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Langmuir 2006, 22, 11034-11040
Control of CaCO3 Crystallization by Demixing of Monolayers Holger Mu¨ller,† Rudolf Zentel,*,† Andreas Janshoff,‡ and Matthias Janke‡ Institutes of Organic Chemistry, Duesbergweg 10-14, and of Physical Chemistry, Jakob-Welder Weg 11, Johannes Gutenberg-UniVersity Mainz, D-55099 Mainz, Germany ReceiVed June 7, 2006. In Final Form: September 19, 2006 In this paper we describe how to template a demixed monolayer into a spatially patterned inorganic replica. For this purpose a new amphiphilic monomer was synthesized which can be polymerized both in solution and in the monolayer of a Langmuir-Blodgett (LB) trough. Since it inhibits the crystallization of CaCO3, it can be usedsin combination with stearic acid (nucleation-promotor)sto control CaCO3 crystals formed under the monolayer. Investigations of the two-component monolayer (Langmuir isotherms and AFM measurements of transferred films) showsin the biphasic regionsdemixing in solid analogue stearic acid domains and the liquid analogue phase of the monomer. Crystallization of CaCO3 starts under the stearic acid domains whose size varies from less than 100 nm to several tens of micrometers. The addition of poly(acrylic acid) into the subphase hinders the three-dimensional growth of CaCO3 crystals from the monolayer into the solution. Thus, it becomes possible to transfer the pattern of the demixed domains into an inorganic replica of CaCO3.
Introduction The investigation of the biomineralization process is of great interest for the better understanding of the formation of inorganic structures in nature (shells, nacres, corals, and others) and for a successful mimicking of crystal growth under ambient conditions. It is known that patterned acidic groups of proteins have a big influence on the crystallization of, e.g., calcium carbonate and many other inorganic compounds mineralized from nature.1,2 To mimic this natural patterning process, Mann and others used the LB technique and investigated the epitaxial influence of monolayers of natural acidic lipids on the crystallization process. It was found that the phase of the monolayer has a great influence on crystallization3-6 and allows a selection of the crystal modification. It is, however, not possible to control the lateral dimension of the crystals and their growth into the third dimension. On the other hand, it is known that mixtures of lipids can undergo a phase separation when they are spread and compressed on an LB trough. This can lead to a regularly shaped twodimensional biphasic system with a complex and even chiral pattern.7-10 The size of the phase-separated regions can thereby be adjusted, e.g., by varying the surface pressure. If it is possible to template such a biphasic monolayer into an inorganic matrix, it would be possible to select not only the * To whom correspondence should be addressed. E-mail: zentel@ uni-mainz.de. † Institute of Organic Chemistry. ‡ Institute of Physical Chemistry. (1) Addadi, L.; Weiner, S. Angew. Chem. 1992, 104, 159. (2) Falini, G.; Albeck, S.; Weiner, S.; Addadi, L. Science 1996, 271, 67. (3) Mann, S.; Heywood, B. R.; Rajam, S.; Birchall, J. D. Nature 1988, 334, 693. (4) Mann, S.; Heywood, B. R.; Rajam, S.; Birchall, J. D. Proc. R. Soc. London 1989, A423, 457. (5) Weissbuch, I.; Majewski, J.; Kjaer, K.; Als-Nielsen, J.; Lahav, M.; Leiserowitz, L. J. Phys. Chem. 1993, 97, 12848. (6) McConnell, H. M. Annu. ReV. Phys. Chem. 1991, 42, 171 (7) Zhang, L. J.; H. G. Liu; Feng, X. S.; Zhang, R. J.; Zhang, L.; Mu, Y. D.; Hao, J. C.; Qian, D. J.; Lou, Y. F. Langmuir 2004, 20, 2243. (8) Chi, L. F.; Fuchs, H.; Johnston, R. R.; Ringsdorf, H. Thin Solid Films 1994, 242, 151. (9) Kuramori, M.; Uchida; N.; Suehiro, K.; Oishi, Y. Bull. Chem. Soc. Jpn. 2000, 73, 829. (10) Heckl, W. M.; Mo¨hwald, H. Ber. Bunsen-Ges. Phys. Chem. 1986, 90, 1159.
