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J . Phys. Chem. 1988, 92, 3492-3500
broadening of micellar peaks and a quantitative analysis of the aggregation properties. Our GFC data do not provide exact aggregation numbers of the small micelle and the large micelle, since no appropriate molecular standard compound was found for the present system. Furthermore, it is still uncertain whether the large micelle is formed by coagulation or coalescence of small micelles and whether much larger micelles are formed at much higher concentrations than at those in this work. Applications of the GFC Method to Molecular Aggregation Systems. The main advantage of GFC is based on separation of species different in size, even for chemically reacting species. GFC has been established as a tool for determining the aggregation number and equilibrium constant of self-associating proteins.31 For instance, this method has applied to hexamerization of chym o t r y p ~ i ndimerization , ~ ~ ~ ~ ~ of carbo~yhemoglobin,~~ and subunit aggregation of hemoglobid3 and provided results close to those from other method^.^' GFC was also used for ionic surfactant^^^*^^ and nonionic surfactants26,36and provided quantitative information (31) Ackers, G. K. Adu. Protein Chem. 1970, 24, 343. (32) Winzor, D. J.; Scheraga, H. A. Biochemistry 1963, 2, 1263. (33) Valdes, R., Jr.; Ackers, G. K. J . Biol. Chem. 1977, 252, 74. (34) Suzuki, H.; Sasaki, T. Bull. Chem. SOC.Jpn. 1971, 44, 2630. (35) Herris, D. G.; Bishop, W.; Richards, F. M. J . Phys. Chem. 1964,68, 1842.
about the solute partition between water and micelle^,^^^^^ mixed micelle formation,26and micellar size.IsJ6 In GFC, it is essential to select the best experimental conditions, e.g., kinds of gel and flow rate. Such a study is under way for a surfactant in detaiLZ4 A too high flow rate might fragment a huge micelle into small micelles and cause the nonequilibrium of secondary micellar aggregation. With these possibilities in mind, we chose Sephadex G-200, the softest Sephadex gel, in this work and consequently carried out the experiments at a very low flow rate. In regard to GFC data interpretation, we encountered difficulties involved in the estimation of absolute micellar weights of DE6 from the elution volumes, since appropriate standards for micellar weights are not available. Therefore, we failed to simulate elution patterns based on the stepwise aggregation model in general form. However, it is again noted that the broadening of micellar peaks (Figure 2) at intermediate concentrations is inexplicable on the basis of the stepwise aggregation model. Acknowledgment. This work was supported by a grant-in-aid from the Scientific Research Foundation of Kyoto Pharmaceutical University. Registry No. DE6, 3055-96-7 (36) Goto, A,; Nihei, M.; Endo, F. J . Phys. Chem. 1980, 84, 2406.
Phase Behavior in a Model Surfactant/Alcohol/Oil/Brine System over Wide Ranges of Conditions Jong-Duk Kimt and John P. O’Connell* Department of Chemical Engineering and Center for Surface Science and Engineering, University of Florida, Gainesville, Florida 3261 1 (Receioed: May 20, 1987; In Final Form: January 20, 1988)
The phase behavior of the sodium stearate/isobutyl alcohol/n-hexadecane/NaCl brine system was studied at unit water/oil volume ratio over all ranges of surfactant/alcohol/salt compositions and from room temperature and pressure up to 50 OC and 70 bar. Though turbidity and viscous phases were found at some conditions, from one to four isotropic phases were found. Sensitivity to state conditions of properties and phase amounts existed, and behavior characteristic of multiple critical regions was observed. The complex phase behavior and properties such as density, electrical resistivity, and interfacial tensions are illustrated. Application of critical scaling relationships yields linear correlations of interfacial tensions, which may be of practical use for systems with ultralow values.
Introduction Mixtures with surfactant, alcohol, oil, and brine show a richness of phase behavior that includes several types of “liquids” and liquid crystal structures.14 The great interest in these phenomena arises from their similarity to elements of biological systems and applications to detergency and surfactant-enhanced oil recovery. In the last, the existence of the “middle phase microemulsion” is of significance because there is a direct connection between the desired ultralow interfacial tension (IFT) and the volumes of the one to four phases that can c ~ e x i s t . ~ - ~ This work describes an investigation of the phase behavior and properties of sodium stearate/isobutyl alcohol/n-hexadecane/NaCl brine mixtures over ranges of surfactant/alcohol/slt compositions, temperature, and pressure. This model system was chosen because it gives the same “salinity scans” as EOR petroleum sulfonates and low interfacial tensions but apparently does not show their order-of-mixing dependence. We find the dominant feature of this system to be several critical lines with varying sensitivity to ‘Present address: Department of Chemical Engineering, Korean Advanced Institute of Science and Technology, Seoul, Korea.
0022-3654/88/2092-3492$01.50/0
thermodynamic state. After giving a brief description of the experimental methods, we describe the phase behavior and variations of properties such as density, electrical resistivity, and interfacial tension. Finally, the application of critical scaling relationships to the properties will be illustrated. (1) Bartolino, R.; Meuti, M.; Chidichimo, G.; Ranieri, G. A. In Physics of Amphiphiles; Micelles, Vesicles and Microemulsions; DeGiorgio, V., Corti, M., Eds.; North Holland: Amsterdam, 1985, p 524. (2) Bellocq, A. M.; Bias, J.; Clin, B.; Gelot, A,; LaLanne, P.; Lemanceua, B. J . Colloid Interface Sci. 1980, 74, 3 1 1. (3) Bothorel, P.In Physics of Amphiphiles: Micelles, Vesicles and Microemulsions; DeGiorgio, V., Corti, M., Eds.; North Holland: Amsterdam, 1985; p 702. (4) Dominguez, J. G.; Willhite, G. D.; Green, D. W. In Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Academic: New York, 1979; p 673. ( 5 ) Fleming, P. D.; Vinatieri, J. E.; J . Chem. Phys. 1977, 66, 3147. (6) Healy, R. N.; Reed, R. L.; Stenmark, D. G. SOC.Per. Eng. J . 1976, 16, 147. (7) Kahlweit, M. E.; Kessner; Strey, R. J . Phys. Chem. 1983, 87, 5032; 1984.88, 1937. (8) Kahlweit, M.; Strey, R.; Haase, D. J . Phju. Chem. 1985. 89, 183 (9) Winsor, P. Chem. Rev. 1968, 68, 1 .
