Molecular Recognition Controls the Organization of Mixed Self

Nov 1, 2007 - Benoit P. Pichon , Cedric Leuvey , Dris Ihawakrim , Pierre Bernard , Guy ... Jeroen A. C. M. Goos , Cong D. Vo , Archan Dey , Chris J. v...
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Langmuir 2007, 23, 12655-12662

12655

Molecular Recognition Controls the Organization of Mixed Self-Organized Bis-Urea-Based Mineralization Templates for CaCO3 Benoıˆt P. Pichon,† Sophie Cantin,‡ Maarten M. J. Smulders,† Matthijn R. J. Vos,† Natalia Chebotareva,† Daniela C. Popescu,† Otto van Asselen,§ Franc¸ oise Perrot,‡ Rint Sijbesma,† and Nico A. J. M. Sommerdijk*,† Laboratory of Macromolecular and Organic Chemistry and Department of Polymer Technology, EindhoVen UniVersity of Technology, P.O. Box 513, 5600 MB EindhoVen, The Netherlands, and Laboratoire de physico-chimie, des polyme` res et des interfaces, UniVersite´ de Cergy Pontoise, 5, mail Gay-Lussac, NeuVille sur Oise, 95031 Cergy Pontoise Cedex, France ReceiVed August 24, 2007 To investigate the role and importance of nondirectional electrostatic interactions in mineralization, we explored the use of Langmuir monolayers in which the charge density can be tuned using supramolecular interactions. It is demonstrated that, in mixed Langmuir monolayers of bis-ureido surfactants containing oligo(ethylene oxide) and ammonium head groups associated with matching or nonmatching spacers between the two urea groups, the organization is controlled by molecular recognition. These different organizations of the molecules lead to different nucleation behavior in the mineralization of calcium carbonate. The formation of modified calcite and vaterite crystals was induced selectively by different phases of mixed monolayers, and they were characterized by SEM, TEM, and SAED. To understand the influence of the mixed Langmuir monolayers on the crystallization process, we studied the mixtures by means of (π-A) isotherms and Brewster angle microscopy observations. Infrared reflection-absorption spectroscopy experiments were also performed on Langmuir-Schaefer films. From these results, we conclude that the local organization of the two systems discussed here gives rise to differences in both charge density and flexibility that together determine not only polymorph selection and the nucleation face but also the morphology of the resulting crystals.

1. Introduction Biomineralization involves the formation of hierarchical inorganic structures controlled by an organic matrix.1 The close association of inorganic and organic components leads to composite biomaterials with often remarkable properties tuned to specific applications. These processes require defined arrangements of proteins or other biomacromolecules bearing specific functionalities. To understand the underlying control mechanisms, many scientists have mimicked crystal growth at interfaces defined by simpler but precisely designed molecular assemblies. For the study of calcium carbonate mineralization, for example, self-assembled,2 Langmuir,3 and LangmuirBlodgett4 monolayers as well as substrate-adsorbed peptides have been used.5 Despite the number of investigations, still different views exist on how a template exposing an organized array of functional groups directs the nucleation of the inorganic crystals.6 Geometrical lattice matching and orientational complementarity between the template functional groups and the ions at the template/crystal interface have been considered key elements in * Corresponding author. Telephone: 0031 40 247 5870. Fax: 0031 40 245 1036. E-mail: [email protected]. † Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology. ‡ Universite ´ de Cergy Pontoise. § Department of Polymer Technology, Eindhoven University of Technology. (1) Mann S. Biomineralization: Principles and Concepts in Bioinorganic Materials Chemistry; Oxford University Press: New York, 2001. (2) (a) Archibald, D. D.; Quadri, S. B.; Gaber, B. P. Langmuir 1996, 12, 538. (b) Wurm, D. B.; Brittain, S. T.; Kim, Y.-T. J. Mater. Sci. Lett. 1996, 15, 1285. (c) Ku¨ther, J.; Nelles, G.; Seshadri, R.; Schaub, M.; Butt, H.-J.; Tremel, W. Chem.-Eur. J. 1998, 4, 1834. (d) Ku¨ther, J.; Seshadri, R.; Tremel, W. Angew. Chem., Int. Ed. 1998, 37, 3044. (e) Aizenberg, J.; Black, A. J.; Whitesides, G. M. Nature 1998, 394, 868. (f) Aizenberg, J.; Black, A. J.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121, 4500. (g) Travaille, A. M.; Donners, J. J. J. M.; Gerritsen, J. W.; Sommerdijk, N. A. J. M.; Nolte, R. J. M.; van Kempen, H. AdV. Mater. 2002, 14, 492.

