12097
J. Phys. Chem. 1994, 98, 12097-12102
Dispersion Morphology Diagrams for Three-phase, "Microemulsion" Emulsions. 2. "Disappearance" of Morphology-Transition Lines Gregory K. Johnson: Dady B. Dadyburjor,$and Duane H. Smith**+?* US.Department of Energy, Morgantown Energy Technology Center, Morgantown, West Virginia 26507-0880, and Department of Chemical Engineering, West Virginia University, Morgantown, West Virginia 26506 Received: May 31, 1994@
The limits over which the oil-rich top phase (7), middle-phase microemulsion (At),or water-rich bottom phase (@ ) is the continuous phase in steady-state, three-phase macroemulsions have been determined by means of electrical conductivity measurements for the amphiphile/oil/"water" system C ~ H ~ ~ ( O C ~ & ) Z O W ~ tetradecane/aqueous 10 m M NaC1. Measurements were made at three different temperatures and apparent wettability conditions: (a) 45 "C, wetting middle phase; (b) 25 "C, no wetting phase; and (c) 12 "C, wetting bottom phase. The results at 25 "C were in accord with expectations from previous predictions and experiments; but for both two-phase and three-phase emulsions no abrupt, "first-order" transitions between J% and %continuous emulsions at 45 "C or between J% and @-continous emulsions at 12 "C were found. Instead, these changes of continuous phase appeared to occur smoothly and continuously between their respective single-phase and two-phase limits. It is not yet clear if the "disappearance" of first-order morphology transitions correlates with phase wettability transitions; the phenomenon suggests the possibility of bicontinuous twophase and three-phase macroemulsions.
Introduction
temperatures also were observable. The observed transition temperatures were T h = 36 "C, TIc = 9 "C, and Tu,, = 49 "C, re~pectively.~~'~ Subsequently, a second wetting transition was reported for this system at 15 "C.14 Hence, for 49 "C = Tu, > T > T h = 36 "C, Mwets the interface between Tand 0,for 36 "C = T a > T > Tfi = 15 "C, no phase is wetting; and for 15 "C = Tfi T > E, = 9 "C, 0 wets the interface between &and 57 The experimental temperatures 45,25, and 12 "C were chosen to fall within the respective wettability temperature ranges. These experimental temperatures, along with the study of the C&i90C&OWn-decane/aqueous 10 mM NaCl system,s allow us to make several comparative tests for the effects, if any, of near-criticality and wettability on the three-phase dispersion morphology diagram: (1) for wetting middle phases, nearcritical condition vs near-optimum; (2) for two systems both near optimum [To,, = (c, Tu,)/2],15wetting middle phase vs nonwetting middle phase; (3) for near-criticality, wetting middle phase vs wetting bottom phase; (4) wetting middle phase vs wetting bottom phase vs no wetting phase, all for the same chemical system.
An oil-rich top phase (T),a middle-phase microemulsion and a water-rich bottom phase often form in systems that contain amphiphile/oil/water and (frequently) inorganic electrolyte(s), as well. These three-phase, "microemulsion" systems have been used in enhanced oil recovery for nearly two decades' and offer a promising means to remediate many ground-water pollutants.2 These systems readily form threephase macroemulsions in bulk, and there is experimental evidence3 that the fluids also can flow through porous media as three-phase macroemulsions. The development of these applications combines studies of phase behavior, three-fluid wettabilities, bulk three-phase dispersions, and flow through microvisual and natural porous media.3-7 A recently proposeds dispersion morphology diagram for bulk emulsions predicts that the three-phase tietriangle should contain three different regions, with A( or respectively, the continuous phase for emulsions formed within each region. This diagram was postulated on the basis of the dispersion morphology diagram for the two phase regions that surround the tietriangle?JO Electrical conductivity measurements on steadyExperimental Section state, three-phase emulsions of C&OC&OWn-decane/aqueThe materials and basic methods of this study have been ous 10 mM NaCl were in accord with the dispersion morphology * ~ ~ J ~ electrical conductivities diagram.* This system is known to have a "strongly ~ e t t i n g " ~ described e l s e ~ h e r e ! ~ ~ ~ Briefly, were measured for steady-state unstable emulsions formed by middle phase; i.e., .