Naked Science Forum
General Science => General Science => Topic started by: DoctorBeaver on 16/11/2005 01:33:05
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My "toast crumbs" post raised the question of washing-up, so here's a follow-up.
What the hell are anionic surfactants? I've done web searches & all I can find is their chemical names & products they're used in.
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I found this "a surface-active chemical widely used in industry and laundering"...well that explains the ' surfactants' part......
Men are the same as women.... just inside out !! (https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.world-of-smilies.com%2Fhtml%2Fimages%2Fsmilies%2Fparty%2Fballoons.gif&hash=f9f40e7ab655ca9089398c3f3719f593)
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An anion is a negatively charged ion, so I imagine an anionic surfactant is a surfactant with an anion at one end of it – I assume the anion is to attach to the water molecule, with some sort of hydrophobic tail attached.
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A surfactant is a posh and probably slightly more general name for detergents, the way these work is they have a head that is ionic (in this case probably -ve) and a tail that is oily (they look a bit like a wobbly pin), this means that the head loves water but the tail hates it. So the tails collect anywhere that water isn't, the surfaces, grease etc. Because the grease gets covered with the detergent molecules (think pincusion) it is now covered with water loving heads so will dissolve well, hence why you use them to wash up. They also have the effect of breaking up the surface tension so foams will form.
Soap is an anionic surfactant. You can also get cationic surfactants, but these are less common.
If you haven't already found it
http://www.kcpc.usyd.edu.au/discovery/9.5.5-short/9.5.5_anionic.html
is quite useful
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Thanks, Dave. I can sleep easy ion my bed now! [:D]
Why the hell couldn't they have just put "contains detergents" on the bottle?
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[:0] Look what I found at that link...
Long-term exposure to anionic surfactants has been linked to swelling of the skin in a conditioned allergic reaction. This swelling is temporary, although it tends to increase the susceptibility of the skin to permeation by other substances. Anionic surfactants are generally avoided in cosmetic products, but their use in shampoos and other products can still lead to irritation (that's why some people suggest changing shampoos every month or so).
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I remember seeing an experiment about this subject,
You can float a needle on the surface of water if you do it carefully - by then adding washing up the needle sinks due to the loss in surface tension.
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I don't have a floating needle, will a floating paperclip do ?
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fi24.photobucket.com%2Falbums%2Fc23%2FSUEDONIM%2Fth_SURFACETENSIONDAVE80K.jpg&hash=4b4854b0c6043880f15f5f178436e08f)
As garlicman said this trick is not possible if the water has any soap/detergent in it as it lowers the surface tension.
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There's a really cute demo you can do with this..
Cut out a piece of paper this sort of shape
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/ /
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Edit... my artwork seems to be broken. It's supposed to be a sort of arrowhead shape, but the spaces have got lost.
Float on some clean water, then spot a little bit of washing up liquid, or soap, or anything else that'll break the surface tension, into the nick in the back and it'll shoot off across the water because there's more surface tension on one side than the other.
I think daveshorts probably knows better than I do which random other things you can use to break the surface tension... I've an idea one was mustard powder, but I think there were more.
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quote:
Originally posted by DoctorBeaver
[:0] Look what I found at that link...
Long-term exposure to anionic surfactants has been linked to swelling of the skin in a conditioned allergic reaction. This swelling is temporary, although it tends to increase the susceptibility of the skin to permeation by other substances. Anionic surfactants are generally avoided in cosmetic products, but their use in shampoos and other products can still lead to irritation (that's why some people suggest changing shampoos every month or so).
Some synthetic surfactants are oestrogenic, particularly when partially degraded by bacteria. Exposure to minute quantities of oestrogenic chemicals can reduce fertility in animals, including humans:-
www.eng.ox.ac.uk/chemeng/people/darton/oestrogens.pdf
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fi24.photobucket.com%2Falbums%2Fc23%2FSUEDONIM%2Fth_DETERGENTFILM3.jpg&hash=0887bac55610394518406a88224890bf)
[magnified image of detergent film, colours due to interference]