determining crystal orientation. More recently, template adaptability7 and the role of nondirectional electrostatic interactions have been emphasized.8,9 To investigate the role and importance of nondirectional electrostatic interactions in more detail, we explore here the use of Langmuir monolayers in which the charge density can be tuned using supramolecular interactions. In addition to the conventionally compressed Langmuir films and self-assembled monolayers, monolayers based on selforganization by hydrogen-bonding interactions have also been used.3h,7b,10 It was demonstrated recently that the incorporation of bis-urea moieties in surfactant molecules can be used not only (3) (a) Mann, S.; Heywood, B. R.; Rajam, S.; Birchall, J. D. Nature 1988, 324, 692. (b) Mann, S.; Heywood, B. R.; Rajam, S.; Walker, J. B. A.; Roger, J. D.; Birchall, J. D. AdV. Mater. 1990, 5, 257. (c) Rajam, S.; Heywood, B. R.; Walker, J. B. A.; Mann, S.; Davey, R. J.; Birchall, J. D. J. Chem. Soc., Faraday Trans. 1991, 87, 727. (d) Heywood, B. R.; Mann, S. Chem. Mater. 1994, 6, 311. (e) Didymus, J. M.; Mann, S.; Benton, W. J.; Collins, I. R. Langmuir 1995, 11, 3130. (f) Volkmer, D.; Fricke, M.; Agena, C.; Mattay, J. Cryst. Eng. Commun. 2002, 4, 288. (g) DiMasi, E.; Patel, V. M.; Sivakumar, M.; Olszta, M. J.; Yang, Y. P.; Gower, L. B. Langmuir 2002, 18, 8902. (h) Cavalli, S.; Popescu, D. C.; Tellers, E. E.; Vos, M. R. J.; Pichon, B. P.; Overhand, M.; Rapaport, H.; Sommerdijk, N. A. J. M.; Kros, A. Angew. Chem., Int. Ed. 2006, 45, 739. (4) (a) Berman, A.; Ahn, D. J.; Lio, A.; Salmeron, M.; Reichert, A.; Charych, D. Science 1995, 269, 515. (b) Lahiri, J.; Xu, G.; Dabbs, D. M.; Yao, N.; Aksay, I. A.; Groves, J. T. J. Am. Chem. Soc. 1997, 119, 5449. (5) Addadi, L.; Moradian, J.; Shay, E.; Maroudis, N. G.; Weiner, S. Proc. Natl. Acad. Sci. U.S.A. 1987, 84, 2732. (6) Co¨llfen, H.; Mann, S. Angew. Chem., Int. Ed. 2003, 42, 2350. (7) (a) Aizenberg, J.; Black, A. J.; Whitesides, G. M. Nature 1999, 398, 495. (b) Champ, S.; Dickinson, J. A.; Fallon, P. S.; Heywood, B. R.; Mascal, M. Angew. Chem., Int. Ed. 2000, 39, 2716. (c) Donners, J. J. J. M.; Nolte, R. J. M.; Sommerdijk, N. A. J. M. J. Am. Chem. Soc. 2003, 124, 9700. (8) (a) Volkmer, D.; Fricke, M.; Vollhardt, D.; Siegel, S. J. Chem. Soc., Dalton Trans. 2002, 4547. (b) Volkmer, D.; Fricke, M.; Huber, T.; Sewald, N. Chem. Commun. 2004, 16, 1872. (c) Volkmer, D.; Fricke, M.; Agena, C.; Mattay, J. J. Mater. Chem. 2004, 14, 2249. (d) Volkmer, D.; Fricke, M.; Gleiche, M.; Chi, L. Mater. Sci. Eng., C 2005, 25, 161. (9) (a) Duffy, D. M.; Harding, J. H. Langmuir 2004, 20, 7630. (b) Duffy, D. M.; Harding, J. H. Langmuir 2004, 20, 7637. (c) Duffy, D. M.; Travaille, A. M.; van Kempen, H.; Harding, J. H. J. Phys. Chem. B 2005, 109, 5713. (10) Buijnsters, P.; 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.

10.1021/la7026225 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/01/2007

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Chart 1. Bis-Urea-Based Surfactants Used To Template Mineralization

