Direct measurement of contact angles of silica particles in relation to double inversion of pickering emulsions.

In an alkane-water system containing submicrometer silica particles at high pH, double emulsion inversion from oil-in-water (o/w) to water-in-oil (w/o) to oil-in-water can be effected by increasing the concentration of a dichain cationic surfactant in water. The contact angle θ of the particles at the planar oil-water interface has been measured directly using freeze-fracture shadow-casting cryo-scanning electron microscopy, enabling single-particle measurements of high accuracy. θ passes through a maximum with respect to surfactant concentration.