SCS Summer School 2019 Emulsion Technology

Russell Cox What is a Macro- Dispersed heterogeneo particles Colloid? us systems dimensions

• comprised of one • may be in the order substance of 10-9m to 10-6m dispersed in (1-1000nm) another substance • Too small to see without a microscope What is a Colloid?

The dispersed substance = internal phase

The material it is dispersed in = the continuous phase What is a Colloid?

The dispersed substance = internal phase

The material it is dispersed in = the continuous phase

Continuous Phase

Internal Phase Types of Colloid?

Dispersed Continuous Common Name Cosmetic Product Phase Phase Example Gas Liquid Solid Liquid Solid Solid Gas Solid Liquid Gas Solid Gas Liquid Liquid Types of Colloid?

Dispersed Continuous Common Name Cosmetic Product Phase Phase Example Gas Liquid Foam Bubble Bath or Mousse Solid Liquid Solid Solid Gas Solid Liquid Gas Solid Gas Liquid Liquid Types of Colloid?

Dispersed Continuous Common Name Cosmetic Product Phase Phase Example Gas Liquid Foam Bubble Bath or Mousse Solid Liquid Soil, Sol, Dispersion Toothpaste or Suspension Solid Solid Gas Solid Liquid Gas Solid Gas Liquid Liquid Types of Colloid?

Dispersed Continuous Common Name Cosmetic Product Phase Phase Example Gas Liquid Foam Bubble Bath or Mousse Solid Liquid Soil, Sol, Dispersion Toothpaste or Suspension Solid Solid Solid Sol Antiperspirant stick Gas Solid Liquid Gas Solid Gas Liquid Liquid Types of Colloid?

Dispersed Continuous Common Name Cosmetic Product Phase Phase Example Gas Liquid Foam Bubble Bath or Mousse Solid Liquid Soil, Sol, Dispersion Toothpaste or Suspension Solid Solid Solid Sol Antiperspirant stick Gas Solid Solid Foam Polystyrene or Sponge Liquid Gas Solid Gas Liquid Liquid Types of Colloid?

Dispersed Continuous Common Name Cosmetic Product Phase Phase Example Gas Liquid Foam Bubble Bath or Mousse Solid Liquid Soil, Sol, Dispersion Toothpaste or Suspension Solid Solid Solid Sol Antiperspirant stick Gas Solid Solid Foam Polystyrene or Sponge Liquid Gas Aerosol Hairspray Solid Gas Liquid Liquid Types of Colloid?

Dispersed Continuous Common Name Cosmetic Product Phase Phase Example Gas Liquid Foam Bubble Bath or Mousse Solid Liquid Soil, Sol, Dispersion Toothpaste or Suspension Solid Solid Solid Sol Antiperspirant stick Gas Solid Solid Foam Polystyrene or Sponge Liquid Gas Aerosol Hairspray Solid Gas Solid aerosol Powder Spray Liquid Liquid Types of Colloid?

Dispersed Continuous Common Name Cosmetic Product Phase Phase Example Gas Liquid Foam Bubble Bath or Mousse Solid Liquid Soil, Sol, Dispersion Toothpaste or Suspension Solid Solid Solid Sol Antiperspirant stick Gas Solid Solid Foam Polystyrene or Sponge Liquid Gas Aerosol Hairspray Solid Gas Solid aerosol Powder Spray Liquid Liquid Emulsion Cream or Lotion What is an emulsion?

A dispersion of one or more immiscible liquid phases in another, the distribution being in the form of tiny droplets

For Example: Oil and Water Simple Oil in Water Water emulsion in Oil O/W W/O types

Water (Continuous Phase) Oil (Continuous Phase)

Oil droplet Water droplet (Dispersed Phase) (Dispersed Phase) Emulsion orientation

The predominant Generally, the The phase in The phase that is solubility of the phase of the which the stirrer added tends to emulsifier tends greatest volume is placed tends to become the to determine the tends to become become the internal phase external phase the external external phase (Bancroft’s rule) phase Feel O/W emulsions tend to have a lighter feel than W/O

Dye penetration Dispersibility Water soluble Identification Drop a small amount dye is easily of emulsion in water taken up in O/W of emulsion – O/W disperses system but not in type easily while W/O W/O remains whole

Conductivity O/W emulsions conduct electricity well showing high levels of conductance Use of sound waves

Laser Particle Analyser Droplet size measurement

Microscopy Microscopy

• Droplet size and size distribution • Quality of manufacturing process • Detecting unwanted crystallisation

• Droplet size and size distribution • Quality of manufacturing process • Detecting unwanted crystallisation • Early indications of instability • Comparison of different emulsions • Liquid crystals • Early indications of instability • Comparison of different emulsions • Liquid crystals What does an emulsion look like? Stability, or rather, Instability of Emulsions

