Ind. Eng. Chem. Res. 1996, 35, 3233-3240
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Behavior of Hydrocarbon/Alcohol Drops Injected into Dilute Solutions of an Amine Oxide Surfactant Moon-Jeong Rang and Clarence A. Miller* Department of Chemical Engineering, Rice University, 6100 South Main Street, Houston, Texas 77005-1892
Heinz H. Hoffmann and Christine Thunig Physikalische Chemie, Universita¨ t Bayreuth, D-95440 Bayreuth, Germany
Videomicroscopy was used to observe intermediate phase formation and other dynamic behavior which occurred when drops containing mixtures of n-decane and a short-chain alcohol were contacted with dilute solutions of an amine oxide surfactant. Some equilibrium phase behavior was also determined with n-heptanol as the alcohol to help in interpreting the contacting experiments. In most contacting experiments the first intermediate phase formed was the lamellar liquid crystal. For systems rich in hydrocarbon it grew very rapidly as short and fluid myelinic figures containing substantial amounts of both hydrocarbon and water. The myelinic figures almost immediately disintegrated into a multitude of drops in a process resembling an explosion. For systems rich in alcohol a highly viscous lamellar phase developed around the drop in a configuration resembling a polyhedron. At intermediate alcohol contents oil drops formed spontaneously as the amount of liquid crystal increased. Both the “explosions” and the spontaneous emulsification are likely to be favorable for detergency. discussed in terms of diffusion and the equilibrium phase behavior of the system.
Introduction Dynamic phenomena involving interfaces are of considerable importance in many practical applications. They have also been of interest to Professor Ruckenstein throughout his career. Among other things, he has made contributions to the understanding of Marangoni flow with simultaneous chemical reaction (Ruckenstein and Berbente, 1964), spreading of liquids on immiscible liquids (Suciu et al., 1970), and, with one of the present authors, on solids (Miller and Ruckenstein, 1974; Lopez et al., 1976), and drainage and instability of thin liquid films (Ruckenstein and Jain, 1974; Sharma and Ruckenstein, 1987; Tsekov and Ruckenstein, 1994). The dynamic phenomena considered in this paper involve simultaneous diffusion and phase transformation. Of particular interest is the formation of one or more intermediate phases at the interface where immiscible liquids not initially in equilibrium are brought into contact. While such behavior is possible in many types of systems, it is particularly likely in systems involving surfactants, which are well-known to exhibit a richness of phase behavior involving various types of micellar solutions, microemulsions, and lyotropic liquid crystals. Intermediate phase formation has been shown to be significant in solubilization-emulsification mechanisms for removal of oily soils from synthetic fabrics (Miller and Raney, 1993) and no doubt is important in other applications as well. We present below dynamic behavior recently observed with videomicroscopy when drops which are mixtures of a hydrocarbon and a short-chain alcohol are injected into dilute solutions of an amine oxide surfactant. The behavior for injection of pure alcohol drops was described previously (Rang et al., 1995). As will be seen, intermediate phase formation is sometimes quite dramatic, causing rapid dispersion of the initial drop into many tiny droplets in a process resembling an explosion. This behavior and other phenomena observed are * To whom correspondence should be addressed.
S0888-5885(96)00077-2 CCC: $12.00
Experimental Section Tetradecyldimethylamine oxide (C14DMAO) was obtained from Hoechst and recrystallized twice from acetone. Reagent-grade n-heptanol, n-hexanol, n-pentanol, and oleyl alcohol were obtained from Sigma, n-octanol from Fluka, n-decanol from BDH, and ndecane from Humphrey Chemical. Water used for solution preparation was double distilled and deionized. Phase behavior studies were conducted in an environmental room maintained at 30 °C. Birefringent phases were identified by viewing samples placed between polarized sheets having perpendicular orientations. Contacting experiments were conducted using the technique described previously (Lim and Miller, 1991). A thin hypodermic needle was used to inject small drops of n-decane/alcohol mixtures (50-100 µm in diameter) into a rectangular glass capillary cell having a thickness of 400 µm and filled with the aqueous surfactant solution of interest. The cell was maintained at constant temperature in a modified Mettler microscope thermal stage. The behavior was observed using a videomicroscopy system and recorded on videotape. Results 1. Contacting Experiments. A series of videomicroscopy experiments was conducted in which drops containing various proportions of n-decane and nheptanol were injected into aqueous solutions containing 0.05 wt % of C14DMAO at 30 °C. For drops of pure n-decane no significant activity or change in drop size was observed over a period of 1 h, although solubilization was presumably occurring at a very slow rate into the dilute surfactant solution. Such behavior is typical (Miller and Raney, 1993) for systems considerably more hydrophilic than “optimal” conditions where hydrophilic and lipophilic properties are balanced and a bicontinu© 1996 American Chemical Society
3234 Ind. Eng. Chem. Res., Vol. 35, No. 9, 1996
Figure 1. Video frames showing dynamic behavior of a drop containing 85/15 n-decane/n-heptanol by weight injected into 0.05 wt % C14DMAO in water at 30 °C: (a) drop shortly after injection with a diameter of about 97 µm; (b) increased diameter and appearance of pattern about 2 s later; (c) further increase in diameter with myelinic figures visible along periphery about 3 s later; (d) breakup into tiny droplets about 8 s later.
