Surface properties of petroleum oil polar fraction as investigated by Zetametry and DRIFT spectroscopy

The polar fractions of various transformer petroleum oils (one new and two used oils) were extracted from the neat oils using silica as solid support. We found that the amount of the polar fraction adsorbed on the silica was higher for the used oils as compared to the new one. Such oil polar fractions are insoluble in low molecular weight paraffins (n-hexane and n-heptane) and are soluble in ethanol. The analysis of the diffuse reflectance infrared Fourier transform (DRIFT) spectra of the silica samples coated with oil polar components indicate the presence of oil polar functional groups such as hydroxyl, aromatic and carboxyl groups resembling asphaltenes or resins. In addition, we studied the electrical properties of the oil polar fraction by determining the ζ potential, in water, of the coated silica. In order to compare the modified and the non-modified silica surface properties, we made blank experiments by measuring the surface charge in water of the bare SiO2 particles. The negative surface charge at the water/oil–silica interface was found to increase with pH, resulting from the increase in ionisation of the oil polar fraction acidic surface groups. The objective of varying the pH was to analyse the possible acid–base interactions at the water–oil interface for substrates having various oil polar fraction contents. The polar components in the neat oil adsorb on the silica surface and increase its surface charge and stabilisation in water. We found a good correlation between the amounts of the oil polar fractions, the surface concentration of their functional groups (carboxylic and phenolic groups) and the magnitude of their ζ potential at the water/oil–silica interface.