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Name: Merili
Status: educator
Grade: K-3
Location: Outside U.S.
Country: USA
Date:  Fall 2011  

Question:
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?

Replies:
Merili,

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) Canisius College


Hi Merili,

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.

Don Yee





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