Crystallization of Silver Stearate from Sodium Stearate Dispersions

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Crystallization of Silver Stearate from Sodium Stearate Dispersions Bin Lin,† Jingshan Dong,† David R. Whitcomb,*,‡ Alon V. McCormick,† and H. Ted Davis† Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, and Eastman Kodak Company, 1 Imation Way, Oakdale, Minnesota 55128 Received May 14, 2004. In Final Form: July 29, 2004 Silver carboxylates, the common silver source used for photothermographic imaging materials, are normally obtained from the reaction between sodium soap (e.g., sodium stearate) and silver nitrate. They form platelet-like crystals with a lamellar structure in water at room temperature. Light microscopy investigations reveal that the formation of silver stearate (AgSt) crystals follows a diffusion-controlled mechanism. The reaction between the sodium soap and silver nitrate preferentially occurs in solution rather than on the soap fiber solid interface. Cryogenic transmission electron microscopy, together with an on-the-grid reaction technique, provides a useful tool to directly image silver stearate microstructures at the initial stages of AgSt precipitation. The AgSt reaction product first forms particles about 5 nm in size, which is similar to the d-spacing of final AgSt crystals. Those particles aggregate to produce larger and loosely packed embryonic crystals, the precursors to the ultimate silver stearate crystals.

Introduction Silver salts of long-chain fatty acids, often called silver soaps by analogy to their sodium counterparts, have been utilized in thermographic and photothermographic imaging materials for over 40 years.1-3 These are extremely important commercial applications where high-quality imaging for medical X-ray diagnoses is combined with complete freedom from the use of hazardous chemicals normally required to develop such black and white, silverbased films.4 The X-ray data file, collected separately, is simply used to drive a laser scanner, in the photothermographic system, to expose the film. The light-induced latent image acts as a catalytic site to initiate image formation, just as in conventional photographic processes. However, simple thermal processing is all that is needed to fully form the image; no wet development chemistry is necessary. In these materials, the silver carboxylate acts as a source of silver ions, which are thermally reduced in the presence of a phenol-based electron source to form metallic silver that comprises the image. The visible component of the image is comprised of nanosized particles of metallic silver, with collections of various sizes and shapes,5,6 whose composite visible absorption spectra give the image its black appearance. * To whom correspondence should be addressed. E-mail: [email protected]. † University of Minnesota. ‡ Eastman Kodak Co. (1) Whitcomb, D. R. In Kirk Othmer Encyclopedia of Chemical Technology, online edition, 2003. http://www3.interscience.wiley.com/ cgi-bin/mrwhome/104554789/HOME (2) Cowdery-Corvan, P. J.; Whitcomb, D. R. In Handbook of Imaging Materials; Diamond, A. S., Weiss, D. S., Eds.; Marcel-Dekker: New York, 2002. (3) Klosterboer, D. H. In Imaging Processes and Materials; Sturge, J. M., Walworth, V., Shepp, A., Eds.; Van Nostrand-Reinhold: New York, 1989; Chapter 9. (4) Willett, B. ACS Symposium on Green Chemistry; Rochester, New York, 2000. (5) Bokhonov, B. B.; Burleva, L. P.; Frank, W.; Mizen, M. B.; Sahyun, M. R. V.; Whitcomb, D. R.; Winslow, J.; Zou, C. J. Imag. Sci. Tech. 1996, 40, 417. (6) Bokhonov, B. B.; Burleva, L. P.; Whitcomb, D. R.; Sahyun, M. R. V. Microsc. Res. Tech. 1998, 42, 152.

