An emulsion is a dispersion of two or more immiscible materials, where one phase, also known as the internal phase, is dispersed in the continuous or external phase

Cosmetic emulsions are classified as oil in water (O/W), water in oil (W/O) and water in silicone (W/Si). Multiple emulsions such as oil water in oil (W/O/W) are also possible. Oil in water emulsions are the most common due to preferable cost and light skin feel.

Emulsification consists of dispersing one fluid into another, no miscible ones, via creation of an interface.

Emulsification methods can be classified in 2 main categories:

a.- Mechanical methods: Energy to break droplet is mechanical

• Mechanical Mixing using different type of impeller
• High Pressure Homogenization
• Membrane Permeation Emulsification

b.- Physico-Chemical methods: energy is coming from modification of chemical and physical parameters.

• Phase Inversion Temperature
• Phase Inversion Composition
  
  

Emulsion is an out of equilibrium system: metastable

The path to make the emulsion has as much impact on emulsion characteristics and properties as the emulsion composition.

Emollients are ingredients found in the oil phase and they provide skin protection and prevent skin dryness.

Key parameters for emollient selection:

• Sensorial:

Spreadability (rub-out), gloss (after-feel), oiliness (after-feel), firmness (pick-up), absorbency(rub-out),…

• Clinical:

Moisturising, substantivity, lubricity, softening,…

• Physical:

Compatibility, viscosity, spreading coefficient, polarity,…

General characteristics of a good emollient:

• Good chemical stability
• Good compability in the formulation

• Ease of incorporation
• Good spreadability (High spreading coefficient, Low interfacial tension, Low surface tension)
• Low toxicity (Non comedogenic)
• Biodegradable
• Global approval as PC ingredient

Esters are biocompatible ingredients that increase the static skin water level when applied and help to restore the natural hydrophobic barrier.

Functions as a cosmetic ingredient:

Biological or Health related functions:

• Lubricants in the stratum
• Restoration of the skin barrier function
• Preventing the penetration of exogenic undesired factors
• Regulation of the skin hydration
• Moisturization
• Skin adhesion= substantivity
• Elasticity
• Refattening
• Occlusivity= film forming=protection
• Skin penetration enhancer

Functional properties:

• Solvent/carrier for actives:
• Chemical sunscreens agents (SPF Boosting)
• Antiperspirant
• Water-repellency
• Dispersing medium:
• Pigments
• Physical sunscreen
• Gloss

Features:

• Spreadability

Spreading on the skin is influenced by:
Viscosity of the emollient
Surface and interfacial tension of the emollient
Water content of the Stratum Corneum
Amount of sebum on the skin

Cascading theory:
Cascading theory means select a number of emollients so that the sensorial feel is spread throughout the time of application.

Perfect combination of emollients



Combination of emollients depending on their spreadability: High, medium and low.

Polarity

Emollients according to their polarity

Applications:

Any type of emulsions (facial creams, body milks, sunscreens, make-up’s, make-up removers, baby products, anti-perspirants, hair mascara & conditioners,…)

Used level is dependent on formulation type: 1 to 30%.

Emulsifiers are compounds able to stabilize the dispersed droplets in the continuous phase. Emulsifiers are molecules consisting of waterloving (hydrophilic) part and water-hating but oilloving (lipophilic) part. With their lipophilic part emulsifiers wrap around and incorporate oil drops thereby preventing them from reunite again to form a separate oily phase. In this way, the oil particles are shielded from each other resulting in a stable emulsion.

Characteristics of a suitable emulsifier:

• High interface activity
• High spreadability at the liquid/liquid interface
• Sufficient diffusion to the interface
• Sufficient solubility in the external phase
• High elasticity of the formed emulsifier film

The functions of surface-active agents to provide stability to dispersed droplets are as following:

1. Reduction of the interfacial tension.
2. Form coherent monolayer to prevent the coalescence of two droplet when they approach each other.
3. Provide surface charge which cause repulsion between adjust particles.

Combination of surface-active agent is used most frequently. The combination should form film that closely packed and condensed.

Emulsifier’s classification:
O/W emulsifiers: High HLB value
W/O emulsifiers: Low HLB value

Emulsifier’s type:

• Anionic Emulsifiers (O/W):
  
  

Anionic O/W emulsifiers can be used to obtain very stable emulsions because they can build an electrical double layer around the droplets, which prevents the droplets from coalescing. They carry a net negative charge in solution and because of that they are sensitive to low pH and electrolytes.

