6/<c/> (1/jj.m)
Figure 5.7. Coupling reactivity as a function of sur-face-to-volume ratio (proportional to the specific surface area) for the coupler M prepared as an NS dispersion. This coupling reactivity is expressed as PCZA in units of g CZA per litre. Both the coupler M and CZA (citrazinic acid) are four-equivalent.
The specific surface-to-volume ratio for monodisperse spheres is 6/d. The average diameters < d > for the various dispersions were measured by disc centrifugation with turbidity detection.
(Data by courtesy of Dr Andrew Sierakowski)
modelled in homogeneous physical organic chemistry.
However, another important contribution to coupling reactivity is the specific surface area of the coupler dis-persion particles. Thus, for certain classes of couplers, the specific surface area increases with decreasing par-ticle diameter, while the coupling reactivity increases as the particle size decreases. Some example data are illustrated in Figure 5.7, where decreasing size yields increased reactivities for "NS" dispersions (i.e. those formulated without (coupler) solvents - see Section 4.1 below) of the coupler M:
in Figure 5.7 of the coupler M dispersions, PCZA, is defined (8) in terms of the amount of added competi-tor, in this case CZA (citrazinic acid), that is required in the developer in order to decrease the molar dye yield by a factor of 50% over that obtained in the absence of added competitor. Both the coupler M and CZA are four-equivalent with respect to their utilization of QDI, so in this case ex = ec = 4. The linearity illustrated in Figure 5.7 shows that interfacial (specific surface) area (particle size) is a kinetically limiting parameter in the heterogeneous coupling of M.
3 PARTICLES AND COLLOIDS IN
on reagent addition in many different types of reactors.
Supersaturation is induced in such solutions when the respective solubility product is exceeded, where the solubility product
^ s p , A g X = ^ A g + ^ X
-in which aAg+ and ax- are the activities of silver ion and halide ion, respectively (cf. Table 5.2 below).
These activities can usually be adequately approxi-mated by concentrations, and when required, augmented with activity coefficients. Nucleation is an important, but as yet incompletely characterized process for sil-ver halides. Homogeneous nucleation from reagent ions and complexes is an important colloidal concept that balances the free energy of silver halide condensation and phase formation with the unfavourable surface free energy of growing nuclei that must be overcome to sustain growth. A very useful review of the balance of nucleation and growth processes has been provided by Leubner (9). Seeded nucleation, a concerted form of heterogeneous nucleation, is an important practical approach in many processes. Typically, nanoparticulate silver halide crystallites, e.g. AgBr, AgI or Ag(BrJ), are added to the reactor and used as growth centres to grow particular types of silver halides. Such approaches are particularly useful for growing core-shell types of particles, wherein the core and shell regions have sub-stantially different halide compositions. Such core-shell approaches are also useful for constructing crystallites that will promote internal latent image sites, wherein defects and internal fog centres can easily be created on the surface of grains of a given size, and then cov-ered with a silver halide shell. Such core-shell par-ticles are used in certain reversal or direct positive applications.
A key to the control of crystal habit is in controlling the electrochemical potential by maintaining a constant halide (alternatively, silver ion) concentration during growth. This is usually achieved by using feedback potential control in batch reactors or by maintaining a given reagent flow rate in continuous reactors.
For the case of AgBr, one can vary the habit from cubic to octahedral to tabular by simply adjusting the excess bromide concentration. This concentration controls how the silver ion in solution is complexed or coordinated. At very low excess bromide levels, the silver ion is tetrahedrally aquated as Ag(H2O)4+ (10).