&apparently wets the interface between T mixing measured volumes of two or three pre-equilibrated and over the complete range of three-phase temperatures." phases. Continuous phases then were identified by comparison We originally chose4J2 the system C ~ H ~ ~ ( O C Z % ) Z ~ W ~ of the measured conductivitieswith the conductivitiespredicted tetradecane/water for our first three-phase morphology experiby equations that contain no adjustable parameters. 16,17 Because ments because this was the first case in which a temperaturethe three-phase, two-morphology hysteresess were very small, induced three-liquid wetting transition3 was found5J3in a system their systematic measurement was omitted. These omissions for which both the lower (T,) and upper (Tu,) critical end-point introduce imprecisions of about 1 volume % into the dispersion morphology boundaries reported below. As before,"JzJ7 all *Author to whom correspondence should be addressed at the US. compound purities were 99% or better. Department of Energy. U.S.Department of Energy. (&,
(a)
a,
* West Virginia University. 5 Also, Institute for Applied
Surfactant Research and School of Chemical Engineering and Materials Science, University of Oklahoma, Norman, OK 73019. @Abstractpublished in Advance ACS Abstracts, October 15, 1994.
+
Results The measured phase conductivities were K a = 1750pS cm-', K a = 1.2 ,US cm-', and KT= 0.03 pS cm-2 at 45.0 "C; K g = 1150 pS cm-', K / N = 50 pS cm-', and KT= 0.8 pS cm-' at
0022-3654/94/2098-12097$04.50/00 1994 American Chemical Society
Johnson et al.
12098 J. Phys. Chem., Vol. 98, No. 46, 1994 :
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Figure 1. Emulsion conductivities (a) predicted (dashed line) for the T/AJmorphology, (b) predicted (dotted line) for the AZ/Tmorphology, and (c) measured (squares, emulsions titrated with 2 diamonds, emulsions titrated with A)along the T/AXtieline (all at 25 "C).
25.0 "C; and KG?= 970 pS cm-', K,g= 280 pS cm-', and KT = 0.3 pS cm-' at 12.0 "C. For nonmultiple, two-phase emulsions formed along side f - z o f a tietriangle,IKone can accurately predict1gthe emulsion conductivities from the phase conductivities, &and K s and the measured phase-volume fraction, 4s by means of the Maxwell equationz0
where
In eq 1 one of the two phases is continuous, while the other is discontinuous, and typically the predicted conductivities strongly depend on which phase is which. These features are illustrated by Figure 1, which shows (at 25 "C) "dimensionless" conductivities (1) measured, ( 2 ) predicted for Y/&emulsions, and (3) predicted for &/Temulsions. (Here and below, dimensionless conductivity is defined to be the measured conductivity divided by the conductivity of phase @, whether physically present or not.) The measurements extend completely along the side of the tietriangle, i.e., from #T= 0, @.n= 1 to 4 ~ 1,=4,&= 0. The agreement between the predictions and measurements indicates that (1) the morphology was V&for 0.0 < 4 ~ 0.47, 5 (2) the morphology was & / m o r 0.47 5 4~ < 1.0, and (3) a change of continuous phase (i.e., inversion) occured at 4~ = 0.47. (In fact, the inversion displayed hysteresis; Le., depending on direction, inversion occurred at 4 7 = 0.46 and 4~ = 0.48, respectively.) This pattern of behavior has been observed for so many different pairs of conjugate phasesg*10.1s~21-23 that we may think of it as "normal" behavior for two-phase emulsions. The phase volume fraction at which the continuous phase changed gives us a single point on the dispersion morphology diagram. For measurements inside a tietriangle, the appropriate conductivity equation depends on the emulsion morphology, and there are at least fourteen different morphologies that might f ~ r m . ' ~ Nevertheless, ,'~ often one can predict16,17the threephase emulsion conductivities, &is, from the two-phase emulsion conductivities (either measured or predicted by eq 1) and the measured phase-volume fractions by means of the equation
0.20
0.40
0.60
0.80
1.00
VOLUME FRACTION TOP PHASE
VOLUME FRACTIONTOP PHASE
= 0.10 at 25 Figure 2. Electrical conductivities (along the path "C), illustrating the sudden change of continuous phase between two
different three-phase emulsion morphologies: dashed line, predicted for the Atcontinuous morphology; dotted line, predicted for the !%continuous morphology.
