Phase Behavior of Surfactant WaterOil Systems

Microemulsions are transparent or translucent, thermodynamically stable "emulsion" systems (Griffin 1949). Forming a middle phase microemulsion (MPM) requires matching the surfactant system's hydrophobicity with that of the oil. The HLB (hydrophilic-lipophilic balance) number reflects the surfactant's partitioning between water and oil phases; higher HLB values indicate water soluble surfactants while lower values indicate oil soluble surfactants (Kunieda et. al. 1980, Abe et. al. 1986). While a balanced surfactant system produces middle phase microemulsions, an under-optimum surfactant system is too water soluble (high HLB) while an over-optimunTSystem is too oil soluble (low HLB).

Figure 1 shows changes in the system phase behavior as its HLB value is systematically adjusted. The left side of the diagram represents a two-phase system with micellar-solubilized oil in equilibrium with an excess oil phase (Winsor Type I) (Winsor 1954). The right side of the diagram represents a different two-phase system with reversed micellar-solubilized water. In-between these two systems a third phase coemerges which contains enriched surfactant with solubilized water and oil. This new thermodynamically stable phase is known as a Winsor Type III middle phase microemulsion.

It is widely recognized that the system IFT reaches a minimum in the middle phase microemulsion region. At the same time, the solubilization parameter (a), defined as mass of oil solubilized per unit mass of surfactant, is maximized in middle phase microemulsion systems (see Figure 1). This inverse relationship between the solubilization parameter (a) and IFT (y)has been defined by the Chun-Huh equation (Huh 1979, Sunwoo et. al. 1992,

When equal volumes of water and oil are solubilized in the middle phase, the system is said to be at its optimum state (Heely et al. 1974, Salager et. al. 1979) (which is denoted by superscript *), so defined because IFTs are minimized and thus the highest oil solubilization occurs.

The goal of surfactant enhanced subsurface remediation is to maximize the contaminant extraction efficiency while optimizing system economics. Since middle phase microemulsions maximize the solubilization while minimizing the oil-water interfacial tension, these systems are highly desirable, especially for NAPLs lighter than water, where downward

migration of released oil is not a concern. Additional factors of concern are system capillary curves, viscosity, density, etc. For more details

Increasing Salinity----->

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Decreasing HLB—>

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