CRYSTAL GROWTH & DESIGN
Vaterite Polymorph Switching Controlled by Surface Charge Density of an Amphiphilic Dendron-calix[4]arene Fricke,†
Marc Dirk Andreas Hirsch#
Volkmer,*,†
Carl E. Krill
III,‡
Michael
Kellermann,#
2006 VOL. 6, NO. 5 1120-1123
and
Department of Chemistry, AC II, UniVersity of Ulm, Albert-Einstein-Allee 11, D-89081 Ulm, Germany, Materials DiVision, UniVersity of Ulm, Albert-Einstein-Allee 47, D-89081 Ulm, Germany, and Institute of Organic Chemistry, UniVersity of Erlangen-Nu¨rnberg, Henkestrasse 42, D-91054 Erlangen, Germany ReceiVed October 11, 2005
ABSTRACT: The highly charged amphiphilic dendron-calixarene (1) forms stable monolayers at the air-water interface. Crystallization of CaCO3 beneath monolayers of 1 selectively leads to growth of the metastable polymorph vaterite at low surface pressure, in distinct contrast to the growth characteristics of calcite or aragonite crystals under less highly charged calixarene monolayers, as we have reported previously. This result lends further support to our hypothesis that surface charge density is the dominant factor governing heterogeneous nucleation of CaCO3 crystals at the monolayer/solution interface. Introduction The biological processes that lead to a wide variety of biomineral structures are largely unsolved.1 According to the widely accepted “template-matrix hypothesis”, the organic matrix of calcified tissues (e.g., bones, mollusk shells) contains specific macromolecules that promote site-selective nucleation of crystals and determine the crystal polymorph and spatial orientation(s) of the overgrowing crystal layer.2 Calcium carbonate (CaCO3) is the most abundant crystalline biomineral, existing in three stable polymorphsscalcite, aragonite, and vateritesall of which are found in calcified tissues.3 Vaterite is thermodynamically the least stable (nonhydrated) CaCO3 polymorph, and it rapidly transforms into calcite or aragonite in aqueous solution. Perhaps for this reason, organisms that are able to control the polymorphism of CaCO3 are far less likely to grow calcified tissues or protective shells of vaterite than of the other two polymorphs.4 To gain insight into the putative mechanisms controlling CaCO3 polymorph selection at interfaces, researchers have employed artificial matrixes such as Langmuir monolayers,5 selfassembled monolayers (SAMs),6 polymers,7 and biomacromolecules.8 From a 4.5 mM Ca solution, vaterite crystals were observed to grow beneath a monolayer of stearic acid at high surface pressure (π ) 20 mN/m), whereas uniformly oriented calcite crystals were obtained from a 9 mM Ca solution.9 The selective nucleation of oriented calcite crystals was discussed in terms of stereochemical complementarity and geometrical matching between the monolayer headgroups and the nucleated crystal face.5a The growth of vaterite from a CaCO3 solution at lower supersaturation was conjectured to be driven by electrostatic and stereochemical matching, which was assumed to overcome the geometrical mismatch between the monolayer and the nucleated crystal face.10 Crystallization of calcite underneath monolayers of a glycine-terminated diacetylene lipid covering a 9 mM Ca solution at low surface pressure (π ∼ 0-5 mN/m) and crystallization of vaterite taking place at high surface pressure (π ) 20 mN/m) were discussed in the same context.5b * To whom correspondence should be addressed. Tel: +49 (0)7315023921. Fax: +49 (0)731-5023039. E-mail:
[email protected]. † Department of Chemistry, AC II, University of Ulm. ‡ Materials Division, University of Ulm. # University of Erlangen-Nu ¨ rnberg.
