Ultrasmall Nanocapsules Obtained by Controlling Ostwald Ripening

We describe here a method to synthesize ultrasmall nanocapsules with a diameter of 6 nm, exhibiting a well‐defined core–shell morphology. Remarkably, the nanocapules are synthesized in a miniemulsion process without the need of large amounts of surfactant as commonly used in the microemulsion process. Ultrasmall nanocapsules with an oil core and a silica shell are formed by the concurrent processes of a sol–gel reaction and Ostwald ripening. Using solvents with different water solubilities and alkoxysilanes with different reactivities, we demonstrate that sizes of obtained nanocapsules depend on the ripening rate and alkoxysilane conversion rate. The method can be also used for encapsulating natural oils such as peppermint oil and limonene. This work shows that the Ostwald ripening phenomenon can be employed beneficially for the preparation of very small colloids. Ostwald ripening is used for reducing particle size in miniemulsions, providing a rapid and versatile method to synthesize silica nanocapsules with an ultrasmall size (ca. 6 nm) and a well‐defined core–shell morphology, even at low surfactant concentration. Aqueous solubility of solvents and reactivity of alkoxysilanes used in the synthesis are the key factors in controlling the formation of ultrasmall nanocapsules.