability e.g.
flocculation, coalescence, synerisis
Comparison of different emulsions
Liquid crystals
What does an emulsion look like?
What does an emulsion look like?
What does an emulsion look like?
What don’t you want to see?
Is it Stable?
What are the stability risks
associated with emulsions?
Take a few minutes in your tables to
come up with some possible answers
Is it Stable?
What are the stability risks associated
with emulsions?
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•
•
•
•
•
Creaming
Sedimentation
Flocculation
Coalescence
Phase Inversion
Ostwald Ripening
Is it Stable?
Why is that?
• Emulsions are thermodynamically unstable
• Their natural tendency is to revert to a state of
least energy i.e. separated into two layers
• The process of emulsification is to produce
droplets but also to maintain them in this state
over a reasonable shelf life
Creaming / Sedimentation
•
•
•
•
No change in droplet size
Reversible
Driven by density difference
Usually results from gravitational forces
Creaming
Sedimentation
Coalescence
• Not reversible
• May lead from flocculation, creaming /
sedimentation or Brownian motion
• Involves 2 drops coming together
• May lead to complete separation
Stokes’ Law
Defined as:Velocity of droplet (v) = (2a2 g (ρ1 – ρ2)) / 9η
Where
a = Radius of dispersed phase droplet
ρ1= Density of continuous (external) phase
ρ2 = Density of continuous (internal) phase
g = Acceleration due to gravity
η = viscosity of the continuous (external) phase
Van der Waals forces
Defined as
F = - Aa
12H
Where
F = Van der Waals forces of attractions
A = Hamaker constant
a = Radius of dispersed phase droplets
H = Distance between two adjacent dispersed
phase droplets
Is it Stable?
What can these two equations tell us
that will help us make formulating
decisions to improve stability?
Take a few minutes in your tables to
come up with some possible answers
• From Stokes’ Law
• Thicken the continuous phase. This increase η
•
and reduces the velocity
• Reduce the particle size by greater or more
effective mixing.This has a major effect as a is
squared
• Reduce the difference in specific gravity
between the phases
Additionally from Van der Waal
• Increase the distance between droplets either
through thickening the water phase to reduce
mobility, reducing particle size, or introducing
materials that avoid droplets getting too close
e.g. polymers
Improving emulsion stability
• Charge stabilisation
• Interfacial film strengthening
• with powders
• with polymers
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•
•
•
•
•
With non-ionic emulsifiers
Steric stabilisation
Continuous phase viscosity
Droplet size
Co-emulsifiers / polar waxes
Liquid crystals
Improving emulsion stability
Charge stabilisation
+
+
- - -- +
- +
+
+
+
+
- - -- +
+
+
+
+
+
- - -- +
+
+
+
+
+
+
+
+
+
+
+
+ +
- -
+
- - -++- -
- - -- +
- -- +
+
+
+
+
+
+
+
- - -+ - - +
Negatively charged
oil droplets repel
each other
Stability affected by
quantity of electrolyte
and whether M+ or M++
Improving emulsion stability
• Interfacial film strengthening
• Reduces the probability of coalescence
when droplets collide
Improving emulsion stability
Interfacial film strengthening with powders
Powder particle size must be
very small
Powder must have an affinity
for both the oil and water
phase
Improving emulsion stability
Interfacial film strengthening with polymers
Polymer sits at emulsion interface
Polar groups orient into the water phase
e.g. Cetyl PEG/PPG-10/1 Dimethicone
Acrylates/vinyl isodecanoate
crosspolymer
Improving emulsion stability
Interfacial film strengthening with non-ionic
emulsifiers
Oil
Interface strengthening is dependent
on the number of molecules that
are packed into the interface
Tighter packing
at interface
Improving emulsion stability
Steric stabilisation
• Polymer molecules adsorb on
•
•
•
•
the surface of oil droplets,
leaving tails and loops
extending into the water phase
Polymer molecules must be
strongly adsorbed at interface
There must be high coverage
of droplet surface with polymer
The 'tails and loops' must be
soluble in the water phase
e.g. Cetyl PEG/PPG-10/1
Dimethicone
Improving emulsion stability
• Continuous phase viscosity
• Thickening the water phase restricts
•
movement of oil droplets
Thickeners with yield points are most
effective
• Droplet size
Increasing stability
Improving emulsion stability
• Co-emulsifiers / polar waxes
• e.g. Cetyl alcohol
• Co-emulsifiers have weaker surface
•
activity than primary emulsifiers
Adds body and helps prevent coalescence
Is it Safe?