x$
and F a r e % and 0 in any permutation and = 1 - @g5F Because the values of Kf/ypredicted by eq 1 depend on which phase ( T 4 or L?) is Y a n d which is $ the three-phase emulsion conductivities (&is) predicted by eq 2 do also. Thus, under a permutation of phases, eq 2 predicts three different conductivity surfaces, Kdis(4&s4d, analogous to the two dispersion conductivity lines illustrated in Figure 1. Comparison of measured values with the three predicted conductivity surfaces allows us to determine which phase is continuous, and the phase-volume fractions at which the continuous phase changes also are plotted on the dispersion morphology diagram. (This procedure has been described in detail e l ~ e w h e r e . ~ * . * ~ ~ ' ~ ) Figure 2 illustrates the sudden change of three-phase emulsion morphology between two different continuous phases, as well as the conductivities predicted by eq 2 for continuous phases &and T respectively. The electrical conductivity data were obtained at 25 "C for the path = 0.10 (parallel to the twophase path of Figure 1). Analogous to the two-phase data, there is a range of phase-volume fractions over which the data are very close to the conductivities predicted for &continuous emulsions, a second range of compositions over which the data agree with the conductivities predicted for 5Gontinuous emulsions, and a discontinuous jump of the measurements between the two predicted curves. Similar behavior has been found at 25 "C for the path 4~ = 0.20.17 The three-phase dispersion morphology diagram for the C&I~3(oC2~)20Hln-tetradecane/aqueous10 mM NaCl pseudotemary at 25 "C,which was obtained from Figures 2 and 3 and similar data,I2 is plotted (in phase-volume fraction coordinates) in Figure 3. As anticipated by previously reported expenments4,K116,17 and by predictionsK based on the two-phase dispersion morphology diagram,9 the emulsion morphology diagram has three regions: Within one region the continuous phase is the oil-rich top phase ( T ) within ; a second region the continuous phase is the middle-phase microemulsion (Af);and within the third region the water-rich bottom phase (0) is the continuous phase. Each region forms three-phase emulsions in its interior; whereas along the sides of the tietriangle the threephase emulsions reduce to the two-phase morphologies T / J ~ AT/T T/G, etc. For all three phases, X= %&or a,phase T i s the continuous phase over the phase-volume fraction range 0.5 S 4 ~ 1.0. 5 Because similar behavior has been found for the three-phase dispersion morphology diagram of C4HgOC2&0Wn-decane/
Here
49449+ 49
@Y/SF
J. Phys. Chem., Vol. 98, No. 46, 1994 12099
Three-phase, "Microemulsion" Emulsions
m
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Figure 3. Three-phase dispersion morphology diagram (in phasevolume fractions) measured for C&3(OC~I-I&OWn-tetradecane/aqueous 10 mM NaCl at 25 "C, exhibiting regions of !%continuous, JKcontinuous, and @-continuoustwo-phase and three-phase emulsions,
respectively.
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0.20
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Figure 4. Comparison of predicted and measured conductivities for ~ 0.20 at 45 "C: diamonds, three-phase emulsions along the path q 5 = emulsions titrated with &and @, squares, emulsions titrated with T and @, dashed line, predicted by eq 2 for the JKcontinuous morphology; dotted line, predicted by eq 2 for the %continuous morphology; solid line, predicted by eq 3."
