© Copyright 1996 American Chemical Society
APRIL 17, 1996 VOLUME 12, NUMBER 8
Letters Macroemulsion Stability within the Winsor III Region: Theory versus Experiment Alexey Kabalnov* and Jeffry Weers Alliance Pharmaceutical Corp., 3040 Science Park Road, San Diego, California 92121 Received November 20, 1995. In Final Form: January 31, 1996X The stability of macroemulsions formed upon mixing the phases of the Winsor III equilibrium was studied. The resulting macroemulsions contained the upper and lower phases, emulsified in each other; the middle phase did not coemulsify with them and separated within the first hour after emulsification. Within ca. 0.2 °C on the both sides of the balanced point, the macroemulsions were very unstable. Within the following 0.15 °C, a spectacular increase in stability by 3 orders of magnitude occurred on either side of the phase inversion temperature. The same pattern of macroemulsion stability was reproduced by varying salinity at constant temperature. The experimental data are in good agreement with a model, relating the rate of coalescence to the free energy penalty of hole nucleation in the emulsion films.1
Introduction There is a strong correlation between the phase behavior of oil-water-surfactant systems and the type and stability of macroemulsions formed upon mixing of the equilibrium phases in each other.2-7 In the Winsor I region, where surfactant micelles coexist with excess oil in a twophase equilibrium, nonequilibrium macroemulsions are an oil in water (O/W) type. In the Winsor II region, where inverse micelles coexist with excess water, macroemulsions are a water in oil (W/O) type. In the Winsor III region, where oil, water, and a “middle” bicontinuous microemulsion phase coexist in a three-phase equilibrium, both O/W and W/O macroemulsions are unstable. Al* Corresponding author. E-mail:
[email protected]. X Abstract published in Advance ACS Abstracts, April 1, 1996. (1) Kabalnov, A.; Wennerstro¨m, H. Langmuir 1996, 12, 276. (2) Shinoda, K.; Friberg, S. Emulsions and Solubilization; John Wiley & Sons: New York-Chichester-Brisbane-Toronto-Singapore, 1986. (3) Baldauf, L. M.; Schechter, R. S.; Wade, W. H.; Graciaa, A. J. Colloid Interface Sci. 1982, 85, 187. (4) Salager, J. L.; Loaiza-Maldonado, I.; Minana-Perez, M.; Silva, F. J. Dispersion Sci. Technol. 1982, 3, 279. (5) Aveyard, R.; Binks, B. P.; Fletcher, P. D. I.; Ye, X. In EmulsionssA Fundamental and Practical Approach; Sjo¨blom, J., Ed.; Kluwer: Amsterdam, 1992; p 97. (6) Binks, B. P. Langmuir 1993, 9, 25. (7) Binks, B. P. Colloids Surf., A: Physicochem. Eng. Aspects 1993, 71, 167.
0743-7463/96/2412-1931$12.00/0
though this phenomenon has a very general character, the reason for it has been unknown. Recently1 we proposed a model of macroemulsion stability, in which we argue, that both the equilibrium phase behavior and the stability of nonequilibrium macroemulsions are dependent on the monolayer bending elasticity parameters. The latter are introduced by the Helfrich equation8
W)
∫
∫
κ (2H0 - H1 - H2)2 dA + κj H1H2 dA (1) 2
Here W is the elastic bending energy, H0 is the spontaneous curvature, κ and κj are the bending and saddle-splay moduli, H1 and H2 are the reciprocal principal radii of curvature of the monolayer, and dA is the surface area of a patch on the neutral surface. According to the sign convention, the curvature of O/W droplets is considered positive. According to our model, the spontaneous curvature strongly affects the free energy penalty for nucleation of a critical hole in an emulsion film W*. The dependence of W* on H0 comes from the fact that the surfactant monolayer is strongly curved at the edge of the nucleation pore and contributes an additional (positive or negative) (8) Helfrich, W. Z. Naturforsch. 1973, 28c, 693.
