(85e) Source of Charges in Petroleum Systems

Asphaltenes, the most aromatic and heaviest weight fraction of petroleum fluids, exhibit electrostatic charging in non-polar systems.  This has been evidenced in literature studies through both electrodeposition and conductivity measurements.  As is the case with electrostatics in non-polar media in general, the source of charge in asphaltene systems remains somewhat mysterious.

Being highly aromatic, asphaltenes are soluble in toluene, xylene, and other aromatic solvents.  In non-polar short-chain alkanes like heptane, asphaltenes precipitate from the molecular scale to full separation.  A colloidal suspension phase forms during this process, allowing for measurements of the electrophoretic mobility of colloidal asphaltenes.  Electrophoretic mobility measurements confirm the presence of charge on colloidal asphaltenes, while at the same time indicating the overall charge-neutrality of colloidal asphaltene suspensions.  The presence of both positive and negative charges in the colloidal phase partially explains the instability of asphaltenes in heptane.  At the same time, the colloidal surface charge can be tuned and manipulated through the addition of surfactants.  Both ionic and non-ionic surfactants enable similar increases in average electrophoretic mobility.  Ionic surfactants increase the overall charge at concentrations beyond their critical micelle concentraion (cmc) in heptane, and non-ionic surfactants increase the overall charge at concentrations less than their cmc.

In this presentation, we discuss measurements of both electrophoretic mobility abd bulk conductivity in non-polar colloidal asphaltene suspensions.  We measure suspensions made by mixing heptane with isolated asphaltenes dissolved in toluene as well as by combining petroleum fluids with heptane directly.  We find that the source of charge in all cases arises from the asphaltenes themselves: charge does not arise from the surfactants, regardless of whether they are present below or above their cmc in heptane.  Rather, both ionic and non-ionic surfactants can enable net charging of the colloids via adsorption.  We discuss the importance of both pi-pi bonding and acid-base interactions in allowing this process to occur.  Furthermore we discuss possibilities for the molecular source of charges in asphaltenes, including heteroatomic and metal content.