for controlling organization of molecules through H-bond formation, but also for controlling the interaction between different compounds based on molecular recognition.11,12 Here, we explore the generation of mixed template surfaces by the self-organization of two hydrogen-bonding surfactants at the air-water interface where only one of these is active in the oriented nucleation of calcium carbonate.13-15 For the inactive component, we selected surfactant 1 containing an oligoethylene glycol head group since this moiety has been demonstrated to show little or no activity in mineralization assays.16 For the active component, we used ammonium surfactants 2 and 3 because their head groups are not expected to display directionality in mineralization experiments. Our strategy further involves the mixing of 1 containing a bisurea heptylene moiety, with the two ammonium surfactants of which 2 contains a matching bis-urea heptylene group and 3 a nonmatching butylene group. In this system, mixtures 1 and 3 are expected to phase separate and induce the formation of the combined crystal faces nucleated under monolayers of the pure components. However, for 1 and 2 which have matching bisurea moieties, we may expect mixing at the molecular level and the formation of monolayers that induce the formation of new calcium carbonate faces depending on the charge density as determined by the ratio of the two components. Chart 1 shows the bis-urea-based surfactants used to template mineralization. 2. Experimental Section 2.1. Crystallization Experiments. CaCO3 crystallization experiments were performed using the Kitano procedure.17 All glassware used was thoroughly cleaned with soap, nitric acid solution (∼10%, overnight), and methanol. The glassware was rinsed three times with ultrapure water (18 MΩ/cm) after each cleaning step. A 9 mM supersaturated Ca(HCO3)2 solution was prepared by bubbling CO2 gas through a suspension of CaCO3 (3.5-4 g) in ultrapure water (1.5 L) for 90 min, followed by filtration and CO2 bubbling another 30 min to dissolve any CaCO3 particles present. A solution of surfactant (1 mg/mL) was spread at the air-water interface of freshly prepared supersaturated Ca(HCO3)2 solution poured in crystallization dishes with a diameter of 60 mm (60 mL/dish). The amount of molecules spread was determined from Langmuir isotherms in the liquid condensed phase and corresponded to a surface pressure of 40 mN/m on the aqueous subphase with a 100% surface coverage. TEM images and selected area electron diffraction (SAED) patterns of crystals collected on carbon-coated TEM grids by Langmuir(11) Chebotareva, N.; Bomans, P. H. H.; Frederik, P. M.; Sommerdijk, N. A. J. M.; Sijbesma, R. P. Chem. Commun. 2005, 4967. (12) Vos, M. R. J.; Jardl, G. E.; Pallas, A. L.; Breurken, M.; van Asselen, O. L. J.; Bomans, P. H. H.; Leclere, P.; Frederik, P. M.; Nolte, R. J. M.; Sommerdijk, N. A. J. M. J. Am. Chem. Soc. 2005, 127, 16768. (13) It is interesting to note that although many Langmuir monolayers of binary mixtures, both miscible and immiscible, have been investigated, few of these have been used as a mineralization template. (14) Stottrup, B. L.; Stevens, D. S.; Keller, S. L. Biophys. J. 2005, 88, 269. (15) For a recent article on the patterning of calcium carbonate mineralization under demixing binary monolayers, see: Muller, H.; Zentel, R.; Janshoff, A.; Janke, M. Langmuir 2006, 22, 11034. (16) (a) Co¨lfen, H.; Qi, L. Chem.-Eur. J. 2001, 7, 106. (b) Pichon, B. P.; Popescu, D. C.; Smulders, M. M. J.; Bomans, P. H. H.; Frederik P. M.; Sommerdijk N. A. J. M. In preparation. (17) Kitano, Y.; Park, K.; Hood, D. W. J. Geophys. Res. 1963, 67, 4873.

Schaefer (horizontal) transfer after 20 min of reaction were performed on a JEOL 2000-FX electron microscope operating at an accelerating voltage 80-120 KV. The ones obtained after 20 h were collected by vertical dipping of glass microscopy slides (diameter ) 15 mm) and examined by using a Jeneval optical microscope and a Philips XL30 scanning electron microscope. The densities of crystals obtained from pure and mixed monolayers were measured by counting crystals collected after 20 h of reaction from pictures obtained by optical microscopy and were plotted as a function of the molar percentage of 2 and 3. 2.2. Langmuir Film Experiments. All the isotherms were recorded in an integrated dust-free cabinet at 20 °C and ambient humidity. Monolayers were formed by spreading solutions of surfactants (1 mg/mL) from CHCl3/MeOH (4:1, v/v) solution on water or on Ca(HCO3)2 subphase in a commercial Teflon Langmuir trough (KSV Minitrough, KSV Instrument Ltd). The monolayer was left undisturbed for 15 min (to allow solvent evaporation) before compression. Computer-controlled symmetrically movable hydrophobic Delrin (polyacetal) barriers were used to regulate the surface area. The trough dimensions were 260 mm × 75 mm × 5 mm. The surface pressure was measured using the Wilhelmy method with a sensitivity of (0.01 mN/m using a 5 × 20 mm2 filter paper. The compression rate was set at 5 mm/min. The Langmuir monolayer stability experiments were achieved by compressing the molecules until a surface pressure of 40 mN/m was reached. The surface pressure was kept constant by continuous adjustment of the surface for 1 h. Monolayer compressibility, C, for each surfactant composition was calculated from isotherms using C ) -(1/A)(dA/dΠ) where A is the limited area per molecule at zero pressure and Π is the monolayer surface pressure.14 2.3. Brewster Angle Microscopy (BAM) Experiments. Experiments were performed with a Brewster angle microscope18 that uses the reflectivity properties of an interface illuminated at the Brewster angle with light polarized in the plane of incidence. The reflected light is sensitive to the density and the thickness of the film. Firstorder phase transitions can be thus observed in this way. Moreover, with an analyzer placed in the path of the reflected light, optical anisotropies due to the tilt of the molecules or to the anisotropy of the unit cell in untilted phases can be detected. On the images, each shade of gray corresponds either to a different tilt-azimuthal orientation of the molecules in tilted phases or to a different orientation of the unit cell in untilted phases. BAM experiments were carried out at the University of Cergy-Pontoise using a homebuilt trough. 2.4. Infrared Spectroscopy and Infrared-Absorption Spectroscopy (IRRAS). Measurements were performed on monolayer films deposited on gold-covered glass substrates. The molecules were compressed to a surface pressure of 40 mN/m and transferred to substrates by the Langmuir-Schaeffer technique. A Biorad FTS6000 FTIR spectrometer equipped with a DTGS detector was used. The spectra were recorded with a resolution of 4 cm-1, and 1500 scans were coadded. For the reflection measurements, a Harrick Seagul accessory was used. The reflection angle was 80°. P-polarized radiation was obtained using a rotatable wire grid polarizer. 2.5. Calculation of Charge Densities at Crystal Surfaces. Calculations were performed according to crystallographic data visualized using Materials Studio 4.0 software. Charge densities of the crystal face attached to the monolayer were calculated from the number of CO32- groups located in the areas of the crystallographic (18) He´non, S.; Meunier, J. ReV. Sci. Instrum. 1991, 62, 936.