ous microemulsion and/or the lamellar liquid crystal are seen at equilibrium when substantial amounts of both oil and water are present. When enough alcohol was present that conditions were somewhat more lipophilic than optimal, the behavior was very different. Drops initially containing 5-10 wt % n-heptanol broke up immediately upon contacting the surfactant solution into several smaller drops and many tiny droplets. For drops containing 15-25 wt % n-heptanol there was a brief period of expansion as surfactant and water were incorporated more rapidly than alcohol was lost. The duration of this behavior was shorter for smaller drops and for those with less alcohol and, in fact, too short to be detected in some cases. For all drops with at least 15 wt % alcohol, however, a regular pattern was suddenly observed covering the drop surface (Figure 1a-c) accompanied by a very rapid increase in the apparent diameter. For example, the rate of increase was about 12 µm/s starting with a diameter of about 100 µm for the experiment of Figure 1. When the alcohol content was only 15 wt %, the drop surfaces exhibited irregularities which appeared to be small myelinic figures of the lamellar phase growing outward (Figure 1c). The myelinic figures were clearly identifiable for the systems containing more alcohol (25 wt %) and less hydrocarbon, presumably because the lamellar phase was more viscous under these conditions. Only a few seconds after the myelinic figures were seen, the drop broke up into many tiny droplets (Figure 1d) in a process resembling an explosion. The droplets persisted for many minutes and indeed were not completely solubilized during the
time scale of the experiment (about 1 h) except for the drops with the lowest initial hydrocarbon content (75 wt %). Drops initially containing equal amounts of n-heptanol and n-decane at first decreased in diameter, in contrast to the behavior just described (Figure 2a,b). Some liquid droplets appeared within the drop near its surface (Figure 2c). As discussed below, we consider that they were an excess oil phase, i.e., mainly hydrocarbon with some dissolved alcohol. A short time later myelinic figures began to grow outward (Figure 2d), although they soon retracted into a layer of the lamellar phase covering the drop surface. The oil droplets increased in size and number during this time and became a rather concentrated dispersion within the larger drop. Suddenly, they were released and drifted into the surfactant solution (Figure 2e). There they disintegrated rapidly into even smaller droplets (Figure 2f), which were quickly solubilized. Similar behavior was observed for drops initially containing 75% n-heptanol except that when the small droplets were released, they did not disintegrate as rapidly as described above. Moreover, during the early stages of drop shrinkage a front that was basically circular but with small irregularities along its surface spread rapidly outward from the drop. Probably, the drop contacted the upper surface of the cell, and a thin film spread along the glass surface. Yet a different type of behavior was observed for drops initially containing 90% n-heptanol. At first they remained spherical and shrank, as seen at 50% and 75% n-heptanol. Later they became nonspherical (Figure 3a), slowly developing into polyhedral shapes which
Ind. Eng. Chem. Res., Vol. 35, No. 9, 1996 3235
Figure 2. Video frames showing dynamic behavior of a drop containing 50/50 n-decane/n-heptanol by weight injected into 0.05 wt % C14DMAO in water at 30 °C: (a) drop at about 21 s after injection with a diameter of about 82 µm; (b) diameter is smaller 1 min, 27 s later; (c) droplets visible inside about 14 s later; (d) larger droplets and myelinic figures about 7 s later; (e) droplets escaping about 13 s later; (f) droplets disintegrating about 16 s later.
