Impact of ionic strength on partitioning of naphthenic acids in water–crude oil systems – Determination through high-field NMR spectroscopy

Abstract

Naphthenic acids and asphaltenes are two classes of materials indigenous to crude oils that are recognized as potential emulsion stabilizers. Reduction in interfacial tension and formation of a viscoelastic interfacial film are the two mechanisms whereby naphthenic acids and asphaltenes can contribute to emulsion stability. Synergistic effects of these two types of materials remain controversial. In this paper, we explore the impact of the aqueous phase ionic strength on the partitioning of the naphthenic acids between water and crude oils utilizing high field nuclear magnetic resonance (NMR) spectroscopy. We calibrated the 1H NMR spectra to eliminate ionic strength effects on NMR signal to determine the relative concentration of the naphthenic components in the aqueous phase resolved from partitioning tests. Our results show that higher ionic strength is, in general, conducive to higher rate of partitioning of the naphthenic acids (NA). However, the NA concentration in aqueous phase as a function of time can be affected by the specific crude oil and type of cation, among other things. In addition, adsorption of asphaltene onto the interface to create a mass transfer barrier to dissolution of naphthenic acids in water is consistent with our results. Furthermore, our observations are consistent with the notion that stability is the outcome of the combined effects of the several competitive interfacial processes, including adsorption of asphaltene onto the interface, partitioning and dissociation of the naphthenic acids in water, diffusion of the surface active material through the oleic or aqueous phase and towards the interface, as well as displacement and rearrangement of the adsorbed material at the interface.