Self-Organized Bis-Urea-Based Templates for CaCO3

Figure 1. (a-f) Modified crystals grown under pure monolayers of 2 or 3 or mixtures of 1 and 3. (g-l) Crystals grown under mixtures of 1 and 2. (a, d, g, j) SEM pictures of mature crystals obtained after 20 h. (b, e, h, k) TEM pictures of early crystals (20 min). (c, f, i, l) SAED patterns of modified crystals grown under the monolayers. (a, b, c) (10.0) Modified calcite (type I) and (d, e, f) (11.0) modified vaterite grown under pure monolayers of 2 and 3 and mixtures of 1 and 3. (g-i) (00.1) Vaterite and (j-l) (10.0) modified calcite (type II) grown under mixtures of 1 and 2. SAED patterns: (c) Reflections: A ) (0 2 4), d ) 1.92 Å; B ) (0 0 6), d ) 2.84 Å; C ) (0 2 10), d ) 1.36 Å; (A∧B) ) 63°; (A∧C) ) 39°; zone axis [10.0] of calcite. (f) Reflections: A ) (1 1 0), d ) 3.57 Å; B ) (0 0-12), d ) 1.41 Å; C ) (1 1 12), d ) 1.43 Å; (A∧B) ) 90°; (A∧C) ) 69°; zone axis [11.0] of vaterite. (i) Reflections: A ) (1 0 0), d ) 3.58 Å; B ) (0 1 0), d ) 3.58 Å; C ) (1 1 0), d ) 3.58 Å; (A∧B) ) 120°; (A∧C) ) 60°; zone axis [00.1] of vaterite. (l) Reflections: A ) (0 0 6), d ) 2.84 Å, B ) (0 2 4), d ) 1.92 Å; C ) (0 2 10), d ) 1.34 Å; (A∧B) ) 82°; (A∧C) ) 38°; zone axis [10.0] of calcite. Accelerating voltage: 100 KV (except for (f): 120 KV), and camera length: 60 cm. planes that were calculated from u and V parameters. For (11.0) vaterite: u ) 8.48 Å, V ) 7.15 Å; (10.0) calcite: u ) 4.99 Å, V ) 17.061 Å and (00.1) vaterite: u ) 4.13 Å, V ) 7.15 Å. The surface areas of the respective 2D unit cells were 60.63, 85.14, and 29.53 Å2 and in all cases contain two carbonate ions in the surface plane.

3. Results Langmuir films were prepared by spreading solutions of 1 containing different amounts of surfactants 2 or 3 (0, 20, 50, 80, and 100 mol %) at the air-water interface. Upon compression, in all cases, high collapse pressures (∼60 mN/m) were observed, indicating the formation of stable monolayers. The ability of these monolayers to control the formation of calcium carbonate was investigated by spreading them on a supersaturated 9 mM Ca(HCO3)2 solution following the Kitano procedure.17 As expected, monolayers of pure 1 produced predominantly unmodified calcite expressing its {10.4} faces (Supporting Information). In contrast, pure monolayers consisting of either 2 or 3 both induced the same mixture of calcite and vaterite. SEM revealed that the calcite in addition to unmodified calcite (approximately 25% of the crystals) consisted of rhombohedral crystals (Type I) exposing an additional triangular truncated facet (Figure 1a). SAED (Figure 1c) of young crystals that appeared rhombohedral in TEM (Figure 1b) suggested that these had nucleated from their (10.0) face. SEM further showed the formation of vaterite crystals displaying a distinct nucleation face from which a flowerlike morphology developed (Figure 1d), similar to those found by Mann et al. under octadecyl amine monolayers.3b In agreement with this, also in the present case, young disklike crystals with outgrowths on opposite sides were observed in TEM (Figure 1e), which according to SAED indeed

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Figure 2. Density of modified crystals grown at the interface of the solution and mixtures of (a) 1 and 2 and (b) 1 and 3.