© 1996 American Chemical Society
1932 Langmuir, Vol. 12, No. 8, 1996
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Figure 1. Logarithm of macroemulsion lifetime, log( τ), s, versus temperature T, °C (upper X-axis), and salinity (lower X-axis) for the C12E5-n-octane-water system. Salinity scan data at T ) 19.91 °C (squares), and the data of the temperature scan experiment with the middle phase present (filled circles) and middle phase removed (unfilled circles) are shown. (A) Overall shape of the curves. Insert schematically shows the dependence of the activation energy on the spontaneous curvature according to ref 1. (B) Detailed behavior in the vicinity of the balanced point.
term into the free energy penalty. The activation energy determines the film lifetime τ as
W* τ ) f exp kT
( )
(2)
where the preexponent f is a constant. We briefly outline the predictions of the model below, with respect to the stability of an oil-water-oil (O/W/O) emulsion film (the behavior of the W/O/W film is a mirror image to that of the O/W/O film, see insert on Figure 1A). At H0 < 0, the O/W/O film breaks without a significant barrier. At positive values of the spontaneous curvature, the barrier steeply increases with H0 as 3/2
W* ) -4πκj + 7.0κ +
35.7κ σ01/2
H0
(3)
where σo is the interfacial tension between the oil and water phases at the balanced point (H0 ) 0). Note that at small positive H0 values, the film is still unstable, because the magnitude of the exponential factor is canceled by the preexponent. Thus, the sharp increase in stability is predicted to be somewhat offset from the balanced point. After this steep increase, the barrier levels off to a constant value, which is a function of the bending and the saddle splay moduli. For instance, if κj ) 0, W* ) ∼64.3κ. The spontaneous curvature of nonionic surfactants of the poly(oxyethylene) type can be controlled by varying temperature due to temperature-induced dehydration of ethylene oxide groups.9,10 Close to the balanced point, H0 can be expanded in series versus temperature (9) Olsson, U.; Wennerstro¨m, H. Adv. Colloid Interface Sci. 1994, 49, 113. (10) Strey, R. Colloid Polym. Sci. 1994, 272, 1005.
H0 ) -R(T - T ˜)
(4)
where T ˜ is the balanced, or phase inversion temperature (PIT), and R is an empirical coefficient. Alternatively, H0 can be finely tuned by adding inorganic electrolytes.11-13 Many inorganic salts are depleted from the O/W interface due to the image charge repulsion effect.14-16 The depleted salt exerts osmotic pressure on the surfactant brush, dehydrates its polar heads, and bends the monolayer towards water.13 As in the case of temperature, the effect is linear in the salt osmotic pressure (concentration), but the proportionality coefficient has a more clear physical sense
∆H0 ) βCsalt ) -
δ2 Π 4κ salt
(5)
where Πsalt ) 2CsaltRT/M is the osmotic pressure of the bulk salt solution, Csalt is the weight concentration in the bulk, M is the salt molecular weight, δ is the thickness of the depletion layer, modeled as a step function, and β ) δ2RT/2κM. The inner side of the depletion layer is assumed to coincide with the neutral surface. Strey10 studied in detail the equilibrium properties of the n-octane-water-n-C12H25(OCH2CH2)5OH [C12E5] system and found the following values for the parameters of this system: κ ) 0.6 kT; κj ) 0.3 kT; σ0 ) 4 × 10-4 mN/m; (11) Kahlweit, M.; Lessner, E.; Strey, R. J. Phys. Chem. 1984, 88, 1937. (12) Kahlweit, M.; Strey, R.; Haase, D. J. Phys. Chem. 1985, 89, 163. (13) Kabalnov, A.; Olsson, U.; Wennerstro¨m, H. J. Phys. Chem. 1995, 99, 6220. (14) Onsager, L.; Samaras, N. T. J. Chem. Phys. 1934, 2, 528. (15) Aveyard, R.; Saleem, S. M. J. Chem. Soc., Faraday Trans. 1 1976, 72, 1609. (16) Aveyard, R.; Saleem, S. M.; Heselden, R. J. Chem. Soc., Faraday Trans. 1 1977, 73, 84.
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Langmuir, Vol. 12, No. 8, 1996 1933
Results and Discussion Our study mostly concerns the macroemulsion stability within the Winsor III region. The Winsor III system in equilibrium contains three phases: transparent upper (oil) and lower (water) phases and a bluish middle phase. The middle phase contains ca. 5.1 wt % of surfactant, and almost equal volumes of oil and water, being slightly waterrich below the balanced temperature and oil-rich above it. The lower phase represents a weak molecular solution of C12E5 (∼2 × 10-3 wt %) in water; the upper phase is a molecular solution of C12E5 (∼1 wt %) in octane.18,19 Adding sodium chloride decreases the balanced temperature (PIT) according to the following empirical equation:
T ˜ ) PIT ) 32.65 - 1.48CNaCl
Figure 2. Typical pattern of macroemulsion breakage after mixing the phases of the Winsor III equilibrium in each other, within (0.5 °C of the balanced point. Here u is the upper (oil) phase, l is the lower (water) phase, and m is the middle (microemulsion) phase.
