Study of Calcium Carbonate Precipitation under a ... - ACS Publications

The crystal polymorph, morphology, and nucleation face were identified, and a Brewster angle microscope (BAM) was employed to determine the location o...
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Study of Calcium Carbonate Precipitation under a Series of Fatty Acid Langmuir Monolayers Using Brewster Angle Microscopy Eva Loste, Eva Dı´az-Martı´, Ali Zarbakhsh, and Fiona C. Meldrum* Department of Chemistry, Queen Mary University of London, Mile End Road, London E1 4NS, United Kingdom Received November 13, 2002. In Final Form: January 10, 2003 Calcium carbonate was precipitated from a supersaturated calcium bicarbonate solution in association with a series of long-chain fatty acid monolayers, and the location of the growing crystals with respect to the monolayer domain structure was determined by in situ analysis using a Brewster angle microscope (BAM). Increase in the chain length from palmitic acid (C16) to triacontanoic acid (C30) strongly affected crystallization, resulting in changes in the crystal polymorphs and morphologies, in the homogeneity of morphology, and in the distribution of crystals over the monolayer surface. Induction times also increased with longer chain lengths, an effect consistent with the decreased rate of carbon dioxide diffusion through monolayers as a function of increasing thickness. While the palmitic acid films supported the growth of high proportions of aragonite, vaterite, and nonoriented calcite, almost exclusive precipitation of an extremely uniform distribution of oriented, regular triangular calcite crystals was produced under triacontanoic acid films. BAM analysis of the system showed that the crystals preferentially nucleated under condensed domains and at domain boundaries. Nucleation at the domain boundaries was attributed to relaxation of the ordered monolayer structure at the domain interface and freedom of the fatty acid molecules to accommodate the growing nucleus. The changes in crystal morphology observed under the different monolayers were attributed to changes in the monolayer structure as a function of chain length.

Introduction A key theme in biological control of mineralization is the use of organic matrixes to regulate crystal nucleation and growth. It is widely considered that oriented crystal growth is the product of nucleation on an organized organic matrix, whose structure dictates the nucleation face and orientation of a growing crystal. This phenomenon has been demonstrated by in vivo study of mollusk-shell nacre1 and in vitro experiments employing biological macromolecules.2,3 Excellent synthetic systems in which to study the control over crystal nucleation and growth by an organized organic matrix are provided by Langmuir monolayers and selfassembled monolayers (SAMs). Considering the important biogenic solid calcium carbonate, Langmuir monolayers of long-chain aliphatic compounds bearing carboxylic acid, sulfate, phosphonate, and amine headgroups supported the growth of oriented calcite or vaterite, according to the surfactant structure and the subphase concentration, although no in-plane alignment of crystals was observed.4-11 Superior control over crystal nucleation to produce arrays of crystals nucleated on identical faces and co-aligned over large areas was achieved using monolayers of well-defined structures. Total alignment * To whom correspondence should be addressed. E-mail: [email protected]. (1) Worms, D.; Weiner, S. J. Exp. Zool. 1986, 237, 11. (2) Addadi, L.; Weiner, S. Proc. Natl. Acad. Sci. U.S.A. 1985, 82, 4110. (3) Addadi, L.; Weiner, S. Mol. Cryst. Liq. Cryst. 1986, 134, 305. (4) Heywood, B. R.; Mann, S. Adv. Mater. 1994, 6, 9. (5) Biomimetic Materials Chemistry; Heywood, B. R., Ed.; VCH: New York, 1996; p 143. (6) Mann, S.; Heywood, B. R.; Rajam, S.; Birchall, J. D. Nature 1988, 334, 692. (7) Mann, S.; Heywood, B. R.; Rajam, S.; Walker, J. B. A.; Davey, R. J.; Birchall, J. D. Adv. Mater. 1990, 2, 257. (8) Mann, S.; Heywood, B. R.; Rajam, S.; Walker, J. B. A. J. Phys. D: Appl. Phys. 1991, 24, 154.

of calcite crystals was achieved with respect to polymeric Langmuir-Scha¨fer films of 10,12-pentacosadiynoic acid (p-PDA)12 and monolayers generated through linkage of the monolayer components by hydrogen bonding.13,14 Oriented thin films of calcite were grown under monolayers of amphiphilic tricarboxyphenylporphyrin iron(III) µ-oxo dimers in the presence of the soluble additive poly(acrylic acid) (PAA).15 SAMs have also provided excellent control over crystal orientation, both as silanes on silica substrates16 and thiols on gold,17-21 depending on the headgroup expressed. The work described in this article investigates the control of the nucleation and growth of CaCO3 by a series of Langmuir monolayers prepared from long-chain fatty acids of varying chain lengths. The crystal polymorph, (9) 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. (10) Heywood, B. R.; Rajam, S.; Mann, S. J. Chem. Soc., Faraday Trans. 1991, 87, 735. (11) Heywood, B. R.; Mann, S. Chem. Mater. 1994, 6, 311. (12) Berman, A.; Ahn, D. J.; Lio, A.; Salmeron, M.; Reichert, A.; Charych, D. Science 1995, 269, 515. (13) Champ, S.; Dickinson, J. A.; Fallon, P. S.; Heywood, B. R.; Mascal, M. Angew. Chem., Int. Ed. 2000, 39, 2716. (14) 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. (15) Xu, G.; Yao, N.; Aksay, I. A.; Groves, J. T. J. Am. Chem. Soc. 1998, 120, 11977. (16) Archibald, D. D.; Qadri, S. B.; Gaber, B. P. Langmuir 1996, 12, 538. (17) Aizenberg, J.; Black, A. J.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121, 4500. (18) Aizenberg, J.; Black, A. J.; Whitesides, G. M. Nature 1999, 398, 495. (19) 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. (20) Ku¨ther, J.; Seshadri, R.; Knoll, W.; Tremel, W. J. Mater. Chem. 1998, 8, 641. (21) Ku¨ther, J.; Nelles, G.; Seshadri, R.; Schaub, M.; Butt, H. J.; Tremel, W. Chem.sEur. J. 1998, 4, 1834.

