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