Wetting and interfacial properties of bitumen at room temperature, and the role of naturally present ionic molecules

Abstract

Bitumen is a complex material with great interest because of technological applications in road construction. The properties of asphalt formulations greatly depend on adhesion of bitumen to gravel particles. In this context, colloidal and adhesion properties of bitumen are related to bitumen composition and surface tension. It is known that bitumen contain ionic species that define its interfacial properties. Many authors have denoted these molecules as “endogenous surfactants”, but nevertheless these ionic molecules might not be true surfactants. Herein, the physicochemical properties of bitumen are investigated at room temperature, which is not an easy task, since the extremely high viscosity of bitumen makes difficult to characterize its surface properties at low temperatures. First, characterization by FTIR, SWAXS, DSC and TGA has been performed. Zeta potential of bitumen-in-water emulsion droplets has also been measured as a function of a cationic surfactant concentration. It was found that zeta potential inverted from negative to positive, with surfactant addition. The point of charge inversion was used as a titration tool to determine the density of acid groups present on bitumen/water interface. Then, surface pH was calculated using the Ionizable Surface Group Model and Grahame equation. Ionic species of bitumen are partly soluble in water, and thus, able to decrease surface tension of water. The diffusion of these bitumen ionic species has been studied by measuring surface tension of water as a function of time, and applying Henry and Gibbs diffusion models. Regarding surface energy, it has been studied by depositing sessile drops of various liquids with different polarity, on bitumen thin films at room temperature, since this is the most appropriate method for highly viscous materials. The surface free energy and their respective polar and dispersive components, calculated by the Owens and Wendt approach, have been related to the chemical composition of bitumen, and the main factor is the presence of the ionic molecular species, which are naturally present in bitumen. All results have shown that important physicochemical parameters of highly viscous bitumen can be studied at room temperature.