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Langmuir 2009, 25, 2258-2263
A Detailed Study of Closed Calcium Carbonate Films at the Liquid-Liquid Interface Michael Maas,*,† Heinz Rehage,† Holger Nebel,‡ and Matthias Epple‡ Chair of Physical Chemistry II, Technical UniVersity of Dortmund, D-44227 Dortmund, Germany, and Institute of Inorganic Chemistry and Center for Nanointegration Duisburg-Essen (CeNIDE), UniVersity of Duisburg-Essen, D-45117 Essen, Germany ReceiVed October 16, 2008. ReVised Manuscript ReceiVed NoVember 21, 2008 In this publication, we describe the growth of coherent thin films of calcium carbonate and stearic acid at the liquid-liquid interface. We present a new method to prepare durable, cohesive films. These extended film structures have a thickness of the order of 10 µm. They were characterized by two-dimensional shear rheology, scanning electron microscopy, X-ray powder diffractometry, infrared spectroscopy and dynamic light scattering.
Introduction In the field of biomineralization1,2 it is of foremost interest to study crystallization processes at fluid interfaces in order to mimic similar processes in nature. Most studies on the process of biomineralization at interfaces were carried out either at the air-water surface3-11 or at solid substrates in the aqueous phase12-15 (for a recent review see16). These investigations are easily performed, as the air-water interface can be monitored by in situ methods as Brewster-angle microscopy17,18 or grazingincidence diffractometry. On the other hand, solid substrates provide well-defined, geometrically stable reaction sites. However, only little effort has been dedicated to the study of biomineralization processes at the planar liquid-liquid interface, though some works exist which use emulsion systems as a framework for mineralization.19-21 The air-water surface in * Corresponding author. Telephone: +49-231-7553935. Fax: +49-2317555367, E-Mail:
[email protected]. † Chair of Physical Chemistry II, Technical University of Dortmund. ‡ Institute of Inorganic Chemistry and Center for Nanointegration DuisburgEssen (CeNIDE), University of Duisburg-Essen. (1) Mann, S. Biomineralization, principles and concepts in bioinorganic materials chemistry; Oxford University Press: Oxford, U.K., 2001. (2) Weiner, S.; Lowenstam, H. A. On Biomineralization; Oxford University Press Inc.: Oxford, U.K., 1989. (3) Benitez, I. O.; Talham, D. R. Langmuir 2004, 20, 8287–8293. (4) DiMasi, E.; Patel, V. M.; Sivakumar, M.; Olszta, M. J.; Yang, Y. P.; Gower, L. B. Langmuir 2002, 18, 8902–8909. (5) DiMasi, E.; Olszta, M. J.; Patel, V. M.; Gower, L. B. CrystEngComm 2003, 5, 346–350. (6) Fricke, M.; Volkmer, D. In Biomineralization II, Kensuke, N., Ed.; SpringerVerlag: Berlin and Heidelberg, Germany, 2007; pp 1-41. (7) Heywood, B. R.; Mann, S. Chem. Mater. 1994, 6, 311–318. (8) Mann, S.; Heywood, B. R.; Rajam, S.; Birchall, J. D. Nature 1988, 334, 692–695. (9) Mann, S.; Archibald, D. D.; Didymus, J. M.; Douglas, T.; Heywood, B. R.; Meldrum, F. C.; Reeves, N. J. Science 1993, 261(5126), 1286–1292. (10) Xu, G.; Aksay, I. A.; Groves, J. T. J. Am. Chem. Soc. 2001, 123, 2196– 2203. (11) Xu, G.; Yao, N.; Aksay, I. A.; Groves, J. T. Langmuir 1998, 120, 11977– 11985. (12) Widawski, G.; Rawiso, M.; Francois, B. Lett. Nat. 1994, 369, 387–389. (13) Hosoda, N.; Kato, T. Chem. Mater. 2001, 13, 688–693. (14) Zhang, S.; Gonsalves, K. E. Langmuir 1998, 14, 6761–6766. (15) Kato, T.; Suzuki, T.; Irie, T. Chem. Lett. 2000, 2, 186–187. (16) Xu, A. W.; Ma, Y.; Co¨lfen, H. J. Mater. Chem. 2006, 17, 415–449. (17) Ma, C. L.; Lu, H. B.; Wang, R. Z.; Zhou, L. F.; Cui, F. Z.; Qian, F. J. Cryst. Growth 1997, 173(1-2), 141–149. (18) Hacke, S. Brewsterwinkel-Mikroskopie zur Untersuchung der Kristallisation von Calciumcarbonaten an Modell-Monofilmen an der Grenzfla¨che Wasser/ Luft. Dissertation; Georg-August-Universita¨t zu Go¨ttingen: Go¨ttingen, Germany, 2001. (19) Walsh, D.; Hopwood, J. D.; Mann, S. Science 1994, 264(5165), 1576– 1578.
