Acylated gelatin
From Silvergrain Research
Acylated gelatin is a collective name of modified gelatin, where amino group of lysin is blocked by acylation. Most common of this type are phthalated gelatin and trimellitiated gelatin. These modified gelatins are made for several applications.
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Applications
Desalting of gelatin
Since acylated gelatin has no ionizable amine group, the gelatin becomes hydrophobic at about pH of 3.2 to 4.5, and gelatin separates from the aquaous phase. This can be used to remove soluble salts from silver-gelatin emulsion (called precipitation method or sedimentation method), because silver halide crystals are precipitated together with the gelatin molecules.
Making of tabular grain emulsion
Tabular grain emulsions are prepared with minimal of gelatin interaction with the AgX crystal surface. That is, the most desirable kind of gelatin has minimum of the functional groups that adsorbs on AgX surface. This is traditionally achieved by gelatin containing very small mole fraction of methionine. This is made by oxidizing gelatin to destroy -S- (thioether) group of methionine, as described in numerous patents given to Maskasky and others at Eastman Kodak. Subsequently, Fujifilm researchers had good luck with making high quality tabular grain emulsions by using phthalated gelatin. Of course, methionine is intact in this type of gelatin (unless there is a separate oxidation stage), but amine is blocked. Amino group also has strong adsorption to AgX surface, and therefore acylated gelatin is superior for preparing tabular grain emulsions.
When acylated gelatin is used for tabular grain preparation, it is most preferred to use small molecular weight gelatin of low viscosity. This is because high quality crystals are prepared at low temperature (25-40C) and small MW gelatin provides lowest gelation temperature and is best to minimize solution viscosity at this low temperature range.
Improvement of adhesion
Phthalated gelatin and, in particular, trimellitiated gelatin have significantly stronger adhesion to hydrophobic substrate such as glass, cellulose film, latex-subbed plastic film, etc. Therefore, these gelatins are advantageously incorporated in the subbing layer, emulsion layer or both to increase adhesion.
Preparation
Reaction
The reaction is a simple nucleophilic acyl substitution reaction. (See also: Amine Reactivity) Nucleophilic primary (and secondary) amines of gelatin molecule are reacted with electrophilic acid anhydrides, such as phthalic anhydride, at alkaline pH. Alkyl halide, sulfonate, etc. are also electrophilic and may be used, but phthalic anhydride and trimellitic anhydride (1,2 anhydride of 1,2,4-benzenetricarboxylic acid) are most commonly used.
It is generally thought that the 4-aminobutyl side chain of lysine is involved in acylation of gelatin. This is the same target as crosslinking of gelatin by common hardeners, such as glutaraldehyde and mucochloric acid.
Procedure
Modifying a gelatin by acylation is simple:
- In a reaction vessel (kettle), 100g of gelatin is soaked in 1.0 liter of deionized water at room temperature, with constant agitation.
- The kettle is fitted with a termometer and a pH probe for continuous monitoring of these parameters, and the solution is continuously stirred until the completion of the reaction.
- The temperature is gradually raised to 40C to completely dissolve the gelatin.
- From a jet apparatus, sufficient quantity of 2.5M NaOH (10% sodium hydroxide) is dropped to the gelatin solution to maintain the pH between 9.5 and 10.0.
- Added to the kettle is a 10g of phthalic anhydride completely dissolved in about 80ml of acetone.
- The pH will gradually drop, but necessary amount of 2.5M NaOH should be occasionally added to maintain the pH in 9.5 to 10 range for 10 minutes, again at 40C.
The reaction is complete, and more than 95% of the amino group is blocked. This gelatin can be purified by usual precipitation method by bringing the pH to 3.5 with sulfuric acid. The supernatant is decanted off, and the solid precipitate washed in deionided water. The resulting gelatin may be dried to make gelatin granules or sheets.
Hardening
Acylated gelatins do not have free amino groups available for crosslinking reactions by common hardeners. Those nitrogen atoms are now in the form of double amide bonds (very strong). However, carboxyl groups are still available for crosslinking by suitable agents, such as bisepoxides and chrome alum.
Acylated gelatin is rarely used alone when hardening is required. For example, emulsion may be prepared in phthalated gelatin, but regular gelatin of high molecular weight is added after desalting or after digestion to improve the mechanical property of the emulsion, such as wet membrane strength. In this case, common hardeners like s-triazines, glutaraldehyde, vinylsulfones, etc. may be used, although, most preferrably, an adjunctive carboxyl-reactive crosslinking agent is used.
