Calorimetric investigation of the precipitation of calcium monohydrogen phosphate in water/AOT/n-heptane microemulsions

The molar enthalpies of precipitation of calcium monohydrogen phosphate (Ca HPO4) in water/sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/n-heptane micr oemulsions were measured at 25 degrees C by a calorimetric technique as a f unction of the molar ratio R (R = [water]/[AOT]) at a fixed AOT concentrati on. Calorimetric data indicate the formation of CaHPO4 nanoparticles encaps ulated in the aqueous core of the AOT reversed micelles displaying a slow g rowing process with time. Their energetic state is initially different from that in bulk water approaching the value in water at longer rimes. The obs erved growing process results to be completely inhibited by the presence of small amounts of bis(2-ethylhexyl) phosphate acid (DEPA) leading to the fo rmation of stable nanosized calcium monohydrogen phosphate: particles coate d by an oriented monolayer of chemically bonded DEPA molecules. After evapo ration of the volatile components of the microemulsions, it was observed by transmission electron microscopy that CaHPO4 nanoparticles embedded in the surfactant matrix are still present. (C) 2000 Elsevier Science B.V. All ri ghts reserved.