were (11.0) oriented (Figure 1f). The nucleation of both modified calcite and vaterite under monolayers of 2 and 3 suggests the presence of domains with different nucleation activity already in the pure monolayer. A similar behavior was found previously for octadecyl amine which promotes the nucleation of both (11.0) and (00.1) vaterite.3b As expected, mixtures of the nonmatching molecules 1 and 3 induced the same crystal orientations as did the pure components, that is, a mixture of (10.4) and (10.0) oriented calcite and (11.0) oriented vaterite was obtained after 20 h. Although the results indeed support the proposition that a mismatch between the bisurea groups leads to phase separation, the total amount of the latter two crystal types was not proportional to the amount of 3 present. Instead, the combined amount of (10.0) oriented calcite and (11.0) oriented vaterite increased from 5 to 45% by increasing the amount of 3 from 20 to 80 mol % (Figure 2b). Moreover, the nucleation density under the mixed monolayers was significantly lower than that of the individual components, suggesting that phase separation between the two components may not have been complete. In contrast to the above, the monolayers formed by mixing 1 with different amounts of 2 led, in addition to some unmodified {10.4} calcite, to the nucleation of two new crystal types: vaterite with a hexagonal floret morphology (Figure 1g) and rhombohedral calcite (Type II) with four facets defining an indentation in one of the {10.4} faces (Figure 1j). Both crystal types have been reported previously,3b,5 and in agreement with these reports, TEM studies on young crystals (Figure 1h,k) in combination with SAED (Figure 1i,1) indicated that these calcite and vaterite nucleated from the (00.1) and the (10.0) faces, respectively. Interestingly, increasing the amount of 2 in the mixtures from 20 to 80 mol % did not lead to the formation of different crystal types. Instead, an increase in the number of (00.1) vaterite and (10.0) calcite crystals (with a constant ratio vaterite/calcite ) 0.45 between these two types) was observed (Figure 2a) at the expense of the proportion of unmodified calcite crystals. This suggests that mixing 1 and 2 in different ratios does not lead to a gradual increase in density of isolated ammonium groups in a matrix of oligoethylene chains but results in the formation of domains with a fixed ratio of the two components in a background formed by the excess of 1. Also in this case, the mixing of the two components leads to a decrease in the total nucleation density (modified and unmodified crystals) compared to that of the pure components. The finding that the mixing-in of small amounts of either 2 or 3 in both cases leads to a decrease in the nucleation density further shows that 1 has a non-negligible activity in the nucleation of {10.4} calcite.

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Figure 3. (a) π-A isotherms of monolayers of 1, 2, and 3 recorded on a water subphase at 20 °C. (b) Mean molecular areas of the mixtures 1,2 and 1,3 at 0 mN/m as a function of the amount (mol %) of ammonium surfactants 2 or 3 in a monolayer of 1. The dashed lines represent the theoretical mean molecular areas for ideal mixtures of 1 with 2 or 3. (c) Compressibility for pure and mixed monolayers as a function of the amount of ammonium surfactants 2 and 3 (mol %). (d) Stability in time at a constant surface pressure (40 mN/m) of mixed monolayers of 1,2 and 1,3 both with a molar ratio of 0.5.

To obtain a better understanding of the organization of the template and its effect in the nucleation of calcium carbonate, we performed a more detailed analysis of the (π-A) isotherms of the three compounds and the mixtures (Figure 3a). The isotherm of 1 exhibits a steep increase in the pressure upon compression, and by extrapolating the slope of the curve to zero pressure, a molecular area of 26 Å2/molecule was determined. For 2 and 3, the increase in pressure was more gradual (6.5 and 8.1 mN/Å2, respectively, compared to 15.2 mN/Å2 for 1), and although extrapolation in both cases yielded the same molecular area of 26 Å2, the isotherms clearly show that these two compounds could be compressed to much lower areas before a collapse was observed. The gradual increase in pressure observed for 2 and 3 was attributed to strong interactions of the ammonium head groups with the aqueous subphase, whereas the steeper slope for 3 compared to 2 may result from the difference in intermolecular interactions arising from the antiparallel vs the parallel organization of the urea groups, respectively.19 The higher area at the collapse point for 1 was attributed to the minimal area of densely packed oligoethylene oxide groups. The isotherms of the binary monolayers of 1 with 2 or 3 are all shifted to larger mean molecular areas compared to those of the pure compounds (Figure 3b). As the limiting molecular areas of mixtures of two ideally miscible or completely immiscible compounds are proportional to the ratio of the components, the present results suggest that in all cases partial phase separation occurs.20 Moreover, the increase in the compressibility of mixtures (19) Bantignies, J. L.; Vellutini, L.; Maurin, D.; Hermet, P.; Dieudonne, P.; Wong Chi Man, M.; Bartlett, J. R.; Bied, C.; Sauvajol, J. L.; Moreau, J. J. E. J. Phys. Chem. B 2006, 110, 15797. (20) The limited surface area for total phase separation or ideal mixtures is given by the formula A ) (1 - xEO5)AEO5 + xANH3. AEO5 and ANH3 are the limited surface areas of 1, 2, and 3 at zero pressure.

