We have modeled the soft permeable interface between a dielectric, rep
resenting the hydrophobic interior of a membrane or protein, and an aq
ueous solution. The intention is to study the electric fields in the a
queous solution around a biomembrane or a protein within the framework
of the ''local'' electrostatic theory, as a function of four paramete
rs: the thickness of the interface; the electric charge and dipole den
sities, sigma and nu, at the interface; the Debye length of the aqueou
s solution. Our principal result is a plot of the sign of the surface
potential as a function of sigma and nu. We show that an electrically
neutral surface (sigma = 0) can set up an electric field if nu not-equ
al 0. For the general case sigma not-equal 0, nu not-equal 0, we show
that we can get various signs of the surface potential depending upon
the relative magnitudes of sigma and nu and that changes in the interf
ace thickness and Debye length can change the sign of the surface pote
ntial. Finally, we relate changes in the interface tension to changes
in sigma and nu.