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Subfractionation of asphaltenes based on their interfacial activity has begun to highlight critical differences between those asphaltenes that are strongly interfacially active (IAA) and the remaining asphaltenes (RA). Following the methods of petroleomics, representative structures of the two asphaltene fractions were determined, reflecting differences in abundant heteroatom groups, carbon number, double-bond equivalents, and single-core/multicore motifs. Using atomistic-potential-based grid-search methods, the intermolecular interactions between asphaltene–asphaltene and asphaltene–solvent (water, heptane, and toluene) were rapidly screened to identify the most favorable and therefore most likely intermolecular interactions to occur. Asphaltene–water interactions were stronger for IAA (abundance-weighted average interaction energy of −9.29 kJ/mol) than for RA (−6.32 kJ/mol), with hydrogen bonding more significant in the IAA–H2O interaction. Dimer interactions of IAA–IAA were stronger than RA–RA, and from the top 100 most favored interactions, the contribution to the total interaction energy was almost exclusively van der Waals for RA–RA (only 3% electrostatic), while for IAA–IAA, electrostatic interactions (9%) and hydrogen bonding (2%) were significant contributors in the most favored interactions. As the relative contribution of the electrostatic interaction increased, the dimer orientation less resembled that of a π–π stack. With the conception of petroleomics and large structural databases, the grid-search method is a useful atomic/molecular screening approach that provides an ideal triaging tool to rapidly assess a wide range of different molecular structures and interactions. The method is complementary to the more computationally expensive precision molecular modeling tools that are not suited to such workflows.

Asphaltenes are a complex mixture of molecular structures with a variety of functionalities, which in turn impacts their physical properties. Discriminating between asphaltenes that are strongly and weakly interfacially active is providing a new direction to mitigate asphaltene-related problems. Whole asphaltenes (WA) were extracted from a South American heavy crude oil, further fractionated into interfacially active asphaltenes (IAA) and remaining asphaltenes (RA), and molecularly characterized by positive-ion (+) atmospheric pressure photoionization (APPI) using a 9.4 T Fourier transform ion cyclotron mass spectrometer (FT-ICR MS). The IAA fraction was found to contain a greater abundance of heteroatoms with >50% of IAA containing two or more heteroatoms as compared to ∼30% for RA. The IAA fraction was enriched in oxygen-containing species, more specifically higher-order Ox and OxSy groups that were predominantly of low DBE. Gas-phase fragmentation of RA and IAA precursor ions (m/z 650) by infrared multiphoton dissociation (IRMPD) revealed an abundance of multi-core motifs in IAA, while RA was found to be a mixture of single-core and multi-core structures. Analysis of the fragmented ions showed a prevalence of nitrogen-containing species of high DBE (aromatic molecular structures), while oxygen-containing species were most likely associated with aliphatic side chains. Extrography fractionation of RA and IAA verified the abundance of multi-core motifs in IAA, which were highly polar and of low DBE and carbon number. These “atypical” structures of IAA are classified as asphaltenes as a result of their functionality and polarity rather than high aromaticity.

Using the extended-SARA method to fractionate asphaltenes based on their interfacial activity, the current study reports the first results on the estimated size and shape of interfacially active asphaltene (IAA) and remaining asphaltene (RA) nanoaggregates. These fractions have been reported to exhibit distinctly different chemical architectures that influence the size of asphaltene clusters in good and poor solvents. However, little is known about the building blocks, commonly referred to as nanoaggregates, which form these clusters and how those subtle differences in chemical architecture impact aggregation of asphaltenes. The nanoaggregate size and shape of IAA and RA was measured using small-angle neutron scattering (SANS). The characteristic length and asymptotic power-law exponent of whole asphaltenes (WAs) extracted from heavy crude oil and dispersed in deuterated toluene were 28.0 ± 0.2 A and 2.86 ± 0.01, respectively, showing negligible variations with changing asphaltene concentration, source of asphaltenes (bitumen and heavy crude oil), and solvent aromaticity. For RA fractions, which account for 98.5 wt % of WA, the characteristic length and power-law exponent of 28.8 A and 2.86 were comparable to that of WA but in contrast to 59.7 A and 2.20 for IAA. A ∼100% increase in the characteristic length and reduced power-law exponent of the IAA fraction confirms that these two asphaltene subfractions form dissimilar nanoaggregate structures.

Hypothesis Improved oil recovery by low-salinity injection correlates to the optimal brine concentration to achieve maximum dewetting of oil droplets on rock surfaces. While interfacial tension and electrical double layer forces are often cited as being determinant properties, we hypothesize that other structural/interfacial forces are more prominent in governing the system behavior. Experiments The sessile droplet technique was used to study the receding dynamics of oil droplets from flat hydrophilic substrates in brines of different salt type (NaCl and CaCl2) and concentration, and were studied at both low and elevated temperatures (60 and 140 °C) and pressures (1, 10, 100 and 200 bar). Findings At 1 bar and 60 °C, the minimum oil droplet-substrate adhesion force ( F A ) was determined at 34 mM NaCl and 225 mM CaCl2. For NaCl this strongly correlated to strengthening hydration forces, which for CaCl2 were diminished by long-range hydrophobic forces. These results highlight the importance of other non-DLVO forces governing the dewetting dynamics of heavy crude oil droplets. At 140 °C and 200 bar, the optimal brine concentrations were found to be much higher (1027 mM NaCl and 541 mM CaCl2), with higher concentrations likely attributed to weakening hydration forces at elevated temperatures.