0 1988 American Chemical Society
Model Surfactant/Alcohol/Oil/BrineSystem
The Journal of Physical Chemistry, Vol. 92. No. 12. 1988 3493
Materials and Methods The systems were studied a t unit water/oil volume ratio while varying the surfactant, alcohol, and salt amounts at various temperatures and pressures. n-Hexadecane (Chemical Sample Co. or Wiley Organics), isobutyl alcohol (Mallinckrodt. Inc.), and sodium stearate (ICN Pharmaceuticals, Inc.) were used as purchased. Brines were formulated by adding pure NaCl (Baker Chemical) to doubly distilled water. Each brine composition was made by dilution of an 80 g/L stock solution, although a few were individually formulated. Except for the surfactant. which was weighed when added, the chemicals were added volumetrically. We estimate these overall compositions (no phase compositions were measured) to be accurate to 0.005 wt % NaCI. 0.5 vol % isobutyl alcohol (except when less than 10% where the accuracy was 0.05%) and 0.0002 g/mL sodium stearate. The mixtires were placed in 15 mL centrifugal t u b a with scmv caps sealed with Teflon tape and thoroughly shaken by hand until no solid phase could be seen. For the samples studied at ambient pressure, the tubes were kept at room temperature (22 f 3 "C) or in ovens a t 34 and 45 "C (fl "C). Observations in ambient light were made a t 24 h and 2 weeks to see the phase volumes, colors, turbidity, and p m m v of precipitates. Reexamination was also made after more than a year of storage. Except for conditions where one or more of the phases scattered blue light or where significant amounts of white solid were found at the interface(s), there were no observable changes with time and no dependence on mixing protocol. At the present time. after over 6 years, the behavior of a sample kept in a centrifuge tube in light is substantially the same as one of the same composition kept in a =led glass tube in darkness. Further, they appear essentially the same as when aged for only a few days. Thus, evaporation and degradation, if present. do not appear to have affected the behavior of this particular set of substances. For ambient-pressure observations at higher temperatures, the samples were examined a t room temperature to ensure consistency with ambient samples, reshaken, and placed in the Ovens for 1 day, reshaken. and observed again after several days for final recording. When these samples were returned to room temperature, they quickly changed to the same behavior as samples that had always remained a t room temperature. However, the samples with slight turbidity often t w k several months to stabilize. This included some of the surfactant-free systems very close to their observed critical point. Variations of pH by using HCI were also made; no significant effect was found within 2 pH units of neutral. Samples of the middle and lower phases used for density. resistivity, and IFT measurements were withdrawn from the multiphase systems by using an 8-in. needle attached to a syringe. A small bubble of air was kept at the tip of the needle as it was moved through the upper phases. More than one sample was drawn in each state to ensure that adsorption on the surface of the needle and syringe could be ignored in the final sample. Usually, these samples were different from the ones used to observe the phase diagram behavior because each type of measurement required up to 5 mL. However, the results should still be r e p resentative. Since equilibration time was short for essentially all of the samples, the measurements of density and electrical resistivity were made on the same day as initial separation was observed. Density measurements were made with a pycnometer of about 2 mL and calibrated with water. The densities were probably aoxrate to 0.002g/mL. Electrical resistivity was measured with a Beckman Model R C l6B2 50-60 H z conductivity bridge with a probe having a cell constant of 0.1 and shielded by ABS resin. Pure isobutyl alcohol and the doubly distilled water were used as references for solution measurements. As mentioned above, some samples showed aging and mixing effects. In such cases, only data from aged samples were accepted. Accuracies ranged from 5% in the low-conductivity middle and upper phases to 10% in the high-conductivity lower phases. Interfacial tensions were measured with the spinning drop interfacial tensiometer developed at the University of Texas. The less dense liquid was injected into the more dense liquid, and the
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measurements. analysis was done in the usual way.'O Equilibrium in the tensiometer was found in a few minutes for well-aged samples. If this time had k e n insufticient for equilibrium, the values would probably be high. At very low interfacial tensions. we found that the temperature inhomogeneities of the tensiometer occasionally caused continuous deformation of the droplet out of the spinning tube. Breakup of large drops into small drops was also seen, probably due to mechanical vibration. No data could be obtained in either of these cases, effectively making 0.0005 dyn/cm the lower bound of IFT. For samples measured at elevated pressures, the apparatus shown in Figure 1 was used. The sample was confined by mercury within a closed precision-bore glass tube of 0.28-cm id.. 0.7-cm 0.d. and 15-cm length. Samples held with mercury a t elevated temperatures up to 60 'C showed changes of clarity and phase volume only after 6 months, so the effect of the confining fluid can be ignored for these short-time measurements. Pressure was changed rapidly by an oil-filled intensifier and was measured on a Bourdon tube gauge accurate to 0.1%. Temperature was kept to within 0.1 OC by manostat control of the pressure over a boiling liquid and measured by a Hewlett-Packard quartz crystal thermometer. Locations of the interfaces between the phases were observed by a cathetometer that could detect changes of 0.01 mm. Stirring was accomplished by a Teflon-coated stirring bar moved by magnets outside the temperature jacket. Samples for these measurements were prepared and stored in the same way as the others. The samples were placed in the tube by inverting it, freezing the sample with liquid nitrogen or acetone and dry ice, and then connecting the upright tube to the base. While melted samples initially showed different behavior from unfrozen samples, after stirring. no difference was seen between frozen and never-frozen samples. This confirms the ease with which this system achieves equilibrium. After equilibration a t the desired temperature, the phase volumes were recorded. The pressure was rapidly changed, the sample was stirred, and, after settling, the phase volumes were recorded. Finally, the pressure was decreased to ambient, stirring was repeated, and final phase volumes were recorded. All measurements showed excellent reproducibility and no hysteresis. Since in many cases one or more of the phases became turbid in the transient period, clarity upon changing pressure was recorded. In drawing the full phase diagrams from individual samples, we have assumed continuity of phase boundaries a s determined by eye. Thus, while the results are not fully quantitative, we feel they are adequate for making significant general conclusions about trends and relative boundaries. We report results only where the observed amount of solid or liquid crystal was small. In the case of critical point observations, we could not maintain adequate temperature control to be precise about the state. However, there (IO) Cayiaa. J. L.;Sehster. R.S.; Wade. W. H.ACSSymp. Ser. 1975. No.8. 234.
