Ask A Scientist Chemistry Archive

   
name         Jim E.
status       other
age          60s

Question -   This question came up in a discussion about using hand
 sanitizing wipes to remove urushiol oil (poison ivy).  Essentially all
 authorities caution against rubbing the oil with anything that will
 spread it around and thus worsen the situation.

An acquaintance stated that germicidal sanitizing wipes are a good,
effective immediate treatment in the field to remove the oil because they
contain alcohol.  In searching the web for information on alcohol and oil
I found this article from Haverford College
http://www.haverford.edu/educ/knight-booklet/propofwater.htm [linked from
a poison ivy website] which says oil is not soluble in alcohol.  I did
similar experiments to those in the article.  I filled a small jar half
full of denatured alcohol and added some household oil. The oil did not
mix. It sank immediately to the bottom forming balls of oil just like the
article said. I shook the mixture and, as the article suggested, the
larger balls broke up into tiny balls, but the oil did not dissolve in the
alcohol.  I repeated this experiment using vegetable oil.  Then again,
with both kinds of oil using isopropyl alcohol.   The results were always
the same.  When I emptied the container I found the oil clung/adhered to
the bottom such that I had to use soap to remove it ­ that is, the
generous amount of alcohol in the container could not be used to remove
the small amount of oil on the bottom.

Yet my friend found web articles recommending the use of alcohol to remove
urushiol oil.

Some articles specifically caution against using water alone as it will
spread the oil, yet oil is lighter than water and thus might ‘float’ the
oil away.  Whereas oil is heaver than alcohol, thus alcohol cannot have
this imaginable positive effect.

If alcohol does not dissolve oil why is it recommended as a solvent to
remove urushiol oil?
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As you have already surmised, alcohol is not a good solvent for urushiol oil.
As the web site on solubilities already indicated to you, and as you found in
your experiments, oils are not particularly soluble in alcohol. Neither are
they soluble in water. I'm guessing that all these ideas about how to treat
poison ivy or sumac is part of the "lore" of such things, or have arisen more
as palliatives rather than actual first-aid treatment.

It is my understanding that urushiol takes anywhere from minutes to hours to
bind to the skin and cause the allergic reaction. Thus, if you suspect that
you have been exposed to this oil, you have this time to try and wash it off.
Since oils do not readily mix with either water or alcohol, you will need to
use soap. Urushiol oil, like soap has a hydrocarbon tail and a polar head 
(see
the like-dissolve-like discussion in this site).

Roberto Gregorious
=====================================================
Jim,

Urushiols are a mixture of compounds referred to as catechols. I cannot 
draw the molecular structure here -- nevertheless, if you look it up in a 
chemistry book, you will note that it has two -OH groups adjacent to each 
other on a benzene ring. In addition, it has a 15 carbon aliphatic 
hydrocarbon tail also attached to the ring structure. The two -OH groups 
enable urushiol oil to partially dissolve in water because water (and 
alcohols of low molecular mass) can form hydrogen bonds with those groups. 
The benzene ring part and the hydrocarbon "tail" makes urushiol oily.

I belong to the group that thinks using soaps or detergents or alcohols 
are NOT proper ways to remove poison ivy juices. Those materials assist 
solublization of the oil and enable it to better penetrate the skin. Thus, 
the rash is worsened.

I prefer to remove the oil by running a generous stream of cold water over 
the contaminated area while gently stroking the area with clean hands. If 
you repeat your experiments using an exceedingly small amount of vegetable 
oil, you will discover that the alcohol will dissolve the oil. Alcohol is 
a poor solvent for vegetable oil. Even so, it will dissolve enough to 
facilitate urushiol's penetration into the skin. In a sense, alcohol is a 
better urushiol solvent than water because the aliphatic part of the 
alcohol molecule shares non-polar properties with the urushiol "tail."

Regards,
ProfHoff 907
====================================================
"Urushiol" refers to a class of 1,2 di-hydroxy, 3 alkyl benzenes. So it is
an "oil" only in a generic sense. I would guess that wipes that contain
alcohol and a surfactant (detergent) would help remove this class of
compound although I do not know of a scientific study that recommends or
measures its effectiveness. the web site:
http://waynesword.palomar.edu/indxwayn.htm
gives some details. Scroll down the index site above to "poison ivy".  I
just found this site and it is pretty informative on a lot of chemical
topics.

Vince Calder
=====================================================
Not all oil is the same.

Broadly used, "Oil" just means a liquid that is not water, does not mix 
with water, is a little more viscous than water, and evaporates slower 
than water.
By that definition, glacial phosphoric acid is almost an "oil".  Pure 
H3PO4, no carbon in it anywhere.
But it is entirely soluble in water or alcohol.

In the narrowest possible sense, "oils" refer only to pure hydrocarbons,
Liquids made only of atoms of carbon and hydrogen (alkanes),
  with boiling points above 150C and freezing points below room temperature.
True, these don't wipe off with water or dry paper towels, and are they 
are only slightly soluble in alcohol.
But add one little change, almost any little group with an atom other than 
C or H, and suddenly isopropanol will dissolve some of it.

I am sure that urushiol oil has several such differences from pure 
hydrocarbon oil.

You mentioned isopropanol would not dissolve your vegetable oil.
When you do your experiment with vegetable oil and isopropanol,
(should be 91%-100% isopropanol, 70% has too much water)
shake them together, then let them settle and separate.
They remain two separate liquids, it is true.
But pour off some of the cleanest-looking isopropanol and let it evaporate.
I think you will have at least a film of vegetable oil remaining.
This proves that a little of the oil had dissolved in the alcohol.
That is called partial solubility, as opposed to total miscibility.

Even for oils which are distinctly insoluble in alcohol,
the alcohol can still help with absorption into a cloth.  And it often does.
I soak up globs of silicone vacuum grease with isopropanol on a paper towel.
This grease is insoluble in isopropanol and only marginally soluble in its 
best solvent, heptane.
But it is "almost" soluble in isopropanol, so that helps the grease 
"evaporate" across very tiny distances
if it can immediately re-attach itself to the high solid surface area in 
something like a paper towel.
The same trick should work with urushiol, and powders like starch or flour 
should work even better than a paper towel.

I'd like to evaluate the difference between urushiol and pure hydrocarbon 
oils,
so I looked around the Internet for its chemical formula.  Difficult to find.
But one excellent web site, http://waynesword.palomar.edu/ww0802.htm,
showed a chemical formula for urushiol C6H3(OH)2(R), where the R group is 
C17H35 or C15H31.
Also called heptadecyl catechol, or 3-heptadecyl 1,2 hydroxy benzene.
With that formula it is probably not very liquid, and it is probably very 
sticky.
Logical, since it is part of the sap of the plant.

With those two hydroxyl (-OH) groups resembling the (-OH) in alcohols,
I am sure that urushiol is partially soluble in isopropanol.

Do not forget that the paper fibers or powder in the paste are important too.
Alcohol evaporates, leaving the urushiol in its place.
The urushiol needs a final destination, some solid surface area other than 
your skin.

Jim Swenson
=====================================================