What are the key components of an
emulsion that contribute to safety
risk?
Is it Safe?
What are the key components of an
emulsion that contribute to safety risk?
Here are some possibilities
•
•
•
•
Preservatives
Fragrance
Actives
Emulsifiers
What is an emulsifier?
Water loving
head
Oil loving
tail
'Hydrophilic'
'Lipophobic'
'Lipophilic'
'Hydrophobic'
Potential irritation
• Emulsifiers, since they are surface active,
may be a factor in increasing the risk of
irritation
therefore
• Excessive levels of emulsifier should be
avoided
• The most appropriate type of emulsfier
should be chosen
Types of emulsifiers
• Anionics
The emulsifier carries a negative charge e.g.
Sodium Stearate soap
C H COO 17 35
Na +
Types of emulsifiers
Pros and Cons
•
•
•
•
•
•
Were very common
Old fashioned
Not as versatile
Cheap
Limitations for actives due to high pH
Give negative charge to the oil droplet
Types of emulsifiers
• Cationic
The emulsifier carries a positive charge e.g.
Palmitamidopropyl Trimonium Chloride
O
CH3(CH2)14C
NH(CH2)3
CH3
+
N CH3
CH3
_
Cl
Types of emulsifiers
Pros and Cons
•
•
•
•
•
Usage is not high in Skincare
Good barrier
Excellent silky skin feel
Give positive charge to oil droplet
Can be used at lower pH
Types of emulsifiers
• Non-ionic
Emulsifier carries no overall charge and
can be made to form both Water-in-oil or
Oil-in-water emulsifiers e.g. Steareth-2
CH3 (CH2 )16 CH2 (OCH2 CH2)2 OH
Types of emulsifiers
•
•
•
•
•
Most common
Wide range
Versatile
Strengthen the emulsion interface
HLB system to predict choice
HLB system
0
10
Lipophilic
Oil loving
Non polar
Oil soluble
20
Hydrophilic
Water loving
Polar
Water soluble
HLB system
Emulsifier HLB 5
Water
phase
Emulsifier HLB 10
Oil
phase
Emulsifier HLB 15
Determining HLB values
Source: Croda ( Taken from Croda’s time saving guide to emulsifier selection” - training course available from Croda PLC)
How can you tell the difference?
• Measure conductivity – conductivity is
higher with an O/W emulsion than a W/O
emulsion
• Dye uptake - a water soluble dye will be
taken up by an O/W emulsion
• Dispersion – an O/W emulsion will easily
disperse in water
HLB system
Required HLB for oil-in-water emulsion
Benzophenone-3
Mineral oil
Caprylic/Capric triglyceride
Cetyl alcohol
Vitamin E
7
10 - 11
5
15 - 16
6
Required HLB for water-in-oil emulsion
Mineral oil
4
Emulsifier selection using HLB
• Oil phase components can be given required HLB
values
• Required HLB and emulsifier HLB are matched up
• Each oil will have 2 required HLB’s, one for oil-inwater emulsions, the other for water-in-oil
emulsions
• The required HLB is published for some oils
HLB Summary
• Cons
• Pros
– Not good for
– Empirical system
anionics and
giving starting
cationics
position
– Need to know HLB
– Can be assessed
of oil which can vary
practically
– Can be time
consuming working
out or measuring
– Does not determine
the amount of
emulsifier needed
Emulsifier blends
In the HLB system the HLB of the emulsifier blend
is additive for example if an oil system had a
required HLB of 10 you could use either
Emulsifier
HLB 10
or
Emulsifier
HLB 5
Emulsifier
HLB 15
Emulsifier blends
For a given blend of non-ionic emulsifiers, where
Emulsifier A is more lipophilic than Emulsifier B
Emulsifier A
Oil
Emulsifier B
Oil
Tighter packing
at interface
Considerations when choosing
an emulsifier
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Type of emulsion
Oils to be emulsified
Processing - hot or cold
Effect on skin
Properties of the emulsion
Cost
Level of electrolyte
Can you make it on a larger
scale consistently?
