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Template-Directed Inorganic Crystallization: Oriented Nucleation of Barium Sulfate under Langmuir Monolayers of an Aliphatic Long Chain Phosphonate Brigid R. Heywood* and Stephen Mann School of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, U.K. Received October 17, 1991. I n Final Form: March 3, 1992 CompressedLangmuir monolayers of an aliphatic long chain (CZO)phosphonate were shown to influence the morphology and crystallographic orientation of barium sulfate nucleated from the aqueous subphase. The crystals were aligned with the [1001crystallographic axis perpendicular to the plane of the monolayer indicating that nucleation occurred on the (100) face. The majority of these oriented crystals exhibited a rhombic habit during the initial stages of crystallization. These subsequently developed into platelike or "bow-tie" morphologies depending upon the growth mechanism under the monomolecular film. Occasionally, small polar laths oriented along the [lo01 axis were observed. For each of these crystal forms, nucleation on the (100)face and their subsequent development can be rationalized in terms of specific electrostatic, geometric and stereochemical interactions at the organic/inorganic interface. Introduction At the present time there is much interest in the use of organized organic assemblies as inductive templates for the controlled crystallization of both organic and inorganic In particular, attention has focused on the the application of highly organized two-dimensional surfaces, such as Langmuir monolayers, as potential templates for regulating the nucleation and growth of inorganic ~rystals.~-gThese studies have shown that the oriented nucleation of both inorganic and organic crystals can be achieved when the two-dimensional domains in the monolayer are structurally compatible with the two-dimensional lattice of specific crystal faces.* This structural compatibility appears to be largely dependent upon geometric and stereochemical factors although electrostatic interactions are also an essential element of the recognition process. A more recent development has been the use of Langmuir monolayers as molecular surfaces for directing the subsequent growth of oriented crystal n ~ c l e i .For ~ example, monolayers of n-eicosyl sulfate induced the nucleation of barium sulfate on the (100) face and selectivelyregulated crystal morphology such that unusual crystal textures were e ~ h i b i t e d . ~ Recent studies have shown that certain macromolecular species act as potent inhibitors of crystal growth when dispersed in the solution phase of the crystallization media, but as active nucleators when immobilized onto a semiorganized substrate.lOJ1 With this in mind, we elected to
* To whom correspondenceshould be addressedat the Department of Chemistry and Applied Chemistry, University of Salford, Salford M5 4WT, U.K. (1) Landau, E.M.; Grayer-Wolf, S.; Levanon, M.; Leiserowitz, L.; Lahav, M.; Sagiv, J. Mol. Cryst. Liq. Cryst. 1986,134, 323-325. (2)Landau, E.M.; Popovitz-Bior, R.; Levanon, M.; Leiserowitz, L.; Lahav, M.; Sagiv, J. J. Am. Chem. SOC. 1989,111, 1436-1439. (3)Mann, S.;Heywood, B. R.; Rajam, S.; Birchall, J. D. Nature 1988, 334, 692-695. (4)M a n , S.;Heywood, B. R.; Rajam,S.; Walker, J. B. W.; Davey, R. J.; Birchall, J. D. Adu. Mater. 1990,2 (06),257-261. (5)Mann, S.;Heywood, B. R.; Rajam,S.; Walker, J. B. W. J.Phys. D: Appl. P h y ~1991, . 3, 154-164. ( 6 ) Zhao, X. K.; Xu, S.; Fendler, J. H. Langmuir 1991, 7, 520-523. (7)Meldrum, F. C.; Wade, V. W.; Nimmo, D.; Heywood,B. R.; Mann, S. Nature 1991, 349,684-687. (8)Heywood,B.R.;Rajam,S.;Mann, S. J.Chem. Soc., Faraday. Trans. 1991, 87 (5),734-741. (9)Heywood, B. R.; Mann, S. J. Am. Chem. SOC.,in press. (IO)Addadi, L.; Moradian, J.; Shay, E.; Maroudaa, N. G.; Weiner, S. Proc. Natl. Acad. Sci. U.S.A. 1987, 84, 2732-2736.
