Top: MD simulations of ethanol and IPA at an uncharged surface show that these molecules orient on average with their methyl groups pointing slightly towards the interface and their electronegative oxygen atoms pointing towards the bulk, giving an interfacial potential φ 0 > 0. Unlike water, the average interfacial molecular dipole moment (green arrow) points towards the surface. Bottom: various contributions to the total interaction free energy for positive (red lines) and negative (blue lines): electrostatic repulsion, ∆ F el (black dotted line; applies to both signs of charge); interfacial solvation contribution, ∆ F int (dashed line, different for positive and negative); total interaction energy, (solid lines); and the vdW contribution, U vdW (dashed-dotted grey line; applies to both cases). When φ 0 > 0, positively charged particles may display a minimum in U tot at an interparticle separation x min (solid red line), whereas negatively charged particles repel monotonically, displaying no minimum (solid blue line) (Supplementary Information, section 3.1 ).