crystal modification but also the crystal shape. Such a laterally controlled templating would be interesting for several reasons: (i) It would allow the self-organization of the mobile organic monolayer to pattern an inorganic material at ambient conditions. (ii) This would serve as a new model system for the lateral control of crystallization in biomineralization processes. (iii) Since the size of the crystallization-inducing domains can be varied on the trough, it might also provide a model system to study the influence of the size of the crystallization-inducing surfaces on the crystallization process. The realization of such a system requires the preparation of a laterally demixed monolayer composed of nucleation-promoting and nucleation-inhibiting domains. In the current study we wanted to pattern a lipid membrane by controlled phase separation into crystallization-promoting domains and a crystallization-inhibiting matrix. As a second stage we wanted to utilize the nucleationpromoting domains to initiate crystallization of inorganic material into an ordered two-dimensional array. As lipid membranes suffer generally from their high fluidity, we wanted to stabilize the phase-separated lipid bilayer by photopolymerization of one phase.11-14 The phase to be polymerized should ideally be the liquid analogue phase, out of which solid analogue domains of the nucleation-promoting lipid emerge. This concept is schematically depicted in Figure 1. A binary mixture consisting of a nucleation-promoting lipid, 2, and a polymerizable nucleationinhibiting lipid, 1, is spread on an LB trough. On compression, the monolayer phase separates into solid analogue domains of the nucleation-promoting lipid 2, which form a regular pattern within the liquid analogue phase of lipid 1 (nucleation-inhibiting and polymerizable). This pattern separates different phases (solid-liquid analogue) and different compositions (lipid 1 or 2). It can be mechanically stabilized by polymerizing lipid 1 in the liquid analogue matrix around the domains of lipid 2. Finally this pattern shall be transferred to an inorganic layer of CaCO3 by limiting crystallization exclusively beneath the solid analogue domains of lipid 2. In addition, it might be necessary to limit crystal growth into the third dimension to keep the surface effects (11) Laschewsky, A.; Ringsdorf, H.; Schmidt, G.; Schneider, J. J. Am. Chem. Soc. 1987, 109 (3), 788. (12) Fichet, O.; Teyssie, D. Macromolecules 2002, 35, 5352. (13) Elbert, R.; Folda, T.; Ringsdorf, H. J. Am. Chem. Soc. 1984, 106, 7687. (14) Laschewsky, A.; Ringsdorf, H. Macromolecules 1988, 21, 1936.
10.1021/la061637k CCC: $33.50 © 2006 American Chemical Society Published on Web 11/16/2006
CaCO3 Crystallization Control by Monolayer Demixing
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Figure 1. Schematic representation of lateral patterning of a mixed lipid layer by demixing, its stabilization by polymerization, and the templating into an inorganic crystal array including the structures of monomer 1 and stearic acid 2.
strong during crystal growth. In this paper we describe the realization of such a system and the laterally controlled templating of a demixed monolayer into an inorganic film.