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TABLE I: Nomenclature for Phases and Phase Regimes" conditions componentsb symbol low alcohol, low surfactant oil (D), alcohol 0 high alcohol, low surfactant alcohol (D), oil A brine high salinity B
other phases and criticals
with with with in low alcohol, high surfactant with oil (D), alcohol, surfactant low alcohol, high surfactant. with alcohol (D), oil, surfactant 0 high salinity with brine (D), alcohol, surfactant low salinity, low surfactant brine (D), alcohol, surfactant low salinity, high surfactant with
O* A*
B* W*
A* and B in region I; upper/middle criticals with A (2) and A* B and 0 in region I; two upper/middle criticals with 0 0, A, or A* in two-phase, with A and 0 in region I, with A * and 0 region 11; lower/middle critical with W* B in two-phase; upper/middle critical with A* B and 0 in region 11, with B in two-phase; upper/middle critical with
0 in two-phase, with A and 0 in region I 0 in two-phase, with B and 0 in region 11; lower/middle critical with
B (D) indicates dominant component.
"An asterisk (*) indicates microstructured phase.
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were diffuse interfaces, light dispersion, and asymptotic approach of density variables such as specific gravity and resistivity, which were consistent in indicating nearness to critical points and lines.
Phase Equilibrium Results Salinity Scans. The system conditions were chosen to examine the phase volumes as a function of salinity at fixed water/oil ratio. We first found the set of states with the most prevalent EOR variation with increasing salinity: a surfactant lower phase in equilibrium with an oil upper phase becomes a three-phase system having a surfactant middle phase, which then becomes a two-phase system with an upper surfactant phase in equilibrium with a lower brine phase. Then the salinity scans for wide ranges of alcohol and surfactant were taken. Figure 2 shows the results at ambient temperature (22 f 1 "Cj presented in columns for various overall surfactant amounts (0, 0.02 and 0.035 g/mL) and in rows for overall various alcohol amounts (1 5 , 20, 40 vol %). The actual samples were made at even integer weight percent NaCI, except close to rapid changes in phase volumes, and the lines indicating the interfacial locations are hand-drawn. The dashed lines show t h e locations t h a t would be f o u n d if no dissolution of t h e liquid components occurred and the surfactant and salt volumes were ignored. The differences between the full and dashed lines may be indicative of the relative solubilities, but we did not measure compositions of any of the phases. The left column shows that at low salinities in the absence of surfactant, the isobutyl alcohol is mostly in the oil phase at low alcohol content but with more alcohol forms a third phase. This is consistent with the observations of Kahlweit et al.'.' and Knickerbocker et al.11s12However, at high salinity, the system (11) Knickerbocker, B . M.; Pesheck, C. V.; Scriven, L . E.; Davis, H. T. J . Phys. Chem. 1979,83, 1984. (12) Knickerbocker. B. M.: Pesheck, C .V.; Davis, H. T.: Scriven. L. E. J . Phys Chem. 1982, 86. 393.
0
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Figure 3. Phase behavior in surfactant-free systems a t various overall alcohol volume fractions and salinities (water/oil volume ratio unity).