H0 ) -1.525 × 105 (T - T ˜ ) [cm-1], T ˜ ) 32.6 °C. On substitution of these values in eq 3, a ∼20-fold increase in stability of macroemulsion is predicted per 0.1 °C increment, or per ∼0.01 M of added salt. These macroemulsion stability coefficients are very unusual. Indeed, in terms of a phenomenological Arrhenius equation (i.e., assuming the activation energy to be temperatureindependent), the temperature coefficient of 20 per 0.1 °C corresponds to the activation energy of 2 × 104 kJ/mol, which is ca. 2 orders of magnitude larger than the energy of a chemical bond. Similarly, salting-out effects at the centimolar level are typically negligible.17 If these temperature/salt effects would indeed be found, it would be a strong argument supporting our model. The objective of this study was to examine the transition region from unstable to stable emulsions in the vicinity of the balanced point and to check if the prediction of our model (eq 3) is true. We control the spontaneous curvature by changing temperature and by adding salt (NaCl). Experimental Section A 0.736 g portion of a 4.33 wt % solution of C12E5 in n-octane and 1.00 g of an aqueous solution of NaCl at the appropriate concentration were placed into a 5-mL screw-cap test tube; the samples were ca. 1:1 oil-to-water by volume. Throughout this paper, the concentration of NaCl is presented in wt % per weight of water only. The test tubes were mounted into a 2-L jacketed beaker filled with water. The constant temperature was maintained with an RTE-221M circulating bath (Neslab Instruments) and controlled by an F250 digital thermometer (Automatic Systems Laboratories) within 0.01 °C. After thermal equilibration of the samples, the beaker with the mounted samples in it was vigorously shaken by hand 3-5 times. A macroemulsion of coexistent phases was formed, that gradually broke over time. Despite this very primitive emulsification procedure, no dependence of the emulsion stability on the shaking procedure was found. On the other hand, temperature control at the level of 0.01 °C was proven to be absolutely essential for the experiment. (17) Collins, K. D.; Washabaugh, M. W. Q. Rev. Biophys. 1985, 4, 323.
(6)
where the concentration of NaCl is in wt % and the temperature is in °C.20 After mixing the three pre-equilibrated phases, the system becomes milky. Very close to the balanced state (within ca. 0.2 °C on each side) the macroemulsion breaks within 1 min and one cannot follow any pattern of the phase separation. At ca. 0.3 °C from the balanced point, the macroemulsion becomes rather stable, and one can notice the following sequence of events (Figure 2). (i) Within the first hour, the bluish microemulsion phase, designated on Figure 2 as m, separates from the milky macroemulsion. The location of the m-phase in the test tube depends on the relative densities of the m-phase and the macroemulsion: Below the PIT, the m-phase is waterrich and is located at the bottom. Conversely, above the PIT, the m-phase is oil-rich and is located at the top. (ii) At the second stage, the “milk” layer starts to cream, and it does so in a very peculiar manner. The excess continuous phase of the macroemulsion drains through the m-phase and forms a separate layer. Above the PIT, this is a layer of free oil (u) at the top of the vial; below the PIT, this is a layer of free water (l) at the bottom (Figure 2). (iii) At the third stage, the cream layer starts to coalesce, forming a layer of free dispersed phase. Above the PIT it leads to formation of a free water layer (l) at the bottom; below the PIT it leads to a free oil layer (u) at the top. The rate of this stage is strongly temperature dependent and is the main interest for our study. We arbitrarily assign the macroemulsion lifetime, τ, as the time necessary for half of the cream layer to coalesce. The value of τ was found to be dependent only on the monolayer spontaneous curvature or, in other words, on the difference between the PIT (T ˜ ) and the actual temperature T: τ ) f(T - T ˜ ). The macroemulsion stability experiment can therefore be run in two versions: at a constant salinity and variable temperature (so that T ˜ ) const and T varied), or at a constant temperature and variable salinity (T ) const and T ˜ varied). The latter was found more convenient because it allows a simultaneous experiment on many samples and does not necessitate time-consuming temperature readjustments. Temperature Scan Experiment. Parts A and B of Figure 1, upper X-axis, show the lifetime of the emulsion cream plotted versus temperature at CNaCl ) 0. Within ca. 0.2 °C on either side of the balanced point, macro(18) Aveyard, R.; Binks, B. P.; Clark, S.; Fletcher, P. D. I. J. Chem. Soc., Faraday Trans. 1990, 86, 3111. (19) Kabalnov, A.; Olsson, U.; Wennerstro¨m, H. Langmuir 1994, 10, 2159. (20) The coefficient 1.48 K/wt % holds below 1 wt % NaCl concentration. It somewhat decreases with CNaCl: thus, at CNaCl ) ∼10 wt %, it equals 1.38 K/wt %. We neglect this small nonlinearity in our estimates of the coalescence barrier.
