Brine-microemulsion interface

In contrast, the brine phase tended to form large pockets in the TRS system, causing extensive movement of the liquid crystal to the brine-microemulsion interface. This behavior is schematically illustrated in Figure 9. Not surprisingly, interface movements were inconsistent with diffusion path analysis. Figure 10 shows such a plot for the TRS/C12 system at 1.5 gm/dl salinity. The nonlinearity results from convection, which speeds up equilibration. Experimentally, the inconsistency with diffusion path theory was evident from the time-dependent appearance of the upper microemulsion interface, an indication of variable interface compositions. 

Another type of interfacial instability occurred in both systems whenever liquid crystal penetrated the brine to contact the brine-microemulsion interface. At high magnification (40x), rapid convection of liquid crystal particles to the interface was observed at volcano-like instabilities (Figure 11). Reported earlier for the same systems (4), this type of instability forms convection currents in the surrounding brine phase. After times ranging from a few seconds to a few hours, the instabilities choke-off." The mechanism by which this small-scale convection is initiated, maintained, and terminated is as yet unknown. 

Figure 8. Brine-microemulsion interface positions (bottom) and microemulsion-oil interface positions (top) for the PDM system at salinities below optimum.

An extended experiment was performed for the 2.0 gm/dl-salinity Pm system to determine the effect on relative interface velocities of the formation of myelinic figures and the C phase. With reference to Figure. 18, the formation of a uniform layer of C phase at t1 = 12 hr1 (6 days) corresponds to a decrease in relative velocity of the brine-microemulsion interface. The layer of C phase grew uniformly after its initial formation. In Figure 18, all positions are plotted relative to the same reference position. The offset of the liquid crystal interface at t = 0 indicates brine formation due to initial mixing. 

Figure 8 shows the microemulsion interface positions for the PDM system in this regime. Convection is not indicated. The dispersion front boundaries were very irregular in shape, and, therefore, those positions are not plotted. However, estimates of the relative dispersion front velocity in each experiment are given in the first two entries of Table III. As is evident from the small difference between these values and those for the brine interface, the brine layer grew very slowly at these salinities. 

As indicated by Figure 16, which shows the positions of the brine-oil interface for two PDM experiments as functions of t1, the oil phase grew in volume with time. This solubilization of brine into the oil contrasts with the behavior at lower salinities where the oil phase was consumed by the microemulsion. Based on equilibrium phase behavior, one can conclude that conversion of oil to a water-in-oil microemulsion was occurring above the brine interface. Also, as shown in Table IV, the position of the interface between brine and this oil-continuous phase varied as the square root of time, indicating no extensive convection in these samples. 

Huh,C. (1979) Interfacial tension and solubilizing ability ofa microemulsion phase that coexists with oil and brine. /. Colloid Interface Sci., 71,408-426. 

Figure 9 shows the effect of surfactant concentration on the volume of the middle phase microemulsion. It is interesting that the plot goes through the origin indicating that even at very low surfactant concentrations, a microscopic amount of the midddle phase microemulsion must exist at the interface between oil and brine. 

The identity of the intermediate phase formed at these conditions can be deduced from the relative movement of the interfaces. Because the phase grew quickly in the direction of the aqueous surfactant solution, it contained predominantly brine. Although small in quantity, some oil did diffuse into it. From this information and from its isotropic appearance, one can conclude that the intermediate phase was an oil-in-water microemulsion. Additional support for this conclusion is that this type of microemulsion is an equilibrium phase at low salinities. 

The increased relative velocities of the microemulsion-oil interfaces at these conditions (as evidenced in Table II) indicate that the microemulsion contained a significant amount of oil. As a result, the microemulsion was probably a middle phase. This conclusion is supported by the formation of the brine phase, which exists in equilibrium with this type of microemulsion at these conditions. 

At approximately optimum salinity, spontaneous emulsification of brine drops in the oil phase began in both systems. This phenomenon resulted from local supersaturation of the oil phase, as explained in the discussion section below. The amount of emulsification tended to increase with increasing salinity. As a result, the cloud of emulsion drops began to obscure the interface between the microemulsion and oil, making interface position measurements difficult. These observations of spontaneous emulsification confirm the results of the earlier contacting experiments performed in the horizontal configuration ( 4). 

