The Effect Of Surfactant Partitioning On The Phase Behavior And Phase Inversion Of The Middle Phase Microemulsions
Authors: Chan K.S. , Shah D.O. 
Published: December 2007

Abstract
For multiphase surfactant-oil-brine systems, it was observed that the phase transition of microemulsion phase from lower to middle to upper phase can be phase from lower to middle to upper phase can be achieved by varying any one of the seven variables:emulsifier or surfactant concentration,oil/ brine ratio,surfactant formulation/oil ratiobrine salinity,alcohol concentration,oil chain length, andtemperature.
The first three variables show a transition from the middle-phase to upper-phase microemulsion in which the middle-phase microemulsion grows and incorporates the excess oil into the middle phase. The resulting upper-phase microemulsion was found to be water-external. However, for the last four variables, there is a true phase inversion where a middle-phase microemulsion phase inversion where a middle-phase microemulsion inverts into an oil-external microemulsion.
It has been established in low surfactant concentration systems that the transition of surfactant-rich phase from lower to upper phase is characterized by a partition coefficient of unity for the surfactant. Similarly for high surfactant concentration systems, we have observed that the phase inversion (i.e., the disappearance of the phase inversion (i.e., the disappearance of the middle-phase microemulsion and formation of oil-external upper-phase microemulsion) occurs at the partition coefficient unity defined as the concentration partition coefficient unity defined as the concentration in the oil phase divided by the concentration of surfactant in the middle and lower phases. This similarity between the effects of salt concentration on low and high surfactant concentration systems supports the view that the middle phase is an integral part of the aqueous phase. Furthermore, a careful examination of the phase-inversion region revealed that as the middle-phase volume decreases, the surfactant concentration in the oil phase increases.
We have found also that at the optimal parameters (e.g., salinity, oil chain length, etc.) the surfactant concentration in the excess oil phase approaches that in the excess brine phase. However, the phase inversion occurs when the surfactant concentration in the excess oil phase approaches the concentration in the combined middle and lower brine phase. The results show the similarity between the phase. The results show the similarity between the interfacial tension behavior of low concentration systems and those of high concentration systems.

Introduction
Systems containing 2 to 10% petroleum sulfonate with or without the addition of alcohol as cosolvent usually exhibit a complicated phase behavior. The surfactant can be predominantly soluble in brine and form a water-external microemulsion in equilibrium with an excess oil phase. On the other hand, if the surfactant has a greater solubility in oil, it can form an oil-external microemulsion in equilibrium with an excess brine phase. The transition of surfactant-rich phase from lower phase (brine phase) to upper (oil phase) generally goes through a region in which a middle phase, which virtually contains most of the surfactant, and remains in equilibrium with an excess oil phase and an excess brine phase. It has been well established that the formation of this middle phase correlates with increased oil displacement efficiency of the process.
The development of the multiphase behavior from two-phase with surfactant in the brine phase through three-phase having most of the surfactant in the middle phase to two-phase with the surfactant in the oil phase can be achieved by changing one of the seven parameters in the direction described as follows:increasing the total surfactant concentration in the system,increasing the brine/oil ratio,increasing the surfactant solution/oil ratio,increasing the salinity of the brine,decreasing the hydrocarbon chain length of the oil,increasing the alcohol concentration, anddecreasing the temperature.
The first three parameters are controlling the phase behavior in quite the same manner. phase behavior in quite the same manner.