The stability of an emulsion is how long the dispersion endures before the mixture separates out into the two phases again Stability, or rather, Instability of Emulsions

The stability of an emulsion is how long the dispersion endures before the mixture separates out into the two phases again

This process is driven by Thermodynamics Stability, or rather, Instability of Emulsions

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

∆퐺 = 훾퐴 − 푇∆푆

Derived from the second law of thermodynamics

This equation describes the energy required for a change to take place in any given system Gibbs free energy equation

∆퐺 = 훾퐴 − 푇∆푆

ΔG is free energy of emulsification

훾 is the interfacial tension

A is the interfacial area

T is temperature or heat

ΔS is the entropy of mixing Gibbs free energy equation

∆퐺 = 훾퐴 − 푇∆푆

What does ΔG mean?

If ΔG is positive, energy is required for a process to occur Indicates spontaneous emulsification is unlikely

If ΔG is negative, energy is not required for a process to occur Indicates spontaneous emulsification will likely occur

The closer ΔG is to zero, the easier the formation of an emulsion Gibbs free energy equation

∆퐺 = 훾퐴 − 푇∆푆

The first thing to overcome is the interfacial tension, 훾, between the two liquids

Put crudely, this is a measure of how insoluble the two liquids are to each other

In emulsions the two liquids will have some interfacial tension, so 훾 is always positive

The only way to overcome interfacial tension without chemical addition is mechanical work

– Stirring! Gibbs free energy equation

∆퐺 = 훾퐴 − 푇∆푆

The second thing the system needs to do is increase the surface area of the oil phase to make droplets

In the equation this is represented by A (change in surface area). In emulsification this is always big and positive Gibbs free energy equation

∆퐺 = 훾퐴 − 푇∆푆

So far; we can see that the energy needed for emulsification ΔG = a positive number (훾) multiplied by a big, positive number (A) Gibbs free energy equation

∆퐺 = 훾퐴 − 푇∆푆

Energy can be stored in an emulsion

Firstly; as Heat, T

Heat can be applied to an emulsion which results in the temperature increasing As this energy is used to heat up the system it is not used to create new surface area, therefore it is additional energy

In emulsification, T is positive and small Gibbs free energy equation

∆퐺 = 훾퐴 − 푇∆푆

Secondly as entropy,

This is the chemical energy in the system

ΔS is the difference in chemical energy from the initial system to the new system

In emulsification, it is relatively small and can be positive or negative Gibbs free energy equation

∆퐺 = 훾퐴 − 푇∆푆

Therefore, 푇∆푆 represents the energy left stored in the system

Now we see that a positive moderate number (푇) multiplied by a small negative or positive number (∆푆 ) Gibbs free energy equation

∆퐺 = 훾퐴 − 푇∆푆

Combining both elements of the equation;

We can see that the energy needed for emulsification ΔG= a positive number (훾) multiplied by a big, positive number (A) minus a positive moderate number (푇) X a small negative or positive number (∆푆)

Therefore, ΔG is big and positive showing that emulsification needs an energy input to proceed and does not happen spontaneously Gibbs free energy equation

∆퐺 = 훾퐴 − 푇∆푆

So what if we consider the reverse process

The emulsion separating out into discrete phases:

Firstly, 훾 remains the same. It is still the same liquids in contact with each other so the interfacial tension between them is the same (still positive)

However, surface area is now reducing from lots of droplets to a single plane. So, A is still big but now negative

TΔS for emulsion collapse can be treated similarly to emulsification, T will be small and positive, ΔS will be small and positive or negative Gibbs free energy equation

∆퐺 = 훾퐴 − 푇∆푆

So for emulsion collapse we can see that the energy needed for the process

ΔG= a positive number (훾) multiplied by a big, negative number (ΔA) minus a positive moderate number (T) multiplied by a small negative or positive number (ΔS)

Therefore, ΔG is big and negative showing that emulsion collapse needs no energy input to proceed and does happens spontaneously Stability, or rather, Instability of Emulsions

So, thermodynamics tells us that all emulsions will separate out

But it does not tell us how fast – for that is the realm of : Kinetics Stability, or rather, Instability of Emulsions

To understand the rate of separation, we have to understand the separation mechanisms

There are 6 mechanisms:

• Coalescence • Flocculation • Creaming • Sedimentation • Oswald Ripening • Phase Inversion Stability, or rather, Instability of Emulsions

Technically an better way to diagrammatically represent this; Creaming and Sedimenting