Results and Discussion Lipids. As a prerequisite for creating a membrane pattern capable of selective mineralization, we first had to choose a suitable pair of lipids consisting of an amphiphilic nucleationinhibiting monomer and a nucleation-promoting amphiphile, which should undergo a phase separation during compression. Therefore, we synthesized monomer 1 (see the Experimental Section, two-step synthesis), which should allow radical photochemical polymerization in the liquid analogue phase12-14 because of its methacrylic group. The nucleation-inhibiting effect of monomer 1 (Figure 1) should result from the hydrophilic headgroup, which possesses only nonionic units such as ester and ether groups. For demixing with the second nucleationpromoting amphiphile, we used a combination of mono- and dialkyl amphiphiles, which tend to phase separate under compression. Stearic acid 2 (Figure 1) was chosen as the nucleation-promoting amphiphile, because it fulfils all the necessary requirements and is well-known in the literature for its epitaxial influence on the calcium carbonate crystallization on an LB trough. Monomer 1 was characterized by monitoring pressure-area isotherms (Figure 2A) at different temperatures. We observed for all temperatures a liquid analogue and a crystal analogue regime. The onset of the coexistence region (lateral pressures) rises with increasing temperature, while its width decreases as expected. In the coexistence region solid analogue domains of uniform size nucleate from the liquid phase and pack regularly. The process can be visualized by fluorescence microscopy (by addition of a fluorescent amphiphile) of the film on the trough (Figure 2C). The minimal area for the amphiphile is found to be 0.44 nm2 at 20 °C. This is slightly larger than that expected for a dialkyl amphiphile (about 0.4 nm2), which may be assumed to result from the big headgroup. Notably, it is also possible to
spread monomer 1 after it has been polymerized in solution to polymer P1. The resulting isotherms are displayed in Figure 2B. The liquid phase and coexistence regime can only be observed at higher temperatures (35 °C). At lower temperatures the polymeric backbone does not allow sufficient mobility to show the liquid analogue phase. Compressing the monolayers of polymer P1 with a rate of 20 cm2/min yields isotherms with a minimal area per monomer unit of about 0.55 nm2. This suggests a hindered packing of the alkyl chains due to the polymer backbone. We found that if the monolayer is compressed at a slower rate the minimal area per monomer unit becomes smaller. Hence, we attribute this to a kinetic effect. The lowest surface area is reached for the polymer P1 prepared on the LB trough by photopolymerization (Figure 2D). In this case, we observed a smaller surface area for the polymer than that for the monomer, and the minimum value per monomer unit (slightly less than 0.4 nm2) corresponds to the densest packing of two alkyl chains. A possible molecular interpretation of this result is illustrated in Figure 3. For monomer 1 the hydrophilic headgroup possesses a large amount of different conformations, which in turn produces a large area per molecule. In polymer P1 this conformational freedom is lost since subsequent hydrophilic spacers leave the PMMA backbone at a distance of only about 0.25 nm. Thus, the headgroup needs less volume. Despite the kinetic effects, the curves of polymer P1 prepared on the trough or in solution are alike. This forms a proof of the successful photopolymerization of the monolayer. Another piece of evidence for the polymerization is a dramatic loss of the fluidity of the monolayer after irradiation. It can be observed by blowing dust on the surface. While the dust particles move quickly on the monolayer of 1, this mobility is lost after photopolymerization. Crystallization Experiments. Further we investigated the influence of monolayers of monomer 1 and stearic acid 2 and mixed monolayers on the crystallization of calcium carbonate. For this purpose the monolayers were spread on a saturated
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Figure 2. Pressure-area isotherms of monomer 1 and polymer P1 at different temperatures on pure water: (A) monomer 1, (B) polymer P1 at a compression speed of 20 cm2/min, (C) growth of the solid analogue domains of monomer 1 during compression at 20 °C as viewed by fluorescence microscopy (size of the photographs about 200 µm), (D) comparison of the isotherms of monomer 1, polymer P1 polymerized in solution, and polymer P1 polymerized on the trough (polymerization at a constant lateral pressure of 20 mN/m) at 25 °C.
Figure 3. Schematic representation of the space requirement of the flexible headgroup in monomer 1 (left) and polymer P1 (right).
solution of Ca(HCO3)2 prepared according to the Kitano method.15 After spreading and compressing the different systems to a surface pressure of 20 mN/m (end of the coexistence region), we allowed crystallization for 20 h. This experiment was carried out according to Mann3,4 and gives a macroscopic correlation between surface composition and crystallization. The results are presented in Figure 4. Crystallization of calcium carbonate under a monolayer of pure stearic acid 2 gives the same results as reported by Mann.3,4 Big calcite crystals (size about 100-200 µm) are formed, and nearly the whole surface is covered with crystals. On the contrary, (15) Kitano, Y. Bull. Chem. Soc. Jpn. 1962, 35, 1980.
