SrCO3 Crystals of Ribbonlike Morphology Grown within Thermally

Bunker, B. C.; Rieke, P. C.; Tarasevich, B. J.; Campbell, A. A.; Fryxell, G. E.; Graff, G. L.; Song, L.; Liu, J.; Wirdem, J. W.; McVay, G. L. Science ...
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Langmuir 2003, 19, 888-892

SrCO3 Crystals of Ribbonlike Morphology Grown within Thermally Evaporated Sodium Bis-2-ethylhexylsulfosuccinate Thin Films Debabrata Rautaray, S. R. Sainkar, and Murali Sastry* Materials Chemistry Division, National Chemical Laboratory, Pune-411 008, India Received August 27, 2002. In Final Form: November 14, 2002 In this paper we demonstrate the crystallization of strontium carbonate in the form of thin sheets/ ribbons within thermally evaporated sodium bis-2-ethylhexylsulfosuccinate (aerosol OT, AOT) thin films by a process of Sr2+ ion entrapment followed by reaction with CO32- ions within the lipid matrix. The morphology of the strontianite crystals is observed to be strongly dependent on the nature of the lipid used in the ion entrapment process; stearic acid thin films result in the formation of the more commonly observed strontianite needles arranged in flowerlike assemblies. Contact angle measurements of the strontianiteAOT films indicated that they were hydrophobic, thus pointing to the possible role of hydrophobic interactions between the AOT-monolayer-covered strontianite crystals in modulating the crystal morphology.

Introduction The presence of inorganic materials in biological organisms has broad implications in the physical sciences, such as geology, mineralogy, physics, chemistry, and materials science, as well as in biological fields, such as zoology, microbiology, physiology, evolution, and cellular biology. The structures of biocomposites are highly controlled from the nanometer to the macroscopic levels, resulting in complex architectures that provide multifunctional properties.1 Development of strategies to grow crystals of controllable structure, size, morphology, and superstructures of predefined organizational order is an important goal in crystal engineering with tremendous implications in the ceramic industry.2,3 Approaches leading to such crystallography/morphology control based on mimicking biological mineralization (biomineralization) procedures have gained in popularity. An additional level of complexity is reached when living systems form composite structures that organize minerals in organic matrixes.4 In this context biominerals have served as an inspiration to crystal engineers. This is not surprising given the exquisite control that biological organisms exert over mineral nucleation and growth (both amorphous and crystalline) encompassing the above-mentioned aspects.5,6 It is now established that an important requirement for biomineralization is epitaxy between the crystal nucleating face and underlying bioorganic surface, and consequently, biomimetic surfaces such as those presented by Langmuir monolayers,7,8self-assembled monolayers * To whom correspondence should be addressed. Phone: + 91 20 5893044. Fax: +91 20 5893952/5893044. e-mail: sastry@ ems.ncl.res.in. (1) Sarikaya M. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 14183. (2) Heuer, A. H.; Fink, D. J.; Laraia, V. J.; Arias, J. L.; Calvert, P. D.; Kendall, K.; Messing, G. L.; Blackwell, J.; Rieke, P. C.; Thomoson, D. H.; Wheeler, A. P.; Veis, A.; Caplan, A. I. Science 1992, 255, 1098. (3) Bunker, B. C.; Rieke, P. C.; Tarasevich, B. J.; Campbell, A. A.; Fryxell, G. E.; Graff, G. L.; Song, L.; Liu, J.; Wirdem, J. W.; McVay, G. L. Science 1994, 264, 48. (4) Mann, S. J. Mater. Chem. 1995, 5, 935. (5) (a) Biomineralization, Chemical and Biochemical Perspectives; Mann, S., Webb, J., Williams, R. J. P., Eds.; VCH: Weinheim, Germany, 1989. (b) Mann, S. In Inorganic Materials; Bruce, D. W., O’Hare, D., Eds.; John Wiley & Sons: New York, 1996; p 255. (c) Mann, S. J. Chem. Soc., Dalton Trans. 1993, 1. (6) Addadi, L.; Weiner, S. Angew. Chem., Int. Ed. Engl. 1992, 31, 153.

