Laundry Science 101: Surfactants

December 14th, 2011 12:18 AM | 1 Comment

how soap works3Surfactant: (pronounced Surf-Fact-Ent: Surf as in Surf’s up baby! Fact as in “Just the facts, Ma’am,” and Ent as in those giant trees walking around in Lord of the Rings)

It means “surface active agent” and is the scientific word for soap at its most basic.

Surfactant = Soap

Surfactants change the surface tension of the water. What this means in terms of laundry is that water can more easily penetrate the fibers which loosens the dirt and then holds it in suspension in the water. I’ll explain that in more detail below.

There’s a bunch of different types of surfactants, but we’ll focus on two for now which are the most common when doing laundry.
Surfactants are shaped like tadpoles. The head loves water, and the tail not only hates water, but it’s attracted to dirt. This is also called Hydrophilic (Greek word meaning Water-Loving) and Hydrophobic (Greek word meaning runs away from water).

 

 

Most types of soaps and detergents are anionic surfactants which means they have a positive and negative charge on either end of the molecule. The head is negatively charged and the tail is positively charged.

To understand charges a bit better, think back to when you played with magnets as a kid. One side of the magnet would attract another magnet, and if you flipped one of those magnets around they would repel each other. That’s because one side of the magnet has a positive charge and one has a negative. Opposites attract.

http://www.howmagnetswork.com/

Back to the surfactant “tadpoles” – the molecules will surround a dirt particle, break it up, and force it away from the fabric surface, holding it in suspension in the water until it’s rinsed away.

The reason why anionic soaps don’t work well in hard water is because the minerals (calcium and magnesium ions) in hard water are positively charged, attracting the heads of the surfactants. The hard water ions deactivate the surfactant by binding to it instead of the dirt. Each hard water mineral molecule has 2 ions – so each hard water molecule can take 2 soap molecules (which has 1 ion). When the ions attract, the soap becomes insoluable and can’t bind to the dirt. This is why you may need up to four times the recommended detergent in hard water. This reaction also causes soap scum.

That’s why it is recommended that if you have hard water, you use a softener – not just for your laundry but also to help keep buildup of soap scum from happening in your machine and to clean out your pipes. Otherwise the machine will break down eventually and the area in your pipes for water to run will get smaller and smaller.

Eco Nuts Soap Nuts and Soap Nuts Liquid is a non-ionic surfactant. The difference is that there’s still a tadpole with a head that loves water and a tail that doesn’t, but there’s no charge. This means it is not affected by hard water ions. There are other factors in hard water which make it harder for the nuts to work in some (not all) cases and why we recommend the liquid for hard water or a water softener.

Surfactants work 3 ways:

from Scienceinthebox.com

Roll-Up mechanism
Picture a pincusion being surrounded on all sides with tin pins. The tiny pins are the surfactant and are essentially “rolling up” a ball of dirt until it’s completely surrounded.

Emulsification
The surfactant breaks up oil particles so that they are dispersed in the wash water and can be rinsed away. Picture two salad dressings – oil and water (not emulsified), vs vinaigrette (emulsified).

Solublization
The surfactant and the water causes the dirt to dissolve into a liquid

The way the surfactant will work on the dirt depends on not only the type of soil it is, but the type of fabric.

Okay, the dirt is now off your clothes swishing in the washing machine.

 

A great video to watch that demonstrates how surfactants work is this Candy Corn In Space video. It shows some experiments carried out by astronauts in zero gravity with candy corn and water and they found that they behaved similarly to how surfactants behave in the presence of soil.

What keeps the dirt from winding up elsewhere on your clothes?

Remember an Anionic surfactant has a charge to it? Once the little tails are all stuck to the dirt, that leaves the heads poking out which have a negative charge to them. The fabric gains a negative charge in the presence of the surfactants so both sides repel each other.

This negative charge builds up on the fabrics as they rub against each other and continues in the dryer – which causes static cling!
When your water is hard, this repulsion doesn’t happen. Yet another reason to use a water softener and why your clothes come out dingy-looking.

With a non-ionic surfactant (like Eco Nuts) it works in a similar way to keep dirt from being re-deposited on fabric – the a term in chemistry called a steric hinderence. In plain English it means that once the surfactant molecules attach the dirt, the particle is too big and bulky to cling to anything because its size was increased by too

 

Is this ball gonna stick to anything? NOPE!

many things sticking out of it. The non-ionic surfactants have a very long water-loving head which sticks out and is attracted to the water, not the fabric. This keeps the dirt from combining with other dirt molecules or going back to the fabric at all. Think of a nubby ball – each little nub is a surfactant tadpole head which wants to be next to water, not fabric.

Remember the stick-um game? You toss a ball and catch it in a velcro mitt. Another way to picture steric hinderence would be if you tried to play stick-um with two different balls – a tennis ball and a nubby ball. The tennis ball sticks to the velcro, but the ball with nubbies won’t. The nubbies aren’t a velcro surface that the velcro can attach to like the tennis ball, so the nubby ball will simply bounce off.

So then if the soap doesn’t stick once it removed the dirt, what causes residue/buildup?

Residue – not such a huge deal for most people unless you cloth diaper or have skin sensitivities, however for cloth diapering and skin-sensitive folks, residue is a nightmare!

Residue/buildup is caused by the following things – ingredients that are not soap in the detergent (including optical brighteners and stain guards or stain lifters), incomplete rinsing caused by an overload of the machine, misuse of liquid fabric softener (or fabric softener in general – that includes dryer sheets used after washing), powdered detergent that did not dissolve properly, and hard water.

To remove detergent residue, wash in the hottest water setting recommended by the clothing manufacturer.

Surfactants also create suds! Artificial surfactants create more bubbles/suds! Suds have nothing to do with cleaning power, and are just a side effect of the soap in the water. What makes a bubble? Read on!

Remember our tadpoles again and how much the head loves water and the tail hates it?

a cross section of a soap bubble from Exploratorium.edu

When a surfactant molecule finds its way to the surface of the water the hydrophobic tails poke out of the water because they are trying to get away from it. Also keep in mind from earlier that surfactants break the surface tension of the water. This is because the distance between water molecules increase because the soap molecules are forcing their way in between, and the surface tension can be bent/broken. In other words, you agitate water a little bit introducing some air and the surfactant molecules line up trapping a thin layer of water molecules in a soap sandwich. This is a bubble! The soap film protects the water from evaporating. As you can see, the bubbles have everything to do with the presence of soap but nothing to do with cleaning. Different types of soap molecules/surfactants create different sized and colored bubbles.

Some surfactants are good at cleaning but don’t make many bubbles (Like Eco Nuts Soap Nuts and Liquid Detergent). Some artificial surfactants are bad at cleaning but make great bubbles. Some surfactants make great bubbles and clean well (usually those are more specialized for grease) Some big brand detergents and cleaners put those artificial foaming surfactants in them just to make bubbles so you think they’re cleaning extra-hard. But now you know better! If you talk to your local water treatment plant they’ll tell you all about have they have to add de-foaming agents to the water when they treat it because there are so many surfactants in our waste water (the majority are the artificial foaming type that we don’t need!). Here is a video demonstrating a de-foamer being added to surfactant-laden water.

 

 

 

 

  • Sandra Cravinda

    Fantastic article on stain removal. Especially for me since I love factual scientific explainations. I’m trying to figure out how to remove fabric softener stains like like here (http://remove-stain.com/remove-fabric-softener-stains), but it’s really tricky because I think it’s a surfactant, or maybe a basic, formula. Detergent just leaves the fabric softener stain looking wet when it has actually been dried.

    Any suggestions?

    Thanks,

    Sandra