Emulsions and foams
• Emulsions and foams
- Both dispersion continuous phase=liquid
- Emulsion: liquid-liquid dispersion (liquid droplets in liquid)
- Foam: air-liquid dispersion (air bubbles in liquid)
- Common characteristics emulsions and foams:
o Both systems have a large amount of interface between the phases
▪ For emulsion: interface
▪ For foams: surface
o Interfaces need to be stabilized by a surface active component
o Both are meta-stable not in stable equilibrium and both will tend to phase
separate can happen in days, weeks, months stable over a certain period of
time (slowly evolving towards complete separation)
o Same type of physical phenomena occur to make the emulsions and foams unstable:
evolve by the same mechanisms (aggregation, coalescence, Ostwald ripening,
creaming)
- Differences emulsions and foams:
o Density difference between the two phases is much larger in foams
▪ Related to the difference in dispersed phase
▪ Emulsions: liquid, foams: air air much lighter than liquid large difference
in density between continuous and dispersed phase in foams
o Droplet size is much smaller in emulsions than air bubbles in foams (µm in emulsions
vs mm in foam)
- As a results of these two differences destabilization (creaming) is much faster in foam
limits shelf-life for foams to large extent
o Shelf life foams: minutes, days, or weeks
o Shelf life emulsions: months or years
Emulsions
• Emulsions
- Two main types of emulsions:
1. Oil in water (O/W): oil droplets in a continuous water phase
o Milk
o Mayonnaise
o Sauces
o Salad dressing
o Ice-cream (also a foam more complex)
o Large variety in foods
2. Water in oil (W/O): water droplets in an oil phase
o Butter (contains small amounts of water, >85% fat)
o Products are more fatty
o Margarine
o Spreads
o Less common in foods
- Hydrophobic oil and hydrophilic water are not miscible one phase can
exist in the other one
o Requires the creation of an oil-water interface and these interfaces
are present in both types of emulsions
o Both emulsions are not in a thermodynamically stable state (meta-stable) phase
separation (oil lower density upper phase)
▪ Process of phase separation will occur for both oil-in-water emulsions as the
water-in-oil emulsion but not at the same speed
• Preparation emulsions
- Oil and water have no affinity for each other contact is not a favourable situation
- To create a surface between oil and water costs energy:
o Energy can be given as Gibss free energy
o The creation of interface in a system costs energy:
56
, o Creating interface (not favourable not spontaneously need do add ingredient):
▪ G (free energy) is positive: Process will not occur
spontaneously and not preferred
▪ System always try to keep the interfacial area to a minimum achieved by
merging droplets total volume will stay the same, but total surface area will
decrease
o Reducing interface:
▪ G (free energy) is negative: Process may occur
spontaneously
- When system contains an interface will always try to reduce the amount of interfacial
area to lower the energy
- Why are emulsions meta-stable?
o Interfacial tension determines how much energy is needed to create an interface
o The energy required to create a surface:
▪ Decrease in surface tension
➢ Lower surface tension = related to how easy it is to
make droplets and create surface/interface
between 2 phases (low = easier)
➢ decrease in energy
➢ Easier to create surface
▪ Surface tension: the change of free energy resulting from a change in surface
area
▪ Interfacial tension between water and oil and surface tension
between water and air is quite large difficult to make small
droplets/air bubbles (requires a lot of interfacial area)
▪ The interfacial/surface tension can be reduced by adsorbing different types of
molecules or particles at the interface for the same energy input more
surface area can be created (as the surface tension is lower)
▪ The energy required to create a surface can be reduced by adsorbing
molecules or particles at the interface that reduce the surface tension
- Surface tension = results of the interactions of water molecules
o Interactions between water molecules quite strong due to hydrogen bonds
o In bulk phase: water molecules surrounded by other water molecules in oil directions
due to strong interactions the surrounding water molecules will pull on the middle
water molecule net force = 0
o When molecules are present at surface (air)/interface (oil) only water molecules in
bulk water will be able to pull on the water molecule greater attraction of water
molecules in the water than to the molecules in air/oil inward force at the surface
surface will acts as membrane
o As the interaction between water molecules is higher surface tension between air
and water is relatively high (72.8 mN/m)
57
, - Role of surfactants
o Hydrophilic side (head group) and hydrophobic side (tail)
able to absorb to interface between oil and water
o Have the capability to reduce the energy needed to form an
interface
o In water:
▪ Unfavourable mixing between water and hydrophobic tails
micelles (hydrophobic interactions)
▪ Hydrophobic tails are clustered on the inside and hydrophilic