The most important anionic O/W emulsifier is soap, which forms the stearate creams. Neutralized with potassium hydroxide, the stearate is alkaline. The use of triethanolamine as a neutralizing agent is more common. A disadvantage of stearic acid is the typical “soap-up” effect, a type of foaming that occurs when the cream is rubbed into the skin. To prevent this, silicon oil is frequently added to the formulation.

In principle, all anionic surfactants can be used. However, due to the fact that the soaps are relatively alkaline and fatty alcohol sulfates are irritant to the skin.

• Cationic Emulsifiers (O/W):
  
  

Cationic emulsifiers have generally been avoided in skin-care products because they are often more irritant compared with anionic emulsifiers. They are important in hair-care formulations, where they act as conditioners and anti-static agents. These are commonly called quaternium ammonium compounds.
Cationic emulsifiers function very well in formulations that are desired to stay on the skin for a long time such as sunscreens, long-wear make-up and barrier creams. This is because they are positive, cationic charge adheres these products strongly to the surface of the skin, resisting wash-off and wear.

A practical example of where cationic emulsifiers has proved very helpful, is in preventing sand from sticking to a freshly-sun-screened body. Thanks to its anti-static capacity.

It is important to note that, in using a cationic emulsifier system the formulator does not rule out the use of pretty much all grades of carbomer, and even anionic thickening agents such as xanthan and unmodified guar.

• Non-ionic Emulsifiers (O/W):
  
  

Non-ionics remain the first choice go-to emulsifiers for most applications due to their flexibility and low potential for chemical interaction. Non- ionic emulsifiers are those that are free from any external electrical charge caused by free ions. Examples of these are:

• Polysorbate 80
• Polysorbate 20

These are familiarly known as Tweens, a common trade name.
A wealth of non-ionic O/W emulsifiers is available. These are mainly PEG derivatives such as ethoxylates or PEG esters. Typical O/W emulsifiers are:

• ethoxylated fatty alcohol
• PEG esters of fatty acids
• Ethoxylated sorbitan esters
• Ethoxylated monoglycerides
• Ethoxylated castor-oil derivates

The degree of ethoxylation determines the primary properties of that part of the molecule that results in the HLB classification. Good W/O emulsifiers can be found in the HLB range between 8 and 18. Contrary to anionic emulsifiers, the non-ionics are unaffected b changes in pH and, in the case of pure ethers, they can also be used at extreme pH values.

The key benefit of a non-ionic emulsifier is its robust salt tolerance.

An emulsion is a combination of two immiscible phases held together by what to many looks and feels like magic.

The surface tension between oil and water is so high that they do not mix unless you add an emulsifier.
Glycerin, Propylene Glycol and Ethanol are common additives in a cosmetics product and can all impact on surface tension. All these ingredients decrease the polarity of the water phase, and the influence of that decrease changes depending on the quantity of additive present, although the relationship between dose and effect is not strictly linear.

Impact of Ions in Water phase:

We often talk about salt and saltiness in formulating, but what we really talk about much of the time is the ionic strength of continuous phase.
Ionic charge in the water phase can help increase intramolecular bonding and can also help in the formation of an electric double layer around the dispersed phase.

Adding too many ions into the continuous phase will give the product a sticky/tacky /salty feel when applied.

Oil in Water emulsion (O/W)

Oil in water emulsifiers create oil-in-water emulsions. In oil-in-water (O/W) emulsion systems, oil droplets are dispersed in water.

• Oils is the internal/dispersed phase whereas water is the external/continuous phase.
• O/W emulsifiers are more soluble in water than in oil.
• O/W emulsifiers have an HLB greater than 15.

Water in Oil emulsion (W/O)

Water-in-oil emulsifiers help in producing water-in-oil (W/O) emulsions. In W/O emulsions, water droplets are dispersed in oil.

• Water is the internal/dispersed phase whereas oil is the external/continuous phase.
• W/O emulsifiers are more soluble in oil that water.
• They have HLB between 2.5-6 are non-ionic or polymeric.

Oil in Water emulsion (O/W)

Water in Oil emulsion (O/W)

Multiple emulsion –double emulsion –W/O/W or O/W/O

Other type of emulsion

HIPE emulsion

High Internal Phase emulsion.

Pickering emulsion

Emulsion stabilized with particles rather than surfactant.

What is not an emulsion?

Micro-emulsion:

They are made of oil/water/surfactant but are at equilibrium. The structure is independent from path taken to make it and are STABLE.

* They usually contain a large amount of surfactant.