Gelatin

[johnfp]

Wow, what a wonderful substance gelatin is. I have been doing copious amount of research on gelatin and have learned quite a bit. I use gelatin to make DiChromated Gelatin (DCG) Emulsion for use in holography. But I have always wanted to understand the mechanics of what actaully happens to the gelatin in the DCG process. I am a little, actually quite weak in chemistry, thus my post here. Here is what I think I understand so far and would love some feedback or corrections.

 

First and to generalize we start with collagen. A triple helix structure of amino acids. The collage is denatured and the hydrogen bonds holding the coiled amino strands are broken and these bond area are now bonded with water. This substance is dried and a few of the water molecules are released and the covalent bonding of the amino strands reoccure but only sparcely. THis is what we call gelatin. Even dried gelatin contains 12 - 14% water. It is this gelatin that I used to record interference fringes. The idea is to get the gelatin to have two different indices of the refraction of light. Greatly crosslinked areas and barely crosslinked area (from the standing wave pattern of destructive and constructive interference fringes).

 

Now we take this gelatin, mix it with water and Amonium DiChromate (AmDi), stir it and heat it (about 120F) at the same time until the gelatin is completely disolved. We then coat a glass plate.

 

Now, it is my understanding that when we perform the above proceedure I assume that most of the covalent hydrogen bonds get broken again and those areas are again bonded with water. We let the plate cure (dry). Once again, as in the drying process after denaturing, some water molecules become detached and once again some, but not many of the hydrogen bonds between two strands reform. This must happen very occasionally. Think of those loose amino strand looking like a pot of boiled spaghetti. All twisted and intertwined but with some covalent hydrogen bonds at various points. I assume this is what gives Jello it's wiggle.

 

Now because the amino chains are only attached at various and sparce points this leave a lot of points that have the potential to reattach to one another but the water molecules being attached to those point prohibits this. Now, remember we have AmDi in the mix. This has to be disolved in the water in between those amino strands. The Chromate is CrVI when disolved. Now we expose to light. This light acts as a catalyst to excite the chromate and caused it to gain an electron, I assume from those points that are connected to the water molecules.

 

Now, how mechanically does that light (actinic radiation) cause the amino strands to give up that water molecule and bond with the CrVI?

And how does that light cause the CrVI to break down to CrV (possible same question as above).

Also, with prolonged exposure to that light the CRV breaks down to CrIII and now has accepted the sharing of 3 electrons. How does that happen?

Do you think the CRIII is now bonding (crosslinking) with three points on potentially 3 differnt amino strands?

 

I do know that it is CRIII that is what we ultimately need to crosslink the gelatin and cause the differnt index of refraction as compared to the gelatin that has not received light.

 

Remember I said I am not that good in chemisty but would love to learn more on the mechanics of what is going on. So if you can give any, any at all, insite as to what is going on in pretty much easy to follow, laymens tearms, it would be greatly appreciated. Thank you in advance,

John

 

PS: Feel free to ask any questions you may have with any part of DCG holography. I could talk about DCG holography all day long and no question is dumb or too easy to answer.

 

Have a great day!

[UncleAl]

Look up chrome tanning of leather. In your case, light promotes reduction of the dichromate by oxidation of the collagen. Chromium cations like coordination by hard Lewis acids - oxygen and nitrogen, crosslinking the collagen. Cr(III) is substitution inert. You can reflux Cr(NH3)6(3+) in concentrated HCl for months without forming any ammonium chloride - if the atmosphere is oxidizing. Mix at room temp and add a tiny granule of zinc metal. Cr(II) is substitution labile. The whole pot catalytically goes right before your eyes.

 

In summary: Light triggers reduction of dichromate, Cr(VI) eventually reduced down to Cr(III) that irreversibly crosslinks the collagen. This is reversible with added reducing agent - nitrite, sulfite, dithonite... Vitamin C - though the oxidative damage remains.

 

In principle you could use blueprint chemistry instead. However, the eventual precipitate of Prussan Blue would be much coarser than the molecular scale of Cr(III) coordination and matrix crosslinking. Almost any lightly crosslinked hydrogel (to swell but not dissolve in water) could substitute for collagen. One is amazed that somebody does not commercialize a holographic Sauflon, poly(HEMA), etc. contact lens.