Any Chemist able to explain Washing Up Liquid to the layman?

Mad Mike · 24-06-2013 09:19AM #1

Can any chemist enlighten me about the ingredients of washing up liquid please?

In the supermarket the other day I was shopping for washing up liquid. Confronted with a range of brands and prices I examined the labels to try and figure out which was better value.

I don't remember the exact pricing but there seemed to be three tiers of products:

1. Fairy Liquid was on its own, the most expensive brand and also had the highest concentration of active ingredients:
15-30% Anionic Surfactants
5-15% Non-ionic Surfactants

2. The second tier had a variety of products from different manufacturers ranging from as little as half the price of Fairy to almost as expensive. They all had similar active ingredients:
5-15% Anionic Surfactants
Either no mention of Non-Ionic Surfactants or less than 5%

3. The third tier had some very cheap generic brands perhaps a quarter the price of Fairy. Their ingredients were:
Less than 5% Anionic Surfactants
Again no mention of Non Ionics.

I understand at a basic level that washing up liquid is a mixture of soap and water and it seems clear that the more expensive brands have higher concentrations of soap. What is the difference between anionic and non-ionic though and how come only Fairy seems to have non ionic? I did study chemistry for my leaving cert many years ago so I remember the difference between ionic and covalent bonds but even with the aid of Google I couldn't figure this out.

In use I can clearly see the difference between the very cheap brands and the mid range. The cheap stuff is very watery. I find it harder to see a difference between the mid range and Fairy though even though Fairy is supposedly twice as concentrated. Of course I generally just slobber in a good squirt of liquid so this is not a very scientific test.

Capt'n Midnight · 24-06-2013 03:16PM

washing up liquid usually doesn't have any soap. Many of them add a foaming agent to make bubbles because soapy water makes bubbles. We associate bubbles with clean because soap does both. For washing machine detergents an anti-foam is used because bubbles would be a bad thing.

Surfactants are molecules one end of which is hydrophobic and so likes oil/fat/grease, the other end is hydrophillic and so likes water. This means the oily dirt can be changed to an emulsion suspended in water.

The water loving hydrophilic part can have a positive or negative charge or be neutral.

https://en.wikipedia.org/wiki/Surfactant#Anionic - this lists some of the compounds that may be used, different recipes for different brands / jobs

Unfortunatley part of the reason for the ingrident list to be a bit vague is to prevent you making a direct comparison

Mad Mike · 24-06-2013 03:25PM

Thanks for the reply Cap'n Midnight.

OK so the surfactants have a head that bonds to water and a tail that bonds to grease. The "ionic bits" are the head that bonds to water. Anionic and non-ionic seem to be two different kinds of head bit.

Anyone know which is better and why Fairy is the only wup liquid that seems to have the non-ionic kind.

Edit: reading further it seems that anionic surfacants are all some kind of "ate" (sulphate, phosphate etc) while non ionic are likely to be some kind of alcohol.

dasanimal · 03-07-2013 12:31PM

the wikipedia link that capt gave you explains it pretty well.
Anionic surfactants had a negatively charged head, as opposed to a neutral charged head for nonionic surfactants.
Anionic surfactants may be better for attaching to dirt/compounds that is positivey charged whereas the nonionic surfactants can attach to neutral dirt/compounds.

So i would imagine the reason Fairy is better is because it contains more anionic and nonionic surfactants and therefore is able to bind to a greater variety of dirt/compounds??

I do know surfactants are often used to lower the hydrophobicity of materials to enable reactions to occur

Any Chemist able to explain Washing Up Liquid to the layman?

Comments