While sodium soaps (long-chain carboxylates having chain lengths between 12 and 24) have been investigated in great detail over many years,7-11 the details concerning the formation of the corresponding silver soaps are far less understood.12 Unbeknownst to the inventors of photothermography, the solid-state structure of the silver soaps plays an important role in the imaging properties of the materials.1,2 Most publications on this topic are patents that typically do not discuss a fundamental understanding of the effect of the reaction conditions on the formation of the silver soap.13-15 Normally, the simple addition of a soluble silver ion, such as silver nitrate, to the sodium soap is sufficient to rapidly exchange metal ions and precipitate the silver soap. Since the sodium soaps exhibit a Krafft temperature near the melting point of the free acid,9 the silver addition can be placed into either a solution or dispersion of the soap, depending only on the reaction mixture temperature or presence of organic solvents. The reaction conditions, such as temperature, concentrations, and reaction flow rates, have been claimed to affect the overall silver soap solid-state structure.15 Consequently, beneficial effects to various thermalimaging products from the silver carboxylate morphology are suggested. The solid-state structure of silver stearate, AgSt, is known to comprise layers of silver stearate dimers, [Ag(O2C18H35)]2, built up from eight-membered rings, as illustrated in Figure 1.16 (7) McBain, J. W.; Vold, R. D.; Frick, M. J. Phys. Chem. 1940, 44, 1013. (8) Madelmont, C.; Perron, R. Colloid Polym. Sci. 1976, 254, 581. (9) de Mul, M. N. G.; Davis, H. T.; Evans, D. F.; Bhave, A. V.; Wagner, J. R. Langmuir 2000, 16, 8276. (10) Laughlin, R. G. In The Aqueous Phase Behavior of Surfactants; Academic Press: San Diego, 1994. (11) Small, D. M. The Physical Chemistry of Lipids. In Alkanes to Phospholipids; Plenum Press: New York, 1986. (12) Hrust, V.; Kallay, N.; Tezˇak, D. J. Colloid Polym. Sci. 1985, 263, 424. (13) Hatakeyama, A. EP Patent 911692, 1999. (14) Toya, I.; Minami, A. EP Patent 1014178, 2000. (15) Defieuw, G.; Janssen, P.; Loccufier, J.; Horsten, B. U.S. Patent 5,817,598, 1998.

10.1021/la048793g CCC: $27.50 © 2004 American Chemical Society Published on Web 09/08/2004

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Figure 1. Eight-membered silver carboxylate ring polymeric structure.

The solid-state structure of sodium soaps exhibits strong similarities to that of silver soaps, the most significant of which is the layered structure in which the layers of metal ions are separated by a double layer of carboxylate chains.17 The bonding differences between the oxygen of the carboxylate and the silver and sodium ions control the physical properties of the corresponding solids. The physical properties of the silver soaps prepared from the sodium analogues, such as thermal stability, poor solubility, and crystallinity, make them quite suitable for thermal imaging applications.1,2 Improved understanding of the preparation of these silver soaps should be useful in improving the imaging properties of films made from them. The 18 carbon chain of silver stearate is one of the chain lengths that comprise the silver source used in the thermal imaging products; the 22 carbon chain of silver behenate, another. Silver stearate was selected in this study as a model for preparation of various chain length silver soaps used in thermal imaging materials because the solid-state structure is known,16 and silver behenate is considered to have a completely analogous structure.18 The object of this study is to reveal the details of the initial stages of reaction of the silver ion with the sodium soap, to understand the processes involved in controlling the formation of the final silver carboxylate crystal. We now report that the reaction between silver nitrate and sodium soap, and the following crystal precipitation and growth of the reaction product, is much faster compared to the mass transport of those two reactants, which is mainly driven by diffusion. The powerful cryogenic transmission electron microscopy (cryo-TEM),19 with a special specimen preparation technique, enabled the initial formation and the transitional crystalline structure of the silver carboxylate to be captured. Nanoparticles, having a size comparable to the d-spacing of the final silver carboxylate crystalline product, are first formed, followed by particle aggregation and recrystallization to produce larger crystals. Experimental Section Materials. Sodium stearate (NaSt, >99%; Sigma Chemical Co., MO) and silver nitrate (AgNO3, crystals; Allied Chemical Co., NJ) were used as received. Water was filtered and deionized (DI) to 18.2 MΩ cm by a Millipore UV coupled with a Millipore Q water purification system. The calculated amount of NaSt was dissolved in water at 85 °C in a Teflon-sealed glass vial placed in a water bath. The mixture was stirred by a magnetic bar placed inside the glass vial to homogenize the concentration and temperature. The resulting clear, isotropic liquid was cooled under ambient conditions as heating was stopped. NaSt gradually crystallized and formed a white dispersion at room temperature. A 0.03 M (0.5 wt %) (16) Tolochko, B. P.; Chernov, S. V.; Nikitenko, S. G.; Whitcomb, D. R. Nucl. Instrum. Methods Phys. Res., Sect. A 1998, 405, 428. (17) Liang, J.; Ma, Y.; Zheng, Y.; Davis, H. T.; Chang, H.-T.; Binder, D.; Abbas, S.; Hsu, F.-L. Langmuir 2001, 17, 6447. (18) Blanton, T.; Lelental, M.; Zdzieszynski, S.; Misture, S. Adv. X-Ray Anal. 2002, 45, 371. (19) Bellare, J. R.; Davis, H. T.; Scriven, L. E.; Talmon, Y. J. Electron Microsc. Tech. 1988, 10, 87.