Sedimentation and Creaming occurs due to gravity

The lower density oil phase will migrate above the water phase resulting in an increase in concentration at the surface (creaming)

Higher density phases will migrate below the water phase resulting in an increase in concentration at the bottom (sedimentation) Stokes Law

Defined as:-

2 V= (2r g (ρc – ρd)) / 9η

Where V = Velocity of a droplet = rate of sedimentation or creaming r = Radius of dispersed phase droplet

ρc= Density of continuous phase ρd = Density of dispersed phase g = Acceleration due to gravity η = viscosity of the continuous (external) phase Stokes Law

2 V= (2r g (ρc – ρd)) / 9η

The velocity of a droplet represents the rate of sedimentation or creaming

The lower the value, in theory, the less likely the emulsion will separate Stokes Law

2 V= (2r g (ρc – ρd)) / 9η r = Radius of dispersed phase droplet

V is directly proportional to the size of the droplets

As r increases so does V

Reducing the droplet size should stabilise the emulsion Stokes Law

2 V= (2r g (ρc – ρd)) / 9η

G is acceleration due to Gravity

It is a Gravitational constant

The only way to change this would be to leave planet Earth Stokes Law

2 V= (2r g (ρc – ρd)) / 9η

(ρc – ρd) is the difference in phase densities

If the difference increases then so does the velocity, therefore making the emulsion less stable

Very difficult to change the relative densities of each phase

Typically water phase has a density of 1

Typically the oil phase has a density of 0.8 Stokes Law

2 V= (2r g (ρc – ρd)) / 9η

η is the viscosity of the continuous (external) phase

Velocity of the droplet is inversely proportional to the viscosity of the external phase, as viscosity increases V decreases Stokes Law

2 V= (2a g (ρc – ρd)) / 9η

Therefore to stabilise a system from sedimentation and creaming, make the smallest droplets you can, try to minimise density difference between phases and increase viscosity of the external phase Ostwald Ripening

Ostwald ripening is dependent on the diffusion of disperse phase molecules from smaller to larger droplets through the continuous phase

It is driven by the phenomenon that smaller droplets have a higher internal pressure than larger droplets. As expressed in the Laplace equation:

2훾 푃 = 푟

Where;

P = Laplace Pressure 훾 = Surface Tension r = droplet radius Ostwald Ripening

2훾 푃 = 푟

So from this we can say the rate of Oswald Ripening is:

Inversely proportional to the difference in radius of the internal phase droplets, i.e. the bigger the difference in particle size the faster Oswald Ripening occurs

Directly proportional to the solubility of the dispersed phase in the external phase

To stabilise this a formulator must create as close to a mono-dispersion of particles as possible, and ensure minimal solubility between the phases Coalescence and Flocculation

Coalescence and flocculation occur due to collisions of the dispersed phase droplets These collisions can result in;

Repulsion, droplets move apart again with no change to the colloidal state (stable)

Coalescence, where the droplets join to make a larger droplet, decreasing overall surface area of the colloid (unstable), or

Flocculation, where the droplets do not move apart but associate and move together through the colloid but overall surface area remains the same (route to instability) Stability, or rather, Instability of Emulsions

In the first few moments of emulsification the system is chaotic and droplet approach and collide constantly

The outcome of these collisions is different for every system and is dependant on the attractive and repulsive forces of the phases; hence the rate of coalescence and flocculation is unique to every system. Two events required for emulsion instability

Physical attraction between droplets

Coalescence of droplets Factors that contribute to emulsion instability

Forces of attraction between droplets

Gravity

Random movement of droplets Simple routes to improve stability

Physical attraction;

Reduce attraction forces -eg introduce charge repulsion between droplets, by adding stearic acid or electrolytes

Reduce the ability for the droplets to move freely; increase viscosity, using viscosity modifiers or waxes and gums Simple routes to improve stability

Coalescence

Strengthen the interface of the droplet using polymers

Strengthen the interface of the droplet using powders

Alter the surface characteristics of the droplet using surfactants and emulsifiers Emulsifiers What is an emulsifier?

Water loving Oil loving head tail

'Hydrophilic' 'Lipophilic' 'Lipophobic' '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

It migrates to the interface between the two phases

They have two affects on the system:

• Lowering the interfacial tension between the phases

• Creating repulsive forces between the internal phase particles to slow the rate of coalescence and flocculation Gibbs free energy equation

Why does an emulsifier work?