becomes two-phase either by disappearance of the middle phase (20% IBA) or by disappearance of the upper phase (40% IBAj. This shows that there is a critical point between the middle and upper phases. From observation of the interface disappearance and decrease of the interfacial tension, we found the critical salinity to be 8.2 f 0.2 wt 7% NaCl and 30 f 2 vol % IBA at unit water/oil volume ratio and 22 O C . The sensitivity of this point to composition and temperature make the values subject to uncertainty. Only a trace of interfacial turbidity was seen with small temperature changes in the vicinity of its disappearance, and a slight hysteresis (f0.5 "C) was seen between lowering and raising the temperature. Our nomenclature of the diagrams indicates molecular solutions of phases dominated by oil (0),alcohol (A), and brine (Bj. Table I shows the full set of identifications we made for the phases and phase regimes. Figure 3 shows some more detail of the number of phases in surfactant-free systems at various alcohol amounts and salinities as well as their sensitivity to temperature. Also noted by a dashed line is the salinity where the interface of two-phase systems showed a dramatic change of location as the alcohol is apparently salted out over a small range of salt concentration. Increasing the pressure to about 70 bar at 20 vol % IBA made the middle phase persist to higher salinities and temperatures by decreasing both t h e upper and lower phase volumes. The middle column of Figure 2 shows the influence of some added surfactant. At low alcohol content, salt makes the middle phase appear and then disappear while the higher alcohol systems show nearly the same behavior as the surfactant-free systems. The added surfactant moves the upper/middle critical to lower alcohol and higher salinity with no significant increase in its turbidity. The effect of surfactant is best seen along the top row of Figure 2. A t low surfactant, increasing salinity makes the middle phase appear a n d then disappear. At higher surfactant, the salinity
Model Surfactant/Alcohol/Oil/Brine System
The Journal of Physical Chemistry. Vol. 92. No. 12. 1988 3495 >.or
0
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makes the lower phase a p p a r and then the upper phase disappear. This implies that there is also a t least one critical line between the lower and the middle phases when sufficient surfactant is present a t low salinity. When we made samples close to this state, there was a considerable amount of turbidity in one or both of the lower and middle phases over a wide range of composition. In daylight the scattered light showed a variation of color with salinity, indicating a variation of the size of the scattering sites (from blue to red to white and back again). The prevalent EOR salinity scan appeared only a t high surfactant and low to intermediate alcohol amounts. Finally, it should be noted that the low-salinity behavior is more annplex than the high-salinity curves, implying that the lower/middle critical line is more complicated than the upper/middle one is. Some of this was observed by Bennett et aI.l3 Our nomenclature for the phases (Table 1) with surfactant involves the use of a n asterisk for cases where we think there is a microstructure with extensive internal interface. None of the phases showed optical anisotropy by rotating polarized light. so these dispersions were assumed to be spherical microemulsions. We m u m e that in the presence of surfactant there are low-salinity, low- and high-surfactant O / W microemulsions, B* and W*. a high-surfactant microstructured, alcohol-rich middle phase, A*, and a W / O microemulsion, 0 '. at high salinities. Such distinctions are important because the phases that become the same a t a critical point must not only attain the same composition and density but also reach the same structure. Phases with different microstructure may show different critical behavior than those where the microstructures are the same. The effects of temperature and pressure depended on all the concentrations, i.e., whether the system was near the lowcr/middle or the upper/middle critical. Near the latter. increased temperature and/or pressure stabilized three-phase systems as in the surfactant-free case. Similar temperature behavior was found by Kahlweit et al.' and Bennett et al." Near the low-salinity lowerlmiddle critical. temperature and pressure destabilized the lower phase, making the three-phase system become two-phase. The effect was significant, considering that the pressure was changed by only 70 bar. Also, it should be noted that when the pressure was changed, turbidity in one or both of the phases near their critical would often appear immediately. It then would disappear over time. The pattern of which phases and whether the turbidity appeared with an increase or decrease of pressure depended on the composition, but it was completely reproducible as had been found with EOR sulfonate systems." Some turbidity upon pressurization was also observed by Rossen and Kohn,l6 though they did not study it in depth. (13) Bennett, K. E.;Phelp. C. H.K.:Davis. H.T.:kriven. L. E.SPEJ, Soc. Per. Eng. 3. 1981. 21. 741. (14) knnctt. K.E.;Davis. H.T.;Serivcn. L. E.3. Phys. Chcm. 1982.86, 3917.
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Figure 5. Phase behavior at 6 wt % NaCl for various overall alcohol volume fractions and surfactant concentrations.
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overall alcohol
Phase Diagrams. Although there are a wide variety of ways
to present data on multicomponent sy~tems,f'.~*'~since we did not measure phase compositions to obtain tie lines and the systems had fixed overall waterloil ratio, we give a simpler representation. Figures 4-8 present the number and names of the phases observed as a function of overall alcohol volume percent and surfactant (IS) Kim, J.-D.FkD. DiMeRation. University of Florida. 1982. (16) Rasscn, W. R.; Kahn. J. P. SPEJ, Soe. Per. Eng. 3. 1%. 24,536. (17) Fleming, P.D.; Vinatimi, J. E. Am. Imr. Chem. Ens, J.1979,25,493. (18) nemin&P.D.:Vinatirri,J.E. 3. ColloidlnrcrfoceSci. 1981.81.319. (19) Francis. A. W. Liqvid.Liquid Equilibn'um; Wiley-lntcrwicncc: New York. 1963.