seemed to be very viscous (Figure 3b). Here too drops of another phase could be seen inside. Subsequently, myelinic figures formed and grew slowly outward (Figure 3c). The behavior of pure n-heptanol drops has been shown and described in detail elsewhere (Rang et al., 1995). In brief, shrinkage occurred until an intermediate L3 or sponge phase containing more water and less alcohol began to develop around the alcohol-rich drop. The former phase increased in volume, while the latter became smaller and eventually disappeared. The diameter of the L3 phase then increased rapidly because its rate of incorporation of surfactant and especially water greatly exceeded the rate of alcohol loss. However, the apparent rate of expansion was not as great as that shown in Figure 1. After a short period of such expansion, the lamellar phase began to form as myelinic figures with a corresponding decrease in volume of the L3 phase. Similar contacting experiments were conducted using drops which were mixtures of n-decane with other
alcohols. When n-octanol, n-decanol, and oleyl alcohol were used, phenomena similar to those described above were observed. However, as alcohol chain length increased, any given phenomenon occurred at higher initial hydrocarbon/alcohol ratios, as shown in Table 1. Moreover, the entire process was slower. Thus, it was possible to observe formation of myelinic figures which broke up into tiny drops even when only part of the drop exploded due to insufficient alcohol content (entries labeled A*). The rates of increase of the apparent drop diameter during “explosions” were about 4 µm/s for a drop having a 90/10 weight ratio of decane/n-octanol and 2 µm/s for a drop having a 95/5 weight ratio of decane/ n-decanol, both considerably below the 12 µm/s mentioned above for Figure 1 with 85/15 decane/n-heptanol. These results are expected since longer-chain alcohols are more effective in making the system lipophilic and also dissolve more slowly in the aqueous phase. One difference seen with oleyl alcohol was that highly viscous phases such as those of Figure 3b (denoted by D in Table 1) were not observed for the composition
3236 Ind. Eng. Chem. Res., Vol. 35, No. 9, 1996
Figure 3. Video frames showing dynamic behavior of a drop containing 10/90 n-decane/n-heptanol by weight injected into 0.05 wt % C14DMAO in water at 30 °C: (a) drop initially about 52 µm in diameter but becoming nonspherical some 3 min, 37 s after injection; (b) drop smaller and exhibiting corners and droplets inside about 24 s later; (c) myelinic figures 57 s later.
range investigated, presumably because the double bond in the oleyl chain made the lamellar phase rather fluid. Alcohols with shorter chains than for n-heptanol were also used. With n-hexanol, for example, smaller initial hydrocarbon/alcohol ratios were needed to produce a given phenomenon, and the process occurred more rapidly than with n-heptanol. When n-pentanol was used, however, water solubility was so high (about 2.7 wt % compared to 0.6 and 0.18 wt % for n-hexanol and n-heptanol, respectively) that alcohol diffusion into the aqueous phase dominated the dynamic behavior, and drop breakup occurred shortly after injection in all cases. Some experiments were conducted where 0.5 wt % n-pentanol was dissolved in the surfactant solution before drop injection to reduce the alcohol mass-transfer rate. As shown by the entries in parentheses in the last column of Table 1, phenomena similar to those found for the longer-chain alcohols were then observed for drops containing at least 50 wt % pentanol.
