  Electro-osmosis (EO) is a powerful tool to manipulate liquids in micro and
nanofluidic systems. While EO has been studied extensively at liquid-solid
interfaces, the case of liquid-vapor interfaces, found e.g. in foam films and
bubbles, remains to be explored. Here we perform molecular dynamics (MD)
simulations of EO in a film of aqueous electrolyte covered with fluid layers of
ionic surfactants and surrounded by gas. Following the experimental procedure,
we compute the zeta potential from the EO velocity, defined as the velocity
difference between the middle of the liquid film and the surrounding gas. We
show that the zeta potential can be smaller or larger than existing predictions
depending on the surfactant coverage. We explain the failure of previous
descriptions by the fact that surfactants and bound ions move as rigid bodies
and do not transmit the electric driving force to the liquid locally.
Considering the reciprocal streaming current effect, we then develop an
extended model, which can be used to predict the experimental zeta potential of
surfactant-laden liquid-gas interfaces.
