pubs.acs.org/Langmuir © 2009 American Chemical Society
Cation-Controlled Crystal Growth of Silver Stearate: Cryo-TEM Investigation of Lithium vs Sodium Stearate Jingshan Dong,*,† Alon V. McCormick,† H. Ted Davis,† and David R. Whitcomb‡ †
Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue SE, Minneapolis, Minnesota 55455 and ‡Carestream Health, Inc., 1 Imation Way, Oakdale, Minnesota 55128 Received July 22, 2009. Revised Manuscript Received November 14, 2009 Cryo-TEM, SAXS, and light microscopy techniques were used to probe the morphology and kinetics of silver stearate self-assembly and crystallization from the reaction of silver nitrate with lithium stearate. Unlike the reaction of sodium stearate with silver nitrate, which proceeds via micelle aggregation, the lithium stearate forms vesicles that drastically change the reaction kinetics of the silver stearate nucleation and self-assembly process. In addition, even with excess silver nitrate present, only about 80% of the lithium stearate can be converted to silver stearate. The presence of the residual lithium stearate inhibits the silver stearate crystal growth process. Consequently, no silver stearate micelle aggregates of any significant size form, unlike the system utilizing sodium stearate. Instead, significantly smaller silver stearate crystals result from lithium stearate compared to the silver stearate crystals from sodium stearate and provide an opportunity to further control silver stearate self-assembly and crystal growth.
1. Introduction For more than four decades, silver carboxylates such as silver stearate, CH3(CH2)16COO 3 Ag (AgSt), have been used as thermographic and photothermographic imaging materials, a very important application of which is X-ray diagnostic film. In these materials, the silver carboxylate acts as a source of silver ions (Agþ), which are thermally reduced to metallic silver that composes the image.1 In addition, silver carboxylates can be used as precursors to the formation of silver nanoparticles, which are important in catalysis and electronics.2-5 Fundamental to all of these applications is the preparation of the silver carboxylate via self-assembly from the aqueous soap micelle or micelle aggregation. Understanding this process is important to the control of the crystallization process and the resulting crystal morphology. Previously, we reported the reaction and crystallization process of silver carboxylates from sodium salts of several fatty acids.6-8 Regardless of the chain length of the carboxylate tail group (molecules with carbon numbers of 18, 20, and 22, i.e., stearate, arachidate, and behenate, were chosen), the silver carboxylate crystallization always started from AgSt spherical micelles and micelle aggregations. Such initial structures, although not thermodynamically stable, play a major role in controlling the final, stable crystal morphology and size of AgSt.7 For example, adding silver bromide (AgBr) nanocrystals bypasses the micelle aggregate formation by attracting micelles directly to the AgBr nanocrystal *Corresponding author. E-mail:
[email protected]. (1) Cowdery-Corvan, P. J.; Whitcomb, D. R. In Handbook of Imaging Materials, Diamond, A. S., Weiss, D. S., Eds.; Marcel-Dekker: New York, 2002. (2) Lee, S. J.; Han, S. W.; Choi, H. J.; Kim, K. J. Phys. Chem. B 2002, 106, 2892– 2900. (3) Liu, X.; Lu, S.; Zhang, J.; Cao, W. Thermochim. Acta 2006, 440, 1–6. (4) Yang, N.; Aoki, K. Electrochim. Acta 2005, 50, 4868–4872. (5) Yoon, S.; Kwon, W. J.; Piao, L.; Kim, S. H. Langmuir 2007, 23, 8295–8298. (6) Lin, B.; Dong, J.; Whitcomb, D. R.; McCormick, A. V.; Davis, H. T. Langmuir 2004, 20, 9069–9074. (7) Dong, J.; Whitcomb, D. R.; McCormick, A. V.; Davis, H. T. Nanotechnology 2005, 16, S592–S600. (8) Dong, J.; Whitcomb, D. R.; McCormick, A. V.; Davis, H. T. Langmuir 2007, 23, 7963–7971.
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surfaces, which results in smaller AgSt crystals.8 In the present paper, the reactant is replaced with a lithium salt of stearic acid, CH3(CH2)16COO 3 Li, lithium stearate (LiSt), to determine its effect on the AgSt self-assembly and crystallization process. In this case, AgSt micelles are formed initially, analogous to the sodium carboxylate conditions, but no obvious micelle aggregation was found. Consequently, the number of nuclei increased, which grew into more, and thus smaller, crystals compared to the systems that produce silver carboxylates from sodium carboxylates.
