Crosslinking Gelatin

Gelatin is used in many products. From medicine capsules to photograhy film. I would like to start a thread so some of the chemistry brains out there may help me understand the full process that gelatin undergoes when it is crosslinked by one means or another. Below is what I have so far and I ask for any corrections, additions or deletions please. I am also favorable to any type of discussion concerning crosslinking gelatin. The writing of mine below pertains to crosslinking gelatin with Ammonium dichromate as a sensitizer and then exposed to light but many other processes/chemicals can be used to crosslink the gelatin. So here is what I have so far. As you read you will come across questions that are questions that I still need help with..hint, hint.

Basics of Gelatin

Type A - Acid process used in production of product. Primarily pork skins.

Type B - Alkaline process used in production of product. Primarily cattle hides.

Type B (bovine) - Alkaline process used in production of product. Specifically cattle bone. According to one supplier, this type may contain small amounts of silver salts which could increase speed of Silver Halide films although it was unknown how much (if any) effect this might have.

Bloom Stength - This is a standard measurement that determines hardness of gelatin after a specific period of time. The higher the Bloom number the harder the gelatin.


The Mechanics of Gelatin in the Dichromated Holography Process
There is a lot of information available on collagen, gelatin and Dichromated Gelatin (DCG) holography but a paper that ties together these facets and can be understood by the amateur holographer is simply hard if not impossible to find. The scope of this paper is to finally bring together a concise understanding of what is happening in the DCG process. As it is impossible to footnote exact portions studied from other works because I intend to combine all research, I will simply put the credit due to the works I studied at the bottom of this paper and leave it up to the reader to research the individual papers for verification of the information I found.




Collagen
Collagen is a protein found in the skin, bones, tendons, cartilage, teeth, ligaments and connective tissue. It is the supporting structure for most body tissue. The collagen molecule is about 300nm long and 1.5nm in diameter. It is made up of three polypeptide strands, each of which is a left handed helix. These three left handed helices are wound together into a right handed triple helix. The strands are stabilized by hydrogen bonds. There is also some covalent crosslinking within the collagen molecule and crosslinking between molecules. The more crosslinking the less soluble to water the collagen is. The smallest amino acid is Glycine and it is this amino acid that resides on the inside of the triple helix structure with its hydrogen atom facing inward. Two more common amino acids are Proline and Hydroxyproline and face outward. This gives the polypeptide chain its characteristic helical shape(2,3,4,5).

If collagen is hydrolyzed, the three amino chains are separated into a random glob, while still being bonded to adjacent chains with a peptide bonds and some hydrogen bonding. This is now the nature of gelatin. Because the structured arrangement has been broken down, the gelatin will have partial triple helices with loose ends bonded to other polypeptide strands and loose polypeptide strands bonded to other loose polypeptide strands forming a matrix of connected fully and partially broken down collagen molecules. It is this Random Coil that give gelatin its springy properties(6,7).

These two images were taken from source (16).



Triple helix of collagen (crosslinked to another molecule from peptides at end of molecule)




Collagen molecules line-up to form a fibril in "quarter staggered" array.




Gelatin
Gelatin is made by using the Hydrolysis process to get water to react with the Collagen. The Collagen undergoes partial hydrolysis and is broken down into the Random Coil Globs. The intermolecular and intramolecular bonds that render collagen insoluble to water has to be broken as well as the hydrogen bonds holding the triple helix together has to be broken. The amount of water bonded directly to the gelatin is about 12% - 14% after hydrolysis and after the gelatin is allowed to dry. As the newly formed gelatin cools, hydrogen bonds reform, forming the Random Coil Globs. Gelatin dehydrated to 2% water becomes insoluble in water because of the extensive crosslinking and is achieved by dehydraion. It is this water bonding to the polypeptide chains that keeps the chains from crosslinking. Crosslinking is the covalent (sharing of 1 or more electrons) bonding of the polypeptide chains. This gelatin can be reheated in water to break down the hydrogen bonds again and then redried. It is this latter part that we use to make emulsion(6,8,9).

Gel Strength of gelatin is a measure of the rigidity of a gel formed from a 6.67% solution and prepared according to certain arbitrary prescribed conditions(13,14).

Bloom (named after Mr Bloom whom invented the measuring device) is a measure of force (weight) required to depress a prescribed area of the surface of the samplee a distance of 4 mm. The more rigid the sample the higher the bloom(13,14).

