Intermolecular Interactions of Isolated Bio-Oil Compounds and Their Effect on Bitumen Interfaces

Bio-oils derived from low-value or waste biomass have been shown to be useful as low-cost, sustainable additives to improve the adhesion or moisture resistance of bituminous asphalt binder. The chemical profile of bio-oil is complex, and identifying the most active compounds in the mixture and understanding how they work to modify bitumen properties is essential for synthesizing bio-based additives with desired characteristics and performance. Hexadecanamide and hexadecanoic acid, two surfactants prevalent in both animal- and plant-derived bio-oils, are notable candidates as active compounds responsible for altering the colloidal or interfacial properties of bitumen. In this study, pure forms of these surfactants were mixed into bitumen to examine their effects on binder composition and morphology. AFM and FTIR data show that the amide strongly separated from the mixture, much of it crystallizing at the exposed surface. The DFT-based studies indicate that self-assembly of amide molecules is energetically preferred to adsorption of amides to asphaltenes or wax crystals in bitumen. The acid seemed to mix in well but induced the growth of a unique morphology at the glass interface. However, in combination, the amide and acid improve each other’s solubility without strongly interacting with each other. Other molecules in bio-oil may similarly work to improve the solubility of the total mixture and make it an effective surface-modifying additive.