2. Materials and Methodologies A 2 wt % LiSt dispersion was prepared by mixing stoichiometric amounts of lithium hydroxide (LiOH) and stearic acid in water and heated at 75 °C in a water bath. The dispersion was cooled to room temperature naturally with the heating stopped. It was diluted to the desired concentration for different experiments. In producing AgSt, an excess molar amount of 1 wt % AgNO3 (Allied Chemical Co., NJ) solution was added. Cryogenic transmission electron microscopy (cryo-TEM) experiments followed the procedures described previously.6 In preparing the specimen for cryo-TEM, an on-the-grid mixing method was used to capture the structures at reaction times of 30 s, 1 min, and 3 min, i.e., a small drop of the 0.1 wt % LiSt dispersion and a small drop of 1 wt % AgNO3 were attached to the same holey carbon grid. They were mixed on the grid and not disturbed for a variety of times before blotting and freezing so that different stages of the crystallization process could be observed with cryo-TEM. For reaction times longer than 3 min, on-the-grid reaction is not necessary because there is enough time to do the mixing outside the controlled environmental vitrification system6 before the reaction time exceeds the desired amount of time. All digital images were recorded with a Gatan 724 multiscan camera (Gatan Inc., CA) that is incorporated with a JEOL 1210 TEM (JEOL Inc., MA). AgSt is highly hydrophobic. Several minutes after 0.1 wt % LiSt and 1 wt % AgNO3 are mixed and stirred with hand shaking, it floats to the water surface. Samples of the floating solid were
Published on Web 12/17/2009
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Figure 1. Cryo-TEM image of 0.1 wt % LiSt. A single-walled vesicle is on the left, and a three-walled vesicle is on the right. The asterisk (*) marks the holey carbon grids.
Figure 3. Cryo-TEM of 0.1 wt % LiSt plus 1 wt % AgNO3; reaction times are about (a) 1 min and (b) 3 min. Tiny ribbons of lamellar crystals are flocculating and linking.
3. Results and Discussion
vesicles: a single-walled vesicle on the left and a three-walled vesicle on the right, the overall size of which is about 300 nm. The multiple micrographs collected demonstrate that these vesicles vary in size from less than 10 nm to about 1 μm. Some irregularly shaped plates can be seen, but they are very rare (not shown here). Therefore, the vesicle is the predominant structure of this LiSt dispersion, which is completely different from a sodium stearate (NaSt) dispersion of similar concentration.9 Although NaSt dispersions can form vesicles if they are quenched quickly enough from above the Krafft temperature, the common NaSt morphologies in water at ambient conditions are ribbons and fibers of lamellar crystals.8-10 The different morphologies of LiSt and NaSt could be due to LiSt having a larger headgroup size, which increases the curvature of LiSt layers thereby favoring the vesicle during
One of the reactants, the 0.1 wt % LiSt dispersion, was observed with cryo-TEM to provide reference information before the reaction process was studied (Figure 1). The image shows two
(9) Dong, J., Cryo-TEM of Morphology and Kinetics of Self-assembled Nanostructures, Ph.D. dissertation, University of Minnesota, 2006. (10) Liang, J.; Ma, Y.; Zheng, Y.; Davis, H. T. Langmuir 2001, 17, 6447–6454.
Figure 2. Cryo-TEM of 0.1 wt % LiSt plus 1 wt % AgNO3. The reaction time is about 30 s.
collected as small-angle X-ray scattering (SAXS) samples. SAXS was performed using a 12 kW Rigaku rotating anode X-ray generator (Rigaku Co., TX,) that produces Cu KR radiation (λ = 1.542 A˚). Two-dimensional images were obtained with a Siemens Hi-Star multiwire area detector (Siemens Corp., NY). Digital light microscopy (DLM) was done to the mixture on a Nikon Optiphot-Pol Microscope (Nikon Inc., NY) with transmitted light at ambient temperature. The digital images were acquired using a Dage-MTI DC-330 3-chip color video camera (Dage-MTI Inc., IN).
