A Ultralow Interfacial Tension

Increasing temperature (for POE nonionics) Increasing salinity (for ionics) 

FIGURE 5-5 Effect of molecular environmental conditions on interfacial tension and phase volumes. Shaded phases indicate locations of the surfactant. 

The maximum volume of the surfactant (middle) phase D at the temperature where all three phases exist is dependent upon the percentage of surfactant in the system. If the percentage is very small, the surfactant phase may not be visible to the naked eye and the system may appear to contain only two phases if the 

From the above discussion, it should be apparent that for POE nonionics, there is a particular temperature where the hydrophilic and lipophilic characters of the surfactant balance each other and yow is at, or close to, its minimum value. It is usually defined operationally, for example, as the temperature where the surfactant phase solubilizes equal volumes of water and nonpolar material or the temperature at which an emulsion (Chapter 8) of the surfactant, water, and nonpolar material inverts. In the latter case, it is known as the phase-inversion temperature (PIT) (Chapter 8, Section IVB). Similarly, there is an electrolyte content at which the hydrophilic and lipophilic characters of ionic surfactants balance. The point at which equal volumes of water and nonpolar material are solubilized into the surfactant is known as the optimal salinity (Healy, 1974) and has been extensively investigated for enhanced oil recovery (Healy, 1977 Hedges, 1979 Nelson, 1980). The optimal salinity or PIT is at or close to the point where the parameter Vh/lcao (Chapter 3, Section II) equals 1 and lamellar normal and reverse micelles are readily interconvertable. 

The larger the volume of water (VV) (or nonpolar material Vo) solubilized into the surfactant phase relative to its volume Vs, the lower the interfacial tensions yDW, yOD, and yow (Robbins, 1974 Healy, 1976). This is understandable, since for both normal and reverse micelles, the interfacial tension against the second liquid phase decreases as the amount of second phase solubilized increases. The greater the amount solubilized in the presence of excess solubilizate, the more closely the natures of the two phases approach each other. 

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