Crude oil/water nanoemulsions stabilized by biosurfactant: Stability and pH-Switchability

Abstract

Emulsification is one of the proposed mechanisms for enhancing crude oil recovery in chemical flooding. The replacement of chemical surfactants in enhanced oil recovery (EOR) applications with biosurfactants is an attractive alternative; yet, there is a lack of studies on the crude oil emulsification using biosurfactants. Particularly, the formulation of crude oil/water (crude O/W) nanoemulsions with an extreme stability and a pH-responsiveness behavior is still not documented in the literature, to the best of our knowledge. Accordingly, we report in this study the preparation of both light and heavy crude oil/water emulsions using different concentrations of the rhamnolipid biosurfactant. The emulsions were prepared in a two-step process: stirring overnight to create coarse emulsions followed by sonication. All the prepared emulsions have a nanoscale size (e.g., about 50% of the formed droplets at 0.1 wt% rhamnolipid are < 200 nm in size), which decreases with increasing the biosurfactant concentration. The zeta potential values of the formed crude O/W nanoemulsions were highly negative (≤−39.3). Increasing the biosurfactant concentration resulted in a more negative zeta potential. The formulated crude O/W nanoemulsions were extremely stable with 0% oil or water separation even after a month of storage. Despite the extreme stability of these crude O/W nanoemulsions, they could be easily switched off (i.e., demulsified) within 1 h by pH-swing via the addition of 50 mM sulfuric acid (H2SO4) regardless of the bioemulsifier (i.e., rhamnolipid) concentration. It was also possible to switch on/off the crude O/W nanoemulsions a number of times (at least five times) through this pH-swing mechanism. We also demonstrated that the crude O/W nanoemulsions can be completely switched off through the addition of NaOH, particularly, when these crude O/W nanoemulsions are stabilized by low rhamnolipid dosages (≤0.5 wt%); such an observation is very unusual. The findings reported in this study clearly demonstrate the ability of creating extremely stable crude O/W nanoemulsions, which is desirable in EOR; however, the emulsified crude oil can be easily and economically recovered via a simple and an economic process (i.e., pH-switch) with the ability to switch off/on the emulsion several times.