Synthesis of calcium phosphate nanofilaments in reverse micelles

Reverse micelles of calcium bis(2-ethylhexyl)phosphate (Ca(DEHP)(2)) were used to synthesize calcium phosphate nanostructures from a reaction mixture initially containing a water-in-cyclohexane emulsion of Ca(DEHP)(2), ammonium hydrogen phosphate, and triblock copolymer, P123. Bundles of DEHP-coated amorphous calcium phosphate nanofilaments, 2 nm in width and > 300 mu m in length were formed in the oil layer of a phase-separated reaction mixture prepared at a [Ca]: [PO4] molar ratio of 1:1 and pH of 8.2. The nanofilament bundles were stable in the reverse micelle phase up to around 5 days, after which they transformed into 5-nm-wide surfactant-coated hydroxyapatite nanorods. In contrast, similar experiments at a higher supersaturation level (pH = 9, [Ca]:[PO4] = 1.66) produced discrete nanofilaments (100500 x 10-15 nm in size) that consisted of a linear superstructure based on the side-on stacking of surfactant-coated amorphous calcium phosphate nanorods. Possible factors giving rise to these ordered nanostructures are discussed.