Abstract--The t-plot method has been applied to the results of nitrogen adsorption at 76~ on sepiolite first heated in vacuo at various temperatures. Heating sepiolite samples in vacuo at 427~ results in a large decrease in surface area compared with samples outgassed at 373~ A change in structure and a consequent collapse of micropores is postulated. However, the t-plots indicate that some microporosity remains. Outgassing at 623~ appears to destroy completely the micropores.
INTRODUCTIONThe t-plot method (Lippens et al., 1964;Lippens and de Boer, 1965;de Boer et al., 1966) for the calculation of the surface areas of solids has been extended by Mikhail et al. (1968a) to the determination of both internal and external surface areas of microporous solids, i.e. solids with pores of radii 1.(L1.5 nm or less (Dubinin, 1966). According to the authors a single gas adsorption isotherm can yield information concerning the extent of the micropore system and the external surface area of solids which is not obtainable by methods such as the BET method (Brunauer et al., 1938). Complete pore structure analyses have been made using the t-plot approach by Mikhail et al. (1968b).In the t-plot method, the adsorption isotherm data (volumes, V~ (STP), or weights, xa, of gas adsorbed/g adsorbent at a series of equilibrium pressures) is converted into a plot of V, (or x,) against t, the statistical thickness of the adsorbed layer, by means of a "master" t-curve for non-porous adsorbents, several of which have been published (Shull, 1948;Cranston and Inkley, 1957;de Boer et al., 1966). Such curves have been derived for a large number of non-porous solids by calculating the statistical thickness, t, from V~/S~I values (where Sj~,~ is the specific BET surface area), assuming close packing of adsorbate molecules, and plotting these against the corresponding relative pressures at equilibrium. Smoothed-out ("universal") t-curves are obtained for groups of solids. Non-porous solids yield linear t-plots (Va vs t) which pass through the origin if no capillary condensation takes place. A surface area, S,, may be calculated from the slope of such a linear plot. However, the choice of the master t-curve influences the results obtained. The interpretation of the results continues to be a matter of controversy (Sing, 1973).