The Microemulsion Phase in the Didecyldimethylammonium Bromide

Didecyldimethylammonium Bromide/Dodecane/Water ... double-chained surfactant didecyldimethylammonium bromide in the presence of water and dodecane...
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Langmuir 2001, 17, 3567-3572

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The Microemulsion Phase in the Didecyldimethylammonium Bromide/Dodecane/Water System. Phase Diagram, Microstructure, and Nucleation Kinetics of Excess Oil Phase Andreas Arvidsson and Olle So¨derman* Physical Chemistry 1, Center for Chemistry and Chemical Engineering, P.O. Box 124, S-221 00 Lund, Sweden Received October 23, 2000. In Final Form: February 28, 2001 We report on a study dealing with the properties of the microemulsion phase formed by the synthetic double-chained surfactant didecyldimethylammonium bromide in the presence of water and dodecane. It is shown that the microemulsion phase in this system connects to the water corner. The upper boundary of the microemulsion at high water contents is concave downward, and an explanation is provided for this effect. The microstructure of the microemulsion is probed using component resolved NMR diffusion methods. The results indicate that upon decreasing water content, the microemulsion becomes bicontinuous and at a rather well-defined concentration there is a discontinuity in the water diffusion implying that at this point the microemulsion becomes truly bicontinuous. Finally, on account of the appearance of the extension of the microemulsion region at high water content, it is possible to study the nucleation kinetics of the excess oil phase as the system is perturbed by the addition of water such that one enters the two-phase region microemulsion/excess hydrocarbon.

Introduction The cationic double-chained molecules with the general appearance (Cx-Cy)-N-(CH3)2 X (where X is an anion) constitute an important class of surfactants.1-4 Both the binary surfactant/water and ternary surfactant/hydrocarbon/water systems have been the topics of numerous studies. Their ability to form microemulsions and the evolution of the microemulsion properties with changes in various parameters is by now well investigated. The surfactants have very little water solubility and virtually zero hydrocarbon solubility, making them excellent compounds for model studies of microemulsions. The symmetric variety with x ) y ) 12 and Br- as the counterion forms microemulsions with a number of different hydrocarbons.5 With dodecane, the microemulsion is connected to the oil corner (where we use the term “oil” as a generic term for a hydrocarbon) of the ternary phase diagram, and the hydrophobic/hydrophilic interface has negative curvature (counting curvature toward hydrocarbon as positive) in its entire region of existence. Interestingly, the exchange of Br- for SO42- (while keeping the other components the same) changes the sign of the curvature of the surfactant film.6,7 As a consequence, the microemulsion phase in the SO42- case is connected to the water corner and the microemulsion is water continuous (either with closed oil-swollen aggregates or with a bicontinuous structure) over its entire region of existence. * Corresponding author. E-mail: [email protected]. (1) Chen, S. J.; Evans, D. F.; Ninham, B. W.; Mitchell, D. J.; Blum, F. D.; Pickup, S. J. Phys. Chem. 1986, 90, 842. (2) Chen, V.; Warr, G. W.; Evans, D. F.; Prendergast, F. G. J. Phys. Chem. 1988, 92, 768. (3) Evans, F.; Wennerstro¨m, H. The Colloidal Domain: where Physics, Chemistry, Biology and Technology Meet; VCH Publishers: New York, 1994. (4) Fontell, K.; Ceglie, A.; Lindman, B.; Ninham, B. Acta Chem. Scand. 1986, A40, 247. (5) Warr, G. G.; Sen, R.; Evans, D. F.; Trend, J. E. J. Phys. Chem. 1988, 92, 774. (6) Kang, C.; Khan, A. J. Colloid Interface Sci. 1993, 156, 218. (7) Nyden, M.; So¨derman, O. Langmuir 1995, 11, 1537.

