1. SCS Summer School 2014
Emulsion Technology
Russell Cox

2. What is an emulsion?
A dispersion of one or more immiscible liquid phases in another, the distribution being in the form of tiny droplets

3. 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”

4. 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

5. Simple emulsion types Oil-in-water Water-in-oil Water
Oil droplet
(dispersed phase)
Water
(continuous phase)
Water-in-oil
Water droplet
(dispersed phase)
Oil
(continuous phase)

6. 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

7. 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

8. Droplet size measurement
Laser method Laser Particle Analyser Audio method Use of sound waves (Malvern) Optical method

9. 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

10. What does an emulsion look like?

11. What does an emulsion look like?

12. What does an emulsion look like?

13. Emulsifiers

14. What is an emulsifier? Water loving head Oil loving tail 'Hydrophilic'
'Lipophobic'
'Lipophilic'
'Hydrophobic'

15. 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

16. 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
Water - in - oil
Oil - in - water

17. 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

18. Types of emulsifiers - Anionics C H COO Na +
The emulsifier carries a negative charge e.g. Sodium Stearate soap
C H COO Na 35 17 - +

19. 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

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

21. 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

22. 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

23. Types of emulsifiers - Non-ionic
Most common
Wide range
Versatile
Strengthen the emulsion interface
HLB system to predict choice

24. HLB system and selecting emulsifiers

25. HLB system
Hydrophile / Lipophile Balance

26. HLB system 10 20 Hydrophilic Lipophilic Water loving Oil loving Polar10 20
Lipophilic
Oil loving
Non polar
Oil soluble
Hydrophilic
Water loving
Polar
Water soluble

27. HLB system Water Emulsifier HLB 5 Emulsifier HLB 10 Oil
phase
Water

28. 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

29. 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

30. Determining HLB values
Source: Uniqema/ Croda2

31. 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

32. Determining HLB value A simple O/W lotion formula Mineral oil 8 %
Caprylic/capric triglyceride 2 %
Isopropyl isostearate %
Cetyl alcohol %
Emulsifiers %
Polyols %
Water soluble active %
Water %
Perfume q.s.
Preservative q.s.
Source: Croda presentation (Croda’s time saving guide to emulsifier selection)1

33. Determining HLB value Mineral oil 8 / 16 = 50%
Caprylic/cap. trig / 16 = 12.5%
Isopropyl isostearate 2 / 16 = 12.5%
Cetyl alcohol / 16 = 25%
Source: Croda presentation (Croda’s time saving guide to emulsifier selection)1

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

35. 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

36. 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
HLB 5
HLB 15 or

37. 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

38. 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

39. 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

40. HLB Summary Pros Cons 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

41. Nothing can go wrong – can it?

42. 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…

43. CREAMING SEDIMENTATION PHASE OSTWALD INVERSION RIPENING COALESCENCE
FLOCCULATION 43

44. Factors that contribute to emulsion instability
Forces of attraction between droplets
Gravity
Random movement of droplets 44

45. Creaming / Sedimentation
No change in droplet size
Reversible
Driven by density difference
Usually results from gravitational forces
Creaming
Sedimentation

46. 46

47. 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 47

48. Coalescence Not reversible
May lead from flocculation, creaming / sedimentation or Brownian motion
Involves 2 drops coming together
May lead to complete separation 48

49. Coalescence Coalescence increases if:- Fat or ice crystals present
Viscosity of continuous phase is decreased
Emulsion is agitated
Interfacial viscosity is decreased 49

50. Van der Waals forces 𝐹=− 𝐴𝑎 12𝐻 Defined as 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 50

51. 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 51

52. Improving emulsion stability
Charge stabilisation - +
Negatively charged oil droplets repel each other
Stability affected by quantity of electrolyte and whether M+ or M++

53. 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

54. Improving Emulsion Stability
Helmholtz double layer effect + -
Oil droplet
Water phase
Electrical double layer

55. 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

56. Improving emulsion stability
Interfacial film strengthening
Reduces the probability of coalescence when droplets collide 56

57. 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 57

58. 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 58

59. Improving emulsion stability
Interfacial film strengthening
with non-ionic emulsifiers
Interface strengthening is dependent
on the number of molecules that
are packed into the interface
Oil
Tighter packing
at interface

60. 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 60

61. 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 61

62. 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

63. 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 63

64. Stability testing -available tests
Freeze thaw cycling
Accelerated stability testing
Tests at various temperatures
Good guidance at
Ultra centrifuge
High speeds (>25,000 rpm) required
Visual assessment
As part of other techniques
Use microscope 64

65. Stability testing Low shear evaluation Other tests as required
Use sophisticated rheology machines
Shake for several days
Other tests as required
Light
Humidity
Microbiological 65

66. Stability testing Examining stability samples
Actual pack and clear container samples
Visual assessment in pack
Microscopic assessment
Viscosity, pH etc 66

67. Emulsion manufacture

68. How are emulsions formed? Chemical energy + Mechanical energy
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)

69. 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

70. Two stages of creating an emulsion
Stage 1 – apply energy to the two phases to create a dispersion
Generally heat to °C
Stage 2 – stabilise the created dispersion
Maintain the small droplet size
Increase the external phase viscosity
Reduce phase density difference 70

71. 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 71

72. 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 72

73. 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! 73

74. Emulsion manufacture

75. Emulsion properties

76. 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!

77. 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

78. 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

79. 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

80. 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

81. 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 0o
75o 20
% emulsifier blend
Temperature oC TU T TL
2 phase
1 phase
3 phase
Source: Kahlweit4

82. Phase Inversion Temperature
Why might this be the case?
Solubility of ethoxylated emulsifiers increases with increasing ethoxylation
Solubility 8 20
Number of ethoxylate groups

83. 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

84. 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

85. 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

86. Emulsion rheology

87. 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 87

88. Emulsion rheology A detailed study can yield information about
Predicted stability
Flow
during application
during pumping
time dependency
effect of temperature on 88

89. Emulsion rheology
accessed 6 July 2010

90. Emulsion rheology
Can pictorially describe the properties that the emulsion might exhibit
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) 90

91. 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 91

92. - - - 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 - - - 92

93. Sources and further reading
“Croda’s time saving guide to emulsifier selection” - training course available from Croda PLC
accessed 22 June 2009
Uniqema technology training document (unpublished)
Kahlweit M: Microemulsions, Science 29 April 1998, p