Surface area estimation from the adsorption isotherm gradient at the linear portion by n-butylbenzene vapour

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Using n-butylbenzene vapour having a comparatively low saturation pressure (0.160 Torr at 0°C), small solid surface areas could be measured from the gradient of the linear portion of the Type II adsorption isotherms which were found for most solid adsorbents. In this “gradient method”, surface areas were obtained from the monolayer volumes equated directly to the gradient values [d v/d( P/ P s); where v is the amount adsorbed, and P/ P s is the relative pressure] and the molecular cross section (0.443 nm 2) of the adsorbate calculated from the liquid density. The surface areas thus obtained agreed with the standard BET-N 2 surface areas, with a difference of about ±20% for over 20 solid powder samples. The gradient method for the plate-like solid samples of glass plates and aluminium foil resulted in a slight overestimation of +3% and +12% compared with the geometrical areas; this was reasonable, considering the remaining surface porosity of the sample. The gradient method using n-butylbenzene vapour as well as toluene and ethylbenzene vapours, which have been reported previously, was ascertained to be useful for surface area estimation, especially for very small surface areas, because of the extremely low adsorbate pressure at room temperature.