Crystal Engineering of Inorganic Materials at ... - ACS Publications

Chapter 3

Crystal Engineering of Inorganic Materials at Organized Organic Surfaces

Downloaded by CORNELL UNIV on August 28, 2016 | http://pubs.acs.org Publication Date: November 26, 1991 | doi: 10.1021/bk-1991-0444.ch003

Stephen Mann, Brigid R. Heywood, Sundara Rajam, and Justin B. A. Walker School of Chemistry, University of Bath, Bath BA2 7AY, United Kingdom The nucleation and growth of the mineral, CaCO , under compressed Langmuir monolayers has been studied by optical and electron microscopy and electron diffraction. The structure and crystallographic orientation of the mineral formed is dependent on a complex interplay of surface and solution parameters. Negatively charged stearate films induce oriented calcite or vaterite nucleation depending on theCaconcentration in supersaturated solution. The crystals are aligned with the [110] and[001]axes perpendicular to the monolayer surface, respectively. The common features of these interactions are Ca accumulation at the stearate headgroups, partial geometric matching of lattice distances and stereochemical correspondence between carboxylate and carbonate groups at the monolayer/ crystal interface. By contrast, crystallization under positively charged octadecylamine monolayers resulted in almost complete vaterite nucleation independent of Ca concentration. Two distinct crystallographic alignments were observed, viz. [001] and[110].AlthoughCabinding is absent, there is the possibility of stereochemical matching between the binding motif of bicarbonate and the crystal surfaces nucleated under the film. These results indicate the potential of organized organic substrates to control inorganic crystal nucleation and suggest that this may be a viable system for modeling biomineralization processes and as a new approach to controlled materials synthesis. 3

The investigation of surface-reactive peptides and polymers involves a wide range of fields and interests. One rather esoteric relationship is that between inorganic solid state chemistry and organic macromolecular chemistry. The impetus for this apparently unrelated connection arises from studies of biological mineralization in which the nucleation and growth of inorganic materials are controlled by 0097-6156/91/0444-0028$06.00/0 at a rate of approximately 0.18 m h~ for 1 hour. The suspension was thenfilteredand thefiltratepurged with C O 2 gas for 0.5 h to dissolve any remaining crystals. The resulting supersaturated solution had a pH of 5.8-6.0. Experiments were done at total Ca concentrations of 8.5-9.0 mM (estimated by EDTA titration) and successive dilutions of these solutions. At total [Ca] = 9 mM, the supersaturation, defined as S = [ H 2 C O 3 * ] / Kp(CU2) where [ H 2 C O 3 * ] represents the overall concentration of aqueous carbon dioxide and carbonic acid, Κ the solubility of C O 2 in gmol Γ atm" at an ionic strength of 2.34 χ 10~ M , and p(C02) the partial pressure of C O 2 , was 3.7 χ 10 atm. Crystals were examined in situ by optical microscopy and on coverslips dipped through the films. These were also mounted on scanning electron microscopy (SEM) stainless steel stubs. The mature crystals were studied after a period of 21 hours. Crystals at early stages of growth were mounted on carbon-coated formvar-covered copper transmission electron microscopy (TEM) grids by dipping procedures. Imaging and electron diffraction studies were undertaken. Bulk samples for X-ray diffraction (XRD) were obtained by collecting the crystals on glass slides dipped through the air/ water interface. Quantitative X R D was undertaken using calibration curves established for known calcite/vaterite mixtures recorded a

1

1

3

1

1

1

2

3

Sikes and Wheeler; Surface Reactive Peptides and Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


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SURFACE REACTIVE PEPTIDES AND POLYMERS

OM/SEM/HRTEM ED/XRD CPBD co

WB

2 ( g )

PH

(à C

a

Uq)

+

2

H

C

C

Î(aq)

5

C

a

C

0

3(8)

+

C

0

2(

f

l

)

+

H

20(„

Figure 1. Experimental procedure for growth of CaCU3 crystals under compressed Langmuir monolayers. O M = optical microscopy, S E M = scanning electron microscopy, HRTEM - high resolution transmission electron microscopy, ED = electron diffraction, X R D = X-ray diffraction, WB = Wilhelmy balance, CPBD = constant perimeter barrier device.


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on a Philips PW 1710 X-ray diffractometer. Intensities measurements were made at the (104) and (118) reflections of calcite and vaterite respectively. It was found that experiments involving fully compressed monolayers could be readily reproduced inrigorouslycleaned glass crystallization dishes. In these experiments compressed films were formed by adding known amounts of surfactant to generate a solid phase film at the vacuum cleaned air/water interface. The advantage of this approach was that a range of experiments could be performed under identical conditions of system preparation. In particular, the effect of successive dilutions of the stock supersaturated solution could be readily assessed.


