(471c) Study of Bitumen Adsorption Onto Mineral Surfaces Using a Quartz Crystal Microbalance

Different methodologies have been developed to measure the adhesion between bitumen and aggregates used in hot asphalt mixtures during the last years. The understanding of the interfacial phenomena in the system is a key issue to obtain better bitumen – aggregate combinations.  The proper selection of materials used in the asphalt mixtures design lead to pavements more resistant to damage caused by different factors therefore with higher service lives and lower maintenance costs.

The properties of the bonds formed between molecules of bitumen and stone aggregates can be related to thermodynamic quantities. Currently, the indirect measurement of properties through techniques like contact angle and the Universal Sorption Device at macroscopic scale allow the characterization of solid-liquid interface. These techniques treated the chemical heterogeneity of bitumen and multi-component surface of aggregates as uniform, regardless the true complexity of aggregate surface and structure at nanometer scale. The presence of nano domains covered by hydrophobic or hydrophilic molecules, determine the type of bond formed with the specific molecules within the bitumen.

Nowadays, despite the current development of experimental techniques to study the characteristics of components of asphalt mixtures and its behaviour, the relationship between the interfacial properties of their main components remains unclear. This work presents a direct study of adsorption and deposition of different bitumens onto minerals surfaces, successfully measured with a quartz crystal microbalance with dissipation (QCM – D) and ellipsometer. The minerals were selected according to the composition of aggregates used for the asphalt mixtures design.

The adsorption process of three different bitumens onto minerals surfaces was monitored using a quartz crystal microbalance. It was noted that adsorption kinetics exhibits a rapid initial response followed by a slow approach to equilibrium. The data was found to follow first order kinetics. The values for the mass adsorbed at equilibrium were used to generate adsorption isotherms for each bitumen - mineral system. Adsorption isotherm analyses were developed with Langmuir, Freundlich and Sips models. Some samples shows a monolayer formation represented by Langmuir model. Others samples display a combined form of Freundlich and Langmuir expressions, wich are properly represented by Sips model. The dissipation obtained with QCM – D shows differences in the properties of film over the mineral surface. Some bitumens exhibited characteristics of rigid films, while for others it was found the formation of viscoelastic films. The thickness of adsorbed bitumen films were measured with ellipsometer and were in good agreement with the data for adsorbed mass obtained with QCM - D.  

The properties of bitumen and minerals are used to explain the adsorption results and to determine the main contributions to the adsorption process. These results have significant impact on the understanding of the bitumen - aggregate behavior at the interface and its relation with the properties of the system.