In fact, it is possible to tune the curvature of the surfactant film by changing the proportion of either ion in mixtures of surfactants with Br- and SO42- counterions.8 The origin of the specific ion effect is most certainly differences in interaction between Br- or SO42- and the surfactant film. Ninham and Yaminsky argue that the effect is due to dispersion interactions acting on the ions.9 An alternative way to tune the curvature is to change the hydrocarbon, while at the same time keeping the surfactant tail length and counterion unchanged. Thus, Warr et al. have investigated the microemulsions formed with various oils in the x ) y ) 10 variety, henceforth referred to as DeDAB.5 With hexane, the microemulsion is connected to the oil corner, whereas for octane it is disconnected from the oil corner (cf. Figure 1). This difference can be understood in terms of penetration of the hydrocarbon molecules into the surfactant film. The rule of thumb used when discussing oil penetration states that the penetration is substantial for straight-chain hydrocarbons of equal or shorter lengths than the surfactant tail. When oil penetration occurs, the curvature of the surfactant film is decreased, and as a consequence microstructures with reversed curvatures are preferred. The preferred microstructure of a surfactant film is often discussed in terms of the critical packing parameter, CPP (sometimes referred to as the surfactant number), v/al where v is the surfactant hydrocarbon volume, a is the (effective) polar head area, and l is the length of the surfactant tail. In terms of this quantity, oil penetration results in an increase of the CPP, a situation that favors reversed structures. Thus, it is conceivable that the microemulsion formed in the DeDAB/water/dodecane system may connect with the water corner, as opposed to the C12 corresponding surfactant with dodecane. The difference between the properties of C10 and C12 surfactant based systems in (8) Nyde´n, M.; So¨derman, O. Langmuir, in press. (9) Ninham, B. W.; Yaminski, V. Langmuir 1997, 13, 2097.

10.1021/la001487s CCC: $20.00 © 2001 American Chemical Society Published on Web 05/11/2001

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Arvidsson and So¨ derman NMR Self-Diffusion Measurements. The water, hydrocarbon, and surfactant self-diffusion coefficients in the microemulsion region were determined by means of the 1H NMR PGSE method.10,11 Depending on the values of the relaxation times T1 and T2, a Carr-Purcell modified Hahn echo or a stimulated echo with square gradient pulses was used. The experiments were performed on a Bruker DMX 100 NMR spectrometer, using a Bruker self-diffusion probe and a gradient driver of in-house design. The echo decays were analyzed by means of the relevant Stejskal-Tanner expression:

I ) I0e-kD

Figure 1. The extension of microemulsion regions in the DeDAB/hydrocarbon/water systems at 25 °C. Data from this work and ref 5.

this regard can then be understood in terms of two effects: the first being the lesser degree of oil penetration in the C10 case and the second being the general effect associated with having a surfactant with a shorter tail. Both of these effects render the C10 surfactant more hydrophilic than the C12 variety, pointing to a surfactant film with curvature toward hydrocarbon rather than toward water for systems with the former surfactant. Expressed in terms of the CPP, this quantity is thus predicted to be smaller for the C10 than for the C12 surfactant. In this paper, we present the extension of the microemulsion region in the DeDAB/water/dodecane system. The microstructure of the microemulsion has been investigated by means of the NMR pulsed gradient spin echo (PGSE) self-diffusion method. Finally, we present some data associated with the kinetics of the phase separation of the microemulsion upon dilution into the two-phase area where the microemulsion is in equilibrium with excess oil. Experimental Section Materials. The surfactant used in this investigation was from Tokyo Kasei and was used as received. Two different batches were used, and there were no noticeable differences in the results regardless of which batch was used. n-Dodecane was from MerckSuchardt. Water was of Milli-Q quality. Phase Diagram. The extension of the microemulsion region in the ternary phase diagram was determined as follows. Samples were first made along the binary water/surfactant axis, each containing approximately 0.2 g in total. Each sample was then titrated with dodecane. After each addition of oil (typically one drop was added), the sample was stirred by a magnetic stir-bar for typically 15 min. When the sample became fully transparent, it was judged to be a microemulsion. The maximum amount of hydrocarbon that each sample could solubilize was then determined by further addition of hydrocarbon, until the sample became nontransparent. A laser beam was shone through the sample to facilitate the detection of large scattering objects (such as excess hydrocarbon in the form of macroscopic oil drops). The water-rich microemulsion samples were left for several days to equilibrate, because it was found that the rate of deemulsification was very slow. The number of samples used in determining the extension of the phase diagram was 65. The phase diagram was determined at 25 ( 1 °C.

