  We investigate the mobility of nanometer-size solutes in water in a uniform
external electric field. General arguments are presented to show that a closed
surface cutting a volume from a polar liquid will carry an effective non-zero
surface charge density when preferential orientation of dipoles exists in the
interface. This effective charge will experience a non-vanishing drag in an
external electric field even in the absence of free charge carriers. Numerical
simulations of model solutes are used to estimate the magnitude of the surface
charge density. We find it to be comparable to the values typically reported
from the mobility measurements. Hydrated ions can potentially carry a
significant excess of the effective charge due to over-polarization of the
interface. As a result, the electrokinetic charge can significantly deviate
from the physical charge of free charge carriers. We propose to test the model
by manipulating the polarizability of hydrated semiconductor nanoparticles with
light. The inversion of the mobility direction can be achieved by
photoexcitation, which increases the nanoparticle polarizability and leads to
an inversion of the dipolar orientations of water molecules in the interface.