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The Journal of Physical Chemistry. Vol. 92. No. 12. 1988
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concentration at several salinities figure 4 is at 7% salinity where the boundaries of a variety of one-, two-, and three-phase regions are estimated from the samples indicated by the circles. In particular. there arc two three-phase regions of dramatically different shape. separated by a two-phase region of B* and 0 at low surfactant and Band A' at higher surfactant amounts. The two three-phase regions are identified as I and 11, respectively; Table I shows our phase regime notation. The higher alcohol three-phase q o n has an upper (O)/middle (A) critical near 0.024 g/mL surfactant and 23 vol % IBA. The existence of the surfactant-free upper (O)/middle (A) critical is evident from the narrowing of region I near the zero-surfactant axis. The lower alcohol threaphase region shows a critical point of 0 and A* near 0.026 g/mL surfactant and 13 vol % IBA. However, though the criticals were between the upper and middle phases. there was significant turbidity in the region of the one at lower alcohol content. We attribute this to the lower three-phase region having another critical point between aqueous phases that cannot be distinctly seen at this salinity and room temperature because of the intervention of solid and liquid crystal phases (though it appears at higher temperatures). The intersection of these two lower alcohol criticals causes considerable complexity in the behavior at lower salinities. Figure 5 shows the phases present at an overall salinity of 6 wt % NaCI. Here there is only a single, apparently continuous three-phase region. However, the distinctive shapes of the hih-alcohol and high-surfactant portions of Figure 4 remain since the only major movement is the lowering the bottom of region I to where it overlaps region 11. We would expect the overlap region to be complex since it involves the disappearance of the two criticals seen in Figure 4. Further, all of the compositions in this area are likely to be in the 'neighborhood" of all three criticals mentioned above. However, other than some regions of diffuse turbidity that remained over many months in both centrifuge tube and sealed tube samples, there were no visual anomalies or interfaces in this region. Increased temperature at the compositions of Figure 5 causes region I to decrease in size. pulling away from the zero-surfactant axis at about 28 "C and Separating from region I1 by 32 'C. Thus, in addition to the reappearance of the two criticals of Figure 4, the upper/middle critical of the surfactant-free system also reappears. The effect of increased pressure at low alcohol (15%) and surfactant (0.02 g/mL) contenu is to stabilize the three-phase region, while at 20% IBA it has minimal effect. Increasing the pressure initially causes turbidity in the upper and middle phases of this region (I), the same behavior as was followed in the surfactant-free case. Unfortunately. measurements were not made at higher surfactant to see if there was a transition to region 11 behavior. Figures 6 and 7 show the phases present at lower salinities (4 and 2 wt % NaCI. respectively). The three-phase region evolves to one with an upper boundary that decreases less dramatically
Kim and O'Connell with surfactant. Further, the critical point between the lower and middle phases appears as the lower boundary at ambient temperature emerges from the region of solids and liquid crystals. Here, these phases appear to be B and W', though at higher salinities at ambient temperature, as in Figure 4, and at higher temperatures. B and A' would have been involved. Thus, as salinity is decreased at room temperature, the critical lines of region 11 apparently intersect. This also confirms that there are four critical lines in this system. three between the upper and middle phases mentioned above and one between the lower and middle phasa. This is more complex than the systems investigated by Kahlweit et al,7.8 The effect of temperature at lower salinities is similar to that at higher salt amounts. Increased temperature separates regions 1 and I1 generally by requiring more alcohol lo obtain the middle phase of 1. Increased pressure increaxs the stability of the middle phase at 20% IBA as in region 1 at higher salinities. However, at lower alcohol amounu, increased pressure eliminates the lower phase, making the system behave like region 11. Consistent with this, the appearance of turbidity with changes in pressure occurs only in the phases near their critical. Thus, there is a distinct behavior observed for the two regions even at salinities well below the amount where they apparently merge. Finally, figure 8 shows the same diagram at I wt % NaCI. The lower boundary of the three-phase region is not only at its highest, a four-phase region appears between about 20 vol 96 IBA and 0.02 g/mL surhctant and nearly 40 vol % IBA and 0.07 g/mL surfactant. The phase of next highest density showed persistent turbidity even in sealed tubes kept for several years. This confirms that the lower/middle phase behavior is complicated by the intersection of the critical lines between B and W* and B and A*, leading lo the appearance of all three along with 0. It may also explain the greater turbidity over a larger range because the possibility of forming several different structures could lead to longer correlation lengths as implied in the dynamic light scattering of Kahlweit et al.? the ultrasonic absorption work of Lang et al.,m and the dielectric study of Senatra and Giubilaro?I It may also be that the micmtructure is more like that described by Wid0m.u where there are aqueous and oleic domains that have larger fluctuations of compcsition than those for the upperJmiddle case. Phase Properties and Interfacial Tension Measurements
While no compositions of the phases were determined, density and electrical resistivity data were taken to gain insight into relative structures, while interfacial tensions were examined to see if ultralow values could be obtained. For the surfactant-free systems, extrapolation of the densities and resistivities to single-phase values confirmed thosc of the phase volumes to indicate the critical salinity and alcohol amount. As Figure 9 shows, densities at 30 vol % isobutyl alcohol with varying salinity showed that in general the middle phase has properties with some aqueous character while the brine appeared to have some alcohol. In measurements made just above the critical salinity of 8.2 wt % NaCl (soonly two phases were observed under all conditions), initial amounts of alcohol added to the oil plus brine phases lowered the density of the latter though further additions caused it to rise nearly to the pure brine value. At the same time, the upper (oil/alcohol) phase density increased from the pure oil value to nearly that of the pure alcohol. These results imply that salting-out of alcohol occurs only when the system contains enough alcohol. The resistivity of the upper phase decreased to below that of pure IBA a l high alcohol amounts. indicating the presence of some water in it. Slightly above the critical alcohol and salinity, we found an order of magnitude difference in resistivity between the top and bottom on the upper phase, indicating gravity stratification in this near-critical phase. As salinity increased at 30 vol % IBA (Figure IO). the lower interfacial tensions increased continuously from 5 to 10 dyn/cm (20) Law. J.: Djavanbkht. A.: 7 "R.1. Phys. Chrm. 1)80.84.1541. (21) Senatra. D.:Giubilaro. G. J. Colloid Inrefme Sci. 1978.67, 448. (22) Widom. B. J . Chem. P h p . 1984.81. 1030.