1934 Langmuir, Vol. 12, No. 8, 1996
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Figure 3. Photographs, showing the typical evolution of the macroemulsions over time in a salinity scan experiment at T ) 31.71 ˜ ) 31.94 °C; CNaCl ) °C. On each photograph, the conditions for the tubes from left to right were as follows: CNaCl ) 0.48 wt %, T ˜ ) 31.91 °C. Initially, the macroemulsions have an O/W type. The sample with the 0.49 wt %; T ˜ ) 31.92 °C; CNaCl ) 0.50 wt %; T highest salinity is closest to the balanced state and breaks first.
emulsions are very unstable and break within 1 min. Within the following 0.15 °C, a drastic increase in the cream stability from ∼1 min to ∼10 h occurs. Similar data were previously reported by others.4,21 Although the cream stability is, to a first approximation, symmetrical with respect to the PIT, the structure of the cream is not. Conductivity measurements on systems containing NaCl revealed that below the PIT the cream is an O/W macroemulsion, whereas above the PIT it is W/O. We found that the middle phase does not affect the cream stability. After the macroemulsion broke, we removed most of the middle phase and reemulsified the phases in each other. Essentially the same cream stability was found after the volume fraction of the middle phase was reduced from ∼0.3 (filled circles) to ∼0.005 (open circles), as shown on Figure 1B. At lower temperatures, over Winsor I region, the O/W macroemulsions are very stable. None of the systems studied showed any significant coalescence within more than a month. There is no apparent discontinuity in the O/W stability in the vicinity of the lower three-phase body end-point temperature. The behavior is different over the Winsor II region, where W/O emulsion stability first increases with temperature, again, with no discontinuity at the upper three-phase body end-point, and then passes (21) Kahlweit, M.; Strey, R.; Haase, D.; Kunieda, H.; Schmeling, T.; Faulhaber, B.; Borcovec, M.; Eicke, H.-F.; Busse, G.; Eggers, F.; Funk, Th.; Richmann, H.; Magid, L.; So¨derman, O.; Stilbs, P.; Winkler, J.; Dittrich, A.; Jahn, W. J. Colloid Interface Sci. 1987, 118, 436.
through a maximum and decreases back to several minutes at 50 °C. Salinity Scan Experiment. Parts A and B of Figure 1, lower X-axis, show the lifetime of the emulsion cream plotted versus salinity at constant temperature T ) 19.91 °C. As in the temperature scan experiment, within the Winsor I the system forms stable O/W macroemulsions, and the sharp change in stability occurs within the Winsor III region, close to the balanced point (Figure 1). In contrast with the temperature scan experiment, over the Winsor II region, all the samples are very stable and no secondary decrease in stability is observed. More detailed experiments were conducted with two sets of NaCl samples: one covering the range from 0.42 to 0.5 wt % and the other from 9.35 to 9.43 wt %. Although the absolute concentrations differed by a factor of ca. 20, the same relative spans of ca. 0.1 wt % and increments of ca. 0.01 wt % were studied. Both sets of samples were used for investigating W/O and O/W emulsions. The temperature was adjusted in such a manner that the spontaneous curvature range of a given set of samples matched the range at which the sharp change in macroemulsion stability occurred. Thus, in the O/W study, the temperature was kept at ca. 0.3 °C below the PIT value of the sample with the highest salinity. All the samples were kept in the same beaker and were mixed simultaneously to keep all factors as constant as possible. In the vicinity of the balanced point, the macroemulsion lifetime changes by a factor of ∼2 per 0.01% of NaCl added
Letters
Langmuir, Vol. 12, No. 8, 1996 1935 Table 1. Spontaneous Curvature Coefficients of Activation Energy E ) dW*/dH0, Experiment versus Theorya
theoryb: ) (35.7κ3/2/σ01/2 experiment: temperature scan,c CNaCl ) 0 salinity scan, T ) 19.91°C salinity scan, T ) 19.61°C salinity scan, T ) 31.71°C salinity scan, T ) 32.24 °C
× 1027 [J m], O/W systems
× 1027 [J m], W/O systems
7.24 5.6 (r ) 0.99)
-7.24 -8.7 (r ) 0.99)
8.5 (r ) 0.98)
-13.2 (r ) 0.98) -13.1 (r ) 0.95)
8.4 (r ) 0.97) 8.7 (r ) 0.97) 9.4 (r ) 0.998) 9.0 (r ) 0.99) -12.0 (r ) 0.998) -11.2 (r ) 0.996)
a The results of double runs are shown, if available. The correlation coefficient of the linear plots is shown in parentheses. b Calculated with κ ) 0.6 kT; σ ) 4 × 10-4 mN/m; R ) -1.525 × 0 107 [m-1 K-1] and β ) -2.26 × 107 [m-1 (wt %)-1], ref 10. c The data are straightened over the ranges 32.36 < T < 32.46 °C (O/W) and 32.84 < T < 33.01 °C (W/O) (temperature scan experiment), and over the ranges shown in Figure 4 (salinity scan experiment).
