Do some films have 'even-ness' additive?

A couple years ago I did some tests where I photographed a light gray card on a dark gray card on various films ,stand developed in Rodinal 1:200 90m 68F and made a 10x enlargement of the edge.
Some films showed a noticeable light line about 1mm wide on the print on the gray side at the edge with the black.These films were: Tri-X, HP5, Plus-X, Adox/Efke 100, Adox/Efke 25.
A second group of films showed little or very little light line, they were more even. These were: T-Max 100, Delta 100, Acros, Delta 400, Pan-F, FP4.
Is it possible that the emulsions in the second group may contain some even-ness additive or is there some other explanation?This information may be proprietary but comment appreciated, thanks.

It is probably due to the edge effects in the emulsion itself. This is usually due to iodide. It is most pronounced in very dilute developers. It can also be due to developer exhaustion along an edge or by bromide effects. Usually, the bromide 'drag' can be seen around the sprocket holes of 35mm film as well as in image areas.

PE

"My film isn't even"
"That's odd"

Thanks P.E.
Noting that Plus-X showed the edge effect and T-Max 100 very little, is the possible explanation that iodide in T-Max 100 inhibits development? IIRC I have read these newer films have iodide in the emulsion.

Alan;

They all contain iodide. It would be hard to speculate without seeing examples and running some tests.

PE

There is some comment that T-Max and Delta probably have more iodide:
http://www.groups.google.com/group/rec.photo.darkroom/msg/708a39c939a6c51c
Iodide seems a candidate.

I have read material by Knoppow before. He is right as often as he is wrong. One example in that reference of yours is that he says that films have up to 50% iodide. Well, geenrally the high figure is 10% or less, not 50%. The second thing is that iodide often inhibits sensitization rather than speeding it up. Thirdly, it can increase fog levels if not controlled properly or if used at high levels due to dislocations in grains.

Also, some very fine grained fast film use iodide, contrary to his assertion but he is right that some slow fine grain print films do not use iodide.

I had better stop there.

PE