The Journal of Physical Chemistry, Vol. 92, No. 12, I988 3497
Model Surfactant/Alcohol/Oil/BrineSystem I
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Figure 11. Interfacial tensions for various salinities at 20 vol % isobutyl alcohol and 0.02 g/cm3 sodium stearate and unit water/oil volume ratio.
is lowered (Figures 4 and 5). On the other hand, the values of the low-salinity middle-phase densities were greater and resistivities lower than with higher salinities, indicating more region I1 behavior and consistent with the higher alcohol amounts at the lower boundary of the three-phase region under these conditions (Figure 5-8). All of this is indicative of the solution complexity in the neighbor& of several critical lines. Unfortunately, the amounts of the middle phases of the four-phase region were too small for us to measure their properties. Interfacial tensions in three-phase systems with surfactant showed ultralow extremes for the interface between the phases close to their critical but relatively little change for the other interface. At high salinity and high alcohol, the upper interface shows an extreme; Figure 10 shows it with varying surfactant at 6 wt % NaCl and 15 vol % IBA. At higher alcohol amounts the upper phase (0)disappears at sufficient surfactant amounts. The 1-2 wt % salinity scan for IFT (Figure 11) and fixed (20 vol W ) alcohol and high (0.02 g/mL) surfactant shows a minimum for the lower interface but slow variation for the upper interface. In the process of passing close to the critical, the surfactant moves from a lower phase B*, leaving it as B, and moves to the middle phase A, making it W*. Finally, a large sample was made to determine properties, near the four-phase region. Two liters of 1.3 wt W NaC1, 20 vol % IBA, and 0.025 g/mL sodium stearate in equal volumes of n-hexadecane and water was allowed to equilibrate in a 40-cm glass cylinder for 2 months at 23 f 1 OC after thorough mixing. The solution formed 40 mL of a clear upper phase divided by a sharp interface from 60 mL of a lower phase that contained three regions only diffusely divided. There were about 30 clear mL at the bottom, 20 slightly cloudy mL on top of that, and 10 clear mL just below the well-defined interface with the upper phase. With better temperature control (*0.05 "C), the lower two interfaces were more distinct, though only one of the phases was turbid. Densities were measured on samples taken every 2 mL from the bottom at 23 "C. They varied gradually from the lowest 20 mL to the sharp interface. Interestingly, I F I (against the upper phase) and resistivity measurements made at a number of the elevations of the sample showed no significant variation with elevation. A slight increase in temperature made the turbidity appear over a greater range of height but did not change its intensity. These results, while of insufficient precision to determine quantitative critical behavior, indicate that subtle variations can occur in these complex systems.
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Figure 10. Interfacial tensions for various overall surfactant concentrations at 6 wt % NaCl and three isobutyl alcohol volume fractions.
while the upper IFT decreased continuously from 0.5 to less than 0.01 dyn/cm as the upper phase disappeared. Because of poor temperature control and other difficulties mentioned above, the measurements had large uncertainties below the latter value. In the presence of surfactant, middle-phase density and resistivity were generally between the upper- and lower-phase values. However, as surfactant was added at low salinities, variations of these middle-phase properties were found over much greater surfactant amounts than at high salinities, where the change was rapid up to 0.01 g/mL surfactant but then much more gradual. This was particularly noticeable at higher alcohol amounts. This behavior is consistent with the greater extent of region I as salinity
1.8
WEIGHT % NaCl
3498
The Journal of Physical Chemistry, Vol. 92, No. 12, 1988
Kim and O’Connell 10’’ r
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Figure 13. Scaling plot of upper interfacial tension versus salinity for surfactant-free system at 30 vol % ’ isobutyl alcohol.
1 1 1 1 1 1
0.1
1.o
UPPERIMIDDLE DENSITY DITFERENCE, lp‘-pyl Kg-Li‘
Figure 12. Scaling plot of upper interfacial tension versus the phase density difference for surfactant-free system at 30 vol % isobutyl alcohol.
Analysis of Phase Properties from Critical Scaling Relations The presence of critical lines in this system suggests that the properties might be correlated according to modern critical scaling l a ~ s . ~Such ~ , ideas ~ ~ have been discussed by Fleming and Vinatieri”.’* and Huang and Kim.25 Here we are interested in variations with composition of such properties as density, IFT, and electrical resistivity. Thermodynamic “density” variables are different in two equilibrium phases and become the same at the critical. These include specific gravity and electrical resistivity and are expected to include composition measures such as mole fraction or concentration. There are also “field” variables such as temperature, pressure, and component chemical potential, which are the same in both phases. Of special interest is the interfacial tension, which goes to zero at the critical. In the critical region there are universal connections between the variables, regardless of the type of system. While a large number of possibilities for correlation of our data exist,I5 we focus here on the relations between IFT and resistivity and measured densities and salinities. Thus, in surfactant-free systems we explore the application of the following proportionalities: Figure 14. Lower interfacial tension versus the phase density difference at 1 wt % NaCl and various overall isobutyl alcohol volume fractions.
where the difference of densities in phases I and 11, p1 and pI1, the resistivity of a phase, r, relative to the value at the critical, rc, and the IFT, o, vary with the salinity, S , relative to that at the critical point, S,. Here we assume that the overall salinity is an appropriate measure of the phase concentration since it is associated with the water which dominates the lower phase. The path to the critical is along fixed temperature, pressure, and alcohol amount. This is chosen because the middle- and upper-phase densities approach each in the same way that phase densities of (23) Ma, S.-K. Modern Theory of Critical Phenomena; W . A. Benjamin: New York, 1976. (24) Rowlinson, J. S.; Swinton, F. L. Liquids and Liquid Mixtures, 3rd ed.; Butterworths: London, 1982. (25) Huang, J. S.; Kim, M. W. In Physics of Amphiphiles: Micelles, Vesicles and Microemulsions; DeGiorgio, V., Corti, M., Eds.; North Holland: Amsterdam, 1985; p 864.
a pure component approach each other along the critical isochore (Figure 9). Under these conditions, depending upon whether salinity is a field or a density variable, the values of the exponents would bez6either the set C N 0.35, C‘ N 0.65, and C” N 1.3 or the set C N 1.0, C’ N 1.6, and C”- 3.7 or 4.0 (the last depends upon whether the critical involves two or three phases simultaneously.) To test whether the IFT and resistivity measurements were consistent with changes in density variables, we plotted our IFT data for the surfactant-free system versus the measured phasedensity difference. Figure 12 shows the IFT versus the difference in the density of the middle and upper phases. This slope is clearly closer to 3.7 than to 1.3. We next determined the effect of salinity on the upper/middle IFT. While our data are subject to some uncertainty, the results all indicate that salinity is a field variable (26) Widom, B. Chem. Soc. Reu. 1985, 14, 121.
