Fig. 1.

(A) The schematic shows an oppositely charged $M^{+}-M^{-}$ ion pair separated by r and situated in a plane at a distance z from the interface. The z axis is normal to the plane of the average vapor–liquid interface $\left(z=0\right)$, with $z>0$ being the liquid side. (B) PMF between $M^{+}$ and $M^{-}$ ions calculated in various z planes. We set $W\left(r|z\right)=0$ in each z plane at r = 1.2 nm for reference. (C) The 3D $W\left(r,z\right)$ PMF landscape for the $M^{+}-M^{-}$ ion pair in which an ion pair separated by 1.2 nm and located at $z=1$ is used as a reference state; i.e., $W\left(r=1.2,z=1\right)=0$. This landscape highlights the work needed to move an ion pair from bulk water to the interfacial region.