When analyzing the interfacial entropies, it should also be noted that in our computational model the interfacial water molecules are rather free to move in the z-coordinate due to the presence of the vacuum layer. The water molecules may eventually drift into the vacuum, but they are captured back into the water layer again for the duration of the simulation. While we have corrected for the formation and solvation free energies by removing the γ lv -term, the free relaxation of the water film does not guarantee a fixed bulk density of 1 g/cm 3 corresponding to standard pressure conditions. As a result, the volume of the water film tends to fluctuate and the average bulk density becomes lower than 1 g/cm . The lower density leads to somewhat higher entropies as can be inferred from experimental data, showing that increasing the water temperature by 25 K decreases water density by ca. 1 % but increases the entropy by 0.06 meV/K from 0.65 to 0.79 meV/K. Therefore, the larger entropy of water in the IBL region, as shown in Table , may at least partially be due to the lower density of water in the simulations. Nevertheless, the effect of the lower water entropy on the (computed) total entropy is modest compared to the experiment.