Quantification of the formation of molecular complexes between simple organic aromatic molecules is straightforward for pure components of definite composition; however, it becomes challenging when the interacting species are complex, intractable mixtures of thousands of polycyclic aromatic hydrocarbons (PAHs). Using the example of asphaltenes, a complex mixture of oil-derived PAHs, we demonstrate the potential of fluorescence spectroscopy for quantification of molecular processes that are at the core of their aggregation. It is found that small electron-deficient aromatic additives, especially surfactant-like additives with electron-withdrawing functional groups, interact strongly with the PAHs and lead to the formation of molecular complexes. Computational modeling revealed that as we increase the number of PAH molecules in the cluster, cohesive π–π stacking interactions between PAHs dominate in preference to adhesive hydrogen bonding or π–π stacking interactions between the PAHs and additives.
In times of increasing crude oil demand, enhanced oil recovery methods, such as CO 2 flooding, are gaining interest as alternative approaches to more efficient oil extraction. In conjunction with this technology, which inevitably affects the oil chemistry, stands the pressing issue of asphaltene precipitation, which results in significant increases in operating expenses for these operations. Here, we report details of an analytical protocol that uses quartz crystal microbalance with dissipation module (QCM-D) for testing the immediate-and long-term effects of chemically modified inhibitors against asphaltene deposit growth onto asphaltene-coated carbon steel, a situation that is often encountered in oil production equipment. The inhibitors were tested within a large concentration range on model solutions of different Middle Eastern asphaltene samples. Unlike the traditionally and commonly used precipitation test, QCM-D provides accurate and time-dependent information on physicochemical processes, such as adsorption, removal, and inhibition, onto/from a desired surface within a single experiment. The results indicate that the inhibitor chemistry and, particularly, the electronic properties of the functional groups in the inhibitor molecules are central to their performance. Specifically, while non-Brønsted-acid-type additives with electron-withdrawing functional groups are ineffective toward removal of previously adsorbed asphaltenes, complex Brønsted-acid-type molecular mixtures having electron-withdrawing functional groups remove deposits at very low concentrations. However, Brønsted-acid-type additives having electron-withdrawing functional groups can also aggravate deposition above a certain concentration threshold, which is not the case for the non-Brønsted-acid-type additives. Surprisingly, the addition of a second electron-withdrawing functional group to an already electron-poor Brønsted-acid-type inhibitor was found to have an adverse effect on the inhibitor's performance. It is also shown that single-component inhibitors can compete with multicomponent systems in removal; however, they are less effective in prevention of precipitation after the removal.
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