(SAMs),9 and functionalized polymer surfaces10 have been studied in great detail. Attempts have also been made to control the morphology of crystals via addition of suitable crystallization inhibitors11 and carrying out crystal growth in constrained environments such as those afforded by microemulsions.12 The anionic surfactant sodium bis-2ethylhexylsulfosuccinate (aerosol OT, AOT) is a twin-tailed surfactant and is most commonly used to make reverse micelles, due to its bulky hydrophobic tail relative to the hydrophilic group. This anionic surfactant has been used in the synthesis of nanoparticles of barium chromate,13 barium sulfate,12b-d calcium sulfate,14 barium carbonate,15 and silica.16 The principal reason for using microemulsions is that particle sizes and the corresponding size distributions can be readily controlled by reaction confinement. In this laboratory, we have developed a process based on thermally evaporated ionizable lipid films for the electrostatic entrapment of inorganic ions,17 surfacemodified nanoparticles,18 proteins/enzymes,19,20 and DNA.20 Electrostatically entrapped ions such as Cu2+ and Ni2+ within lipid matrixes such as stearic acid may be reduced (7) Heywood, B. R.; Mann, S. Adv. Mater. 1994, 6, 9 and references therein. (8) BaSO4: (a) Heywood, B. R.; Mann, S. Langmuir 1992, 8, 1492. (b) Heywood, B. R.; Mann, S. J. Am. Chem. Soc. 1992, 114, 4681. CaCO3: (c) Litvin, A. L.; Valiyaveettil, S.; Kaplan, D. L. Mann, S. Adv. Mater. 1997, 9, 124. (d) Buijnsters, P. J. J. A.; Donners, J. J. J. M.; Hill, S. J.; Heywood, B. R.; Nolte, R. J. M.; Zwanenburg, B.; Sommerdijk, N. A. J. M. Langmuir 2001, 17, 3623. (9) CaCO3/SrCO3: (a) Kuther, J.; Nelles, G.; Seshadri, R.; Schaub, M.; Butt, H.-J.; Tremel, W. Chem.sEur. J. 1998, 4, 1834. CaCO3: (b) Aizenberg, J.; Black, A. J.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121, 4500. (10) (a) Feng, S.; Bein, T. Science 1994, 265, 1839. (b) Falini, G.; Gazzano, M.; Ripamonti, A. Adv. Mater. 1994, 6, 46. (11) BaSO4: (a) Bromley, L. A.; Cottier, D.; Davey, R. J.; Dobbs, B.; Smith, S.; Heywood, B. R. Langmuir 1993, 9, 3594. (b) Qi, L.; Coffen, H.; Antonietti, M. Angew. Chem., Intl. Ed. 2000, 39, 604. (c) Uchida, M.; Sue, A.; Yoshioka, T.; Okuwaki, A. CrystEngComm. 2001, 5. (12) (a) Hopwood, J. D.; Mann, S. Chem. Mater. 1997, 9, 950. (b) Li, M.; Mann, S. Langmuir 2000, 16, 7088. (c) Hopwood, J. D.; Mann, S. Chem. Mater. 1997, 9, 1819. (d) Summers, M.; Eastoe, J.; Davis, S. Langmuir 2002, 18, 5023-5026. (13) Li, M.; Schnablegger, H.; Mann, S. Nature 1999, 402, 393. (14) Rees, G. D.; Evans-Gowing, R.; Hammond, S. J.; Robinson, B. H. Langmuir 1999, 15, 1993. (15) Qi, L.; Ma, J.; Cheng, H.; Zhao, Z. J. Phys. Chem. B 1997, 101, 3460. (16) Arriagada, F. J.; Osseo-Asare, K. J. Colloid Interface Sci. 1995, 170, 8.

