Molecular Characterization of p-Alkyl Phenol−n-Heptane Interactions and Their Implication as Asphaltene Dispersants

The interaction between a series of p-alkyl phenols with the solvent n-heptane (n-C7) is studied by means of theoretical methods. The solvation energies of these molecules are computed using quantum mechanical methods in which the solvent is considered as a dielectric medium. Accordingly, the interaction of a solute molecule with the reaction field of the solvent yields a solvation free energy. The electrostatic, dispersion, and repulsion contributions to the solvation energies are analyzed as a function of the alkyl chain length and the relationship with the electronic structure of the molecules, as accounted for by the properties of the quantum topological atoms defined within the formalism of the atoms in molecules theory, is also discussed. Implications of the solvation energies, computed at the microscopic level, on the efficiency of p-alkyl phenols as asphaltene dispersants are discussed. The quantitative aspects of the solubility of these amphiphiles are correlated with their experimentally reported dispersing power of an asphaltene of a given oil source in n-C7. These observations support the hypothesis that the relative ability of a series of p-alkyl phenols to dissolve a given asphaltene in n-C7 is dominated by the amphiphile−solvent interactions, which are sufficient to offset the repulsion and cavitation contributions.