Definition of emulsifier or emulsifying agent

Also called an emulsifier and emulsifier, an emulsifier is a chemical substance or compound used to help prepare and stabilize emulsions. An emulsion is a milky-looking liquid that contains small particles or drops of another insoluble substance in suspension. This means that they are substances that prevent liquids that do not normally mix from separating into phases that are easily distinguishable with the naked eye.

Emulsions are very common in different areas of daily life, as well as in chemistry, biology and other sciences. Many food products that we consume every day are emulsions. Some common examples are milk, ice cream, mayonnaise, mustard, and vinaigrettes.

On the other hand, many paints are also emulsions between water, oils and inorganic pigments. All of these emulsions require the addition of an emulsifying agent to maintain stability. If not, they will eventually separate into two different phases.

The term emulsifier comes from the Latin emulsus which, depending on the context, can be understood as milked or mixed. This refers to the fact that milk is an emulsion of water and fat with dissolved proteins and sugars and also to the fact that emulsions are stable mixtures of immiscible liquids.

Types of emulsifying agents

Emulsifiers can be classified according to three main criteria:

• According to its electrical charge in aqueous media.
• According to its solubility or hydrophilic/lipophilic balance
• According to the functional groups they contain.

Types of emulsifiers according to their electrical charge in aqueous media

When dissolved in water, emulsifiers can ionize giving rise to four different types of chemical species:

• Cationic emulsifiers: these are those that acquire a positive charge when dissolved in water. These emulsifiers are effective at acidic pH but are not good in solutions with high salt concentrations, since the negative ions of the salt tend to counteract the positive charge of the emulsifier, limiting its effectiveness.
• Anionic emulsifiers: they are very common. They form negatively charged ions and are particularly useful in solutions with alkaline pH. They are also not effective in media with a high salt content for the same reasons as cationics.
• Non-ionic or neutral emulsifiers: as their name indicates, they do not form ions when dissolved in water. Their effectiveness is not affected by pH, the presence of salts or the presence of other emulsifying agents, which is why they are some of the most widely used in the industry, including popular emulsifiers such as mono- and diglycerides.
• Amphoteric emulsifiers: these can have both a positive and a negative charge in aqueous solution, which makes them useful in a wide range of pH values, with the exception of the pH corresponding to its isoelectric point. Soy lecithin is one of the most popular.

Types of emulsifiers according to their solubility or hydrophilic/lipophilic balance

Emulsifiers can be classified according to a scale that indicates how water-soluble or fat-soluble they are. This scale, called hydrophilic/lipophilic balance (HLB), goes from zero (0) to around twenty (20) and allows, in turn, to classify emulsifying agents into two large groups:

• Hydrophilic emulsifiers: They have an HLB between 10-18, so they are much more soluble in water than in oil. For this reason, the greatest interaction occurs with the aqueous phase, reducing its surface tension to almost 0, which facilitates the formation of oil-in-water emulsions in which water is the continuous phase and oil is dispersed in it. in the form of small drops.
• Lipophilic emulsifiers: they are just the opposite of hydrophilic ones. They usually have an HLB between 3 and 6 and are therefore more soluble in oil than in water. They are used to prepare water-dispersed-oil emulsions.

Types of emulsifiers according to the functional groups they contain

There is a very wide variety of emulsifiers with different compositions and structures and for different applications. In the food industry alone, more than 14 families of emulsifiers can be distinguished according to the functional group they contain, among which polysorbates, monoglycerides, diglycerides, etc. can be mentioned.

How do emulsifiers work?

Emulsifiers are amphiphilic molecules, which means that one part of the molecule (the polar or ionic end of the molecule) is hydrophilic (soluble in water), while the other part is hydrophobic or, what is the same, lipophilic ( fat soluble). This allows emulsifiers to interact with water and fats and oils at the same time, so it is always located at the interface between the two.

The net effect is that the emulsifying agents are able to lower the surface tension at the interface of the two immiscible liquids, allowing the formation of smaller droplets of the dispersed phase within the continuous phase. Simply put, the emulsifier facilitates the initial formation of the emulsion.

In addition to this, the emulsifier also helps to keep the emulsion stable. It does this by preventing the small droplets from coalescing through coalescence to form larger droplets that ultimately coalesce to permanently separate the two phases and thus break the emulsion.

The way emulsifiers achieve this depends on the type of emulsifier. In the case of ionics (cationic or anionic), this is achieved by electrostatic repulsion between dispersed phase droplets that are covered by a layer of equally charged molecules. In other cases, coalescence is prevented by steric hindrance of the emulsifier molecules that does not allow the liquid of one drop to come into contact with that of another in order to fuse. Finally, there are cases in which a layer of water molecules firmly attached to the emulsifier layer is formed, which also makes it difficult for two nearby droplets to merge.

Examples of emulsifiers

References

Miller, R. (2016). Emulsifiers: Types and Uses. Encyclopedia of Food and Health, 498–502. https://www.researchgate.net/publication/301702384_Emulsifiers_Types_and_Uses

Everything in Polymers (July 24, 2020). Emulsifiers. Recovered from https://todoenpolimeros.com/2020/07/24/emulsificantes/