Figure 2. Schematic of specimen preparation and imaging spots for light microscopy observation. Observation area 1 is in the NaSt dispersion adjacent to the liquid-liquid interface. Observation area 2 is about 500 µm farther away from observation area 1. aqueous AgNO3 solution was prepared by dissolving the appropriate amount of AgNO3 crystals in DI water at room temperature. Light Microscopy. Aqueous NaSt dispersions or mixtures of NaSt dispersions and AgNO3 solution were viewed by a Nikon Optiphot-Pol microscope with transmitted light at 25 °C. Nomarski differential interference contrast (DIC) illumination was used. Each specimen was prepared on a glass slide with a coverslip. The digital images were acquired using a Nikon monochrome camera. The reaction between the NaSt and AgNO3 is illustrated in Figure 2. Two reactant droplets, one of dilute NaSt aqueous dispersion and the other of dilute AgNO3 aqueous solution, are placed on the glass slide side by side, without direct contact. The coverslip is gently placed to squeeze both droplets so that the reactants meet each other and diffusion begins through the interface. The slide is immediately put under the prefocused lens. The observation area is quickly adjusted, and images are quickly taken at the selected spot. Small-Angle X-ray Scattering (SAXS). The microstructures of selected samples were determined by X-ray diffraction. SAXS was performed on specimens using a 12 kW Rigaku rotating anode X-ray generator producing Cu KR radiation (λ ) 1.542 Å). Two-dimensional images were obtained with a Siemens Hi-Star multiwire area detector. The results were analyzed with Jade 5.0 X-ray pattern processing software (Materials Data Inc.). Cryo-TEM. Cryogenic transmission electron microscopy19,20 was used to image the microstructure of NaSt and reaction products with silver nitrate in a dilute aqueous dispersion. Onthe-grid reaction was carried out by mixing the two reactants, followed by blotting excess liquid from the mixing in the specimen preparation process. The reaction time was controlled by adjusting the hold time period between blotting and plunge freezing. In this study, about 2.5 µL (half the volume of the normal droplet used in cryo-TEM specimen preparation for one species) of one of the reactants (e.g., NaSt) was withdrawn from the bulk phase of a dilute dispersion and deposited on one side of a 200mesh TEM copper grid (held vertical) having a supported, perforated carbon film with hole sizes ranging from 1 to 10 µm in diameter (Ted Pella, Inc.). Another droplet with about the same volume (2.5 µL) of the second reactant (e.g., AgNO3) was withdrawn from the bulk solution and deposited on top of the sodium stearate drop on the same grid. Filter paper was used to blot the excess liquid away from the backside of the grid, leaving thin liquid films (50-300 nm in thickness) with the mixed reactants spanning the holes of the carbon film. The sample preparation was done in a controlled environment vitrification system saturated with water vapor at 25 °C.19 A schematic of the specimen preparation is illustrated in Figure 3. The reaction time in the thin liquid films prior to blotting ranged from 1 to 10 s. The reaction was stopped by plunging the specimen into liquid ethane at its melting point (-182.8 °C) and cooled by liquid nitrogen. The vitrified specimen was mounted (20) Talmon, Y. In Modern Characterization Methods of Surfactant Systems; Binks, B. P., Ed.; Marcel Dekker: New York, 1999; Chapter 5, pp 147-178.