10.1021/la026486+ CCC: $25.00 © 2003 American Chemical Society Published on Web 01/10/2003

SrCO3 Crystals of Ribbonlike Morphology

in situ to yield the corresponding metal nanoparticles and thereafter alloyed at relatively low temperatures in situ.21 It is clear that metal ions entrapped in such lipid bilayer stacks can be chemically reacted to yield a variety of materials within the confines of the bilayer stacks. One interesting possibility is the growth of minerals within the bilayer stacks by suitable reaction of entrapped metal cations. We have recently reported on the growth of BaSO4,22a CaCO3,22b and SrCO322c crystals in thermally evaporated stearic acid thin films and have observed highly oriented growth of calcite crystals22b and interesting assembly of strontianite needles mediated by the fatty lipid host.22c In this paper, we investigate the role of the thermally evaporated lipid host in determining the morphology and assembly of SrCO3 crystals grown in situ. More specifically, we demonstrate the crystallization of strontium carbonate within thermally evaporated AOT thin films by a process of sequential entrapment of Sr2+ and CO32- ions and compare the morphology of the strontianite crystals formed with that of crystals grown within thermally evaporated stearic acid thin films by a similar process of ion entrapment.22c It is observed that the SrCO3 crystals grown in AOT thin films have a ribbonlike morphology with interesting texture, this morphology being significantly different from that of the more commonly observed strontianite needles formed both in solution and in stearic acid films as the host under identical supersaturation conditions.22c To the best of our knowledge, plate- and ribbonlike morphologies of strontianite crystals have not been reported thus far. Presented below are details of this study. Experimental Details Aerosol OT, strontium chloride, and sodium carbonate were obtained from Aldrich Chemicals and used without purification. Thin films of aerosol OT (C20H37NaO7S, MW ) 444.56) of 500 Å thickness were thermally vacuum deposited in an Edwards E306 vacuum coating unit operated at a pressure of better than 1 × 10-7 Torr onto gold-coated AT-cut quartz crystals [for quartz crystal microgravimetry (QCM) studies] and onto glass and Si(111) substrates for X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements. The film thickness was monitored using a quartz crystal microgravimeter fitted to the deposition chamber and cross-checked by ellipsometry measurements. After deposition of the AOT films, the QCM crystal was immersed in 50 mL of 10-3 M aqueous SrCl2 solution (pH 5.5) and the frequency change of the crystal was monitored ex situ, as a function of time of immersion in the electrolyte solution, taking care to wash and dry the crystals thoroughly prior to frequency measurement. Frequency measurements were made on an Edwards FTM5 frequency counter (stability and resolution of 1 Hz). For the 6 MHz crystal used in this study, the mass resolution was 12.1 ng/cm2 and the frequency changes were converted to mass loading using the Sauerbrey equation.23 The optimum immersion time determined from the QCM kinetics measurements was used to load the AOT films on glass and Si(111) substrates with Sr2+ ions by similar immersion in 10-3 (17) (a) Ganguly, P.; Sastry, M.; Pal, S.; Shashikala, M. N. Langmuir 1995, 11, 1078. (b) Mandal, S.; Sainker, S. R.; Sastry, M. Mater. Res. Bull. 2002, 37, 1613. (18) Sastry, M.; Rao, M.; Ganesh, K. N. Acc. Chem. Res. 2002, 35, 847. (19) Gole, A.; Vyas, S.; Sainkar, S. R.; Lachke, A.; Sastry, M. Langmuir 2001, 17, 5964. (20) Sastry, M. Trends Biotechnol. 2002, 20, 185. (21) Damle, C.; Kumar, A.; Sastry, M. J. Mater. Chem. 2002, 12, 1860. (22) (a) Rautaray, D.; Kumar, A.; Reddy, S.; Sainkar, S. R.; Sastry, M. Cryst. Growth Des. 2002, 2, 197. (b) Damle, C.; Kumar, A.; Bhagwat, M.; Sainkar, S. R.; Sastry, M. Langmuir 2002, 18, 6075. (c) Sastry, M.; Kumar, A.; Damle, C.; Sainkar, S. R.; Bhagwat, M.; Ramaswami, V. CrystEngComm. 2001, 21. (23) Sauerbrey, G. Z. Phys. (Munich) 1959, 155, 206.