10.1021/la026837k CCC: $25.00 © 2003 American Chemical Society Published on Web 02/27/2003

BAM Study of Calcium Carbonate Precipitation

morphology, and nucleation face were identified, and a Brewster angle microscope (BAM) was employed to determine the location of the growing crystals with respect to the domain structure of the monolayers. Previous studies have examined the effect of the surfactant headgroup only, and very few have considered the influence of the monolayer domain structure on inorganic crystal growth.22-25 We demonstrate that the crystal morphology and polymorph are both affected by the chain length. This can be related to the rate of carbon dioxide diffusion through the monolayer,26 and thus the kinetics of CaCO3 precipitation from supersaturated bicarbonate solutions, and to changes in the organization and domain structure of the monolayers according to the chain length.27,28 Experimental Section Preparation of Supersaturated Calcium Bicarbonate Solutions. Supersaturated bicarbonate solutions were prepared by suspension of CaCO3 (99.95%+, Aldrich) in Milli-Q water (resistivity, 18.2 MΩ cm) at a concentration of 0.75 g/dm3. Carbon dioxide gas, which had been passed through a flask of Millipore water, was bubbled through the stirred aqueous suspension for 6-8 h at room temperature. The solution was then filtered to remove any undissolved CaCO3 particles, and CO2 was bubbled through the solution for a further 30 min. The solutions formed were filtered once more prior to each experiment to ensure the absence of CaCO3 particles. Total calcium concentrations were determined by EDTA titration29 and ranged from 7 to 8 mM. Preparation of Surfactant Solutions. Palmitic acid CH3(CH2)14-COOH (PA), stearic acid CH3-(CH2)16-COOH (SA), and triacontanoic acid CH3-(CH2)28-COOH (TA) were purchased from Aldrich, and arachidic acid CH3-(CH2)18-COOH (AA) and lignoceric acid CH3-(CH2)22-COOH (LA) from Sigma. All surfactants were purified by recrystallization from ethanol prior to use. Spreading solutions were prepared by dissolution in chloroform to give concentrations of 10-4 ( 2 × 10-5 M. Crystallization under Langmuir Monolayers. The prepared saturated calcium bicarbonate solutions were poured into a Langmuir trough (Nima Technology, model 611M; maximum working area, 220 cm2), and the air-water interface was swept and aspirated before deposition of the surfactant solution. The surfactant solution (700 µL) was carefully deposited onto the solution surface, and the monolayer was left for 15 min prior to compression. Pressure-area isotherms were recorded while compressing the monolayer at rates of 10 cm2/min until surface pressures corresponding to a condensed film were reached (target pressure of π ) 30 mN m-1). Experiments were also performed in Pyrex crystallizing dishes (volume, 1 L; diameter, 13.5 cm), yielding results very similar to those obtained with the trough (a slight increase (3-5%) in the number of nonoriented crystals was noted). The volume of surfactant solution required to give a condensed film was spread. Each experiment was repeated three times with each surfactant, and in the absence of a monolayer as a control experiment. Analysis of Crystals. Crystals grown in association with the monolayers were removed after 22 h by carefully dipping hydrophilic glass slides in-and-out through the air-water interface. The crystal face growing into the solution is therefore directly deposited on the glass slide. To view crystals from below (22) Whipps, S.; Khan, S. R.; O’Palko, F. J.; Backov, R.; Talham, D. R. J. Cryst. Growth 1998, 92, 243. (23) Backov, R.; Lee, C. M.; Khan, S. R.; Mingotaud, C.; Fanucci, G. E.; Talham, D. R. Langmuir 2000, 16, 6013. (24) Schief, W. R.; Dennis, S. R.; Frey, W.; Vogel, V. Colloids Surf., A 2000, 171, 75. (25) Litvin, A. L.; Sammuelson, L. A.; Charych, D. H.; Spevak, W.; Kaplan, D. L. J. Phys. Chem. 1995, 99, 12065. (26) Hawke, J. G.; Alexander, A. E. In Retardation of Evaporation by Monolayers: Transport Processes; La Mer, V. K., Ed.; Academic Press: New York, 1962. (27) Ramos, S.; Castillo, R. J. Chem. Phys. 1999, 110, 7021. (28) Dutta, P. Colloids Surf., A 2000, 171, 59. (29) Belcher, R.; Nutten, A. J. Quantitative Inorganic Analysis; Butterworth: London, 1962.

