Science Buddies: "Ask an Expert"

My project is about making a marker ink from dragon fruit skin as it has a pigment color.
But as I turn it into a dye and try to write it on a paper, the dye didn't absorb into the paper. It is like the dye just floating on it.Probably I am missing something. So please, could you tell me how to solve this problem.

Hi,
That seems a bit strange, perhaps you could give us some more detail on the procedure you used, and then we'll be able to pinpoint the problem more easily. Thanks!!

Hi Unwaniah,

This is a very interesting chemistry project. To understand why your experiment didn't work, you need to understand the chemistry of the dragon fruit pigments, and the chemistry of marker ink. Here is some information that should help:

Here is a website that includes a brief description of the chemistry of ink:

http://www.rsc.org/chemistryworld/Issue ... mistry.asp

In reading this, you will find that water solubility, particle size, and stability are important characteristics of ink, and that most red inks contain a pigment called eosin. Pigments used in inks are not water soluble.

Here is a website that describes eosin, which is derived from petroleum, and the second shows the structure of eosin. Notice that there are lots of carbon/hydrogen ring structures in this molecule, with alternating single and double bonds. This characteristic of dyes is responsible for the color.

http://www.chemistrydaily.com/chemistry/Eosin

http://omlc.ogi.edu/spectra/PhotochemCA ... Y-JZL.html

Now for the chemistry of dragon fruit. The red dyes in dragon fruit are called betanin, betacyanin, and lycopene. Betanin and betacyanin are water soluble glycosidic (sugar derived) dye molecules that degrade with exposure to heat, light, and oxygen. Look at the chemical structure of betanin and betacyanin in the following sites. Can you find the sugar part of the molecule? Also, notice that the rest of the molecule has the ring structure with alternating single and double bonds.

http://en.wikipedia.org/wiki/Betanin
http://en.wikipedia.org/wiki/Betalain


Now, how about lycopene? Lycopene is a large hydrophobic pigment molecule, which is not soluble in water and contains only carbon and hydrogen atoms. Here is a website the shows the structure of this molecule:

http://www.phytochemicals.info/phytoche ... copene.php

Notice that lycopene does not have the ring structure like eosin, but it does have lots of alternating single and double bonds.

Now, can you use all of this information to decide what to do next on your project? Please let us know if you have any questions about the chemistry.

Donna Hardy