Asphaltene aggregation under vacuum at different temperatures was obtained using classical molecular dynamics (MD) simulations under nonperiodic boundary conditions in a monodisperse system of 96 hypothetical asphaltene molecules. Identical asphaltenes were originally set as an array, where the separation between each other was ∼40 Å. Simulations under the canonical ensemble at NVT conditions, using the Verlet numerical method to solve the motion equations, were conducted. Aggregated systems formed by several asphaltene monomers after 100 ps of classical MD simulations were found. The structure of the solution was analyzed using the radial distribution function. Simulations at four different temperatures (273, 312, 342, and 368 K) were accomplished. Another similar MD simulation at a temperature of 310 K for 300 ps was performed, to validate the stability in the previous systems. After this run, good structures for explaining asphaltene interactions were also observed; these structures were never before proposed. The following effects can be observed from the results: (i) aggregates that have different structures indicate different types of interactions; (ii) decreasing aggregation number values with increasing temperature is consistent with experimental reality; (iii) the average molecular weights obtained for different temperatures agree with the expected range of experimental values; and (iv) the minimum of the potential energy well, in the range of 3.5-4.0 Å, is consistent with the Yen model.

The potential energy surface of Zn O clusters (n = 2, 4, 6, 8) has been explored by using a simulated annealing method. For n = 2, 4, and 6, the CCSD(T)/TZP method was used as the reference, and from here it is shown that the M06-2X/TZP method gives the lowest deviations over PBE, PBE0, B3LYP, M06, and MP2 methods. Thus, with the M06-2X method we predict isomers of Zn O clusters, which coincide with some isomers reported previously. By using the atoms in molecules analysis, possible contacts between Zn and O atoms were found for all structures studied in this article. The bond paths involved in several clusters suggest that Zn O clusters can be obtained from the zincite (ZnO crystal), such an observation was confirmed for clusters with n = 2 - 9,18 and 20. The structure with n = 23 was obtained by the procedure presented here, from crystal information, which could be important to confirm experimental data delivered for n = 18 and 23.

No abstract

No abstract