emulsion technology russell cox scs summer school 2014
TRANSCRIPT
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Emulsion Technology
Russell Cox
SCS Summer School 2014
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What is an emulsion?
• A dispersion of one or more immiscible liquid phases in another, the distribution being in the form of tiny droplets
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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”
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Gibbs free energy equation
ΔG is free energy of emulsificationγ is the interfacial tensionA is the interfacial area T is temperature ΔS is entropy of mixing
If ΔG is positive, the spontaneous emulsification is unlikelyIf ΔG is negative, spontaneous emulsification will likely occurThe closer ΔG is to zero, the easier the formation of an emulsion
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Simple emulsion types
Water-in-oil
Water droplet(dispersed phase)
Oil(continuous phase)
Oil-in-water
Oil droplet(dispersed phase)
Water(continuous phase)
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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
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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
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Droplet size measurementLaser method Laser Particle Analyser
Audio method Use of sound waves
(Malvern)
Optical method
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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
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What does an emulsion look like?
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What does an emulsion look like?
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What does an emulsion look like?
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Emulsifiers
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What is an emulsifier?
Water lovinghead
Oil lovingtail
'Hydrophilic''Lipophobic'
'Lipophilic''Hydrophobic'
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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
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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 - waterWater - in - oil
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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
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Types of emulsifiers
Anionics
The emulsifier carries a negative charge e.g. Sodium Stearate soap
C H COO Na3517
- +
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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
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Types of emulsifiers
Cationic
The emulsifier carries a positive charge e.g. Palmitamidopropyl Trimonium Chloride
_ClCH3(CH2)14C NH(CH2)3
O
CH3
CH3
N CH3+
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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
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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
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Types of emulsifiers - Non-ionic
• Most common• Wide range• Versatile• Strengthen the emulsion interface• HLB system to predict choice
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HLB system and selecting emulsifiers
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HLB system
Hydrophile / Lipophile Balance
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HLB system
0 10 20
LipophilicOil lovingNon polarOil soluble
HydrophilicWater lovingPolarWater soluble
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HLB system
Emulsifier HLB 5
Emulsifier HLB 10
Emulsifier HLB 15Oilphase
Waterphase
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• 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
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• 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 value
Source: Croda presentation (Croda’s time saving guide to emulsifier selection)1
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Determining HLB values
Source: Uniqema/ Croda2
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• 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
Determining HLB value
Source: Croda presentation (Croda’s time saving guide to emulsifier selection)1
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• 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)1
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• 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)1
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Determining HLB value
Source: Croda presentation (Croda’s time saving guide to emulsifier selection)1
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
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• 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
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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
EmulsifierHLB 10
EmulsifierHLB 5
EmulsifierHLB 15or
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Emulsifier blendsFor a given blend of non-ionic emulsifiers, where Emulsifier A is more lipophilic than Emulsifier B
Emulsifier A Emulsifier B
Oil Oil
Tighter packingat interface
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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
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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
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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
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Nothing can go wrong – can it?
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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…
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CREAMING
SEDIMENTA
TION
COALESCENCE
OSTWALDRIPENING
PHASEINVERSION
FLOCCULATION
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Factors that contribute to emulsion instability
Forces of attraction between droplets
Gravity
Random movement of droplets
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Creaming / Sedimentation• No change in droplet size• Reversible• Driven by density difference• Usually results from gravitational forces
Creaming Sedimentation
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Stokes’ Law
Defined as:-
Velocity of droplet (v) = (2a2 g (ρ1 – ρ2)) / 9η
Wherea = Radius of dispersed phase dropletρ1= Density of continuous (external) phaseρ2 = Density of continuous (internal) phaseg = Acceleration due to gravityη = viscosity of the continuous (external) phase
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Coalescence
• Not reversible• May lead from flocculation, creaming /
sedimentation or Brownian motion• Involves 2 drops coming together
• May lead to complete separation
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Coalescence
Coalescence increases if:-
• Fat or ice crystals present• Viscosity of continuous phase is decreased• Emulsion is agitated• Interfacial viscosity is decreased
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Van der Waals forces
Defined as
Where
F = Van der Waals forces of attractionsA = Hamaker constanta = Radius of dispersed phase dropletsH = Distance between two adjacent dispersed phase droplets
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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
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Improving emulsion stabilityCharge stabilisation
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Negatively charged oil droplets repel each other
Stability affected by quantity of electrolyte and whether M+ or M++
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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
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Improving Emulsion StabilityHelmholtz double layer effect
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Oil droplet Water phase
Electrical double layer
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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
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Improving emulsion stability
• Interfacial film strengthening• Reduces the probability of coalescence when
droplets collide
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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
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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
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Interfacial film strengthening• with non-ionic emulsifiers
Oil
Tighter packingat interface
Interface strengthening is dependent on the number of molecules that are packed into the interface
Improving emulsion stability
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• 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
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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
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• 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
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• 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 stability
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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
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Stability testing• Low shear evaluation
• Use sophisticated rheology machines• Shake for several days
• Other tests as required
• Light• Humidity• Microbiological
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Stability testing
Examining stability samples
Actual pack and clear container samples Visual assessment in pack Microscopic assessment Viscosity, pH etc
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Emulsion manufacture
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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)
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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
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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
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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
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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
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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 mixingSize and distance
– Cold water passed through water jacket with gate stirring
Takes hours!
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Emulsion manufacture
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Emulsion properties
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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!
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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
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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
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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
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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
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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
0 20% emulsifier blend
Tem
pera
ture
oC TU
T
TL
2 phase
1 phase
2 phase
3 phase
Source: Kahlweit4
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Phase Inversion Temperature Why might this be the case?
Solubility of ethoxylated emulsifiers increases with increasing ethoxylation
8 20
Sol
ubil
ity
Number of ethoxylate groups
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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
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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
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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
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Emulsion rheology
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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
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Emulsion rheology• A detailed study can yield information about
• Predicted stability
• Flow• during application• during pumping• time dependency• effect of temperature on
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http://en.wikipedia.org/wiki/File:Rheometer.jpg accessed 6 July 2010
Emulsion rheology
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Emulsion rheology
0
100
200
300
400
500
600
700
800
900
10001
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
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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
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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 reading1. “Croda’s time saving guide to emulsifier selection” - training course
available from Croda PLC2. www.crodalubricants.com/download.aspx?s=133&m=doc&id=267
accessed 22 June 20093. Uniqema technology training document (unpublished)4. Kahlweit M: Microemulsions, Science 29 April 1998, p671-621