Screened electrostatics of charged particles on a water droplet.

We study the electrostatic properties of charged particles trapped at an interface in a water-in-oil microemulsion. The electrostatic potential and the counterion distribution in the water droplet are given in terms of the ratio of the Debye screening length kappa(-1) and the droplet radius R. In the limit R-->infinity we recover the well-known results for a flat interface. Finite-size corrections are obtained in terms of the small parameter 1/kappaR. Part of the counterions spread along the interface and form a charged layer of one Debye length thickness. In particular, there is a uniform surface charge contribution. We derive explicit expressions for the electric field, the mobile charge density, and the charge-induced pressure on the interface.