Ultralow tensions

Microemulsions became well known from about 1975 to 1980 because of their use ia "micellar-polymer" enhanced oil recovery (EOR) (35). This technology exploits the ultralow iaterfacial tensions that exist among top, microemulsion, and bottom phases to remove large amounts of petroleum from porous rocks, that would be unrecoverable by conventional technologies (36,37). Siace about 1990, iaterest ia the use of this property of microemulsions has shifted to the recovery of chloriaated compounds and other iadustrial solveats from shallow aquifers. The latter appHcatioa (15) is sometimes called surfactant-enhanced aquifer remediation (SEAR). 

Process effectiveness depends on maintaining an ultralow (ca 10 ° N/m (10 dynes/cm)) interfacial tension between the injected surfactant slug and the cmde oil (213). The effect of petroleum composition on oil solubilization by surfactants has been the subject of extensive study (214). 

Since Thin Film Spreading Agents do not produce ultralow interfacial tensions, capillary forces can trap oil in pore bodies even though the oil has been displaced from the surface of the porous medium. Therefore, recovery of incremental oil is dependent on the formation of an oil bank. Muggee, F. D. U.S. Patent 3 396 792, 1968. 

This transition may j-.e. reducing the specific surface energy, f. The reduction of f to sufficiently small values was accounted for by Ruckenstein (15) in terms of the so called dilution effect". Accumulation of surfactant and cosurfactant at the interface not only causes significant reduction in the interfacial tension, but also results in reduction of the chemical potential of surfactant and cosurfactant in bulk solution. The latter reduction may exceed the positive free energy caused by the total interfacial tension and hence the overall Ag of the system may become negative. Further analysis by Ruckenstein and Krishnan (16) have showed that micelle formation encountered with water soluble surfactants reduces the dilution effect as a result of the association of the the surfactants molecules. However, if a cosurfactant is added, it can reduce the interfacial tension by further adsorption and introduces a dilution effect. The treatment of Ruckenstein and Krishnan (16) also highlighted the role of interfacial tension in the formation of microemulsions. When the contribution of surfactant and cosurfactant adsorption is taken into account, the entropy of the drops becomes negligible and the interfacial tension does not need to attain ultralow values before stable microemulsions form. 

C. A. Miller, R.-N. Hwan, W.J. Benton, and T.J. Fort Ultralow Interfacial Tensions and Their Relation to Phase Separation in Micellar Solutions. J. Colloid Interface Sci. 61,554(1977). 

T. Sottmann and R. Strey Ultralow Interfacial Tension in Water-N-Alkane-Surfactant Systems. J. Chem. Phys. 106, 8606 (1997). 

Fotland P, Skauge A (1986) Ultralow interfacial tension as a fimction of pressiue. J Dispers Sci Technol 7 563-579... 

Ward, A. D., Berry, M. G., Mellor, C. D., Bain, C. D., Optical sculpture controlled deformation of emulsion droplets with ultralow interfacial tensions using optical tweezers. Chem. Commun. 2006, 4515-4517. 

Several theories have been proposed to account for the thermodynamic stability of microemulsions. The most recent theories showed that the driving force for microemulsion formation is the ultralow interfacial tension (in the region of 10 4-10 2 mN m 1). This means that the energy required for formation of the interface (the large number of small droplets) A Ay is compensated by the entropy of dispersion —TAS, which means that the free energy of formation of microemulsions AG is zero or negative. 

The ultralow interfacial tension can be produced by using a combination of two surfactants, one predominantly water soluble (such as sodium dodecyl sulfate) and the other predominantly oil soluble (such as a medium-chain alcohol, e.g., pentanol or hexanol). In some cases, one surfactant may be sufficient to produce the microemulsion, e.g., Aerosol OT (dioctyl sulfosuccinate), which can produce a W/O microemulsions. Nonionic surfactants, such as alcohol ethoxylates, can also produce O/W microemulsions, within a narrow temperature range. As the temperature of the system increases, the interfacial tension decreases, reaching a very low value near the phase inversion temperature. At such temperatures, an O/W microemulsion may be produced. 

