Microscopic picture of the liquid surface

A surface is not an infinitesimal sharp boundary in the direction of its normal, but it has a certain thickness. For example, if we consider the density p normal to the surface (Fig. 2.1), we can observe that, within a few molecules, the density decreases from that of the bulk liquid to that of its vapor [10]. 

The density is only one criterion to define the thickness of an interface. Another possible parameter is the orientation of the molecules. For example, water molecules at the surface prefer to be oriented with their negative sides out towards the vapor phase. This orientation fades with increasing distance from the surface. At a distance of 1-2 nm the molecules are again randomly oriented. 

Which thickness do we have to use This depends on the relevant parameter. If we are for instance, interested in the density of a water surface, a realistic thickness is in the order of 1 nm. Let us assume that a salt is dissolved in the water. Then the concentration of ions might vary over a much larger distance (characterized by the Debye length, see Section 4.2.2). With respect to the ion concentration, the thickness is thus much larger. In case of doubt, it is safer to choose a large value for the thickness. 

The surface of a liquid is a very turbulent place. Molecules evaporate from the liquid into the vapor phase and vice versa. In addition, they diffuse into the bulk phase and molecules from the bulk diffuse to the surface. 

---