  Two collective properties distinguishing the thin liquid water vapour
interface from the bulk liquid are the anisotropy of the pressure tensor giving
rise to surface tension and the orientational alignment of the molecules
leading to a finite dipolar surface potential. Both properties can be regarded
as capillary phenomena and are likely to be coupled. We have investigated this
coupling by determining the response of the tangential component of the surface
tension to the application of an electric field normal to the surface using
finite field molecular dynamics simulations. We find an upside down parabola
with a maximum shifted away from zero field. Comparing the molecular dynamics
results to an elementary electromechanical continuum model we relate the zero
field derivative of the tangential part of the surface tension to the
electrostatic potential generated by the spontaneous dipole alignment. The
calculations show that these quantities have similar values but are not and in
fact need not be identical. The electromechanical model also allows us to
convert the absolute curvature of the quadratic field dependence to an
effective dielectric constant of the water interface which is found to be much
lower compared to the bulk value as expected.