Results Control Experiments. In all experiments, crystallization was governed by the slow loss of C O 2 gas from unstirred supersaturated solutions according to the reaction: Ca

2+

+

+

(aq) 2HC03 (aq) - CaCC>3(s) CC>2(g)

+

H20(1)

In the absence of monolayer films crystals grew randomly at the air/water interface. The crystals were calcite, intergrown, non-oriented and had rhombohedral and truncated rhombohedral morphologies (Figure 2a). The particle size distribution was heterogenous (mean = 30 μτη, σ- 12.5 //m). Vaterite was present at the air/water interface in varying amounts (maximum 25 wt%). Crystals formed at the bottom of the containers were invariably discrete non- oriented rhombohedral calcite. The above observations were not significantly changed by lowering the total Ca concentration. Stearic Acid Monolayers, (a) [Ca] - 9 mM. Crystallization under fully and partially compressed stearic acid monolayers at total [Ca] = 9 mM resulted in a white sheet of oriented calcite crystals (70 wt% from XRD). The crystals were of two related morphological types (Figure 2b). Type I crystals were discrete capped rhombohedral plates of pseudo C2V symmetry with four rhombohedral {104} basal edges (Figure 3a) and a roughened upper (104) surface. The mean particle size was variable in different experiments, typically, 60 μτη with a relatively narrow size distribution (σ - 10.5 //m). Type II crystals were triangular in projection with only two {104} basal edges (Figure 3b). Again, the surface apposed to the monolayer was elevated but no upper plate-like (104) face was expressed. The relative ratio of type I to type II crystals was variable. Crystallization under liquid phase stearic acid monolayers at [Ca] = 9 mM gave similar results to those observed for fully compressed films except that the nucleation density was reduced by approximately 30%. Detailed SEM studies suggested that the type I crystals arose from a realignment and subsequent secondary growth of type II crystals at the monolayer surface. The evidence for this was as follows. Firstly, although the type I crystals had smooth rhombohedral {104} side faces they were wedge-shaped (Figure 3a).




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Figure 2. Optical micrographs of (a) control crystals, bar = 50 μτη, (b) oriented calcite under stéarate monolayers [Ca] = 9 mM; arrows highlight different morphological types, bar = 100 μτη, (c) oriented vaterite under stéarate monolayers [Ca] = 4.5 mM, bar = 50 μτη, (d) oriented vaterite under octadecylamine monolayers; arrows highlight different morphological types, bar = 100 μτη.



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3. MANN ET AL

Figure 3. SEM micrographs of calcite crystals grown under stéarate monolayers. (a) type I, (b) type II and (c) intermediate type crystal. Bars in all micrographs = 10 μτη.



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Furthermore, there were consistent differences in crystal texture at different sides of the long diagonal of the upper surface; the thicker half was well defined whilst the thinner side contained cracks and stepped edges (Figure 3a). As these effects were not symmetry related, they must arise from anisotropic growth effects due to the (changing) spatial relationships of the crystals and the membrane surface. Secondly, the extensive elevation of type II crystals was identical to that of the corresponding smaller elevated feature on the upper surface of type I crystals. The elevation comprised three inclined faces, two of which were related by reflection symmetry. The ridges formed by intersection of these three faces ran parallel to the short and long diagonals of the basal rhombohedral plate of the type I crystals. Thus, if we assume that the apex of the elevated faces represents the initial point of attachment of the type I and II crystals, it is clear that both morphological forms nucleate along the same crystallographic direction. Thirdly, some crystals were imaged which were intermediates between the type I and II end members. Figure 3c shows a type II crystal in which there is secondary growth perpendicular to the a axis (long diagonal). The consequence of this further growth is to establish two additional {104} edges and the plate-like morphology of type I crystals. The filling-in of one side of the crystal in this way gives rise to the wedged {104} side faces and the localised structural irregularities seen in the mature type I crystals (Figure 3a). The orientation of the central elevated features of type I and II crystals was determined by morphological examination of mature crystals (SEM) and imaging and electron diffraction studies of early crystals (TEM). Both these approaches (data not shown) gave results consistent with the [110] crystallographic axis aligned perpendicular to the monolayer surface. (b) [Ca] - 4.5 mM. A marked change in the structure of the crystals was observed when the [Ca] concentration was reduced to 4.5 mM. Whereas the metastable polymorph, vaterite, was a minor component of the crystals nucleated under stearic acid at 9 mM, dilution of the solution resulted in oriented vaterite almost exclusively across the monolayer surface (Figure 2c). (At intermediate [Ca] levels, 5-8 mM, both oriented calcite and vaterite were observed). The crystals were discrete, of narrow particle size distribution (e.g., mean=50 μτη, σ-1.6 μτη) and exhibited hexagonal symmetry with the six-fold rotation axis perpendicular to the monolayer surface. This latter observation was consistent with the crystallographic c axis being aligned perpendicular to the monolayer surface. The mature crystals were floret-shaped with a central disk-like core and pseudohexagonal arrangement of plate-like radial outgrowths (Figures 4a and b). The direction of outgrowth was into solution such that the surface of the central disk in contact with the monolayer surface was smooth and partially elevated (Figure 4a). Vaterite crystals at early stages of growth were studied by electron diffraction (16,17) and shown to be single crystals oriented with the c axis aligned perpendicular to the monolayer surface. The size distribution of vaterite disks removed from the trough after 35 minutes was influenced by the degree of film compression. Interestingly, the size distribution for crystals



