Infrared Spectroscopic Analysis of the Composition of an Oil/Water Interfacial Film

Interfacial tension plays an important role in reservoir mechanisms and in the production and processing of crude oil with coproduced water. Natural surfactants adhere to the oil/water (O/W) interface, which reduces the interfacial tension and in some cases leads to a solidified interfacial film with a significant impact on emulsion stability. These technical challenges are a result of the chemistry of the petroleum system, and hence optimization of production depends on a better understanding of this. The properties and composition of the interfacial film have been the subject of many studies. In this study, films of oil-related components were generated at the interface between oil and water. The interfacial material was recovered by skimming the interface remaining after repeated solvent washings. The recovered material was analyzed using Fourier transform infrared spectroscopy by attenuated total reflectance (FTIR ATR) to obtain both quantitative and qualitative information about the composition of the film. When compared to the spectrum of the total precipitated n-heptane asphaltenes, which show many aromatic features, the interfacial film shows surprisingly few of these. There is a substantial difference between the interfacial film material and the asphaltenes, especially in the aromatic C-H vibration range between 700 and 900 cm(-1). It is observed that oxygenated compounds, sulfoxides and carbonyls, and one specific type of aromatics are concentrated in the film. The latter also shows up in the oil and in the maltenes but less prominently. In a recent communication [Andersen et al. Energy Fuels 2016, 30 (6), 4475-4485], we showed that homologous series of fatty acids can be found in the interfacial film, and the current detailed examination of the FTIR spectra also indicates that the water interface serves as a substrate for specific adsorption of aromatic molecules that are not representative of the bulk asphaltenes.