(1)

In eq 1, I0 represents the peak amplitude in the absence of gradients and k is the field gradient parameter, defined as k ) (γδg)2(∆ - δ/3), where γ is the gyromagnetic ratio of the proton, g is the magnitude of the field gradient, δ the length of the gradient pulse, and ∆ is the separation (from leading edge to leading edge) of the two gradient pulses in the echo experiment. D is the selfdiffusion coefficient. The 1H echo NMR spectrum showed four peaks, originating from the following molecules/groups: H2O, N-CH3, (CH2)n, and ω-CH3. Of the three latter peaks, the N-CH3 originates solely from the surfactant, whereas the other two have contributions from both surfactant and hydrocarbon. The echo decays of H2O and N-CH3 were always monoexponential (when plotted vs k, see eq 1), whereas the data from the (CH2)n and ω-CH3 peaks were sometimes biexponential, implying that in those samples the hydrocarbon and surfactant do not diffuse with the same diffusion coefficient. For those samples, the surfactant diffusion coefficient was determined from the decay of the N-CH3 peak, and the hydrocarbon diffusion was obtained from a fit of a sum of two exponentials (with appropriate weighting factors), where one of the diffusion coefficients was locked to the value obtained from the N-CH3 peak. The uncertainties of the presented diffusion coefficients are better than (5%, somewhat smaller for the water than for the surfactants. The temperature was controlled at 298 K by a stream of temperature-controlled air. The flow of air was kept high in order to minimize the temperature gradients over the sample, because these introduce artefacts in the diffusion measurements.12

Results and Discussion Phase Diagram. Presented in Figure 1 is the extension of the microemulsion region in the DeDAB/dodecane/water system. Also given are the extensions of the (main) microemulsion regions for hexane and decane, as redrawn from results presented in ref 5. With hexane, the microemulsion region extends from the oil corner, whereas with decane the microemulsion connects with neither the water nor the oil corners. On the basis of conductivity data,5 Warr et al. conclude that there is a transition to closed water droplets as the hexane system is diluted with oil, occurring at around 90 wt % oil content. At lower oil contents, the hexane system appears to be bicontinuous, whereas the decane microemulsion is bicontinuous in its entire region of existence. With dodecane, the microemulsion extends all the way to the water corner, whereas at lower water contents it solubilizes less oil than the octane, although its boundaries are fairly similar to those of the decane system. We will start by considering the water-rich part of the dodecane system. Given in Figure 2 is an enlargement of the water-rich area, covering the range from 90% to 100% water. As noted above, the microemulsion region connects with the water corner. Above the region, a two-phase area of microemul(10) Stilbs, P. Prog. Nucl. Magn. Reson. Spectrosc. 1987, 19, 1. (11) Callaghan, P. T. Principles of Nuclear Magnetic Resonance Microscopy; Clarendon Press: Oxford, 1991. (12) Hedin, N.; Furo´, I. J. Magn. Reson. 1998, 131, 126.

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Figure 3. The quantity ΦO/ΦS vs the surfactant number. See text for details.

Figure 2. The extension of microemulsion regions in the waterrich corner of the DeDAB/dodecane/water systems at 25 °C.

sion and pure dodecane exists. Thus, the upper boundary indicates the maximum amount of oil that can be solubilized for a given amount of surfactant. Such boundaries are often termed emulsification failure boundaries.13 This fact implies that along the upper line, oilswollen spherical micelles exist. An interesting observation is that the upper line is concave when seen from above. This means that the quantity ΦO/ΦS on the upper boundary decreases as the system is diluted with water. The reason for this, we maintain, follows from the following state of affairs. Consider a spherical oil-swollen micelle in equilibrium with excess oil (the situation at hand along the upper boundary). The relation between the CPP and the ratio of the volume fraction of hydrocarbon to the volume fraction of surfactant is given by7

ΦS v 1 v - + ) ΦO al 3 al 1 1 + 1+ Φ ΦS 1 1 v S v 3 1+ 27 1+ al ΦO 3 al ΦO 3

(

)

((

) ) ((

) )

2

(2)

Solving eq 2 for ΦO/ΦS and plotting this quantity as a function of the value of the CPP () v/al) produces the data shown in Figure 3. As can be inferred from the figure, a decrease in the CPP leads to a decrease in ΦO/ΦS. Thus, in the present system the CPP decreases upon water dilution, which is the expected behavior for an ionic surfactant. For such systems, electrostatic effects have a major influence on the phase behavior and the surfactant aggregate structure.3,14-16 From an electrostatic point of view, the addition of water decreases the concentration of ions close to the surface, which leads to a decreased shielding between the charged headgroups at the hydrophobic/hydrophilic interface and consequently to a larger effective headgroup area. Thus, in terms of the quantities of the CPP, a increases while v and l remain unchanged, leading to a decrease in the CPP. The emulsification failure boundary for the microemulsion formed in the corresponding C12 surfactant with SO42- as the counterion is (13) Safran, S. NATO ASI Proceedings; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1991; p 1. (14) Jo¨nsson, B.; Wennerstro¨m, H. J. Colloid Interface Sci. 1981, 80, 482. (15) Jo¨nsson, B.; Wennerstro¨m, H. J. Phys. Chem. 1987, 91, 338. (16) Hayter, J. B. Langmuir 1992, 8, 2873.

