Occlusion of Foreign Molecules in Primary Asphaltene Aggregates from Near-UV–Visible Absorption Studies

Abstract

This study examined the states of vanadyl porphyrins in toluene solutions of n-heptane solid asphaltenes via measurements of near-UV−visible absorption spectra. Low intensity of the characteristic Soret absorption peak of porphyrins in most samples with various asphaltene concentrations C A indicated that porphyrins are bonded to individual basic (one to three ring) molecules of asphaltenes (at C A ≤ 0.5 mg/L), within porous supramolecular structures of most primary asphaltene aggregates (at C A = 0.6−30 mg/L) as well as in all colloidal-size complexes at higher asphaltene concentrations, up to C A = 1880 mg/L. However, porphyrin−asphaltene bonds appear to be weak and may be disrupted merely by dilution to some proper final asphaltene concentrations. Namely, for some specific values of C A (close to 4 and 12 mg/L) we observed sharp increases of porphyrin Soret absorption peaks attributed to the appearance of free, nonbonded, porphyrin molecules in these samples. We suggest that these specific dilutions of solid asphaltenes result in such equilibrium molecular systems where the active centers of asphaltenes are effectively engaged in internal bonds of primary asphaltene aggregates and, hence, are not available for aggregation with any foreign molecules.

Further evaluation of the Re-Os systematics of crude oil: Implications for Re-Os geochronology of petroleum systems

“…It is possible that the Re and Os bearing species in crude oil can be adsorbed and occluded into the asphaltene aggregates like many other compounds (Mujica et al, 2000;Liao et al, 2005;Liao et al, 2006aLiao et al, , 2006bLiao et al, 2009;Acevedo et al, 2012;Derakhshesh et al, 2013;Zhao et al, 2013;Liu et al, 2015;Castillo & Vargas, 2016;Snowdon et al, 2016;Evdokimov et al, 2017). The molecules absorbed at the periphery of asphaltene aggregates, i.e.…”

Asphaltenes: Separations, structural analysis and applications

“…Usually, the metal ions are chelated with organic groups, and some are located at the defect centers of an imperfect aromatic sheet as porphyrins [30][31][32] , and these possible structures were always hypothesis [32,33] . Meanwhile, amount of experiments confirmed metal compound to be nonporphyrin structure, existing as free molecule, or captured one in asphaltene nanostructure, or a component of asphaltene [34] . Recently, metallic nanoparticles and dissolved metals in asphaltene solutions have been distinguished more clearly by single-particle inductively coupled plasma mass spectrometry (spICP-MS) with oxylene.…”

“…Recently, near-UV-vis spectroscopy was used to illustrate how vanadyl porphyrins existed in different concentrations of asphaltene solution. As concentration of asphaltenes increasing, porphyrins were gradually attached to asphaltene molecules with one to three rings, wrapped in porous supramolecular aggregates and micelles, and free porphyrins existed at concentration from 4 to 12 mg/L [34] . Besides, the combination of UV-vis and EPR proved that the non-porphyrinic structure of V-containing asphaltene molecules also existed [152] .…”

Density Anomalies in Crude Oil Blends Reflect Multiple Equilibrium States of Asphaltene Colloidal Aggregates

“…Recently, it was experimentally proven [48][49][50][51][52] that basic asphaltene molecules (unimers, monomers) typically include very small, 1-3 ring, aromatic systems, in contrast to the popular notions about the predominance of large multiring fused systems [41,42]. These basic molecules become predominant equilibrium species only after dissolution of solid asphaltenes in "good" solvents (benzene, toluene, etc.)…”

“…The latter may include organic clathrates, in which occluded guest molecules stabilize the assembly of a cage, as methane does in gas hydrates [53]. With increasing asphaltene concentration (still below CNAC), several structural phases of soft, voluminous, primary aggregates are observed in solutions, which may be accompanied by a release of some occluded "guests" [51]. Finally, at C ≈ 100-130 mg/L, primary aggregates shrink and may be described as "solid-like" asphaltene colloidal nanoparticles [52].…”