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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.
Heywood et al. (1994) studied this question.