Fractionation of Interfacial Material Reveals a Continuum of Acidic Species That Contribute to Stable Emulsion Formation

Abstract

The acidic components of Athabasca bitumen interfacial material (IM) were isolated and subsequently fractionated based on hydrophobicity by a modified aminopropyl silica (MAPS) method to determine whether low-molecular-weight IM acids are preferentially ionized in negative-ion electrospray ionization (ESI (-)) and, thus, bias the compositional information obtained by direct infusion (dilute and shoot) mass spectral analysis. Characterization by negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) revealed that MAPS fractionation of IM acids extends the detection of high-m/z (>600-Da) IM compounds by 2-fold, yields an approximate 10-fold increase in the number of assigned formulas, and exposes a continuum of acidic species that includes the first definitive identification of doubly charged acids in interfacial material. Comparison of the heteroatom contents of singly and doubly charged Ox species, combined with the acid-targeted extraction procedure, strongly suggests that the chemical functionalities are similar for the two ion types and are largely composed of mono- and dicarboxylic acids. Excitation emission matrix spectroscopy (EEMS) revealed that the most hydrophobic IM fractions approach the size and aromaticity of bitumen asphaltenes, but despite the increase in hydrophobicity, these asphaltene-like acids form the tightest emulsions, as revealed in simple bottle tests. Thus, the most surface-active material from Athabasca bitumen comprises low-molecular-weight, acidic, resin-like species, as well as larger (higher-molecular-weight) acidic asphaltene-like compounds.