Thermodynamically Stable Vesicle Formation and Vesicle-to-Micelle

Apr 5, 2013 - The aggregation behavior of sodium 3,6,9,12,15-pentaoxa-heptacosanoate (AEC4-Na) in aqueous solution with increase of the concentration ...
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Thermodynamically Stable Vesicle Formation and Vesicle-to-Micelle Transition of Single-Tailed Anionic Surfactant in Water Takaya Sakai,*,† Risa Ikoshi,‡ Natsuko Toshida,† and Mariko Kagaya† †

Research and Development, Eco-Innovation Research, Kao Corporation, 1334, Minato, Wakayama-shi, Wakayama 640-8580, Japan Research and Development, Beauty Research, Kao Corporation, 2-1-3 Bunka, Sumida-ku, Tokyo 131-8501, Japan

‡

ABSTRACT: The aggregation behavior of sodium 3,6,9,12,15-pentaoxaheptacosanoate (AEC4-Na) in aqueous solution with increase of the concentration at 25 °C was investigated using differential scanning calorimetry, equilibrium surface tension, solubilization of an oil-soluble dye, steady-state fluorescence, dynamic light scattering, and freezefractured transmission electron microscopy. Vesicle formation of AEC4Na preceded micelle formation below the critical micelle concentration (cmc). The vesicle-to-micelle transition was observed through a narrow concentration region above the cmc. The mean diameters of the vesicles and micelles were not affected by the concentration. All solutions over a wide range of concentrations were homogeneously transparent with a low Krafft point below 0 °C. These results indicate that the AEC4-Na vesicles have a thermodynamically stable structure. Vesicle formation may be caused by a pseudobinary mixed surfactant system composed of monomeric AEC4-Na and an acid soap that consists of a dimer complex formed between AEC4-Na and unneutralized AEC4-Na. The thermodynamic stability would then result from the inhibition of close intermolecular aggregation and flexibility of the molecular shape in the vesicles due to the oxyethylene units in AEC4-Na.

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INTRODUCTION Vesicles are a type of aggregate formed by certain surfactants in water that have attracted attention with respect to their morphology and properties, which could provide functional applications in biomimetics, drug delivery, and microarray technologies.1−3 The fundamental morphological structure of vesicles is comprised of surfactant bilayers is widely known. The type of aggregates formed by surfactants in water is thought to be predicted by the packing parameter (P): P=

ν a0lc

tailed surfactants with an ionic headgroup generally tend to exhibit much smaller P values than 0.5 because they have a small volume of tail group and a large surface area around the headgroup, which should lead to difficulty in spontaneous vesicle formation. Therefore, to form thermodynamically stable vesicles in a single surfactant system, it is important that the molecular structure of the surfactant has conformational flexibility for the morphological environment in the vesicles.6 Moreover, although many vesicles that appear to be thermodynamically stable have been reported, most of them should finally deposit as precipitates in water because almost all of these vesicles were metastable solid aggregates.7 The two most well-known surfactants that easily exhibit vesicle formation at dilute concentrations are dipalmitoyl phosphatidyl choline (DPPC) and dioctadecyl dimethylammonium bromide (DODAB). Vesicle formation of these surfactants requires the input of mechanical energy such as sonication.8 In addition, the Krafft points of these surfactants are much higher than room temperature,9,10 so that they should exist as hydrated solids separated from water at room temperature, even though they have the structures of vesicular aggregates. This suggests that these vesicles are thermodynamically metastable, and that the formation of thermodynamically stable vesicles is difficult, even more so for single-component systems. There have been only a few reports of spontaneous vesicle formation.11−13 Recently, there has been much research regarding the spontaneous formation of thermodynamically stable vesicles