As the bromide concentration increases, AgBr(H2O)S, AgBr2(H2O)2", AgBr2(H2O)"2 and AgBr4"3 complexes compete for silver ion complexation. The interactions of these complexes with the various AgBr crystalline facets dictates the growth rates of such facets, while
the competition in growth rates of the different facets determines the resulting habit. Facets that grow more quickly tend to disappear, and those facets we see are those that grew more slowly. At bromide concentrations less than 10~3 M, primarily cubic AgBr is obtained (or less than 0.1 M chloride in the case of AgCl). The AgBr(H2O)3 and Ag(H2O)4+ complexes predominate under these conditions. As bromide is increased and the AgBr2(H2O)"2 complex becomes competitive in concentration with AgBr(H2O)3, the octahedral habit predominates. At bromide concentrations greater than 10"2 M, tabular crystals of AgBr predominate. Under these high halide conditions, the largest exposed surfaces are (111) surfaces and are essentially covered with hexagonally close packed halide. More rapid growth occurs parallel to these planes, along one or more (111) twin planes.
The solubility of silver halides varies markedly because of the common ion effect and because of the formation of various mononuclear complexes, such as those articulated above for AgBr. The solubility of AgBr as a function of pAg (— log[Ag+]) is illustrated in Figure 5.8. Along the right side of the parabolic sol-ubility curves, excess bromide provides ever increas-ing solubilization from the silver bromide complexes articulated above. On the left-hand side of these parabo-las where there is excess free silver ion, multinuclear (positively charged) silver complexes provide a mecha-nism for increasing solubilization. These solubility vari-ations with pAg have also repeatedly been observed
PAg
Figure 5.8. AgBr solubility (M) in water as a function of pAg at various temperatures. (Date by courtesy of Dr Jacob Cohen)
Solubility [M)
for silver halide growth rates, because Ostwald ripen-ing is a very important mechanism of growth. Ostwald ripening is a growth mechanism wherein larger crys-tallites grow at the expense of smaller cryscrys-tallites. The greater solubility of the smaller crystallites results in the mass of these smaller particles being redeposited on larger particles having a larger mean radius of curvature.
Transport of material is generally ionic or molecular, and anything that increases the continuous-phase sol-ubility of the condensing materials will increase the growth rate.
In addition to the surface chemical effects of intrinsic components such as these complexes, growth restraint can be effected by many kinds of specifically and non-specifically adsorbing species. Organic ligands that tend to form stable and soluble complexes with silver ion, will generally act to promote Ostwald ripening rates.
Sulfur-containing ligands generally complex silver ion to some extent, and this tendency makes such species surface active with respect to silver halides. The thiolate thione (1) and thioether (2) species provide, respectively, relative growth rates about 30-fold and 100-fold greater at 10~3 M bromide (pBr 3), relative to a bromide-only control, so that cubooctahedral crystallites twofold and tenfold larger, respectively, were obtained relative to the control. The thioether compound binds silver ion about four orders of magnitude more strongly than does the thiolate thione species. These size and growth rates are simple functions of the solution-phase silver ion solubility.
The crystal size of silver halides ranges from less than 10 nm to more than several microns. High speed X-ray films typically contain large crystallites, since sensitivity to X-ray radiation is intrinsically a volume effect. The crystallites used for blue light detection in colour negative films are also large, often of the order of 1 jim in diameter or larger (cf. the fast blue silver halide in Figure 5.5). Particle sizes fall to 100-300 nm for print materials, where there is more control of exposure and high speed is not an issue. Particle sizes less than 100 nm are used for microfilm products, which typically are very slow but require high spatial resolution. Finally, astronomical plates requiring the greatest spatial resolution utilize the smallest crystallites in the 8-50 nm size range. Control of crystal size distribution is often achieved by Ostwald ripening to eliminate small crystallites.
3.2 Surface charge
Many of these growth effects can be related to surface chemistry. Honig and Hengst(ll) made the important discovery that all of the silver halides are negatively charged at their equivalence points. These data are sum-marized in Table 5.2. One would expect the equivalence point to be an isoelectric point. However, all of the silver halides exhibit a preference for halide adsorption at their equivalence points. The corresponding isoelectric points (IEPs) are also given in Table 5.2. Specific adsorption studies show experimentally that nearly twice as much bromide can adsorb on octahedral (111) AgBr surfaces than on cubic (100) surfaces. This is as expected for the corresponding lattice layers, wherein bromide is hexagonally close packed in (111) layers and cubically arranged in (100) planes.
trimethylammonium pendant chains, on the other hand, tend to promote octahedral AgBr formation. A myriad of organic compounds have been found to promote one sort of habit or another. The surface-active species 3 has been found to restrain growth on (111) AgCl surfaces, and promote octahedral grain growth.
