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 ID: 491589
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Slide1
Emulsion Technology
Russell Cox
SCS Summer School 2014Slide2
What is an emulsion?
A
dispersion
of one or more
immiscible liquid phases
in another, the distribution being in the form of
tiny dropletsSlide3
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”Slide4
Gibbs free energy equation
Slide5
Simple emulsion types
Water-in-oil
Water droplet
(dispersed phase)
Oil
(continuous phase)
Oil-in-water
Oil droplet
(dispersed phase)
Water
(continuous phase)Slide6
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 phaseSlide7
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/OSlide8
Droplet size measurement
Laser method
Laser Particle Analyser
Audio method Use of sound waves (Malvern)
Optical methodSlide9
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 crystalsSlide10
What does an emulsion look like?Slide11
What does an emulsion look like?Slide12
What does an emulsion look like?Slide13
What don’t you want to see?Slide14
EmulsifiersSlide15
What is an emulsifier?
Water loving
head
Oil loving
tail
'Hydrophilic'
'Lipophobic'
'Lipophilic'
'Hydrophobic'Slide16
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 coalescenceSlide17
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 - oilSlide18
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 stabilitySlide19
Types of emulsifiers
Anionics
The emulsifier carries a negative charge e.g. Sodium Stearate soap
C H COO Na
35
17
-
+Slide20
Types of emulsifiers
- Anionic
Pros and ConsWere very commonOld fashioned
Not as versatile
Cheap
Limitations for actives due to high pH
Give negative charge to the oil dropletSlide21
Types of emulsifiers
Cationic
The emulsifier carries a positive charge e.g. Palmitamidopropyl Trimonium Chloride
_
Cl
CH
3
(CH
2
)
14
C
NH(CH
2
)
3
O
CH
3
CH
3
N
CH
3
+Slide22
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 pHSlide23
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
CH
3
(CH
2
)
16
CH
2
(OCH
2
CH
2
)
2
OHSlide24
Types of emulsifiers - Non-ionic
Most common
Wide range
Versatile
Strengthen the emulsion interface
HLB system to predict choiceSlide25
HLB system and selecting emulsifiersSlide26
HLB system
H
ydrophile
/
L
ipophile
B
alanceSlide27
HLB system
0
10
20
Lipophilic
Oil loving
Non polar
Oil soluble
Hydrophilic
Water loving
Polar
Water solubleSlide28
HLB system
Emulsifier HLB 5
Emulsifier HLB 10
Emulsifier HLB 15
Oil
phase
Water
phaseSlide29
Calculate the water loving portion of the surfactant on a
molecular weight percent basis
and then divide that number by 5Dividing by 5 keeps the HLB number scale limited to a maximum of 20 which makes the scale smaller, thus a bit more manageableOnce calculated assign this number to the non-ionic surfactantThis assigned number is the HLB
VALUE
Determining HLB value
Source: Croda presentation (Croda’s time saving guide to emulsifier selection)
1Slide30
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 instructionsand a little bit of time (actually a lot of time!)
Determining HLB value
Source: Croda presentation (Croda’s time saving guide to emulsifier selection)
1Slide31
Determining HLB values
Source: Uniqema/ Croda
2
Slide32
Look at your formula
Determine which are the oil soluble ingredients
this does not include the emulsifiersWeigh each of the weight percents of the oil phase ingredients together and divide each by the totalMultiply these answers times the required HLB of the individual oils
Add these together to get the required HLB of your unique blend
Determining HLB value
Source: Croda presentation (Croda’s time saving guide to emulsifier selection)
1Slide33
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.