combination with lipid or fatty acid Langmuir films is often considered as a good model system for biomineralization processes occurring on the surface of cells or vesicles.22 In nature, of course, the mineralization takes place within the organisms without contribution of the gas phase. Therefore our approach was the transfer of the experiments which we previously performed at the air-water interface23 to the liquid-liquid (oil-water) interface. This, again, is a simplification compared to the bilayer membranes of cells which separate two aqueous phases. The results of our new experiments which we performed at the liquid-liquid interface were notably different from mineralization processes which we first discovered at the air-water interface. At the interface between water and air, only transient precursor films were observed which formed isolated crystals after a short period of time. At the liquid-liquid interface, however, coherent inorganic films formed spontaneously within about 12 h. Similar solid films of calcium carbonate composite materials were already prepared by other groups using additives such as hydrophilic polymers4,10,11,13-15,24-29 (mainly poly(acrylic acid), PAA) or high concentrations of magnesium salts.30-32 These additives served as crystallization inhibitors in the bulk phase and stabilized the amorphous CaCO3 precursor phase in proximity to the interface. In contrast to these experiments, we here report the synthesis of coherent films without any additives. The structure and the dynamic properties of these films were analyzed by two-dimensional rheological experiments, scanning electron microscopy (SEM), X-ray powder diffraction (XRD), infrared spectroscopy (IR) and dynamic light scattering (DLS). (20) Walsh, D.; Mann, S. Nature 1995, 377, 320–323. (21) Estroff, L. A.; Hamilton, A. D. Chem. Mater. 2001, 13, 3227–3235. (22) Co¨lfen, H. Curr. Opin. Colloid Interface Sci. 2003, 8, 23–31. (23) Maas, M.; Rehage, H.; Nebel, H.; Epple, M. Colloid Polym. Sci. 2007, 285, 1301–1311. (24) Yu, S.-H.; Co¨lfen, H. J. Mater. Chem. 2004, 14, 2124–2147. (25) Co¨lfen, H.; Mann, S. AdV. Mater., Int. Ed. 2003, 42, 2350–2365. (26) Xu, X.; Han, J. T.; Cho, K. Chem. Mater. 2004, 16, 1740–1746. (27) Volkmer, D.; Harms, M.; Gower, L.; Ziegler, A. AdV. Mater., Int. Ed. 2005, 44, 639–644. (28) Wada, N.; Suda, S.; Kanamura, K.; Umegaki, T. J. Colloid Interface Sci. 2004, 279, 167–174. (29) Kato, T.; Sugawara, A.; Hosoda, N. AdV. Mater. 2002, 14(12), 869–877. (30) Jiao, Y.; Feng, Q.; Li, X. Mater. Sci. Eng. C-Biomimetic Mater. Sensors Syst. 2006, 26, 648–652. (31) Kitamura, M. J. Colloid Interface Sci. 2000, 236, 318–327. (32) Aizenberg, J.; Lambert, G.; Weiner, S.; Addadi, L. J. Am. Chem. Soc. 2002, 124, 32–39.