Langmuir, Vol. 19, No. 7, 2003 2831 the air/solution interface, slides were placed on the solution surface and were picked up horizontally. The crystals were imaged using a JEOL 6300F scanning electron microscope (SEM) fitted with a field emission source and operating at 5 kV. The slides supporting the crystals were mounted on aluminum sample stubs with conducting carbon tape and were sputter-coated with gold prior to viewing. The proportions of the different CaCO3 polymorphs, which were identified morphologically, were determined by counting crystals from SEM images; 400-600 crystals were sampled in each experiment. Morphological analysis of the crystals was carried out by imaging the crystals in a direction normal to, and from above the monolayer. The angles between the crystal faces meeting at the apex of the crystal were measured in projection on the plane of the monolayer, and the face parallel to the monolayer was indexed by comparing the experimental angles with those obtained from calculated orientation of a regular calcite rhombohedron.16,19 The trigonal-hexagonal unit cell (a ) 4.99 Å; c ) 17.06 Å; symmetry, 3-C3) was employed to describe the calcite structure. The orientation of the calcite crystals nucleating under the monolayer was determined using a JEOL 2000EX transmission electron microscope (TEM), operating at 120 kV. Crystals were transferred to a C-coated, Formvar-covered Cu TEM grid 15-30 min after the beginning of the experiment by passing the grid in-and-out through the solution surface. Individual crystals were then analyzed by bright field imaging and selected area diffraction. In situ morphological analysis of the calcite crystals growing in association with triacontanoic acid monolayers was also carried out using optical microscopy. A monolayer was spread on a saturated calcium bicarbonate solution in a transparent glass placed on the sample stage of an optical microscope which could be operated in both transmitted and reflected light modes. The crystals were imaged after 24 h using both operational modes of the microscope, a process that enabled the nucleation face to be distinguished. While the overall morphologies of the crystals were apparent in transmitted light, the crystal face apposed to the monolayer was observed by viewing the crystals with reflected light. Brewster Angle Microscopy. All BAM experiments were performed using the Langmuir trough described above together with a homemade BAM. The optical elements of the microscope were mounted on optical rails located centrally under two hinged arms. A He-Ne laser (5 mW, 638.2 nm) was used in conjunction with a CCD camera (COHU; sensitivity, 0.65 lux), which was connected to a PC via a frame-grabber card (National Instruments, model PCI-1407). A lens (f ) 3 cm) was employed to focus the image, and an iris to eliminate parasitic light. The incident and reflective angles were precisely adjusted to the Brewster angle of water to attain minimal reflection at the air-water interface. A black nonreflective sheet was also placed on the base of the trough during the experiments, to minimize reflection of the refracted beam. BAM Characterization of Crystal Growth under Langmuir Monolayers. The general procedure described above for the preparation of monolayers in the trough was used during BAM studies. The surfactant solution was spread on the solution, and the BAM was adjusted and focused during the 15 min required for solvent evaporation. The monolayers were then compressed with the moving barrier to a maximum surface pressure of 30 mN/m such that all compressed monolayers covered similar areas (∼80 cm2). Image quality was maximized by excluding parasitic light using the iris. BAM images were recorded during monolayer compression and continuously after the selected surface pressure of π ) 30 mN m-1 had been attained. The location of crystals growing under the monolayers was determined with judicious use of the analyzer. CaCO3 crystals grown under the monolayers scatter light strongly and therefore appear as bright spots. When the analyzer is positioned such that the condensed domains appear bright, the crystals located under these domains are masked. Rotation of the analyzer allows removal of polarized light derived from these domains such that at particular positions of the analyzer, the domains appear dark. The light scattered by the crystals is not polarized, and thus the crystals are imaged at any analyzer position. Crystals under

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Figure 1. Calcium carbonate crystals grown (a) at the air/water interface in the absence of a monolayer and under monolayers of (b) palmitic acid, (c) stearic acid, (d) arachidic acid, (e) lignoceric acid, and (f) triacontanoic acid. All crystals were sampled after 22 h and are oriented such that they are viewed from above the monolayer. Scale bar ) 100 µm. condensed domains were therefore located by rotation of the analyzer such that the domains appeared dark and the crystals bright. An estimate of the induction time for crystal growth was also made using the BAM and was recorded as the time at which images contained over ten CaCO3 crystals, measured from when the solution was poured into the trough. The induction times varied according to the surfactant used and ranged from 25 to 65 min. Control experiments were also carried out in the absence of a monolayer.

