Structured solvent effects on precipitation: Microscopically organized cerium oxalate particles

The synthesis of cerium oxalate, a reaction intermediate in the preparation of high performance ceramics, is studied in sessile droplets. We compare here the effect of alcoholic solvents with the usage of structured solvents, namely ultra-flexible microemulsions in the water-rich and oil-rich configurations. The nucleation and the growth of cerium oxalate particles are the result of local mixing by the Marangoni flow of droplets loaded with oxalic acid and cerium nitrate. The morphology of the particles is very different depending on the composition of the solvent and on the mixing process. We show that the variation of the particle morphology results from the difference of the structured solvent microphases during the precipitation process. When the precipitation occurs in a water continuous phase, usual needle-like oxalate particles precipitate. When the precipitate is formed in the dispersed phase of the emulsion, structured aggregates, as close-packed aggregates of needles, are compacted by microcapillarity effects.