Specific Surface Area of Nanopowders from Argon Adsorption at 77 and 87 K: Zeta Adsorption Isotherm Approach

Abstract

The surface characterization of nanopowders is widely attempted by both N2 and Ar vapor adsorption methods. They are used as complementary probe molecules because of their similarities and certain differences. However, the respective reported analyses, using the Brunauer–Emmett–Teller method, suffer from inconsistencies between the determined values of the specific surface areas, A s. We examine this issue by using the zeta adsorption isotherm approach on the reported data for both vapors on two different samples from the same batch of a nanopowder material. By assuming the equivalence of the A s determined by both of the vapors, we obtain the average occupied area of an adsorbed argon atom, σA(Ar), on silica and carbon black at the solid- and liquid-state temperatures of 77 and 87 K, respectively. We then examine the determined value of σA(Ar) by obtaining A s for other samples of the same material with varying specific areas. By transforming the adsorption data from mass-specific to area-specific, the respective thermodynamic isotherms, n SV, of Ar vapor adsorbing on silica and carbon black are obtained. The determined n SV are then used with the Gibbs adsorption equation to determine the solid surface energy, γS0, and solid–vapor interface energy, γSV, for both substrates. The obtained γSV(x V) on both substrates is used to interpret the phase transition state of the Ar adsorbate at 77 K. The determined thermodynamic isotherms for Ar and N2 vapors are further used in a comparative study to show that the Ar vapor adsorption is negligibly affected by the change in silanol concentration on the silica surface in contrast with N2 vapor adsorption.

Determination of the surface properties and adsorption states of nanoporous materials using the zeta adsorption isotherm

“…20,44 Table 6 also lists the surface parameters of argon adsorption by three kinds of silica calculated by the BET method BJH method, and the surface area values of three kinds of porous samples calculated by the zeta method in the Ar-silica system are obtained from eqn (11). The percentage difference in the specific surface area obtained using the same procedure on nitrogen and argon is described by eqn (19). Table 6 includes the precise calculation results.…”

“…Although the shortcomings of the BET method have been reported by many researchers, 12,19,40 it has become the standard method for measuring the specific surface area of various powders and porous materials. Therefore, we compared the specific surface area values calculated by this method and the BET method.…”

“…This model has also been successfully used to determine the surface parameters of porous materials, including surface area, pore diameters, and pore distributions. 11,17 For further study of the zeta adsorption isotherm, Narayanaswamy and Ward 18,19 analyzed several groups of experiments for different adsorbents and adsorbates and found that due to the heterogeneity of the material (the uneven physical properties of the material surface led to the phenomenon of micropore filling at the beginning of adsorption and capillary condensation in the range of high-pressure ratio), a certain amount of deviation between the predictions and measurements still appeared at a low pressure ratio and near the saturation pressure. Therefore, a method for selecting the heterogeneity-free range from the measured isotherm is proposed and the corresponding isotherm constants of the materials are obtained, which provides a possibility to relate the adsorption processes of nonporous and porous materials in the same system.…”

Investigation of the Zeta Adsorption Model and Gas-Solid Adsorption Phase Transition Mechanism Using Statistical Mechanics at Gas-Solid Interfaces

“…Narayanaswamy and Ward [43] conducted extensive linear fitting of the specific surface areas of different samples of given materials using the zeta isotherm model’s four parameters. He investigated the adsorption of nitrogen gas on five different materials: silica, α -alumina, γ -alumina, carbon black, and graphitized carbon, resulting in a total of 19 different samples.…”