10.1021/la803446q CCC: $40.75 2009 American Chemical Society Published on Web 01/06/2009
CaCO3-Films at the Liquid-Liquid Interface
Materials and Methods Chemicals. All chemicals were purchased at analytical grade from VWR and used without further purification. Millipore grade water was prepared by an Elsa Purelab Ultra with 18.2 MΩ final resistivity. Preparation of the Ca(HCO3)2 Solutions. For the preparation of the Ca(HCO3)2 solutions, several grams of CaCO3 were suspended in 0.5 L water and flushed with gaseous CO2 for about 2 h. This led to dissolution of most of the solid material. In order to remove any undissolved CaCO3, the solution was filtered with a white ribbon paper filter. In addition, the solutions were filtered again before each application. The concentration of the Ca(HCO3)2 solutions prepared by this method was approximately 8 mM (determined by titration with EDTA). The pH of this solution was 7.2. General Experiment. A 50 mL aliquot of a freshly filtered 4 mM Ca(HCO3)2 solution was put into a 100 mL crystallization dish (diameter 70 mm, height 40 mm). On top of this aqueous solution, 40 mL of a 1 mM solution of stearic acid in dodecane or toluene was given, leading to a liquid oil-water interface. The volumes and concentrations were determined after a series of preliminary experiments. Samples for SEM, XRD and IR were taken after specific amounts of time. Control experiments were systematically carried out by exchanging the aqueous phase with 4 mM Na(HCO3)2, 4 mM CaCl2 and pure water, respectively, while maintaining the organic phase. Additionally the stearic acid was exchanged by different lipids or was omitted completely while the aqueous Ca(HCO3)2 solutions were kept constant. No films could be obtained under any of these conditions. All measurements were carried out at room temperature. Scanning Electron Microscopy. For scanning electron microscopy at specified reaction times, a silicon wafer was quickly dipped with the sharp edge through the film and carefully pulled out again in a way that the lower side of the film was attached to the surface of the wafer. Samples were prepared after various growth times at room temperature. These samples were investigated by a FEI ESEM Quanta 400 scanning electron microscope which was equipped with energy-dispersive X-ray spectroscopy (EDAX EDS Genesis 4000). X-ray Powder Diffractometry. The material was obtained by collecting film fragments out of the crystallization dishes with a spatula. These fragments were fixed with petrolatum (Vaseline) on a glass carrier and measured with a Siemens D 500 diffractometer, using Bragg-Brentano optics. Cu KR radiation (λ ) 1.54056 Å) was used at a voltage of 40 kV and a current of 20 mA. Additionally, samples were measured with pure CaCO3 powder (calcite) and stearic acidswhich were the same as used for the preparation of the samplessand calcium stearate. Interfacial Rheology. The surface shear rheological properties of the films were determined by a Rheometrics fluid spectrometer (RFS II), which was equipped with a modified shear system.33 The measuring cell consisted of a quartz dish (diameter 83.6 mm) and a thin biconical titanium plate (angle 2°, diameter 60 mm) which could be placed exactly at the interface between hydrophobic organic solvent and water. The dish was first filled with the aqueous phase (100 mL). The titanium plate was then positioned at the water surface and the stearic acid solution was added (40 mL). We measured the torque required to hold the plate stationary as the cylindrical dish was rotated with the sinusoidal angular frequency ω. In such experiments, the two-dimensional storage modulus µ′(ω) and the two-dimensional loss modulus µ′′(ω) can be computed from the amplitudes and phase angles of the stress and deformation signals. Additional measurements were performed with a torsion pendulum (Sinterface ISR-1). The measurements were carried out with a biconical disk at the dodecane/water interface with both pure and 1 mM stearic acid containing dodecane solutions. In the aqueous phase, water, 4 mM Ca(HCO3)2 and 8 mM CaCl2 were used. The surface viscosity η and the surface elasticity G were obtained by this approach. (33) Pieper, G.; Rehage, H.; Barthe`s-Biesel, D. J. Colloid Interface Sci. 1998, 202, 293–300.