aqueous 10 mM NaCl at 35 oC,8we might think of Figure 3 as representing the "normal" three-phase dispersion morphology diagram. However, we have found other interesting phenomena for three-phase emulsions which indicate that Figure 3 may not adequately illustrate the three-phase emulsion morphology diagram for all conditions. Figure 4 illustrates the conductivities measured for three-phase emulsions along the path @a = 0.20 at 45 "CI7 (at which temperature the system has a wetting middle phaseL3). In Figures 1 and 2 part of the data are in accord with the predicted conductivities for one morphology, while the rest of the data are in accord with the predictions for a second morphology; and there is a discontinous jump in the data between the two predicted curves. By contrast, in Figure 4 neither the A& continuous (dashed line) nor the Tcontinuous (dotted line) prediction of eq 2 agrees with the data. Furthermore, there seems to be no discontinuity: Instead, all of the data fall close to the predictions of a single-valued function. Analogous behavior at 45 "C was found'* along the experimental path 4~ = 0.10, which is the compositional path (at a different temperature) of Figure 2 and parallel and close to the experimental path of Figure 4. For three-phase emulsions that exhibit behavior of the type shown in Figure 4 we have that the measured conductivities are accurately predicted by the equation16
I;, . \..
".- '.
*.,
\
I 0.00
0.20
0.40
0.60
0.80
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Figure 5. Emulsion conductivities at 45 "C: (a) measured (diamonds, emulsions titrated with 4 squares, emulsions titrated with T) along the T-ACtieline, (b) predicted by eq 1 for T/A(dashed line) and &/T(dotted line) morphologies, and (c) predicted (solid line) by eq
4.
As in eq 1, the two-phase emulsion conductivities in eq 3 may be either measured or predicted by the Maxwell equation. (If the morphology of either of the two-phase emulsions is more complexI8 than that assumed by eq 1, either the measured conductivity or the conductivity predicted by another theoretical equation must be used instead of that predicted by eq 1.) The values of eq 3, based on the two-phase conductivities predicted by eq 1, also have been plotted in Figure 4. They are in good agreement with the measurements. The agreement between experiment and prediction is even better if, in eq 3, the measured values of the two-phase emulsion conductivities are used in place of the values predicted by eq 1. In the two-phase limit +F= 0.0, eq 3 reduces to16J7
Kdis= (4gKi'3
+ 43Kj'3)3
(4)
The conductivities measured for emulsions of phases Tand A at 45 "C are plotted in Figure 5. The compositional path is the same as in Figure 1, but for a different temperature and wettability condition. As in Figure 1, Figure 5 includes the conductivities predicted by eq 1 for Fcontinuous (dotted line) and &continuous (dashed line) emulsions, respectively. However, analogous to the differences between Figures 2 and 4,two important differences between Figures 1 and 5 may be noted: (1) The conductivity data in Figure 5 exhibit no discontinuity. ( 2 ) The data are not accurately predicted by eq 1 for either the A I T o r the T/Amorphology. As illustrated by Figure 5, the predictions of eq 4 are in much better accord with the experimental data than is either of the curves predicted by eq 1. The similar differences between Figures 1 and 5 (two phases) and between Figures 2 and 4 (three phases) suggest that for this system the morphologies of two-phase and three-phase emulsions formed for small volume-fractions of bottom phase may undergo some fundamental change between 25 and 45 "C. At 45 "C three-phase emulsions and emulsions of top-bottom and middle-bottom phase-pairs exhibited the milkiness that typifies most such dispersions. However, we observed that the two-phase emulsions of Figure 5 were not cloudy but instead were transparent (Le., "water-white"). These observations correlate with the fact that at 45 "C &and T a r e near-critical (Tuc = 49 "C) but neither of the other two phase-pairs is. Although at 45 "C we could not detect transitions between &continuous and Fcontinuous emulsions, we had no difficulty
Johnson et al.