Results Morphological Study of CaCO3 Precipitation under Fatty Acid Monolayers. Precipitation at the airwater interface in the absence of a monolayer was observed by eye after 2 h and produced nonoriented, intergrown CaCO3 crystals (Figure 1a). Calcite crystals formed 66% of the precipitate, while vaterite (21%) and aragonite (3%) were also observed in smaller quantities. PA monolayers had little effect on the product of CaCO3 crystallization and gave results similar to those of the control experiments, although aggregation was somewhat less marked (Figure 1b). In contrast, condensed monolayers of all of the longer chain fatty acids supported the growth of nonaggregated crystals, which were uniformly distributed over the solution surface. The homogeneity and nucleation density within the CaCO3 film increased with increase in length of the hydrocarbon chain (Figure 1). Longer induction times (as judged by eye) were also obtained with the longer chain length surfactants: TA (∼1 h 45 min) > LA, AA (∼1 h 30 min) > SA (∼1 h 15 min) > PA (∼55 min). In all cases, precipitation was accelerated as compared with the control experiments. Calcite was the principal polymorph produced under all of the monolayers tested, and it markedly increased in proportion with increase in the chain length (Table 1). While significant quantities of vaterite and aragonite were observed under PA and SA films, the monolayers with longer aliphatic chains principally promoted calcite, together with small amounts of vaterite and aragonite. Calcite was almost exclusively nucleated across TA

Table 1. Proportions of Calcium Carbonate Polymorphs and Morphologies Produced under Fatty Acid Monolayers of Varied Aliphatic Chain Length monolayer crystal form

PA

SA

AA

LA

TA

aragonite vaterite nonoriented calcite type-I calcite type-II calcite type-III calcite total calcite

20 28 32 12 0 8 52

9 23 9 39 1 19 68

0 12 2 26 50 10 88

2 8 3 28 48 11 90

1 5 5 10 0 79 94

monolayers. The morphologies and orientations of the calcite crystals were also affected by the surfactant tail lengths. PA monolayers predominantly generated nonoriented rhombohedral calcite crystals, together with a small number of triangular calcite crystals and rhombohedral platelike crystals with an elevated central region (Table 1, Figure 1b). These oriented crystal forms were the majority product in association with the SA monolayers (Figure 1c). The platelike rhombohedral crystals (Figure 2a) have been described in the literature as type-I calcite, display a basal and side {104} type planes, and are oriented with the (104) face parallel to the monolayer.7-9 The triangular crystals fall into two categories, namely, those which display an elevated central region and have been described as type-II calcite7-9 (Figure 2b) and those which exhibit a planar triangular face parallel to the plane of the image and display a basal and two side {104} type planes and one rough side plane and will be termed here type-III calcite (Figure 2c). It has been suggested from electron diffraction of early growth forms that both type-I and type-II calcite nucleate from a (11 h 0) face and that realignment of type-II crystals during growth produces type-I calcite.7-9 Morphological analysis of the type-III calcite crystals by measurement of the angles between the crystal faces meeting at the apex of the crystal demonstrated that in

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Figure 2. Figures of calcite crystals grown under fatty acid monolayers that display morphologies (a) type I, (b) type II, and (c) type III. Scale bar ) 10 µm. Figure 4. BAM images of uncompressed monolayers (Π ) 0 mN/m) of (a) palmitic and (b) lignoceric acid and compressed monolayers of (c) lignoceric acid at Π ) 29 mN/m and (d) triacontanoic acid at Π ) 27 mN/m. Scale bar ) 0.5 mm.

Figure 3. Crystals grown under a triacontanoic acid monolayer after 24 h incubation time, as imaged in situ using an optical microscope using (a) reflected light and (b) transmitted light. Type III calcite crystals are arrowed.

the orientation shown in Figures 1f and 2c, the upper triangular plane and two planar side faces were of {104} type. Tilting experiments carried out in the SEM further confirmed that the upper triangular face was planar and that it was parallel to the basal {104} face. Individual type-III crystals produced under a TA monolayer at early stages of growth (when crystals were 200-500 nm in size) were also examined by electron diffraction. The vast majority of diffraction patterns corresponded to a 〈421〉 zone, which is representative of a crystal lying on a {104} face. In situ optical microscopy of the crystals, however, strongly suggested that the upper {104} face was not the nucleation face. When viewed in reflected light, the one rough side plane of the crystal appeared parallel to the monolayer while the other crystal faces, which all corresponded to {104} planes, were directed into the solution (Figure 3). The crystals are therefore reoriented on transfer to a solid substrate for SEM and TEM imaging, such that they lie on the large {104} basal plane. Interestingly, the angle measured in the SEM tilting experiments between the unique rough face and the upper/basal triangular faces was almost constant at 137° ( 2°. Given that (11h 4)∧(11 h 0) is 135°, our data are consistent with the assignment of the nucleation face as (11 h 0) by Mann et al.7-9 The calcite precipitated under SA monolayers was almost exclusively type I (39%) and type III (19%) (Table 1). Increase of the chain length by a further two carbons (AA films) resulted in the additional growth of type-II calcite (50% of the calcite crystals present) (Figure 1d, Table 1). Elongated, triangular type-III crystals (characterized by a relatively large, planar upper face) were also observed in relatively high proportions. CaCO3 precipitation in association with LA monolayers produced calcite with morphological features similar to those described for AA (Figure 1e) with elongated type-II calcite and irregularly shaped type-I calcite being the principal forms (Table 1). The crystals produced under the TA

