Molecular construction of oriented inorganic materials: Controlled nucleation of calcite and aragonite under compressed Langmuir monolayers

The oriented nucleation of the calcium carbonate polymorphs calcite and aragonite under compressed langmuir monolayers of anionic surfactants was studied by optical and analytical electron microscopy. Whereas n-octadecanoic acid (CH₃(CH₂)₁₆COOH) monolayers induced the nucleation of the {1-1.0} face of calcite, n-eicosyl sulfate (CH₃(CH₂)₁₉OSO₃H), and n-eicosyl phosphonate (CH₃(CH₂)₁₉PO₃H₂) promoted nucleation of the calcite (00.1) face. The precipitation of the CaCO₃ polymorph araognite is favored by the presence of Mg²⁺ in the crystallization medium. Aragonite nucleation under compressed monolayers of these surfactants showed a similar dependence on headgroup stereochemistry. The (100) face was nucleated under eicosanoic acid monolayers, whereas the (001) face was observed in the presence of eicosyl sulfate or eicosyl phosphonate. The nucleation selectivity observed in these experiments can be explained by an interfacial mechanism that involves stereochemical complementarity between the oxygen atoms of the anionic headgroup and those of carbonate ions located in lattice positions in the crystal faces of nuclei forming at the monolayer/solution interface.