Modeling of adsorption isotherms of probe vapors on aggregates for accurate determination of aggregate surface energy components

Abstract

The vapor adsorption method is a commonly-used method to determine the surface energy components of aggregates for paving asphalt mixtures. It is crucial to appropriately model the adsorption isotherms of the probe vapors on the aggregates. However, the traditionally-used five-parameter modified Toth model was limited to monolayer adsorption and its fitting parameters were highly dependent of seed values.This study performed the adsorption and desorption of nitrogen on four types of aggregates to characterize the surface pores of the selected aggregates. Micropores, mesopores and macropores in a wide size range were identified at the aggregate surface. Adsorption tests of three probe vapors were then performed on the selected aggregates using the Gravimetric Sorption Analyzer. The measured adsorption isotherms were modeled using multiple isotherm models, including the five-parameter modified Toth model, Dubinin-Astakhov (DA) model and the modified Brunauer-Emmett-Teller (BET) model, for the purpose of comparison. The DA model was identified to be the most appropriate option for modeling the adsorption isotherms of the probe vapors on the aggregates in terms of both physical significance and goodness of model fit. The DA model was able to address the volume filling of micropores and the adsorption in mesopores and macropores; this model also provided the best model fit while its model parameters were independent of the seed values.It was proved that the appropriate modeling of the adsorption isotherms was critical for the accurate determination of the equilibrium spreading pressure of the probe vapor on the aggregate surface, which then directly affected the accuracy of the calculated surface energy components of the aggregates. A deviation in the determined spreading pressure would cause significantly larger variations in the calculated surface energy components. This fact further demonstrated the importance of the accurate modeling of the adsorption isotherms of the probe vapors on the aggregates.