under a pure monolayer of monomer 1 we do not observe any calcium carbonate crystals at all. This proves the nucleationinhibiting effect of monomer 1. Under a 50 mol % mixture of 1 and 2 we observe calcite crystals that are differently shaped and smaller than those for pure stearic acid (size about 50-100 µm). The crystal yield is also smaller than it is for pure stearic acid. By analyzing crystallization under monolayers from mixed systems with different molar ratios, we observe that the crystal yield rises with increasing stearic acid content. The size of the crystals investigated by optical microscopy is, however, much larger than the expected domain size of the nucleation-promoting solid analogue phase (several micrometers). This is also found for 2 dispersed in the polymer matrix of P1 (polymerized on the trough). Thus, the amount of 2 controls the amount of crystals formed, but the correlation between the phase-separated pattern of different lipids and the inorganic crystals is lost, at least during the late stages of crystallization. Demixed Monolayers on a Ca(HCO3)2 Subphase. To learn more about mixtures of 2 and 1, we performed (i) pressure-area isotherm measurements on the Ca(HCO3)2 subphase relevant for the crystallization experiments and (ii) atomic force microscopy (AFM) measurements of mixtures, transferred at a pressure of 20 mN/m at 20 °C onto mica sheets. The pressure-area isotherms in Figure 5A represents results for different mixtures of 2 and 1 on a Ca(HCO3)2 solution as the subphase. It should be noted that they are different from curves obtained on a pure water subphase. We observe a loss of the coexistence region with increasing content of stearic acid. At the same time the specific solid-solid transition of stearic acid can be observed in all mixtures. The isotherms can be interpreted as an ideal superposi-
CaCO3 Crystallization Control by Monolayer Demixing
Figure 4. Optical micrographs of CaCO3 crystals obtained from crystallization under monolayers of (A) pure stearic acid 2, (B) a mixture of stearic acid 2 and polymer 1 (molar ratio 2:3) polymerized on the trough, and (C) pure monomer 1 (the micrograph shows no crystal formation) (scale bar 100 µm).
tion of the two isotherms for the pure lipids. This indicates a perfect phase separation in nucleation-promoting domains of stearic acid and in nucleation-inhibiting domains of monomer 1. The concept of a complete phase separation of 2 and 1 on a Ca(HCO3)2 subphase is supported by plotting the area per molecule against the molar ratio of the mixture (see Figure 5B). The almost linear relation suggests again a perfect phase separation. By transferring the lipid membrane onto mica sheets (Figure 6), the phase separation can be investigated in detail with the AFM technique on the level of the individual domains. This transfer was done at 20 mN/m at the end of the solid-liquid coexistence region, because this pressure could easily be controlled and kept constant during transfer. Due to local pressure changes happening during the transfer process, the structures investigated by AFM may, however, correspond to slightly different pressure values. Figures 7 and 8 show measurements of 50 mol % mixtures before and after polymerization. In addition, mixtures made from a solution of prepolymerized P1 and stearic acid are presented. As expected, large flat domains of one material in a matrix are found. The assignment of the molecular composition can be done on the basis of the height analysis (Figures 7 and 8). The height from the bottom of the film (small holes, which have the highest hardness in phase contrast and extend to the mica substrate) to the flat domains of 1.8 nm is found in line scans (see Figure 7 for the monomer). This corresponds to the expected distance from the substrate to the surface of a monolayer of stearic acid 2 (see Figure 1) which is tilted. The distance between the flat domains and the surrounding matrix is about 1.0 nm, corresponding to a thickness of the surrounding matrix of 2.8 nm (Figures 7 and 8). This is a reasonable thickness for a monolayer of monomer 1 (see Figure 1). An evaluation of the relative area
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Figure 5. (A) Pressure-area isotherms for pure stearic acid 2 and monomer 1 and their mixtures on a saturated Ca(HCO3)2 solution. Values were taken at 20 °C and a compression speed of 20 cm2/min. (B) Surface area at 20 mN/m as a function of the composition of stearic acid.
