Desalting
From Silvergrain Labs
After emulsion is precipitated and grown to a desired size, the emulsion usually contains too much salts, such as free halide and nitrate ions. Except for some chloride emulsions, the halide concentration must be reduced to less than 0.1mM. The process of removing excess halide ions is called desalting.
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Noodle washing method
The most primitive method of desalting is to set emulsion to a stiff jelly, and cut it out to noodle shape, or sometimes cubes. Then these noodles are immersed in cold water with good agitation and several replacement of water.
The washing water is often added with magnesium sulfate or calcium sulfate to minimize gelatin swelling, which can decrease washing efficiency and also dilutes emulsion.
This method does not require sophisticated chemistry, but the process is labor intensive and time consuming.
Sedimentation method
Sedimentation method takes a different approach. Gelatin is made hydrophobic, so that the silver halide crystals and gelatin are separated from water phase together. The supernatant is removed, and sediment is washed in water twice. The washed coagulum is redispersed to make desalted emulsion.
Some coagulating agents are more effective than others in sedimentation. However, this is not necessarily good for emulsion making, because redispersing the coagulum in the next step becomes more difficult. There is a trade off among the compounds used to modify the gelatin, ease of sedimentation process, and ease of redispersion process. The optimal trade-off depends hugely on the kind of gelatin being used. The optimal balance among these factors is one area of know-how that is never described in adequate details in academic or patent literature.
There are two major methods in making gelatin separate from water phase temporarily.
acylated gelatin method
This method uses acylated gelatin, such as phthalated gelatin, and adjust the solution pH to about 4, so that carboxyl groups are not dissociated. Since amino groups are blocked with hydrophobic groups by acylation, the acylated gelatin is insoluble at this pH.
polymeric sulfonic acid method
This method may be used with unmodified gelatin, but requires addition of soluble, ionizable polymeric sulfonic acid into the emulsion, so that the added sulfonic acids block amino groups. The emulsion pH is adjusted to about 4 to precipitate the gelatin in the same way as acylated gelatin method above.
The advantage of this technique over acylated gelatin technique is that, the normal gelatin can be used for nucleation phase of the emulsion, and then the same gelatin can be used for desalting process, with a simple addition of the suitable agent to modify gelatin. Selection of the gelatin for the nucleation process is extremely important, because this has influence on the crystal habit (e.g., cubic, octahedral, tabular) and emulsion fog. Gelatin can be acylated after performing nucleation step in a conventional gelatin, but this process requires the pH be raised to 10 for 5-10 minutes. This can be a serious disadvantage in many high speed emulsions, since the fog may be elevated if acylation is performed at the conclusion of growth stage and the emulsion is gold sensitized. On the other hand the acylation-in-emulsion can be useful when very mild reduction sensitization is desired at that particular stage of grain growth. Most coagulants used for desalting, such as those described below, can react with gelatin much faster than acylation reaction, and without the need to elevate the pH to 10.
Classic choice of such polymer is poly(stylene-co-stylene sulfonic acid) of molecular weight about 2000. These molecules are synthesized from stylene and stylene sulfonic acid in suitable solvent (such as toluene or acetone) by radical polymerization method. Radical scavenger is added to the mixture in solvent, and the mixture is stirred while radical initiator (such as AIBN) is added. To control the average molecular weight, concentrations, agitation, and temperature must be tightly controlled. It is not a very easy operation to perform in typical darkroom.
Instead of polystylene sulfonic acid, 2-naphthalene sulfonic acids may be used. Sodium salt of 2-naphthalene sulfonic acid and alkyl naphthalene sulfonic acid are commercially available as dispersants and specialty surfactants, and they may also be used. Examples include: Demol N (Kao), Darvan 1, Tamol SN (Roam and Haas), Daxad 11.
Some of these compounds make gelatin extremely hydrophobic and easy to separate from the water phase. However, the difficulty awaits in the redispersion stage.
Dialysis method
Emulsion may be desalted by placing the emulsion in dialysis bag, and immersing it in washing water. The salt is gradually removed to the wash water due to osmolarity, but the process is rather slow and inefficient, compared to the sedimentation method.
Ultrafiltration method
