Influence of pH on Stability and Dynamic Properties of Asphaltenes and Other Amphiphilic Molecules at the Oil−Water Interface

TLDR

Oil‑in‑water emulsions are being explored to transport viscous heavy oils, and their behavior is governed by oil‑water interfaces where surface‑active components such as asphaltenes and naphthenic acids compete with each other and with added surfactants. The study aims to investigate how pH affects the dynamic interfacial tension of water–toluene interfaces containing varying amounts of asphaltenes. Dynamic interfacial tension measurements and micropipette experiments were performed on water–toluene drops with asphaltenes, varying pH to observe changes in surface activity and droplet coalescence. The results show that pH strongly alters asphaltene interfacial properties, with charged functional groups at extreme pH enhancing surface activity, an interaction with maltenes that promotes molecular arrangement, minimal droplet coalescence, and a correlation between microscopic interfacial behavior and macroscopic emulsion stability.

Abstract

Oil-in-water emulsions are currently being investigated to facilitate the transport of viscous heavy oils. The behavior of these emulsions is largely controlled by the interfaces between oil drops and water. The surface-active components of crude oil, such as asphaltenes and naphthenic acids, compete among themselves at these interfaces and also with possibly added synthetic surfactant emulsifier. Here, we present a study of dynamic interfacial tension of interfaces between water and a model oil (toluene) in which variable amounts of asphaltenes are solubilized. We show that pH has a strong influence on interfacial properties of asphaltenes at the oil/water interface. At high or low pH, asphaltenes functional groups become charged, enhancing its surface activity. The influence of lower-molecular-weight surface-active species, such as the natural naphthenic acids contained in maltenes (crude oil without asphaltenes), has been investigated, and an interaction between asphaltenes and maltenes that facilitates molecular arrangement at the interface was detected. Several micropipette experiments, in which micrometric drops have been manipulated, are also described and indicate that very little coalescence of water droplets is observed at high or low pH. The microscopic properties of the interface and the macroscopic behavior of the emulsion are determined to be correlated.