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3. MANN ET AL.

Figure 4. S E M micrographs of oriented vaterite. (a) view from above a stéarate monolayer, [Ca] = 4.5 mM, (b) view from below a stéarate monolayer, [Ca] = 4.5 mM, (c) type II crystal viewed from above an octadecylamine monolayer, (d) type II crystal, octadecylamine monolayer, side view. Bars in all micrographs = 10 μτη.


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grown under compressed films was increased and the mean size decreased compared with crystals grown under partially compressed monolayers (17). These results suggest that films of increased dynamical freedom may aid uniform nucleation. Octadecylamine Monolayers. Substituting a positively charged amine headgroup for the negative carboxylate headgroup of stearic acid had a pronounced affect on the crystallization process. At [Ca] = 4.5-9 mM, oriented vaterite was the predominant polymorph deposited. Some calcite (30%) in all experiments. Observations of crystals growing in situ showed that both the type I and II crystals maintained their respective orientations throughout the growth process. These crystal types, therefore, develop independently, unlike the type I and II calcite crystals on stéarate films. Type I crystals consisted of a series of overlying radial outgrowths with hexagonal end faces comparable to the vaterite florets formed under stéarate films (Figure 4a and b). Viewed from below the monolayer, the outgrowths were observed to originate from the peripheral edge of a central disk. The symmetry of these crystals is consistent with the crystallographic c axis aligned perpendicular to the monolayer surface. Figures 4c and d show SEM micrographs of type II vaterite crystals imaged from above the monolayer and from the side, respectively. The type II crystals had a pseudo C2v symmetry when viewed from above the monolayer (Figure 4c). The crystals also contained a central disk which was elongated and extended in four directions (± 12° and 24° to the disk long axis) in the plane of the monolayer. The outgrowths were aligned almost directly into the solution subphase in the form of thin hexagonal plates (Figure 4d). These plates were oriented along the crystallographic a axis of vaterite indicating that the type II crystals were oriented with the vaterite (110) face parallel to the monolayer surface. This morphological inference was confirmed by electron diffraction studies of type II crystals at early growth stages 18). Discussion Stéarate films. The above results clearly show that the crystallization of calcium carbonate from supersaturated solution is profoundly influenced by the presence of organized charged surfactant monolayers. Both oriented calcite and vaterite



