Assessing salt-surfactant synergistic effects on interfacial tension from molecular dynamics simulations

In the recent years, many efforts have been carried out trying to comprehend how surfactants and salts interact among each other at the oil/brine interface to reduce the interfacial tension (IFT). To that end, the interfacial properties of several combinations of surfactants, salts and oils have been measured experimentally confirming the existence of a synergistic effect. Unfortunately, many of the proposed mechanisms for that effect arise from experimental observations, so this work, based on molecular dynamics simulations, intends to reproduce and explain this kind of phenomenon from a molecular point of view. The correct understanding of these phenomena can have application in many fields, especially in Enhanced Oil Recovery, where reducing IFT can potentially increase oil production. In this article we evaluate the effect of adding three different salts (i.e., NaCl, CaCl2 and MgCl2) on the IFT of a water/oil system with different non-ionic surfactants. We have evaluated the effect that the ions of salt produce to surfactants, as well as the perturbation that surfactants produce on the ions. From our results, we can assess that salts (especially NaCl) and surfactants are able to interact with each other, being both active species in reducing the IFT of the system.