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Figure 5. 0.1 wt % LiSt þ AgNO3; reaction time is ∼13 min.
Figure 4. Cryo-TEM of 0.1 wt % LiSt plus 1 wt % AgNO3; reaction time is about 5 min: (a) a single AgSt crystallite that has many white spots caused by electron beam damage, (b) another area that has linked AgSt crystallites.
its self-assembly. The headgroup size effect will be discussed further later in this section. Using the on-the-grid sample preparation method, the 30 s reaction product of 0.1 wt % LiSt and 1 wt % AgNO3 was imaged with cryo-TEM (Figure 2). Although some spherical micelles of about 5 nm in diameter are loosely gathered together, the majority of them are just free micelles. What is important is that most AgSt micelles do not aggregate in this time frame. This phenomenon is entirely different from what we found in the previous NaSt þ AgNO3 system,6 in which AgSt micelles form dense aggregates, about 50 nm in diameter and in cubic, spherical, or irregular shapes. In addition, free AgSt micelles that are not aggregated can only be seen sporadically in the NaSt þ AgNO3 system. When the reaction time is 1 min, embryonic lamellar crystals form (Figure 3a). In this figure, several tiny ribbons are entangling (and possibly linking with each other) to form a round pattern. Some ribbons are suitably oriented, in which the layers of their (11) Tolochko, B. P.; Chernov, S. V.; Nikitenko, S. G.; Whitcomb, D. R. Nucl. Instrum. Meth. A 1998, 405, 428–434.
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lamellar structure are parallel to electron beam, so that these layers can be seen. It is known that AgSt possesses a lamellar structure,8,11,12 and Figure 1 shows that the reactant LiSt dispersions form vesicles, so Figure 3 demonstrates that AgSt has been produced. The status of the 3 min reaction is nearly the same (Figure 3b) in which the layers of tiny ribbons can be seen as well, but they seem to have better crystallinity. The flocculation and linkage of the tiny ribbons is possibly a path for these embryonic crystals growing into big, plate-like lamellar crystals. In the case of NaSt converting to AgSt, although the path from micelle to crystal is different, the fusion of small crystals also happens at an early stage.9 After about 5 min, the AgSt crystals are fully developed, one of which appears as a circular plate of several hundreds of nanometers in diameter (Figure 4a). It is very vulnerable to electron beam radiation damage. The white spots that look like blown bubbles, large and small, are probably due to mass loss caused by electron beam damage. Because the plate-like crystal has a relatively small lateral size, it is always positioned with its lamellae parallel to the TEM grid. The layers of this lamellar structure, which are now perpendicular to the electron beam, cannot be seen (compare to Figure 3). After 5 min of reaction, linkage between plates can be observed (Figure 4b), but such coalescence among crystals, presumably a ripening process, is much more common in the reaction time of 13 min (Figure 5). In this figure, two branched crystals are shown, inside which the precursor circular plates that build them are still distinguishable. The overall crystal size is several micrometers, which is close to the size observed by a larger-scale characterization methodology, such as light microscopy (Figure 7, below), so the crystallization process can be estimated to be complete at around 15 min. The severe beam damage in Figures 4 and 5 is worthy of some consideration. It is true that cryo-TEM specimens normally show some radiation damage during observation, but it should generally not change the appearance of structures so dramatically. In Figures 4 and 5, the flat crystals, individual circular plates, or linked multiple plates were burned so badly that they looked like foams, which is not a common phenomenon, the cause of which (12) Binnemans, K.; Deun, R. V.; Thijs, B.; Vanwelkenhuysen, I.; Geuens, I. Chem. Mater. 2004, 16, 2021–2027.
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Figure 6. SAXS patterns of LiSt, AgSt made from NaSt, and AgSt made from LiSt.
