Following an equilibration period, snapshots were extracted that include two solvation shells around the ion, according to the distance criteria described in Ref. . The number of water molecules varies from one snapshot to the next, with the average number N w depending on both the size of the ion and how tightly hydrated it is. In bulk water, these averages range from N w ≈ 28 for Cl -(aq) up to N w ≈ 43-44 for Br -(aq) and I -(aq), with N w = 35-37 for the remaining ions. The interfacial snapshots contain fewer water molecules, on average, as the water density is smaller in the interfacial region. In the analysis that follows, we consider interaction energies (E int ) between the ion and its first two hydration shells. The quantity E int is intensive with respect to system size and this insulates our analysis against the step-to-step fluctuations in the number of water molecules that are included in these calculations. Ensemble averages reported below represent 51 snapshots for each ion in bulk water as well as 51 snapshots for each ion at the air/water interface, with each individual snapshot separated by 5 ps in time in the corresponding MD simulation. Coordinate files for these data sets are provided in the Supplementary Material.