heads stick out
▪ Micel formation Occurs naturally
o In emulsion:
▪ Tails are present in oil prevents contact with water
▪ Favourable mixing of hydrophobic tails with hydrophobic oil
o Such surfactants will therefore be preferably present on the interface
between oil and water
▪ Surfactants adsorb to the interfaces between oil and water as much as
possible
o Comparing emulsions with and without surfactants:
▪ No surfactant: only energy costs for creating unfavourable contact between oil
and water Gibbs energy positive (not preferred) unstable situation
▪ Surfactant: energy costs for creating unfavourable contact between
hydrophobic oil and hydrophilic water positive value for Gibbs energy +
energy gain by avoiding contact of hydrophobic tails and water favourable
situation
▪ Presence of surfactants lowers the surface or interfacial tension more
favourable less energy needed to create the same interfacial area
o Energy cost for creating unfavourable contact between hydrophobic oil and
hydrophilic water ΔG > 0 not stable
▪ Energy gain by avoiding contact of hydrophobic tails and water ΔG < 0
o Surfactants will decrease the surface tension at low concentration concentration
too high molecules already covered surfactants will stay in bulk phase (and
surface tension will not further decrease)
o How much interfacial tension will decrease depends on:
▪ The size of the molecules
▪ The hydrophobicity
▪ The distance between the molecules (as a result of the mutual interactions)
o Interfacial tension does not say anything about the stability of emulsions stability
related to other properties of the interface and the interactions between oil droplets
- Emulsification process
o To make emulsion stable have to create a lot of interactions requires energy
input commonly added by mixing process: stirring provides energy to create the
interfaces
▪ How much energy depends on stirring speed (harder = more)
▪ This amount of energy determines how much interface can be created
depending on the interfacial/surface tension
58
• Emulsions and foams
- Both dispersion continuous phase=liquid
- Emulsion: liquid-liquid dispersion (liquid droplets in liquid)
- Foam: air-liquid dispersion (air bubbles in liquid)
- Common characteristics emulsions and foams:
o Both systems have a large amount of interface between the phases
▪ For emulsion: interface
▪ For foams: surface
o Interfaces need to be stabilized by a surface active component
o Both are meta-stable not in stable equilibrium and both will tend to phase
separate can happen in days, weeks, months stable over a certain period of
time (slowly evolving towards complete separation)
o Same type of physical phenomena occur to make the emulsions and foams unstable:
evolve by the same mechanisms (aggregation, coalescence, Ostwald ripening,
creaming)
- Differences emulsions and foams:
o Density difference between the two phases is much larger in foams
▪ Related to the difference in dispersed phase
▪ Emulsions: liquid, foams: air air much lighter than liquid large difference
in density between continuous and dispersed phase in foams
o Droplet size is much smaller in emulsions than air bubbles in foams (µm in emulsions
vs mm in foam)
- As a results of these two differences destabilization (creaming) is much faster in foam
limits shelf-life for foams to large extent
o Shelf life foams: minutes, days, or weeks
o Shelf life emulsions: months or years
Emulsions
• Emulsions
- Two main types of emulsions:
1. Oil in water (O/W): oil droplets in a continuous water phase
o Milk
o Mayonnaise
o Sauces
o Salad dressing
o Ice-cream (also a foam more complex)
o Large variety in foods
2. Water in oil (W/O): water droplets in an oil phase
o Butter (contains small amounts of water, >85% fat)
o Products are more fatty
o Margarine
o Spreads
o Less common in foods
- Hydrophobic oil and hydrophilic water are not miscible one phase can
exist in the other one
o Requires the creation of an oil-water interface and these interfaces
are present in both types of emulsions
o Both emulsions are not in a thermodynamically stable state (meta-stable) phase
separation (oil lower density upper phase)
▪ Process of phase separation will occur for both oil-in-water emulsions as the
water-in-oil emulsion but not at the same speed
• Preparation emulsions
- Oil and water have no affinity for each other contact is not a favourable situation
- To create a surface between oil and water costs energy:
o Energy can be given as Gibss free energy
o The creation of interface in a system costs energy:
56
, o Creating interface (not favourable not spontaneously need do add ingredient):
▪ G (free energy) is positive: Process will not occur
spontaneously and not preferred
▪ System always try to keep the interfacial area to a minimum achieved by
merging droplets total volume will stay the same, but total surface area will
decrease
o Reducing interface:
▪ G (free energy) is negative: Process may occur
spontaneously
- When system contains an interface will always try to reduce the amount of interfacial
area to lower the energy
- Why are emulsions meta-stable?