monolayers were the most uniform in terms of distribution over the monolayer surface and morphology. The vast majority of calcite crystals were type-III calcite (79%), which was present in combination with small quantities (10%) of type-I calcite (Figure 1f, Table 1). Brewster Angle Microscopy. Monolayers were imaged before, during, and after compression. The rapid time scale of the crystallization experiments (nucleation occurred in under 30 min under PA monolayers) prevented equilibration of the monolayer prior to imaging. Before compression, BAM images demonstrated the coexistence of LC domains and either LE or gaseous (G) phase (Figure 4a,b). In the case of the LA and TA films, the ordered domains coexist with a dilute gas phase.30 After compression (to π ) 30 mN m-1), monolayers comprised LC and LE/G domains in which the LC domains were predominant (Figure 4c,d). The LC domains were anisotropic in structure, as demonstrated by reversal of the contrast between the LE/G and LC domains on rotation of the analyzer.31,32 Differences in the organization of the monolayers formed were observed according to the chain length of the fatty acids. The size of the LC domains formed prior to compression tended to increase with increase in the surfactant chain length, being in the order of 100 µm for PA monolayers (Figure 4a) and reaching lateral dimensions of 1 mm for LA (Figure 4b) and TA. Increase in the length of the aliphatic chains results in stronger intermolecular interactions, which favors the spontaneous formation of large condensed domains. The structure of all of the compressed monolayers was studied and related to the growth of CaCO3 under the monolayers. In the case of PA monolayers, the compressed film comprised both spherical LC domains surrounded by the LE/G phase and aggregated condensed domains that remained in constant movement after compression. Due to the relatively short length of the PA surfactant chains, the scattering from crystals was quite strong compared with the reflectivity of the condensed monolayer domains. CaCO3 precipitation was therefore observed by identifying (30) Schwartz, D. K.; Schlossman, M. L.; Pershan, P. S. J. Chem. Phys. 1992, 96, 2356. (31) Hosoi, K.; Ishikawa, T.; Tomioka, A.; Miyano, K. Jpn. J. Appl. Phys. 1993, 32, L135. (32) Wolthaus, L.; Schaper, A.; Mobius, D. J. Phys. Chem. 1994, 98, 10809.

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Figure 5. BAM images of calcium carbonate crystals grown under (a) a palmitic acid monolayer after 30 min, (b) a stearic acid monolayer after 1 h 43 min, and (c) the air-water interface, in the absence of a monolayer after 3 h. The inset shows the absence of crystals after 1 h.

bright spots that grew with time. Crystal growth under the PA films appeared to take place randomly with respect to the monolayer structure, such that crystals were distributed uniformly across the film (Figure 5a). Similar results were obtained for SA monolayers, although uncompressed films contained a higher proportion of LC phase than PA monolayers. Compressed SA monolayers were fairly uniform LC films with discontinuities due to entrapped LE/G phase. Rotation of the analyzer demonstrated that the LC domains were anisotropic, such that the crystals growing under LC domains could be viewed by selecting analyzer angles at which the LC domains appeared dark. Rotation of the analyzer also readily enabled small spherical LC domains which developed during compression (18 mN m-1) and which appear as bright spots to be distinguished from CaCO3 crystals. The CaCO3 crystals primarily grew under LC domains and were often located at the domain boundaries (Figure 5b). Few crystals were present under LE domains. AA monolayers displayed larger condensed domains than PA and SA films at low surface pressures. The proportion of LC phase was also larger at low pressures, and little LE/G phase was observed at surface pressures higher than 15 mN/m. The LE/G phase progressively reduced in quantity as LC domains aggregated while the surface pressure was increased. The final compressed AA monolayers were more uniform than the PA and SA films and exhibited a lower percentage area of LE/G domains.

Loste et al.

Some formation of striplike domains was observed in compressed AA films, although this was less marked than in LA monolayers (discussed below). Rotation of the analyzer again demonstrated that the LC domains were anisotropic. As with SA monolayers, crystals preferentially grew in association with the LC domains and were frequently located at the edges of LC domains. This effect was somewhat less pronounced than for the SA monolayers. LA monolayers exhibited structures similar to those of the AA monolayers during compression. Large macroscopic domains were observed at low surface pressures (Figure 4b), and compression resulted in a film that was principally LC phase, containing enclosed G domains. The LC domains frequently displayed texture in the form of striplike subdomains, with the long axis of the strip lying perpendicular to the direction of compression, and were again shown to be anisotropic (Figure 4c). Subdomains with these morphologies can arise due to the influence of the external shear flow on the monolayer domains.33,34 The pattern of CaCO3 growth under LA monolayers was identical to those observed for SA and AA films with crystals being principally located under condensed domains and at the boundaries of domains. The same monolayer structure and CaCO3 crystallization pattern were observed for TA as for AA and LA films, although in this case the LC domains were principally uniform in light intensity (Figure 4d). Measurement of Induction Times Using Brewster Angle Microscopy. The surface of a supersaturated calcium bicarbonate solution was imaged by Brewster angle microscopy to determine CaCO3 induction times in the absence of a monolayer. Prior to crystal growth, the interface was featureless, and a very low light intensity was recorded (Figure 5c, inset). A subtle texture composed of bright spots, corresponding to CaCO3 crystals, began to be discerned after 1 h 45 min; this was recorded as the induction time. The spots grew with time into well-defined particles homogeneously distributed along the interface (Figure 5c). The presence of the fatty acid monolayers catalyzed CaCO3 crystallization. Crystals started to appear under PA films after 25 min, while SA monolayers yielded induction times of 45 min. A further increase in induction time was observed for AA, LA, and TA films, giving values of 50, 60, and 65 min, respectively. Induction times therefore systematically increased with increasing chain length, and a plot of the induction times versus the number of carbons forming the aliphatic chain of the fatty acids followed an exponential relationship (Figure 6). Discussion The experiments reported here demonstrate that CaCO3 precipitation under fatty acid monolayers is significantly influenced by the aliphatic chain length. This is demonstrated by changes in the distribution of crystals over the monolayer, in the proportions of the polymorphs produced, and in variations in the morphologies of calcite crystals. Induction times were also higher for longer chain lengths, which can be attributed to the decrease in rate of CO2 diffusion with increasing aliphatic chain length. The resistance to diffusion offered by monolayers with identical headgroups increases exponentially with the film thickness.35 That the variation in experimental induction times with increasing chain length as measured here using the BAM similarly followed an exponential relationship (33) Ignes-Mullol, J.; Schwartz, D. K. Langmuir 2001, 17, 3017. (34) Maruyama, T.; Fuller, G.; Frank, C.; Robertson, C. Science 1996, 274, 233.