a straight line.7 Thus, the process of dilution does not decrease the concentration of SO42- close to the micellar surface to the same extent. Keeping in mind that iSO42is divalent, this is perhaps an expected result, as the interaction between a divalent ion and a charged surface is much stronger than that between a monovalent ion and a charged surface. The fact that the extension region of the microemulsion region in the water-rich part of the phase diagram is “bent” means that it is possible by a mere dilution with water to enter the excess oil/microemulsion region. Thus, it is possible to investigate the kinetics of the phase transition in a straightforward manner (see further discussion below). Turning to the main extension of the microemulsion region, it is observed to form an extended area, which lies approximately parallel to the water/surfactant binary axis. Below the microemulsion region, there is a two-phase area. Samples from this region are bluish. Presumably vesicles are formed here, because there is a lamellar phase below the microemulsion phase. Before leaving this section, we note that the extension of the phase boundaries of the microemulsion changes considerably if D2O is used instead of H2O. This is an unusual feature in a charged surfactant system and corroborates the findings of Warr et al., who noticed a similar behavior in the binary DeDAB/H2O,D2O system.5 Microstructure from PGSE NMR Measurements. To investigate the microstructure of the microemulsion, the self-diffusion coefficients of the water, dodecane, and surfactant (denoted below as DW, DO, and DS, respectively) were determined. The values of the self-diffusion coefficients convey easily interpretable data about the microstructure found in surfactant solutions (see, e.g., ref 17 and references therein). The basis for the method is the fact that the self-diffusion coefficients are measured on a time scale of around 100 ms. As a consequence, the mean-squared displacements of the molecules are in the micrometer regime, and the values of the microemulsion component diffusion coefficients are sensitive to micellar size and obstruction effects. Plotted in Figure 4 are the values of DS and DO versus the volume fraction of oil plus surfactant (Φ(O+S)). It should be noted that the samples are not on a dilution line toward the water corner (i.e., they do not have a constant value of ΦO/ΦS). The compositions of the samples used are given as an inset in Figure 4. At high water contents (corresponding to samples in the narrow section of the micro(17) So¨derman, O.; Stilbs, P. Prog. Nucl. Magn. Reson. Spectrosc. 1994, 26, 445.

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Figure 6. The water diffusion coefficient vs the volume fraction of oil and surfactant for the same samples used in Figure 4.

Figure 4. DO and DS vs the volume fraction of oil and surfactant. The inset shows the position in the phase diagram of the samples used. Note that the schematic phase diagram in the inset corresponds to the water-rich corner (covering from 100 to 40% water).

Figure 5. The hydrodynamic radius vs the volume fraction of oil and surfactant as derived from the data in Figure 4.

emulsion region which extends down toward the water corner), DS and DO are equal. This implies that the oil and surfactant diffuse together in one entity and reflects the fact that oil-swollen discrete micelles are present in this area. The hydrodynamic radius RH of these micelles can be obtained from

DS,O )

kT 6πηRH

(3)

where k, T and η have their usual meanings. Assuming spherical aggregates with no hydrocarbon penetration into the surfactant film, the area per polar headgroup can be obtained from RH and ΦO/ΦS by means of the following relation:

a)

(

)

ΦO 3vS 1+ RH ΦS

(4)

where vS is the surfactant molecular volume (for DeDAB, we use vS ) 718 Å3). Using the data in Figure 4 and eqs 3 and 4, we present in Figure 5 RH as a function of Φ(O+S) for the extension of the microemulsion where there are discrete aggregates. As can be inferred from Figure 5, the aggregates are rather small, implying large headgroup areas at high dilution. For the highest water content