Langmuir, Vol. 25, No. 4, 2009 2259 IR Spectroscopy. The FTIR measurements were performed with a Bruker Vertex 70. The samples were collected on glass cover slides after one to seven days. These samples were scraped off the cover slides and then mixed with KBr in a mortar. Pellets were prepared with a hydraulic press at a pressure of 10 tons. After that the pellets were measured in transmission mode from 400-4000 cm-1. Dynamic Light Scattering. The light scattering experiments were performed with a Brookhaven BI-200SM instrument and confirmed with a Malvern Nano-ZS. A 4 mW He-Ne laser (633 nm wavelength) with a fixed detector angle of 173° was used for the measurements. The measurements were carried out both in the organic and the inorganic phase of a two-phase system containing pure water or 4 mM Ca(CHO3)2-solution underneath dodecane or 1 mM stearic acid in dodecane. The pure solutions and the respective solvents were measured as well. Both glass and plastic cuvettes were used in order to rule out any wall effects. All solutions were filtered (cellulose acetate, pore size: 100 nm) before the measurements. The autocorrelation function of the scattered intensity was analyzed using the inverse Laplace transform program CONTIN. The ζ-potential of the 4 mM Ca(HCO3)2 solution was measured with a Malvern Nano-ZS instrument at 20 °C.
Results and Discussion After a growth time of 12 h, the films became visible to the unaided eye. They appeared as a translucent, slightly glossy membrane at the liquid-liquid interface. After drying, the films appeared white with a crystalline shining. The drying procedure often led to breaking and sometimes to furling processes which were probably induced by osmotic or capillary forces34 or different features of the upper and lower sides.35 As derived from the experimental setup, the upper side of these films represents the surface which was exposed to the organic phase. In turn, the lower side was exposed to the aqueous phase. Scanning Electron Microscopy (SEM). SEM provided a detailed insight into dried versions of the films. Figure 1a shows a large fragment of a film that furls on the right side so that the lower side becomes visible. Note the difference in roughness between both sides. Figure 1b shows the lower side of the same film and Figure 1c allows the estimation of the film thickness which is of the order of 10 µm after 7 days of growth. Figure 2 shows a film that was left in the reaction vessel for 2.5 months. In contrast to the films which grew over 1 week, here, crystalline shapes which were embedded inside the membranes could be observed directly. As suggested by the rhombohedral shapes of these crystals and according to the XRD analysis, they consist of deformed calcite crystals. If the toluene was replaced by dodecane (Figure 3), the films became more porous and the upper side seemed to consist of an agglomeration of small rods (Figure 3a). Again, the lower side (right side of Figure 3b) is much rougher than the upper side. The side view (Figure 3c) allows an estimation of the films thickness which is about 5 µm after 7 days of growth. EDX. Elemental analysis of the samples yielded comparable results for all films. The EDX-data for the film shown in Figure 1 (growth time 7 d) is presented in Table 1. From the high amount of carbon it becomes clear that the films consist mainly of stearic acid while calcium constitutes only about 2% to the overall composition of the films. The amount of calcium is only slightly increased to about 5% for the film shown in Figure 2 (growth time 52 d). The film grown in toluene exhibits a calcium ratio of about 3%. Due to the low fraction of oxygen in all films, it (34) Nagayama, K. Colloids Surf. A-Physicochem. Eng. Asp. 1996, 109, 363– 374. (35) Freund, L. B. J. Mech. Phys. Solids 2000, 48, 1159–1174.
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Figure 1. Scanning electron micrographs of thin solid films obtained with 4 mM Ca(HCO3)2 and 1 mM stearic acid in toluene after a growth time of 7 d. Key: (a) upper side (toward organic phase) with furled section; (b) lower side (toward water phase); (c) upper side with side view.
Figure 2. Scanning electron micrographs of thin solid films obtained with 4 mM Ca(HCO3)2 and 1 mM stearic acid in toluene after a growth time of 52 d. Key: (a) upper side and (b, c) close ups of part a.