12100 J. Phys. Chem., Vol. 98, No. 46, 1994
m
m
L.U 4
,'
I
J
,I
a Figure 6. Three-phase dispersion morphology diagram (in phasevolume fractions) measured for C&I13(0C2H&OWn-tetradecane/aqueous 10 mM NaCl at 45 "C.
n
8
Figure 8. Dispersion morphology diagrams at 25 and 45 "C superimposed, showing the relative insensitivity of the "inversion" phasevolume fractions to temperature.
m
a Figure 7. Three-phase dispersion morphology diagram (in phasevolume fractions) at 12 "C, illustrating "disappearance" of the A2
continuous
-
@-continuoustransition line.
in finding transitions between the @-continuous morphology and emulsions in which 68 was not the continuous phase. The three-phase dispersion morphology diagram (in phasevolume fractions) measured at 45 "C is shown in Figure 6. In many ways it resembles the diagram at 25 "C: The AT- 68 side of the tietriangle is divided into regions of a / @ and @/AT morphologies, respectively; the T- 0limiting tieline is divided and 681Temulsions; and the interior of into regions of the tietriangle has a line marking the smallest volume fractions of @ for which @?-continuous emulsions can form. This latter boundary may even have a cusp pointing to where the "missing" boundary between &continuous and Tcontinuous emulsions would be expected from the diagram at 25 "C. (See Figures 3 and 8.) However, in Figure 6 the boundary between &continuous and -7continuous emulsions is absent. This absence implies that three-phase emulsions which contained small amounts of W changed smoothly between the two-phase 0 / T a n d @/AT morphology limits, without any abrupt transition between the two continous phases; similarly, emulsions which contained no LZ changed smoothly between T a n d i7/Mmorphologies, without any discemable "first-order" transition. After failing to find any &-continuous Fcontinuous morphology transition at 45 "C, we attempted to find acontinuous @-continuous transitions at 12 "C, again performing experiments along both two-phase and three-phase paths. Figure 7 illustrates the dispersion morphology diagram at 12 "C. (The morphology transition lines at 12 "C are less exact, because fewer experimental paths were used than at the two higher temperatures.) As at 25 and 45 "C, at 12 "C transitions between % ! continuous and @-continuous morphologies were easily found. Furthermore, at 12 "C transitions between J$$continuous and
-
-
*-C,,H,rJ
H2°
Figure 9. Superimposed dispersion morphology diagrams of Figure
7-replotted in component concentrations-showing how morphology boundaries can appear to be very sensitive to temperature because of the temperature dependence of the composition of the middle phase. Tcontinuous morphologies also were found. However, at the lowest temperature we could not detect first-order &continuous @-continuous transitions in either three-phase or two-phase emulsions. Thus the dispersion morphology diagram reflected the (approximate) symmetry of the phase and physical properties about the To,, = (Tic -I- TUJ2 plane. In all cases abrupt transitions of continuous phase could not be found between pairs of near-critical phases, whereas such transitions were readily detected when the two continuous phases were not close to critical. Although temperature had a dramatic effect on our ability to find dispersion morphology boundaries between &-continuous and either Fcontinuous or 68-continuous emulsions, it had relatively little effect on the locations of the boundaries that were observed. This finding is illustrated by Figure 8, which shows the dispersion morphology diagrams measured at 25 and 45 "C superimposed. The maximum boundary shift found was about 10 phase-volume %. However, if the isothermal morphology boundaries and tietriangles have not been measured, experiments performed by variation of temperature at constant system composition can appear to indicate that the three-phase morphology boundaries are very temperature-sensitive.24 This is illustrated by Figure 9, which shows the dispersion morphology diagrams of Figure 8 converted to component concentrations by means of measurem e n t ~of~ the ~ phase compositions. (Phase densities were estimated from component densities25and the assumption that component volumes were additive.) In component-concentration space the transition line between 5continuous and GJcontinuous emulsions is relatively insensitive to temperature because the compositions of phases Tand 68 change relatively
-
J. Phys. Chem., Vol. 98, No. 46, 1994 12101
Three-phase, “Microemulsion” Emulsions MIDDLE
TOP
BOTTOM
Figure 10. Three-phase emulsion morphology diagram (in phasevolume fractions) measured for C4H~OC2H40Wn-decanelaqueous10 mM NaCl at 35 OC,* for comparison with Figure 3: triangles, limits of ‘?continuous emulsions; circles, limits of JZcontinuous emulsions;
squares, limits of @-continuousemulsions. little. However, the lines for isothermal transitions between &-continuous and Fcontinuous emulsions and between A& continuous and @-continuous emulsions change considerably with temperature, because the composition of the middle phase changes greatly between temperatures TlCand Tuc.