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can be nucleated under stéarate films whilst oriented vaterite is predominant under monolayers comprising amine headgroups. Although two morphological types of calcite are observed under stéarate films, the S E M results indicate that these are related through a realignment of the initial orientation (type II) followed by secondary growth to give the plate-like form (type I). The crystals are nucleated with the [110] face parallel to the monolayer surface such that rhombohedral growth gives rise to stable {104} faces aligned initially at 45° to the monolayer/solution interface. As the surface area of these faces increases, there is the possibility of realignment due to surface tension effects acting on the well-developed crystal faces with the result that the {104} faces become opposed to the monolayer surface. Why some crystals undergo this realignment whilst others do not is unclear. Nucleation of calcite on the [1Î0] face can be explained in terms of charge, stereochemical and geometric factors. Changes in the compression isotherms (17) were indicative of Ca binding prior to nucleation. Furthermore, the limiting area of these films was 22 λ which is consistent with a hexagonal packed layer of molecules with an interheadgroup spacing of ca. 5Â. This distance is commensurate with a Ca-Ca distance in the plane of the [1Î0] face of calcite. Thus ion-binding may aid nucleation of this face by restricting the arrangement of Ca atoms in two-dimensions. However, there are other calcite faces with 5Â Ca-Ca spacings implying that additional factors are responsible for the preferential stabilization of the [1 Γθ] face. An important feature of the [110] face is the presence of alternate rows of carbonates lying perpendicular to the crystal surface. All these anions are equivalent with a bidentate motif at the surface. Thus the stereochemistry of the carboxylate headgroups match those of the anions in the [110] crystal face (Figure 5). In this respect the Ca-stearate layers represent a sub-unit cell motif which is closely related to the structure of the [1Î0] face. The switch from calcite to vaterite nucleation on stéarate films at low [Ca] suggests that the extent of Ca binding is important for calcite nucleation. Furthermore, there is the possibility of HCO3" intercalation in the Stern layer at higher concentrations. This would require a change in the bonding stoichiometry of Ca at the headgroups from bridging (CaSt2; St = stéarate) to non-bridging (CaSt*). with the excess positive charge balanced by HC03~. The presence of carbonate in thefirstlayer under the film would assist [1Î0] nucleation as carbonates are almost coplanar with Ca atoms in this face. At lower [Ca], both the reduced Ca binding and absence of intercalated HC03~ favour vaterite nucleation on the (001) face. This face contains a unicharged layer of Ca atoms with a second layer of carbonates oriented in a bidentate motif perpendicular to the surface (Figure 6). However, the in plane Ca-Ca distances are 4.2Â which is incommensurate with the inter-headgroup spacing (5 Â). This mismatch can be accomodated particularly at low [Ca] when Ca binding is not extensive over the whole monolayer. It may also be offset by the close stereochemical 2



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Figure 5. Possible structural and stereochemical relationships between the calcite (1Î0) face and a compressed stéarate monolayer. Inter-headgroup spacing, X=5Â, Ca-Ca and carbonate-carbonate distances, Y=4.96Â.

Figure 6. Possible organization at the interface between a vaterite (001) face and stéarate headgroups. Note that the Stern layer Ca atoms (é) have a spacing equivalent to that of the stéarate molecules (a - 8.6 A) whereas the corresponding Ca atoms in the unit cell (o) have a - 7.1 Â. The distance between adjacent Ca or CO3 " layers, 1 /4 c - 4.25 Â. s

2



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Crystal Engineering ofInorganic Materials

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correspondence of the carboxylates and carbonates in the second layer of the incipient crystal surface. The increased uniformity of the immature vaterite crystals on partially rather than fully compressed stéarate films suggests that nucleation on the former can proceed by localized organization of the surfactant molecules (induced by localized Ca localized binding ?) and that this is a single autocatalytic event. Compressed films, on the other hand, possibly remain catalytic over a relatively long time course resulting in episodic nucleation and a range of particle sizes. This is an important observation because it suggests that a degree of dynamical freedom may be beneficial in controlling nucleation on organized organic surfaces. One can envisage a synergistic effect in which ion binding induces local conformational changes in the organicfilmwhich in turn induce further ion binding and consequent oriented nucleation. Such effects may be important in biomineralization and in the development of synthetic organic substrates for controlled crystallization. Octadecylamine Films. The formation of oriented vaterite under positively charged amine monolayers indicates that Ca binding is not a prerequisite for the deposition of this metastable polymorph. This suggests that interactions involving HCO3" and the amine headgroups may inhibit calcite and/or promote vaterite nucleation. Interestingly, vaterite formation in aqueous solution is favoured under conditions of high HC03~/Ca ratio (19). Under such conditions, the normally positive charged surfaces of CaC03 are rendered negative by the surplus of adsorbed anions. Lippmann (20) has suggested that this effect could preferentially stabilise vaterite nuclei over calcite clusters due to the more open structure of vaterite and the increased tolerance of the carbonates in this lattice to disorientation. In this regard accumulation of carbonate under amine films would be consistent with the preferential stabilization of negatively charged nuclei and hence vaterite formation. Why are two distinct vaterite orientations, viz. the a and c axes, observed on the amine films? Clearly, there is no direct stereochemical correspondence between headgroup molecules and lattice ions. However, at pH = 6, the mode of carbonate binding will be through bidendate HCO3" interactions with the amine - N H 3 headgroups. Thus there is the possibility of indirect stereochemical control mediated by the structure of the underlying carbonate-containing boundary layer. Significantly, the common feature of both the (001) and (110) faces nucleated parallel to the organic film lies in the perpendicular orientation of their carbonates with respect to the crystal surfaces. The (001) face has all carbonates perpendicular whilst (110) has a subset in this orientation. Thus, the possibility of weak bidentate binding of HC03~ provides both the general requirement of orthogonal carbonates and the stereochemical flexibility to accommodate the nucleation of both (110) and (001) faces. 2+

