The control of water flow in Electrostatic Double Layers (EDL) close to charged surfaces in solution is an important issue with the emergence of nanofluidic devices. We compare here the zeta potential governing the electrokinetic transport properties of surfaces, to the electrostatic potential directly measured from their interaction forces. We show that on smooth hydrophilic silica these quantities are similar, whereas on OTS-silanized hydrophobic surfaces the zeta potential is significantly higher, leading to an enhanced electro-osmotic velocity. The enhancement obtained is consistent with an interfacial water slippage on the silanized surface, characterized by a constant slip length of approximately 8 nm independent of the salt concentration in the range 10(-4)-10(-3)M.