Interfacial tension measurements between oil fractions of a crude oil and aqueous solutions with different ionic composition and pH

Dynamic and equilibrium interfacial tensions between crude oil fractions and aqueous solutions of various compositions and pH were measured. The basic oil components seemed to determine the interfacial tensions at pH 2, while the non-dissociated and dissociated acidic components governed the interfacial tension at the natural pH and pH 9, respectively. The ionic composition of the aqueous phase influenced the degree of dissociation of the acidic components at pH 9: Na+ ions in the aqueous phase promoted dissociation of the interfacial acidic components (compared to pure water), while Ca2+ ions resulted in complexation with the dissociated acids and most likely formation of stable interfacial films. The amount of Ca2+ determined which of these phenomena that dominated when both ions were present in sea water solutions. Generally, the interfacial tensions of the oil fractions were lower when measured against the high salinity aqueous solutions than against the corresponding low salinity solutions.