Vaterite also crystallized on SAMs of carboxylate-terminated alkylthiols adsorbed on a Au surface. In this system, a remarkable even-odd effect was reported for the number of CH2 groups separating the carboxylate from the thiol groups: an odd-numbered derivative (HS-(CH2)11-COO-, 10 mM Ca solution) predominantly led to vaterite crystals, whereas the even-numbered derivative (HS-(CH2)16-COO-, 10 mM Ca solution) selectively promoted growth of calcite crystals.6 However, with structurally similar SAMs, CaCO3 crystallization from a more concentrated Ca solution (20 mM Ca) was found to lead exclusively to uniformly oriented calcite crystals.11 In light of such widely varying results and the significant differences in the underlying experimental conditions, it is apparent that no clear-cut evidence in favor of a specific mechanism for CaCO3 polymorph switching can be drawn from prior studies. To address this problem, we have synthesized a set of amphiphilic calixarene12 and resorcarene13 molecules, designed in such a manner as to permit the variation of a single structural parameter, namely, the average number of carboxylic acid residues per area exposed to the aqueous subphase, while keeping all other experimental conditions as constant as possible. Experimental Section Monolayer experiments were performed with a double-barrier NIMA trough using a compression speed of 15 cm2/min to ensure reproducibility. The surface pressure of the monolayers was measured using a Wilhelmy plate. Langmuir monolayers were formed on aqueous subphases by spreading compound 1 from a trichloromethane/methanol (9:1) solution (50 µL, 0.5 mg/mL). Compression was started after 10 min. Simultaneously, the surface potential was recorded using a vibrating plate located approximately 2 mm above the water surface. The reference electrode, made from stainless steel, was placed in the aqueous subphase. Each isotherm was measured at least five times. Brewster angle microscopy was performed with a NIMA Langmuir trough (NIMA BAM702) using a BAM2plusLE (NFT, Go¨ttingen). Solutions of calcium bicarbonate were prepared by bubbling carbon dioxide gas through a stirred aqueous (double de-ionized H2O, resistance 18.2 MΩ‚cm) solution of CaCl2/NaHCO3 (c ) 9/18 mM) for a period of 2 h. Compressed films were formed by spreading the trichloromethane/methanol (9:1) solutions of surfactants to generate liquidlike films at the air-water interface. Crystals were studied at several time intervals either in situ by optical microscopy (PZO Biolar upright microscope) or on cover slips laid on the film (Olympus IX70). The crystal-growth experiments were repeated at least five times. The cover
10.1021/cg050534h CCC: $33.50 © 2006 American Chemical Society Published on Web 03/29/2006
Vaterite Polymorph Switching
Crystal Growth & Design, Vol. 6, No. 5, 2006 1121
Figure 1. From left to right: Surface pressure-area (π-A) isotherms (dashed curve) and surface potential-area (∆V-A) isotherms (solid curve) of 1 on H2O (22 °C) and on CaCl2 (c ) 10 mM, 22 °C).
Scheme 1
slips were also mounted on scanning electron microscope (SEM) specimen holders for investigation with a Zeiss DSM 962 scanning electron microscope operating at 15-20 keV. The CaCO3 crystals were sputtered with Pd/Au prior to examination. Bulk samples for x-ray diffraction (XRD) were obtained by collecting the crystals from the air-water interface and filling a Mark capillary of 0.2 mm diameter. X-ray diffraction scans were recorded in θ-θ geometry using Cu KR radiation (λ ) 1.54 Å) with a PANalytical X’Pert PRO MPD X-ray diffractometer.
Results and Discussion Our previous investigations of the growth of polycrystalline aragonite beneath monolayers of octacarboxy-resorcarene suggest that monolayers generating a sufficiently high surface charge density are able to induce and stabilize metastable crystal phases at the air-water interface.14 To test this hypothesis, we employ in this study a monolayer of the highly charged amphiphilic molecule dendron-calix[4]arene (1),15 which is comprised of 18 carboxylic acid residues per molecule (Scheme 1). Langmuir monolayers were formed on different aqueous subphases by spreading compound 1 from trichloromethane/ methanol (9:1). Figure 1 shows the surface pressure-area (π-A) isotherms and the simultaneously recorded surface potential-area (∆V-A) isotherms of compound 1 covering aqueous subphases containing either pure water or CaCl2 (10 mM). Compound 1 forms stable monolayers that begin to collapse upon compression at a surface pressure above ∼50 mN/m on pure water and above ∼38 mN/m on CaCl2 (10 mM)sremarkably high values, considering the high degree of water solubility of 1.15 The featureless isotherms and the monotonic pressure increase upon compression indicate that the