is steeper for 2 than that for 3 when small amounts of these compounds were incorporated in a monolayer of 1 (Figure 3c). This indicates that there is indeed a better mixing of 1 with 2 than with 3 because of the molecular recognition of the bis-urea groups. A stability measurement at a constant pressure of 40 mN/m demonstrated a larger decrease in average molecular area for the mixture of 1 and 3 compared to the mixture of 1 and 2 (Figure 3d). This points to a faster reorganization of surfactants 1 and 3 within the monolayer and is in good agreement with the nonmatching behavior of both surfactants which promotes the exclusion of one phase from the other. With the aim of studying the behavior of the monolayers under mineralization conditions, we recorded all three pure monolayers and both mixtures of (1,2) and (1,3) also on a supersaturated calcium bicarbonate subphase (Supporting Information). All isotherms exhibited steep increases in the pressure upon compression similar to those recorded on a water subphase. They showed similar collapse pressures, but they all were shifted to higher surface areas. The extrapolation of the slope of the curve to zero pressure indicated an increase in mean molecular areas of 3-5 Å2/molecule (Table 1). This indicates that the monolayers respond to the presence of calcium ions. Nevertheless, the MmA values of both mixtures of (1,2) and (1,3) containing 50% of 1 were still higher than those of the pure monolayers, implying that the partial phase separation still remains also under mineralization conditions. To establish the extent of phase separation, pure and mixed monolayers were imaged by BAM. Before compression (surface pressure π ) 0), all mixtures revealed condensed domains coexisting with an analogous gas phase showing the selforganization of these molecules through H-bonding (Figure 4ac). 1 and 2 were organized in small anisotropic domains that

Self-Organized Bis-Urea-Based Templates for CaCO3 Table 1. Mean Molecular Areas of Pure and Mixed Monolayers on Water and Calcium Bicarbonate Subphases MmA (Å2/molecule)

compressibility on Ca(HCO3)2

monolayers

water

Ca(HCO3)2

(∆Sp/∆MmA)

1 2 3 (1,2) (1:1) (1,3) (1:1)

26 26 26 30 28

29 29 31 32 33

0.002931 0.004568 0.004623 0.003834 0.004536

were clearly circular for 1 (diameters 30-50 µm), whereas for 2, their size (∼5 µm) was close to the limit of resolution of the microscope (∼1.5 µm). This indicated that molecules of both 1 and 2 have a non-negligible tilt angle with respect to the airwater interface and have long range order. Upon compression, these domains remain anisotropic and keep their orientation without fusing, showing the strong self-organization of these molecules (Figure 4d,e). In contrast, 3 formed large isotropic plates up to 1.0 mm2 in size at π ) 0 (Figure 4c) that upon compression fused into a single continuous sheet. This suggests an orientation of the molecules perpendicular to the interface, both before and after compression. The orientation of the molecules with respect to the air-water interface was confirmed by IRRAS on Langmuir-Schaefer films of the pure monolayers transferred to gold-coated glass substrates (Figure 5). As IRRAS is only sensitive to vibrations that have a component oriented perpendicular to the substrate surface, for all compounds only very small residual NH stretching (∼3320 cm-1) and amide I (∼1614 cm-1) vibrations were observed, demonstrating that the urea groups were preferentially oriented parallel to the air-water interface. In addition, the presence of the amide II vibration at ∼1578 cm-1 confirmed their involvement in strong intermolecular hydrogen bonds. Significantly, the asymmetric CH2 vibration (∼2923 cm-1) associated with the all-trans conformation of the alkyl chain was present for all three compounds, whereas the contribution at ∼2932 cm-1 that corresponds to the less ordered gauche-trans conformation was observed only for the monolayers of 1 and 2.21,22 This agrees well with the BAM observation of isotropic domains of 3 in which the dense packing of the molecules confines the alkyl chains in an orientation normal to the air-water interface where the lower degree of organization of 1 and 2 allows the alkyl chains to adopt a tilted orientation. BAM images of compressed mixtures of 1 and 2 showed domains corresponding to two phases with different anisotropies (Figure 6a-c, inset). By increasing the amount of 2, the phase with the highest anisotropy became more dominant, which is consistent with the proposal that in this mixture domains composed of a fixed ratio of the two components are present in a matrix of 1. However, in this respect, it is important to note that this phase-separated structure apparently still exists in the mixture containing 80 mol % of 2, which suggests that within this new phase the content of 1 must be lower than 20 mol %. Compressed mixtures containing 20 mol % of 3 exhibited small domains similar to that described for the mixtures containing 2 (Figure 6d). However, increasing the amount of 3 to 50 and 80 mol % led to the formation of less dense and almost isotropic plates coexisting with smaller anisotropic domains (Figure 6e,f). The formation of plates at higher concentrations of ammonium compound supports the expected phase separation of 1 and 3. (21) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. B 1992, 86, 5145. (22) Vaia, R. A.; Teukolsky, R. K.; Gianneli, E. P. Chem. Mater. 1994, 6, 1017.

