Sundara Rajam

Controlled crystallization of CaCO3 under stearic acid monolayers

A fundamental concept in the study of biomineralization concerns the molecular recognition of inorganic materials at organized organic macromolecular substrates1. Here we investigate this concept through the use of stearic acid monolayers in the controlled crystallization of CaCO3 from supersaturated solutions. Whereas crystallization in the absence of a monolayer results in rhombohedral calcite crystals, the presence of an organized monolayer gives rise to oriented vaterite formation. The vaterite nuclei are aligned with their (0001) face parallel to the plane of the organic substrate and develop initially in the form of disk-shaped single crystals. The degree of compression of the monolayer dictates the homogeneity of vaterite nucleation. In particular, partially compressed films are optimal for controlled crystallization, suggesting that the mobility of organic surfaces may be of general importance. Our results can be explained by electrostatic and stereochemical interactions at the inorganic–organic interface and these observations support current theories of biomineralization, as well as being of potential significance in the crystal engineering of microscopic inorganic assemblies2.

Oriented crystallization of CaCo3 under compressed monolayers. Part 2.—Morphology, structure and growth of immature crystals

Transmission electron microscopy and electron diffraction have been used to determine the structure, orientation and morphology of CaCO3 crystals at the early stages of growth under compressed Langmuir monolayers of stearic acid [CH3(CH2)16CO2H] and octadecylamine [CH3(CH2)17NH+3]. At [Ca]≈ 9 mmol dm–3, elongated plate-like calcite single crystals of narrow particle size distribution were nucleated under fully compressed stearate monolayers. The crystals were elongated along the c axis and oriented with a {1text-decoration:overline10} face parallel to the monolayer/solution interface. Subsequent development of the crystals resulted in rhombohedral outgrowth into the bulk solution. Calcite crystals nucleated under partially compressed monolayers showed the same crystal chemical properties but had a more uniform size population. Nucleation under octadecylamine monolayers resulted in disc-shaped vaterite single crystals of narrow particle size distribution. Two types of disc were observed: (a) circular/hexagonal crystals oriented with their c axis perpendicular to the monolayer, and (b) oval-shaped crystals oriented with their a axis perpendicular to the monolayer. Changes in monolayer surface pressure did not influence these properties. The preferred orientation of calcite can be rationalized in terms of geometric and stereochemical matching between the carboxylate headgroups and the ions in the {1text-decoration:overline10} crystal face. There is no geometric match for vaterite but a stereochemical relationship exists on stearate monolayers with respect to the (00.1) face. For octadecylamine monolayers, bidentate binding of HCO–3 may be important in determining the preferential orientation of nuclei formed at the organic surface.

Oriented crystallization of CaCo3 under compressed monolayers. Part 1.—Morphological studies of mature crystals

The crystallization of CaCO3 under compressed Langmuir monolayers of stearic acid [CH3(CH2)16CO2H], octadecylamine [CH3(CH2)17NH2], octadecanol [CH3(CH2)17OH] and cholesterol (C27H45OH) has been studied over a range of supersaturation conditions by optical and scanning electron microscopy and X-ray diffraction. At total [Ca]= 9 mmol dm–3, negatively charged stearate monolayes induced the oriented nucleation of calcite. The crystals were of two related morphological types, one of which was formed through the physical realignment of the oriented crystals at the organic surface during growth. Reducing the total [Ca] to 4.5 mmol dm–3, resulted in oriented vaterite nucleation on the stearate monolayers. Vaterite was the major product on positively charged octadecylamine films, independent of [Ca]. Unlike the vaterite crystals of stearate films, these crystals were of two distinct morphological forms which represented two specific nucleation orientations. Neutral monolayers of octadecanol inhibited crystallization and those of cholesterol gave random non-oriented calcite deposition analogous to the control experiments. Morphological analyses indicated that the crystals were oriented with their [1text-decoration:overline1.0](calcite) and [00.1] and [11.0](vaterite) axes perpendicular to the monolayer surface.

Structural and stereochemical relationships between Langmuir monolayers and calcium carbonate nucleation

The structural and stereochemical factors involved in the oriented nucleation of CaCO3 under compressed Langmuir monolayers have been studied by optical and electron microscopy, and X-ray and electron diffraction. The effect of headgroup charge, stereochemistry and changes in supersaturation have been investigated. At total (Ca)=9 mM, monolayers with carboxylate headgroups induce the oriented nucleation of calcite with the (11.0) axis perpendicular to the organic surface. At (Ca)=4.5 mM, oriented vaterite with the (00.1) axis normal to the monolayer, is deposited. The common features of these interactions are: Ca accumulation at the stearate headgroups, geometric matching of lattice distances (calcite only) and stereochemical correspondence between carboxylate and carbonate groups at the monolayer/crystal interface. In contrast, nucleation under positively charged octadecylamine monolayers was independent of (Ca) and gave vaterite aligned either along the (00.1) or (11.0) crystallographic axes. Although Ca binding is absent on these films, there is the possibility of stereochemical matching between bidentate binding of bicarbonate and the corresponding anions in the (00.1) and (11.0) crystal surfaces. Crystals (calcite and vaterite) grown under liquid-phase stearate monolayers were of increased size and uniformity in particle size distribution compared with those nucleated under fully compressed films. CaCO3 crystallization under octadecanol monolayers was inhibited and crystals grown in the presence of cholesterol films were the same as in the control experiments.

Interfacial control of nucleation of calcium carbonate under organized stearic acid monolayers

The nucleation and crystal growth of calcium carbonate from supersaturated bicarbonate solutions in the presence of stearic acid monolayers has been studied by electron microscopy and electron and X-ray diffraction. Whereas rhombohedral crystals of calcite are randomly deposited in the absence of the monolayer, regiospecific crystallization of vaterite florets occurs at the organic surface. The florets develop initially as single crystalline biconvex discs oriented with respect to the negatively charged headgroups of the organic membrane such that the radial axis, which corresponds to the crystallographic [0001] direction, is aligned perpendicular to the monolayer interface. Subsequent radial outgrowth results in the polycrystalline floret morphology. The degree of compression of the monolayer results in significant changes in nucleation. Solid-phase films are highly catalytic and generate vaterite discs with a wide size distribution. In contrast, nucleation on liquid-phase monolayers results in discs of more uniform particle diameter. Increased supersaturation and uncompressed films gave both calcite and vaterite polymorphs. Preferential orientation of the ab vaterite crystal face parallel to the monolayer surface is discussed in terms of the electrostatic accumulation of a Stern layer of Ca ions. Polymorph selectivity is determined by kinetic factors involving charge accumulation and possibly stereochemical recognition between the alignment of the stearic acid carboxylates and the orientation of carbonate anions in the vaterite unit cell. Geometric (epitaxial) matching at the interface is not a primary factor in regulating nucleation.

Selective stabilization of the (001) face of calcite in the presence of lithium

Rhombohedral calcite crystals grown from supersaturated solutions undergo an unusual and specific morphological change in the presence of Li; the doped crystals are structurally invariant but exhibit well-developed basal (001) faces indicating that these relatively high energy crystal surfaces are stabilised by the incorporation of Li into surface interstitial sites.