  The origin of the apparent negative charge at hydrophobic-water interfaces
has fueled one of the biggest debates in physical chemistry for several
decades. The most common interpretation given to explain this observation is
that negatively charged hydroxide ions (OH-) bind strongly to the interfaces.
Here, using first principles calculations of the air-water and oil-water
interfaces consisting of thousands of atoms, we unravel a mechanism that does
not require the presence of OH-. We show that small amounts of charge transfer
along hydrogen bonds and asymmetries in the hydrogen bond network, associated
with local topological defects can lead to the accumulation of negative surface
charge at both interfaces. For water near oil, we show that there is also some
spillage of electron density into the oil leaving it negatively charged. The
surface charge densities at both interfaces is computed to be approximately
$-0.015$ e/nm$^{2}$ in agreement with electrophoretic experiments. We also
show, using an energy decomposition analysis, that the electronic origin of
this phenomena is rooted in a collective polarization and charge transfer
effect.

Chemical Physics;Soft Condensed Matter