60th Annual Report on Research 2015

Molecular dynamics simulations provide a powerful computational tool for investigating the interactions, structure, and thermodynamic stability of asphaltenes and their aggregates.  Unfortunately, conventional atomically detailed models remain prohibitively inefficient for investigating the relevant length and time scales that are industrially relevant for asphaltene aggregation.  Because they are far more efficient, lower resolution coarse-grained (CG) models provide a far more promising means for simulation studies of asphaltene aggregation.  However, it remains challenging to develop CG models that accurately model both structure and thermodynamic properties.   In particular, bottom-up models typically provide a more accurate description of structural properties, while top-down models generally provide a more accurate description of thermodynamic properties.

During the past funding period, we have continued our efforts to develop bottom-up CG models that accurately model both structure and thermodynamic properties.  By extending the previous work of Das and Andersen (DA), we (Dunn and Noid, DN) have demonstrated that rigorous, bottom-up approaches can accurately describe the structure, average density, and compressibility of atomically detailed models for industrially relevant asphaltene solvents.  The first figure below compares for three site models of heptane (left) and toluene (right), the site-site radial distribution functions (rdfs) sampled by an atomic model (AA), with the rdfs sampled by the DA and DN bottom-up models.  In each case, the bottom-up model quite accurately reproduces the AA structure.  In contrast, a comparable 3-site top-down model (SDK) for heptane provides a less accurate description of the AA structure. 

Figure 1: Comparison of site-site radial distribution functions (rdfs) for heptane (left) and toluene (right).  In each panel, the solid black curves represent rdfs for the OPLS-AA model, while the green and red curves present rdfs for the DA and DN bottom-up models.  In the right column, the dashed-dotted purple curve presents rdfs for the top-down SDK model. 

More importantly, the following figure demonstrates that the resulting bottom-up models accurately reproduce the density fluctuations, which reflect the equilibrium liquid density and compressibility, as well as the pressure equation of state (near the simulated conditions of 1-bar pressure) for the atomic models.  In particular, this figure demonstrates that the CG models reproduce these thermodynamic properties with similar accuracy for 1, 2, and 3-site heptane models (left) and 1 and 3-site toluene models (right).  This is particularly important since it paves the way for performing simulation studies of asphaltenes at a variety of different resolutions.

Figure 2:  Density fluctuations and simulated pressure equations of state for heptane (left) and toluene (right).  In each panel, the solid black curves represent results for the OPLS-AA model, while the blue, green, and red curves present rdfs for 1, 2, and 3-site DN bottom-up models.

We are currently finalizing results that appear to demonstrate similar success for a transferable bottom-up model of heptane-toluene mixtures at a range of compositions that vary from pure heptane to pure toluene.  Our initial work focused on these particular systems because asphaltenes are operationally defined on the basis of their relative solubility in toluene and heptane.   By applying the methodology that we have developed during past funding periods, we are also working to parameterize bottom-up CG models for model asphaltene compounds in these solvents. 

In addition, we have also initiated simulation studies with phenomenological top-down models of model asphaltene compounds.  As indicated below, preliminary results of these studies demonstrate the impact of solvent quality and asphaltene architecture upon the nanoaggregates that form.  The insights from these studies with top-down models will guide future investigations with bottom-up models that more accurately model asphaltene-solvent interactions.

Figure 3:  Representative asphaltene nanoaggregates that form under different simulated conditions.