Emulsion technology

Emulsion
Technology
Russell Cox
SCS Summer School 2014

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
∆𝐺 = 𝛾𝐴 − 𝑇∆𝑆
ΔG is free energy of emulsification
γ is the interfacial tension
A is the interfacial area
T is temperature
ΔS is entropy of mixing
If ΔG is positive, the spontaneous emulsification is unlikely
If ΔG is negative, spontaneous emulsification will likely occur
The closer ΔG is to zero, the easier the formation of an
emulsion

Simple emulsion types
Water-in-oil
Water droplet
(dispersed phase)
Oil
(continuous phase)
Oil-in-water
Oil droplet
(dispersed phase)
Water
(continuous 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
• O/W emulsions tend to have a lighter feel than W/O
• 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

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 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 Na
3517
- +

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

Cationic
The emulsifier carries a positive charge e.g.
Palmitamidopropyl Trimonium Chloride
_
ClCH3(CH2)14C NH(CH2)3
O
CH3
CH3
N CH3
+

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

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 20
Lipophilic
Oil loving
Non polar
Oil soluble
Hydrophilic
Water loving
Polar
Water soluble

HLB system
Emulsifier HLB 5
Emulsifier HLB 10
Emulsifier HLB 15Oil
phase
Water
phase

• 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
Determining HLB value
Source: Croda presentation (Croda’s time saving guide to emulsifier selection)1

• 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!)

Determining HLB values
Source: Uniqema/ Croda2

• 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

• A simple O/W lotion formula
• Mineral oil 8 %
• Caprylic/capric triglyceride 2 %
• Isopropyl isostearate 2 %
• Cetyl alcohol 4 %
• Emulsifiers 4 %
• Polyols 5 %
• Water soluble active 1 %
• Water 74 %
• Perfume q.s.
• Preservative q.s.

• Mineral oil 8 / 16 = 50%
• Caprylic/cap. trig. 2 / 16 = 12.5%
• Isopropyl isostearate 2 / 16 = 12.5%
• Cetyl alcohol 4 / 16 = 25%

Oil phase
ingredient
contribution X required
HLB of
ingredient
equals
Mineral oil 50.0% 10.5 5.250
Caprylic cap.
Trig.
12.5% 5 0.625
Isopropyl
isostearate
12.5% 11.5 1.437
Cetyl alcohol 25.0% 15.5 3.875
Total 11.2

• 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
Emulsifier selection using HLB

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
Emulsifier
HLB 5
Emulsifier
HLB 15or

Emulsifier blends
For a given blend of non-ionic emulsifiers, where
Emulsifier A is more lipophilic than Emulsifier B
Emulsifier A Emulsifier B
Oil 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…

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
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

• 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

Charge stabilisation
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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

Helmholtz double layer effect
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Oil droplet Water phase
Electrical double layer

• 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

• Interfacial film strengthening
• Reduces the probability of coalescence when
droplets collide

Interfacial film strengthening
• with powders
Powder particle size must be
very small
Powder must have an affinity for
both the oil and water phase

• 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

• with non-ionic emulsifiers
Oil
Tighter packing
at interface
Interface strengthening is
dependent
on the number of molecules that
are packed into the interface

• 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
Interface stabilisation using non-ionic
emulsifiers

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

• Continuous phase viscosity
• Thickening the water phase restricts
movement of oil droplets
• Thickeners with yield points are most
effective
• Droplet size
Increasing 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

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:-
 For long term stability both forms are needed
Chemical energy + Mechanical energy
(emulsifier) (homogeniser)

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

 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

 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!

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

(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

 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
0o
75o
0 20% emulsifier blend
Temperatureo
C
TU
T
TL
2 phase
1 phase
2 phase
3 phase
Source: Kahlweit4

 Why might this be the case?
 Solubility of ethoxylated emulsifiers
increases with increasing ethoxylation
8 20
Solubility
Number of ethoxylate groups

 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
• 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

 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

Emulsion rheology
• A detailed study can yield information about
• Predicted stability
• Flow
• during application
• during pumping
• time dependency
• effect of temperature on

http://en.wikipedia.org/wiki/File:Rheometer.jpg accessed 6 July 2010
Emulsion rheology

Emulsion rheology
0
100
200
300
400
500
600
700
800
900
1000
1
2
34
5
Significant Yield Stress Pa (x10)
Phase Angle, Delta (x100)Viscosity with Shear
(rubbing) Pa (x1000)
Complex Modulas,
G* (Pa)
Rate Index (from Power
Law model)
 Can pictorially describe the properties that the
emulsion might exhibit

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
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Sources and further reading
1. “Croda’s time saving guide to emulsifier selection” - training course
available from Croda PLC
2. www.crodalubricants.com/download.aspx?s=133&m=doc&id=267
accessed 22 June 2009
3. Uniqema technology training document (unpublished)
4. Kahlweit M: Microemulsions, Science 29 April 1998, p671-621

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