Surfactants,
Part I

When at least a minimum amount of surfactant is  dissolved in a solvent (like water), surfactant molecules want to get together - the water-hating ends hang out on the inside, the water-loving ends hang out on the outside.  This forms a structure known as a micelle, and micelle formation is predicated on reaching the “Critical Micelle Concentration”. Below, an illustration of a nonionic surfactant intended to remove oily soils. The water-loving heads face out, the water-hating ends get together in the middle to escape the water.

When a micelle encounters a soil that the hydrophobic tail is attracted to, the micelle breaks up, the tails grab the soil and drag it into the water (thus removing it from the textile)  and the micelle re-forms, keeping the soil up in the water to be drained or diluted away.   Let’s look at this in the context of removing a common soil from textiles:

Here we have the start of the wash process; surfactant micelles have formed in the wash water and there is soil attached to the fabric substrate.

Now the hydrophobic tails of the surfactant molecules have found themselves more attracted to soil than each other and they're bonding to the soils. The hydrophillic heads are dragging the molecules towards the water.

The micelles re-form as the soil detaches from the substrate - they reorganize into groups of their own kind (more on this in a moment).

When all the soils are removed from the substrate and floating in the water, the textiles are clean and it's time to remove the soil-surfactant combo from the drum.

The Chemistry of Attraction (It’s Not Just A Bottle of Chanel No. 5)

While all surfactants work the same general way, there are differences in what kind of soils the hydrophilic ends are attracted to, because the hydrophilic ends differ.  One primary difference between surfactants is the electrical charge the hydrophilic end carries.    If the business end has a negative charge, it’s an anionic surfactant, and it’s attracted to soils with a cationic (positive) charge.  If the business end has no charge, it’s a nonionic surfactant and is most attracted to soils without an electrical charge.  If the business end has both a positive and negative charge in balance, it’s an amphoteric or zwitterionic surfactant, and the behavior changes based on the pH of the wash as a whole.  

There are also surfactants with positive charges, the cationic surfactants.  These aren’t used for cleaning - they’re what makes fabric softener work, and will be discussed in a (much) later post.

Why Charge Matters: 

The difference in which soils a given surfactant is attracted to is a critical determinant of cleaning performance.   Soils that lack an ionic charge like petroleum oils or intact sebum are much less visible to anionic surfactants and are removed better by nonionic surfactants.   Conversely, soils that are highly cationic like soot and mud and dust, and thus attracted to textiles with a negative charge may be neglected by nonionics and remain electrically connected to the textiles.   For those soils?  Anionics in the mix improve cleaning performance. 

Almost all finished detergent products contain anionic surfactants and most contain nonionic surfactants.   Amphoteric surfactants are relatively uncommon in conventional detergents but often appear in green/biobased formulas.  

Other Differences Between Surfactants:  Tail Length And Single vs Double Tails.

Aside from the electrical charge differences in the head, two aspects of surfactant structure that affect their action against soil are the tail length and whether they are single tail (common) or double-tail (less common).   I’ll talk more about this in Part II, as it’s common to include surfactants of various tails to optimize performance against specific soils and in specific wash conditions.

Coming Up In Surfactants Part II - Curling Up With A Good Jug Of Detergent

In the next installment, we’ll look at common surfactants found in conventional and plant-based detergents, and how they’re manufactured, along with the differences in soil removal capabilities and environmental impacts.