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Figure 3. Illustration of on-the-grid mixing and reaction. Small droplets of reactant solutions were first attached on the perforated carbon film (microgrid) supported by a normal TEM copper grid held vertical. The reactants mix on the grid by capillary action and shear flow produced by blotting. on a Gatan cryo-holder and transferred into a JEOL 1210 TEM. Specimens were imaged with the objective lens at a nominal underfocus of 0.7-4 µm to produce maximum phase contrast. The specimen temperature was maintained below -170 °C throughout the observation. All images were recorded with a Gatan 724 MultiScan camera. High-magnification images were recorded with low electron doses (fewer than 10 electrons/Å2) to minimize electron-beam radiation damage to the specimen.

Results and Discussion Recrystallized AgSt Crystals from Dilute NaSt Dispersions. As noted above, silver carboxylates are the silver source for the vast majority of photothermographic imaging materials, and they are obtained from the reaction between sodium soap (e.g., NaSt) and silver nitrate: NaSt + AgNO3 f AgSt + NaNO3. Like NaSt, AgSt has very poor solubility in water at room temperature.1,2 Therefore, AgSt produced from the reaction rapidly precipitates and crystallizes in water. The recrystallization and growth of AgSt crystals occur over a longer period of time, although the initial metallic ion exchange reaction can be observed to finish in the order of seconds. When stoichiometric, dilute solutions of NaSt and AgNO3 are mixed and allowed to age for several months, not only is the AgSt formation reaction completely finished, but the system has also reached AgSt crystal growth equilibrium. The morphology of the final AgSt crystals from this process, observed by light microscopy, is shown in Figure 4. Only platelet crystals having a size ranging from several micrometers to dozens of micrometers are observed, and NaSt fibers completely disappeared in the mixed system. The SAXS data show (see the Supporting Information) that these crystals have a lamellar structure with a d-spacing value of 4.868 nm, which is significantly larger than the d-spacing of NaSt crystals (4.467 nm) in the freshly made dispersion (see the Supporting Information) and is consistent with the previously reported d-spacing of AgSt.21 Therefore, AgSt, produced by the reaction between NaSt and AgNO3, crystallizes in water and forms platelets with a lamellar structure. The Ag+ layers are separated by the zigzag chains16 and, thus, form a lamellar structure with a d-spacing that is roughly double the fully extended hydrocarbon chain length. (21) Vand, V.; Atkins, A.; Campbell, R. K. Acta Crystallogr. 1949, 2, 398.

Figure 4. Morphology of AgSt observed by light microscopy at 25 °C in the aqueous reaction system of a stoichiometric mixture of NaSt and AgNO3 after aging for 6 months.

AgSt Crystals Formed in the Reacting System. Light microscopy, as a simple but effective direct viewing tool, is the first probe used to reveal the initial stage of the reaction between sodium stearate and silver nitrate and the subsequent AgSt crystallization-recrystallization processes. To capture the quick changes in the reaction system within a short period of time, specimen preparation becomes a very challenging yet important process. Figure 5 shows a sequence of AgSt crystal appearance at the same spot, right beside the liquid-liquid interface (observation area 1 in Figure 2), with increasing time observed by light microscopy. The observation spot is intentionally selected where there are no NaSt fibers. The diffusion-controlled mechanism for this system can be verified by scaling analysis, based on the light microscopy images shown in Figure 5. The average distance, x, traveled by Ag+ ions within a certain period of time, t, can be approximated by the mean-root square displacement of Ag+ in a very dilute aqueous solution driven by pure diffusion:

x ) x〈x2〉 ) x2Dt

(1)

where D is the effective diffusion coefficient of Ag+ with a value of 1.648 × 10-5 cm2/s at 25 °C,22 and t is the total

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Figure 5. A series of light microscopy images taken every 40 s at the same spot in observation area 1, after mixing the two reactants by the method illustrated in Figure 2.

