Classical molecular dynamics simulations point to an anisotropy of water-water hydrogen 
bonding at the water surface. Approaching from the gas phase, a region of primarily 
dangling hydrogens is followed by dangling oxygens before the isotropic bulk region. 
Using ab initio calculations we translate this hydrogen bonding anisotropy to charge 
transfer between water molecules which we analyze with respect to both instantaneous 
and averaged positions of the water surface. Similarly to the oil/water interface we 
show that there is a region of small net negative charge extending 0.2-0.6 nm from the 
Gibbs dividing surface in the aqueous phase. Using a simple continuum model we translate 
this charge profile to a zeta potential which acquires for realistic positions of the 
shear surface the same negative sign as observed experimentally, albeit of a smaller 
absolute value.