2. Phase Behavior Results. Partial phase diagrams for this system, determined at 30 °C for n-decane/ n-heptanol ratios of 95/5, 90/10, and 75/25, are shown in Figures 4-6. They are based on the results of 33 samples for the 95/5 diagram and about 80 samples for each of the 90/10 and 75/25 diagrams. All the multiphase regions shown were observed, but, except in the immediate vicinity of the coexistence curve, little effort was made to determine precise phase boundaries or to assure that all small multiphase regions were detected. Nevertheless, the phase behavior shown is adequate for interpreting the experiments described above. For the contacting experiments we expect, based on the quasi-steady-state analysis of similar experiments presented elsewhere (Rang et al., 1995; Lim and Miller, 1991), that, after a short transient, drop composition will follow a path on the L2 coexistence surface with L1 for this four-component system. Here, L1 is the aqueous surfactant solution and L2 an isotropic phase which, in the systems of interest here, extends continuously from pure alcohol/hydrocarbon mixtures through compositions containing some water and surfactant where the microstructure is that of a water-in-oil microemulsion to more water-rich compositions where the microemulsion becomes bicontinuous or perhaps even watercontinuous. Figures 4-6 emphasize compositions near the L1-L2 coexistence region and show that it is terminated at high water contents by a three-phase region in which the third phase is the lamellar liquid crystal LR over a wide range of decane/alcohol ratios. As might be expected, the ratio of the lipophilic alcohol to the more hydrophilic surfactant at the point on the coexistence curve where the single-phase L2 region ends and the three-phase region begins is roughly the same in all three casessabout 0.55 and 1.2 on weight and molar bases, respectively. However, the surfactant concentration is greater for the 75/25 system than for the others, and separation into distinct L1, L2 (ME), and LR phases occurs. For the 90/10 and 95/5 systems the L2 (ME) and LR phases are seen as a stable dispersion in equilibrium with L1. The presence of the lamellar phase is evident, however, from the streaming birefringence of the dispersion. The L2 phase in this three-phase region has been desiginated ME on Figures 4-6 to emphasize that it is a microemulsion, as discussed above. Figures 4-6 also confirm the above statement that the L2 phase near the end of the coexistence curve is a microemulsion and indeed contains more water than hydrocarbon. It is noteworthy that both the appearance of the lamellar liquid crystal as the first intermediate phase and the high water content of the L2 phase when the LR phase begins to form are consistent with the behavior found for pure nonionic surfactants with n-hexadecane/oleyl alcohol mixtures (Lim and Miller, 1991). In all three phase diagrams the system becomes more lipophilic as the hydrocarbon/alcohol mixture is added at constant surfactant concentration. For the 90/10 system at about 8 wt % surfactant, one finds, for example, transformation from initially hydrophilic conditions with an oil-in-water microemulsion (L1) to L1 in equilibrium with excess oil (the latter present in an emulsion) to the microemulsion alone to two-phase equilibrium between the microemulsion (now probably bicontinuous) and the lamellar phase to another singlephase microemulsion region (L2) to coexistence between this microemulsion and excess water. At lower surfac-
Ind. Eng. Chem. Res., Vol. 35, No. 9, 1996 3237 Table 1. Contacting Experiments with Drops of Decane/Alcohol Mixtures in C14DMAO Solutionsa decane:alcohol (wt %)
decane + oleyl alcohol
decane + n-decanol
decane + n-octanol
decane + n-heptanol
decane + n-hexanol
decane + n-pentanol
100:0 95:5 90:10 85:15 80:20 75:25 50:50 25:75 10:90
no activity A* C** C** C** C**
no activity A* C** C** D D
no activity A* B B* B* B* D
no activity A A B B B* C C* D
no activity
no activity
A
E
A* B* C*
E(E) E(A) E(B)
a (1) All experiments involve injection of drop into 0.05 wt % C DMAO solutions at 30 °C, except letters in parentheses represent 14 behavior with 0.5 wt % pentanol added to the surfactant solution. (2) Notation. A: Drop breaks up into a few smaller drops and many tiny droplets just after contacting the surfactant solution. A*: Similar to A but breakup is not immediate and involves formation of myelinic figures (see text). B: Similar to Figure 1. B*: Similar to B except that myelinic figures are more distinct and tiny droplets are completely solubilized. C: Similar to Figure 2. C*: Similar to C except that an irregular circular front moves outward along the cell surface early in the experiment (see text). C**: Similar to C except that myelinic figures develop before droplets from within the drop. D: Similar to Figure 3. E: Drop breaks up into a few smaller drops just after contacting the surfactant solution.