+


40



Conclusions The results presented in this paper indicate that charged compressed Langmuir monolayers have the ability to control the oriented nucleation of CaC03 from supersaturated solution. That this is in essence a charge effect is shown by the absence of oriented nucleation under neutral alcohol films. Calcium binding is important for oriented calcite formation. In all cases to date, the orientation of the crystals is such that the carbonate groups are oriented perpendicular to the organic surface and this may be augmented by direct stereochemical matching with surfactant headgroups and through bidentate binding of anions. Geometric relationships may play a role but are secondary effects. Finally, we note that the use of organic films of precise molecular design could be a potential method of tailoring crystal synthesis in general. Thus surface reactive organic surfaces may have an important role in future advances in materials science. Acknowledgments We thank Professor J.D. Birchall and Dr. R. J. Davey for interesting discussions and SERC and ICI pic for financial support. Literature Cited 1. Wheeler, A. P.; Rusenko, K. W.; Swift, D. M.; Sikes, C. S. Mar. Biol. 1988, 98, 71-80. 2. Sikes, C. S.; Wheeler, A. P. In Chemical Aspects of Regulation of Biomineralization, Sikes, C. S.; Wheeler, A. P., Eds.; Univ. of South Alabama Publication Services: Mobile, 1988; pp 15-20. 3. Wheeler, A. P.; Sikes, C. S.; In Biomineralization: Chemical and Biochemical Perspectives, Mann, S.; Webb, J.; Williams, R. J. P., Eds.; VCH Publishers: Weinheim, 1989; pp 95-132. 4. Berman, Α.; Addadi, L.; Weiner, S. Nature 1988, 331, 546-8. 5. Hay, D. I.; Moreno, E. C.; Schlesinger, D. H. Inorg. Persp. Biol. Med. 1979, 2, 271-85. 6. Addadi, L.; Weiner, S. Proc. Natl. Acad. Sci. USA 1985, 82, 4110-4. 7. Campbell, Α. Α.; Ebrahimpour, L.; Perez, S. Α.; Smesko, Α.; Nancollas, G. H. Calcif. Tissue Int. 1989, 45, 122-8. 8. Addadi, L.; Moradian, J.; Shay, E.; Maroudas, N . G.; Weiner, S. Proc. Natl. Acad. Sci. USA 1987, 84, 2732-6. 9. Kallitsis, J.; Koumanakos, E.; Dalas, E.; Sakkopoulos, S.; Koutsoukas, P. G. J. Chem. Soc. Chem. Commun. 1989, 1146-7. 10. Mann, S.; Williams, R. J. P. J. Chem. Soc. Dalton Trans. 1983, 311-6. 11. Mann, S.; Hannington, J. P. J. Colloid. Interface Sci. 1988, 122, 326-35. 12. Bhandarkar, S.; Bose, A. J. Colloid. Interface Sci. 1990, 135, 531.



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13. Landau, E. M.; Levanon, M ; Leiserowitz, L.; Lehav, 8 M.; Sagiv, J. Nature 1985, 318, 353-6. 14. Landau, E. M . ; Popovitz-Bior, R.; Levanon, M . ; Leiserowitz, L.; Lehav, M.; Sagiv, J. Molec. Cryst. Liq. Cryst. 1986, 134, 323-35. 15. Landau, E. M.; Grayer, Wolf, S.; Levanon, M.; Leiserowitz, L.; Lehav, M.; Sagiv, J. J. Am. Chem. Soc. 1989, 111, 143645. 16. Mann, S.; Heywood, B. R.; Rajam, S.; Birchall, J. D. Nature 1988, 334, 692-5. 17. Mann, S.; Heywood, B. R.; Rajam, S.; Birchall, J. D. Proc. R. Soc. Lond. A. 1989, 423, 457-71. 18. Mann, S.; Heywood, B.R.; Rajam, S.; Walker, J.B.A.; Davey, R.J.; Birchall, J.D. Adv. Materials 1990, 2. 257-261. 19. Turnball, A. G. Geochim. Cosmochim. Acta 1973, 37, 1593-1601. 20. Lippmann, F. Estudios geol. 1982, 38, 199-208. RECEIVED August 27, 1990


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