When the separation between molecules of a liquid is of the order of 10-9m, force of attraction between them becomes zero. Suppose a molecule is at point A on the free surface of liquid. Now imagine a hemisphere of radius 10-9m with point A, at its centre. The molecule at A cannot attract other molecules lying outside this hemisphere. The lower surface of the surface film is the plane which is tangent to the hemisphere at its lower most point (as shown in figure). so thickness of surface film is of the order of 10-9m.