Island versus Archipelago Architecture for Asphaltenes: Polycyclic Aromatic Hydrocarbon Dimer Theoretical Studies

Abstract

To shed some light on the problem of the most likely architecture, island or archipelago, of the polycyclic aromatic hydrocarbon (PAH) core in asphaltenes, molecular orbital (MO) calculations have been carried out considering two PAHs per asphaltene molecule cross-linked (archipelago model) and the same two PAHs aggregated in a parallel fashion and not cross-linked (island model of interacting PAHs). The calculated highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gaps (H―L gaps) are compared to the 0―0 band experimental fluorescence emission data of asphaltenes. The calculations of the H―L gap were performed at the semi-empirical ZINDO approach using density functional theory (DFT) dimer-optimized structures (relaxed structures). This methodology was validated against experimental data and it was proven to have a good predictive power. A total of 48 PAHs with 5―10 fused aromatic rings (5FAR―10FAR) were calculated, as well as all of their possible combinations in dimers formed by large core (5FAR―10FAR)―large core (5FAR― 10FAR) systems (large―large core dimers). In total, 1176 systems in the archipelago architecture, crossed linked with a C 5 alkane chain, and 1176 systems in the island-stacked interaction were calculated. Also, island and archipelago models were constructed with the 48 PAHs, interacting in both architectures, with 10 PAHs that contain 1―4 fused aromatic rings in the structure (1FAR―4FAR), thus comprising 960 calculations of large core (5FAR―10FAR)―small core (1FAR―4FAR) dimers (large-small core dimers). Therefore, a total of 3312 systems are calculated in this work. For both cases, large-large and large―small dimer systems, and for both architectures, island and archipelago, statistical analysis of the results was carried out. It is found that the calculated H―L gap of the dimers with the archipelago architecture is similar to the H―L gap of the monomers. For the case of the island dimers (stacked dimers), there is a dependency of the H―L gap upon both the size of the monomers, in terms of the number of fused aromatic rings, and the type of interlayer geometries or stacking arrangement between the monomers composing the island (stacked) dimer. The stacked dimers are more stable in most of the cases than the archipelago dimers. The higher stability in turn produces larger H―L gaps. The remarkably broad variation of the energy gap, of 0.9 eV, between the island and archipelago architectures for the large―large dimers strongly depends upon the dimer size and the stacking arrangement. In the later, the π―π interactions play a role but have a minor influence. The stacked island systems that fall inside the experimental range of asphaltenes are, in general, dimers composed with the same monomer in hexagonal (H) arrangement and 6FAR―10FAR combinations with parallel displacement (PD) arrangement, mainly without fjords or coves in the structure. Of all of the PAH systems that fall inside the experimental range of asphaltenes when interacting with any other PAH, to form a dimer, in either island or archipelago architecture, the 7FAR PAHs are more abundant in agreement with the former literature. When the percentage of all the calculated dimers, large-large and large―small, is compared, in both architectures, and whose H― L gap falls inside the experimental asphaltene fluorescence range, the large―large dimers are more abundant than the large―small dimers. The large-large dimers are more abundant by 20.6% in island architecture, and by 17.9% in archipelago architecture. It is concluded that indeed, if two asphaltene core PAHs are directly bound via a single bond, then optical methods, such as time-resolved fluorescence depolarization (TRFD) studies, would identify this as a single chromophore, thereby blurring the distinction between island versus archipelago. Some decomposition studies might find this single-bonded pair of PAHs as a single entity as well.