Formation of monetite nanoparticles and nanofibers in reverse micelles

Reverse micelles solution of water and cyclohexane containing either cetyltrimethylammonium bromide (CTAB) or polyoxyethylene-8-dodecyl ether (C₁₂E₈) surfactants and n-pentanol as co-surfactant have been used as organized reaction microenvironments for monetite (dicalcium phosphate anhydrous, DCPA) precipitation. Well-crystallized monetite nanoparticles with various morphologies such as spheres, nanofibers and bundles of nanowires were obtained in CTAB reverse micelles solution. The molar ratio of water and surfactant (W _o) and the molar ratio of co-surfactant and surfactant (P _o) have great influence on the structure and morphology of the final products. A generalized mechanism for the growth of monetite in reverse micelles is proposed, in which the interaction between the surfactant molecules and PO ₄ ³⁻ ions leads to the formation of a surfactant/CaHPO₄ complex. It is because of this central complex that the further fusion with reactant ions containing reverse micelles will occur only in one direction. Changing the content of water and co-surfactant has great influence on the morphology of reverse micelles and on the interaction between the surfactant/CaHPO₄ complex leading to a fine tuning of the morphology of products. By contrast, lacking of this interaction in the C₁₂E₈ system only tablet amorphous calcium phosphate can be formed.