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The stability or interracial naphthenate films of three synthetic naphthenic acids and four different divalent cations is measured by means of the Langmuir technique with a trough for liquid-liquid systems. The chloride salts of Ca2+, Mg2+, Sr2+, and Ba2+ were dissolved in water at two different pH levels: ultrapure water (pH 5.6) and buffered uttrapure water at pH 8.0. The water phase was covered by the oil phase consisting of n-decane and dissolved naphthenic acid. The systems were then equilibrated for 45 minutes before the compression of the interfacial films was initiated. The results, plotted as interfacial pressure (IP) versus compression area, showed clear distinctions depending on acid structure, type of metal salt, and pH of the aqueous phase. The differences in film stability by introducing various salts are assumed to be a result of different degrees of hydration of metal cations, which according to the theory increases with decreasing cationic size. Raising the pH from 5.6 to 8.0 generally caused decreasing Him stability due a higher ionization of the interfacial layer and a higher water solubility. Furthermore, introducing naphthenic acids with bulky structures increases the molecular distance, which in turn may hinder the monomers to be bound in 2: 1 ratio with the divalent cations. [ABSTRACT FROM AUTHOR]

Dynamic interfacial tension (JET) measurements were used to investigate the interactions between a dissociated model naphthenic acid (p-(n-dodecyl) benzoic acid) and various divalent metallic cations (Mg2+, Ca2+, Sr2+, and Ba2+) across a toluene/hexadecane-water interface. The measurements were performed by using the pendant drop technique. The results obtained, plotted as IFT vs. time gave curves with similar shapes but different slopes and levels of the equilibrium IFT, depending upon the acid and salt concentrations and the type of cation added. Due to differences in degree of hydration of the various cations, the products of the reaction between dissociated acid monomers and the cations showed differences in solubility, which, in turn, affected the IFT. Based on the shapes of the curves, the mechanisms of the reactions involved in the process are discussed. [ABSTRACT FROM AUTHOR]