A variety of experimental techniques were applied to a single source asphaltene sample at the same experimental conditions in order to reveal the possible size distributions of asphaltene monomers and aggregates. The asphaltene sample was divided into solubility cuts by selective precipitation in solutions of heptane and toluene. Asphaltene self-association was assessed through a combination of density, vapor pressure osmometry (VPO), elemental analysis, Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry, and time-resolved fluorescence emission spectra measurements performed on each cut. The physical dimensions of the asphaltenes were assessed using SAXS, DLS, membrane diffusion, Rayleigh scattering, and nanofiltration measurements. Molecular and nanoaggregate dimensions were also investigated through a combination of interfacial tension, interfacial adsorption, and surface force measurements. All of the measurements indicated that approximately 90 wt % of the asphaltenes self-associated. Ultrahigh resolution spectrometry suggests that the nonassociated asphaltenes are smaller and more aromatic than bulk asphaltenes indicating that the associating species are larger and less aromatic. On the basis of VPO, the average monomer molecular weight was approximately 850 g/mol, while the molecular weight of the nanoaggregates spanned a range of at least 30000 g/mol with an average on the order of 10000 to 20000 g/mol. SAXS and DLS gave molecular weights 10 times larger. The physical dimensions of the nanoaggregates were less than 20 nm based on nanofiltration and with average diameters of 5 to 9 nm based on diffusion and Rayleigh scattering. SAXS and DLS gave average diameters of 14 nm and indicated that the nanoaggregates had loose structures. Film studies were consistent with the lower molecular weights and dimensions and also demonstrated that asphaltene monolayers swell by a factor of 4 in the presence of a solvent. The most consistent interpretation of the data is that asphaltenes form a highly polydisperse distribution of loosely structured (porous or low fractal dimension) nanoaggregates. However, the discrepancy between VPO and SAXS molecular weights remains unresolved.

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Calcium and sodium naphthenates are solid deposits and emulsions formed by the interaction of naphthenic acids with divalent (Ca2+, Mg2+) or monovalent (Na+, K+) ions in produced waters. Calcium naphthenate formation, an interfacial phenomenon, is thought to depend largely on tetraprotic naphthenic acids known as “ARN” acids (∼C80) in the crude oil, whereas sodium naphthenates originate from lower molecular weight (C15 to C35) monoprotic naphthenic acids. Here we present detailed chemical heteroatom class composition analyses of calcium and sodium naphthenates from the field based on high resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). In all cases, calcium naphthenate deposits consist predominately of tetraprotic acids with a C80 hydrocarbon skeleton whereas sodium naphthenate emulsions consist mainly of specific monoprotic saturated carboxylic acids. Furthermore, low molecular weight tetraprotic (ARN) acids with C60−77 hydrocarbon skeletons were identified in the calcium naphthenate deposit. The high resolution and mass accuracy of FT-ICR MS provide detailed acidic speciation for the analyzed deposits and emulsions.

Crude oil contaminated soil cores were collected from a basin that contained oily solids left from three decades of oil production. Hydrocarbon biomarker analyses revealed that the soil extracts were moderately biodegraded compared with the non-degraded source oil. The degree of biodegradation also decreased with core depth (7 cm to 1 m). These data were correlated to compositional changes observed in acidic NSO-compounds that were selectively ionized and mass resolved by negative ion electrospray Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS). Among the NSO-compounds ionized, the increase in naphthenic acid concentration (e.g., acyclic and alicyclic carboxylic acids) best correlated with the increase in biodegradation (e.g., from non-degraded to moderately degraded) as determined by the hydrocarbon biomarker analyses. The most biodegraded surface extracts (7 cm) exhibited an 80% increase in the abundance of acids relative to the source oil. Use of an internal standard allowed the semi-quantitative determination of the total naphthenic acid concentration, which decreased significantly (P < 0.05) with soil depth. Furthermore, the shift to higher double bond equivalents (DBEs), from acyclic to alicyclic acids, indicated that the increase in acids in the soil extracts was predominantly due to biotic processes. This work demonstrates the potential of ESI FT-ICR MS as a semi-quantitative tool to monitor the production of naphthenic acids during crude oil biotransformation in the environment.