Clarifying gelatin swell
In this post, I will attempt to clarify some items with respect to gelatin swell as governed by isoelectric point and hardening.
So that I don't have to repost the graph that gives the profile of swell vs pH I hope you all avail yourselves of that first in a previous thread.
Ok, now, lets consider gelatin to being like a coil spring. Gelatin can compress and extend based on pH, temperature and the like.
Let us assume that each coil is 1 cm wide and 1 cm apart when there is no compression. Each coil is thus 2 cm apart when no compression is applied. Lets then clip on a metal clip 6 cm long across coils. This will be the 3rd coil attached to the first coil. Now, let us take a second spring and compress it fully and attach a similar clip. It will now be the 1st coil attached to the 6th coil.
Let this analogy represent gelatin at different conditions. Gelatin can be many thicknesses depending on pH, temperature or hardness or method of hardening.
Let me explain. At the normal isoelectric point of bone gelatin, the gelatin is more tightly compressed and therefore hardening leaves less space between the coils so to speak. In an alkaline solution, there is more distance between the coils and the hardening takes on a different form.
The example here might be an unhardened film which is hardened in a developer (high pH and maximum swell) vs a film hardened in an acid hardening fix (low pH and minimum swell). Not only is the gelatin different in internal form, but swell is different even for identical gelatins.
Now, consider the fact that just as the metal clip above could be 1 cm or 12 cm instead of 6 cm, we can have different hardeners. Formalin (1 carbon) forms a smaller gap in hardened gelatin than glyoxal (2 carbons) vs succinaldehyde (4 carbons) and glutaraldehyd (5 carbons). Each gives a progressively more 'relaxed' bonding between gelatins.
Now lets examine hardening developers. Hardening here is done with larger molecules that are more rigid than the aldehydes above. These are developing agents which are doing the crosslinking of gelatin. This is what happens in matrix and pan matrix film for example. An alkaline swollen film is hardened by a large sized molecule. Hardness can therefore only be loosely compared with other hardeners or hardening done at other pH values.
Of course, metal salts such as chrome alum and the like can further confuse this picture as they harden by other means than a direct chemical bond like an aldehyde or a hardening developing agent.
With all of this complexity, it is no wonder why everyone argues about how hard, how swollen or how brittle a given film or process yields. In effect, a given gelatin maintains the same profile of swell vs pH regardless of hardening, but the amount of swell varies considerably based on hardness.
To take this a bit further, the amount of swell influences the ability of developer, fixer and silver halide complexes throughout the coating. The diffusion of acid or base is much faster but still is not instantaneous. But, the bottom line is that different developer and fixer formulas affect the rate of diffusion of chemistry and the rate of washing as well.
As a base measure, try dipping a strip of film into a strong fixer and watch the rate of clearing from the back. With any fixer, it takes an average of about 15 - 20 seconds to see any change on the back. You will see the same if you dip a strip of exposed film into developer. Watch the blackening of the silver on the front and the back. The difference is due to diffusion of the chemistry and this is based on thickness and swell, as well as the emulsion type.
There are agents that can decrease swell and agents that can increase swell. For example, sodium sulfate will decrease swell while sodium hydroxide will increase swell. One is due to salt effects and the other is due to pH effects, so these are not good chemicals to test theories with, merely to give some examples of chemicals that change swell and why they do it.
This applies to hardening in both processing and in coating.
Hope this helps explain why there are so many opinions and results out there.