Surfactant-free polyurethane nanocapsules via inverse Pickering miniemulsion.

We report on a surfactant-free synthesis of Pickering-stabilized submicrometer-sized capsules in inverse miniemulsion. Functionalized silica nanoparticles are able to stabilize water-in-cyclohexane miniemulsions to form stable polyurethane shells via interfacial polyaddition. The effect of the type of silica functionalization on the stabilizing properties is demonstrated by varying the hydrophobicity and, therefore, the contact angle between silica and the two liquid phases. Addition of small amounts of salt leads to a reduction of the capsule size and to a narrow size distribution. The impermeability of the formed capsule shell is proven by encapsulation of an organic fluorescent dye and release studies in aqueous environment. In addition, we show the possibility to encapsulate large amounts of inorganic salts without negative effects concerning the stability of the emulsion, which enables the application for phase-change materials.