Figure 3. Scanning electron micrographs of thin solid films obtained with 4 mM Ca(HCO3)2 and 1 mM stearic acid in dodecane after a growth time of 7 d. Key: (a) upper side; (b) left half of the picture, upper side, and right half, lower side; (c) film fraction on the right half of the picture, upper side with side view, and left half, lower side.
can be concluded that calcium is only partly bound in CaCO3 while the other (probably larger) fraction of calcium can be attributed to the formation of calcium stearate. XRD. Because of the very small amounts of material and the large fraction of organic components (mainly stearic acid and residues of organic solvent; see the discussion of the EDXspectra) the powder diffractograms of the films could only be qualitatively analyzed. Note that the samples were only airdried. In all samples we observed different amounts of crystalline calcium stearate (as confirmed with the blank tests) and calcite (ICDD#83-0578) (see Figure 4 for a representative diffractogram). This shows that the films consisted of an inorganic/organic composite material. The intensity ratio (maximum minus background) between the strongest reflexes of calcium stearate
(5.3° 2Θ) and calcite (29.4° 2Θ) is 2.4:1 after 2 days, 1.5:1 after 7 days, and 0.34:1 after 14 days for measurements with toluene as organic solvent (the results for dodecane were comparable). Although this ratio does not reflect the actual ratio of the two phases in the films, it indicates that the amount of calcite increases over ongoing time. It is important to note that the crystals indeed formed within the films. The SEM micrographs indicate that the external morphology structures of the films did not change during the first weeks after the formation of a closed film; therefore, possibly a recrystallization occurs within the film. IR Spectroscopy. The IR measurements were performed with the films grown at the toluene water interface. The evaluation of the spectra is difficult because of the superposition of the IR bands of the different compounds in the characteristic region for
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Figure 4. X-ray powder diffractograms of a thin solid film obtained with 4 mM Ca(HCO3)2 and 1 mM stearic acid in toluene after a growth time of 2 d.
Figure 5. IR spectrum of a solid film grown in toluene (same sample as in Figure 4).
Figure 6. 2d-couette measurements at the liquid-liquid interface: (a) time (γ ) 0.2%, ω ) 0.0016 Hz), (b) frequency (γ ) 0.1%), and (c) deformation (ω ) 0.016 Hz) tests for 4 mM Ca(HCO3)2, 1 mM stearic acid in toluene. The frequency test was performed after 3 days of growth time, the deformation test after 4 days.
Table 1. Elemental Analysis of the Film shown in Figure 1 Where the Presence of Silica Is Due to the Silicon Substrate element
wt %
at %
CK OK SiK CaK total
75.59 6.82 12.36 5.23 100
86.32 5.85 6.04 1.79 100
calcium stearate and calcite. Because the different spectra showed no distinct changes after different growth times, we will discuss only one representative spectrum (Figure 5). The bands at 1580 cm-1, 1540 cm-1, 1472 cm-1, and 1113 cm-1 can be clearly attributed to calcium stearate.36,37 Furthermore, various bands of different -CH2-vibrations can be identified in the region between 700 and 800 cm-1. The bands at 1472 cm-1, 1435 cm-1, and 1423 cm-1 belong to the ν3-vibration of calcium carbonate;38 however, the band at 1472 cm-1 is superposed by a band of calcium stearate. The (36) Simons, W. W. The Sadtler Handbook of Infrared Spectra; Sadtler Research Laboratories: Philadelphia, PA, 1978.
splitting of the band of the ν3-vibration into three or more smaller bands suggests that next to calcite another phase of calcium carbonate could be present. Because only calcite could be identified via XRD measurements, this may be a hint that ACC exists inside the films. At 1170 cm-1 a broad band can be barely distinguished which could be the ν1-band of ACC (no ν1-band exists for calcite). The ν4-vibration is superposed by CH2vibrations from calcium stearate in the region of 700-800 cm-1, the ν2-vibration (about 870 cm-1) vanishes in the background. Interfacial Rheology. At constant deformation and frequency, the two-dimensional viscoelastic properties of the films were measured as a function of time. With this experiment the growth of the films could be followed. In case of the toluene films (Figure 6a), an elastic film formed within 12-24 h after the start of the experiments. After this initial growth, the moduli decreased and the distance between storage modulus (µ′) and loss modulus (37) Dreamer, D. W.; Meek, D. W.; Cornwell, D. G. J. Lipid Res. 1967, 8, 255–263. (38) Nebel, H.; Neumann, M.; Mayer, C.; Epple, M. Inorg. Chem. 2008, 47, 7874–7879.
