Partition Coefficients and Interfacial Activity for Polar Components in Oil/Water Model Systems.

Partition coefficients, surface tension, and interfacial tension for some polar organic components dissolved in oil/water model systems have been investigated. The systems consist of isooctane modeling the oil phase and of water solutions of NaCl and CaCl2 modeling the water phase. The organic compounds examined were 1-naphtoic acid, 5-indanol, and quinoline, all well-defined molecules known to be representative of polar components in crude oil. The dependence on pH, salinity, and ionic strength in the water phase was investigated. The surface tension and interfacial tension were also examined as a function of component concentration. The results show a connection between the distribution of the polar components and the interfacial tension. Correspondence between the partition coefficient and the pKa value for the components is also reported. For 1-naphtoic acid none of the two ionization forms of the molecule are found to be surface active in aqueous solution. For 5-indanol both forms are surface active, and for quinoline only the nonionic form of the molecule is found to be surface active. The results indicate that the aqueous phase is the one that governs the interfacial tension. Increasing salinity increases the concentration of the component in the oil phase and decreases the interfacial tension between the oil phase and the aqueous phase. The results are explained due to the "salting-out" effect and to changes in the electrostatics for the various systems. Copyright 1999 Academic Press.