Why don't laundry detergents rinse clean?
October 8, 2013 4:59 PM   Subscribe

If I spill tide laundry detergent on my hand it doesn't just rinse off with water like it should, you have to actually wash it off with hand soap or dish soap to get it off. Why is this?
posted by john123357 to Science & Nature (14 answers total) 5 users marked this as a favorite
A lot of detergents now contain fabric softeners as well. Perhaps that is what isn't simply rinsing off? I stick with Ivory powder detergent, I have never really used Tide or Gain or any of those.
posted by kellyblah at 5:11 PM on October 8, 2013

No, the tide I buy has no fabric softener in it. Also the residue on your hands is drying to the skin so it is detergent residue.
posted by john123357 at 5:17 PM on October 8, 2013

Laundry detergents contain chemicals called surfactants. What are surfactants? Short answer: they lower the surface tension of water, making it better able to interact with oil and grime for cleaning. (Longer answer here, via HowStuffWorks.) Surfactants make water behave differently than we're used to seeing, causing it, among other things to stick to your skin and resist being washed off.

I work with detergents in my job and can tell you that there is a trick, if you ever get them on you. Take a dry paper towel and pull off as much of the detergent as you can. Then wash your hands. This way, you get a much better ratio of water to surfactant, and it's easier to wash the stuff off.
posted by DirtyOldTown at 6:19 PM on October 8, 2013 [4 favorites]

There are Surfactants in any kind of detergent, including hand soaps and dish detergents all of which rinse clean. Laundry detergent does not rinse clean.
posted by john123357 at 6:32 PM on October 8, 2013

That is true.

I was trying to keep my answer simple and not go too out on a limb, since I'm not a legit chemist, just a guy who works for a chemical company.

It is the surfactants, though, as I understand it. What I probably should have said was that the kinds of surfactants in laundry detergent provoke a different kind of interaction between oil (even in the oils in your skin) and water.

There are longer explanations available online, including differentiating the various types of surfactants (I believe there are three or four main ones) and the various chains they end up making. I can sort of nod my head along when I hear those conversations, but the simplified version (as has been related patiently to me) is that certain surfactants are sort of made to hold together under more vigorous (i.e. mechanical) washing and don't break up quite so quickly under plain old hand washing.

The pulling-the-stuff-off-with-a-dry-paper-towel trick is still the best way, though. That much I can say from personal experience.

I'd love it if a legit chemist would check in, though, as I feel I'm not quite explaining this right. Failing that, I promise to call the laundry tech service people at my company and ask one of the chemists on our mutual behalf.
posted by DirtyOldTown at 6:51 PM on October 8, 2013 [5 favorites]

You probably have hard water. Tide rinses off my hands fairly easily. Not as easily as hand soap or dishwashing detergent, but not terrible either.

Also, laundry detergent is very concentrated, so a little of it is going to be "soapier" than other soaps. And they have to formulate it to behave nice if it doesn't all get rinsed out correctly. That's why it feels slippery. Otherwise our clothes would feel sticky and gross.

Lastly, handsoaps and dishwashing detergents are designed to feel like they rinse clean. So they formulate it to rinse away better than average. So to speak.
posted by gjc at 6:53 PM on October 8, 2013

When I asked the same question, I was happy with "because of the surfactants." I can see you're looking for a more detailed answer, from a chemistry standpoint. I will call tech service at the company where I work and ask one of the nice chemists there to give me a more detailed answer.
posted by DirtyOldTown at 7:05 PM on October 8, 2013 [1 favorite]

It's designed to have more staying power than hand soap. It has to survive a 20+ minute long wash cycle with very hot water. And it's designed to be rinsed off of fabric by the rinse/agitation cycle of a washing machine, not off of human skin in a sink. Laundry detergents also have enzymes to dissolve different types of stains and other chemicals designed to preserve/brighten fabric dyes. These might rinse out of fabric easier than off of skin.
posted by kimberussell at 7:18 PM on October 8, 2013 [1 favorite]

It could be that the proteolytic enzymes/proteases found in most commercial laundry detergents start, to some extent, breaking down your skin? I'd consider that unlikely as they are formulated to work under entirely different conditions (think the inside of a running washing machine), so I'm just throwing ideas out there.
posted by halogen at 7:24 PM on October 8, 2013

Clothing detergents are designed to not wash out completely. That's because they contain "brighteners", which are compounds which absorb ultraviolet and radiate blue. If you have an ultraviolet light handy, try shining it on your clothing detergent; you see it glow quite brightly.

The whole point is to leave some of the brightner in the cloth because it makes whites look cleaner. If they washed out completely, that would defeat the purpose.

Obviously, hand detergents don't have any equivalent.
posted by Chocolate Pickle at 9:10 PM on October 8, 2013

I was thinking enzymes, too—that's one of the things laundry detergent like Tide has that hand and dish soaps, body wash, and shampoo don't.
posted by limeonaire at 9:55 PM on October 8, 2013

So this is going to be kind of vague and imprecise, but you know how everybody says 'oil and water don't mix'? Surfactants are just a kind of molecule with one side that 'likes water' and one side that doesn't (that likes oil). If you dump a whole bunch of surfactant into water, after a certain concentration, bunches of surfactant molecules form little balls which have the parts that 'like water' on the outside and the parts that don't 'like water' on the inside. The idea is that the dirt that won't dissolve in water will instead dissolve into these little balls. Hopefully this gets your stuff cleaner than just water by itself.

Now the important part to remember is that there are all sorts of different surfactants which have all sorts of properties. Like a common surfactant in hand soap/shampoo is Sodium Dodecyl Sulfate (SDS), which dissolves fairly easily in cold water (and is cheap!), but it's not the same surfactant in Tide Laundry detergent. See: Tide Laundry Detergent Ingredients.

If I had to guess, I'd guess that the the amount of Tide Laundry detergent surfactant that dissolves in water is less than say, SDS. That is, it would wash off, with enough water. I couldn't quickly find solubilities for these surfactants, but you can try to google them. For more information about surfactants in general, I recommend Hiemenz.
posted by Comrade_robot at 9:47 AM on October 9, 2013

I'm back and I spoke to a nice chemist in our tech service department.

He told me that laundry detergents like Tide have anionic surfactants, which tend to create more foam than the nonionic surfactants found in hand soap and manual dish detergents. They also contain another class of chemicals called amines, which act as foam stabilizers. The combination of the two things means that laundry detergents are built to have long lasting interaction with water and agitation (the mechanical action of washing) without breaking down too quickly.
posted by DirtyOldTown at 1:01 PM on October 9, 2013 [2 favorites]

[OP please don't threadsit.]
posted by jessamyn (staff) at 3:23 PM on October 9, 2013

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