Alcohol, Solvent, and Polarity
Location: Outside U.S.
Date: Fall 2011
My daughter tested six solvents to see which would best >remove permanent marker. Denatured alcohol (and MEK close behind) did the best across all trials. We know that like dissolves like and we suspect that something in the marker is nonpolar. However, in our research we have come across that alcohol is polar, non-polar, and both! Which is it? If it is polar, why would it dissolve permanent marker, since water does not?
The rule of thumb that likes dissolves like is just that - a rule of
thumb, a quick way of making decisions on solubility. However, as
you have already discovered, it does not always work.
In order for something to be soluble (as opposed to whether
something dissolves or not, more on this distinction later), the
energy produced from the interaction between the solvent and solute
must be comparable to the energy required to break all solute-solute
interactions and some solvent-solvent interactions. Think of the
case of trying to dissolve oil and water. The water-water
interaction, being very strong, requires a lot of energy, the
oil-oil interaction requiring a little energy, and the oil-water
interaction producing just a little energy. So the production of
energy does not match the energy requirement - and the two do not
dissolve into each other.
This may be what is happening in your experiment. Water interacts
strongly with itself and so the energy required to break this
interaction (so that the water will interact with the permanent
marker materials instead) is much too large to be produced from the
water-permanent marker materials interaction alone. But it works
well for the denatured alcohol (a mixture of isopropanol, methanol, and water).
The other issue is dissolution. While solubility tells us whether
something can eventually form a solution, dissolution takes into
account how quickly the solution can form. Most plastics are
non-polar, but most of them do not dissolve readily in oil (also
non-polar). This is because oil is composed of very large molecules
that do not readily insert themselves into plastic so that
interactions between oil and plastic develop. So, while plastics may
be soluble in oil, they do not appreciably dissolve in reasonable times.
This may be happening in your experiment as well. The permanent
marker materials, may actually dissolve in some of the solvents you
tried, but the dissolution is slow so you don't notice it when you
try to wipe a board with the solvent.
Greg (Roberto Gregorius)
Calling something polar versus nonpolar is an artificially binary
construct, the reality is that polarity is a continuum, with lots of
gradations in between. Denatured alcohol, which is mostly ethanol,
has a polar end (the -OH to make it an alcohol) and a non polar end
(ethane) which helps it dissolve the marker ink.
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Update: June 2012