Mineralization of water-in-oil emulsion droplets
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Mineralization of water-in-oil emulsion droplets Giulia Fornasieri, Stéphane Badaire, Rénal Vasco Backov, Philippe Poulin, Cécile Zakri and Olivier Mondain-Monval* Centre de Recherche Paul Pascal - CNRS, Avenue Albert Schweitzer, 33600 Pessac, France [email protected] Using reverse emulsion systems, we were able to trigger mineralization confined at an oil-water interface. In this process, the alcoxide silica precursor is dissolved in the oil continuous phase of the emulsion and diffuses through the bulk to the interface where it starts to hydrolyze and condense as soon as a certain concentration threshold is attained. The process takes place only in the presence of a water soluble surfactant inside the droplet. This surfactant leads to the presence of a controlled mesoporosity inside the silica shells. The obtained objects could be used in different encapsulation applications. Introduction While the mesostructure of porous minerals grown via surfactant-templated processes is well established [1],[2], the control over the structure on larger scale remains more problematic. Free bulk mineral growth generally leads to microparticles with random shapes. The morphology of the particles results from a complex interplay between reaction-diffusion mechanisms, elasticity, interfacial and topological constraints of the surfactant mesophase.[3] To achieve both a higher degree of control and a hierarchically organized structure, mineralization can be confined at a macroscopic interface.[4] For instance, flat mesoporous films have been realized onto solid[5] or liquid interfaces.[6] Some years ago, Schacht et al. have used oil/water interfaces of oil droplets in water (direct emulsions) to produce spherical mesoporous capsules filled with oil.[7] The mineral precursors, initially dissolved in the oil droplets, condense after hydrolysis at the oilwater interface. The continuous water phase contains a single water soluble surfactant that governs the mineral mesoporosity and stabilizes the droplets against coalescence.[8] A feature of such a process is that mineralization not only takes place at the droplets surface but also occurs, in an uncontrolled manner, in the bulk phase. In a recent paper[9], we presented a new approach that overcomes these problems by using reverse emulsions instead of direct ones. In particular, we showed that mineralization takes place only at the droplets surface. Here, we describe this synthesis and the experimental parameters that govern the structure of the obtained mineralized objects. Results and discussion To perform the synthesis, we first emulsify water (at a controlled pH) in a surfactantorganic phase mixture. In our process, a second surfactant, that is soluble in the water phase only, is also required to control the structure and morphology of the finally obtained objects. As we demonstrate, the choices of this surfactant and of the surfactant-organic mixture are critical. Also, we have examined the influence of the used process. A first set of experiments were performed
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