Biocides
From Silvergrain Research
Biocide is used in photography, mostly to protect gelatin and other binders from bacterial and fungal attack. Attack from microbes is harmful because bacteria and fungi secrete enzymes that break down gelatin and other polymer molecules to change its chemical, rheological and mechanical properties. Of course, microbiological deterioration of the polymer macromolecules often accompany foul odor and other unplesant experience to be avoided by effective use of biocides.
Another place where biocide is commonly used is washing water used in automatic processing machines (minilab machines). Minilab machines recycle washing water for a long time by removing thiosulfate and other chemical contaminants by ion exchange resin or reverse osmosis membrane. Bacterial growth in this washing water is a very common problem faced by minilab machine engineers. Therefore, almost all minilab manufacturers use antiseptic agent in the washing water. This is, of course, unnecessary if the film or print is washed in fresh running water, as in ordinary darkroom.
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Bacteria, yeasts and fungi
There are four major classes of microbes:
- Gram positive bacteria
- Gram negative bacteria
- Yeasts
- Molds and fungi
No biocide agent is perfect. For example, one agent may be effective against both bacteria but not against fungi. Many agents are effective only in certain pH zones (e.g., in alkaline or acidic condition). Therefore, proper choice of biocide depends on the type of the medium, pH, surfactant used in the solution, etc. When high efficacy of disinfection is required, it often happens that two or more biocides are preferrably used in combination.
Note also that the term bactericide or biocide is used to indicate agents that are effective in killing the microorganisms, but it does not mean that the agent is capable of killing ALL or almost all of them. For the latter meaning, the term sterilizing agent is used. Sterilization is a much more rigorous process than disinfection or antisepticizing. The term bacteriostat is used for agents that suppress further growth of bacteria, although the agent may not be effective in killing them. Many biocides posses this bacteriostat property when used in smaller concentration, and this may be sufficient for preservation of photographic emulsions, photographic gelatin, and various other binders such as gum arabic.
Biocide agents
Recommendation for photographic applications: 2-phenylphenol, thymol, methyl paraben.
Second recommendation: isothiazolones, p-chloro-m-cresol.
Phenol
Phenol is a classic germicide but not very effective against spores and hydrophilic viruses. More effective in weakly acidic than alkaline solution.
Thymol
(5-methyl-2-(1-methylethyl)phenol, 2-isopropyl-5-methylphenol, 6-isopropyl-m-cresol, etc.)
benzoic acid
A common preservative for highly acidic food and beverages (carbonated soda, fruit juice, sauerkraut, pickles). Its effective spiecie is undissociated form, and so this agent is most effective in the pH range from 2.5 to 4.0. The most common commercial form of benzoic acid is sodium benzoate, but because of the reason above, the benzoate must be used in highly acidic medium to be effective. It is most effective against bacteria, then yeasts, and least effective against molds.
methyl paraben
Methyl paraben is also called methyl-p-hydroxybenzoate, Nipagin M (Nipa Laboratories, UK), Tegosept M (Thos. Goldschmidt, FRG), Solbrol, etc.
A common preservative, effective against molds and yeasts, somewhat effective against gram positive bacteria in acidic condition (pH below 7). Methyl paraben is effective in a wider window of pH compared to benzoic acid, which requires strongly acidic condition to be effective against gram negative and gram positive bacteria. Methyl paraben is my preferred choice for acidic solutions.
Because methyl paraben is relatively nontoxic, stable at baking or sterilization temperature, and it is effective in acid to neutral pH, methyl paraben is commonly used in baked goods, beer, olives, pickles, syrups, fruit preserves, soft drinks, etc.
The alkyl group can be increased to make propyl paraben, heptyl paraben, etc. As the alkyl group is made longer, the compound becomes more potent, but also less water-soluble. Methyl paraben is least expensive, most water-soluble and most common.
p-chloro-m-cresol
Effective antiseptic, once very popular, still used, but slowly being replaced by other options. Used in adhesives, latex, etc.
2-phenylphenol
Relatively nontoxic to mammals (used as postharvest treatment of citrus fruits). Very effective against bacteria and fungi in alkaline condition. This is my most preferred biocide, whenever the solution is alkaline. Used at the level of 1% of gelatin weight or 0.05% of the solution volume. Common form is sodium salt, sodium 2-phenylphenolate, sold as Preventol ON Extra Flakes (Bayer) or Dowcide A (Dow). Used in many applications, including: adhesives, paints, textiles, leather, metalworking fluids, cooling waters, caulks, construction products.
Kremer Pigmente (Germany and New York City) sells sodium 2-phenylphenolate as "Preventol ON Extra Flakes" in 100g bags (Kremer English top page). In my experience, their web order form maybe buggy but if you call them, they ship products promptly. Product list for the US market (see near the bottom) They also provide some information on this agent. German description of the product
4-tert-amylphenol
Extremely effective against gram-positive bacteria. Since 2-phenylphenol is not as effective against gram-positive bacteria, these two agents are often used in combination to broaden the spectrum.
isothiazolones (isothiazolinones)
Commonly used for antiseptic of processing/washing water and emulsion additives, as well as many personal care products (lotions, shampoos, etc.) due to excellent efficacy, broad spectrum and low toxicity.
Usual usage of isothiazolinone biocide in aquaous media is to blend 1 part of 2-methyl-4-isothiazolin-3-one and 3 parts of 5-chloro-2-methyl-4-isothiazolin-3-one. Sometimes, 4,5-dichloro derivative is used. For nonaquaous media, alkyl substitution at the 2-position is made longer, such as in 4,5-dichloro-2-(n-octyl)-4-isothiazolin-3-one.
Weakness of this type of agents is susceptibility of nucleophilic attack on the isothiazolone ring. In particular, amines (particularly secondary amines at alkaline pH), sulfites, some other reducing agents, thiols, etc. can be deleterious to isothiazolones. Isothiazolones are also unstable at higher temperature. A combined use of magnesium salt is a common technique to alleviae some of the weakness in personal care products, but this is not desirable in photographic applications.
Major producer is Roam and Haas.
glutaraldehyde
Glutaraldehyde is a very effective biocide with wide spectrum at pH of 8, but because glut can crosslink many macromolecules having amino group residues, it is used as a hardener and not as a biocide in photography.
2-propanol
2-propanol or isopropyl alcohol is a decent biocide at 50% or higher concentration. Because of this and its very volatile nature, propanol is used as a disinfectant for surface and skin, but it is hard to utilize in photographic applications.
mercury compounds
There are many useful mercury compounds, but because of toxicological and environmental reasons, they are no longer used except for a few specialized applications, such as contact lens solutions, multiple-use vials, etc. In particular, mercury compounds have not been used in photography for many years.
Most common preservative containing mercury is thimerothal, a.k.a. Merthiolate Sodium (Eli Lilly, USA).
Bacteriostat agents
Bacteriostats are usually not as strong as bactericide in killing bacteria and spores, but they are effective in suppressing further growth of the bacteria.
salicylic acid
Salicylic acid has useful bactericide and bacteriostat function, but its use is not permitted for food related applications. For non-food uses, other agents such as isothiazolinones, phenylphenol, etc. are more common.
layryl dimethylbenzyl ammonium
Zephiran, 0.05ml per liter
