Here we report evidence for substantial changes in the separation efficiency of nanoporous anodic alumina membranes with nanochannel diameters ranging from 10-100 nm modified with octadecylphosphonic acid in the transitional flow regime. Softening of surface by alkyl groups with a surface density similar to 2 groups/nm(2) leads to a general permeance decrease in 3-500 times, depending strongly on the penetrant gas nature and the channels diameter. The divergence of the permeance, for different gases, increases with the decreasing diameter of the pores. For a surface-functionalized membrane, with 10-nm channel diameters, it results in n-C4H10/CH4 ideal and mixed gas separation factors up to 32.3 and 9.0 respectively at a n-C4H10 permeance up to 3.5 m(3)/(m(2).barh). The effect is related to the changes of the ratio of molecule travelling time to residence time in the adsorbed state, as well as a strong influence of surface saturation by the absorbed molecules on the tangential momentum accommodation coefficient, which is supported by the derived model. Synergetic contribution of these two factors allows to enhance the separation factor of permanent and condensable gases strongly beyond the Knudsen limit, while maintaining a high permeance of porous membranes.