Figure 4. Partial phase diagram at 30 °C for a system containing water, C14DMAO, and a n-decane/n-heptanol mixture which is 95/5 by weight. L1 is a water-continuous micellar solution or microemulsion; L2 is an oil-continuous or bicontinuous microemulsion; ME is a microemulsion; LR is the lamellar liquid-crystalline phase; E is an oil-in-water emulsion.
tant concentrations of 4-5 wt % the last single-phase microemulsion region is replaced by three-phase coexistence of the microemulsion, lamellar phase, and excess water. It should be noted that Figures 4-6 are results for system compositions on three planes through the tetrahedron representing the overall phase behavior of the four-component system at 30 °C. Accordingly, the various single-phase microemulsion regions shown in each figure are almost certainly not isolated from each other but instead joined by pathways which are outside the plane of the diagram. In separate experiments not shown in Figures 4-6 we observed four-phase coexistence of water, microemulsion, liquid crystal, and oil over narrow concentration ranges at low surfactant and alcohol contents, specifically at 1 wt % C14DMAO, 20 wt % n-decane, and a 96.5/3.5 ratio of decane/heptanol and at 2 wt % C14DMAO, 20 wt % decane, and a 94.5/5.5 ratio. In both cases microemulsion and liquid crystal did not separate but occurred as a stable dispersion which was somewhat turbid. A slight iridescence of the dispersion could be detected since the lamellar phase in such systems is iridescent at low surfactant concentrations (Hoffmann, 1990). It is considered that further experiments would
reveal a similar four-phase region over a small composition range in Figure 4 for the 95/5 ratio. It is noteworthy that, in our phase behavior studies with this system, we have not seen the usual behavior where a bicontinuous microemulsion coexists with excess oil and water. The existence of a four-phase region as described above instead of the usual threephase region occurs when the surfactant film becomes too rigid to form a microemulsion (Hackett and Miller, 1988; Kegel and Lekkerkerker, 1993). When n-heptanol was replaced by n-pentanol, the films were more flexible, and the water/microemulsion/oil coexistence was observed. Discussion The behavior described above for the drops containing 15-25% n-heptanol can be explained as follows. At first, heptanol, which has an appreciable solubility in water, diffuses into the aqueous phase while surfactant and water diffuse into the drop, converting it into a water-in-oil microemulsion. Assuming as a first approximation that the quasi-steady-state approach of Lim and Miller (1991) is applicable, we find that drop
3238 Ind. Eng. Chem. Res., Vol. 35, No. 9, 1996
Figure 5. Partial phase diagram at 30 °C for a system containing water, C14DMAO, and a n-decane/n-heptanol mixture which is 90/10 by weight. Symbols as in Figure 4.
Figure 6. Partial phase diagram at 30 °C for a system containing water, C14DMAO, and a n-decane/n-heptanol mixture which is 75/25 by weight. Symbols as in Figure 4.
composition moves along the L2 portion of the L1-L2 coexistence surface for this system. This surface is generated by the L1-L2 coexistence curves of Figures 4-6 and similar diagrams for other hydrocarbon/alcohol ratios. Since loss of alcohol from the drop requires that its hydrocarbon/alcohol ratio increase with time, its composition continuously shifts toward coexistence curves such as those of Figures 4 and 5 which are richer in hydrocarbon. During this time the surfactant/alcohol ratio in the microemulsion, i.e., the drop, increases, giving its surfactant films a smaller spontaneous curvature toward a water-in-oil configuration and thereby increasing its ability to solubilize water. Eventually, drop composition reaches the boundary of the three-phase region denoted by L1/ME/LR on Figures 4-6, at which time a rather fluid lamellar liquid-crystalline intermediate phase containing con-
siderable n-decane and a high content of water begins to develop as myelinic figures, causing the drop to have a patterned appearance and an irregular surface (Figure 1b,c). Just before the pattern develops, the radii of drops initially containing 15% n-heptanol have increased by a factor of about 1.5 or 1.6 from their initial values, according to the videotapes. The corresponding volume increase is in the range of 3.5-4, which means that the oil content of the drops has decreased to some 25-30%, a composition consistent with that of the L2 phase at its boundary with the L1/ME/LR region in Figures 4 and 5. This agreement supports the use of the quasi-steady-state approach to interpret this portion of the experiments. Once the pattern appears, rapid expansion is observed owing to the large amount of water taken up in forming the dilute lamellar phase. The increase in volume is
