(540c) Strong Ion-Pairing of Asphaltenes with Ionic Surfactant Enable Charge-Stabilization in Non-Polar Systems

The π-conjugation of asphaltene molecules provides an interesting and unique platform for the investigation of charges, ion-pairing, electrostatics and electronics in non-polar systems.  Asphaltenes are the largest molecular weight and highest aromaticity fraction of petroleum fluids.  Their aromaticity and propensity to π-stack causes asphaltenes to be insoluble in alkanes and non-polar systems, and fully soluble in aromatic solvents, such as toluene and xylene.  The high degree of asphaltene instability causes problems in the petroleum industry, including deposition and clogging in wellbores and pipelines.

Asphaltenes are polyaromatic hydrocarbons, and are molecular relatives of compounds like graphene and hexabenzocoronene, which are highly sought after for molecular devices due to their unique electronic properties.  At the same time, asphaltenes contain electronic defects including heteroatoms and metal porphyrin compounds, which cause them to be charged with both positive and negative ions simultaneously.  While this bimodal charge on asphaltene molecules increases their instability in non-polar systems, we can exploit these charges to enable both colloidal stability and molecular-scale dissolution of asphaltenes in non-polar systems.

We have previously found that non-ionic polymeric surfactants can enhance the surface charge of asphaltenes to provide electrostatic stabilization of colloidal suspensions in heptane^1-3.  Non-ionic surfactants that contain double bonds can interact with asphaltenes through π-bonding, adsorbing to the source of negative charges.  This adsorption does not generate additional charges, but enhances the positive surface charges already present on the colloidal asphaltenes, generating strong electrostatic repulsion which stabilizes them against further aggregation.

In this presentation we discuss the use of ionic surfactants to provide charge stabilization for asphaltenes in non-polar systems.  We use a combination of experimental methods, including light scattering and UV-vis spectroscopy, to confirm the action of ionic surfactants on asphaltenes and the importance of electrostatic stabilization.  While non-ionic polymer surfactants can stabilize asphaltenes as colloidal suspensions, ionic surfactants not only stabilize colloidal asphaltenes but also can fully dissolve asphaltenes to the molecular scale, even in non-polar systems.  We find that ionic surfactants can create strong ion-pairs with asphaltene ions.  When the ionic surfactants contain long-chain tails, their pairing with asphaltenes creates a solvation shell, thereby enabling solubility in heptane.

Our results have many operational implications for improving hydrocarbon energy production in the petroleum industry, and also provide insights for molecular electronics devices.  While π-conjugated systems are highly desirable due to their electronic properties, they are often difficult or impossible to dissolve without further functionalization, adding a layer of complexity to solution processing methods.  Here we exploit the natural heteroatomic functional groups in asphaltenes to enhance the solubility of these π-conjugated molecules. 

References

1 – SM Hashmi, A Firoozabadi. “Field- and Concentration-Dependence of Electrostatics in Non-polar Colloidal Asphaltene Suspensions,” Soft Matter 8, 1878 (2012).

2 – SM Hashmi, A Firoozabadi. “Tuning size and electrostatics in non-polar colloidal asphaltene suspensions by polymeric adsorption,” Soft Matter 7, 8384 (2011). 

3 – SM Hashmi, A Firoozabadi. “Effect of Dispersant on Asphaltene Suspension Dynamics: Aggregation and Sedimentation,” Journal of Physical Chemistry B 114, 15780 (2010).