∆퐺 = 훾퐴 − 푇∆푆

Thermodynamically the effect is that the interfacial tension between the phases is reduced

Which means that with 훾 now lower the work needed to create a colloid is less

This does not change the fact that the system is thermodynamically unstable, but it does mean the emulsion is easier to create. 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 Cationic

Anionic Types of Emulsifier

Non-Ionic

Amphoteric Types of emulsifiers

Anionics

The emulsifier carries a negative charge e.g. Sodium Stearate soap

C H COO - Na + 17 35 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 Types of emulsifiers

Cationic

The emulsifier carries a positive charge e.g. Palmitamidopropyl Trimonium Chloride

O CH3 + _ CH3(CH2)14C NH(CH2)3 N CH3 Cl

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 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 Types of emulsifiers - Non-ionic

Pros and Cons

• Most common • Wide range • Versatile • Strengthen the emulsion interface • HLB system to predict choice How does an Emulsifiers work?

Emulsifiers create repulsive forces by:

• Creating a physical barrier to collision

• Electrostatic repulsion How does an Emulsifiers work?

Emulsifiers create repulsive forces by:

• Creating a physical barrier to collision

• Electrostatic repulsion How does an Emulsifiers work?

Creating a physical barrier to collision

Prevents coalescence and flocculation by extending tails into the continuous phase

Stops collision by entanglement of the molecules How does an Emulsifiers work?

Electrostatic repulsion

Creates a surface charge on each droplet

Results in electrostatic repulsion as the droplets approach each other

Relevant for polar continuous phases (eg Water) as there is an abundance of dissociated ions to interact with How does an Emulsifiers work?

Electrostatic repulsion

Once a charge is acquired at the surface , counter ions from the continuous phase are attracted to the surface, creating a tight layer of ions on the surface (Stern Layer)

Double layer has an electric potential that extends into the continuous phase –Zeta Potential

The higher the Zeta Potential the more repulsion between particles HLB system and selecting emulsifiers HLB system

Hydrophile / Lipophile Balance HLB system

0 10 20

Lipophilic Hydrophilic Oil loving Water loving Non polar Polar Oil soluble Water soluble HLB system Determining HLB value • 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.

Source: Croda presentation (Croda’s time saving guide to emulsifier selection)1 Determining HLB value • Mineral oil 8 / 16 = 50%

• Caprylic/cap. trig. 2 / 16 = 12.5%

• Isopropyl isostearate 2 / 16 = 12.5%

• Cetyl alcohol 4 / 16 = 25%

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

Oil Phase Ingredient Contribution X Required HLB of Equals Ingredient Mineral Oil 50% 10.5 5.25 Capric Caprylic Triglyceride 12.5% 5 0.625 Isoprpyl Isostearate 12.5% 11.5 1.437 Cetyl Alcohol 25% 15.5 3.875 Total 11.2

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

Oil Phase Ingredient Contribution X Required HLB of Equals Ingredient Mineral Oil 50% 10.5 5.25

Capric Caprylic Triglyceride 12.5% 5 0.625

Isoprpyl Isostearate 12.5% 11.5 1.437

Cetyl Alcohol 25% 15.5 3.875

Total 11.2

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

• Pros • Cons – Empirical system – Not good for anionics and cationics giving starting – Need to know HLB of oil position which can vary – Can be assessed – Can be time consuming practically working out or measuring – Does not determine the amount of emulsifier needed 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 Emulsifier Emulsifier HLB 10 or HLB 15 HLB 5 Emulsifier blends

This would look like: 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 Potential Irritation

Emulsifiers, since they are surface active, may be a factor in increasing the risk of irritation

Therefore

H319 Eye Irritation H318 Eye Damage

Excessive levels of emulsifier should be avoided Van der Waals forces

Defined as

퐴푎 퐹 = − 12퐻 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 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 stability

Charge stabilisation 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 Improving Emulsion Stability

We can see how the Double Layer can come into play: 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 Improving emulsion stability

Interfacial film strengthening • with powders Improving emulsion stability

Interfacial film strengthening • with polymers 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 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 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 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 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 Stability testing -available tests

• Freeze thaw cycling

• Accelerated stability testing • Tests at various temperatures

• 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 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:-

Chemical energy + Mechanical energy (emulsifier) (homogeniser)

• For long term stability both forms are needed 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 • Factory • Oil phase added with • Oil phase added with Silverson mixing stirring followed by homogeniser mixing Size and distance

• Beaker placed in bowl • Cold water passed of cold water and stir through water jacket with cooled stirring

• Takes approx 15 min • 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 o

complex three phase 75 C mixture is found o T U 2 phase

Above TU a W/O emulsion T 3 phase 1 phase exists, below T O/W L Temperature 2 phase TL This temperature and band will be different for different 0o systems 0 % emulsifier blend 20

Source: Kahlweit4 Phase Inversion Temperature

• Why might this be the case?

• Solubility of ethoxylated emulsifiers increases with increasing ethoxylation 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 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