The placement of Iodide in emulsions

[Photo Engineer]

Fuji did some work on this IIRC but it really was only useful in the slower ranges AFAIK.

PE

 

[kb3lms]

Many of the formulas we see start off with Iodide in the kettle. Since this traps the iodide at the core (because it precipitates first), why is this done? It seems to me that the effects of iodide would be more pronounced in that it would produce more dislocations if it were added throughout precipitation. Or, add iodide near the end of precipitation, thereby making a shell of AgBrI over an AgBr core. Is this something that is done in modern emulsions?

 

[Photo Engineer]

Well, if you run Iodide, Bromide and Silver, you get a gradual progression to AgBr and thus make a core shell which you can treat as you wish. Or, you can make high Iodide emulsions by just running Silver into this mixture with the right halide.

PE

 

[thio]

afaik the modern tabular grain emulsions consists of core-shell microcrystals.

 

[guangong]

PE, could you relate your comments on iodine to Bergger panchro 400 film?

 

[Photo Engineer]

afaik the modern tabular grain emulsions consists of core-shell microcrystals.

The ones I saw were graded Iodide with discrete divisions. I have seen smooth gradation as well.

PE

 

[Photo Engineer]

PE, could you relate your comments on iodine to Bergger panchro 400 film?

No, as I have no means to analyze emulsions now. I could have done it years ago, but even then we at Kodak only examined products from the major manufacturers.

PE

 

[kb3lms]

I read some information last night in a Fuji patent from 1987 where they made a high iodide core (up to 45 mol %) and a low iodide shell (about 5 mol%) with the aim being an average iodide of 7%. The theory they gave in this patent was that "bending" (I think they meant crystal dislocations) at the core/shell interface would contribute to high sensitivity because of the high iodide and the low iodide shell would provide better development. Then "holes" created as photons knocked electrons into the conduction band would migrate into the high iodide core leaving the electrons in the better developing, low iodide shell to reduce Ag ions. I guess that by having the holes move into the core that recombination was discouraged which would contribute to sensitivity.

Light absorption increases in the order of AgCl, AgBr, AgI but development activity increases in the order AgI, AgBr, AgCl.

The Fuji patent is US 4,668,614. They seemed to feel that this was all a novel idea so maybe this was one of the 1st core/shell patents. Didn't really discuss prior art other than research work by Klien and Moisar.

Later work talked more about graded and multiple shells.

-- Jason

 

[Photo Engineer]

Jason, I have a formula for a 9 - 10% core shell emulsion. I was told by a prominent APUG member that I was out of my mind. Well, these are common. There are several by Konishiroku as well as Fuji. You can start with Iodide or dump it later on. Either can work, but give different results.

This type of formula was used in Kodak 800 speed film.

PE

 

[kb3lms]

Thanks, PE. Is that 9% core/shell formula the one you spoke about in "A very high speed formula" some years back?

If the theory put out by Fuji in the above patent was correct and not disproved later, I could see why the core/shell idea would get a formula to higher speeds. T. Tani, in his last book, presents the idea that increases in film speed (sensitivity) really come from preventing recombination.

Core/Shell is now on my list of things to try. If one can use pumps (ie a syringe pump) to control the silver and halide flow, it seems that it should be possible to control iodide placement and make core/shell emulsions. I have a syringe pump set up I have been working at but it's not quite ready yet. The pump works with very nice control. I want to build a second pump and then complete some programming.

 

[Photo Engineer]

Tadeki Tani has the right idea. I've actually met him several times and had dinner with him and his wife. At Kodak we had several theories about what is possibly going on here. Of course things are quite secret in this area and chemical sensitization plays a large part in this. For example, many use Steigman's solution, but a simple modification can give a huge speed gain, and the best speed gain is seen by using Selenium compounds, not that I suggest anything of the sort for the home user. Chemical sensitization can thus alter the picture totally.

PE

 

[thio]

PE,

What do you know about the 'second iodide' for emulsion stabilization (physical ripening inhibition)? As example see PAGI gaslight emulsion.
And what do you know about iodate as emulsion (chlorobromide) stabilizer? I read that only in the Steigmann paper (Brit. J. Phot. 1946).

thanks to Denise for this paper!

 

[kb3lms]

+1, thanks for the paper Denise!

 

[Photo Engineer]

Well, the Steigmann paper looks similar to the Kodak Azo formula. The Agfa formula (Lupex) used Iodide instead of Bromide.

As for this second Iodide, it sometimes adds speed by broadening blue sensitization or it can adsorb to the grains with the most surface area (finer grains) and thus increase contrast. Is that what you are looking for Thio? Oh, BTW, Bromide can have a similar effect.

PE