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Figure 7. Damped oscillation curves of the different interfaces. The dotted curves are the same in respect to the reproducibility of this method. Figure 8. Particle size distribution in a 4 mM Ca(HCO3)2 solution as measured by dynamic light scattering.
(µ′′) increased in favor of the storage modulus. The local minimum and the increase in elasticity are probably induced by crystallization processes inside the films. After three days, the moduli remained stable. The reason for the relatively low values of the two-dimensional moduli is probably the small thickness of the films. The observation that the rheological moduli increased rapidly by many orders of magnitude immediately after starting the experiments indicates a two-dimensional sol-gel transition. If the films are sufficiently stable (after three days) it is possible to measure µ′ and µ′′ at very low frequencies (Figure 6b). Such measurements typically take 4-8 h. As a result of these experiments we conclude that the films consisted of permanent networks, exhibiting mainly elastic properties. This follows from the fact that the moduli are parallel over the whole frequency spectrum. As an exception, the loss module rises at very high angular velocities, showing the glassy state of the films. The brittleness and crystalline or ceramic character of the films is emphasized by the deformation test (Figure 6c). The linear viscoelastic regime (the deformation range at which the films can be deformed reversible) is constricted to threshold deformations of about 0.1%, which is extremely low.39 As rubberelastic materials have extended regimes of linear-viscoelastic properties up to deformation limits of 100%, the measured data point to the presence of energy-elastic films. As a compromise to the resolution of the measuring device, in all our rheological experiments the applied strain was set to 0.1 or 0.2%. The films grown with dodecane exhibited a very similar viscoelastic behavior. The only difference was the initial growth state of the films as seen in the time dependent experiment. In this case the kinetics of film formation were faster, i.e. the structures are formed after about 2-4 h. By torsion pendulum measurements, the rheological properties of the liquid-liquid interface with and without Ca(HCO3)2, CaCl2 or stearic acid were compared (Figure 7). The moduli of the pure liquid-liquid interface were similar to the moduli of the interface with the addition of stearic acid to the organic phase. This behavior was not significantly changed after the addition of CaCl2, even in the presence of stearic acid in the organic phase. Under these conditions, the interfacial viscosity was around η ) 0.018 mN m-1 with a surface elasticity of G ) 0. When Ca(HCO3)2 was added (at the presence of stearic acid in the organic phase), the moduli were so high that the oscillation of the pendulum turned instantly into creep behavior. This suggests that before the addition of Ca(HCO3)2 only a very loose adsorption layer of stearic acid is present at the interface. If a microemulsion or similar phenomena19 had occurred at the interface, this would have been (39) Mezger, T. G. Das Rheologie Handbuch, 2nd ed.; Vincentz Network: Hannover, Germany, 2006.
clearly visible with this method. Additionally, even in the presence of calcium ions which at the air-water interface leads to steep rise of the moduli,40 the interface is not significantly altered. This suggests that calcium carbonate particles (and not calcium ions) are necessary for the formation of a closed adsorption layer of stearic acid and eventually a thin film. This also means that the surface is not specifically ordered before the formation of the precursor particles, which would exclude a formation mechanism by templating and epitaxical overgrowth of a stearic acid monolayer. Dynamic Light Scattering. The pure 4 mM Ca(HCO3)2 solution was investigated by means of dynamic light scattering (DLS). Particles with broad size distribution were detected (Figure 8), with an average size of about 300 nm. The ζ-potential of the formed calcium carbonate particles was ζ ) 0. Despite the broad size distribution the ζ potential measurement displayed a very sharp peak, which indicates that it is practically independent from the size of the particles. The presence of particles in the Ca(HCO3)2 solution indicates the presence of a precursor phase. The broad size distribution could be explained with a highly hydrated and instable precursor phase. Additionally, a ζ-potential of zero strongly favors for the aggregation of the particles because no electrostatic repulsion occurs. However, the ζ-potential may change in the presence of stearic acid due to adsorption on the surface. In addition, we studied the organic solution (dodecane with stearic acid) above the Ca(HCO3)2-solution and a pure water phase below stearic acid in dodecane by dynamic light scattering. No particles or micelles were detected in these experiments.