Figure 7. Light micrograph of AgSt produced from (a) 0.1 wt % LiSt and (b) 0.1 wt % NaSt with 1 wt % AgNO3. The reaction time is about 2 h.
could be due to the samples themselves. By contrast, such severe beam damage was never observed in our previous studies on AgSt formation from sodium carboxylate.6,7 The current system has changed in only one variable: LiSt is the reactant rather than NaSt. In order to understand this beam damage mechanism, three different systems (LiSt, AgSt from LiSt, and AgSt from NaSt) were investigated with SAXS, and the results are shown in Figure 6. In the top curve, AgSt produced by mixing LiSt with excess AgNO3 has peaks that correspond to both AgSt (arrow in Figure 6) and LiSt (arrowhead). This means that it has a 2266 DOI: 10.1021/la902697t
significant amount of residual LiSt, even though excess AgNO3 was used for the purpose of converting all LiSt to AgSt. By comparing the intensity of the two peaks, the molar ratio of AgSt to residual LiSt is approximately 4.4:1, which gives a conversion rate close to 80%. On the other hand, if AgSt is produced from NaSt, no residual reactant (NaSt) is seen (middle curve in Figure 6), as was the case in the previous work.8 The residual LiSt content leads us back to the discussion about electron beam damage in Figures 4 and 5. The presence of a significant amount of LiSt that is not converted by AgNO3 could make the AgSt crystals more vulnerable to electron radiation. In fact, when LiSt was intentionally added to NaSt, NaSt crystals were found to be more easily damaged compared to pure NaSt in the cryo-TEM observation (not shown here). As will be discussed below, the hydration of the lithium ion (Liþ) may explain its electron beam vulnerability because its coordination with water can be easily broken with input energy (electron beam radiation in this case) and generate highly diffusive small Liþ that weakens the structure containing it. In addition, another question may be asked: in the cryo-TEM image of Figure 2, why is there no vesicle of the residual LiSt, as shown in Figure 1? The answer is probably that the concentration of the residual LiSt is too low to form vesicles. Instead, LiSt micelles might be formed, which could be hard to differentiate from the predominant AgSt spherical micelles. If the structures intended to be imaged are in the order of 1 μm, light microscopy can also be used. Figure 7 shows some DLM images of AgSt produced from LiSt (Figure 7a) or NaSt (Figure 7b) upon reaction with AgNO3. Both specimens were prepared after about 2 h of reaction with excess AgNO3, so that the reactants were converted to AgSt as much as possible, and AgSt crystals were fully developed. A single AgSt crystal (marked by the black arrow) from the LiSt dispersion is less than 5 μm, and it is much smaller than that from NaSt, which is more than 10 μm. A further consideration about the effect of LiSt on the AgSt crystallization is inspired by a note in any inorganic chemistry textbook: Liþ occupies the tetrahedral site of four neighboring water molecules,13 which makes Liþ have a larger hydrated radius than Naþ. Consequently, Liþ in water diffuses slower than Naþ. On a molecular scale, solid AgSt has a lamellar crystalline structure, while on an atomic scale, adjacent Agþ and carboxylate groups form an eight-membered ring9 that is stronger than the forces (13) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th ed.; John Wiley & Sons: New York, 1988; p 131.
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commonly existing at self-assembly and not seen in NaSt, so AgSt should be thermodynamically stable. However, kinetics also play an important role: the AgSt formation from NaSt is a diffusioncontrolled process, as we reported previously.4 In the current case, the presence of Liþ slows down the AgSt self-assembly and crystallization, especially the steps involving the solution phase: micellization and micelle aggregation.4-7 The lower rate of micelle aggregation is important because it could be overtaken by the sequential crystal ripening and thus depressed kinetically. Without a considerable number of micelle aggregates, the alternative nuclei, the micelles themselves, result in smaller crystals. The slower diffusion of Liþ could be also a major reason for so much residual LiSt.
4. Conclusions Cryo-TEM, SAXS, and light microscopy techniques were used to probe the morphology and kinetics of AgSt self-assembly and
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crystallization from the reaction of silver nitrate with LiSt. Unlike the case of the reaction of NaSt lamellar crystals with AgNO3, the LiSt forms vesicles and changes the reaction kinetics of the AgSt self-assembly. Also, even with excess AgNO3 only about 80% of the LiSt can be converted to AgSt. The presence of residual LiSt slows down the AgSt crystal growth processes. Consequently, no silver stearate micelle aggregates of any significant size form, unlike the system utilizing NaSt. Instead, significantly smaller AgSt crystals result from LiSt compared to the AgSt crystals from NaSt. Acknowledgment. We acknowledge the use of the facilities of the Characterization Facility Center at the University of Minnesota and funding support from IPRIME (Industrial Partnership for Research in Interfacial and Materials Engineering).
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