The Interaction of Synthetic Porphyrins with Asphaltenes: Role in Self Assembly

The Interaction of Synthetic Porphyrins with Asphaltenes: Role in Self Assembly

McKay Rytting and Peter Kilpatrick

Department of Chemical and Biomolecular Engineering

University of Notre Dame

Notre Dame, IN  46556

Figure  SEQ Figure \* ARABIC 1: Cu(II) Pheophytin a          Chlorin e6 6-NH2 Hexanoic Acid (P10299)                               Amide (C40286)

Asphaltenes are known to aggregate at very low concentrations to form nanoscale aggregates that are oblate cylindrical in shape (disk-shaped) and that have characteristic disk diameters in the 2-50 nm size range.  Aggregate sizes vary with temperature, solvent, asphaltene chemistry, and with added solvating agents such as demulsifiers and surface-active agents.  These aggregates also adsorb onto interfaces (oil-air, oil-water, oil-solid) and molecularly rearrange to form elastic solid films that stabilize foams and water-in-oil emulsions, as well as form deposits during subsea production.  Naturally occurring asymmetric petroporphyrins are known to alter asphaltene aggregation and modify the size and interfacial activity of asphaltenes.  In an effort to understand this solvating process better, we have performed interfacial tension and rheology experiments, small angle neutron scattering (SANS), and pulsed field gradient spin echo (PFGSE) diffusion NMR experiments on asphaltenes to which two very different synthetic porphyrins (see Figure 1) have been added: one in which there are multiple carboxylic acid functional groups grafted to the exocyclic porphyrin ring and one in which a long branched alkyl tail has been grafted to the exocyclic ring.  As one might anticipate, the results are dramatically different. The first synthetic porphyrin with the grafted branched alkyl tail (P10299) solvates the asphaltenic aggregates and reduces aggregate size and molecular weight at all concentrations (see Figure 2).  The second synthetic porphyrin with the multiple carboxylic acid grafts (C40286) appears to incorporate into the aggregates and increase aggregate size with increasing concentration at low temperature, but solvates and decreases the size of aggregates at higher temperature (see Figure 3).  Results from PFGSE-NMR diffusion measurements show similar trends. These results offer valuable clues into the precise molecular mechanisms of asphaltene self assembly.  In this talk, we will present comprehensive data from a battery of experiments, including interfacial tensiometry and oscillating drop dilatational rheometry, as well as the SANS and PFGSE-NMR results.

Figure  SEQ Figure \* ARABIC 2: Effect of P10299 on size and weight of Maya asphaltenic aggregates

Figure  SEQ Figure \* ARABIC 3: Effect of C40286 on size and weight of Maya asphaltenic aggregates