Alternatively, organic ligands and compounds that tend to form sparingly soluble silver salts will generally act as growth restrainers. It was mentioned above that the range of 10~3 to 10~2 M excess bromide (pBr 2) is a transition region from the cubic to the octahedral habit for AgBr. Within this region, other additives can be used to modify the crystal habit. The use of gelatin as a peptizing agent, at pH > 5 where most of the carboxy groups are ionized, tends to maintain the cubic or cubooctahedral habits. Cationic copolymers having
Table 5.2. Solubility products, equivalence points (EPs) and isoelectric points (IEPs) for silver halides
AgX pKsp EP(pAg) IEP (pAg)
AgCl 9.8 4.9 4.6 AgBr 12.3 6.1 5.3 AgI 16.1 8.1 5.6
The surface charge of silver halides, as well as the electrokinetic charge, may be varied by various surface-active species. For example, the effects of adsorbed gelatin on the electrokinetics of AgBr are illustrated in Figure 5.9. For both gelatin types, the same IEP is obtained on AgBr and on glass beads. Glass (silica) typically has an IEP that is very low, usually at a pH less than 3. Moreover, for the pig gelatin, an identi-cal IEP was measured by a streaming current detector on polyethylene. These measured electrophoretic mobil-ity data show that the electrokinetic charge reflects the charge state of the adsorbed gelatin, not of the under-lying solid surfaces. The zero mobility points occur at the isoelectric points of the corresponding gelatins, at about pH 4.8 for bone gelatin and at about pH 8.5 for pig gelatin.
Various charged and neutral species have been shown to modify the surface charge or the electrokinetic charge of silver halides. The adsorption of positively charged quaternary species at pBr 3 has been shown to reverse the sign of the electrophoretic mobility of cubic AgBr.
The adsorption of triazolium thiolates (4), molecules
3.3 Chemical sensitization
Most latent image sites for negative-working silver halide systems are believed to be surface sites. Sur-faces offer the highest frequency of defects and other sites that may stabilize a latent image cluster. This pre-ponderance is increased by chemical sensitization, by forming latent image precursors or an increased num-ber of surface or near-surface sites that will stabilize the latent image. Typical chemical sensitization chemistries involve treatment with sulfur, gold and reducing agents, such as hydrogen and hydrazine. Many other metals and diverse complexes and species have been investigated with large dipoles that carry no net charge, has been shown to lead to electrokinetic charge reversal by an adsorption mechanism that displaces adsorbed halide ions, and so such reversals are due to surface charge reversals (12). Such triazolium thiolates also strongly bind silver ion and are very effective growth promoters (ripening agents).
pH pH
Figure 5.9. Electrophoretic mobility effects of (a) bone and (b) pig gelatins adsorbed on cubic AgBr, glass beads and polyethylene at 24°C. Reproduced by permission from J.I. Cohen, W.L. Gardner and A.H. Herz, Advances in Chemistry Series, 145, 198-217 (1975); © 1975 by The American Chemical Society
/ uVs \ Electrophoretic mobility \y/crn) Electrophoretic mobility l^v/cm/ Streaming current (JJA)
Pig Gelatin Bone Gelatin
Cubic AgBr Glass beads
Cubic AgBr Glass beads Polyethylene
as well. The most prevalent form of chemical sensiti-zation is the use of sulfur and gold. Sulfiding, often by chemical decomposition of thiosulfate, is believed to form latent image precursors involving Ag2S and AgnSm n + n~m clusters or aggregates. Gold is often intro-duced as an auric complex (e.g. KAuCl4) or as an aurous complex (AuSCN, Au2S or as an Au-triazolium thiolate salt). It is believed that Au+ occupies Ag+ vacancies and forms mixed AuAgn latent image specks that are supe-rior catalysts to homogenous silver specks. Reduction sensitization can be achieved with a variety of reducing agents, such as with hydrogen gas, hydrazine deriva-tives and reducing gelatins, to name just a few. It is believed that such treatments produce latent image pre-cursors involving silver atoms that require only one or a couple of additional silver atoms to produce a stable latent image speck. Chemical sensitization processes, for negative-working emulsions, are usually carried out as a separate operation after crystal formation. Temperature, pH and pAg are the parameters that are typically var-ied, and often cycled, to promote surface sensitization chemistry.