o Interfacial tension determines how much energy is needed to create an interface
o The energy required to create a surface:
▪ Decrease in surface tension
➢ Lower surface tension = related to how easy it is to
make droplets and create surface/interface
between 2 phases (low = easier)
➢ decrease in energy
➢ Easier to create surface
▪ Surface tension: the change of free energy resulting from a change in surface
area
▪ Interfacial tension between water and oil and surface tension
between water and air is quite large difficult to make small
droplets/air bubbles (requires a lot of interfacial area)
▪ The interfacial/surface tension can be reduced by adsorbing different types of
molecules or particles at the interface for the same energy input more
surface area can be created (as the surface tension is lower)
▪ The energy required to create a surface can be reduced by adsorbing
molecules or particles at the interface that reduce the surface tension
- Surface tension = results of the interactions of water molecules
o Interactions between water molecules quite strong due to hydrogen bonds
o In bulk phase: water molecules surrounded by other water molecules in oil directions
due to strong interactions the surrounding water molecules will pull on the middle
water molecule net force = 0
o When molecules are present at surface (air)/interface (oil) only water molecules in
bulk water will be able to pull on the water molecule greater attraction of water
molecules in the water than to the molecules in air/oil inward force at the surface
surface will acts as membrane
o As the interaction between water molecules is higher surface tension between air
and water is relatively high (72.8 mN/m)
57
, - Role of surfactants
o Hydrophilic side (head group) and hydrophobic side (tail)
able to absorb to interface between oil and water
o Have the capability to reduce the energy needed to form an
interface
o In water:
▪ Unfavourable mixing between water and hydrophobic tails
micelles (hydrophobic interactions)
▪ Hydrophobic tails are clustered on the inside and hydrophilic
heads stick out
▪ Micel formation Occurs naturally
o In emulsion:
▪ Tails are present in oil prevents contact with water
▪ Favourable mixing of hydrophobic tails with hydrophobic oil
o Such surfactants will therefore be preferably present on the interface
between oil and water
▪ Surfactants adsorb to the interfaces between oil and water as much as
possible
o Comparing emulsions with and without surfactants:
▪ No surfactant: only energy costs for creating unfavourable contact between oil
and water Gibbs energy positive (not preferred) unstable situation
▪ Surfactant: energy costs for creating unfavourable contact between
hydrophobic oil and hydrophilic water positive value for Gibbs energy +
energy gain by avoiding contact of hydrophobic tails and water favourable
situation
▪ Presence of surfactants lowers the surface or interfacial tension more
favourable less energy needed to create the same interfacial area
o Energy cost for creating unfavourable contact between hydrophobic oil and
hydrophilic water ΔG > 0 not stable
▪ Energy gain by avoiding contact of hydrophobic tails and water ΔG < 0
o Surfactants will decrease the surface tension at low concentration concentration
too high molecules already covered surfactants will stay in bulk phase (and
surface tension will not further decrease)
o How much interfacial tension will decrease depends on:
▪ The size of the molecules
▪ The hydrophobicity
▪ The distance between the molecules (as a result of the mutual interactions)
o Interfacial tension does not say anything about the stability of emulsions stability
related to other properties of the interface and the interactions between oil droplets
- Emulsification process
o To make emulsion stable have to create a lot of interactions requires energy
input commonly added by mixing process: stirring provides energy to create the
interfaces
▪ How much energy depends on stirring speed (harder = more)
▪ This amount of energy determines how much interface can be created
depending on the interfacial/surface tension
58