Conclusions The formed solid films have typical thicknesses of about 5-10 µm and consist of calcium stearate and calcite and possibly amorphous calcium carbonate. The viscoelastic data point to permanent network structures with a crystalline or ceramic hardness. The choice of the organic solvent influences the film thickness and the growth time of the films. This can be due to many different properties of the organic phase, e.g., polarity, density, viscosity, solubility of stearic acid and calcium carbonate etc. It is important to note that the solid films do not consist only of calcium stearate, but instead the occurrence of CaCO3 is required for film formation. Control experiments with CaCl2 solutions did not show any evidence for two-dimensional network formation. Therefore we propose that the film formation is due to the specific interaction of calcium carbonate nanoparticles (40) Ghaskadvi, R. S.; Carr, S.; Dennin, M. J. Chem. Phys. 1999, 111, 3675– 3687.
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Figure 9. Simplified model of the formation of the solid thin films: (a) aggregation of calcium carbonate nanoparticles at the liquid-liquid interface; (b) crystallization of the calcium carbonate particles to larger calcite crystals.
and stearic acid at the liquid-liquid interface. In contrast to the air-water surface, we can expect transport processes of the nanoparticles across the liquid-liquid interface. As calcium carbonate nanoparticles and crystals are evidently embedded in a calcium stearate framework (see Figure 2c), we have to consider the diffusion of calcium carbonate nanoparticles from the water phase into the organic phase or the diffusion of stearic acid into the aqueous phase. These diffusion processes are probably constrained to a very small region of mutual permeation of the phases, the so-called interfacial layer. This is due to the lack of particles in the organic phase and an insignificant solubility of stearic acid and the organic solvents in the aqueous phase. These permeation processes are most likely started not until calcium carbonate nanoparticles are present as the torsion pendulum measurements suggest. In analogy to our previous studies we suggest a growth mechanism involving calcium carbonate nanoparticles that serve as filler particles and cross-linking compounds which add stability to the stearate framework (Figure 9a). Former investigations at the air-water interface showed that Ca(CO3)2 nanoparticles are evidently stabilized underneath the stearic acid monolayer.23 When these particles diffuse into the organic phase, they can be stabilized by adsorption processes of stearic acid molecules at the surface of these particles.
Subsequently, aggregation of particles occurs which produces the quasi-crystalline film structure (Figure 9b). Also alternative mechanisms for the formation of the films can be considered and are a topic of ongoing research in our groups. For example, the interfacial face could be seen as a kind of middle phase (Winsor type III) microemulsion framework19,41 in which mineralization could take place. Indeed, from the outward appearance of the films, they look similar to frozen microemulsions. On the other hand, as was already discussed concerning the results of the torsion pendulum measurements, it is very unlikely that a microemulsion is present at the interface. The aggregation and self-organization that happens at the interface can also be described applying the concept of meso-crystal formation.16,22,25 In this case, the thin films could be seen as a meso-scale structure, which is formed by the aggregation of stearic-acid-stabilized CaCO3 nanoparticles at the liquid-liquid interface. These aggregates are then stabilized by the calcium stearate framework. Supporting Information Available: Figure showing an XRD diffractogram of calcium stearate. This material is available free of charge via the Internet at http://pubs.acs.org. LA803446Q (41) Sasthav, M.; Cheung, H. M. Langmuir 1991, 7, 1378–1382.
