(163f) 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

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