Chemical Insights into Interfacial Materials from Brazilian Crude Oils via Comparative Extraction Methods

The formation and stability of water-in-oil (W/O) emulsions in petroleum are strongly influenced by species that accumulate at the oil-water interface, collectively referred to as interfacial material (IM). Understanding the composition of IM is essential for clarifying the mechanisms responsible for emulsion stability and interfacial rigidity, although its high chemical complexity makes the identification of its major constituents challenging. In this study, IM fractions from four Brazilian crude oils were isolated using two methods, centrifugation followed by Dean-Stark distillation (IMR) and adsorption on wet silica (IMW), and then characterized by negative-ion electrospray Fourier-transform ion cyclotron resonance mass spectrometry (ESI(-) FT-ICR MS), gas chromatography with flame ionization detection (GC-FID), elemental analysis (CHNS-O), and mid-Fourier transform infrared spectroscopy (mid-FTIR). Their ability to stabilize emulsions was also evaluated. The results showed that both extraction methods enriched the IM fractions with highly polar oxygenated classes and reduced the relative contribution of pyrrolic species (N[H]) compared with the original crude oils. The wet silica method was more selective for highly polar compounds, particularly the O₂[H] class, commonly associated with naphthenic acids and rigid interfacial films. In contrast, the centrifugation/Dean-Stark method recovered an interfacial fraction with broader compositional coverage, including lower-polarity classes such as NO₂[H] and NO₃[H], where GC-FID, elemental analysis, and FTIR analyses also confirmed a more aliphatic character for the IMR fractions obtained by centrifugation. Model emulsion tests further showed that the IM fractions obtained by centrifugation/Dean-Stark conferred greater stability to the W/O system than those isolated using wet silica, indicating that emulsion stabilization depends on the combined action of polar and nonpolar species.