(1)

where ν is the volume of the hydrophobic tail, lc is the hydrophobic chain length, slightly inferior to that of a fully extended one, and a0 is the area of the polar headgroup on the surface of the aggregate.4,5 Thus, P is determined according to the nature of surfactants. The P parameter for double-tailed surfactants with a large headgroup could be between 0.5 and 1.0 and such surfactants tend to form vesicles. Surfactants with P = 0.5−1.0 have a preferential orientation in the outer layers of the bilayers of the vesicle that cause a slight curvature of the vesicle surface due to the head groups. However, in the case of a single-component vesicular system, the surfactants form the inner surface of the bilayers, which have the opposite curvature to that of the outer surface. Therefore, the surfactants must exist with a geometry (P > 1.0) that is unfavorable with respect to stability. Moreover, in the case of ionic surfactants, the more unfavorable orientation must be forced by the electrostatic repulsion between ionic head groups, so that half of the surfactant molecules that build vesicles should exist with unfavorable instability in the structures. In addition, single© 2013 American Chemical Society

Received: March 6, 2013 Revised: April 4, 2013 Published: April 5, 2013 5081

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that results from oppositely charged single-tailed surfactant mixtures in water.14−16 With respect to the packing parameter P, this method is reasonable because different compositions of mixed surfactants can be applied to the inner and outer layers of vesicle bilayers. In such binary surfactant systems, vesicles are formed first and then often become micelles with an increase in the surfactant concentrations, which is referred to as vesicle-tomicelle transition. This transition is of significant interest because the more highly ordered vesicle structure spontaneously transforms into a less highly ordered micelle structure with increase in the surfactant concentration. This is completely opposite to the general theory regarding the self-assembly of surfactants. Recently, vesicle-to-micelle transition in single surfactant systems has been reported, where the surfactants are single tailed and have weakly acidic carboxylate or phosphate headgroups.17−19 Although the mechanism for this aggregation behavior has not yet been determined, it has been assumed to result from double-tailed dimer formation between a neutralized anionic derivative and unneutralized acidic surfactant by hydrogen bonding. This dimer has also been referred to as acid soap. The precipitates in these systems also appear when the vesicles are formed in the bulk, which indicates that these vesicles are also thermodynamically metastable. Sodium 3,6,9,12,15-pentaoxa-heptacosanoate (AEC, Figure 1) is an anionic surfactant with a weakly acidic nature that has

Therefore, completely neutralized AEC is also expected to form vesicles spontaneously as well as the weakly acidic single-tailed anionic surfactants that are completely neutralized, even though it has a single-tailed anionic surfactant structure.17−19 In this study, the aggregation behavior of AEC in water was investigated, where AEC is completely neutralized, with an increase in the concentration. It is expected that the flexibility and hydrophilicity of the polyoxyethylene unit of AEC should provide the vesicles with thermodynamic stability, without the deposition of precipitates.

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EXPERIMENTAL SECTION Materials. 3,6,9,12,15-Pentaoxa-heptacosanoic acid (AEC4H), as shown in Figure 1, was prepared by the Williamson reaction of tetra(oxyethylene)dodecylether, of which the ethyleneoxide groups are not distributed along the oxyethylene chain length, but has a single tetra(oxyethylene) unit, and sodium monochloro acetate (SMCA) with NaOH as a catalyst at 75 °C. Tetra(oxyethylene) dodecylether (>99%; Nikko Chemical), NaOH (>97%; Sigma-Aldrich Corporation, St. Louis, MO, U.S.), and SMCA (>97%; Kanto Chemical Co., Inc., Tokyo, Japan) were used as received. The reaction mixture was diluted with ion-exchanged water, followed by extraction with hexane/isopropyl alcohol to remove unreacted tetra(oxyethylene)dodecylether. To the aqueous phase, 35% HCl aqueous solution (Kishida Chemical Co. Ltd., Osaka, Japan) was added to obtain pH 2−3. Hexane/isopropyl alcohol was added again to remove NaCl. After the organic phase was separated and washed with ion-exchanged water several times, the product was evaporated and dried to obtain high purity AEC4-H (>99.99%). The purity was confirmed by gaschromatography. An oil-soluble dye, o-toluene-azo-β-naphthol (Wako Pure Chemical Industries. Ltd., Osaka, Japan) to measure oil solubilization. and pyrene (>98%; Wako Pure Chemical Industries. Ltd.) as a fluorescence probe were used as received to determine the aggregation states of AEC in water. For other measurements, ultrapure water produced by distillation (RFD250RB, Advantec Toyo Kaisha, Ltd., Tokyo, Japan) was used to prepare sample solutions. The water had a surface tension of 72.4 mN m−1 at 25 °C. AEC4-H completely neutralized with Na (AEC4-Na; Figure 1) was prepared in solution by mixing AEC4-H and a molar equivalent of 1 mol dm−3 NaOH solution for quantitative analysis (Wako Pure Chemical Industries. Ltd., Osaka, Japan). All sample solutions were left to stand for one week at 25 °C prior to further measurements. Differential Scanning Calorimetry (DSC). DSC (Discovery DSC, TA Instruments, New Castle, PA, U.S.) was performed using approximately 75 mg of 12.5 wt % AEC4Na solution placed into a 100 μL stainless sealed-pan (900825.902, TA Instruments). Measurements of AEC4-Na samples were performed at a scan rate of 0.5 °C min−1 in the temperature range from −30 to 80 °C after the temperature was lowered to −30 °C at the same rate and held for 6 h. Transmittance (T%). Transmittance of aqueous solutions at 460 nm was measured using a UV−vis spectrophotometer (U-3300, Hitachi Corp., Tokyo, Japan). Equilibrium Surface Tension. The surface tension of the aqueous AEC4-Na solutions was measured at 25 °C using the Wilhelmy Pt plate technique with a tensiometer (K-100MK2 Krüss GmbH, Hamburg, Germany). Measurements were performed in the equilibrium state; i.e., the surface tension