investigate the precipitation of barium sulfate under Langmuir monolayers of long alkyl chain phosphonates since soluble phosphonates are known to be potent inhibitors of barium sulfate cry~tallization.'~-'~ Despite their inhibitor potential, phosphonate monolayersinduced barium sulfate precipitation at the aidwater interface. Furthermore, the nanodimensional crystals so formed are preferentially oriented with respect to the organic surface. It is proposed that the interaction of ions in solution with the highly-ordered, negatively charged monolayer explicitly controls this event and that geometric and stereochemical complementarity at the organic-inorganic interface are key factors in the process. The oriented crystals have unique morphologies which appear to be the result of monolayer-crystal interactions limiting rather than promoting interfacial growth. Materials and Methods Barium sulfatewas precipitatedfromasupersaturatedsolution in either the presence or absence of compressed Ca-phoepho-
nate monolayers. The supersaturated solution (S = 30) was prepared by mixing equal volumes of equimolar solutions of barium chloride and sodium sulfate (3.126X l(r M, pH 7.0,293 K). Under these conditions the induction time (determinedby light scattering techniques) exceeds 2 h at room temperature. Within these time limits the surfactant can be applied and compressed to the desired, limit before crystallization in bulk solution is initiated. Langmuir monolayera of an alkyllong chain phosphonate (Cm*lPO(OH)2) synthesized and donated by BP Research International, Ltd., were formed at the air/solution interface of freshly prepared supersaturated barium sulfate solutions. Monolayers were prepared in a Nima Technology circular trough by first spreading the surfactant/solvent(1 mg mL-l of surfactant dissolved in a hexane, chloroform, and methanol mixture (3l:l))over the solution surface and then reducing the surface area to the required limit by slow compression. The monolayer phase, surface pressure, and limiting area per molecule were determined from P A isotherms recorded during compression. (11)Lussi, A.; Crenshaw, M. A.; Linde, A. Arch. Oral B i d . 1988,33, 685-689.
(12)Weijenen, M. P. C.; van Rosamlen, G. M. in Znduatrial Crystallization84;Jancic, s.J., de Jong, E. D., Eds.; Elesvier: Amsterdam, 1984; p 61. (13) Gardner, G. L.; Nancollaa, G. H. J.PhY.9. Chem. 1983,87,469+ 4702. (14)Weijenen, M. P. C.; Marchee, W. G. C.; van Rosmden, G. M. Desalination 1983, 47, 81.
0143-7463J92124081492$03.OOJO 0 1992 American Chemical Society
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The crystals were removed by collection on glaps coverslipsfor XRD measurements and the nucleation density determinations and particle distributions determined by optical microscopy (Zeiss Axiophot).* Crystals grown in the absence of a monolayer were collected on glass coverslipsplaced at the bottom of the reaction vessel. Crystals grown on the monolayer were collected for transmission electronmicroscopical (TEM) examinationby slowly dipping formvar-coated, carbon-reinforced copper electron microscope grids (3 mm) through the monolayer. Excess fluid collectedduring the dipping procedure was removed immediately with absorbent paper. The grids were then allowed to dry in air. The structure and crystallographic orientation of the particles were analyzed by high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction using established procedures? The crystals were generally collected after t = 8 h. In some cases, however, the crystals were harvested at earlier time points t = 0.5-4 h. All samples were examined using a JEOL 2000FX highresolution analytical transmission electron microscope operating at 200 kV. Selected area diffraction data were recorded from individual crystals. The electron diffraction patterns and lattice imageswere indexed by comparing their d spacings and interplanar angles with calculated values assuming, for barium sulfate (BaS04), an orthorhombic unit cell (space group Pnma) with unit cell dimensions a = 8.878 A,b = 5.450 A,and c = 7.152 A.15
Results Control Experiments. The formation ofBaSO4 in the absence of a surfactant monolayer has been described pre~iously.~ In brief, the control crystals were of relatively narrow size distribution (mean length = 3.7 pm, s = 0.67 nm) and tabular in morphology (Figure 1). The crystals were preferentially elongated along the [Ool] axis and terminated by well-defined (001) faces. The large top and bottom faces were noticeably rounded rather than smooth and clearly faceted, suggesting that these surfaces were of composite ((hkO))f ~ r m . ~Very J ~ few crystals were collected from the air/water interface as sedimentation of the particles was complete within 8 h. Monolayer-GrownCrystals. At the operating pH and ionic strength the phosphonate monolayer was stable in the solid phase even a t low surface pressure (15 mN m-l) (Figure 2). The collapse of the monolayer occurred at surface pressures in the range of 55-58 mN m-l. Once formed the monolayers were stable and remained coherent for long periods (>8 h). Some expansion of the monolayer occurred when Ba2+was present in the solution subphase but the effects were minimal. The limiting area per molecule was 22-23 A2 a t a surface pressure of 51 mN m-l (Figure 2). These observations were in agreement with earlier published work.17J8 The presence of a phosphonatemonolayer had two major effects on barium sulfate precipitation; firstly, the induction time was reduced from 2.5 to
