The behaviour of the PAO4 fluid also varies with water coverage. For the anhydrous (not pictured) and the hydroxylated system (Figure (a)), strong layering is observed in the PAO4 density profile, with a pronounced first density peak that is strongly dependent on the strength of the solid-liquid interactions. However, as the thickness of the water film increases the PAO4 structure becomes progressively less ordered. By 3.0 and 4.0 ML coverage, the first PAO4 density peak is substantially reduced, and density oscillations in the PAO4 layer are no longer apparent. This may be due to the screening of the hematite-PAO4 interactions by the hydroxyl groups and water molecules. However, thermal capillary wave (CW) fluctuations, which are reduced when the water film is in contact with the solid surface, may also play a role. These fluctuations smooth out the apparent layering of water and PAO4 molecules, especially for thick water films, even though the underlying structure remains intact. For the 0.5 ML water system (Figure direct contact between the PAO4 molecules and the surface, with the in-plane density profile revealing a homogeneous interface. For water coverages above 1 ML (Figure )), there is an apparent overlap in the density profiles for the water and PAO4 molecules, suggesting an interface width of ∼5 Å. However, this may also result from CW fluctuations, which blur the sharpness of the interfacial structure despite the presence of molecular layering. To determine the mobility of the confined water molecules, we tracked the mean square displacement, for all water molecules contrained in the film, in the surface plane to calculate the self-diffusion coefficient at 325 K, D xy , as shown in Figure . For further details of the calculation, refer to the Supporting Information. D xy decreases significantly from 4 ML (1.72 nm 2 ns -1 ) to 0.5 ML (0.23 nm 2 ns -1 ), where a higher proportion of the water molecules are immobilized close to the surface. This observation is consistent with previous MD simulations of hydrophilic surfaces. In contrast, the self-diffusion coefficient for bulk water at 325 K was 4.58 nm 2 ns -1 , which is close to the experimental value at this temperature, D = 4.1 nm 2 ns -1 . Despite their reduced mobility near the surface, the water molecules remain diffusive, suggesting that they do not solidify as has been observed for some other surfaces such as mica. This is further confirmed by the oxygen-oxygen radial distribution functions (RDFs)