Langmuir, Vol. 19, No. 3, 2003 889 M SrCl2 solution. After formation of Sr-AOT films, the films on glass were subjected to XRD analysis. Thereafter, the Sr-AOT film on the QCM crystal was immersed in 50 mL of 1.584 × 10-3 M Na2CO3 aqueous solution (pH 6), and the frequency change of the crystal was monitored as a function of time of immersion in the electrolyte solution. The superstaturation ratio (SR) for this experiment is calculated to be 60. The growth of strontium carbonate in AOT thin films for other studies was carried out using the optimum immersion times determined from the QCM kinetics measurements mentioned above. The SrCO3 crystals grown within 500 Å thick AOT films were subjected to FTIR measurements in the transmission mode at a resolution of 4 cm-1 on a Shimadzu FTIR-8201 PC instrument. XRD measurements of the Sr-AOT and SrCO3-AOT films on glass were carried out in the transmission mode on a Philips PW 1830 instrument operating at 40 kV voltage and a current of 30 mA with Cu KR radiation. SEM measurements of the SrCO3-AOT films on Si(111) substrates were carried out on a Leica Stereoscan-440 instrument equipped with a Phoenix energy dispersive analysis of X-rays (EDAX) attachment. Prior to XRD and SEM studies, these films were subjected to mild ultrasonic agitation in water for ca. 5 min to dislodge any SrCO3 crystals that may have nucleated in solution and bound weakly to the AOT film surface. To determine whether the growth of these crystals was purely a surface process, contact angles were measured at various points on the films. Contact angle measurements were carried out on a 1 µL sessile water drop using a Rame-Hart 100 goniometer on at least 10 different points on the film surface. In control experiments, the crystallization of SrCO3 was accomplished directly in solution by a mixture of aqueous solutions of SrCl2 and Na2CO3 by maintaining SR at 60. The crystals were formed predominantly in the bulk of the solution and slowly settled down at the bottom of the container. The crystals were separated by filtration, washed with water, and separately placed on Si(111) substrates for further analysis. To understand better the role played by AOT molecules in the thermally evaporated film on SrCO3 crystal growth, in yet another control experiment the crystallization of SrCO3 was carried out in aqueous solutions of SrCl2 (50 mL, 10-3 M) and AOT (50 mL, 10-3 M) and by passing CO2 gas [produced by decomposition of (NH4)2CO3] through the solution for 36 h. The crystals were formed predominantly in the bulk of the solution and slowly settled down at the bottom of the container. The crystals were separated by filtration, washed with water, and drop-coated onto Si(111) substrates for further analysis.

Results and Discussion The kinetics of incorporation of Sr2+ ions in a 500 Å thick AOT film was followed by QCM, and the data recorded as a function of time of immersion in the SrCl2 solution and thereafter in the Na2CO3 solution are shown in Figure 1. During immersion in the SrCl2 solution, a fairly large mass increase is seen and is attributed to electrostatically controlled diffusion (and entrapment) of Sr2+ ions in the AOT matrix. At pH 5.5, the sulfosuccinate ions of the AOT matrix are expected to be fully negatively charged, leading to maximum attractive electrostatic interaction with the Sr2+ cations. After entrapment of the Sr2+ ions, the Sr-AOT-covered QCM crystal was immersed in Na2CO3 solution (pH 6) and the frequency change of the crystal was monitored as a function of time of immersion in the electrolyte solution. A large mass increase is observed during this cycle of immersion as well and is due to electrostatically controlled entrapment of the CO32- ions in the Sr-AOT film. From the measured Sr2+ and CO32- ion mass uptake values, the Sr2+:CO32molar ratio within the AOT film is calculated to be 1.3:1, in reasonable agreement with the expected 1:1 ratio. Figure 2 shows FTIR spectra recorded from the asdeposited 500 Å thick aerosol OT film on a Si(111) substrate (curve 1), the aerosol OT film after entrapment of Sr2+ ions (curve 2), and the strontium sulfosuccinate film after reaction with Na2CO3 (curve 3). Prominent