In order to vary interfacial tension over more than four orders of magnitude, several fluid systems were chosen that ranged from high tension surfactant-free formulations to middle phase microemulsions that were at optimal conditions for enhanced oil recovery and had ultralow tensions with the excess brine and oil. Table I lists the specific components used along with their corresponding physical properties. In each case a red water-soluble food coloring dye was added before equilibration to enhance the contrast between phases during microscopy. 

An ultralow tension system was needed to study mechanisms of high Ca flow at relatively low velocities. A Witco petroleum sulfonate was used that had previously been studied by several researchers including Qutubuddin (15). The tension between the middle phase microemulsion and the excess oil phase was only 0.0015 dyne/cm. Surfactants were used as received, and all other chemicals were reagent grade. 

For Ca > 0.5, no major change in basic flow mechanism is foreseen until the onset of inertial effects. Care should be taken, however, when dealing with ultralow tension fluids in formations of appreciable thickness since buoyant forces could become important (21). 

Micellar-polymer flooding relies on the injection of a surfactant solution to lower interfacial tension to ultralow levels, on the order of 10 mN/m. The resulting increase in capillary number allows the recovery of residual oil from porous media. The term micellar is used because the concentrations of injected surfactant solutions are always above their critical micelle concentration. That is, they are always above the concentration at which micelles form. 

Dynamic Interfacial Tension. Crude-oil-alkali systems are unusual in that they exhibit dynamic interfacial tension (Figure 11). A solution of 0.05 wt% sodium hydroxide in contact with David Lloydminster crude oil initially produces ultralow values of IFT. A minimum value is reached, after which IFT increases with time by nearly 3 orders of magnitude, measured in the spinning drop tensiometer. Taylor et al. (57) showed that dynamic inter-facial tension can also occur in crude-oil-alkali-surfactant systems. Figure 11 shows interfacial tension versus time for a solution containing 1 wt% sodium carbonate, and the same solution containing 0.02 wt% of Neodol 25-... 

Microemulsions provide ultralow interfacial tensions and large interfacial areas as well as the ability to concentrate and localize significant amounts of both oil-and water-soluble materials within the same isotropic medium. Over the years, attention has been focused on their potential use as novel reaction media for a wide range of chemical, biochemical, and photochemical reactions, and as carriers for chemicals and small particles, reviewed by Eccleston. Inverse microemulsions of the w/o type are the subject of particular interest because of the rapidly emerging range of... 

When the appropriate precautions are taken the method appears particularly suited for measuring very low tensions 10 mN m sometimes even as low as 10 mN m ). Such ultralow tensions are for example encountered in micro-emulsion systems and in just phase-separated polymeric or micellar solutions. For phase-separated colloid-polymer systems de Hoog and Lekkerkerker ) even reported values down to a few pN m , reproducibly being obtained after implementing a number of methodical improvements. (Alternatives for low tensions are the sessile and pending (micro-) drop but these do not usually go below 10 mN m ) Commercial apparatus are nowadays available. A variant proposed by Than et al. J employs a thin rod in the axis of the cylinder, to reduce spin-up time and suppress drift. Another variant, proposed by Kokov, analyses the centrifugal field required to squeeze liquid out of an orifice" ). 

Chan and Shah (26) proposed a unified theory to explain the ultralow interfacial tension minimum observed in dilute petroleum sulfonate solution/oil systems encountered in tertiary oil recovery processes. For several variables such as the salinity, the oil chain length and the surfactant concentration, the minimum in interfacial tension was found to occur when the equilibrated aqueous phase was at CMC. This interfacial minimum also corresponded to the partition coefficient near unity for surfactant distribution in oil and brine. It was observed that the minimum in ultralow interfacial tension occurs when the concentration of the surfactant monomers in aqueous phase is maximum. 

The effect of alcohol concentration on the solubilization of brine has been studied in this laboratory (41). It was observed that there is an optimal alcohol concentration which can solubilize the maximum amount of brine and can also produce ultralow interfacial tension. The optimal alcohol concentration depends on the brine concentration of the system. The effect of different alcohols on the equilibrium properties and dynamics of micellar solutions has been studied by Zana (42). 

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