What are the differences that would
impact on the manufacture of an
emulsion between lab and factory?
Take a few minutes in your tables to
come up with some possible answers
Can you make it on a larger scale
consistently?
What are the differences that would
impact on the manufacture of an emulsion
between lab and factory?
Heating/ Cooling rates
Stirring/ Shear rates
How are emulsions formed?
• In order to overcome the barrier between the oil
and water we need to add energy
• This is derived from two sources:-
Chemical energy
(emulsifier)
+
Mechanical energy
(homogeniser)
• For long term stability both forms are needed
Two key requirements for
creating a stable emulsion
• Apply enough energy to the two phases to
create a dispersion
• Stabilise the created dispersion
• Maintain a small droplet size
• Increase the external phase viscosity to
reduce movement
• Reduce phase density difference
Two stages of creating an
emulsion
• Stage 1 – apply energy to the two phases to
create a dispersion
• Generally heat to 70 - 75°C
• Stage 2 – stabilise the created dispersion
• Maintain the small droplet size
• Increase the external phase viscosity
• Reduce phase density difference
Emulsion manufacture
• Heating to this temperature can change the
level of the oil phase e.g. Cyclomethicone
• If you need to add sensitive ingredients hot e.g.
sunscreens, then do it just prior to
emulsification
• Avoid post emulsification addition of
preservatives etc that partition between oil and
water
Emulsion manufacture
• After cooling the remaining ingredients are
added e.g. heat sensitive preservatives,
perfumes.
• For W/O emulsions if you have to add
preservatives these MUST be added prior to
emulsification
• Only Oil-in-water emulsions can be made to
weight easily
• BUT you must start thinking about scale up
from the first formulation attempt
Emulsion manufacture
• Laboratory
– Oil phase added with
Silverson mixing
– Beaker
placed in
bowl of cold water
and stir cooled
Takes approx 30 min
• Factory
– Oil phase added with
gate stirring followed
by homogeniser
mixing
Size and distance
– Cold
water passed
through water jacket
with gate stirring
Can take hours!
Emulsion manufacture
Phase Inversion Temperature
(PIT)
• Occurs in some non-ionic emulsifier systems
• Linked to solubility of emulsifier in the
respective phases
• At different temperatures
• In the presence of electrolyte
• Mostly used to transition water in oil to oil in
water at a given temperature to produce
desired small particle size
Phase Inversion Temperature
(PIT)
• Unique for any given emulsifier or blend of
emulsifiers
• Useful for explaining behaviour of emulsion
systems
• Helps to understand formation of differing types
of emulsion observed for a given blend of
emulsifiers
Phase Inversion Temperature
• Within the marked band a complex three phase mixture
is found
• Above TU a W/O emulsion exists, below TL O/W
• This temperature and band will be different for different
systems
Temperature oC
75o
TU
2 phase
T
3 phase
1 phase
2 phase
TL
0o
0
% emulsifier blend
Source: Kahlweit M: Microemulsions, Science 29 April 1998, p671-621
20
Phase Inversion Temperature
Solubility
• Why might this be the case?
• Solubility of ethoxylated emulsifiers increases with
increasing ethoxylation
8
20
Number of ethoxylate groups
Phase Inversion Temperature
• Bancroft’s rule suggests that the emulsion formed
will depend on where the emulsifier is most soluble
• Oil in water where most water soluble
(hydrophilic)
• Water in Oil where most lipid soluble (lipophilic)
• Consequently changes the effective HLB
observed
• By correct choice of emulsifier conversion from a
W/O to an O/W is possible