The preparation of photographic emulsions begins with the formulation of a dispersion of microcrystals of silver halide in a protective dispersing medium. Subsequent to or concurrent with the formation of these microcrystals, a silver halide solvent is introduced to permit dissolution, recrystallization, and growth of individual silver halide particles to a desired crystal (grain) size. This process is known as physical ripening and is typically carried out to increase the size of the silver halide crystals, because photographic sensitivity increases with increasing grain size. A wide variety of chemical substances function as solvents for silver halides; many are listed in T. H. James, ed., The Theory of the Photographic Process, 4th ed., Macmillan, New York, 1977, p. 9. Silver halide solvents are also known as Ostwald ripeners, ripening agents, crystal growth modifiers, fixing agents, and growth accelerators.
In addition to enhancing silver halide crystal size, recrystallization reactions by ripening agents at apparently fixed crystal dimensions are also known to modify silver halide morphology, to alter the concentration of crystal defects and to promote the incorporation in the silver halide crystal lattice of sensitizing species such as silver or silver sulfide clusters. These ripener-induced changes tend to increase the photographic sensitivity of silver halide emulsions, and since all these changes involve recrystallization phenomena which also participate in silver halide growth, these phenomena are included hereafter in the discussion and claim regarding silver halide growth.
Among the substances reported to be effective ripening agents are excess halide ion and ammonia, as described in G. F. Duffin, Photographic Emulsion Chemistry, Focal Press Ltd., London, 1966, pp. 60-62, and thiocyanate ion, as disclosed in U.S. Pat. No. 3,320,069 to Illingsworth. Many organic compounds have also been reported to function as ripeners. For example, U.S. Pat. Nos. 3,271,157 to McBride and 3,574,628 to Jones disclose the use of thioether compounds as ripening agents for silver halide photographic materials. U.S. Pat. No. 4,782,013 to Herz et al. discloses the use of macrocyclic ether compounds containing oxygen, sulfur, and selenium atoms for this purpose.
Silver halide solvents or ripening agents are generally ligands for Ag.sup.+ ions that combine with Ag.sup.+ ions to form soluble Ag.sup.+ adducts or complex ions. Although ripening agents are very useful for controlling the size, dispersity, and morphology of silver halide grains and for determining the location of specific halide components in mixed silver halide compositions, they also cause problems in emulsions during keeping or storage. Specifically, ripeners that are retained in an emulsion after formation and growth of the silver halide grains can change the rates of chemical sensitization, interfere with spectral sensitization, and promote fog formation during storage of emulsions, particularly those coated on a support.
To avoid these undesirable effects, efforts have been made to remove organic ripeners from emulsions after formation and growth of silver halide grains by purification procedures such as washing. However these ripening agents cannot be completely removed from emulsions even by extensive wash procedures, most likely because of their relatively low aqueous solubility and their affinity for silver halide. U.S. Pat. No. 4,665,017 to Mifune et al., proposes to circumvent this difficulty by deactivating residual ripeners through an oxidation process. This approach, however, has the disadvantage that gelatin in the emulsion also undergoes irreversible changes on oxidation. Furthermore, some ripening agents, e.g., thiourea compounds, upon oxidation yield products of increased activity with respect to desensitization and fog formation.
Another approach to countering the undesirable effect of residual silver halide solvent is the addition of emulsion stabilizers and antifoggants. However, such additives tend to interfere with spectral sensitization and can lead to loss of emulsion sensitivity.
Organic silver halide solvents or ripening agents can be classified into two types: neutral and acid-substituted. A neutral ripening agent is a compound which either is uncharged or carries an equal number of positive and negative ionic charges, i.e., a zwitterionic compound. An acid-substituted ripening agent is a compound that incorporates a covalently bonded acidic function which, upon deprotonation at about pH 7 or below, confers a negative charge on the molecule. These two classes of ripening agents are exemplified by the neutral compound ethanolamine and its acid-substituted analog, glycine. Both compounds yield Ag.sup.+ complexes of similar stability and are capable of ripening AgBr emulsions. However in dilute alkaline solution, where its acidic function is deprotonated, glycine dissolves AgBr much more slowly than does the neutral ethanolamine (D. Shiao, L. Fortmiller, and A. Herz, J. Phys. Chem., 1975, 79, 816).
Similarly, U.S. Pat. No. 4,749,646 to Herz et al. discloses that N,N,N',N'-tetramethylthiourea accelerates silver halide grain growth, as measured by equivalent circular diameter, more than its N,N'-dicarboxymethyl-N,N'-dimethylsubstituted analog. On the other hand, the high level of storage fog induced by tetramethylthiourea is somewhat diminished when it is replaced by its N,N'-dicarboxyethyl-N,N'-dimethyl analog.
U.S. Pat. Nos. 4,695,535 to Bryan, et al., and 4,865,965 to Friour et al., also disclose acid-substituted ripening agents. The ripeners disclosed in U.S. Pat. No. 4,695,535 are acyclic thioether compounds containing carboxy substituents; the acid-substituted ripening agents disclosed in U.S. Pat. No. 4,865,965 are cyclic ethers.
The cited art makes it apparent that, when coated under a conventional condition at pH values above about 4.6, acid-substituted ripeners interfere less with dye sensitization and cause less storage fog than their neutral analogs. However, under such pH conditions the acid-substituted ripeners exist substantially in their anionic state and often suffer from the distinct disadvantage of exhibiting low activities as accelerators of silver halide growth. Hence, it is the major purpose of the present invention to overcome this barrier for the convenient application of acid-substituted ripeners in photographic systems as useful promoters of silver halide dissolution, recrystallization and growth by using them in combination with a relatively low level of a neutral organic ripener.