Characterization of Asphaltenes by Nonaqueous Capillary Electrophoresis

Nonaqueous capillary electrophoresis was used for the separation and characterization of asphaltene samples from different sources. For the separation medium (background electrolyte), mixtures of tetrahydrofuran and a high-permittivity organic solvent could be used. The best results were obtained with an 80:20 mixture of tetrahydrofuran and acetonitrile, containing 1-10 mM of lithium perchlorate. In this separation medium, asphaltene samples were found to be composed of two fractions that could be clearly separated: one fraction of neutral species and a fraction that carries a positive charge in the solvent mixture employed. Between samples of different origin, differences were found in the relative amounts of the neutral and the charged fractions and in the average electrophoretic mobility of the charged components. Taylor dispersion analysis was applied to estimate the average diffusion coefficient of the asphaltene species in the solvent mixture used. From the results, it is concluded that the asphaltenes are present as nanoaggregate clusters of 3000-4000 Da and that the charged aggregates carry a net charge of approximately +1. The possible correlation between the electrophoretic properties of asphaltenes in crudes of different origin and their field behavior is discussed.