Interfacial interactions and mass transfer at the interfacial region of bituminous hydrocarbon mixtures

Abstract

The adhesion between bitumen and aggregate is a complex process with numerous of variables. To improve the understanding of the bond between bitumen and aggregates in road applications, this research focuses on preferential adsorption, which is one aspect of bitumen-aggregate adhesion. The main objective is to find out whether there is preferential adsorption or segregation at the aggregate and the air interface. The investigation of the preferential adsorption was done on the bitumen as it is used in road applications. So in this research the bitumens were not diluted and the bitumens were not characterized by selected molecules in the bitumen. To determine whether preferential adsorption can occur, the surface energy of common road construction aggregates was measured by means of sorption experiments. The measurements of the aggregates showed that the dominant polar component is significantly large enough. So if bitumen would consist of non-polar and polar components, then the potential interaction energy of the polar components will be larger than the non-polar components with the measured aggregates. The difference in interaction energy could lead to preferential adsorption of specific bitumen particles on the aggregates. The next step was to determine the surface energy of bitumens and bitumen components. This was done by means of contact angle measurements. It was found that the components have a difference in surface energy relatively to each other. This means that preferential adsorption can occur and that, if preferential adsorption really takes place, the interaction energy at the bitumen surface could be different at different interfaces. The adsorption of bitumen at an aggregate was investigated by means of refractometric measurements. Comparison of the calculated index of refraction of the not adsorbed reference bitumen with the measured index of refraction of adsorbed reference bitumen suggested that preferential adsorption took place on the crystal, but that it was certainly not clear. The index of refraction of bitumen at the crystal interface, at both 20°C and 90°C, has a high correlation with the penetration grade. Ellipsometric measurements were used to investigate the bitumen-air interface. In general the bitumens showed a larger difference in results between the ellipsometric measurements and the refractometric measurements compared to materials that are not expected to show interfacial mass transfer. Most bitumens have on average more molecules that give smaller indices of refraction at the bitumen-air interface in comparison with the bitumen-aggregate interface. It was found that the results of the ellipsometric measurements for bitumens were depending on the cooling rate of the bitumen samples during preparation and on time. The preparation temperature also showed to have effect on the results for some bitumens. The difference found between the bitumen at the crystal interface and the bitumen at the air interface was investigated by means of Fourier transform infrared spectroscopy. A chemical characterization for the difference between the bitumen at the crystal interface and the bitumen at the air interface could not be made. The measurement inaccuracies made the chemical characterization of the reference bitumen at the air interface unreliable. Also no preferential adsorption of bitumen onto the crystal surface was observed with the infrared measurements.