Figure 1. Molecular structures of 3,6,9,12,15-pentaoxa-heptacosanoic acid salt. General structure (AEC): R shows the hydrophobic tail group, n is the addition number of ethylene oxide units, and M represents the countercation; AEC4-Na and AEC4-H: the hydrophobic tail groups are dodecyl only. n = 4.0 without distribution.

long been widely applied to cosmetics, detergents, and industrial chemical products.20 The characteristics of AEC are excellent detergency for sebum, low irritation to skin and eyes, excellent water-solubility, and excellent resistance to hard water.20−22 However, although some physicochemical properties of AEC have been investigated,22,23 the aggregation behavior of the single-component AEC system with change of the concentration has not been studied in detail, despite its long history. Kunz and colleagues have reported that AEC spontaneously forms vesicle aggregates in water only around its pKa (≈ 4.5) and this system appeared as a turbid dispersion. There are equivalent amounts of unneutralized AEC and ionic AEC at pKa; therefore, vesicles may also be formed by a doubletailed dimer between a neutralized anionic derivative and an unneutralized acidic surfactant through hydrogen bonding.23 5082

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was taken after measurement fluctuation was less than 0.1 mN m−1 for 1.0 h. Solubilization of Oil-Soluble Dye. Five mL of test solution and the required amount of oil-soluble dye, ο-tolueneazo-β-naphthol, were mixed in a 10 mL centrifuge tube using a vortex mixer. The tube was then gently shaken for a few days in a water bath at 25 °C, followed by filtering to remove the insoluble excess dye with a 5 mL syringe and a disposable filter unit DISMIC-13P (Tokyo Roshi Kaisha, Tokyo, Japan). A 4.00-mL portion of ethanol was added to 1.00 mL of the filtrate and the absorbance (ABS) of the solution was measured at 486 nm using a UV−vis spectrophotometer (U-3300, Hitachi Corp., Tokyo, Japan) to determine the amount of solubilized dye. Steady-State Fluorescence. Steady-state fluorescence measurements employing pyrene (1.0 × 10−6 mol dm−3) as a probe were conducted using a spectrofluorometer (F-7000, Hitachi Corp., Tokyo, Japan). Emission spectra were recorded at an excitation wavelength of 339 nm. Dynamic Light Scattering (DLS). DLS was measured (DLS-7000 system, Otsuka Electronic, Osaka, Japan) and the time-correlation functions were analyzed using the double exponential method.24 Measurements were performed at more than three different scattering angles and the diffusion coefficient was calculated from the slope of the straight line for the decay rate G, as a function of q2, where q is the scattering vector (Å−1). Freeze-Fractured Transmission Electron Microscopy (TEM). A sample replica for freeze-fractured TEM observation (JEM-1011, Jeol Ltd., operating voltage 100 kV) was prepared using a freeze etching apparatus (JFD-9010, Jeol Ltd., Tokyo, Japan) with a drop of 3.0 × 10−4 mol dm−3 AEC4-Na aqueous solution that was placed on a copper disk and then frozen with liquid nitrogen. Platinum and carbon deposition onto the sample was conducted at an angle of 45°. The replica was obtained by removal of the sample using a methanol/ chloroform mixture.

