Ultralow surface tension

R. Avcyard, Ultralow Surface Tensions and Mieroemulsions , Chem. Ind. (London), 1987, 474. 

Moreover, the presence of solvents in the detergent always brings effects such as interfadal turbulence, diffusion, and ultralow surface tension. Thus, for cleaning efficiency of the detergents, solvents play an important role. 

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. 

Chan, K.S., Shah, D.O., 1981. The physico-chemical conditions necessary to produce ultralow interfacial tension at the oil/brine interface. In Shah, D.O. (Ed.), Surface Phenomena in Enhanced Oil Recovery. Plenum, pp. 53-72. 

Elastic and inelastic light scattering are nowadays widely used techniques for the characterization of fluids. In particular, these techniques have been extensively and successfully used with microemulsion systems to obtain information about droplet sizes. Surface light scattering is a less common technique but has been used with microemulsion interfaces, in particular to measure the ultralow interfacial tensions found in these systems. In this chapter we discuss these aspects, first recalling the theoretical background and illustrating the potential of the techniques with experimental results. 

It is well established that ultralow interfacial tension plays an important role in oil displacement processes [16,18]. The magnitude of interfacial tension can be affected by the surface concentration of surfactant, surface charge density, and solubilization of oil or brine. Experimentally, Shah et al. [23] demonstrated a direct correlation between interfacial tension and interfacial charge in various oil-water systems. Interfacial charge density is an important factor in lowering the interfacial tension. Figure 6 shows the interfacial tension and partition coefficient of surfactant as functions of salinity. The minimum interfacial tension occurs at the same salinity where the partition coefficient is near unity. The same correlation between interfacial tension and partition coefficient was observed by Baviere [24] for the paraffin oil-sodium alkylbenzene sulfonate-isopropyl alcohol-brine system. 

In order for a solvent to wet a substrate, its surface energy (surface tension) must be lower than that of the substrate. Since many common substrates such as polyester, polyimide, and polyacrylate all have surface energy values around 45 dynes cm", screen printing solvents are normally chosen to have surface tension values around 40 dynes cm . In this context, it should be noted that some common laboratory contaminants, such as silicone oils, have ultralow surface energy values (<22 dynes cm ), which means they must be excluded from screen printing apparatus. 

Figure 2 summarizes the effect of NaCl concentration, oil chain length and surfactant concentration on the interfacial tension in this system. It is evident that ultralow interfacial tension minimum occurs at a specific salinity or specific oil chain length or specific surfactant concentration. Using light scattering, osmotic pressure, surface tension, dye solubilization and various other techniques, we have shown that the ultralow inter facial tension minimum in this system correlates with the onset of micellization in the aqueous phase and the partition coefficient of surfactant near unity (14). Baviere (15) has also shown that the partition coefficient is near unity at the salinity where minimum interfacial tension occurs. 

Figure 6 shows the effect of surfactant concentration on interfacial tension and electrophoretic mobility of oil droplets (14). It is evident that the minimum in interfacial tension corresponds to a maximum in electrophoretic mobility and hence in zeta potential at the oil/brine interface. Similar to the electrocapillary effect observed in mercury/water systems, we believe that the high surface charge density at the oil/brine interface also contributes to lowering of the interfacial tension. This correlation was also observed for the effect of caustic concentration on the interfacial tension of several crude oils (Figure 7). Here also, the minimum interfacial tension and the maximum electrophoretic mobility occurred in the same range of caustic concentration (17). Similar correlation for the effect of salt concentration on the interfacial tension and electrophoretic mobility of a crude oil was also observed (18). Thus, we believe that surface charge density at the oil/brine interface is an important component of the ultralow interfacial tension.

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. 

The phase behavior of surfactants in water and hydrocarbon is the key to understanding the water- and oil-dissolving power of certain surfactant systems and the interfacial tension between the phases that form in these systems (1, 2). Ultralow tensions less than lOyN/m (0.01 dyn/cm) are required by one of the important mechanisms in various processes for enhancement of petroleum recovery. Much information is now in the literature (3 r4 r5 r6) t but most of the data are for commercial surfactants which are complex mixtures of surface-inactive as well as surface-active components (7 ). ... 

Several theories of surfactant phase are available. Following Scriven (1976), this phase is assumed to be bicontinuous in oil and water, and the interface is assumed to have zero mean curvature, hence the pressure difference between oil and water is zero. Talmon and Prager (1978, 1982) divided up the medium into random polyhedra. The flat walls ensure no pressure difference between oil and water. They placed oil and water randomly into the polyhedra so that both oil and water were continuous when sufficient amounts of both phases were present. As in the earlier models of oil-in-water microemulsions, this randomness gave rise to an increased entropy which overcame the increased surface energy to yield a negative free energy of formation, reached only when the interfadal tension is ultralow. Such structures can form spontaneously. This random structure is characterized by a length scale. This led Jouffrey et al. (1982) to postulate that... 

The above discussion has centered on wave motion imposed on a surface by, for instance, an oscillating bar. But thermal fluctuations cause wave motion of small amplitude even on interfaces that are not disturbed by external means. With laser light scattering techniques it is possible to measure interfadal tension from analysis of surface fluctuations. This method has been applied to the measurement of ultralow interfacial traisions between liquid phases (Bouchiat and Meunier, 1972 Cazabat et al., 1983 Zollweg et al., 1972). Presumably it could also be used to determine surface compressibility or other rheological properties. 

The interfacial tension can undergo significant changes if the polarity of the medium is altered, such as in the stability/coagulation transition caused by the addition of water to hydrophobic silica dispersions in propanol or ethanol [44,52,53]. Also, the addition of small additives of various surface-active substances can have a dramatic effect on the structure and properties of disperse systems and the conditions of transitions [14,16,17,26]. The formation and structure of stable micellar systems and various surfactant association colloids, such as microemulsion systems and liquid crystalline phases formed in various multicomponent water/hydrocarbon/surfactant/alcohol systems with varying compositions and temperatures, have been described in numerous publications [14-22,78,79,84-88]. These studies provide a detailed analysis of the phase equilibria under various conditions and cover all kinds of systems with all levels of disperse phase concentration. Special attention is devoted to the role of low and ultralow values of the surface energy at the interfaces. The author s first observations of areas of stable microheterogeneity in two-, three-, and four-component systems were documented in [66-68],... 

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