Model Surfactant/Alcohol/Oil/Brine System 100
The Journal of Physical Chemistry, Vol. 92, No. 12, 1988 3499
7
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SURFACTANTiALCOHOL RATIO DIFFERENCE, I(S/A), S/Al
-
EFFECTIVE SURFACTAN5 DIFFERENCE, IS,-SI Kg-Li
Figure 15. Lower interfacial tension versus effective surfactant concentration in surfactant-rich phase (A* or W*) for various alcohol volume fractions at 1 wt % NaCI.
for the upper/middle critical in surfactant-free systems. For example, Figure 13 shows the scaling plot variation of the IFT with salinity for the surfactant-free system; the slope C”is definitely more consistent with 1.3 than with 3.7. We interpret this result to mean that the effect of adding salt is not directly on the upper and middle phases according to its concentration but indirectly through its chemical potential. This is consistent with the idea that most of the salt remains in the lower phase, which is not adjacent to the interface of interest. For systems with surfactant, the variations with salinity and density follow in a similar way. Thus, Figure 14 shows a scaling plot for IFT versus density differences of the lower and middle phases at low salinity, where the IFT is low because of the lower/middle critical. Except for the highest IFT at the highest alcohol amount, all the data for a variety of alcohol contents are consistent with a density variable slope of 3.7. However, finding a proper path to the critical to find the effect of salinity alone was not successful. Thus, it is likely that in the presence of surfactant the salt moves to at least the middle phase. If phase concentration measurements had been made, such a plot should have shown phase salinity to be a density variable for this case. Construction of a scaling plot involving overall surfactant and alcohol amounts was complex because these components are not confined to a single phase. We can choose an effective measure of surfactant for the lower/middle IFT as an estimate of the surfactant concentration in the “surfactant” phase (W* or A*), S. While the critical was not actually observed, the resistivity, which changes rapidly in the same range of surfactant as does the IFT, provided a value of the effective critical surfactant concentration, S,, leading to the IFT/surfactant scaling plot of Figure 15 for several overall alcohol amounts. It is possible that the 40% alcohol data are not consistent with this particular concept; the resistivity values closely followed a scaling plot only for the 20% and 30% alcohol amounts. This may indicate that the middle phase is in region I1 for the lower alcohol amounts and in region I for the 40% system. At the higher salinities (6%), the above scaling plot for surfactant does not work, indicating that overall surfactant amount does not give a proper path to the critical for the linear scaling plot. Rather, we found that the ratio of overall surfactant to overall alcohol amount, S / A , gave a straight line as in Figure 16. Here, the upper IFT is plotted for various alcohol amounts versus S / A relative to the critical, (SIA),, estimated from resistivity measurements. The slope is consistent with 3.7 (or 4). This suggests that the alcohol and surfactant associate in the middle phase, a
Figure 16. Upper interfacial tension versus overall surfactant/alcohol amounts for various alcohol volume fractions at 6 wt % NaCl.
behavior different from the upper/middle critical of the surfactant-free systems and the lower/middle critical of the low salinity systems. Another indication of the effects of the intersecting regions I and I1 is that the resistivity behavior of the lowest alcohol concentration (1 5%) was different from the ones at 20% and 30%. We would expect those with higher alcohol content to have the middle phase in region I, while the 15% case would have the region I1 middle phase. Interestingly, the impact of this possible shift of region on the IFT plot is minimal for both Figures 15 and 16. Apparently IFT is not very sensitive to this variation, suggesting that robust correlations might be developed from these ideas.
Discussion and Conclusions The present systems show a variety of behavior that might be missed by measurements over narrower ranges of surfactant and alcohol amounts. In particular, the different multiphase regions and their criticals are much more completely shown than in previous work. Thus, though the salinity scans at high surfactant (Figure 2) are simple and typical of EOR systems, these are the result of complex phenomena that are best appreciated only within the context of the complete phase behavior. Our interpretation of the high salinity upper/middle phase behavior and IFT arises from the basic alcohol/oil critical due to adding salt whether surfactant is present or not. While the dominant alcohol phase probably has microstructure, the solution phenomena are largely independent of it. The low-salinity lower/middle phase behavior is somewhat more complicated due to both its microstructure and the fact that more than one type of critical line is involved. Thus, while the upper/middle transition involves at least one phase without microstructure, the lower/middle transition involves more microstructured phases. The latter is qualitatively different, and this appears in the different number of phases, in the phase volume variations (simple or more complex salinity scans), in the amount of turbidity at critical and low IFT states, and in the kind of scaling law relations that are successful. The use of critical scaling law relations might lead to two benefits. The first is that the kind of variable that leads to a linear correlation emphasizes the differences between the phase transitions and structures and indicates their evolution with alcohol amount. The second is that simple predictive correlations of IFT might arise from such analyses. For example, as Huang and Kim2s imply, carbon chain length can also be chosen as a scaling variable. We have found that the complex curves of the EACN2’ and (27) Cayias, J. L.; Schecter, R.S . ; Wade, W. H. SOC.Pet. Eng. J . 1976, 16, 3 5 1 .