Figure 2. Equilibrium surface tension (γ) versus log C for AEC4-Na aqueous solution at 25 °C. C1, C2, and C3 indicate the break point concentrations. Regions I, II, III, and IV are concentration regions divided by each break point. The pH values of the aqueous solution at each concentration region are also shown.

begins to decrease almost linearly with the increase in the amount of the surfactant molecules that adsorb on the surface as the concentration increases. In Region II, a sudden change in the slope appears at C1 and γ does not decrease linearly, but is curved toward the lower surface tension region. γ plots in this dilute concentration region where γ steeply declines are generally linear and follow the Gibbs adsorption equation: Γ=−

⎛ ∂γ ⎞ 1 ⎜ ⎟ 4.606RT ⎝ ∂log C ⎠

T

(2)

where Γ = the surface excess concentration of surfactant, R = the gas constant, and T = the absolute temperature. Therefore, a constant slope γ plot indicates that the surface excess concentration, which is often regarded as the amount of surfactant adsorbed on the surface, does not change. However, the AEC4-Na aqueous solution does not follow eq 2. The amount of surfactant molecules that adsorb on the surface or the components of surfactants in this system keep changing with the increase in the AEC4-Na concentration. Although the AEC4-Na aqueous solution is a single-component system, a sufficient amount of AEC4-H can be produced to affect the interface properties in the chemical equilibrium because it is a weakly acidic surfactant. In Region III, γ reaches a minimum of 30.5 mN m−1 at C2 and starts increasing gradually again until a constant value is achieved at approximately 39.1 mN m−1, which almost agrees with the γcmc for some general anionic surfactants, such as SDS or sodium di(oxyethylene)dodecyl sulfate.25 Such a minimum value of the γ plot can be thought to be due to a small amount of hydrophobic impurities. However, the purity of the sample used here was confirmed to be over 99.99% by gas chromatography. Moreover, the reproducibility of the γ plots was confirmed by remeasurement with another AEC4-Na sample. Therefore, it is difficult to consider that the downward bulge and minimum are the result of impurities. Villeneuve et al. reported a similar γ curve for a mixed surfactant aqueous solution of SDS and decyltrimethylammonium bromide.26 From thermodynamic analysis, they proposed that the break point at the lower concentration was the critical vesicle concentration (cvc) and the other break point was the cmc, where the vesicle-to-micelle transition occurs. Such complex aggregation behavior has been thought to be caused

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RESULTS DSC. DSC traces for AEC4-Na aqueous solutions did not show any significant endothermic peak above 0 °C, which indicates that the Krafft point of EC-4Na is below 0 °C. Visible T% of 460 nm. Prior to surface tension measurements, T% (460 nm) was measured at 25 °C to determine the presence of precipitates. For samples of all concentrations, no decrease in T% was detected. Therefore, all of the samples are homogeneous solutions at 25 °C, which is consistent with the DSC results. All of the other measurements in this study were performed at 25 °C; therefore, all of the results are discussed as phenomena in aqueous solutions. Equilibrium Surface Tension. Figure 2 shows the equilibrium surface tension (γ) against log C (where C is total concentration) for AEC4-Na in water at 25 °C in the concentration range from 1.0 × 10−6 to 1.0 × 10−2 mol dm−3. The resultant curve is clearly different from a typical surface tension curve, which has only one break point at the critical micelle concentration (cmc), for conventional surfactants such as sodium dodecyl sulfate (SDS). The plot can be divided into four concentration regions (I−IV) according to specific concentrations designated as C1 (= 5.0 × 10−5 mol dm−3), C2 (= 9.0 × 10−4 mol dm−3), and C3 (= 4.0 × 10−3 mol dm−3), from lower to higher concentration. In Region I below C1, γ maintains the γ value of pure water (72.4 mN m−1) and then 5083