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Figure 1. QCM mass uptake recorded ex situ during immersion of a 500 Å thick AOT film sequentially in Sr2+ and CO32- ion solutions. The inset shows a diagram of the lamellar structure of the thermally evaporated AOT film after entrapment of Sr2+ counterions.

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Figure 3. XRD pattern recorded from SrCO3 crystals grown within a thermally evaporated AOT film at an SR of 60. The inset shows the XRD pattern recorded from a 500 Å thick AOT film after entrapment of Sr2+ ions. The (0 0 l) Bragg reflections are identified in this figure.

absorption bands are seen at 1049, 1700, 2850, and 2920 cm-1 in the case of the as-deposited aerosol OT film (Figure 2B, curve 1). The band at 1049 cm-1 is assigned to the SdO stretching vibration of the sulfonate group present in the AOT molecules.24 The band at 1700 cm-1 is due to carbonyl stretch vibrations in the AOT molecules, and the two bands at 2850 and 2920 cm-1 have been assigned to the methylene symmetric and antisymmetric stretching vibrations in the hydrocarbon chains, respectively. After entrapment of Sr2+ ions in the AOT thin film, the absorption band at 1049 cm-1 (Figure 2A, curve 1) has shifted to 1104 cm-1 (Figure 2A, curve 2), clearly indicating that the Sr2+ ions have complexed electrostatically with the sulfonate groups in the film. The FTIR spectrum recorded from the strontium sulfosuccinate film after reaction with Na2CO3 (Figure 2A, curve 3) is essentially featureless, indicating substantial reorganization of the lipid molecules around the mineral crystals. As briefly mentioned in the Introduction, it has been shown in this laboratory that thermally evaporated fatty

acid films when immersed in electrolyte solutions such as PbCl2 and CdCl2 resulted in the electrostatic entrapment of the metal cations and the spontaneous ordering of the lipid films into a lamellar c-axis-oriented structure.17 We have used this approach to form strontium sulfosuccinate films by immersion of 500 Å thick AOT films in SrCl2 solution. The formation of lamellar strontium sulfosuccinate was ascertained by the presence of characteristic odd-even intensity oscillations in the (0 0 l) Bragg reflections in the XRD pattern of the film17 (Figure 3, inset). These Bragg reflections are not observed in as-deposited AOT films (data not shown for brevity). The c-axis-oriented lamellar structure of a strontium sulfosuccinate bilayer is illustrated in the inset of Figure 1. SEM pictures recorded from a 500 Å thick AOT film on a Si(111) substrate after formation of SrCO3 at an SR of 60 are shown in Figure 4 at different levels of magnification. The low-magnification image (Figures 4A) shows densely populated bundles of well-formed SrCO3 sheets/ ribbons. The length of the SrCO3 ribbons is often in excess of 100 µm, while the widths are typically in the range 2-7 µm (Figure 4B). EDAX analysis of the SrCO3 ribbons within this bundle yielded a Sr:C:O atomic ratio of 1:1.4: 9. While the Sr:C ratio is in fair agreement with the expected stoichiometry, the excess oxygen is likely to be from the underlying silica layer on the Si support. The XRD pattern recorded from the SrCO3 crystals shown in Figure 4A is displayed in the main part of Figure 3. A number of Bragg reflections are identified and have been indexed with reference to the unit cell of the strontianite structure (a ) 5.107 Å, b ) 8.414 Å, c ) 6.029 Å, space group Pmcn).25 An interesting feature of the XRD pattern of the strontianite crystals grown in AOT matrixes is the presence of intense (0 2 0), (2 2 1), and (2 0 2) reflections, indicating some degree of oriented growth of the strontianite sheets/ribbons. At higher magnifications, the texture of individual ribbons is more clearly seen (Figure 4C,D). The stronianite crystals are quite flat and less than 1 µm in thickness (Figure 4C). The surface of the crystals shows interesting texture with a number of regularly organized gaps being observed (Figure 4D). This observation taken together with the fact that the Bragg reflections from the strontianite crystals are fairly broad (Figure 3) indicates that the flat, highly textured ribbons observed