Figure 6. Schematic illustration of the transfer process of monolayers and CaCO3 crystals onto a mica sheet.
of the flat domains in the surrounding matrix done for various monolayer compositions (see Table 1) shows that the area of the flat domains corresponds to the amount of stearic acid present. Thus, pure domains of 2 have been phase separated from a liquid analogue matrix of 1 in the coexistence region (considering that 1 has two alkyl chains, whereas 2 has only one). After photopolymerization on the trough and transfer onto the mica sheet a very similar situation is found (Figure 8). On the basis of height analysis and the relative ratio of both components, it
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Figure 7. (A) Topography (SFM, TappingMode) of an LB monolayer of a mixture of stearic acid and monomer 1 (molar ratio 1:1) spread on a subphase of saturated Ca(HCO3)2 solution and transferred onto a mica sheet. (B) Cross-section along the line shown in (A): level a, small holes extending to the bottom of the film; level b, solid analogue domains of 2 and level c, liquid analogue matrix of 1.
can again be concluded that pure domains of 2 are dispersed in the matrix of P1. However, the dispersed domains of 2 are generally more elongated in shape. We assume that this is a result of the higher viscosity of the P1 matrix during transfer, which leads to a deformation of the stearic acid domains. As a third experiment we investigated a mixture (molar ratio 1:1) of P1 (polymerized in solution) and stearic acid 2 (see Figure 9). We observe again a separation into domains of 2 dispersed in a polymeric environment of P1, but now the size of these domains is much smaller (80 nm). This is a result due to the high viscosity of P1, which hinders the growth of the crystalline domains of 2. Thus, it is possible to obtain domains of the nucleation-promoting stearic acid which vary in size between 100 nm and 10 µm depending on the nucleation-inhibiting matrix of 1 or P1. By further analyzing the AFM measurements (Figures 7 and 8), we observe small, very bright points which we attribute to small CaCO3 crystallites attached to the monolayer. This is evident from line scans (Figure 8), which confirm that the crystallites are significantly higher than any lipid layer structure (10-20 nm). Such small crystallites start their growth under a stearic acid domain and subsequently grow rapidly into the subphase. This is evident from the statistical evaluation of AFM measurements, which gives an 82% chance to locate the small crystallites on a stearic acid domain in polymerized systems. This proves that the nucleation of CaCO3 crystals (early stage of crystallization) takes place predominately beneath the solid analogue domains of 2. As the crystals grow into the third dimension of
Mu¨ller et al.
Figure 8. (A) Scanning force microscopy image showing the topography (TappingMode) of a mixture of stearic acid and polymer 1 (molar ratio 1:1) obtained by polymerizing monomer 1 on the trough on a subphase of a saturated Ca(HCO3)2 solution and transferring it subsequently onto a mica sheet. (B) Cross-section along the line shown in (A): level a, liquid analogue matrix of polymerized P1; level b, small CaCO3 crystallite; level c, solid analogue domains of 2. Table 1. Ratio of the Area of Stearic Acid Domains to Monomer (or Polymer) Domains Obtained by Evaluation of AFM Images and Comparison to the Expected Value (Based on Dense Packing of the Alkyl Chains) molar ratio monomer 1:2 1:1 3:2 9:1
monomeric polymeric monomeric polymeric monomeric polymeric
domain ratio found (%)
domain ratio expected (%)
28 27 22 21 5.5 5
33 33 25 25 5.2 5.2
the subphase their lateral extension is not limited by the size of the stearic acid domains. According to crystallization experiments described at the beginning (Figure 4), large crystals are already formed during the time elapsed between preparation of the monolayer and their transfer (30-60 min). We were, however, never able to observe these large crystals by AFM measurements of transferred films prepared according to the process described in Figure 7. The AFM images of monolayers prepared on a Ca(HCO3)2 subphase showed numbers of holes of micrometer dimensions in the monolayer (see Figure 7). We propose, therefore, the following scenario during LB transfer and handling prior to AFM measurements. After about 1 h various crystals have formed under the demixed monolayer. They have started their growth under the stearic acid domains several micrometers in size. This includes small crystals,
CaCO3 Crystallization Control by Monolayer Demixing
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Figure 9. Topography (SFM, TappingMode) of an LB monolayer composed of 50 mol % stearic acid and polymer P1 (prepolymerized in solution) transferred onto a mica sheet from a subphase of saturated Ca(HCO3)2 solution. The z scale is 4 nm black to white.