BAM Study of Calcium Carbonate Precipitation

Figure 6. Graph of the induction time for calcium carbonate crystal growth under fatty acid monolayers, plotted against the number of carbon atoms in the fatty acid chain, where induction times were estimated using the BAM. The solid line represents an exponential fit of the data.

supports this hypothesis. Notably, nucleation under all of the monolayers occurred considerably faster than at the air/water interface in the absence of a monolayer. Ionbinding to the monolayer promotes the local concentration of the ions at the monolayer, catalyzing crystal nucleation and growth.4 Polymorph variation under the monolayers can also be related to the differences in induction times, rather than to structural changes in the monolayers with chain length. While aragonite and vaterite formed in significant quantities under the short-chain PA and SA films, calcite was almost exclusively precipitated under the longer-chain AA, LA, and TA films. Crystallization of CaCO3 occurs on buildup of CO32- in solution, a process which accompanies loss of CO2 from the solution surface. Loss of CO2 and therefore increase in supersaturation occurs more rapidly under the short-chain monolayers. When crystal growth occurs under nonequilibrium conditions, the supersaturation at which nucleation occurs increases with the rate of increase in supersaturation.36 CaCO3 would therefore be expected to precipitate at higher supersaturations under the short-chain monolayers, which favors the metastable phases aragonite and vaterite, as was observed. CaCO3 precipitation under all of the fatty acid monolayers (with the one exception of the short-chain PA) resulted in a uniform distribution of discrete crystals over the monolayer area. The PA monolayer was in constant motion, even when compressed, thus facilitating subsequent aggregation of the crystals. All of the other compressed monolayers exhibited little macroscopic movement, and crystals therefore remain dispersed across the monolayer surface. Similarly, in the absence of a monolayer, although the initial nucleation process occurs uniformly over the water surface (Figure 5c) the crystals do not have a fixed nucleation site and aggregate after growth. Under all of the monolayers (excluding PA in which crystallization took place homogeneously across the film), CaCO3 crystals preferentially formed under the LC (35) Barnes, G. T.; La Mer, V. K. The evaporation resistances of monolayers of long-chain acids and alcohols and their mixtures. In Retardation of Evaporation by Monolayers: Transport Processes; La Mer, V. K., Ed.; Academic Press: New York, 1962.

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domains and were frequently located at domain boundaries. Observation of crystals at the domain boundaries is consistent with BAM studies of calcium oxalate monohydrate precipitation under phospholipid monolayers, where crystals preferentially nucleated at the perimeter of domains.22,23 Simultaneous light-scattering microscopy and fluorescence microscopy of calcium oxalate growth under phospholipid monolayers again demonstrated the location of crystals at domain boundaries, although it was suggested that the particles may migrate to these regions after nucleation.24 In the only BAM study of CaCO3 precipitation under Langmuir monolayers, calcite was the principal polymorph formed under monolayers of diacetylene modified by glycine at surface pressures of π ) 0-5 mN m-1, where the monolayer comprised smectic domains.25 At π ) 20 mN m-1, the monolayer adopted a pseudo-focal-conic texture, and the crystals precipitated were solely vaterite. Notably, the distance between the foci of the monolayer pseudo-focal-conic textures almost exactly equalled the separation of the vaterite crystals, suggesting a direct spatial correlation between the monolayer template and growing crystals. No evidence was obtained in our experiments of the redistribution of crystals associated with monolayers of chain length C g 18, after growth. This suggests that the observed location of crystals at the domain boundaries and under LC domains corresponds to their nucleation sites. That crystal nucleation was promoted by domain boundaries may derive from a number of sources. It has been demonstrated by Brewster angle microscopy37 and Fourier transform infrared (FTIR) studies12 that monolayer organization can alter during crystal growth, accommodating to optimize interaction between the surfactant headgroups and the crystal lattice. This process may occur more readily at the boundary between monolayer domains where the fatty acid molecules have greater freedom of movement. That the proportion of nonoriented calcite is very low for all of the monolayers except PA again supports this hypothesis. A reduction in the line tension between the LC and LE phases23 and/or in the repulsive electrostatic interactions between the LC/LE domains on crystal growth has also been suggested.24 The experiments demonstrate that the morphologies of the calcite crystals varied according to the surfactant chain lengths. Differences in monolayer organization, which occur under identical conditions as a function of the aliphatic chain length,38 may be responsible for these differences. BAM analysis of the compressed monolayers demonstrated that the LC domains of the SA, AA, LA, and TA monolayers were anisotropic in structure. The condensed phases of fatty acid films comprise a rich variety of mesophases, of which the Ov, L2, L2′, and S mesophases exhibit a rectangular lattice and are therefore anisotropic.27,28 The Ov, L2, and L2′ mesophases have additional anisotropy due to molecular tilt, while the S phase is untilted. The LS phase is a rotator phase with a hexagonal lattice and is therefore isotropic. Of the crystalline phases, which possess long-range positional order, both the CS and L2′′ phases have anisotropic rectangular lattices, while only the L2′′ phase contains tilted molecules. As a final source of anisotropy, the orientation of the backbone of the fatty acid molecule about its long axis must also be (36) Ferna´ndez-Dı´az, L.; Putnis, A.; Prieto, M.; Putnis, C. V. J. Sediment. Res. 1996, 66, 482. (37) Zhang, L.; Liu, J.; Pan, Z.; Lu, Z. Supramol. Sci. 1998, 5, 577. (38) Bibo, A. M.; Knobler, C. M.; Peterson, I. R. J. Phys. Chem. 1991, 95, 5591.