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Nevertheless, the presence of some anisotropy in the plates indicated that they do not consist of pure 3 and that some 1 did become mixed into this phase. This interpretation agrees with the partial phase separation suggested by the increase in mean molecular surface area indicated by the (π-A) isotherms (Figure 3b). The fact that increasing the amount of 2 does not lead to a gradual increase of the density of the ammonium groups in a homogeneous monolayer of 1 may be due to attractive forces between the ammonium head groups and the oligoethylene oxide groups of 1 leading to complexes with a preferred stoichiometry. The same type of interactions may also be active to some extent between 1 and 3, explaining the only partial phase separation in this mixture.

4. Discussion From the crystallization results, it is clear that the different monolayers behave as different templates in mineralization experiments. Both surfactants 2 and 3 lead to a continuous monolayer with similar surfaces that are decorated with ammonium head groups at the interface with the aqueous subphase. Although the overall charge densities of these pure monolayers are the same, they will be different at the nanoscopic level. In fact, these monolayers are not continuous and appeared to be built from domains with different packing and orientation. Two different crystal types, (10.0) calcite and (11.0) vaterite, were induced under both monolayers. Although it is often reported that calcite grows from a Stern layer of Ca2+ cations bound to the organic template, our finding that in the present case it nucleates on a positively charged monolayer implies that it also can occur through carbonate binding. Mann et al. reported the formation of (11.0) vaterite under octadecylamine monolayers and proposed that nucleation occurred by indirect stereochemical correspondence.3b However, in the present case the monolayers induce the formation of two distinctly different crystal faces. In the (10.0) face of calcite, the carbonate ions are perpendicular to the surface, whereas they are tilted in the (11.0) face of vaterite (Figure 7). Importantly, pure monolayers of 2 or 3 have the same mean molecular area (26 Å2) although they do not have the same packing and orientation according to BAM and FTIR experiments. Still they induce the same two crystal faces. This implies that crystal nucleation is not influenced by the orientation of the molecules and cannot be related to the positional ordering of carbonate ions with respect to the template. In contrast, it is interesting to note that under the mixed monolayer of 1 and 2 a mixture of (00.1) vaterite and (10.0) calcite is formed. These crystal types both contain carbonate groups perpendicular to the nucleation plane (Figure 7), making it tempting to assume that in this case nucleation may be related to a positional ordering of the first layer of carbonate ions with the monolayer organization. However, as Volkmer et al. proposed that nucleation can be controlled simply by the average charge density of the template,8 it is of interest to look at the relation between the density of the ammonium groups at the monolayer water interface and the density of the carbonate ions in the nucleating crystal faces. From the mean molecular area for the pure monolayers of 1 and 2, a charge density of 3.84 NH3+ groups /nm2 can be derived. The observed crystal faces (Figure 7a,b) have a similar, though not equal, density in CO32- ions (3.33 CO32-/nm2 for (11.0) vaterite and 2.35 CO32-/nm2 for (10.0) calcite). Interestingly, these two crystal types are also observed in mixtures of 1 and 3. This may be expected in the case of a complete phase separation between the two surfactants. However, it follows from BAM experiments that also in this case a partial mixing of the phases

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Figure 4. BAM images recorded on a water subphase. Pure monolayers of 1, 2, and 3, respectively, (a), (b), and (c) at 0 mN/m and (d), (e), and (f) at 40 mN/m. The insets show the optical (an)isotropy of the same region with an analyzer. Very bright spots come from dust.

Figure 5. FT-IR spectra of pure monolayers of 1, 2, and 3 showing (a) the ν CO (amide I) and δ NH amide II bands and (b) the asymmetric CH2 vibrations.

occurs (i.e., a noncomplete phase separation). Although this altered phase composition may explain the observed lowered nucleation density, it is not at all obvious that the type of crystals formed remains unchanged when the charge density under the ammonium-rich domains is lowered by the mixing in of surfactant 1. This therefore indicates that only a small amount of phase mixing takes place. Furthermore, the observed lowering of the nucleation density is significant at all amounts of 1, suggesting that even few molecules induce the formation of small unproductive domains. In contrast, when mixtures of 1 and 2 are used, the resulting ammonium-rich domains (containing >80% of 2) induce the formation of (00.1) oriented vaterite. This crystal plane has a higher charge density (6.81 CO32-/nm2) compared to that of the other crystal planes. This is surprising as, because of the mixing in of 1, the charge density of the mixed phase must be lower (but >3.07 NH3+ groups/nm2) compared to the one of a pure monolayer of 2 (3.84 NH3+ groups/nm2). Moreover, in the

