What causes the preferential development of the latent image?

[laingsoft]

During my lunch break I was doing some reading. Mainly I was trying to find out why the amine group was important in some developers like metol or rodinal. My original understanding was that the amine groups in these molecules allowed the developer to adsorb to the surface of the silver, which would in turn reduce more silver halide, allowing more development to occur. HQ would then reduce the Metol adsobed on the surface and the reaction would continue.

I came across this paper, that talks about the chemical process, with a focus on the action of hydroquinone: https://pubs.acs.org/doi/abs/10.1021/cr60095a001

In it, the author talks about several different possible explanations of how the development of a latent image works, and acknowledges the adsorption of amine-containing developers as a potential explanation for why developer develops the image at exposed areas vs unexposed areas. However, he goes into detail, suggesting that reduction of the silver halide is catalyzed by the silver itself. and that "Its interface with the silver halide provides the necessary ionic deformation for reactivity; on the other hand, it provides a break in the adsorption layer, a platform for displacement processes" In this case, the silver ions would be adsorbed to the silver, which would then be reduced, and the bromine would be kicked out into solution. This would sufficiently explain the superadditivity of metol and hydroquinone, wouldn't it? As even very small amounts of metol would adsorb on the surface, performing the first reduction, which would exponentially increase the amount of interface available for the hydroquinone to work?

The author also talks about how "treatment with iodide can restore develop-ability which has been destroyed by treatment with chromic acid", as well, the darkroom cookbook also suggests the potential of using KI (potassium iodide) as an antifoggant/accelerant.

If these two things, would it be reasonable to suggest that a pure hydroquinone or ascorbic acid developer could be sufficiently sped up with the addition of 2 things: A solvent to expose more latent image centers + a source of silver ions in solution?

For the solvent, the obvious choice in this case would be Potassium Iodide, as it is economical and readily soluble in water.

The source of silver ions could be a Thiosulfate. But if my understanding is correct, wouldn't Oxalate also work?

Sorry if I've gone completely off the rails. If my understanding is completely off I would love to learn more.

 

[Rudeofus]

1942 was a year with lots of active research into photographic development, however, it was far from the last. There was active research into this very topic until well into the 70ies, and drawing conclusions from a 1942 paper may not include very relevant insights gained and published since then:

1. If you treat exposed silver halide grains to chromic acid, surface latent image centers will be bleached back, effectively eliminating these latent image centers. If you then treat this emulsion in a solution of Potassium Iodide, some of these grains will be broken up, and some internal latent image centers are brought to the surface, which renders the grains developable again. There are some advanced developers, which add silver solvent to take advantage of these internal latent image centers, however: iodide will cost more speed (from disabling surface image centers) than it will gain from revealing internal latent image centers.
2. If you mix a Silver Thiosulfate complex with larger than trivial amounts of iodide, you will precipitate Silver Iodide. If you use oxalate, even trivial amounts of iodide will do that.
3. The darkroom cookbook (hopefully) did not advertise iodide as development accelerator. I am not aware of any developer, which would use iodide to boost development. Some patents suggested addition of some iodide to speed up fixers, but this could not be verified by practical results.
4. There are two groups of compounds, which can enable compounds like HQ or Ascorbate to act as active developers: primary development agents (i.e. development agents which adsorb to silver halide), and some tensidic compounds (e.g. N-Dodecyl-Pyridinium). These are have been thoroughly researched, and I see little chance of breakthrough discoveries on that front.

 

[laingsoft]

Interesting about the Iodide. I didn't consider the precipitation of silver iodide. You were also right about the darkroom cookbook, after looking at the chapter heading it appears that I was looking at the fixer chapter, any other mention of KI is as a restrainer.

Could you take advantage of the silver iodide precipitation to decrease the sludging in a monobath? If the silver were to precipitate out, recycling the monobath would just be a matter of decanting, right?

As for the tensidic compounds, google is telling me these are surfactants? If that's the case, then would I be correct in assuming that one end of the molecule adsorbs to the surface of the silver halide, and the developers act on the other tail?

I'm not hoping for any breakthroughs, just understanding

 

[Rudeofus]

Could you take advantage of the silver iodide precipitation to decrease the sludging in a monobath? If the silver were to precipitate out, recycling the monobath would just be a matter of decanting, right?

If you saturate a mono bath with iodide to make all silver precipitate out as Silver Iodide, then you have created the exact condition, where no more silver will be fixed by that mono bath.

As for the tensidic compounds, google is telling me these are surfactants? If that's the case, then would I be correct in assuming that one end of the molecule adsorbs to the surface of the silver halide, and the developers act on the other tail?

I have looked at these surfactants, there are some articles about them in conjunction with photographic development, but I have not really understood their mode of operation. I have done some tests with N-Dodecyl-Pyridinium and Hydroquinone Monosulfonate, and they do indeed facilitate photographic development (i.e. N-DP + HQMS develops much faster than N-DP or HQMS on their own), however, you still end up with a foggy speed losing developer. The only practically used developer I have seen using surfactants is IMHO Caffenol and its variants. Coffee contains some compounds which fit the pattern, and this theory would also explain why addition of ascorbate speeds up things.