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Written by Administrator
Monday, 01 February 2010 11:49

Tomorrow's penetrants

February 2010

In our January, 2010 mail inbox, we answered the e-mail sent by a well known American manufacturer.

We wrote about fluorescence:
‘‘... Basically, in PT, fluorescence is achieved using 2 dyes: a yellow, yellow-green or green dye which emits a fluorescent colour accordingly and an optical brightener which emits a blue/white fluorescence under UV-A radiation.

Both dyes are heterocyclic molecules with N (nitrogen) and C (carbon) atoms with both σ and π linkages. Fluorescence is due to the excitation of the π electrons of the π linkages.
Contrary to σ electrons, π electrons are free electrons moving on an electrons track.
π linkages are not so strong as σ linkages. If some π linkages are destroyed the fluorescent brightness under UV-A irradiance decays.

π linkages may be destroyed by:
• Heat.
• Photolysis.
• Curing (polymerisation) reactions.
• Chemical reactions: as examples, with acids, basic compounds, oxidisers such as chromic acid.’’

A chemical engineer of a PT/MT materials/equipments supplier sent us the following comment:

"Regarding the molecules structure and more exactly π electrons, the sentence "Contrary to σ electrons, π electrons are free electrons moving on an electrons track", implies that it is an absolute truth. In fact this is true only if the molecule comprises a conjugated system (electrons s and electrons p “alternation” as in butadiene) or if the molecule is aromatic.

The 3 conditions for a molecule to be aromatic are: it must be cyclic, have a conjugated system and have 2n+2 electrons π, n being a whole number. Note that π linkages of an aromatic system, though weaker than the σ linkages, are stronger than the classic π linkages.

So in this example, the π electrons are mobile (better to state they are mobile than they are “free” because, in my opinion, this could lead to some confusion with the free electrons of free radicals.  So the p electrons are mobile in the conjugated system).

I guess that the heterocyclic molecules you mentioned are aromatic molecules.

A π linkage contains more energy, hence is more fragile than a σ linkage.
But this is true even for ethylene (non cyclic and non aromatic).

In your chapter about causes of the p linkages break, I would use the word “light” instead of photolysis, because light is the cause and photolysis is the process.
We may even add that the cause is the free radicals of the light."

Our answer:

‘‘The yellow dye and the optical brightener are aromatic heterocycles. In our mind, it was a given but we understand that we should have stated it.

So, when we write: "Contrary to σ electrons, π electrons are free electrons moving on an electrons track", this is correct because, as you said, the molecules under scrutiny are aromatic.

We agree with your statement that π electrons are mobile (instead of free).

Agreed also your statement about the causes of the π linkages break: ‘‘light’’ should be used instead of ‘‘photolysis’".

Within a forty year-span, theoretical concepts of organic and quantum chemistry have been improved.

As an example: in University, in the ‘60s, our teachers of quantum chemistry talked of “the Heisenberg uncertainty principle’’: it was inaccurate. As a matter of fact, nowadays, they teach ‘‘the Heisenberg’s indetermination principle’’. Indeed, this indetermination is due more to the nature of the particles (we should even talk of “quantons”) than to some insufficient accuracy of our methods of measurement.

A little bit of chemistry cannot hurt our readers.

Thank you very much for the attention you gave to our website”.

N.B. The teacher of quantum chemistry of one of us, in these ‘60s, let us know the mnemonic he asked students to use so as to remember the HEISENBERG’s indetermination principle:

“You only have to answer this question:

- Why did HEISENBERG have no child?

The answer:
Because:
When he had the position, he had no more the impulse.
When he had the energy, he had no more the time.

Last Updated ( Wednesday, 01 June 2011 09:49 )