Mesoporous and homothetic silica capsules in reverse-emulsion microreactors
Mesoporous and homothetic silica capsules in reverse-emulsion microreactors
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DOI：
10.1002/adma.200306229
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发表时间：
2004-07-05
影响因子：
29.4
通讯作者：
Poulin, P
中科院分区：
文献类型：
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作者：
Fornasieri, G;Badaire, W;Poulin, P
通过表面模板化过程生长的多孔矿物的介观结构今天已经很好地建立。([1])表面活性剂模板中的溶胶-凝胶合成通常涉及协同热力学和动力学机制的微妙平衡。虽然还没有清楚地了解，介孔织构可以很好地控制经验，并已观察到各种介孔结构具有不同的对称性和孔隙率。([2])自由块状矿物生长通常导致具有随机形状的微粒。([3])颗粒的形态是由反应扩散机制、弹性、界面和表面活性剂中间相的拓扑约束之间复杂的相互作用引起的。为了实现更高程度的控制和分级组织的结构，矿化可以被限制在宏观界面处，并且可以通过各种方法用于生长空心球或核-壳颗粒。([4])在这种情况下，Schacht等人已经使用油滴在水中的油-水界面（直接乳液）来生产填充有油的球形中孔胶囊。([5])矿物前体最初溶解在油滴中，水解后在油-水界面凝结。连续水相含有单一的水溶性表面活性剂，其控制矿物中孔性并稳定液滴以防止聚结。([6])所获得的系统是具有潜在的封装应用的分层组织的矿物的极大兴趣。然而，在直接乳液中，矿化发生在本体连续相中。限制的缺乏使得扩散机制和颗粒形状的控制变得困难，因为这些也会受到流体动力学流动的影响。([2，3，5]）在本文中，我们使用反相乳液，而不是直接的，其中矿化只发生在液滴内，液滴作为模型，微反应器。这允许更多的控制条件，并有助于提供合作二氧化硅/表面活性剂生长机制的见解。事实上，在我们的情况下，矿物前体的通量受到它们在粘性连续相中的扩散和在液滴的整个表面积中的渗透的限制。由于在水相中的扩散快得多，我们的系统可以被视为微反应器，其中一种反应物的浓度随着时间的推移而逐渐增加。观察该系统作为时间的函数，以及所得到的系统，揭示了矿化只发生在液滴内的明确定义的水解前体浓度以上。这种行为表明，该系统达到相边界，对应于形成由二氧化硅低聚物和表面活性剂，矿化之前的中间相。
The mesostructure of porous minerals grown by surfactant-templated processes is today well established.([1]) Sol-gel synthesis in surfactant templates usually involves a subtle balance of cooperative thermodynamic and kinetic mechanisms. Although not yet clearly understood, the mesoscale texturing can be well controlled empirically, and various mesoporous structures have been observed with different symmetries and porosities.([2]) Free bulk-mineral growth generally leads to microparticles with random-shapes.([3]) Morphology of the particles results from a complex interplay between reaction-diffusion mechanisms, elasticity, and interfacial and topological constraints of the surfactant mesophase. To achieve both a higher degree of control and a hierarchically organized structure, mineralization can be confined at a macroscopic interface and can be used to grow hollow spheres or core-shell particles by various methods.([4]) In this context, Schacht et al. have used oil-water interfaces of oil droplets in water (direct emulsions) to produce spherical mesoporous capsules filled with oil.([5]) The mineral precursors, initially dissolved in the oil droplets, condense after hydrolysis at the oil-water interface. The continuous water phase contains a single water-soluble surfactant that governs the mineral mesoporosity and stabilizes the droplets against coalescence.([6]) The obtained systems are of great interest as hierarchically organized minerals with potential encapsulation applications. Nevertheless, in direct emulsions mineralization takes place within the bulk continuous phase. The absence of confinement makes the control of diffusion mechanisms and particle shapes difficult, as these can also be affected by the hydrodynamic flow.([2,3,5]) In this paper we use reverse emulsions, instead of direct ones, where mineralization takes place only within the droplets, which act as model, microreactors. This allows more controlled conditions and helps in providing insights on cooperative silica/surfactant growth mechanisms. Indeed, in our case, the flux of mineral precursors is limited by their diffusion in the viscous continuous phase and penetration throughout the surface area of the droplets. As diffusion in the aqueous phase is much faster, our systems can be viewed as microreactors where the concentration of one reactant is progressively increasing as time elapses. Observing this system as a function of time, as well as the resultant systems, reveals that mineralization takes place only above a well-defined hydrolyzed precursor concentration within the droplets. This behavior suggests that the system reaches a phase boundary, corresponding to the formation of a mesophase made of silica oligomers and surfactants, prior to mineralization.