(24) Spectrometric Identification of Organic Compounds, 6th ed.; Silverstein R. M., Webster F. X., Eds.; John Wiley & Sons: New York; p 107.

(25) The XRD patterns were indexed with reference to the unit cell of the strontianite structure from the ASTM chart (a ) 5.107 Å, b ) 8.414 Å, c ) 6.029 Å, space group Pmcn, ASTM chart card no. 5-0418).

Figure 2. FTIR spectra recorded from a 500 Å thick aerosol OT film on a Si(111) substrate (curve 1), the aerosol OT film after incorporation of Sr2+ ions (curve 2), and the strontium sulfosuccinate film after incorporation of CO32- ions (curve 3) in different spectral windows.

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Figure 4. SEM images of SrCO3 crystals formed in a thermally evaporated aerosol OT film (500 Å thick) at different magnifications at an SR of 60.

Figure 5. (A) SEM image of SrCO3 crystals formed in a thermally evaporated stearic acid film. (B) SEM image of SrCO3 crystals grown in solution by a mixture of aqueous solutions of SrCl2 and Na2CO3 at an SR of 60.

in the SEM images are assemblies of smaller strontianite crystallites. We mention here that the ribbonlike morphology of SrCO3 crystals formed within thin AOT thin films as templates has not been observed in the numerous earlier studies on strontianite crystallization.9a,22c,26 To understand better the role of the AOT matrix on the strontianite growth and assembly process, control experiments were performed wherein strontianite crystals were grown both in solution (in the presence and absence of AOT molecules as an additive) and in thermally evaporated stearic acid films under supersaturation conditions similar to those used for the AOT thin film studies. Stearic acid films also spontaneously assemble into lamellar structures upon electrostatic binding of ions and may thus be used to grow barite22a and strontianite22c crystals. Figure 5A shows an SEM image of SrCO3 crystals grown within a 500 Å thick stearic acid film under conditions identical to those adopted for AOT studies (see ref 22c for experimental details), wherein the flowerlike assembly of strontianite crystals can clearly be seen. In this case, there is no evidence of formation of strontianite sheets/ ribbons as observed for AOT matrixes. To determine whether the growth of these crystals within the stearic acid matrix was purely a surface process, we measured the contact angle of a sessile water drop at various points (26) (a) Kuther, J.; Seshadri, R.; Tremel W. Angew. Chem., Int. Ed. 1998, 37, 3044. (b) Kuther, J.; Bartz, M.; Seshadri, R.; Vaughan, G. B. M.; Tremel, W. J. Mater. Chem. 2001, 11, 503.

on the film surface and found that the surface was quite hydrophobic (mean contact angle of 86°). This result clearly shows that the flowerlike strontianite crystals of Figure 5A nucleate and grow within the stearic acid matrix and that the mature crystals are eventually covered with a monolayer of stearic acid that renders them hydrophobic. The strontianite crystals grown in solution in the absence of AOT are shown in Figure 5B, and in this control experiment as well, needle-shaped strontianite crystals are obtained. Unlike in the case of SrCO3 crystals grown in the stearic acid matrix (Figure 5A), there is no evidence of assembly of the crystallites in solution (Figure 5B). The crystallization of SrCO3 was also carried out in solution in the presence of 10-3 M AOT. Figure 6A shows an SEM image of SrCO3 crystals obtained in this control experiment wherein a number of open aggregates of the crystals are observed. At higher magnification, the structure of the aggregates is seen in greater detail to consist of quasilinear assemblies of strontianite needles (Figure 6B). It is thus clear that the AOT molecules in solution coat the strontianite needles and direct their assembly in solution. An important observation in these control experiments is that there is no evidence of formation of strontianite sheets/ ribbons as observed in the case of crystals grown in thermally evaporated AOT films. These experiments clearly underline the special role played by the thermally evaporated AOT molecules in directing the unusualmorphology of the strontianite crystals obtained in this study.