Figure 6. A typical light microscope image in observation area 2 30 min after mixing the two reactants.

diffusion time. Using the time period between taking images a and d in Figure 5, the diffusion time is 120 s. Based on eq 1, the calculated value of x is 0.0629 cm. Alternatively, the distance x′ traveled by Ag+ ions can be directly measured from the light microscopy images from the first-appearing particle to the last-appearing particle during the diffusion time period (120 s), which is about 0.03 cm. These values are comparable, which verifies that the reaction and the subsequent crystal precipitation are much faster than the diffusion of reactants. Thus the whole process is mainly driven by the mass transport of reactant species. Within observation area 2, sodium stearate crystalline fibers are easily identified before reactant mixing. After 30 min, in addition to the newly formed AgSt crystals (dots), the NaSt fibers are still visible, as shown in Figure 6. The black dots are scattered throughout the observation area. It is important to note that there is no evidence of (22) CRC Handbook of Chemistry and Physics; Lide, D. R., Ed.; CRC Press: New York, 1996.

preferred crystal formation along the soap fibers. That is, the sodium soap does not act as a template for the silver soap formation. Therefore, the reaction occurs primarily in solution as a diffusion-controlled process, and any solidliquid reaction along the soap fiber surface is coincidental. Initial Stage of AgSt Crystallization. The high magnification capability of TEM is a powerful supplement to the light microscopy investigation of the formation of silver stearate while still in its original aqueous matrix. It provides direct high-resolution imaging of the microstructure of a reacting system at any stage of the reaction because of the special, fast cryogenic freezing technique that can easily “stop” the reaction and “save” the structure. In this study, the reaction time on the cryo-TEM microgrids was about 30 s, which is controlled by adjusting the time period between blotting and plunge freezing during the specimen preparation. High-magnification cryo-TEM images captured the initial AgSt crystallization process. Representative images are shown in Figure 7. In Figure 7a, several dark solid aggregates attached onto the perforated carbon film (microgrids, marked by *) and along the NaSt fibers (marked by #) are clearly observed. Because silver has a larger atomic number than sodium, AgSt scatters the electron beam more strongly than NaSt, thus appearing darker in contrast in TEM images. Therefore, fibers in lighter contrast can be attributed to NaSt crystals, while those darker aggregates are AgSt. As can be seen in Figure 7, these AgSt aggregates are about 50 nm in size. While many aggregates are irregularly shaped, AgSt aggregates can be seen as wellshaped crystals, such as the black-filled square in the cryo-TEM image. Because the image is a two-dimensional projection of the real material, this crystal is most likely a cubic AgSt aggregate. In addition to the 50 nm aggregates, many small dots about 5 nm in size are also visible, Figure 7b. Note that the asterisk indicates the supporting perforated carbon film. Even the larger aggregates are collections of these much smaller particles. The aggregate formation pattern illustrates the crystals’ texture and edge morphology. The packing of those aggregates is not very dense because each small individual particle can still be identified among

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Figure 7. Representative cryo-TEM images of AgSt aggregates formed after on-the-grid mixing and reacting of AgNO3 and NaSt for 10 s.

the aggregate. The cubic nature of this aggregate might be a little surprising considering the ultimate lamellar structure of the dried silver carboxylate. However, the cubic form can be attributed to the consequence of the initial packing of the 5 nm spherical building blocks, which eventually rearrange with water loss and recrystallization to create the final lamellar structure. Since this reaction is a diffusion-controlled process, the very first stage of the reaction must be

Ag+(H2O)x(NO3-) + NaSt(H2O)y f (H2O)zAgStsolution (2) where x ) 4,23 y is variable because of the dynamic nature of the soap micelle, and z ∼ 2. A silver-containing solution species would not be seen by TEM, but the smallest particles observed in these images are generally spherical, on the order of 5 nm in diameter. As noted above, the d-layer spacing in AgSt is about 4.8 nm. It is tempting, therefore, to describe the ultimate, small particles seen in the figures above as the very initial ion exchange (23) Seward, T. M.; Henderson, C. M. B.; Charnock, J. M.; Dobson, B. R. Geochim. Cosmochim. Acta 1996, 60, 2273.

