N2 Desorption Method

To determine the distribution of pores with diameters smaller than 20 nm, a nitrogen desorption technique is employed which utilizes the Kelvin equation to relate the pore radius to the ambient pressure. The porous material is exposed to high pressures of N2 such that P/Po 1 and the void space is assumed to be filled with condensed N2, then the pressure is lowered in increments to obtain a desorption isotherm. The vapor pressure of a liquid in a capillary depends on the radius of curvature, but in pores larger than 20 nm in diameter the radius of curvature has little effect on the vapor pressure however, this is of little importance because this region is overlapped by the Hg penetration method. 

The Kelvin equation shows that the smaller the pore radius, the lower the vapor pressure, P, in the pore  

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