This image was taken from source (16).



Denaturation of collagen




CrVI
Hexavalent chromium CrVI compounds are a group of chemical substances that contain the metallic element chromium in its positive-6 valence (hexavalent) state and can be found naturally in rocks but is most commonly produced by industrial processes. It has the ability to gain electrons from other elements (a strong oxidizer), which means it can react easily with them(10,12).

Research is needed using vitamin C with CrVI(11).




DCG
When Dichromate is added to a gelatin emulsion and then dried the compound is in a clear dissolved up state in a gelled solution. The Chromium is still in the CrVI state. On exposure to the appropriate light source (actinic radiation) the Chromium gains an electron by oxidizing some of the amino acid groups (Where from and how does it gain this electron?) and becomes CrV very quickly and easily. This CrV is bound more tightly then CrVI to the gelatin and cannot be easily washed away with just water. With continued exposure some of the CrV gains more electrons and becomes CrIII but this happens much more slowly then the creation of CrV from CrVI. After exposure the, in the light struck areas, there is a large amount of semi-strong bounded CrV and traces of CrIII causing crosslinking. If this latent hologram is allowed to sit in the dark, the CrV continues to gain electrons (from where?) and converts to CrIII causing additional crosslinking. Because the dark reaction of CrVI to CrV is also slow, more CrIII and more crosslinking in formed in the light struck areas CrV to CrIII, then in the non light struck areas, CrVI to CrV to CrIII (15).

During the first step of processing (reducing agent: Fixer or Sodium Metabisulfite) the CrV is very quickly changed to CrIII and ultimately causes more crosslinking in the light struck areas of the gelatin. The CrVI is washed out as the reducing agent works much more slowly on CrVI to CrV to CrIII. So we have now just increased the crosslinking much more in the light struck areas then in the non light struck areas. And it is this highly crosslinked area of the gelatin that has a higher index of refraction then the uncrosslinked areas yielding us our phase hologram(15).

The DCG hologram is then washed to remove all traces of the reducing agent, unbound Cr. and any loose gelatin. Remember, gelatin is soluble in water unless it is crosslinked. The water also has the effect of swelling the gelatin and thus the fringes so a hologram is still not visible until the gelatin and fringes have been shrunk back to their original size or at least shrunk to a size able to replay the visible wavelengths.

The Hologram is then put into an alcohol bath. Many techniques have yielded good results in varying the temperature, duration, concentration and the number of these alcohol baths with each variable changing the final appearance of the hologram. The goal of the alcohol is to remove the water bound in the gelatin structure without allowing a collapse of the delicate fringe lattice structure. (Does alcohol bond where the water was bonded?) (How does alcohol absorb water?) Once the water has been unbound the hologram can be dried with forced or latent heat thus evaporating the alcohol and more of the now scarce water. Again, the more moisture that is taken out of the emulsion, the more crosslinking there is (even in unexposed regions) and the more insoluble the emulsion is. When taken below 2% water content the emulsion is insoluble at room temperature due to being fully crosslinked.

References
1. Dark self-enhancement in dichromated-gelatin grating: a detailed study. Roma Grzymala and Tuula Keinonen
2. Collagen - Wikipedia, the free encyclopedia
3. protein :: Collagen --* Britannica Online Encyclopedia
4. Gelatin
5. http://www.stanford.edu/~spark7/
6. Gelatin - Wikipedia, the free encyclopedia
7. Gelatin
8. The structure and properties of solid gelatin and the principles of their modification
9. Gelatin Information
10. Hexavalent Chromium - NIOSH Topic Page
11. Hexavalent chromium - Wikipedia, the free encyclopedia
12. 3M US: OSHA Hexavalent Chromium Standard - An overview of the Chromium Six (CrVI) standard; Impacts of the New Hex Chrome standard
13. http://www.gelatin-gmia.com/PDFs/2.1...20Strength.pdf
14. Gelatin information, news, history and more
15. Improving the remarkable photosensitivity of dichromated gelatin for hologram recording in green laser light. Jeff Blyth, Christopher R. Lowe, John F. Pecora
16. The Effects of Relative Humidity on Some Physical Properties of Modern Vellum
17. Gel: a short word with a long meaning
18. Broad Range of Types of Polymers and Composites Polymer and Composite Consulting, Contract Research, and Software