Discussion The dispersion morphology diagram, Figure 3, observed for three-phase emulsions of the CsH13(0C~H4)~0Wn-tetadecane/ aqueous 10 mM NaCl (“C6E2”) system at 25 “C conformed to our expectations.* These expectations were based on the morphology diagram for the two-phase dispersions that surround the tietria~~gle,~ which predicted the experimental results for three-phase emulsions of the CJ-IgOC2H40Wn-decanelaqueous 10 mM NaCl system at 35 “C8For comparison with Figure 3, the three-phase emulsion morphology diagram for the C4H9OC~H~OWn-decane/aqueous 10 mM NaCl (“C4E1”) system at 35 “C is shown in Figure 10. Like Figure 3, Figure 10 contains regions of @-continuous, &$continuous, and Fcontinuous twophase and three-phase emulsions, respectively. The diagrams of Figures 3 and 10 were measured near the optimum temperatures of their respective chemical systems, so that for each system the interfacial tensions y pTand y ~ s were g about equal. (For C6H13(OC~H4)~0W~-tetradecane/aqueous 10 mM NaCl at 25 “C y p =~ 0.2 dyn cm-1.z6) However, the middle phase of the C6E2 system is nonwetting at 25 0C,3,5,13 whereas for the C4E1 system” the middle phase wets the interface between the top and bottom phases. Thus, the similarity between Figures 3 and 10 indicates that any role of middle-phase wettability in three-phase emulsion morphologies is, at most, secondary. This is in accord with the existence of three different morphologies in each diagram, which by itself shows4 that a simple spreading-coefficient theory for dispersion morphologiesz7 is invalid for these conditions. The behavior observed for three-phase emulsions of the C6H13(0C~b)~0Wn-tetradecane/aqueous 10 mM NaCl system at 12 and 45 “C was not predicted, however, and this behavior suggests phenomena and experiments that eventually may require fundamental modifications to the dispersion morphology diagram for both two-phase and three-phase emulsions. Possible reasons why no discontinuities were observed in the electrical conductivities for some experimental paths at 12 and 45 “C include the following: (a) Discontinuous morphology transitions-with small amounts of hysteresis-occurred at very large volume fractions of one of the phases, where emulsion conductivity differences among different morphologies are small. (b) Discontinuous transitions-with very wide amounts
of hysteresis-occurred at very small -Jolume fractions of the continuous phase, for both continuous phases of the transition. (c) Direct inversion did not occur, but instead multiple emulsions formed. (d) Conductivity discontinuities occurred near $I = 0.5 but were too small to be detected because of unfavorable values of the phase conductivities. (e) “New” morphologies formed in two-phase and compositionally nearby three-phase emulsions, for which-as the data indicate-abrupt changes of electrical conductivity simply did not occur. Possibility a is unlikely, because theory and experiments indicate that the ratio of the phase-volume fraction of the dispersed phase(s) to the phase-volume fraction of the continuous phase approaches 1.0 as a critical point is approached (Le., for two-phase emulsions, the inversion phase-volume fraction approaches 0.510,21,28929). (Such behavior is observed along both the AT- ?and &Fatransition lines for emulsions of three phases (Figures 3 and 6 and ref 8), as well as for emulsions of two phases with10,28,z9 or withoutz1 oil. Case b is the most difficult to detect, because with it morphology transitions occur only at very small fractions of the discontinuous phase. However, case b also seems unlikely, because of the above-described objection to case a and because theory and experiments also indicate that the width of the morphology hysteresis should approach zero as a critical point is For some ways of forming emulsions it may appear that direct Z/Yinversions do not occur; instead of inversion, multiple emulsions of the type (case c) are formed. However, this appearance can be deceiving. In fact, inversion does occur so long as steady-state emulsions are maintained while the volume fraction of the dispersed phase is increased; the multiple emulsions are formed when mixing is stopped and creaming is allowed to O C C U ~ . ~(A~ ,mechanism ~ ~ for this has been described e l s e ~ h e r e . ~ ~We ) did not allow multiple creamings to occur after each phase addition, as is sometimes done;31hence, formation of multiple emulsions is unlikely in our experiments. Case d is an inherent problem, because conductivity differences between phases approach zero as they approach criticality. We attempted to alleviate this problem