crazyboy
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Why are hydroxides slippery and acids grippy?
I'm sure I'm not alone in my observation that when you get a solution of sodium or potassium hydroxide on your skin it feels slippery. Preliminary
research suggests that the popular opinion is that this is due to the ability of strong bases to saponify the fatty acids present on the skin or
cleave lipids to produce fatty acids for saponfication. I have also notice that a small amount of acid on the hand causes a feeling of "gripiness"
that is an increase in the friction of the skin. In the past I attributed this to the ability of acid to somehow strip away all the oils on the skin.
However, the other day I spilled some 6N NaOH on a linoleum floor and even after wiping up the mess with wet paper towels the floor was incredibly
slippery to the soles of my shoes. I remedied this by wiping the floor with a small amount of HCl and the floor was no longer slippery, in fact the
floor seemed even more grippy than the clean floor untouched by acid or base.
So what's going on here? In my view saponifcation is out of the question, try putting concentrated sodium hydroxide on a gloved hand and you will find
it more lubricated. Any thoughts?
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ave369
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The floor could be subtly greasy, covered by a thin layer of fat (particularly common if it's a kitchen floor, due to condensation of various frying
oils and fats). The alkali could saponify this fat, and the acid remove the soap.
Smells like ammonia....
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hyfalcon
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The base would have also cut through any wax or surface coating if it was oil based, and like ave369 says, the HCl will have neutralized any of the
base left and the wet paper towels wiped all the contaminants off the floor.
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BromicAcid
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My reasoning would be that sodium hydroxide is hygroscopic. If you have a solution of a salt in water, normally you can wipe it up and it will leave
a thin film of the solution on the linoleum, this would then evaporate and leave behind the salt but the in the case of NaOH it's never really going
to evaporate, it will keep pulling water from the air and stay a solution and feel slippery until every bit is up. But your second bit with the
hydrochloric acid would lead me to believe your NaOH was stripping off the coating on your linoleum and you quenched it off leaving behind a textured
surface. Look at that section of the floor in the glare, does the rest of the floor look shiny and that spot dull? If so you've lost your finish.
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Fulmen
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Linoleum is made from linseed oil and would probably be saponified by strong alkali. The increased friction after neutralizing with acid could come
from a roughened surface.
We're not banging rocks together here. We know how to put a man back together.
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deltaH
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I don't think any of these explanations are correct.
Even baking soda solutions feels slightly soapy to the touch, so you expect me to believe that baking soda is saponifying anything at room
temperature... instantly?  Nonsense.
I think this very much has to do with surface science theory and more specifically the charging (or not) of the surface as a function/effect of pH.
One possibility might be that the surfaces of most materials exposed to air are weakly acidic in character due to surface oxidation.
So under basic, even mildly basic conditions, surfaces bear a slight negative charge and a electric double layer forms with counter ions from the
solution immediately above that and these ions and water of solvation lubricate the surface and are held in place by electrostatic forces. Under
acidic conditions, slightly acidic surfaces are less charged, less 'lubricated' and so more friction occurs by way of solid to solid contact.
Certainly linoleum, being a polymer of linseed oil is well known to oxidise very readily in air, is thus expected to have a highly oxidised surface
and considerable surface acidic character.
A surface "hygroscopic effect theory" does not explain the proportionality to pH fully. A surface covered with pH 8 solution and pH 12 solution are
both equally [and completely] wet, yet the pH 12 covered surface will feel much more slippery. Only more surface charge can account for that.
[Edited on 26-2-2016 by deltaH]
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Fulmen
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It might not be the entire truth, but that does not mean it's wrong. Saponification of skin is a very real effect, and I doubt any other effect can
compare to it. It might not be the explanation for every weak alkaline solution, but it is for strong ones and skin.
We're not banging rocks together here. We know how to put a man back together.
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