Initial mass uptake dynamics and diffusivity of cyclohexane vapor in nano-scale bitumen films coated on substrates with different degrees of hydrophilicity

Abstract

Absorption of cyclohexane vapor into thin bitumen films with 0.5 wt% and 2 wt% bitumen coated on samples containing large spherical particles made up of borosilicate glass and fine particles with different chemical compositions (borosilicate glass versus kaolin clay) was studied using a gravimetric technique. Samples containing glass fines and clay fines are denoted as SA and SB, respectively. Fines refer to the particles with sizes less than 45 µm and their concentration varied from 5 to 20 wt% in SA and SB. The results clearly showed that the chemical composition of the fines significantly affected the cyclohexane initial mass uptake rate and that this rate exhibited a negative bitumen content dependence with the dependence of SA much stronger than that of SB. This is because glass (SA samples) is much more hydrophilic than clay (SB samples). As a result, SA rendered a stronger concentration gradient of the polar bitumen molecules in the film thickness direction than SB, thereby stronger negative bitumen content dependence. Diffusion coefficients were estimated using the cyclohexane initial uptake rates and the surface area average film thicknesses of the samples and were found that the diffusion coefficient increased with increasing bitumen content (i.e., film thickness) in both SA and SB cases. Varying fines content did not have any effect on the cyclohexane initial mass uptake rate. It was found that increase in the fines content decreased the bitumen film thickness, thereby decreasing the diffusion coefficient that is offset by the increase in the amount of cyclohexane dissolved at the interface.