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considered. Such herringbone ordering is a source of anisotropy for the untilted L2′′, CS, and S phases.39 Tentative assignment of the phases exhibited by the range of monolayers employed can be made on the basis of published data and phase diagrams, noting that addition of a CH2 group to the aliphatic tail of a fatty acid displaces the phase diagram to higher temperatures by 5-8 °C.40,41 However, the experiments described here were performed on a subphase containing 8-9 mM Ca2+ ions at a pH ≈ 6. This will have a significant effect on the monolayer structures as compared with films spread on water subphases at low pH. Calculations have suggested that under the experimental conditions employed about 60% of the monolayer will be ionized and that a significant fraction of the monolayer will be covered by calcium ions.42 Grazing incidence X-ray diffraction (GIXD) studies of TA and AA monolayers43 and heneicosanoic acid (C20H41COOH) monolayers44 spread on a subphase of CaCl2 have demonstrated that the Ca2+ ions have a strong influence on the monolayer according to the pH. While the monolayer is unaffected at low pH values, increase in the pH causes stronger interaction between the amphiphilic molecules, which results in greater crystallinity and a reduction in the molecular tilt angle.43,44 Compressed TA and LA monolayers at 20 °C and π ) 30 mN m-1 on a water subphase would be assigned as CS phase on the basis of extrapolation of phase diagrams.38,45,46 Structural investigation by electron diffraction and scanning force microscopy of LA monolayers spread at the air/water interface at 293 K showed that domains of L2 phase formed spontaneously on spreading the solution and that a transition from L2 to CS phase occurs on compression beyond π ) 10 mN m-1.47 Assignment of a CS phase to both the TA and LA monolayers in our experiments is consistent with the anisotropic character of the domains (due to long-range herringbone structure39). For π ) 30 mN m-1, 20 °C, and a water subphase, an isotropic LS phase would be predicted for SA and AA monolayers,41 which is inconsistent with the BAM demonstration of anisotropic domains. However, binding of Ca2+ ions at the experimental pH would be predicted to shift the surface pressure-temperature phase diagram of fatty acid monolayers to lower temperatures, suggesting that the AA condensed domains could be the anisotropic S phase.45,48 The PA monolayers could not be analyzed on the bicarbonate solution due to rapid CaCO3 precipitation under this monolayer. However, extrapolation of phase diagrams of fatty acids spread on water at pH 2 at 20 °C and π ) 30 mN m-1 suggests an LS phase,27 an assignment consistent with GIXD studies of PA films deposited onto water at pH 2, 24 °C, and π ) 25 mN m-1.49 Increase in the fatty acid chain length results in stronger interactions between the surfactant chains, which is (39) Johann, R.; Vollhardt, D.; Mo¨hwald, H. Langmuir 2001, 17, 4569. (40) Knobler, C. M.; Desai, R. C. Annu. Rev. Phys. Chem. 1992, 43, 207. (41) Bibo, A. M.; Peterson, I. R. Adv. Mater. 1990, 2, 309. (42) Duffy, D. Personal communication. (43) Bo¨hm, C.; Leveiller, F.; Jacquemain, D.; Mo¨hwald, H.; Kjaer, K.; Als-Nielsen, J.; Weissbuch, I.; Leiserowitz, L. Langmuir 1994, 10, 830. (44) Shih, M. C.; Bohanon, T. M.; Mikrut, J. M.; Zschack, P.; Dutta, P. J. Chem. Phys. 1992, 96, 1556. (45) Rivie`re, S.; He´non, S.; Meunier, J.; Schwartz, D. K.; Tsao, M.W.; Knobler, C. M. J. Chem. Phys. 1994, 101, 10045. (46) Durbin, M. K.; Malik, A.; Ghaskadvi, R.; Shih, M. C.; Zschack, P.; Dutta, P. J. Phys. Chem. 1994, 98, 1753. (47) Kajiyama, T.; Tominaga, R.; Kojio, K.; Tanaka, K. Bull. Chem. Soc. Jpn. 2001, 74, 765. (48) Kenn, R. M.; Bohm, C.; Bibo, A. M.; Peterson, I. R.; Mo¨hwald, H. J. Phys. Chem. 1991, 95, 2092. (49) Weidemann, G.; Brezesinski, G.; Vollhardt, D.; Bringezu, F.; de Meijere, K.; Mo¨hwald, H. J. Phys. Chem. B 1998, 102, 148.

