SCS Summer School 2014
Emulsion
Technology
Russell Cox
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?
•
•
Emulsions are metastable –from a
thermodynamic standpoint they can exist in a
form that is not the state of lowest energy
Gibbs stated that “the only point in time
where an emulsion is stable, is when it is
completely separated”
Gibbs free energy equation
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
Identification of emulsion type
•
Feel
•
Dispersibility
• Tested by dropping a small amount of emulsion in water –
O/W disperses easily while W/O remains whole
•
Conductivity
• O/W emulsions conduct electricity well showing high levels
of conductance
•
Dye penetration
• Water soluble dye is easily taken up in O/W system but not
in W/O
• O/W emulsions tend to have a lighter feel than W/O
Droplet size measurement
Laser method
Laser Particle Analyser
Audio method
Use of sound waves
(Malvern)
Optical method
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?
Emulsifiers
What is an emulsifier?
Water loving
head
Oil loving
tail
'Hydrophilic'
'Lipophobic'
'Lipophilic'
'Hydrophobic'
What is an emulsifier?
•
•
An emulsifier is a surface active agent with an
affinity for both the oil and the water phases on
the same molecule
An emulsifier reduces the surface tension at the
oil / water interface and protects the newly
formed droplet interfaces from immediate
coalescence
Droplet structures
 Within a droplet structure the emulsifier forms
a monomolecular layer on the surface of the
droplet
 The orientation of the emulsifier depends on
the type of emulsion formed
Oil - in - water
Water - in - oil
Improving emulsion stability
Clearly the ability of the emulsifier to completely cover
the surface area of the droplet will be dependent on;
• The concentration of emulsifier in the formulation
• The size of the emulsifier
• The size of the droplet
Good coverage is vital to ensure longer term stability
Types of emulsifiers
Anionics
The emulsifier carries a negative charge e.g. Sodium
Stearate soap
C H COO 17 35
Na +
Types of emulsifiers - Anionic
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 - Cationic
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 - Non-ionic
•
•
•
•
•
Most common
Wide range
Versatile
Strengthen the emulsion interface
HLB system to predict choice
HLB system and selecting
emulsifiers
HLB system
Hydrophile / Lipophile Balance
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 value
• Calculate the water loving portion of the surfactant on
a molecular weight percent basis and then divide that
number by 5
• Dividing by 5 keeps the HLB number scale limited to a
maximum of 20 which makes the scale smaller, thus a
bit more manageable
• Once calculated assign this number to the non-ionic
surfactant
• This assigned number is the HLB VALUE
Source: Croda presentation (Croda’s time saving guide to emulsifier selection) 1
Determining HLB value
• Run a simple practical test based on nine small
experiments
• Materials needed for this test:
• an HLB “kit”
• about 200 grams of your oil
• eight small jars
• the instructions
• and a little bit of time (actually a lot of time!)
Source: Croda presentation (Croda’s time saving guide to emulsifier selection) 1
Determining HLB values
Source: Uniqema/ Croda2
Determining HLB value
• Look at your formula
• Determine which are the oil soluble ingredients
– this does not include the emulsifiers
• Weigh each of the weight percents of the oil phase ingredients
together and divide each by the total
• Multiply these answers times the required HLB of the individual
oils
• Add these together to get the required HLB of your unique
blend
Source: Croda presentation (Croda’s time saving guide to emulsifier selection) 1
Determining HLB value
• A simple O/W lotion formula
•
•
•
•
•
•
•
•
•
•
Mineral oil
Caprylic/capric triglyceride
Isopropyl isostearate
Cetyl alcohol
Emulsifiers
Polyols
Water soluble active
Water
Perfume
Preservative
Source: Croda presentation (Croda’s time saving guide to emulsifier selection) 1
8%
2%
2%
4%
4%
5%
1%
74 %
q.s.
q.s.
Determining HLB value
• Mineral oil
8 / 16 = 50%
• Caprylic/cap. trig.
2 / 16 = 12.5%
• Isopropyl isostearate
2 / 16 = 12.5%
• Cetyl alcohol
4 / 16 = 25%
Source: Croda presentation (Croda’s time saving guide to emulsifier selection) 1
Determining HLB value
Oil phase
ingredient
contribution
X required
HLB of
ingredient
equals
Mineral oil
50.0%
10.5
5.250
5
0.625
11.5
1.437
15.5
3.875
Total
11.2
Caprylic cap. 12.5%
Trig.
12.5%
Isopropyl
isostearate
Cetyl alcohol 25.0%
Source: Croda presentation (Croda’s time saving guide to emulsifier selection) 1
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-in-water
emulsions, the other for water-in-oil emulsions
The required HLB is published for some oils
HLB system
Required HLB for oil-in-water emulsion
•Benzophenone-3
7
•Mineral oil
10 - 11
•Caprylic/Capric triglyceride
5
•Cetyl alcohol
15 - 16
•Vitamin E
6
Required HLB for water-in-oil emulsion
•Mineral oil
4
HLB impacts on Viscosity
• For the same formulation viscosity increases with
decreasing emulsifier HLB
1,600,000
1,200,000
1,000,000
800,000
600,000
400,000
200,000
0
16
14
12
10
HLB
Source: Uniqema technical training document (unpublished)3
8
6
Viscosity (mPa.s)
1,400,000
HLB impacts on Viscosity
HLB
• The effect is seen to be linear when log viscosity is
considered
16
15
14
13
12
11
10
9
8
7
6
4
4.5
5
5.5
Log viscosity
Source: Uniqema technical training document (unpublished)
3
6
6.5
Concentration impacts on
Viscosity
• Increasing concentration has a linear impact when log viscosity is
considered but may vary with emulsifier form
6
Log viscosity
5.5
5
4.5
4
3.5
4
6
8
10
12
Weight percentage of emulsifier
Source: Uniqema technical training document (unpublished)
3
14
16
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







