Modifying the BET model for accurately determining specific surface area and surface energy components of aggregates

Abstract

When adopting the vapor adsorption method to measure the surface energy components of aggregates, a critical step is to determine the specific surface area (SSA) using the traditionally-used BET (T-BET) model. However, this model is demonstrated to be valid only for a limited relative pressure range of 0.05–0.40. Furthermore, the determined total SSA from this model is arbitrarily utilized to calculate the spreading pressure of probe vapors on aggregates, which does not comply with the multilayer adsorption theory. All these would definitely lead to the erroneous values of SSA and surface energy components of aggregates.To overcome these deficiencies, this study develops a modified BET (M−BET) model in the light of the multilayer adsorption theory, based on which five steps are designed to determine the SSA and the surface energy components for aggregates. The vapor adsorption tests of three probe vapors are performed on four types of aggregates. The SSA, spreading pressure and surface energy components are then separately calculated using the two BET models. It is observed that the linear fit of M-BET model applies satisfactorily to the entire adsorption isotherm with the R2 value larger than 0.98. The adsorption heat of the probe vapor in the second to the ith layer is demonstrated to be less than the heat of liquefaction, which accounts for the derivations of the data points of the T-BET model from linearity over the limited pressure range. The accompanying calculation results of the M-BET model reveal significant differences from those of the T-BET model, while the separation of SSA of the non-zero adsorption site from the total SSA also produces a non-negligible effect on the calculated surface energy components, identifying the significance of accurate measuring the SSA. Thus, in the vapor adsorption method it is imperative to employ the M-BET model to determine the SSA of the non-zero adsorption site so as to obtain the accurate surface energy components for aggregates.