Interfacial behavior of naphthenic acids and multivalent cations in systems with oil and water. II: Formation and stability of metal naphthenate films at oil-water interfaces

摘要

The stability of interfacial 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+ Sr2+, and Ba2+ were dissolved in water at two different pH levels: ultrapure water (pH 5.6) and buffered ultrapure 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 film 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.