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66 interaction between tetrahydrofuran solvent and zinc oxideface, defined as the regions where dρ/dz ̸= 0, turns out tobe as thin as 1 nm and it consists of the two regions labeledC and L ′ in Figure 5.6. C region corresponds to the crystallineTHF layer wetting the ZnO surface. L ′ has width 0.5nm and it corresponds to region where the liquid densityis smaller than ρ THF . A visual inspection of the moleculardistribution in L ′ shows that there is an empty spaceseparating the wetting layer from the remaining liquid. Inconclusion, the interface gives rise to a sharp transition inthe THF density corresponding to an order/disorder discontinuityin the molecules distribution.Figure 5.6.: Density profile of ZnO-THF system with respect to theaxys perpendicular to the surface. (For clearness in thepicture we do not represent the hydrogens of THF.)The interface between a Van der Waals liquid and a hardwall (i.e. solid a surface) has been previously studied [110].Density fluctuations within the liquid phase are expecteddepending on its ρ ∗ bulk packing density. ρ ∗ is defined asρ ∗ = ρ THF κ 3 , where ρ THF is the liquid density, and κ isthe Van der Waals diameter of the liquid molecules. Liquidsthat are characterized by high bulk packing density(ρ ∗ > 0.8) show a densified region next to the substrate,followed by an oscillating exponentially decaying densityprofile. This is the case, for example, of cyclohexane on siliconsurface, where a densified close-packed liquid layer