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

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

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 measurementLaser 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?

Emulsifiers

What is an emulsifier?

Water lovinghead

Oil lovingtail

'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 - waterWater - in - oil

Improving emulsion stabilityClearly 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 Na3517

- +

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

LipophilicOil lovingNon polarOil soluble

HydrophilicWater lovingPolarWater soluble

HLB system

Emulsifier HLB 5

Emulsifier HLB 10

Emulsifier HLB 15Oilphase

Waterphase

• 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

EmulsifierHLB 10

EmulsifierHLB 5

EmulsifierHLB 15or

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

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…

CREAMING

SEDIMENTA

TION

COALESCENCE

OSTWALDRIPENING

PHASEINVERSION

FLOCCULATION

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

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

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

Where

F = Van der Waals forces of attractionsA = Hamaker constanta = Radius of dispersed phase dropletsH = Distance between two adjacent dispersed phase droplets

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 stabilityCharge 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

Improving Emulsion StabilityHelmholtz double layer effect

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Oil droplet Water phase

Electrical double layer

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

Interfacial film strengthening• with powders

Powder particle size must be very small

Powder must have an affinity for both the oil and water phase


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


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


• 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

http://www.ich.org/

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:-

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

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 mixingSize 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

0o

75o

0 20% emulsifier blend

Tem

pera

ture

o C TU

T

TL

2 phase

1 phase

2 phase

3 phase

Source: Kahlweit4

Phase Inversion Temperature Why might this be the case?

Solubility of ethoxylated emulsifiers increases with increasing ethoxylation

8 20

Solu

bilit

y

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• 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

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

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

http://en.wikipedia.org/wiki/File:Rheometer.jpg

http://upload.wikimedia.org/wikipedia/commons/9/9c/Rheometer.jpg

Emulsion rheology

0

100

200

300400

500

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700

800

900

10001

2

34

5Significant 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 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