Oriented crystallization of CaCO3 under compressed monolayers. Part 2. - Morphology, structure and growth of immature 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 [11.0](calcite) and [00.1] and [11.0](vaterite) axes perpendicular to the monolayer surface.