which are still much smaller than the stearic acid domains (see the example in Figure 7B) and macroscopically large crystals (several hundred micrometers; see Figure 4). For the large crystals it is not clear whether they are transferred at all to the mica due to their size and gravity. If they would be transferred, they would lead to large holes (several hundred micrometers), too large to be studied. Small crystals are transferred to the mica and lead during drying (evaporation of the water film between mica and the monolayer) to a rupture of the thin monolayer. Some crystals stick to the mica and can be imaged. However, especially somewhat larger crystals (several micrometers) fall mostly off the mica during transport. Crystals which are smaller than the stearic acid domains (several hundred nanometers) thereby leave characteristic holes inside the solid analogue domain. CaCO3 crystals of the size of the stearic acid domain lift the domain completely during drying, and rupture occurs inside the softer liquid analogue matrix. This leaves less characteristic holes with fringed boundaries inside the liquid analogue matrix. Control of Crystallization with Poly(acrylic acid). According to the experiments reported so far, we achieved a phase-separated structure of a nucleation-promoting solid analogue phase consisting of 2 and a nucleation-inhibiting liquid analogue phase composed of 1. This structure can be stabilized by polymerizing the liquid analogue phase (monomer 1 to P1) serving as a matrix for the solid analogue phase. Moreover, we could confirm that crystallization is initiated predominately beneath the stearic acid domains, which serve as nucleation sites for CaCO3. What breaks the correlation between the monolayer pattern and the size and arrangement of the inorganic crystals is the fact that the crystals grow into the third dimension and become macroscopic in size (Figure 4). Thus, it is necessary to control the growth of CaCO3 into the third dimension (subphase). One possibility to achieve control over crystal growth is the addition of poly(acrylic acid) (M ) 2000 g/mol, concentration 1.26 mg/mL) into the subphase of a saturated Ca(HCO3)2 solution. We found that under these conditions a thin film of CaCO3 forms on the stearic acid domains. The CaCO3 film formed under these conditions is likely to be a hybrid material as described by DiMasi16 composed of small CaCO3 crystals and poly(acrylic acid). Macroscopic crystals were not formed even after 24 h. The transfer of monolayers from such solutions shows that it is really (16) DiMasi, E.; Patel, V. M.; Sivakumar, M.; Olszta, M. J.; Yang, Y. P.; Gower, L. B. Langmuir 2002, 18, 8902.
Figure 10. AFM height images of mixtures of stearic acid and (A) monomer 1 and (C) prepolymerized polymer P1 after allowing crystallization in the presence of poly(acrylic acid) (saturated Ca(HCO3)2 subphase) and transfer onto a mica sheet after a crystallization period of 24 h. (B) Cross-section along the line shown in (A): level a, solid analogue domain of 2 covered with a CaCO3 film; level b, the matrix of 1. It has to be noted that now the solid analogue domain of 2 is higher than that of the matrix of 1.
