Validity of the Kelvin equation in estimation of small pore size by nitrogen adsorption

 We have investigated a practical lower limit of a pore-size estimation by the nitrogen desorption isotherms at 77 K using the Kelvin equation. Changes in pore size of porous silica glasses before and after the monolayer preadsorption of n-propylalcohol were estimated by measuring the nitrogen adsorption and desorption isotherms. These changes should correspond to the thickness of monolayer of adsorbed n-propylalcohol. The thickness of monolayers obtained for the samples whose pore sizes are below ca. 2 nm were underestimated, when the Kelvin equation was applied to the nitrogen desorption isotherms using the values of surface tension and molar volume of bulk liquid nitrogen at 77 K. Below ca. 2 nm pore radius a careful application of the Kelvin equation is required to estimate a pore size. These results suggest that a change in the physical properties of liquid nitrogen in such a small pore occurs. It is supposed that the interaction between the solid surface and adsorbate molecules causes the changes in the surface tension and density of liquid nitrogen in such a narrow pore. Received: 21 March 1997 Accepted: 18 July 1997