Figure 4. Salinity scan experimental data: logarithm of macroemulsion lifetime, log(τ), s, versus salinity, CNaCl, wt %. The data for two sets of samples are shown: CNaCl ) 0.42-0.50 wt % (lower X-axis, circles and squares) and CNaCl ) 9.35-9.43 wt % (upper X-axis, triangles). Note that the scale of the both axes is the same, only the origin is different. The temperature at which the experiment was conducted is shown on the plot. Experiments at 32.24 and 31.71 °C were run twice to check the reproducibility: (A) O/W emulsions (systems below PITs); (B) W/O emulsions (systems above PITs).
as illustrated in Figure 3. On each photograph, the tubes from left to right have CNaCl ) 0.48, 0.49, and 0.50 wt % salt concentrations, respectively. The systems are kept below their balanced temperatures and macroemulsions initially have O/W type. One can see that despite only minor differences in salinity, there is a considerable difference in the coalescence rates. The sample with the highest salinity is closest to the balanced state and breaks first. Parts A and B of Figure 4 show the plots of the macroemulsion lifetime in a logarithmic scale versus salinity for O/W and W/O emulsions, respectively. The plots are essentially linear and the slope of the lines is close for the both sets of the samples: 0.42-0.5 wt % (filled squares and circles) and 9.35-9.43 wt % (open triangles); they are however somewhat different between O/W and W/O systems. See Table 1. Theory Versus Experiment. According to theory,1 the emulsion coalescence barrier is a linear function of the monolayer spontaneous curvature in the vicinity of the balanced state, with the slope, determined by eq 3. As can be seen from eqs 2, 4, and 5, the slope can be evaluated from the semilogarithmic plots of the macroemulsion lifetime versus temperature and salinity. The linearity is indeed clearly observed in the salinity scan experiment and, with a higher scatter, in the temperature scan experiment (see Figures 4 and 1B). Table 1 compares the experimental values of the slopes ) dW*/dH0 with theory.
The agreement is reasonable in view of the uncertainty in the value of the monolayer bending modulus κ. There are some trends in the data, however, which do not fit the theory. First, the absolute value of the slopes for inverse emulsions are ca. 30% larger than those for O/W emulsions. Second, the absolute values of the slope in the temperature scan experiment is ca. 30% smaller than that in the salinity scan experiment. Clearly, more work is needed to understand these effects. Another aspect of the theory to be compared with experiment is the emulsion stability over the Winsor I and Winsor II regions. The theory1 predicts a very high coalescence barrier over these regions, which is equal to W* ) 64.3κ, provided that saddle splay and entropy contributions are negligible. The data of the salinity scan experiment qualitatively agree with this prediction: over the Winsor I and Winsor II regions, the macroemulsions do not show any visible coalescence. When analyzing the data of the temperature scan experiment, there is an additional effect, which must be taken into account: as the temperature increases, not only does the spontaneous curvature decrease, but the thermal energy, kT, increases. We attribute the secondary decrease in the O/W emulsion stability at high temperatures to the classical thermal activation effect. The activation energy, calculated from the last four points of the temperature scan experiment with the Arrhenius equation, is however larger than expected (∼150 kT). A possible explanation for this observation may be a decrease in the activation energy barrier with temperature, with concomitant decrease in the bending modulus. This would result in overestimating the apparent activation energy. Conclusions We studied the stability of macroemulsions formed on mixing the equilibrium phases in each other. Over the Winsor III region, the experimental data are in a good quantitative agreement with a model for macroemulsion stability, relating the rate of coalescence to the free energy penalty of hole nucleation in emulsion films.1 Over the Winsor I and Winsor II regions, the agreement is qualitative but not quantitative. Acknowledgment. A.K. is thankful to Reinhard Strey for the personal communication regarding the n-octaneC12E5-water phase diagram. LA951053A