3.4 Sensitizing dye adsorption
Silver halides are spectrally sensitized to visible light by adsorbed sensitizing dyes. Aspects of this electronic coupling are discussed below. A requisite for this pro-cess is to get the sensitizing dyes in a condensed state on the surfaces of the silver halide microcrystals. This process is often facilitated by surface aggregation pro-cesses. These processes are exemplified by distinguish-ing monomeric dye adsorption states from dye aggregate adsorption states. The adsorption of pseudoocyanine 5 on to AgBr surfaces is illustrated in Figure 5.10. The monomeric absorption envelope is that which is char-acteristic of the isolated dye molecule in an adsorbed state. As the solution-phase dye concentration increases, co-operativity in dye-dye interactions come into play, and a condensed-surface dye aggregate, in this case the J-aggregate, forms. This class of aggregates is char-acterized by intense long-wavelength absorption max-ima at longer wavelengths than the adsorbed monomer
Wavelength (nm)
Figure 5.10. Pseudocyanine aggregation on AgBr illustrating the adsorbed J-aggregate spectrum generated from adsorbed monomeric dye. The solution spectrum of the dye is illustrated as the dashed curve. Each spectrum for adsorbed dye is annotated with the number of millimoles of dye added per mole of AgBr. Relative reflectance (R) measurements were made on turbid suspensions relative to a control suspension containing no added dye. Increasing absorbance corresponds to a decreasing R and an increasing —logR. Adapted from A.H. Herz, Adv. Colloid Interface ScL, 8, 237-298 (1977)
absorption band. The so-called H-aggregate produces an absorption maximum at shorter wavelengths than the adsorbed monomer.
Adsorption isotherms can conveniently be measured for sensitizing dyes on silver halides. The formation of a J-aggregate indicates a close-packed condensed state that corresponds to a certain limiting area per dye molecule. Such areas are conveniently measured by performing adsorption measurements on monodisperse emulsions of a given silver halide type. Limiting areas can then be used to estimate surface areas of polydis-perse emulsions of the same halide type.
3.5 Spectral sensitization
Silver halides have intrinsic spectral sensitivities that correlate with their absorption cross-sections. However, these intrinsic sensitivities are essentially limited to the UV region for AgCl, and the blue and UV regions for AgBr and AgI. In order to make silver halides really useful over the visible spectrum, and to make them
-log R (relative)
useful for capturing the red, green or blue components of coloured scenes, spectral sensitizing dyes are adsorbed and electronically coupled to silver halide valence and conduction bands (13). Various mechanisms of spectral sensitization have been described, although no single mechanism covers the behaviour of all sensitizing dyes.
The details depend on the electronic properties, includ-ing the photoelectrochemical properties, of these dyes.
Such details affect how a particular dye, or dye aggre-gate, couples into the electronic structure of the silver halide microcrystal. In simple terms, photoexcitation raises an electron to the lowest unoccupied molecular orbital (LUMO) of the sensitizing dye. This electron can then tunnel into the conduction band of the silver halide and be trapped at a latent image site. Alternatively, this electron can be competitively returned to the sensitizing dye molecule (electron-hole recombination).