reaction between solution species, most likely a small collection of a few of these initial, individual species. This structure can be visualized as a typical sodium soap micelle, but one with the spherical collection of hydrocarbon chains of the soap with Na+/H2O covering the carboxylate surface that is exposed to the water matrix containing Ag+. In this case, at the very initial stages of the exchange reaction, the sodium ions are replaced with silver ions. The resulting structure, nominally [(H2O)zAgSt]n, would be seen in TEM as a spherical structure having a diameter on the order of 5 nm. Considering the strong driving force to form silver carboxylate dimers and the polymeric silver carboxylate crystalline network,1 these intermediate 5 nm particles would be very shortlived and condense with others to aggregated structures. Figure 7b,c may be this condensation step in process. The loose packing generally seen at this point may be a consequence of the H2O carried along at the hydrated silver/sodium soap surface, which is lost in subsequent rearrangement, repacking, and recrystallization processes. Alternatively, a “simple” combination of (H2O)nAgNaSt2 intermediate components can also account for the 5 nm particles observed, but it is hard to distinguish between these and [(H2O)zAgSt]n at this time. Neverthe-

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Figure 8. AgSt crystallites deposited on the carbon film support of the TEM microgrid.

less, any silver-containing nanoparticle of this size cannot contain many individual dimeric AgSt building block units. In addition to the AgSt cubic aggregate, spherical aggregates, such as the one marked by an arrow in Figure 7d, were also observed. The fact that the contrast gradually increases from the outside boundary to the center of the aggregate supports the spherical geometry. Some of those AgSt aggregates like to attach on the perforated carbon film (microgrids, marked by *) and along the NaSt fibers (marked by #), while others are formed and stay in the aqueous phase, as demonstrated in Figure 7e,f. Commonly, the small AgSt crystallites can be seen to deposit extensively on any available surface, such as the holey microgrid itself, Figure 8. Considering the widely different types of surfaces the AgSt is exposed to, it is apparent that any nucleation site seems to be sufficient for the AgSt to begin depositing. Eventually, however, as shown in Figure 4, the AgSt crystallites undergo Ostwald ripening. This process is further illustrated in Figure 9. Under these conditions, both AgSt (large, flat plates) and NaSt are present in the mixture, having dimensions common to those observed in the typical, dried silver soaps. Conclusions The initial stages of the formation of silver stearate from sodium stearate dispersions have been elucidated. Light microscopy investigations reveal that the formation of AgSt crystals is the result of a diffusion-controlled mechanism when the reactants are transported within a

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Figure 9. AgSt after aging in solution.

liquid film. The silver nitrate reacts with solvated NaSt molecules and micelles in solution. In general, the silver ion does not exchange with the sodium that is in the NaSt lattice, other than that adventitiously meeting at the sodium soap fiber/ribbon surface. Cryo-TEM is an effective probe to observe the formation of the silver stearate microstructures at the initial stages of reaction that precede any significant crystal formation. Within 30 s, the precipitated AgSt first forms small particles less than 5 nm in diameter. The nanoparticle is proposed to comprise metastable (H2O)zAgSt or (H2O)nAgNaSt2 intermediate components. Those particles tend to aggregate to produce larger (50-100 nm in size), loosely packed embryonic crystals that are subject to further Ostwald ripening processes to form the final, platelike silver stearate crystals. Acknowledgment. We acknowledge financial support from Eastman Kodak Company and IPRIME (Industrial Partnership for Research in Interfacial and Materials Engineering) at the University of Minnesota. We thank Professor Reinhard Strey at the University of Cologne, Germany, for useful discussions. Supporting Information Available: Light microscopy images of a 0.5 wt % NaSt dispersion, freshly prepared, after aging for 15 days, and after aging for 6 months; cryo-TEM image of a single fiber in a freshly prepared 0.5 wt % NaSt aqueous dispersion; SAXS patterns of NaSt crystals, in a freshly prepared dispersion and after aging for 6 months. This material is available free of charge via the Internet at http://pubs.acs.org. LA048793G