by performing experiments near both critical end-point temperatures, so that very different absolute conductivities would be obtained for the respective near-critical phase-pairs. More directly, the conductivity discontinuitiespredicted for abrupt, first-order morphology transitions in two-phase and three-phase emulsions at 12 and 45 “C were sufficiently large compared to the experimental sensitivity that our failure to find them may simply indicate that the conductivity discontinuitieswere not there. Furthermore (more so at 45 “C, somewhat less so at 12 “C), for experiments in which no discontinuity was detected, the measured conductivities were not accurately predicted by equations that had proved very accurate.9~10.16-19 for emulsions that undergo “normal” morphology inversions. Thus, we tentatively conclude that the morphology transitions of these emulsions were, indeed, “anomolous”. If this conclusion is correct, for the system of the present study the critical point temperature for continuous Fcontinuous morphology transitions apparently occurs not at the upper critical end-point temperature for phases, Tuc(phase) = 49 OC,13 but between 25 and 45 “C; likewise, the critical point temperature for ,&continuous @-continuous morphology transitions would occur not at the phase lower critical end-point temperature, qc(pbe) = 9 OC13but between 12 and 25 “C. These distinctions between phase critical points and emulsion morphology critical points would obtain for both two-phase and three-phase emulsions.
f/z+
9/s/tT...
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12102 . I Phys. . Chem., Vol. 98, No. 46, 1994
a
Figure 11. Emulsion morphology diagram in phase-volume fraction
a-
T a n d 4 - g faces of the threeand temperature space for the phase tieprism, illustrating emulsion inversion hysteresis lines meeting at critical temperatures different from the phase critical end-point temperatures.
Figure 11 (in temperature and phase-volume fraction coordinates) illustrates the dr T a n d M- @ faces of the assumed tietriangular prism; the M/T- T/& TI&- M I T a n d MJ@ @/& @Id- &/68 pairs of inversion lines in the two respective faces; and the &IT- T/&and @/A critical points. As plotted, the emulsion critical points occur at 4 ~ 0.5 = and @ g= 0.5, respectively, but the &IT- TIM (Le., upper) and MI@ @/A (lower) emulsion inversion critical temperatures are below and above the upper and lower phase critical end-point temperatures, respectively. In this model the upper and lower inversion critical temperatures could be at the middle-phase13and bottom-phase14wetting transition temperatures, respectively, but at present this is only an interesting speculation (made less likely by the known failures4.* of spreading-coefficient theory for emulsion morphologies). It is also interesting to speculate about the morphologies of two-phase and three-phase emulsions that apparently did not undergo first-order morphology transitions. One interesting hypothesis" is that they are bicontinuous. If this speculation is correct, true (Le., macro-) emulsions might make it possible to see structures that have been proposed for microemulsion phases. +
+.
Conclusion The limits over which the oil-rich top phase (T),middlephase microemulsion (&, or water-rich bottom phase ( @ ) is the continuous phase in steady-state, three-phase macroemulsions have been determined by means of electrical conductivity measurements for the amphiphile/oiY"water" system C6H13(OC2&)20Wn-tetradecane/aqueous 10 mM NaCl at 12,25, and 45 "C. All three morphologies were observed a t 25 "C;hence, those results were in accord with the expected dispersion morphology diagram previously observed*for another chemical system. Near the critical end-point temperature for phases &and however, abrupt transitions of the type usually found for
Johnson et al. emulsion inversion could not be found between M-continuous and Fcontinuous morphologies in either two-phase or threephase emulsions. Likewise, near the critical end-point temperature for phases &and 68, no first-order transitions between &-continuous and @-continuous morphologies were found. Further experiments are needed to determine if the disappearance of these two transitions is merely an experimental problem of detection or a "real" phenomenon. The latter possibility suggests several interesting speculations, including dispersion morphology diagrams in which critical points for morphology transitions do not coincide with phase critical points (but, possibly, with wetting transitions), and the possibility of macrodispersions in which more than one phase is continuous.
