Capillary condensation in pores with r<10 nm is often difficult to describe using the Kelvin equation.  This is because the Kelvin equation underestimates the size of the pore radius when working on the nanometer scale.  To account for this underestimation, the idea of a statistical film thickness, t, has often been invoked.  The idea centers around the fact that a very small layer of adsorbed liquid coats the capillary surface before any meniscus is formed and is thus part of the estimated pore radius.  The figure to the left gives an explanation of the statistical film thickness in relation to the radius of curvature for the meniscus.  This adsorbed film layer is always present; however, at large pore radii the term becomes so small compared to the radius of curvature that it can be neglected.  At very small pore radii though, the film thickness becomes an important factor in accurately determining the pore radius.