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, As. 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 As 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 As 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, nSV, of Ar vapor adsorbing on silica and carbon black are obtained. The determined nSV 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(xV) 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.