Acknowledgment. We thank Dr. Yau-Hsin Wang for making the measurements at 12 "C. This work was supported by the Office of Fossil Energy, U.S. Department of Energy. References and Notes (1) Improved Oil Recovery by Surfactant and Polymer Flooding; Shah, D. O., Schechter, R. S., Eds.; Academic Press: New York, 1977. (2) Transport and Remediation of Subsugace Contaminants; Sabatini, D. A., Knox, R. C., Eds.; American Chemical Society: Washington, DC, 1992. (3) Smith, D. H.; Covatch, G. L.; Lowry, W. E. SPEFonn. Eval. 1992, 7, 323. (4) Smith, D. H.; Johnson, G. K.; Dadyburjor, D. Lungmuir 1993, 9, 2089. ( 5 ) Smith, D. H.; Covatch, G. L. J . Chem. Phys. 1990, 93, 6870. (6) Wellington, S. L. U.S. Patent 4,380,266, 1983. (7) Schramm, L. L.; Turta, A. T.; Novosad, J. J. SPE Reservoir Eng. 1993, 8, 201. (8) Smith, D. H.; Wang, Y.-C. J. Phys. Chem. 1994, 98, 7214. (9) Smith, D. H.; Lim, K.-H.; J . Phys. Chem. 1990, 94, 3746. (10) Smith, D. H.; Covatch, G. L.; Lim, K.-H. Langmuir 1991, 7, 1585. (11) Kahlweit, M.; Busse, G. J . Chem. Phys. 1989, 91, 1339. (12) Johnson, G. K. A Study of Three-phase Emulsion Behavior. Ph.D. Dissertation, West Virginia University, 1993. (13) Smith, D. H.; Comberiati, J. R. U S . Department of Energy unpublished report, 1989. (14) Chen, L.-J.; Yan, W.-J. J. Chem. Phys. 1993, 98, 4830. (15) Smith, D. H. J . Colloid Interface Sci. 1985, 108, 471. (16) Smith, D. H.; Johnson, G. K.; Wang, Y.-C.; Lim, K.-H. Lungmuir 1994, IO, 2516. (17) Johnson, G. K.; Dadyburjor, D. B.; Smith, D. H. Langmuir 1994, 10, 2523. (18) Smith, D. H.; Nwosu, S.N.; Johnson, G. K.; Lim, K.-H.; Langmuir 1992, 8, 1076. (19) Lim, K.-H.; Smith, D. H. J. Dispersion Sci. Technol. 1990, 11, 529. (20) Maxwell, J. C. A Treatise on Electricity and Magnetism; Dover: New York, 1954; p 485. (21) Smith, D. H.; Lim, K.-H. Langmuir 1990, 6, 1071. (22) Smith, D. H.; Lim, K.-H. SPE Prod. Eng. 1990, 5, 265. (23) Smith, D. H.; Reckley, J. S.; Johnson, G. K. J . Colloid Interface Sci. 1992, 151, 383. (24) Smith, D. H.; Johnson, G. K. Temperature Dependence of Emulsion Morphologies and the Dispersion Morphology Diagram. 2. Three-phase Emulsions. J. Phys. Chem., submitted for publication. (25) Smith, D. H.; Covatch, G. L. (a) J . Colloid Interface Sci. 1994, 162, 372; (b) J . Colloid Interface Sci., in press. (26) Wang, Y.-H.; Smith, D. H. Unpublished. (27) Torza, S.; Mason, S. G. Science 1969, 163, 813. (28) Ross, S.; Kombrekke, R. E. J . Colloid Interface Sci. 1981, 81, 58. (29) Lim, K.-H.; Smith, D. H. J . Colloid Interface Sci. 1991. 142, 278. (30) Salager, J.-L. In Encyclopedia of Emulsion Technology; Becher, P., Ed.; Marcel Dekker: New York, 1988; Vol. 3, p 79. (31) Brooks, B. W.; Richmond, H. N. Colloids Surf. 1991, 58, 131.
