Biosurfactant surfactin with pH-regulated emulsification activity for efficient oil separation when used as emulsifier

In the present study, the pH-regulated emulsification activity of surfactin was studied and its potential application in oil separation towards enhanced oil recovery (EOR) was investigated. As demonstrated, surfactin can stabilize emulsions quite well beyond pH 7.4. An oil emulsification ratio of about 98% was obtained at pH 11.0; while this emulsification activity was rapidly and completely lost when pH decreased to below 3.0, having an oil separation ratio of over 98%. This pH-sensitive property is probably due to surfactin dissolution-precipitation induced by the ionization-protonation of a carboxyl group in its structure under alkaline or acidic conditions. This property allows oil emulsification or oil separation to be readily achieved via simple pH adjustments when surfactin is used as an emulsifier. Furthermore, surfactin sustained its activity after demulsification and can be readily reused many times. The above obtained results indicated surfactin-based EOR processes have great application feasibility.