Type of emulsion
Oils to be emulsified
Processing - hot or cold
Effect on skin
Properties of the emulsion
Cost
Level of electrolyte
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
HLB Summary
• Pros
– Empirical system
giving starting
position
– Can be assessed
practically
• Cons
– Not good for anionics and
cationics
– Need to know HLB of oil
which can vary
– Can be time consuming
working out or measuring
– Does not determine the
amount of emulsifier
needed
Nothing can go wrong – can it?
Nothing can go wrong – can it?
• Emulsions are thermodynamically unstable
• This means that 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
• Accelerated stability testing may reveal some of
the following horrors…
PHASE
INVERSION
Factors that contribute to emulsion
instability

Forces of attraction between droplets

Gravity

Random movement of droplets
Creaming / Sedimentation
•
•
•
•
No change in droplet size
Reversible
Driven by density difference
Usually results from gravitational forces
Creaming
Sedimentation
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
Coalescence
Not reversible
May lead from flocculation, creaming /
sedimentation or Brownian motion
Involves 2 drops coming together
•
•
•
•
May lead to complete separation
Coalescence
Coalescence increases if:-
•
•
•
•
Fat or ice crystals present
Viscosity of continuous phase is decreased
Emulsion is agitated
Interfacial viscosity is decreased
Van der Waals forces
Improving emulsion stability
• Charge stabilisation
• Interfacial film strengthening
• with powders
• with polymers
•
•
•
•
•
• 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
• In this system
• The negatively charged Stearate groups migrate to
the interface
• The positively charged Sodium ions in solution
(counter ions) are attracted to these now charged
droplets
• A layer is formed where the impact of the charge is
reduced
• This layer, called the Helmholtz double layer, can
reduce the repulsive effect and so stability
Improving Emulsion Stability
Helmholtz double layer effect
-
+
-
+
-
+
-
-
+
-
-
- -
+
+
+
-
-
-
+
Electrical double layer
Water phase
-
+
+
+
-
+
-
+
+
+
-
+
-
+
-
+
-
Oil droplet
+
-
+
-
+
-
+
-
-
+
-
Improving Emulsion Stability
• The double layer is likely to be more diffuse the further
away from the droplet you go (Gouy and Chapman and
Stern)
• Can the same happen for cationic and non-ionic
emulsifiers?
• The effect is impacted by the presence of electrolytes
• Adding electrolyte increases instability by reducing the
shielding effect
• The extent of this depends on the amount of
electrolyte added and the valency of the electrolyte
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
Interface stabilisation using non-ionic
emulsifiers
• Stabilises both oil-in-water and water-in-oil emulsions
through reducing interfacial forces
– Aids dispersion
– Reduces particle size
• Appropriate blends optimise stabilisation
– Reducing the energy imbalance
– Providing a barrier to coalescence
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
Stability testing -available tests
• Freeze thaw cycling
• Accelerated stability testing
• Tests at various temperatures
• Good guidance at www.ich.org
• Ultra centrifuge
• High speeds (>25,000 rpm) required
• Visual assessment
• As part of other techniques
• Use microscope
Stability testing
•
Low shear evaluation
•
•
•
Use sophisticated rheology machines
Shake for several days
Other tests as required
•
•
•
Light
Humidity
Microbiological
Stability testing