presence of this mixed monolayer, (10.0) calcite with a much lower charge density (2.35 CO32-/nm2-) is also still induced. Taken together, the above observations indicate that in the present system there is not a simple relation between the average charge density of the monolayer and the nucleating crystal plane. Importantly, we observed that in all cases after 4 h both types of vaterite crystals are bigger (up to 70 µm) than those of the calcite types (∼10 µm), suggesting that the former nucleates before the latter. Indeed, in the early stages of crystal formation (between 10 and 30 min), the vaterite crystals were dominant as became apparent from the TEM analysis of young crystals. This is in full agreement with the Oswald rule of stages, which would demand that the metastable vaterite nucleates first under kinetic control, whereas calcite is formed later under thermodynamic control. Previously, we related the compressibility of the template to its ability to nucleate the kinetic product ({01.2} oriented calcite) versus the thermodynamic product ({10.0} oriented calcite).3h It was shown that a more compressible monolayer leads to a larger proportion of the thermodynamic product. This observation was discussed in terms of the higher possibility of a flexible template to adapt to the requirement of a nucleation crystal leading to a “true” templating effect. Also in the present case, the proportion of the thermodynamic product ({10.0} oriented calcite) vs that of the kinetic product (vaterite) is significantly higher for the (1,2) mixture than that of the (1,3) mixture. The order induced by the hydrogen bonds is also likely to be responsible for the formation of {10.0} calcite under the pure monolayers of 2 and 3, in addition to the {11.0.} vaterite that is also formed under monolayers of octadecyl amine that are similar but lacking the structure directing bis-urea moieties. In addition, we note that although two distinct vaterite orientations are nucleated by the different phases, (10.0) calcite is formed in all systems. We therefore conclude that the formation of (10.0) calcite is not strictly dependent on the density of the ammonium groups nor on the precise arrangement of the surfactant molecules. We note that a common feature in all monolayers is the fact that there is a positional ordering of the surfactant molecules with a spacing of approximately 4.6 Å2 that is dictated by the formation of hydrogen bonds between the bis-urea groups. Remarkably, many examples of templates inducing the nucleation of (10.0) calcite consist of linear chains of surfactant molecules connected

Self-Organized Bis-Urea-Based Templates for CaCO3

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Figure 6. BAM images recorded on a water subphase at 20 mN/m. Mixed monolayers of 1 with (a) 20, (b) 50, and (c) 80 mol % of 2, and (d) 20, (e) 50, and (f) 80 mol % of 3. The insets show the optical (an)isotropy of the same region with an analyzer.

Figure 7. Computer models of crystallographic plans corresponding to the faces attached onto the monolayers. Top views (a,b,c). Side views (d,e,f). (11.0) Vaterite (a,d). (10.0) Calcite (b,e). (00.1) Vaterite (c,f).

through hydrogen bonds formed by either amide or bis-urea groups with a repetitive distance of 4.5-5.0 nm. Although unexplained for the moment, the present results also suggest that this hydrogen bond-determined distance is the key in producing the (10.0) face of calcite. Interestingly, the development of these crystals leads to different morphological types under different monolayers. Whereas type I calcite is produced under pure monolayers of 2 or 3 and mixtures of (1,3), the second type is produced under monolayers of (1,2). Recently, we related the development of indented crystals to the flexibility of the template.3h Also in the present case, we observed that the formation of the indentation is associated with the more compressible monolayers (i.e., those consisting of mixtures of 1 and 2; Table 1).

5. Conclusion The results discussed above show that molecular recognition between bis-urea groups can be used to control the phase separation in a binary monolayer system and thereby their nucleation behavior in crystal growth experiments. Using different matching and nonmatching mixtures of the three surfactants, we produced monolayers consisting of domains with different

distributions of ammonium groups. From these two different types of vaterite crystals with different orientations were obtained as well two types of calcite crystals with the same orientation but with different morphological forms. No direct relation can be drawn between the charge density of the template and the density of the carbonate ions in the nucleating crystal planes. However, the observation that the vaterite crystals are always nucleated before the calcite crystals suggests that the two polymorphs are formed under domains with different compositions or structures as the template was found to rearrange during the mineralization process. The fact that in all cases calcite with a (10.0) orientation is found must be related to the only common feature in the different monolayers: the hydrogen bond directed ∼4.6 Å spacing between the molecules. This relation is supported by the fact that, in many other cases where calcite nucleates from its (10.0) plane, a similar (4.5-5.0 Å) hydrogen bond enforced intermolecular distance was present in the template. The monolayers formed for the matching pair of surfactants showed a higher degree of compressibility compared to the mixtures formed from the nonmatching pair. This difference in flexibility is tentatively related to the larger amount of oriented calcite as

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relative to the amount of oriented vaterite observed for the mixtures (1,2) compared to that of the mixtures (1,3). By analogy to previous reports, the higher flexibility of the matching pair may also be related to the formation of indented crystals. Overall, the present results underline that, in biomimetic mineralization of calcium carbonate, not only the structure and charge distribution of the template but also its evolution during the mineralization process plays an important role not only in the determination of the nucleation face but also for the morphological development of the crystals.

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Acknowledgment. We thank NWO CW for financial support and E.W. Meijer for stimulating discussions. Supporting Information Available: SEM image of unmodified calcite, crystal density of unmodified calcite, and (π-A) isotherms of monolayers on water and calcium bicarbonate subphase. This material is available free of charge via the Internet at http://pubs.acs.org. LA7026225