Photographic Emulsion Stabliizers

[Kirk Keyes]

Here's something I ran across on Google Books:
http://books.google.com/books?id=CPmKIAPqhmEC&pg=PA63&lpg=PA63&dq=tetraazaindene&source=web&ots=GTahYSuefS&sig=6AwtA60skIQcjiUQ2lhYC0ia9i0&hl=en#PPP1,M1

It looks like about half of the book is there.

Kirk

 

[Ryuji]

This book is actually pretty good, but it is also highly advanced for most amateur emulsion makers.

Birr also wrote a book on emulsion stabilizers, but his book is more like collection of quotes from others' work. Fischer's book is more consolidated knowledge in expository style.

However, Fischer often stops at description of the chemical compound, and does not get into why this compound is better than others for a particular application, etc. Those are left for the reader to find out elsewhere, and for that matter, I don't know of a good single book to recommend.

 

[Ryuji]

Incidentally, I use the following to stabilize my emulsions:

KBr
4-hydroxy-6-methyl-1,3,3a,7-tetrazaindene
1-phenyl-5-mercaptotetrazole
2-mercaptobenzimidazole
6-nitrobenzimidazole
5-methylbenzotriazole
nucleic acids

You might want to look these up in Fischer's book. If you are shopping for chemicals, I listed them roughly in the order from the most essential to optional.

It may also be of interest to note that, in recent practice, chemical digestion is often carried out in presence of agents that enhance the reaction yield, so as to minimize the amount of excess agents when the optimal sensitivity is obtained. This makes the emulsion performance more stable over the course of shelf life. (For example, in traditional technique of sulfur sensitization, thiosulfate is added to the emulsion and then the emulsion heated for so many minutes. In this scenario, only a small fraction of thiosulfate is used to make effective sulfur sensitization center, and the excess can slowly react to increase fog and drop speed over the course of storage period. If you have a way to obtain the optimal speed with more complete reaction yield, and then start with less sensitizer, then less excess reagent will result, and that's a much better strategy.)

 

[Kirk Keyes]

Ryuji - could you give a list of sources that we can obtain these from and appoximate costs?

KBr is a non-issue, but some of these others are rather obscure chemicals for a lot of people.

I've found 1-Phenyl-5-mercapto-1234-tetrazole at artchemicals for $36 for 5g:
http://www.artchemicals.com/1-Phenyl-5-mercapto-1234-tetrazole-5g-Spectum-Branded-P37051C585.aspx

Any suggestions on the rest?

 

[Ryuji]

These compounds are rather easily available from scientific chemical supply houses, although their price is high. (But we are buying such a small qty anyway)

I'll list common acronyms to facilitate discussion.

TAI: 4-hydroxy-6-methyl-1,3,3a,7-tetrazaindene
PMT: 1-phenyl-5-mercaptotetrazole
2-MBI: 2-mercaptobenzimidazole
6-NBI: 6-nitrobenzimidazole
MBTA: 5-methylbenzotriazole

TAI is used in a relatively large quantity, 100-1500mg per mole silver. Other compounds are used very small quantities, like 1-50mg per mole silver.

MBI is widely available and it is cheap, but like above, it is used in such a small qty.

NBI is not that essential. MBTA is also very common for color emulsions but not that essential.

I started my emulsion making with KBr, PMT and TAI. These are definitely the most often used stabilizer cocktail.

Nucleic acids are not a single compound. You may find a mixed nucleic acids or partial RNA hydrolysis products. Or you might look into individual ones. These are used only in advanced formulas. Not that essential.

 

[Ryuji]

I should also note that, these compounds, particularly TAI and PMT have some other names for the same compound. You also want to search by aliases and CAS number.

If your goal is a good plate emulsion, TAI is pretty important.

If your goal is to make chlorobromide or chloride printing emulsion, PMT is pretty important.

Also, plain old benzotriazole (a.k.a. 1H-benzotriazole or 1,2,3-benzotriazole) is usable at a somewhat increased quantity (like 30-300mg per mole silver) but not as effectively. Definitely better than not using any, tho.

 

[Kirk Keyes]

Thanks - that helps narrow the list a bit.

 

[Kirk Keyes]

I'll definitely get some TAI and PMT (I've got a bit now, but no stocks for long term).

I notice that Jim Browning's dye matix fomula uses not only Potassium Bromide and 7-Hydroxy-5-Methyl-1, 3,4-triazaindolizine (a variation of TAI I assume), and 1-Phenyl-5-Mercapto Tetrazol (PMT), but he also adds Manganous Sulfate and Sodium Azide.

Care to comment on the Manganous Sulfate and the NaZ?

 

[rmazzullo]

reference regarding Sodium Azide (which is an explosive):

(there was a url link here which no longer exists)

Bob M.

 

[Kirk Keyes]

Bob - That thread was pretty lean on info on why and how one would use sodium azide as a stabilizer. Seems like it broke down into a dry transfer vs. who knows what the hell thread a couple of times. It was downright painful to read at times. I even saw your name in there as it was disintegrating...