Examining stability samples




Actual pack and clear container samples
Visual assessment in pack
Microscopic assessment
Viscosity, pH etc
Emulsion manufacture
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
• Watch out for tea breaks and shift changes and
build these into your considerations!
• 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 15 min

Factory
– Oil phase added with
gate stirring followed by
homogeniser mixing
Size and distance
– Cold
water passed
through water jacket
with gate stirring
Takes hours!
Emulsion manufacture
Emulsion properties
Phase ratio
• In simple terms the ratio of one phase to
another
• BUT, in order to accurately describe the phase
ratio you need to know the type of emulsion
you are dealing with so
• For an o/w emulsion a 30:70 ratio is 30%
oil/ 70% water
• But for a w/o emulsion the opposite is true!
Phase inversion
 It is possible to influence the orientation of an
emulsion in a number of ways including
 Change the phase ratio of the emulsion
 Influencing the behaviour of the emulsifier in the
emulsion
 Phase inverted emulsions tend to have smaller
particle size and so improved chances of longer
term stability
 Often used in wipes systems where low viscosity is
required
Phase inversion - phase ratio
• In practical terms this could happen if
• Phases are mixed opposite to convention e.g.
adding water to oil is expected to give a water
in oil emulsion but could give oil in water
• Deliberately making a water in oil emulsion
then adding water to increase the internal
phase and causing inversion e.g. low energy
emulsification
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
Source: Kahlweit4
% emulsifier blend
20
Phase Inversion Temperature
Why might this be the case?
•
Solubility of ethoxylated emulsifiers increases
with increasing ethoxylation
Solubility
•
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
Emulsion rheology
• Shear deformation
• Is a change due to force F
being applied across the
top surface of area A.
• The ratio of force F to
area, A gives us a shear
stress across the liquid
• The liquid's response to
this applied shear stress
is to flow
Shear Deformation
Emulsion rheology
• Shear deformation
• The medium behaves as a
pack of cards
• At velocity V the liquid
spread and thins (T falls)
• It is this velocity gradient
that gives us the shear
rate
• Viscosity is simply the
ratio of the shear stress to
the shear rate
Shear Deformation
Emulsion rheology
Emulsion rheology
•
•
•
Thixotropy
•
Reduced viscosity when shear applied
•
Viscosity recovers when shear removed
Dilatancy
•
Increased viscosity when shear applied
•
May recover when shear removed
Shear thinning
•
Complete loss of viscosity when shear
or excess shear applied
Emulsion rheology
• A detailed study can yield information about
• Predicted stability
• Flow
• during application
• during pumping
• time dependency
• effect of temperature on
Emulsion rheology
Emulsion rheology
Can pictorially describe the properties that the emulsion
might exhibit
1000
Complex Modulas,
1 G* (Pa)
900
800
Rate Index (from Power
Law model)
700
600
500
400
300
5
Significant Yield Stress Pa (x10)
2
200
100
0
4
Viscosity with Shear
(rubbing) Pa (x1000)
3
Phase Angle, Delta (x100)
Emulsion rheology
•
Observed rheology is linked to extent of
continuous phase
•
•
Large, major continuous phase/ small dispersed
phase
•
Properties similar to that of continuous phase
Small continuous phase/ large dispersed phase
•
Interparticle reactions more important
•
High resting viscosity observed
•
Exhibits yield point
Emulsion rheology
•
Electroviscous effect
•
The apparent increase in viscosity when shear is
applied to charged particles
•
Pulling charged particles between two others
requires greater force
-
-
Sources and further reading
1.
2.
3.
4.
5.
6.
7.
8.
9.
“Croda’s time saving guide to emulsifier selection” - training course
available from Croda PLC
www.crodalubricants.com/download.aspx?s=133&m=doc&id=267
accessed 22 June 2009
Kahlweit M: Microemulsions, Science 29 April 1998, p671-621
Griffin WC: "Classification of Surface-Active Agents by 'HLB,'"
Journal of the Society of Cosmetic Chemists 1 (1949): 311.
Griffin WC: "Calculation of HLB Values of Non-Ionic Surfactants,"
Journal of the Society of Cosmetic Chemists 5 (1954): 259
Gibbs JW: “On the equilibrium of heterogeneous substances” (1878)
ICI Americas Inc: “The HLB system –a time saving guide to Emulsifier
selection” (1980)
W.D. Bancroft, “Theory of emulsification” Journal of Physical
Chemistry, Vol. 17, p501 - 519 (1913)
J. Woodruff, “Energy efficiency” SPC Asia, p27 – 29 (2011)