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 (Wo) and the molar ratio of co-surfactant and surfactant (Po) 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 PO43− 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.