"a substance which tends to reduce the surface tension of a liquid in which it is dissolved."
Basically they lower the interfacial tension between two liquids, a liquid and a gas, or between a liquid and a solid, hence the name surfactant or surface actives.
So let's briefly relate it to function; They are used within skin care such as cleansers, when added to a liquid, surfactants affect it's surface tension or surface action, increasing the spreading and wetting properties of the liquid, their functions in include acting as wetting agents, detergents, emulsifiers, foaming agents, and dispersants. If you try to wash a dirty oily substance from your skin with just water for example it doesn't work, because if you take 2 compounds that don't mix, such as oil and water due to their differing molecular properties (water being a polar molecule and oil being non polar) The addition of a surfactant ingredient breaks this molecular barrier, tension is reduced and the two materials can then be successfully combined. In order to do this, the surfactant molecule must be partly hydrophilic (water-soluble) and partly hydrophobic and lipophilic (soluble in lipids, or oils). They can then form an interaction between the polar and non polar substances. The surfactant molecules form a relatively spherical structure or micelle, the hydrophobic or lipophilic tails face inward, and the hydrophilic heads face outward. Oils and fats are contained within the micelle enabling an emulsion action of both substances rather than 2 seperated substances. This micellar action is also relevant to the detergent abilities to lift and trap oil or dirt. We can see therefore how in skincare they perform not only the function of detersion but also in stabilising and enabling the emulsion of ingredients within a formula that would not ordinarily be able to mix and in increasing performance and spreadability through their ability to combine polar and non polar molecules.
But what are they?
A surfactant is generally an organic compound of amphiphilic molecules, meaning molecules with a head and tail that are composed both of hydrophobic tail or lipophilic parts (unable to dissolve in water and oil soluble compounds) and hydrophilic head (able to dissolve in water) parts. They are identified and classified according to their polarity, or polar charge generally by the head polarity as the hydrophobic tails are mostly the same.
When the head group has no charge, the surfactant is called non-ionic.
When the head group has a negative charge it is called anionic.
When the head has a positive charge, it is called cationic.
When the head group contains both positive and negative groups, that are dependant on pH of water then it is amphoteric.
What is polarity?
Polarity is determined at atomic level, in basic terms everything is composed of atoms, which are either attracted or repelled by each other according to electronegativity; the tendency of an atom to attract electrons towards it.
This then determines whether a molecule is polar or non polar, it's polarity is due to the difference or levels in electronegativity between atoms.
When 2 same atoms which have the same electronegativity, they can form a bond (non polar covalent bonds) and pull the electrons in the same way, sharing them equally.
When 2 atoms are different, with different electronegativities this causes 1 of the atoms more than the other atom to pull the bonded electron and subsequently an uneven distribution of electrons between the 2 atoms making the bond (polar covalent bonds)
The uneven share of electrons within an atomic bond will then determine whether the atoms have a negative or positive charge, they have obtained a partial negative or partial positive charge, higher electronegativity means partial negative and lower means partial positive.
In a molecule, the polarity is determined by the contained atomic bonds, some bonds are non polar while others non polar, in a polar molecule all the atomic bonds collectively produce an uneven charge distribution within it.
Polar molecules = asymmetric bonding
Non polar molecules = symmetric bonding
Nonpolar molecules electrons are shared equally between atoms or when polar bonds in a larger molecule cancel each other out.
If we go back to the example of oil and water;
Water has polar molecules, one end of the molecule has a positive charge and the other end has a negative charge. This allows water molecules to bond together and other polar molecules to dissolve, for example salts.
Oil has non polar molecules, in a nonpolar molecule, there are no positive or negative ends. Any charges are distributed evenly across the molecule. Oil will blend with other non polar molecules such as oils and fats.
Polar + non polar = not immiscible (unable to mix)
The water molecules attract to each other polar to polar and the oil molecules stick together non polar to non polar.
The oil and water forms seperate layers. The assymetrical polar water molecules are packed closer together and sink to the bottom with symmetric non polar sitting looser on top.
Polar + non polar + surfactant agent = miscible (can mix)
Because the surfactant molecules have a polar end and a non polar end...
So back to surfactants...
The Hydrophilic–Lipophilic balance (or HLB) is the balance of the size and strength of the hydrophilic and lipophilic moieties of a surfactant molecule an therefore whether it is more Hydrophilic and less Lipophilic. Each surfactant is rated on a scale according to its HLB value.
Higher HLB are more hydrophilic and less lipophilic = more foaming and emulsifiying ability
Lower HLB are less hydrophilic and more lipophilic = less foaming ability
When used in emulsions the selection of a higher HLB surfactant (more hydrophilic) is more suited to use where water is in higher quantity than oils or lipids.
The selection of a lower HLB surfactant (less hydrophilic) is more suited to use where oil is in a higher quantity.
By combining surfactants in skincare depending on their polarity we can either increase or decrease the action of one or more other surfactant ingredients due to their hydrophilic / lipophilic value for example the addition of one to increase or decrease foaming or to increase and decrease the detergent abilities by lowering surface tension or the selection of certain surfactant ingredients can be used to decrease harshness of another.
Surfactants use within skincare is beyond simply detersion, but when a cleanser, such as Dermogenera® Gentle Cleanse for example, states milder surfactant system, this does not necessarily indicate less cleansing function.
But rather can refer to the choice of ingredients being highly tolerated for skincare and that the balance of those ingredients within the formulation is created to ensure that there is effective surface tension with detergent and foaming action and the use of each of the surfactant ingredients to compliment each other.
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