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Dynamic Dilational Properties of Oil–Water Interfacial Films Containing Surface Active Fractions from Crude Oil

Abstract The dilational viscoelasticity behaviors of oil–water interfaces formed by surface active fractions of different average molecular weights and H/C atom ratios, which were distilled by supercritical fluid extraction and fractionation (SFEF) methods from Iranian heavy oil, and influences of pre‐equilibrium time on them were investigated. The interfacial relaxation processes were investigated by interfacial tension relaxation methods. The dilational moduli, elasticities and viscosities all increased and phase angles also regularly changed along with increasing number average molecular weights and decreasing H/C atom ratios. Along with increasing pre‐equilibrium time, the dilational moduli of samples increased and the phase angles decreased to the equilibrium values, as a result of the enrichment of surface active fractions into the interface. It took longer to reach the equilibrium for relatively high molecular weight samples than for low molecular weights samples. Along with increasing average molecular weights and decreasing H/C atom ratios of the samples, slow relaxation processes gradually appeared and their contribution increased. The characteristic relaxation times of similar processes of different samples also increased. The results of interfacial tension relaxation experiments and dilational viscoelasticity parameters determined by sinusoidal oscillation of interfacial area perfectly coincided with each other. The results for interfacial dilational viscoelasticity measurements, number average molecular weight and H/C atom ratios, and interfacial tension relaxation experiments illustrated that the fractions containing large condensed ring aromatic compounds that have large conjugated structures played a more important role in film‐forming and film rigidity than fractions of smaller molecules.