Akira Nonaka

Surface area estimation from the adsorption isotherm gradient at the linear portion by benzene and toluene vapors on mica and aluminum foil samples

Abstract Adsorption isotherms of benzene and toluene vapors on mica and aluminum foil samples, whose apparent surface areas were known geometrically, were measured at 30 to 40°C by gravimetry, in order to inspect the relation of the surface area of the samples to the gradient at the linear portion or at the inflexion of the Type II (or Type III) adsorption isotherms to be obtained. The surface areas of the mica and aluminum foil samples having respective geometrical surface areas of 500 and 790 cm 2 were estimated as slightly larger values by about +9 and +7% from the values of the linear portion gradient of the adsorption isotherms obtained (adsorption isotherm gradient method). However, the BET surface area of the aluminum sample by the nitrogen adsorption was 12% larger than the geometrical one. It was shown in this study that the adsorption isotherm gradient method using the benzene or toluene vapor at room temperature may be useful as well as the standard BET method for the estimation of the surface area.

Stepwise multilayer adsorption of water vapor on silica at temperatures from 130 to 150°C

Abstract At temperatures from 130 to 150°C and in a pressure range from 0.05 to 3.5 atm, typical stepwise adsorption isotherms were found in the study of multilayer adsorption of water vapor on silica (Porasil) having 6.02 m 2 g of specific area and the structure of α-cristobalite. The adsorption amounts were measured by a gas-chromatographic method. The adsorption isotherms had two or three plateaus and rose to infinity at 0.6–0.7 relative pressure. The adsorption seemed to increase layer by layer up to four layers. From the disagreement in the corresponding step heights found in the isotherms at different temperatures, it was assumed that there were two kinds of adsorption sites which had monolayer capacities of 0.20 and 0.55 mg g . Isosteric heats decreased with increase of adsorption from a rather high value (>25 kcal mole ) in the first step formation to the liquefaction heat (9.5 kcal mole ). The values of differential entropy suggested that the molecules adsorbed up to the second step might be in a solid state and the outer molecules in a liquid state. The molecular area (24 A2) of the adsorbed water agreed approximately with the area per silanol group observed on the adsorbent. The adsorbate molecules might pile up constructing a quasi-ice structure based on the silanol groups on the crystalline surface of the adsorbent.

Adsorption of toluene vapor on glass plates and surface area estimation from the adsorption isotherm gradient at the linear portion

Abstract In order to evaluate a novel method for estimating solid surface areaas from the adsorption isotherm gradient at the linear portion of type II isotherms, the adsorption of toluene on glass plates having a geometrical surface area of 1152 cm2 at 0° and 10°C was studied. The almost constant isotherm gradient value ( dv d ( P P s ), v : amount adsorbed) in a relative pressure ( P P s ) region from 0.15 to 0.45, as well as in the cases of aluminum foil and mica plates, showed a good fit with the monolayer capacity calculated using the average, proper molecular cross section of toluene. For various materials whose geometrical surface area were not known, the standard BET areas were compared with the areas estimated from the isotherm gradient of toluene adsorption and their reasonable agreement suggested the usefulness of the gradient method for the surface area estimation. Based on these results a theoretical basis and conditions for the use of the method were discussed by considering an alternative two-dimensional gas model on the low-energy adsorption surface. In addition, a small type B hysteresis found in the region of low relative pressure and at submonolayer adsorption on the glass plate sample pile was reported.

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

Abstract 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 [dv/d(P/Ps); where v is the amount adsorbed, and P/Ps is the relative pressure] and the molecular cross section (0.443 nm2) of the adsorbate calculated from the liquid density. The surface areas thus obtained agreed with the standard BET-N2 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.

A novel method for surface area estimation from the linear portion gradient of the type II adsorption isotherms using toluene and ethylbenzene vapors

Abstract Previous works (A. Nonaka, J. Colloid Interface Sci. 99, 335 (1984), and A. Nonaka, J. Colloid Interface Sci. 112, 548 (1986)) indicated that the gradient of the linear portion of Type II isotherms of toluene and benzene might be equal to the monolayer capacity of the adsorbent. In the present study the surface areas of a range of solids estimated by the “gradient method” using toluene and/or ethylbenzene vapors (the molecular cross-sectional areas being calculated from the liquid density) have been found to agree within ±20% with the BET surface areas using nitrogen. It is suggested that the gradient method may be useful for the estimation of surface area, particularly if the area is small, because of the low vapor pressure of the adsorptive at room temperature.

“Gradient method” for surface area estimation by benzene vapour adsorption at temperatures below the freezing point

Abstract It was found that benzene vapour, as well as low alkylbenzene vapours, might be applicable to the “gradient method” of surface area estimation in which the gradient of the linear portion in Type II adsorption isotherms obtained for most samples is equal to the monolayer capacity of adsorption. This was ascertained by comparing the surface areas estimated by the method with N 2 /BET areas, using several typical solid samples. Although it is theoretically necessary that the adsorbed molecules used for the method are liquid-like on the multiadsorption layer, the benzene vapour could be used at 0 C, i.e. below the freezing point, 5.5°C. To clarify this discrepancy, the adsorption experiments for benzene vapour were carried out at temperatures much lower than the freezing point, i.e. down to −45°C, using typical solid samples (gold and silicate glass). The experimental results showed that in the relative pressure range 0.2–0.4, where the saturation vapour pressure of the supercooled benzene calculated from the Antoine equation was adopted as the relative pressure standard, the adsorbed benzene molecules might be in a supercooled liquid state, because it was found that the isotherms at various temperatures below the freezing point coincided with each other in the relative pressure range 0.2–0.4, and that the isosteric heats of adsorption were nearly equal to the heat of liquefaction of benzene vapour.