Zeta potential and langmuir films of asphaltene polar fractions

In this work, we study the surface and electrokinetic properties of asphaltenes and some of their polar fractions. Using different mixtures of pentane/methylene chloride as a precipitant, we obtain four fractions (F1-F4), which contain different proportions of iron, nickel, copper, cobalt, aluminum, and vanadium. Modifications of asphaltenes and fractions F1 and F2 are performed by treatment with the salt form of ethylenediamine tetraacetic acid (EDTA) and, after this, with aqueous solutions of copper sulfate. The results indicate differences in the metal content between fractions before and after the treatments. Besides these differences, we observed differences in the zeta potential measurements and differences in the isotherms at the air/water interface. Our results show that, after the EDTA treatment, asphaltenes and fractions F1 and F2 can retain copper (>200% of the original copper content of the samples). This high copper content of the treated asphaltenes and the F1 and F2 samples is reflected in a change of the electrical charge from a negative charge to a positive charge. The hydrophobic behavior of the samples was analyzed with a Langmuir balance. The results show that the asphaltenes and the F1 fractions are more hydrophilic, with a smaller molecular area, and the F4 fractions are more hydrophobic, with a larger molecular area, before and after EDTA and copper sulfate treatments. Both the zeta potential measurements and Langmuir films of the samples indicate the possibility that EDTA molecules remain attached to the asphaltene molecules. © 2005 American Chemical Society.