condensed monolayer phase manifests fluidlike properties. The onset of the pressure increase occurs at a surface area of 230 to 240 Å2/molecule on all subphases, with the estimated mean area per molecule amounting to 200 Å2. For comparison, amphiphilic calix[4]arenes comprised of the same hydrophobic alkyl chains as 1 but dihydroxyphosphonyl headgroups yield surface areas of ∼90 Å2 on water or Ca-containing subphases.15 For a subphase of aqueous HCl (pH 2) the surface potential isotherms manifest a steplike variation that is not seen in the case of pure water; the step corresponds to protonation of 1 (∆VC ) 200 mV on H2O and 325 mV on pH 2, see Supporting Information). On a subphase containing CaCl2 (10 mM CaCl2), the surface potential increases to values similar to those reached on pH 2, which can be attributed to electrostatic/coordinative interactions between the hydrophilic headgroups of 1 and the Ca ions in the aqueous subphase (∆VC ) 325 mV on CaCl2).16 Thus, we deduce that the measured values of surface potential arise from the molecules of 1 being oriented normal to the plane of the monolayer, with the bulky and polar headgroups directed toward the aqueous subphase and the hydrophobic tails exposed to air. There is no experimental evidence for alternative packing arrangements, such as formation of an interdigitated layer.18 Protonation of 1 as well as coordination of Ca ions by 1 give rise to an increase in surface potential. For comparison, the surface potential of stearic acid at collapse pressure is 285 mV (H2O), 390 mV (pH 2) and 345 mV (10 mM CaCl2).17,19 Furthermore, the Langmuir isotherms and Brewster-angle microscopy (BAM) images of 1 indicate that, at the onset of pressure, the monolayers are in a liquid-expanded state on water and in a liquid-condensed phase on a Ca-containing subphase
1122 Crystal Growth & Design, Vol. 6, No. 5, 2006
Figure 2. Optical micrograph of vaterite crystals grown beneath a monolayer of 1 on CaCl2/NaHCO3 (c ) 9/18 mM) after 12 h (π ) 0.5 mN/m).
Fricke et al.
Figure 4. X-ray powder diffraction (XRD) pattern of CaCO3 crystals grown beneath a monolayer of 1 on CaCl2/NaHCO3 (c ) 9/18 mM) after 15 h (π ) 0.5 mN/m). Indexing of the diffraction peaks corresponds to the hexagonal lattice parameters of vaterite.25
0.1-0.5 mN/m). On the other hand, surface charge densities in the range of 4.65-5.00 COO-/nm2 lead to selective crystallization of aragonite, as we and others have shown for monolayers of rccc-4,6,10,12,16,18,22,24-octakis-O-(carboxymethyl)-2,8,14,20-tetra(n-undecyl)-resorc[4]arene14 and 5-hexadecyloxy-isophthalic acid,22 respectively. Finally, the formation of uniformly oriented calcite crystals with the highly polar {01.2} face23 oriented toward the monolayer was observed on many structurally different monolayers, all sharing similar charge densities of 2.0-2.4 COO-/nm2.12,13,24 Conclusions Comparing these systems, we conclude that monolayer packing densities and the resulting surface charge densities play a crucial role in the selection of CaCO3 polymorphs. We are currently extending our investigations to CaCO3 growth beneath monolayers of appropriately designed amphiphiles providing charge densities not yet considered in previous investigations. Figure 3. Scanning electron micrograph of vaterite crystals grown beneath a monolayer of 1 on CaCl2/NaHCO3 (c ) 9/18 mM) after 18 h (π ) 0.5 mN/m).
(see Supporting Information). CaCO3 crystallization experiments show that regions of homogeneous contrast in the BAM images (corresponding to a pressure range of 0-0.5 mN/m) provide optimal conditions for the growth of vaterite crystals. Crystallization of CaCO3 beneath a monolayer of 1 at low surface pressure (π ) 0-0.5 mN/m) leads to formation of vaterite crystals showing a typical floret morphology (Figures 2 and 3),20 the crystal phase of which was confirmed by X-ray diffraction (Figure 4). Our experiments indicate that it is possible to manipulate the surface charge densities in monolayers by the appropriate design of amphiphilic molecules. Considered along with the results of our previous investigations, the evidence presented here illustrates that monolayers generating a sufficiently high surface charge density are able to induce and stabilize metastable crystal phases at the air-water interface. The switching between polymorphs of calcium carbonate possessing different thermodynamic stabilities occurs above a critical surface charge density of the monolayer molecules at which vaterite or aragonite forms as opposed to calcite, primarily as result of a kinetically controlled nucleation process.21 Thus, in the present investigation monolayers of 1 induce the formation of vaterite at a surface charge density corresponding to 6.7-7.2 COO-/nm2 (π )
Acknowledgment. Financial support for this work was provided by the Deutsche Forschungsgemeinschaft (DFG Schwerpunktprogramm 1117, “Prinzipien der Biomineralization”; DFG Grant Vo829/3). A.H. thanks the DFG for financial support (DFG grant HI 468/13-1). Supporting Information Available: Surface pressure area isotherm and surface potential area isotherm of 1; BAM micrographs of monolayers of 1. This material is available free of charge via the Internet at http://pubs.acs.org.