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not as great as one might initially expect from comparing apparent drop diameters in Figure 1a-c because the decrease in interfacial tension to very low values during the expansion permits gravity to flatten the drop so that its diameter in the horizontal plane is greater than its thickness. This behavior is very similar to that seen by Lim and Miller (1991) when drops containing mixtures of nhexadecane and oleyl alcohol contacted dilute solutions of nonionic surfactants. However, the next part of the behavior for the present system, the almost explosive breakup into small droplets shown in Figure 1d, was not observed in the earlier work. The lamellar phase here is evidently so dilute (as mentioned above, iridescent phases have been observed) that the myelinic figures break up rapidly into small droplets, presumably by the usual capillary instability mechanism for fluid cylinders. Probably some oil droplets form and the droplets of the lamellar phase continue to lose alcohol and hence become unable to solubilize all the oil they contain. Oil droplets may also be produced by the spontaneous emulsification mechanism discussed below. The breakup of the drop occurs so quickly that the relative importance of myelinic figure instability and spontaneous formation of oil droplets cannot be determined. For the drops initially containing only 5-10 wt % n-heptanol, it is considered, based on the phase behavior results presented above and the definite occurrence of myelinic figures under similar conditions for other alcohols (behavior A* in Table 1), that an intermediate lamellar phase forms in this case as well even though drop breakup occurred so soon after injection that it could not be observed directly. The presence of some larger drops after breakup indicates that not enough alcohol was present to convert all the hydrocarbon to the tiny droplets. When the injected drops contained nearly equal amounts of decane and heptanol, their diameters initially decreased with time, the opposite behavior to that observed for hydrocarbon-rich drops. The higher alcohol content was responsible for the initial shrinkage because it increased the rate of alcohol diffusion from the drops into the surfactant solution. As before, the drops became microemulsions whose compositions moved continuously along the L1-L2 coexistence surface toward higher hydrocarbon/alcohol ratios and higher water contents. However, because they initially contained more of the lipophilic alcohol, they had to incorporate more of the hydrophilic surfactant over a longer time period to reach the nearly balanced conditions where the lamellar phase could develop. As indicated above, we observed that droplets formed near the surface of the drop and that shortly thereafter myelinic figures grew into the aqueous solution. We also could see a layer of the lamellar phase LR at the drop surface. Although it was not possible to determine the phase constituting the droplets from the videotape alone, it was clear that they were immiscible both with the microemulsion making up the drop and with water. Since the microemulsion was becoming ever more hydrophilic, we believe that its solubilization limit for oil was reached and that the droplets were oil. In a simplified manner, one can imagine that the size of water droplets in the microemulsion increased until their attraction produced separation into a concentrated microemulsion phase and excess oil (Miller et al., 1977). In view of the phase behavior of Figures 4-6, which
indicates that the first intermediate phase should again be LR, we consider that a thin layer of LR formed before the oil droplets. As the layer thickened, it became visible and served as a site from which myelinic figures could develop. Because the lamellar phase normally has less capability of solubilizing oil than a balanced microemulsion (Hackett and Miller, 1988), it would not be surprising for spontaneous emulsification of oil to occur shortly after formation of the lamellar phase. When the oil droplets, having become larger, were later released into the aqueous solution, presumably because the layer of LR at the drop surface either dissolved or broke as it continued to take up water, they likely had compositions in the same range as the hydrocarbon-rich drops discussed above. Accordingly, they quickly disintegrated on contacting the surfactant solution by the same “explosion” process as described previously. For the experiments with drops initially containing 75% n-heptanol, the oil droplets when released presumably contained more alcohol, and they were solubilized more slowly. Spontaneous formation of oil droplets in conjunction with the growth of another intermediate phase has been observed in systems containing triolein, oleyl alcohol, nonionic surfactants, and water (Tungsubutra and Miller, 