studied (99 wt % water), the obtained value of RH is 24 Å which (together with the relevant value of ΦO/ΦS) results in a value of a ) 130 Å2 and an aggregation number of 56 surfactants per micelle (and roughly 50 dodecane molecules per micelle). This is a rather large area per polar headgroup. Areas usually quoted for DDAB in the liquid crystalline phases range from 62 to 70 Å2.18,19 For DDAS, a value of 76 Å2 was obtained from the (normal) hexagonal phase.7 This again points to the fact that at these high water contents, the degree of shielding of the headgroup/ headgroup interactions by counterions is relatively small, although it should be recalled that the analysis above assumes that the micelles are spherical in shape. For other conceivable shapes, such as prolate or oblate shaped aggregates, the area per headgroup would be smaller. After the initial decrease in DS and DO upon increasing Φ(O+S), signaling growth of discrete micelles, the diffusion coefficients go through a minimum and then start to increase. Moreover, the values of DS and DO are no longer equal, the latter increasing considerably more rapidly than the former. This implies that additional diffusion paths for the surfactants and oil come into play. This is the onset of a structural change into a bicontinuous structure, and at about 35 wt % water both DS and DO level off and remain constant up to the highest investigated value of Φ(O+S). The values of DW are presented in Figure 6. Up to Φ(O+S) of around 0.35, there is a mild linear decrease of DW with increasing values of Φ(O+S). Extrapolated to pure water, we recover the bulk value of water.20 This is typical for a microstructure which is water continuous and in which the volume fraction of micelles increases. The decrease in the water diffusion with increasing volume fraction of aggregates is caused by the combination of two effects: the obstruction effect of the aggregates (which depends on the volume fraction and shape of the obstructing aggregates) and the hydration of the polar headgroups.21,22 At around Φ(O+S) ≈ 0.35, there is a discontinuity in the value of DW. Above this point, the water diffusion continues to mildly decrease with roughly the same rate, but now the extrapolated value to Φ(O+S) ) 0 is 1.5 × 10-9 m2 s-1, which is almost exactly 2/3 of the bulk water value of DW. It should be noted that the DW values measured for samples with compositions close to and around the discontinuity were highly reproducible, and the effect is certainly real. (18) Skurtveit, R.; Olsson, U. J. Phys. Chem. 1992, 96, 8640. (19) Dubois, M.; Zemb, T.; Fuller, N.; Rand, R. P.; Parsegian, V. A. J. Chem. Phys. 1998, 108, 7855. (20) Mills, R. J. Phys. Chem. 1973, 77, 685. (21) Jo¨nsson, B.; Wennerstro¨m, H.; Nilsson, P. G.; Linse, P. Colloid Polym. Sci. 1986, 264, 77. (22) Andersson, D.; Wennerstro¨m, H. J. Phys. Chem. 1990, 94, 8683.

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Our interpretation of the discontinuity is based on the following line of reasoning. One common model for bicontinuous microemulsions not too far from the balanced state (i.e., a microemulsion with equal volumes of oil and water) describes them in terms of a minimal surface separating the hydrocarbon and water volumes. The surfactant film is then draped around the minimal surface. The diffusion is now effectively taking place in a twodimensional geometry (see discussion in ref 22), and therefore the expected value of DW is around 2/3 of the relevant bulk value of DW (hydration effects and effects due to the fact that the surfactant film occupies a nonnegligible volume reduce the value of DW further). We also note that in the regime where the dodecane diffusion coefficients are approximately constant, the value of DO is roughly 2.6 × 10-10 m2 s-1. The diffusion coefficient for neat dodecane is 8.3 × 10-10 m2 s-1. Thus, the reduced hydrocarbon diffusion coefficient in this area is roughly 1/ . For the balanced microemulsion (with zero volume 3 fraction of surfactant film), one would expect the reduced diffusion coefficient of both the oil and the water to be 2/3. This is an upper limit, because obstruction effects and solvation effects at finite film volume fractions would decrease this value. Thus, the values of the oil reduced diffusion coefficients are also in agreement with a structure based on a connected monolayer film. Taken together, the values of the measured diffusion coefficients indicate that in the water-rich section of the microemulsion there are discrete aggregates that grow in size when the volume fraction of oil and surfactant increases. The system then undergoes a structural transition, and in the section of the microemulsion which lies parallel to the water/surfactant axis, the microstructure is bicontinuous. Most likely, the structure can be derived from an infinite minimal periodic surface which is draped with a surfactant film, separating the oil and water domains. Nucleation Kinetics of Excess Oil Phase. The fact that the boundary to the oil-rich side of the microemulsion in the water-rich area is concave implies that it is possible to go from a situation of discrete oil-swollen micelles to a two-phase area of micelles and excess oil by a mere dilution with water. This resembles the situation found in nonionic surfactant based microemulsions, although in that case the surfactant film curvature is tuned by the temperature, and consequently the jump from the microemulsion region to the two-phase region is brought about by a sudden depression of the temperature. This feature allows one to study the nucleation kinetics of the excess oil phase. Such studies have been performed by Olsson, Wennerstro¨m, and co-workers for the nonionic system.23 Here, we present a corresponding study of the nucleation kinetics in the ionic system described above. Shown in Figure 7 is a schematic phase diagram, which describes how the experiments were performed. Starting from points well inside the microemulsion phase (a and b in Figure 7), the samples were diluted with water until three points were reached in the two-phase region microemulsion/excess oil. The radii of the droplets are now different from the equilibrium radii, the values of which are given by the points on the upper boundary of the microemulsion region where the tie-lines connecting the oil corner and the points (1,2, and 3 in Figure 7) originate, and the systems respond by expelling oil in order to shrink the microemulsion droplets to the new equilibrium sizes. This process was followed by measuring