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Figure 6. SEM images of SrCO3 crystals grown in solution in the presence of 10-3 M AOT.

While the exact mechanism leading to the morphology variation in the strontianite crystals within the AOT bilayers is not fully understood and may be a consequence of inhibition of growth along certain crystallographic directions by the anionic molecules in the bilayer, another factor that could contribute is the kinetics of crystal growth and the built-in anisotropy of the bilayer geometry. Yet another factor could be the difference in periodicity of the carboxylate ions in stearic acid and the sulfate groups in AOT that would clearly affect face-specific nucleation of the mineral in both cases. The issue of crystal morphology variation in different thermally evaporated templating matrixes is important and requires more study. As mentioned earlier, the XRD and SEM results suggest some hierarchical assembly of small strontianite crystals into ribbonlike superstructures, and we speculate that this may arise by the following mechanism. During the initial stages of growth of the crystals, the AOT bilayers would distort to accommodate the large structures and eventually rupture to form a monolayer around the crystals. To determine whether the growth of these crystals was purely a surface process, we measured the contact angle at various points on the film surface and found that the surface was quite hydrophobic (mean contact angle of 93°). This is to be contrasted with a contact angle of 64° measured for films of strontianite crystals grown in solution and picked up on to a Si(111) wafer coated with a 500 Å thick AOT film. This interesting result clearly shows that the strontianite ribbons of Figure 4 are covered with a monolayer of AOT that renders them hydrophobic. The likely mechanism is therefore nucleation and growth of the strontianite crystals within the hydrophilic regions of the bilayers in the strontium sulfosuccinate film accompanied by expansion of the lipid matrix (and consequent surface coating) to accommodate the large crystals. The hydrophobic nature of the crystallites points to a possible reason for formation of thin sheets of strontianite. Since the growth of the crystals occurs in an

aqueous environment, hydrophobic forces between the AOT-monolayer-covered strontianite crystals (at least in the very early stages of crystal growth) could lead to aggregation of the crystals into sheetlike/ribbonlike structures as observed. The fact that AOT possesses such organizational capability has been shown by Mann and co-workers in their study of the assembly of prismatic barium chromate crystals grown in AOT microemulsions into linear superstructures.13 We recollect that strontianite needles grown in solution in the presence of AOT as an additive also assembled into quasi-linear superstructures (Figure 6). The fact that AOT molecules in a highly condensed solid thin film state result in a similar hydrophobic association-driven assembly indicates considerable swelling of the strontianite-AOT films during immersion in water. In conclusion, the crystallization of strontium carbonate within thermally evaporated AOT films by a process of Sr2+ ion entrapment and thereafter reaction with CO32ions has been demonstrated. The SrCO3 structures grown in the AOT films exhibit an unusual ribbonlike morphology that possibly arises due to AOT-directed assembly of smaller strontianite crystallites. The ability to assemble strontianite crystals into superstructures on a solid support suggests possible extension to more intricate superstructures by suitable patterning/templating of the substrate surface. Acknowledgment. D.R. thanks the Department of Science and Technology (DST), Government of India, for financial assistance. The assistance of Ms. Supriya Inamdar in some of the experimental work is acknowledged. This work was partially funded by a grant from the DST, Government of India, and this support is gratefully acknowledged. LA026486+