Emulsifiability of Bitumens: The Required Properties of Their Endogenous Surfactants

The fabrication of aqueous bitumen emulsions requires the presence of surface-active molecules to minimize interfacial energy. The natural surfactants present in bitumen, especially those belonging to the asphaltene fraction, play an important role, but hydrophilic synthetic surfactants are also added to promote the formation of the desired oil-in-water emulsion. We demonstrated, in a previous paper (Jatav, S.; Bouriat, P.; Anaclet, P.; Hung, Y.; Rondelez, F.; Dicharry, C. Proceedings of the RILEM ISBM 2020, 2022, 27, 1635), that these two types of surfactants, endogenous and exogenous, compete for adsorption onto the interface, which can be detrimental for the long-term stability of the emulsion. In the present work, we studied two different bitumens A and B with opposite emulsion-forming behaviors and used the "wet silica chromatography method" (Jarvis, J.M.; Robbins, W.K.; Corilo, Y.E.; Rodgers, R.P. Energy & Fuels, 2015, 29 (11), 7058) to separate their endogenous surfactants into two categories, referred to as the extracted fraction and the eluted fraction. The viscoelastic and tensiometric properties of these systems diluted with Heptol 10 (a 50 vol % mixture of n-heptane and toluene) were investigated using interfacial shear rheology (ISR) and surface tensiometry (IFT). Our measurements revealed that the extracted endogenous surfactants are the main contributors to the high elastic and viscous moduli necessary to ensure the long-term mechanical stability of the bitumen emulsions. A higher proportion of extracted endogenous surfactants adsorbed at the interface, to the detriment of eluted endogenous surfactants, correlates with greater stability of bitumen emulsions. In addition, our data showed that the extracted endogenous surfactants have to compete with the exogenous surfactants when they first reach the interface and rapidly displace them. These two requirements are fulfilled better in the case of bitumen A than in the case of bitumen B.