Surfactants are a unique type of organic compound. They help liquids to spread and mix more easily by reducing surface tension.
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, sometimes, 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 group 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. A little more on this later.
Surfactants are chiefly applied in cosmetics and cleaning formulations. Their detergency properties are of particular value: by breaking down the interface between water and dirt, they hold it in suspension, 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.
All types of surfactant are all used to lower surface tension, but they have differing compositions that provide properties allowing them to serve different purposes. Surfactants are classified according to their ‘head’.
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. This effectiveness, particularly against particulate soils, makes them 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. One of the most popular types of anionic surfactant is sodium lauryl ether sulphate (SLES).
These types of surfactants have a neutral charge, resulting from a head without a net charge. Nonionic surfactants represent around 45 percent of all industrial surfactant production.
Although both types can be used together, 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.
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.
What sets apart cationic surfactants is the positive charge in their head. This makes them useful for interacting with negatively-charged substances, and for neutralising static charges.
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 other surfactants in household and industrial cleaners and fabric softeners.
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 as anionic or cationic based on surrounding pH. They are often added to formulations to help prevent skin irritation and to help condition hair.
They are fairly new to the market and are still undergoing development. They are significantly more expensive than other types of surfactant, limiting their use to smaller quantities in more premium cosmetics products. 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 to build formulations. To give a few examples of how the different types of surfactants may be worked into various products:
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