Aromaticity-specific interaction of fluid molecules with asphaltene aggregates dissolved in crude oil by means of DNP and NMR relaxometry

The relation of NMR relaxation times and diffusion coefficients with overall viscosity and molecular size in a typical fluid composition has been established for bulk oils [1]. A major obstacle for the task of compositional analysis remains, however, the widely unknown role of molecular shape and chemistry, most importantly aromaticity, on the NMR relaxation behaviour. This is particularly important in asphaltene-containing oils where the relaxation of solvent molecules, so-called maltenes, is significantly affected by interactions with radical-containing asphaltenes. However, aromatic and aliphatic maltenes are expected to interact differently with asphaltene aggregates, and size-dependent residence time variations within the porous aggregates become important. While fundamental studies have attempted to provide a molecular dynamics description of relaxation times that take advantage of data obtained at variable magnetic fields [2], they still suffer from a lack of distinction of dynamics between molecules of various architecture, and from the generally broad relaxation times distribution in natural oils.

In this study, we have applied 19F containing tracer molecules at low concentrations to natural oils of different asphaltene content [3], and investigated the tracer's relaxation time ratio T1/T2 and the field dependence of relaxation times, T1(w). This strategy has the advantage of specifically determining the behaviour of different tracers, where molecular weight and aromaticity are considered as variables. One main finding of this study is the significant increase of T1/T2 for aromatics in the presence of asphaltenes compared to alkanes. The results are interpreted in terms of selective maltene-asphaltene interaction based on frequency dependent relaxation results. The strong contrast of relaxation times allows for a simplified quantification of either asphaltene concentration or maltene aromaticity in crude oils. The role of asphaltene has further been quantified by concentration-dependent measurements of protonated and fluorinated test molecules in a deuterated solvent, and of non-aggregating polycyclic radical and neutral molecules mimicking asphaltene, respectively. First results of the enhancement in dynamic nuclear polarization (DNP) experiments, making use of microwave saturation of the naturally occurring radicals in oils, confirm the significant difference in interaction strength of asphaltenes with aromatic and aliphatic molecules.

[1] D.E. Freed, M.D. Hürlimann, Comptes Rendues Physique 11, 181-191 (2010).
[2] L. Zielinski, M.D. Hürlimann, Energy Fuels 25, 5090-5099 (2011).
[3] S. Stapf, A. Ordikhani-Seyedlar, N. Ryan, C. Mattea, R. Kausik, D. E. Freed, Y.-Q. Song, M. D. Hürlimann, Energy Fuels 28, 2395-2401 (2014).