Detailed compositional analysis of nine crude oils and their saturate, aromatic, resin, and asphaltene fractions were carried out by elemental analysis and Fourier transform infrared spectroscopy. The crude oils had a broad range of physicochemical properties. The largest difference in composition was found in the asphaltene fractions of the oils, followed by the resin fractions. These fractions had significantly higher abundance of oxygen and nitrogen heteroatoms than the saturate and aromatic fractions. Furthermore, the lightest crude oils had the highest oxygen content in the asphaltene fraction, while it was the highest in the resin fraction for the heaviest oils. Partial least-squares regression models suggested that the total acid number was primarily associated with carboxylic acids in aliphatic structures and that high abundance of nitrogen and oxygen particularly in the resin fraction enhanced the total base number. It is believed that the detailed structural information provided here will help to improve the understanding of the interfacial properties of the crude oils.

The significance of interfacial chemistry for the oil removal efficiency during flotation was demonstrated in a series of laboratory flotation and pilot-scale compact flotation unit (CFU) tests. Three crude oils with different physicochemical properties were used in the investigations. The differences in drop size distributions and densities of the oils could not fully account for the observed oil removal. However, taking the time for drainage and rupture (i.e., induction time) of the thin aqueous film separating the drops and bubbles into consideration resulted in good agreement with the oil removal. Moreover, it was demonstrated in a modified CFU setup that water-soluble hydrocarbons adsorbed onto the bubbles and reduced the oil removal. This was most likely as a result of increased induction times caused by the adsorbed components.

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The interactions between crude oil droplets and air bubbles were studied by the droplet-bubble micromanipulator technique. Eight crude oils were investigated, and some aspects of the involved mechanisms were discussed. The induction time was measured for air bubbles approaching crude oil droplets in different aqueous phases. Distinct differences were observed in the presence and absence of salts, which showed the importance of long-ranged electrostatic repulsive forces on thin-film stability. The results also suggested that adsorption of dissolved hydrocarbons at air bubble surfaces may increase the potential energy barrier in the thin liquid film. Furthermore, the time needed for crude oil droplets to spread over the air bubble surfaces (referred to as coverage time) was determined for the crude oils. The results showed that the spreading velocity decreased with increasing viscosity of the crude oil. The detailed understanding of this type of interaction is considered to be a precursor for improving the oil removal efficiency during the flotation process.

A matrix of 18 synthetic produced water samples was analyzed by partial least-squares (PLS) regression modeling. This has been done to investigate the correlation between crude oil properties and water solubility and affinity of the dissolved hydrocarbons for air/water interface which is referred to as produced water quality. The synthetic produced water samples were prepared with nine crude oils and two aqueous phases in the presence and absence of divalent cations (Ca2+, Mg2+). The total organic carbon content and dynamic surface tension was determined for the samples. The findings of the PLS modeling are summarized as follows: produced water quality was not correlated to the amount (wt %) of aromatics, resins, and asphaltenes in the crude oils but with the amount (wt %) of heteroatoms present in the different fractions. The water solubility and affinity for air/water interfaces of the crude oil hydrocarbons was promoted by oxygen-containing compounds and reduced by nitrogen- and sulfur-containing compounds. Heteroatom-containing molecules of similar class originating from aromatic, resin, and asphaltene fractions of crude oils contributed to different degrees of water solubility and affinity for adsorption at air–water interfaces. This can be related to the differences in the molecular structures of the compounds. Furthermore, the presence of divalent cations showed more influence on the quality of produced water samples of crude oils with high total acid number and nitrogen content.