Analysis of nitrogen adsorption isotherms on porous and nonporous silicas by the BET and αs methods

Abstract Nitrogen adsorption isotherms were determined on a wide range of porous and nonporous silicas. Isosteric heats of adsorption were calculated from the isotherms (over the temperature range −192° to −178°C) on representative materials. Standard data for nitrogen adsorption at −196°C on nonporous hydroxylated silica are tabulated for the p p o range 0.001–0.90. The results indicate that certain high-area silicas are truly nonporous, but some grades of commercial Aerosil are porous. Surface areas are calculated from the isotherms by means of the BET method and the new αs-method. The latter is a graphical procedure in which the amount adsorbed is plotted against αs for the standard adsorption data, where αs is the ratio of the amount adsorbed (at the given p p 0 ) to the amount adsorbed at p p 0 = 0.4. Deviations of the a,-plots from linearity are explained in terms of micropore filling and capillary condensation. In the absence of micropore filling, the surface areas calculated from the slope of the αs-plots are in excellent agreement with the BET-areas. Enhanced isosteric heats and C values are associated with micropore filling; the isotherm is therefore distorted in the BET range and the BET-area is not valid. In certain cases, when micropore filling and monolayer coverage at low p p 0 are followed by multilayer formation and capillary condensation at higher p p 0 , a nearly linear αs-plot results, but again neither the BET-area nor the αs-area can provide a meaningful value of the internal surface area.

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