Any other suggestions?

 

[Ryuji]

Manganese (II) sulfate is mentioned in some old literature like Duffin, but I do not see it used as a stabilizer in vast number of modern emulsion patents. I have tested it but it has little power to halt the sulfur sensitization reaction. Jim Browning may have a different intention, but I am not certain if that's a useful agent as a stabilizer on optimally sulfur sensitized emulsions. Manganese ion may adsorb on crystal surface to restrain the ripening effect, though, and it may be effective for this type of problems, but organic antifoggants (like PMT and 2-MBI) are so much more effective.

These days, manganese compounds are rather used as an electron trap as a dopant. However, for this application, some other metals are more common (iron, ruthenium, iridium, etc.). Rhodium and palladium are also effective dopants for chloride emulsions.

Sodium azide is used as a biocide, not as an emulsion stabilizer. This agent is toxic and also can get involved in a number of other reactions, so I prefer some other agents, such as sodium salt of 2-phenylphenol, methylparaben, any of the isothiazolinone agents. Look up biocide on my website and you'll find a whole story.

 

[Ryuji]

Addendum.

In photographic chemistry, terms "stabilizer," "antifoggant" and "biocide" are given some specific meaning, although some authors use the terms less distinctly.

Emulsion stabilizer is an agent used to minimize the fluctuation in emulsion speed and fog level over the course of storage time by inhibiting any further chemical sensitization. TAI is a very good example of this. This agent was introduced by Birr in 1920s in Germany and it is still VERY commonly used today. This agent somehow binds to sulfur sensitizing agent in the gelatin-water phase and prevents them from entering sulfur sensitization reaction.

Emulsion antifoggant is an agent that forms very insoluble silver compound and covers the surface of silver halide crystals. Some compounds make less soluble silver salt than others. These antifoggants also vary in the strength of adsorption on the silver halide crystals. PMT and MBI are very good examples of antifoggants that form very insoluble salt (comparable to AgI) and they also adsorb very strongly on silver halide as well as metallic silver. 1H-benzotriazole (familiar in developer use) makes silver salt that is more soluble than silver-PMT or MBI, and adsorption is also considerably weaker. Plain benzotriazole can be desorbed by color developers (which has stronger adsorption on AgX crystals) and this agent is not very effective at all in color materials. 5-methylbenzotriazole is improved in this regard . Plain benzotriazole is usable for b&w emulsions, but not as effectively.

Some of the effective antifoggants make very dense assembly of antifoggant molecules on the crystal surface, and they can practically halt ostwald ripening process. Some are not as drastic but greatly slow the ripening process. Therefore, these compounds are also called "restrainer" in precipitation, growth and ripening contexts.

Biocides are self-explanatory.

Extra material.

What if you have a restrainer that preferentially adsorbs on one surface over the others? These compounds are used to control the shapes of crystals in highly sophisticated emulsions. Silver chloride usually does not make tabular crystals, but some of these "crystal habit modifiers" can be used to make silver chloride tabular grains. Joe Maskasky is the pioneer in this field. Some of these compounds, as well as some dyes, are used to make exceptionally thin tabular crystals of very large diameters (very high aspect ratio). These crystals have some useful properties, but they are also easy to break by mechanical stress (and can fog or lose sensitivity) and they are hard to use for practical applications.

 

[AgX]

Birr

To my information it was in the late twenties that by two other Agfa people (Mathies/Wendt) the predecessor of that stabilizer was found, a sulphur containing heterocycle. Birr came to Agfa only in ’34 and found TAI in 1937 (after the presentation of the New Agfacolor film).
But I admit finding these dates is difficult and the results are not necessarily trustworthy as long one did not see the original documents, which I did not.

 

[Ryuji]

That's right, it was in 30s. I have the literature detailing these but didn't consult it when I wrote the above. Birr has a couple of famous quotes and I remember one from 20s, so I often get confused...

Helmut Mueller worked with Koslowski and Birr at Agfa before the war, and moved to Ansco/GAF after the war. So I trust his accounts of pre-war AGFA Wolfen research. Fritz Wentzel is another German chemist I respect but I think he moved to the US in 30s. Ira Current worked under him and I heard a lot of stories about how they worked together.

I have his photographic chemistry book from 20s (in German) but haven't had it translated. His memoir (in English) has lots of bits of very interesting information.

 

[Ryuji]

Note for chemical shoppers: 6-nitrobenzimidazole and 5-nitrobenzimidazole are the same compound. 5 and 6 positions refer to the same thing in benzimidazole.

 

[Hologram]

This book is actually pretty good, but it is also highly advanced for most amateur emulsion makers.

Glafkidès, Chimie et physique photographique (there's an English translation available) and Frieser et al., Grundlagen der photographischen Prozesse mit Silberhalogeniden (I think the authors are somehow related to Agfa-Leverkusen – I am not aware of an English translation though) have nice chapters on emulsion stabilization.

