Surfactants are a unique type of molecule that reduce the surface tension between two substances – either between two liquids, a gas and a liquid or a liquid and a solid.
All surfactants are amphiphilic — meaning they have a hydrophilic (water attracting) head and a hydrophobic (water repelling) tail. This allows them to be absorbed into air-water (or oil-water) interfaces, with the hydrophobic tail in the air and the hydrophilic head in the liquid. This results in a decrease in surface (or interfacial) tension.
What allows us to classify and categorise surfactants is the ‘charge’ of the head — the tails are usually very similar across all types of surfactant. The two most commonly produced types of surfactant are anionic and nonionic, but there are also cationic and amphoteric surfactants, which are produced in smaller volumes.
Surfactants are chiefly applied in cosmetics and cleaning formulations. They are particularly prized for their cleaning mechanism: surfactants can break down the interface between water and dirt, holding it in suspension and making it easier to remove.
But they also have wetting, emulsifying, foaming and antifoaming properties, giving them industrial uses for biocides, firefighting foams, insecticides, the deinking of recycled papers and even across food production.
When surfactants are present in a mixture in sufficient numbers, they group together (or ‘aggregate’) to form a micelle (or a ‘ball’) – a type of structure. The point at which these structures begin to form is the critical micelle concentration (CMC). A surfactant with low CMC does not need as much to reach this point; a surfactant with high CMC needs more surfactant to work.
In micelles, the water-loving head points towards the water. The water-repellent tails group together at the centre, protected from the water. The micelles work together; the tails dislike water, and are therefore attracted to soils and surround them, whilst the heads are attracted to water. This mechanism dislodges the soils and pulls them into the surrounding mixture. The soils are then suspended in the mixture by the tails.
In simple terms, surfactants stir up activity on the surface, trapping and removing dirt. The micelles are crucial to the function of surfactants; they allow for soils that are insoluble in water to be broken down and removed.
Surfactants also have an antimicrobial effect – the surfactant’s hydrophobic tail lodges itself in the lipid layer of the cell membrane of a bacteria or virus, causing it to be destroyed and washed away.
All types of surfactant have a tail and a head, but the head is what allows us to classify them and is what gives surfactants their properties. The head is electrically charged – this charge can be negative (anionic surfactants), neutral (nonionic), positive (cationic) or dual (amphoteric).
Anionic surfactants contain a head with a negative charge. They represent around 50 percent of all industrial production. These types of surfactants excel at emulsification — they help to break down dirt. The negative charge helps the surfactant to lift and suspend a wide range of soils.
This effectiveness, particularly against particulate soils, makes anionic surfactants popular for inclusion in household laundry detergents, dishwashing liquids, shampoos, soaps, toothpaste and bath products. They are adept at keeping the dirt, once dislodged, away from fabrics.
One characteristic of anionic surfactants is that they tend to generate a higher level of foam, but are not so good at emulsifying oily soils. One of the most popular types of anionic surfactant is sodium lauryl ether sulphate (SLES).
Nonionic surfactants have a neutral charge – their hydrophilic head does not have a net charge. This type of surfactant represents around 45 percent of all industrial surfactant production.
Nonionic surfactants can be hydrophilic (more attracted to water) or lipophilic (more attracted to oil). This affects their application.
Compared to anionic surfactants, nonionic surfactants have limited sensitivity to water hardness — they don’t react with calcium and magnesium — and do not generate as much foam. Where anionic surfactants excel at removing particulate soils, nonionic surfactants are useful for removing oily soils.
For this reason, both types of surfactant are commonly used together in dual-action cleaners marketed as attacking oily and particulate soils.
Something that distinguishes nonionic surfactants is that they have a ‘cloud point’. This is the temperature at which the surfactant separates from the solution and becomes cloudy – considered to be the ideal temperature for cleaning effectiveness and minimisation of foam. Cloud point depends on the makeup of the nonionic surfactant; a lower cloud point can be seen as preferable for minimising energy use.
They can be found in low-foam formulations and for shampoos, perfumes and beauty products. In make up, they can aid in dispersing pigment across the skin. This type of surfactant is commonly used as a wetting agent in coatings, as a food ingredient and as polish, cleaner and fragrance carrier.
Cationic surfactants have a positive charge in their head. This makes them useful for interacting with negatively-charged substances, and for neutralising static charges. This property gives cationic surfactants popular uses in anti-static formulations, like hair conditioners and fabric softeners.
This type of surfactant is manufactured in a far smaller quantity than its anionic and nonionic counterparts. It is typically more expensive.
For their antimicrobial and antifungal properties — they can disrupt the cell membranes of bacteria and viruses — cationic surfactants are often paired with nonionic surfactants in household and industrial cleaners. They cannot be used with anionic surfactants; the positive and negative charges cause the surfactants to fall out of the mixture.
Also known as zwitterionic surfactants, amphoteric surfactants are unique in that they contain both a positive and a negative charge in their heads. This zero net charge makes them particularly versatile and effective across a wide pH range.
They are generally mild and combine the properties of both anionic and cationic surfactants; they will behave differently depending on surrounding pH – in acidic solutions, they will develop a positive charge, behaving like anionic surfactants. In alkaline solutions, they will develop a negative charge, and behave like anionic surfactants.
Amphoteric surfactants are fairly new to the market and still undergoing development. They are typically more expensive than other types of surfactant, limiting their use to smaller quantities in premium-end cosmetics products – they are often added to hair conditioning and anti-irritation skin formulations. They also have quick dry properties, lending them to use paint products for quicker coagulation.
Anionic and nonionic surfactants account for the vast majority of surfactants manufactured. They are often used together with other elements – hydrotropes, builders and carriers – to create formulated cleaners which have a very effective cleaning mechanism. These products are able to lift, emulsify, disperse, suspend and decompose soils.
Surfactants’ properties also give them widespread use in personal care products, along with elements like chelates.
Airedale Group’s Life Science division specialises in formulation chemistry for cosmetics, personal care, hygiene and detergency product solutions.
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