A two-dimensional mapping of salt density and water orientation within the first density layer reveals that water molecules are particularly strongly aligned in front of the divalent SO 2- 4 ions (Fig. ). This strong alignment of water molecules induced by the SO 2- 4 ions can be rationalized due to sulfate partial charges and the corresponding strong quadrupole moment: the sulfur atom has a charge of +2 e, and the four oxygen atoms each bear a charge of -1 e, for a total charge for the SO 2- 4 ions of -2 e. The alignment could also be interpreted in terms of hydrogen bonds that form between the water molecules and the oxygen atoms of the SO 2- 4 ions. In contrast, hydrogen bonds are quasi absent between water and the NaCl surface: water molecules sometimes form hydrogen bonds with chlorine atoms, but the typical number of hydrogen bonds per water molecule between the water and the solid surface is two orders of magnitude lower in the case of NaCl, as compared to Na 2 SO 4 (Fig. ).