possible to produce an inorganic replica of the stearic acid domains (see Figure 10). AFM images (Figure 10) of monolayers transferred after 24 h provide the following results: At first, the morphology (domains in a matrix) is unchanged compared to the situation without poly(acrylic acid) (see Figures 7-9). This applies also to the area occupied by the domains. However, line scans show that now the domains (stearic acid) are 10 nm or more higher than the surrounding matrix of 1 or P1 (see Figure 10). This increase is homogeneous over the full size of the domains. This proves that indeed a film of CaCO3 has been formed only under the
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stearic acid domains, occupying the complete domains, and hence, the crystallization pattern is a replica of the demixed monolayer. This result is found for demixed monolayers composed of 1 or P1 and 2, and it applies to the large domains (micrometers) obtained by cospreading monomers 1 and 2 as well as to the small domains (e100 nm) obtained by cospreading prepolymerized P1 and 2. By controlling the growth of the CaCO3 into the third dimension with poly(acrylic acid), the replication of the stearic acid domains is allowed. Under these conditions the CaCO3 “films” formed are much wider than thick. Such replica films can be transferred without the rupture of the monolayer (see Figure 10). Without poly(acrylic acid) in the subphase, the CaCO3 crystals grow quickly in the third dimension (see Figure 8). Such crystals tend to break through the monolayer during transfer, thereby creating holes (Figures 7 and 8). Due to their three-dimensional growth, their lateral dimension is not limited by the size of the stearic acid domains. Conclusion. A two-component amphiphilic system consisting of a nucleation-promoting unit (stearic acid) and a polymerizable nucleating-inhibiting unit (monomer 1 with an ester headgroup) could be compressed on an LB trough to yield a patterned biphasic monolayer having regular pure stearic acid domains. Crystallization of CaCO3 was observed under the solid analogue stearic acid domains, whereas the surrounding liquid analogue matrix having ester groups hinders crystallization. In the absence of poly(acrylic acid) the CaCO3 crystallization could not be controlled by the size of the stearic acid domains and was observed to grow macroscopically into the subphase (third dimension). On the contrary, by addition of poly(acrylic acid) to the subphase, it was possible to confine the CaCO3 crystallization into “films” that formed in the pattern of the template monolayer. Experimental Section Stearic acid was purchased from Acros and used without any further purification. Monomer 1 was prepared by refluxing dioctadecylamine with 2,6-dioxodioxane and pyridine in CH2Cl2 for 8 h. After purification
Mu¨ller et al. by extraction with water the crude product (white crystals, mp 74.5 °C) was dissolved again in CH2Cl2, and oxalyl chloride was added. After removal of the oxalyl chloride the product was again dissolved, and 2-hydroxyethyl methacrylate was added. The product was purified by column chromatography (EE:hexane ) 1:4), yield 42%. 1H NMR (300 MHz, CDCl3): δ ) 0.85 (t, 3H), 1.22 (m, 64H), 1.9 (s, 3H), 3.2 (2t, 4H), 4.26 (s, 4H), 4.35 (s, 4H), 5.6 (d, 1H), 6.1 (d, 1H) ppm. Polymer P1 was polymerized in a toluene-methanol solution with 3 mol % AIBN at 70 °C. The polymer was precipitated by dropping in cold methanol. 1H NMR (300 MHz, CDCl3): δ ) 0.85 (t, 6H), 1.2-1.5 (m, 64H), 1.6 (m, 3H), 1.9 (s, 3H), 3.2 (2t, 4H), 4.26 (m, 4H) ppm. Monolayer Experiments. A chloroform solution of typically a 1 mg/mL concentration of the lipid or lipid mixture was spread on ultrapure water or on a Ca(HCO3)2 solution, prepared according to the method of Kitano.15 The pressure-area isotherms were measured at constant temperature on an NIMA trough at a compression speed of 20 cm2/min. Polymerization of the lipid membranes was carried out by irradiating the monolayer for 20 min with an Ar-Xe arc lamp (200 W) at constant pressure and temperature at a distance of 30 cm between the lamp and monolayer. The transfer of the monolayers was carried out at constant pressure and a dip speed of 1 mm/min. Crystallization Experiments. The lipids were spread on a Ca(HCO3)2 solution and compressed to a lateral pressure of 20 mN/m. After 16-40 h the crystallization was stopped by transferring the crystals onto a glass or mica slide. In experiments with poly(acrylic acid) added to the subphase, the concentration was 0.1 mg/100 mL of Ca(HCO3)2 solution. Scanning Force Microscopy. Measurements were carried out with a commercial scanning force microscope (Dimension 3100 with a Nanoscope IIIa A/D controller, Veeco Digital Instruments, Santa Barbara) and silicon cantilevers (OMCL-AC 160 TS, Olympus, Japan). All topographs were measured in the tapping mode with a scan speed according to the scan size, about 1-2 lines per second for a frame size of 10-40 µm.
Acknowledgment. Financial support by the DFG via Grant SFB 625 is gratefully acknowledged. LA061637K
