Effects of the asphaltene and resin contents of the bitumens on the water–bitumen interface properties

Abstract

The contact angles between various bitumen solid surfaces and aqueous solutions having pH values equal to 3, 6, 9 and 12 were measured. The objective of this work was to analyse the possible acid–base interactions at the water–bitumen interface for substrates having various asphaltenes and resins contents. In addition, the work of adhesion at a bitumen–water interface, W A, was calculated from the measured contact angle. This allows us to analysis the surface acidity and basicity of the bitumens. The results obtained indicate that the work of adhesion decreases when the bitumen asphaltenes content increases at any aqueous solution pH value. Further, the variation of the work of adhesion versus the pH for various bitumens shows the presence of acidic, basic and amphoteric groups in the surfaces of these components. The nature and the amount of acidic and basic functionalities present in the bulk bitumen were evaluated by a non-aqueous potentiometric titration method and by comparison with acid–base models compounds. No relationship was found between the acid and base contents of the bulk bitumens and the bitumen surface character determined from the work of adhesion. In order to find any correlation between the adhesion work and the stability of the bitumen aqueous dispersions, zeta potentials of the bitumen droplets dispersed in acidic aqueous solutions were measured. Stable bitumen dispersions having zeta potential values higher than 50 mV and work of adhesion values higher than 55 mJ/m 2 were obtained with bitumens having high values of the resin/asphaltenes ratios. The determination of a surface's acidity and basicity by measuring the contact angles at a water–bitumen interface seems to be a more useful method than the non-aqueous potentiometric titration. Hence, a good correlation is found between the surface charge of the bitumen droplet as measured by zeta potential and the work of adhesion at water–bitumen interface.