The transition from the three-phase to two-phase region in the PDM system was marked by a sudden increase of spontaneous emulsification in the oil phase. Because formation of an intermediate microemulsion ceased at this point, the emulsion drops remained near the brine interface rather than rapidly moving away to form a single-phase region above the brine. An example of this behavior is shown in Figure 15 for the 2.1 gm/dl-salinity PDM system. 

The ratio of the relative diffusion constants of brine and oil in the microemulsion phase was found to be very important. Figure 19 shows the effect of varying this ratio. When oil diffusion is less than that of brine, as would be expected in an oil-in-water microemulsion, the fraction of liquid crystal at the dispersion front Increases over that in the bulk dispersion. This situation corresponds to diffusion path 1 in Figure 19. Essentially, brine diffuses out of the dispersion faster than oil can diffuse in, causing a decrease in overall brine concentration at the interface and hence the formation of additional liquid crystal, the phase having the lower brine content. As mentioned previously, this buildup of liquid crystal was observed experimentally. 

The effect of 750 ppm Xanthan gum on the microemulsion phase behavior is shown in Figure 5(b). The observed phase behavior is similar except that the extent of the three-phase region is widened. Thus at both 0.8 and 1.0 gm/dl salt concentrations there exists a polymer-containing brine phase in equilibrium with the microemulsion phase. When no polymer is present, the microemulsion phase is in equilibirum with only excess oil. The volumes of the polymer phases are small and the interface between the polymer phase and microemulsion is diffipult to detect in Figure 5(b). However, phase separation is clearly visible in Figure 5(c), which illustrates the oil-equilibrated phase behavior at a higher polymer concentration of 1500 ppm. 

Huh, C., Interfacial tensions and solubilizing ability of a microemulsion phase that coexists with oil and brine, J. Colloid Interface Set, 71, 408, 1979. 

Kunieda, H. and Shinoda, K. (1980) Solution behaviour and hydrophile-lipophile balance temperature in the Aerosol OT-isooctane-brine system-correlation between microemulsions and ultralow interfacial tensions. /. Colloid Interface Sci., 75, 601-606. 

Figure 7 Interfacial tension of the planar interface between the microemulsion and the excess phase as a ftinction of the salt concentration for systems composed of AOT (sodium diethylhexylsulfosuccinate), brine, and linear alkanes of varying chain length. Points are experimental data obtained for Cg (O, ), Cio ( , ), and C12 (O, ) linear alkanes making up the oil phase. Open symbols refer to Winsor II systems corresponding filled symbols indicate the Winsor III region where the theory is no longer valid. The lines were calculated according to Eqs. (60) and (61), the fixed parameters listed in Table 2, and suitably chosen values of K/kT — 0.8 (Cg systems), 0.39 (Cio systems), and 1.2 (C12 systems). (Experimental data from Ref 46.)...

There are two bulk interfaces in middle phase microemulsions and one in lower or upper phase microemulsions. Thus, one or three values of interfacial tension (IFT) may be measured depending on system composition (1) ymo between microemulsion and excess oil phase, (2) between microemulsion and excess brine phase, and (3) >Vm between excess oil and brine phases. Phase volumes and consequently the volumes of oil (Vo) and brine () solubilized in the microemulsion depend on the variables that control the phase behavior. The solubilization parameters are defined as Vg/Vs and V JV, where Vs is the volume of the surfactant in the microemulsion phase. These parameters are easily determined from phase volume measurements if all the surfactant is assumed to be in the microemulsion phase. The magnitude of decreases as Vg/Vs increases, i.e., as more oil is solubilized. Similarly, the magnitude of decreases as Vg/Vs increases. The salinity at which the values of ymo and are equal is known as the optimal salinity based on IFT. Similarly, the intersection of Vg/Vs and V. /Vs defines the optimal salinity based on phase behavior. The optimal salinity concept is very important for enhanced oil recovery. 