1994). The intermediate phase was the sponge phase L3 instead of LR. Experiments with single polyester fibers indicated that such behavior was favorable for detergency because the oil drops were dispersed in the washing bath while the L2 phase, which was initially in contact with the fiber, was consumed. In any case little oil remained on the fiber at the end of the experiment (Tungsubutra and Miller, 1994). The lamellar phase shown in Figure 6 at surfactant concentrations above 10 wt % for the 75/25 system was highly viscous. Presumably, the same is true for smaller hydrocarbon/alcohol ratios, where surfactant concentrations should be even larger and hydrocarbon concentrations even smaller in the lamellar phase. Thus, the observed phase behavior is consistent with the appearance of the highly viscous lamellar intermediate phase in the contacting experiments for drops having an initial 10/90 ratio of hydrocarbon to alcohol (Figure 3 and behavior D in Table 1). Of course, the ratio would be considerably higher than 10/90 at the time of intermediate phase formation, owing to diffusion of alcohol into the surfactant solution. Summary A combination of videomicroscopy observations and equilibrium phase behavior results has allowed interpretation of the behavior observed when drops of hydrocarbon/alcohol mixtures were injected into dilute solutions of an amine oxide surfactant. Initially, the drops took up water and surfactant and became microemulsions. Subsequently, an intermediate lamellar liquid-crystalline phase was formed. For systems initially rich in hydrocarbon the lamellar phase developed shortly after injection as many small myelinic figures which almost immediately broke up into tiny droplets in a process resembling an explosion. For drops initially rich in alcohol the lamellar phase formed later and was highly viscous. The few myelinic figures observed grew rather slowly and were considerably longer and larger in diameter. For drops with intermediate initial compositions spontaneous formation of oil drops was seen at about the same time as when the lamellar phase began to form. Both the explosions, which seem to be
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associated with the very dilute lamellar phase formed with this surfactant, and the spontaneous emulsification should be favorable for detergency. Acknowledgment NSF Grant INT-9014032 for cooperative research supported certain travel and communications expenses associated with this collaborative research. Literature Cited Hackett, J.; Miller, C. A. Microemulsion to Liquid Crystal Transition in Two Anionic Surfactant Systems. SPE Res. Eng. 1988, 3, 791. Hoffmann, H. H. Fascinating Phenomena in Surfactant Chemistry. Adv. Colloid Interface Sci. 1990, 32, 123. Kegel, W. K.; Lekkerkerker, H. N. W. Phase Behavior of an Ionic Microemulsion System as a Function of the Cosurfactant Chain Length. Colloids Surf. A 1993, 76, 241. Lim, J. C.; Miller, C. A. Dynamic Behavior and Detergency in Systems Containing Nonionic Surfactants and Mixtures of Polar and Nonpolar Oils. Langmuir 1991, 7, 2021. Lopez, J.; Miller, C. A.; Ruckenstein, E. Spreading Kinetics of Liquid Drops on Solids. J. Colloid Interface Sci. 1976, 56, 460. Miller, C. A.; Ruckenstein, E. The Origin of Flow During Wetting of Solids. J. Colloid Interface Sci. 1974, 48, 368. Miller, C. A.; Raney, K. H. Solubilization-Emulsification Mechanisms of Detergency. Colloids Surf. A 1993, 74, 169. Miller, C. A.; Hwan, R.; Benton, W. J.; Fort, T., Jr. Ultralow Interfacial Tensions and Their Relation to Phase Separation in Micellar Solutions. J. Colloid Interface Sci. 1977, 61, 554.
Rang, M. J.; Lim, J. C.; Miller, C. A.; Thunig, C.; Hoffmann, H. H. Dynamic Behavior of Alcohol Drops in Dilute Solutions of an Amine Oxide Surfactant. J. Colloid Interface Sci. 1995, 175, 440. Ruckenstein, E.; Berbente, C. The Occurrence of Interfacial Turbulence in the Case of Diffusion Accompanied by Chemical Reaction. Chem. Eng. Sci. 1964, 19, 329. Ruckenstein, E.; Jain, R. K. Spontaneous Rupture of Thin Liquid Films. J. Chem. Soc., Faraday Trans. 2 1974, 70, 132. Sharma, A.; Ruckenstein, E. Critical Thickness and Lifetimes of Foams and Emulsions; Role of Surface-Wave-Induced Thinning. J. Colloid Interface Sci. 1987, 119, 14. Suciu, D. G.; Smigelschi, O.; Ruckenstein, E. The Spreading of Liquids on Liquids. J. Colloid Interface Sci. 1970, 33, 520. Tsekov, R.; Ruckenstein, E. Dimple Formation and its Effect on the Rate of Drainage in Thin Liquid Films. Colloids Surf. A 1994, 82, 255. Tungsubutra, T.; Miller, C. A. Effect of Secondary Alcohol Ethoxylates on Behavior of Triolein-Water-Surfactant Systems. J. Am. Chem. Soc. 1994, 71, 65.
Received for review February 13, 1996 Revised manuscript received May 13, 1996 Accepted June 4, 1996X IE960077P
X Abstract published in Advance ACS Abstracts, August 15, 1996.