3500
J . Phys. Chem. 1988, 92, 3500-3504
EPACNUS2*for lower/middle IFT form straight lines with slope of 1.3, implying that carbon number is a field variable for this interface. A number of details of other work on this system can be found in the Ph.D. dissertation of Kim.Is
the U S . Department of Energy under Contracts EW-78-8-S19-0008 and DEAC-1979-BC10075 and to the University of Florida Consortium for Enhanced Oil Recovery Research. The measurements with varying temperature and pressure were made by Michel Hourani and Laurie Knash on an apparatus mostly assembled by Russel K. Code.
Acknowledgment. We are grateful for financial support from
Registry No. NaCI, 7647-14-5;sodium stearate, 822-16-2; isobutyl alcohol, 78-83-1; n-hexadecane, 544-76-3.
(28) Salager, J. L.; Morgan, J. C.; Schecter, R. S.; Wade, W. H. SOC.Pet. Eng. J. 1979, 19, 271.
Moblllty Measurements in Microemulsion Gels Donatella Capitani, Anna Laura Segre,* Istituto di Strutturistica Chimica “Giordano Giacomello” casella Postale 10, I-0001 6 Monterotondo Stazione, Roma, Italy
Gabriel Haering, and Pier Luigi Luisi Institut fur Polymere, Uniuersitatstrasse 6. ETH- Zentrum, 8092 Zurich, Switzerland (Received: May 12, 1987; In Final Form: November 9, 1987)
Gelatin can be solubilized in the water pool of water-in-oil microemulsions formed by AOT (bis(2-ethylhexyl) sodium sulfosuccinate) in isooctane and, under certain conditions, the whole system can be transformed in a transparent, stable gel. A partial physical chemical characterization of these gels is presented, based mostly on viscosity (energy dissipation) and NMR relaxation time measurements, with the aim of investigating the mobility and, more generally, the structure of these novel materials. The viscosity is very high, typical values ranging around 1000 P. However, when the molecular mobility is investigated by pulsed NMR spectroscopy, it is found that isooctane is characterized by T I ca. 1.695 s, a value which is very close to the value of the neat solvent. As far as the surfactant is concerned, NMR spectra are characterized by a very high resolution with relaxation times close to those found for AOT in liquid reverse micelles. 13Cresonances of AOT suggest a picture of the surfactant in the gel phase, according to which the two ester moieties of the molecules assume a closed form. Gelatin, conversely, is highly rigid and cannot be properly studied by NMR spectroscopy. The chemical shift of the water signal is very similar to data previously published on micelles at very high water content (4.8 ppm); however, the line width in gels is larger by an order of magnitude and increases linearly with an increase in the gelatin content in the water phase. From all of these data taken together, in particular from the apparent contradiction between the high viscosity of the gel and the high mobility of the major component (hydrocarbon), a picture for these gels emerges, according to which very large droplets of organic solvents (400-2000 A in diameter in the rough assumption of spherical droplets) are entrapped by rigid gelatin networks.
Introduction In a previous communication, this group has described the preparation of a novel family of gel.’ The material is prepared from water-in-oil microemulsions; in particular from a solution of AOT (bis(2-ethylhexyl) sodium sulfosuccinate) in a hydrocarbon (e.g., isooctane) containing water, up to a Wo (Wo = H,O]/[AOT]) of ca. 30-40. Gelatin is dissolved in the water microphase and, following a simple process of warming up and cooling, the whole organic solution is transformed in a transparent gel. Other groups have also started investigating such systems.2 The term “microemulsion gels” can be used to define this novel material; more generally, the term “organogels” (or in particular, ”hydrocarbon gel”) conveys the qualitative information that the largest component (80-90% v/v) is an organic solvent, specifically a hydrocarbon. Preliminary physicochemical studies have been performed on these gels, but neither their structure nor the mechanism of formation is understood. In particular, one would like to know how the relatively large amount of organic solvent (ca.80% v/v of the system) can ‘gelify” as a result of the presence of a relatively small amount of gelatin which is localized in the remaining water microphase. One would also like to know the distribution of water in the microemulsion gel and establish, for example, whether we are dealing with a
bicontinuous system, or whether water remains localized in droplets as in the starting microemulsion solutions. These questions are also important in view of the application of the gels. Compartmentation of hydrophilic biomolecules, of enzymes and/or bacteria in the gels, for example (studies on this line are in progress in this group), can be successful only if water is present in the gel material in discrete pools which are large enough to host the guest molecules. In this paper, the structure of the microemulsion gels has been studied by pulsed N M R spectroscopy. This technique is, in fact, particularly suitable for studying solids whose components have different m ~ b i l i t i e s . ~We will show that by investigating the dynamic properties of the various components of the gel (AOT, isooctane, water, and gelatin), a first insight of the overall structure can be gained. ‘H and 13CNMR studies of AOT reverse micelles have been already reported in the literature.”” TI measurements have been (3) Noack, F. NMR: Basic Princ. Prog. 1971, 3. (4) Eicke, H.F. Chimia 1982, 36, 241. (5) Wong, M.; Thomas, J. K.; Nowak, T. J . Am. Chem. SOC.1977, 99,
4.7. 3 0
(6) De Marco, A.; Menegatti, E.; Luisi, P. L. J . Biochem. Biophys. Methods 1986, 12, 325. (7) De Marco, A.; Zetta, L.; Menegatti, E.; Luisi, P. L. J. Biochem. Biophys. Methods 1986, 12, 335. (8) Llor, A.; Rigny, P. J . A m . Chem. SOC.1986, 108, 7533. (9) Carnali, J.; Lindman, B.; Siiderman, 0.;Walderhaug, H. Langmuir 1986, 2, 51.
(1) Haering, G.; Luisi, P. L. J . Phys. Chem. 1986, 90, 5892 (2) Quellet, C.; Eicke, H.-F. Chimia 1986, 40(7-8), 233.
0022-3654188 , ,12092-3500%01.50/0 0 1988 American Chemical Societv I
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