Loste et al.

reflected in the formation of phases with reduced molecular area and greater long-range order.43 This trend is reflected in the increased uniformity of crystals produced under monolayers with longer chains. Of the monolayers studied here, the TA films support the growth of a highly uniform population of calcite crystals, as reflects the fact that they form the largest condensed domains of CS phase, which is crystalline and exhibits long-range order. They may provide larger areas where a good spatial match occurs between the organic film and the nucleation face of calcite, together with a small headgroup area such that the concentration of Ca2+ ions associated with the monolayer will be greater. In addition, a higher proportion of crystals grow under the condensed domains than at domain boundaries under TA monolayers, which may be due to the larger LC domain size and corresponding reduction in length of the domain boundary. In the case of the AA and LA monolayers, the growth of a high proportion of elongated crystals may reflect the formation of striplike subdomains during monolayer compression. Studies on the epitaxial nucleation of ice in association with monolayers of long-chain alcohols have similarly demonstrated that subtle changes in the monolayer structure which occur on variation of the chain length can affect how the monolayer directs crystal growth.50 While the induced freezing point for n odd monolayers reaches an asymptote just below 0 °C, the freezing point for n even chains is asymptotic toward -8 °C. GIXD studies of the long-chain alcohol monolayers demonstrated that a gradual change in the monolayer crystal structure occurred with chain length and that the chains became more tilted with decreasing chain length. This was reflected in an increase in average molecular area and molecular motion and the decrease in the size of ordered domains with shorter chains. The difference in behavior of the n odd and n even series of alcohols was attributed to changes in the headgroup orientation. From the results obtained in the current experiments, it is not possible to determine whether the nucleation face of the crystals also varies according to the fatty acid chain length. It has been suggested that calcite nucleates under stearic acid monolayers from a (11h 0) face, a process which may be promoted by geometrical and stereochemical complementarity between the monolayer and the nucleation face of the nascent crystal.4,9,11 This face was assigned on the basis of morphological analysis of the product crystals (assuming that nucleation was from the rough crystal face) and from electron diffraction of crystals at early stages of growth, when crystals were in the order of 1-3 µm in size.8,10 We have demonstrated here that determination of the nucleation face after transfer of the crystals from the monolayer to a solid substrate has to be treated with care. In the case of the calcite crystals grown under the TA monolayers, the SEM images and electron diffraction analysis of nascent crystals both superficially suggest that the crystals nucleate from a {104} face. However, in situ analysis of these crystals under the monolayer clearly demonstrated that nucleation was from the one rough face and that reorientation of the crystals occurred on transfer to a solid substrate, such that they lay on a large flat {104} face. Even at a very small crystal size of 200-500 nm, the particles reoriented on transfer to a TEM grid for diffraction analysis. Therefore, conclusive assignment of the nucleation face can only be made using in situ analysis techniques, or if there is one unique, well(50) Rapaport, H.; Kuzmenko, I.; Berfeld, M.; Kjaer, K.; Als-Nielsen, J.; Popovitz-Biro, R.; Weissbuch, I.; Lahav, M.; Leiserowitz, L. J. Phys. Chem. B 2000, 104, 1399.

BAM Study of Calcium Carbonate Precipitation

defined face present on an isolated crystal, such as has been observed for CaCO3 growth under hydrogen-bonded monolayers13,14 or monolayers of porphyrin amphiphiles15 or eicosyl sulfate.4 Conclusions Growth of calcium carbonate in association with a series of long-chain fatty acid monolayers clearly demonstrated that variation in the chain length while maintaining a constant headgroup can influence crystal growth. Increase in the chain length from palmitic (C16) to triacontanoic (C30) acid affected the polymorphs produced, the morphologies and homogeneity of morphology of the crystals, the distribution of crystals over the monolayer surface, and the induction time. While palmitic acid monolayers supported the growth of high proportions of aragonite, vaterite, and nonoriented calcite, an extremely uniform distribution of regular triangular crystals grew under the triacontanoic acid monolayers. Polymorph selectivity was attributed to changes in the rate of carbon dioxide diffusion

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through the monolayer, and thus the kinetics of crystal growth. Similarly, the increase in induction time with increase in chain length could be fitted to an exponential relationship, which is consistent with the decreased rate of carbon dioxide diffusion through monolayers as a function of chain length. In situ analysis of crystallization under the monolayers using Brewster angle microscopy demonstrated that the crystals preferentially nucleated under condensed domains and at domain boundaries and that the proportion of crystals growing under the LC domains increased with the chain length. The changes in crystal morphology observed under the different monolayers were attributed to changes in the monolayer structure as a function of chain length. Acknowledgment. We thank London University Central Funds, the Nuffield Foundation, the Royal Society, and the EPSRC (Grant No. GR/N65585/01) for funding. LA026837K