An oil/brine/surfactant/alcohol system often forms a middle phase microemulsion in an appropriate salinity range. The salinity at which the middle phase microemulsion contains an equal volume of oil and brine is defined as the optimal salinity (9). At the optimal salinity, the interfacial tension is in the millidynes/cm range at both oil/microemulsion and microemulsion/brine interfaces, and the oil recovery is maximum (6,9). Moreover, we have shown (10) that at optimal salinity, the coalescence time or phase-separation time is minimum for oil/brine/surfactant/alcohol systems. When these systems are pumped through porous media, a minimum pressure drop or apparent viscosity is observed at the optimal salinity (10). All these phenomena occurring at optimal salinity are summarized in Figure 11. In a recent study, we have also found that the surfactant loss in porous media is minimum at the optimal salinity. Therefore, besides ultralow interfacial tension, a favorable coalescence process for mobilized oil ganglia and the minimum apparent viscosity (or minimum AP) of the oil bank and the minimum surfactant loss are the other factors contributing towards the maximum oil recovery at the optimal salinity. 

In high surfactant concentration systems, a middle phase microemulsion forms in equilibrium with excess oil and brine in a given salinity range. The middle phase microemulsion contains equal volumes of oil and brine and practically all of the surfactant at a specific salinity defined as the optimal salinity of the given system. The interfacial tension of the two interfaces, middle phase/brine and middle phase/oil, depends on the extent of solubilization of oil and brine in the middle phase microemuIsion. The higher the solubilization of oil and brine in the middle phase, the lower is the interfacial tension at both these interfaces. We have... 

Table 1 shows the effect of the addition of isobutanol on various properties of oil/brine/surfactant systems for TRS 10-410 and TRS 10-80. Because the same IFT values were obtained for the systems with and without IBA (Table 1), the observed differences in oil recovery cannot be explained in terms of any change in IFT. The presence of alcohol did not significantly influence the partition coefficient of surfactant in n-dodecane or n-octane. It is important to emphasize that the partition coefficient changes sharply near the ultralow IFT region (19). Thus, the partition coefficient does not appear to correlate with the oil displacement efficiency. However, the presence of isobutanol decreases the interfacial viscosity and markedly influences the flattening time of the oil droplets. It has been suggested (18) that a rigid potassium oleate film at the oil/water interface can be liquefied by the penetration of the hexanol molecules in order to produce spherical microemulsion droplets. It has been shown (14) also that for a commercial petroleum sulfonate-crude oil system, the oil droplets with the alcohol coalesce much faster than the ones without alcohol. For the systems studied here, IBA is believed to have penetrated the petroleum sulfonate film as seen by the decrease in IFV. The decrease in interfacial viscosity would presumably promote the coalescence in porous media. 

The surfactant in Systems A and B was Witco TRS 10-80, while in System C, an Exxon C-12 orthoxylene sulfonate was used. The phase behavior exhibited by A and B was that observed with most oil-brine-petroleum sulfonate systems. At low salt concentrations, two phase systems are observed with the surfactant residing in the water phase. (Note that in Table 1 the phase that contains the highest concentration of surfactant is considered the microemulsion (ME) phase.) At intermediate salt concentrations, a three phase region is observed with the ME phase being the middle phase. The lower values of interfacial tension are observed with these three phase systems, with the lowest tensions, at least for A and B, being those associated with the ME-W interfaces. System C does not exhibit a three phase region, but still, a rather low tension is observed for the ME/W interface of the 1.3% NaCl case. 

Abstract We introduce a new technique using small-angle neutron scattering (SANS) to measure the average Gaussian curvature and the average square-mean curvature of the oil-water interface in a three-component, nearly isometric (equal volume fractions of water and oil) ionic microemulsion system. The microemulsion is composed of AOT/brine/decane. SANS measurements are made as a function of both the volume fraction of surfactant and salinity at a constant temperature,... 

The front of a microemulsion slug is approximately 25 microns from the back of the drop. The trailing interface of the nonane suddenly retracts and contacts the slug of microemulsion. This is probably due to a film of brine surrounding the front of the... 

---