Emulsifier
- Also known as
- emulsifying agent, emulsification
An emulsifier is a substance that helps form an emulsion and keeps it stable by collecting at the boundary between the oil and water droplets and preventing them from merging. Most emulsifiers are surfactants, though some polymers, proteins and fine particles also act as emulsifiers.
What is an emulsifier?
Oil and water do not mix; shaken together, they separate within moments. An emulsifier makes it possible to disperse one in the other as droplets, forming an emulsion, and to keep those droplets from separating. Emulsifiers are used in foods, cosmetics, paints, agrochemical concentrates, lubricants and cleaning products.
How emulsifiers work
An emulsifier does two jobs:
- Helps form droplets. By adsorbing at the oil-water boundary and lowering interfacial tension, it reduces the energy needed to break one liquid into small droplets.
- Keeps droplets apart. It forms a film around each droplet that resists coalescence, by electrical repulsion between charged films, by a physical barrier of bulky molecules, or both.
Types of emulsifiers
- Surfactants, the most common emulsifiers, including anionic, nonionic and amphoteric types, and biosurfactants.
- Polymers and proteins, which form thick, protective films around droplets.
- Fine solid particles, which sit at the droplet surface and physically block coalescence, producing what are called Pickering emulsions.
Choosing an emulsifier
The first guide is emulsion type. Emulsifiers that are more water-soluble favor oil-in-water emulsions; more oil-soluble ones favor water-in-oil emulsions, a tendency known as Bancroft’s rule. The hydrophilic-lipophilic balance system quantifies this for nonionic surfactants, and each oil has a required HLB that an emulsifier or blend should match. Blends of two emulsifiers often give more stable emulsions than one.
Formulators then test for stability across temperature, dilution, salt and time, and for compatibility with the other ingredients.
Emulsifiers and demulsifiers
In oil production the goal is often the opposite: to break unwanted emulsions of oil and water. The chemicals used for that are demulsifiers.
Frequently asked questions
A surfactant is a type of molecule; an emulsifier is a role. Most emulsifiers are surfactants, but not every surfactant is a good emulsifier, and some emulsifiers, such as proteins and particles, are not classic surfactants.
Lecithin in foods, polysorbates and other nonionic surfactants in cosmetics and agrochemicals, and glycolipid biosurfactants such as sophorolipids in some industrial formulations.
Sources
- emulsifier; emulsion, IUPAC Compendium of Chemical Terminology (Gold Book)
- Surfactants and Interfacial Phenomena, 4th edition, M. J. Rosen and J. T. Kunjappu, Wiley, 2012