the turbidity of the samples. The measurements started immediately after the dilution process. Each sample was initially transparent and without color. With time, they became bluish and then white and less transparent. After several days, the white region started to move upward, leaving a clear colorless solution at the bottom of the cuvette. The turbidity data are presented in Figure 8, with the data for the first 3000 min given in the inset. Following the discussions in refs 23 and 24, the turbidity data can be interpreted as follows. The excess oil expelled from the micelles will be collected by some micelles that grow in size. The free energy cost of increasing the growing micelles by making their radius deviate more and more from the spontaneous radius will be overcome by the free energy gain when the shrinking micelles adopt a radius closer to the spontaneous radius. If the oil volume and surfactant film area are constant, then the process can occur only if the number of small micelles increases.24 In effect, there will be a flow of oil molecules from the small to the large droplets, in a process that is related to an Ostwald ripening process.24 The rate of formation of the big droplets is expected to depend on the magnitude to the spontaneous curvature jump, such that the rate increases with the size of the jump.24 This is also borne out by the data in Figure 8, that show that the turbidity increases faster for the sample which has the biggest curvature jump (sample 3, cf. Figure 7). Once the big droplets reach a certain size, they will start to cream. This explains the decrease in the turbidity after longer periods of time. We can make a simple estimate of the size of the droplets that cream. The “active area” of the turbidity (as defined by the size of the light beam passing through the sample) experiment is approximately 1 cm in height, starting 1 cm above the bottom of the

(23) Morris, J.; Wennerstro¨m, H.; Olsson, U. Langmuir 1997, 13, 606.

(24) Wennerstro¨m, H.; Morris, J.; Olsson, U. Langmuir 1997, 13, 6972.

Figure 7. Dilution of two samples initially inside the microemulsion (a and b) area produces three samples in the twophase region (1,2, and 3) in which the new equilibrium droplet radii are given by the points on the upper boundary of the microemulsion region where the tie-lines connecting the oil corner and the points originate.

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Figure 8. The results of the turbidity measurements: sample 1 (]), sample 2 (0), and sample 3 (O). In the inset, data for the first 3000 minutes are given.

cuvette. This means that a creaming droplet starting from the bottom has to travel 2 cm before leaving the light path. On the basis of the information in Figure 8, this movement takes approximately 1 × 104 min and thus the velocity of the rising droplet is roughly 3 × 10-7 m s-1. Now, the creaming velocity v for an isolated droplet of radius R is given by

v)

2R2∆Fg 9η

(5)

where ∆F is the difference in density between the continuous medium (water) and the creaming droplet and g is the acceleration of free fall. Using the value estimated for v above, we obtain R ) 0.2 µm. After a creamed layer has formed at the top of the solution, the emulsion breaks and the excess oil phase is finally formed.

The initial turbidity increase in the nonionic surfactant system studied in ref 23 is considerably faster than for the ionic system in the present study. There may be several reasons for this fact. First, the rate depends strongly on the magnitude of the spontaneous curvature jump, which may be different for the two cases. Second, in our investigation dodecane was used as an oil, whereas in ref 23 decane was used. The former has a lower oil solubility (by roughly a factor of 2025) which would certainly affect the rate of flow of oil from the smaller to the larger micelles in the initial step. Acknowledgment. This work was financially supported by the Swedish Natural Science Foundation (NFR). We are grateful to Bjo¨rn Håkansson for technical assistance in connection with the NMR experiments. LA001487S (25) McAuliffe, C. J. Phys. Chem. 1966, 70, 1267.