Determining HLB value
Source: Croda presentation (Croda’s time saving guide to emulsifier selection)
1Slide34
Mineral oil 8 / 16 = 50%
Caprylic/cap. trig. 2 / 16 = 12.5%
Isopropyl isostearate 2 / 16 = 12.5%
Cetyl alcohol 4 / 16 = 25%
Determining HLB value
Source: Croda presentation (Croda’s time saving guide to emulsifier selection)
1Slide35
Determining HLB value
Source: Croda presentation (Croda’s time saving guide to emulsifier selection)
1Slide36
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 HLBSlide37
HLB system
Required HLB for oil-in-water emulsion
Benzophenone-3 7
Mineral oil 10 - 11Caprylic/Capric triglyceride 5Cetyl alcohol 15 - 16
Vitamin E 6
Required HLB for water-in-oil emulsion
Mineral oil
4
Slide38
HLB impacts on Viscosity
For the same formulation viscosity increases with decreasing emulsifier HLB
Source: Uniqema technical training document (unpublished)
3Slide39
HLB impacts on Viscosity
The effect is seen to be linear when log viscosity is considered
Source: Uniqema technical training document (unpublished)
3Slide40
Concentration impacts on Viscosity
Increasing concentration has a linear impact when log viscosity is considered but may vary with emulsifier form
Source: Uniqema technical training document (unpublished)
3Slide41
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 15
orSlide42
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 interfaceSlide43
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 electrolyteSlide44
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 avoidedSlide45
HLB Summary
Pros
Empirical system giving starting positionCan be assessed practically
ConsNot good for anionics and cationicsNeed to know HLB of oil which can varyCan be time consuming working out or measuringDoes not determine the amount of emulsifier neededSlide46
Nothing can go wrong – can it?Slide47
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 layersThe 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…Slide48
CREAMING
SEDIMENTATION
COALESCENCE
OSTWALD
RIPENING
PHASE
INVERSION
FLOCCULATIONSlide49
Factors that contribute to emulsion instability
Forces of attraction between droplets
Gravity
Random movement of dropletsSlide50
Creaming / Sedimentation
No change in droplet size
Reversible
Driven by density difference
Usually results from gravitational forces
Creaming
SedimentationSlide51Slide52
Stokes’ Law
Defined as:-
Velocity of droplet (v) = (2a
2
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) phaseSlide53
Coalescence
Not reversible
May lead from flocculation, creaming / sedimentation or Brownian motion
Involves 2 drops coming together
May lead to complete separationSlide54
Coalescence
Coalescence increases if:-
Fat or ice crystals present
Viscosity of continuous phase is decreased
Emulsion is agitated
Interfacial viscosity is decreasedSlide55
Van der Waals forces
Slide56
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 crystalsSlide57
Improving emulsion stability
Charge stabilisation
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Negatively charged oil droplets repel each other
Stability affected by quantity of electrolyte and whether M+ or M++Slide58
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 stabilitySlide59
Improving Emulsion Stability
Helmholtz double layer effect
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Oil droplet
Water phase
Electrical double layerSlide60
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 electrolyteSlide61
Improving emulsion stability
Interfacial film strengthening
Reduces the probability of coalescence when droplets collideSlide62
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 stabilitySlide63
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 stabilitySlide64
Interfacial film strengthening
with non-ionic emulsifiers
Oil
Tighter packing
at interface
Interface strengthening is dependent
on the number of molecules that
are packed into the interface
Improving emulsion stabilitySlide65
Stabilises both oil-in-water and water-in-oil emulsions through reducing interfacial forces
Aids dispersion
Reduces particle size
Appropriate blends optimise stabilisationReducing the energy imbalanceProviding a barrier to coalescence
Interface stabilisation using non-ionic emulsifiersSlide66
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
Slide67
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 stabilitySlide68
Co-emulsifiers / polar waxes
e.g. Cetyl alcohol
Co-emulsifiers have weaker surface activity than primary emulsifiers
Adds body and helps prevent coalescence
Improving emulsion stabilitySlide69
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
Slide70
Stability testing
Low shear evaluation
Use sophisticated rheology machines
Shake for several days
Other tests as required
Light
Humidity
Microbiological
Slide71
Stability testing
Examining stability samples
Actual pack and clear container samples
Visual assessment in pack
Microscopic assessment
Viscosity, pH etcSlide72
Emulsion manufacture Slide73
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)Slide74
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 differenceSlide75
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 differenceSlide76
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 Slide77
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 attemptSlide78
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!Slide79
Emulsion manufactureSlide80
Emulsion properties Slide81
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!Slide82
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 requiredSlide83
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 emulsificationSlide84
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 sizeSlide85
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 emulsifiersSlide86
Phase Inversion Temperature
Within the marked band a complex three phase mixture is found
Above T
U
a W/O emulsion exists, below T
L
O/W
This temperature and band will be different for different systems
0
o
75
o
0
20
% emulsifier blend
Temperature
o
C
T
U
T
T
L
2 phase
1 phase
2 phase
3 phase
Source: Kahlweit
4Slide87
Phase Inversion Temperature
Why might this be the case?
Solubility of ethoxylated emulsifiers increases with increasing ethoxylation
8
20
Solubility
Number of ethoxylate groupsSlide88
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 possibleSlide89
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 flowSlide90
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 rateSlide91
Emulsion rheologySlide92
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 rheologySlide93
Emulsion rheology
A detailed study can yield information about
Predicted stability
Flow
during application
during pumping
time dependency
effect of temperature onSlide94
Emulsion rheologySlide95
Emulsion rheology
0
100
200
300
400
500
600
700
800
900
1000
1
2
3
4
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 exhibitSlide96
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
Slide97
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
-
-
-Slide98
Sources and further reading
“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)