(261a) Evaluation of Surface Area and Porosity from Physisorption Isotherms

Physical sorption can be presented as a function of: 1. Relative pressure (isotherms); 2. Logarithm of relative pressure (Characteristic curve); or 3. AutoShielding Potential (ASP ---- Logarithm of Polyani sorption potential). For nonporous materials the general shape of these curves are sigmoidal, exponential and linear, respectively, with an enhanced sorption component for interparticulate condensation (IPC) in the pressure region where capillary condensation occurs. ASP plots for physisorption on known homogenous nonporous materials indeed are rectilinear for multilayer coverage. The ASP assumes that there is a finite sorption potential for the first sorption increment followed by the decrease exponential decrease to zero at saturation pressure. Intermolecular condensation becomes more prominent for greater specific surface areas (smaller particle size, more interparticulate voids) Graphical presentations

Sorption Isotherm Characteristic Curve ASP Plot

show the merits of the alternate methods of analyses for Nitrogen sorption on alpha alumina at 77K: Phys. Chem. Chem. Phys. 3 5078(2001). The ASP plot slope = monolayer capacity (6.734 µmol/m2, based on BET) and Intercept = threshold sorption potential (1501 cal/mol, or P= 5.14e-5 P(0)) fitted to the data with a correlation coefficient of 0.9997. The lower isotherm curve is the excess ICP, over and above the extrapolation prediction based on ASP. These ASP parameter would serve well as a method of calculating a reference (t. n, s. etc) isotherm although it is more informative to plot the sorption in the ASP format directly and attribute additional effects to micropores, mesopores, chemisorption, morphology, surface chemistry, etc. One is hard put to assure that the reference isotherm has the same surface chemistry and structure. By comparison, the BET methodology is considerably less definitive since although it is referred to as a multilayer theory it is: ? is only applicable in a restricted range (0.05-0.35P(0)), ca 0.8 to 1.2 monolayers ? does not quantitatively delineate the amount and mechanism of the inevitable ICP ? does not give an estimate of the threshold sorption energy (pressure),