Mordant dyes

From the Draft Folder

Mordant: Derived from Latin "to bite". Through centuries trial-and-error the distaff side of the house learned that certain pre-treatments of yarn not only increased the color intensity of the dyed wool but increased its resistance to washing out.

Some time ago, a reader challenged me to write a post about "Mordant dyes". This reader enjoyed fiber arts and wondered if I could come up with an intuitively pleasing explanation of how they worked. That challenge appealed to me because I am an "Explainer" by nature. Unfortunately, I am also a "Forgetter".

The search for new antibiotics

The search for new antibiotics might seem like an unusual place to start explaining mordant dyes but it is "grabby" and modern and interesting.

It is relatively easy to test a chemical for antibiotic activity. Make a culture of the target organism, stir in the chemical. The organism either dies, stops reproducing or ignores the chemical. Since bacteria in laboratory conditions can double every 20 minutes, it does not take long to determine if either of the first two conditions are met.

Since antibiotics must be able to penetrate infected tissues, there is a practical limit to how large of a molecule the antibiotic can be. Furthermore, the molecule must be small enough to diffuse through the cell wall of the bacteria. If it is too large it diffuses too slowly to be useful. The size limitation means that there are a finite number of chemicals in the universe of candidates for new antibiotics.

Between the ease-of-testing and the finite universe of candidates, a very large percentage of the likely candidates for antibiotics have already been tested.

Then some brilliant person had the idea, "What if we develop binary systems where each half is soluble and small but they combine inside the bacteria to form a much larger molecule?" The idea is analogous to a team smuggling contraband into a country by each member bringing in a single component.

Mordant dyes use the same idea.

The fiber, often wool, is soaked in a solution that contains a "metal" salt like iron sulfate (FeSO4). The iron sulfate dissolves in the water and the iron takes the form of Fe++ ions which are soluble and small enough to penetrate the wool fiber.

Most likely it happened accidentally. Water from certain natural springs was rich in Fe++ and certain colors were an order-of-magnitude sharper than when dyed from springs low in Fe++

It was almost like magic. The Fe++ ion does not interact with visible light. That involves the second part.

Complexes

When you were a small child, was their an adult in your life who absolutely smothered you when they gave you a hug? They just wrapped their arms around you. Perhaps their skirt or apron was voluminous and it wrapped around you as well?

That is what chelating agents do. In most cases chemists use chelating agents to protect the process from contamination with metal ions like Mg++, Ca++ and Fe++. Common chelating agents include citric acid and EDTA which is a synthetic amino acid.

I have personally used Kool-aid Lemonade (which is mostly citric acid) in a pinch to pre-treat well water I then used with the amide formulation of 2,4-d herbicide. I didn't care if the Ca++ was soluble as long as it was tied up by the citric acid and not available to tie-up the expensive 2,4-d molecules.

The interesting thing is that there are some chelating agents that are very water soluble before they hug the kid but become insoluble after they tie-up the kid. Those are the ones that are useful for mordant dyes.

But where does the color come from?

Color is light. It is either the absorbing everything except one frequency or it is a case of a molecule absorbing higher-energy, shorter wavelength light and re-radiating it as a lower energy, longer wavelength color and heat.

Metal ions have a bunch of electrons swarming around the nucleus. Each electron has the option of orbiting (or vibrating) around the nucleus in several different modes. Each mode has a characteristic energy associated with it.

It might be helpful to think about tossing a ball up a flight of stairs. Depending on how much kinetic energy you can impart to the ball you might be able to throw it up one tread or six treads or over the entire flight. Or, you might not even be able to lift the ball up even one tread.

When an incoming photon hits an electron orbiting the ion, it might not have enough energy to lift it to that first tread and it will pass through unaltered.

If the photon has enough energy, it will kick the electron up however-many treads it can and the energy that is left over typically is lower than that of visible light.

But that electron does not want to stay in that higher energy state, just like the baseball does not want to stay on the sixth tread. Something has to happen to the excess energy when the electron drops back to is preferred, lower energy state. The molecular energy checkbook has to balance. It does that by creating whose wavelength/energy corresponds to energy the electron accepted from the original light wave.

Shamelessly mixing metaphors

That chelating agent in the mordant dye does more than make the metal ion insoluble in water and thus part of the fiber. It also influences the modes and characteristic energy levels the electrons of the ion can attain. It is very analogous to how a guitar player presses a string against a fret on the neck to change the characteristic frequencies the string wants to vibrate. The player can (effectively) shorten the string and raise the frequency.

The chelating molecule is hugging and stretching the metal ion in the same way the guitarist's finger and frets interacts with the guitar string. There are some combinations of chelating agents and metal ions that not only create insoluble complexes (so the mega-molecule stays put) but the chelating agent modifies the metal ion's electrons have energy acceptance or dump-states that correlate to visible light.

For example, that Fe++ with tannin as the chelating agent can absorb all visible light and dumps out infrared. It makes a black dye.

Chlorophyll and Hemoglobin

Hemoglobin on left. Chlorophyll on right

 

Chlorophyll is a mordant dye. To the causal observer, the molecule looks like a gong (a magnesium ion) suspended in the middle of a framework of a porphyin molecule with some additional molecular doo-dads hanging off to the side.

The "gong" analogy goes even farther. The incoming photon hits the Mg ion and makes it ring. The unique construction of the chlorophyll molecule siphons off some of the excess energy from the energy checkbook balancing to "latch" the start of the glucose molecule together.

Chlorophyll is arguably the most important single dye in the universe.

Hemoglobin, the chemical that transports oxygen in our blood, is arguably the second most important dye. The core of the hemoglobin molecule is identical to chlorophyll except it has an Iron (Fe) ion in the gong-position rather than Magnesium.

How weird is that?