Oil/Water‐Partitioning and Interfacial Behavior of Naphthenic Acids

Abstract

Partitioning and dissociation constants for naphthenic acids from crude oils and several naphthenic acid model compounds are reported. The pK a for naphthenic acids from crude oil was found to be 4.9. The logarithm of the partition coefficients varies linearly vs. the number of carbons in the naphthenic acid molecules. The naphthenic acids with three rings were found to be more hydrophilic than acids with one or two rings. The data can be used to predict the content of different naphthenic acids in produced water at a given pH, given the acid content and mole weight distribution in the crude oil. By accounting for the dissociation in water and partitioning of undissociated acid a model for the total acid concentration in water was derived. Experiments with synthetic naphthenic acids were performed and the model proved to describe the experimental points at pH < 8. At higher pH there was a deviation between the model and the experimentally determined concentrations. This was suggested to be due to formation of ordinary and reversed micelles. The correlation between interfacial tension and pH was studied for several naphthenic acid systems. A decrease in interfacial tension was seen at high pH; this was explained by an increase in the total concentration of ionized naphthenic acids at the interface. The fraction of ionized naphthenic acids at the interface was lower than the fraction in the bulk phases. This effect is explained by the fact that interfacial pH is lower than the bulk pH due to electrical double layer effects.