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14, 582-589. (c) Naka, K.; Tanaka, Y.; Chujo, Y. Langmuir 2002, 18, 3655-3658. Falini, G.; Fermani, S.; Vanzo, S.; Miletic, M.; Zaffino, G. Eur. J. Inorg. Chem. 2005, 162-167. (b) Lakshminarayanan, R.; Chi-Jin, E. O.; Loh, X. J.; Kini, R. M.; Valiyaveettil, S. Biomacromolecules 2005, 6, 1429-1437. Rajam, S.; Heywood, B. R.; Walker, J. B. A.; Mann, S.; Davey, R.; Birchall, J. D. J. Chem. Soc., Faraday Trans. 1991, 87, 727-734. It should be noted that a monolayer of arachidic acid leads to predominant formation of calcite crystals under experimental conditions identical to the present and previous investigations of CaCO3 crystal growth beneath calixarene monolayers (conc. of CaCl2 (NaHCO3) ranging from 4.5 to 9 mM (9-18 mM)). Han, Y.-J.; Aizenberg, J. Angew. Chem., Int Ed. 2003, 42, 36683670. Volkmer, D.; Fricke, M.; Vollhardt, D.; Siegel, S. J. Chem. Soc., Dalton Trans. 2002, 4547-4554. Volkmer, D.; Fricke, M.; Agena, C.; Mattay, J. CrystEngComm 2002, 4, 288-295. Volkmer, D.; Fricke, M.; Agena, C.; Mattay, J. J. Mater. Chem. 2004, 14, 2249-2259. Kellermann, M.; Bauer, W.; Hirsch, A.; Schade, B.; Ludwig, K.; Bo¨ttcher, C. Angew. Chem., Int. Ed. 2004, 43, 2959-2962.
Crystal Growth & Design, Vol. 6, No. 5, 2006 1123 (16) Houel, E.; Lazar, A.; Da Silva, E.; Coleman, A. W.; Solovyov, A.; Cherenok, S.; Kalchenko, V. I. Langmuir 2002, 18, 1374-1379. (17) Volkmer, D.; Fricke, M.; Gleiche, M.; Chi, L. Mater. Sci. Eng. C 2005, 2, 161-167. (18) Dynarowicz-Latka, P.; Kkita, K.; Milart, P.; Dhanabalan, A.; Cavalli, A.; Oliveira, O. N., Jr. J. Colloid Interface Sci. 2001, 239, 145-157. (19) Oliveira, O. N., Jr.; Bonardi, C. Langmuir 1997, 13, 5920-5924. (20) At a higher surface pressure (π ) 15-25 mN/m), all three polymorphs of CaCO3 form, which might be due to insufficient longterm stability of the monolayer at high compression. (21) Recently, the crystallization of CaCO3 beneath fatty acid monolayers was discussed in terms of kinetic effects studied by in-situ grazing incidence X-ray diffraction: DiMasi, E.; Oltsza, M. J.; Patel, V. M.; Gower, L. CrystEngComm 2003, 5, 346-350. (22) Litvin, A. L.; Valiyaveettil, S.; Kaplan, D. L.; Mann, S. AdV. Mater. 1997, 9, 124-127. (23) Duffy, D. M.; Harding, J. H. Langmuir 2004, 20, 7630-7636. (b) Duffy, D. M.; Harding, D. M. Langmuir 2004, 20, 7637-7642. (24) Champ, S.; Dickinson, J. A.; Fallon, P. S.; Heywood, B. R.; Mascal, M. Angew. Chem., Int. Ed. 2000, 39, 2716-2719. (b) Volkmer, D.; Fricke, M. Z. Anorg. Allg. Chem. 2003, 629, 2381-2390. (25) Kamhi, S. R. Acta Crystallogr. 1963, 16, 770-772.
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