Molecular Characterization of Alkyl Phenols and their Implication as Asphaltene Dispersants
I. García-Cruz1*, J. J. Hernández-Trujillo1,2 and J. Manuel Martínez-Magadán1
1 Programa de Ingeniería Molecular, Instituto Mexicano del Petróleo,
Eje Central Lázaro Cárdenas 152, Colonia San Bartolo Atepehuacán,
México D. F., 07730, México
2 Departamento de Física y Química Teórica, Facultad de Química, Universidad Nacional Autónoma de México, México, D. F., 04510, México.
In agreement with experimental observations, the computed solvation free energies of the series of p-alkylphenols studied in this work allow anticipating that n-pentyl phenol has the shortest alkyl tail necessary in order to efficiently dissolve an asphaltene in n-heptane. The non-electrostatic contribution to DGsolv is the leading factor that differentiates the relative solubility of these compounds. From the dissection of the non-electrostatic term, it is obtained that even though the repulsive contribution (which could be associated to a steric destabilization) is offset in an important manner by the stabilizing effect of the van der Waals forces. Beyond the semantic use of the term “steric estabilization” used by some authors[1] to explain the dispersing character of these compounds, the present results lead to dissent from such an interpretation of the experimental observations. The leading stabilizing effect comes from the van der Waals interaction because of the minor variance of the electron distribution as explained by the topological properties of the electron density and of the atoms in the molecules. Moreover, the trend observed along the series of molecules analyzed is due to the long-range stabilization of the alkyl tail. From these results, the amphiphile - solvent interaction explains the ability of p-alkyl phenols to disperse an asphaltene in n-C7. The quantification of the solvent - solute interactions by means of a dielectric model provides detailed insight on the nature of one of the stabilizing factors involved on the amphiphilic character of p-alkyl phenols used as asphaltene dispersants in n-C7.
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