 

[AgX]

Hologram,

Frieser was holding chair for scientific photography from 1936-45 and was head of research at Agfa-Leverkusen 1952-?

 

[dyetransfer]

Sodium Azide

reference regarding Sodium Azide (which is an explosive):

(there was a url link here which no longer exists)

Bob M.

Sodium Azide was an additive used by Fotokemika as a preservative - that is to prevent disintegration of the uncoated emulsion prior to coating and drying. I never used it, but I updated the formulation to reflect the FK additions of Sodium Azide, and the PMT. I should probably replace this with Thymol as it is safer to use.

As far as the Manganous Sulfate, since it was mentioned in the Duffin book, I used it from the beginning. I don't know if it has any effect on the emulsion, and can probably be omitted.

Regards - Jim Browning

 

[Photo Engineer]

Antifoggants such as PMT must be used with care. Taking Jim's emulsion as an example, if one were to spectrally sensitize it, there might be an interaction between adsorbed species such as PMT and the dye. I have seen PMT unsensitize an emulsion by 'kicking' the sensitizing dye off the grain either immediately or gradually with resulting speed losses.

If mixed emulsions are used such as in a mixed R/G/B set for pan sensitivity, or if coating a VC emulsion set, interactions between emulsions are common. It is therefore necessary to use ballasted versions of many of these compounds to isolate them to the preferred emulsion. An alternative is to use a very strongly adsorbed chemical which is tailored to have the desired activity, but then this has implications on use of the sensitizing dye.

When coating on RC or film, the formulas can be virtually the same, but on FB papers, the equillibration of all chemistry into the paper can lead to changes in speed and fog. Therefore, as is often the case, FB materials use a different formula, or keep more poorly than their RC counterparts.

PE

 

[Kirk Keyes]

"Ballasted"? Adding a larger ligand (in the biochem sense, I think) or other compound to make the functional part of the compound heavier/less mobile so it doesn't leave the intended layer?

 

[Kirk Keyes]

Jim - thanks for your reply. It's great to be able to get info from the author of the original.

 

[Photo Engineer]

Kirk;

Imagine a dodecyl group or two attached to PMT. That is the direction to go. These are not available commercially AFAIK. All the ones I used were custom made in the EK labs or scale up facilities.

PE

 

[Ryuji]

Kirk,

Ballast substitution is very common among cyanine dyes. Substituton with bulky groups on the heterocyclic groups enhances stability of the dye against oxidation, although ethyl group is often sufficient for blue/green dyes. Red and particularly infrared dyes could use more effort in this area. Infrared dyes are usually very susceptible to oxidation and have very short shelf life. Methyl, ethyl, nitro or other substitution on some emulsion stabilizers is used, but the main purpose of such modification is to increase the adsorption of the stabilizer on AgX crystal surface. 5-methylated benzotriazole is a prime example of this. Nitro substitution is also effective on effective desensitizing dyes, because nitro group is more electron withdrawing while alkyl would do the reverse.

PMT and MBI have very strong adsorption property on AgX crystals and they can displace many other molecules. That is always a possibility to keep in mind in emulsion design. Even weaker adsorber like adenine is known to prevent adsorption of spectral sensitizer. However, in B&W emulsion making, this is not a major problem, since there are always dyes that work well without the need of keeping intricate balancing of competitive adsorption. It is also not something you need to worry about your short term goal of making a usable negative plate emulsions of good speed and low fog.

One area where this may become a big issue is when you want to make a chloride emulsion using spectral sensitization. Color papers are a prime example of this. This is because chloride emulsions can take quite a lot of PMT without decreasing speed (compared to bromide emulsion).

Speaking of competitive adsorption of spectral sensitizer dyes, color couplers (in oil-in-water emulsion system) are also a big concern in desorbing sensitizing dyes. Improvements are made in many aspects of this problem, but this is far more tricky issue than PMT displacement. Again, we need not worry about this problem.

 

[Kirk Keyes]

Imagine a dodecyl group or two attached to PMT.

Now that I've read the book a little - and I know what to look for, I see in Chart 3.19 on page 17, they list for TAI "homologs having medium-long alkyls groups, for instance 2-heptyl or 5-hexyl" along with a list of other groups that can be added like phenyl or cyano groups.

It does look like a good book.

 

[Photo Engineer]

Now that I've read the book a little - and I know what to look for, I see in Chart 3.19 on page 17, they list for TAI "homologs having medium-long alkyls groups, for instance 2-heptyl or 5-hexyl" along with a list of other groups that can be added like phenyl or cyano groups.

It does look like a good book.

Kirk;

You are now at the heart of the matter as far as sensitized emulsions, mixed emulsions and color systems are concerned. PMT is not used, BTAZ is not used (BenzoTriAZole), and most others mentioned. TAI is used because it has generic beneficial effects. I have used it in many color products with no bad effects, but generally, none of the other compunds mentioned have been used for years.

PE