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hydrophobic core of micelles above the cmc, I1/I3 falls abruptly at the cmc and maintains a low constant value above the cmc. Therefore, this method is often used to determine the cmc of surfactant solutions or confirm the presence of surfactant aggregates with hydrophobic cores. Figure 4 shows plots of I1/

by the formation of catanionic complexes. However, the result in Figure 2 basically differs from this example, in that it was measured for a single-component AEC4-Na system. We have recently reported similar γ plots and aggregation behavior including the vesicle-to-micelle transition for the potassium monododecylphosphate (MAP) system as a single-component anionic surfactant aqueous solution.19 MAP is also a weak-acid salt that exhibits complex chemical equilibria that includes the formation of a hydrophobic dimer in water. This dimer is the acid soap of MAP, which consists of MAP and the unneutralized form. Generation of the acid soap causes specific aggregation behavior, as if the system were a binary surfactant system. Consequently, the downward bulge including the minimum value of γ curve can be attributed to the hydrophobic dimer. Figure 2 suggests that AEC4-Na can form vesicle aggregates in concentration Region II because it is also a weakacid type surfactant. The one different aspect from the MAP solution is that AEC4-Na forms a thermodynamically stable homogeneous solution without the appearance of precipitates. Solubilization of Oil-Soluble Dye. The solubilization of the oil-soluble ο-toluene-azo-β-naphthol dye in AEC4-Na aqueous solutions was investigated to determine the concentration where the formation of micelles begins. The ABS of the solubilized dye at 486 nm was observed at 25 °C and the results are shown in Figure 3. The ABS begins to increase around C1

Figure 4. I1/I3 versus log C for AEC4-Na in aqueous solution with pyrene at 25 °C. C1, C2, and C3 (Regions I−IV) are indicated in Figure 2.

I3 vs log C for AEC4-Na aqueous solutions at 25 °C. In Region I, the value of I1/I3 remains constant at ca. 1.85, which is almost the same value as that for the saturated pyrene aqueous solution. The onset of a gradual decrease in I1/I3 occurs at C1 until the AEC4-Na concentration reaches C3 (I1/I3 = ca. 1.25). A plateau in the I1/I3 vs log C plot appears again beyond C3, where all the pyrene molecules in the system are solubilized in the aggregate cores. The most interesting feature of these plots is the gradual decrease in I1/I3 between C1 and C3, which is unlike the behavior observed for typical surfactant solutions, in which a sharp drop of I1/I3 is observed at the cmc. This gradual decrease indicates the presence of premicelles that begin to form at C1, and then micelle formation at C2,29 which is the cmc of AEC4-Na. It has been recently reported that pyrene exhibits a broad emission spectrum in a dilute surfactant solution including premicelles, below the cmc.29,30 The appearance of this band is attributed to the formation of an excited state dimer (excimer) that is formed by the reaction of an excited pyrene molecule with another pyrene molecule in the ground state.31 Although the excimer can be formed only at higher concentrations of pyrene in isotropic solvents, a much lower pyrene concentration, such as 10−6 mol dm−3, is sufficient for efficient excimer formation in an aqueous dispersion of vesicles. The reason for this is that in organized media, pyrene is dissolved in microdomains within vesicles, where the local concentration is much higher than the bulk concentration.32 This suggests that the hydrophobic cores in vesicle bilayers are smaller than those in micelles because the excimer spectrum is not observed in micelle solutions. The maximum intensities of the excimer emission band around 470 nm (IE) were normalized for a variety of AEC4-Na solution concentrations using the intensities of the first peak of the steady state pyrene monomer, I1. Figure 5 shows the IE/I1 values plotted as a function of the AEC4-Na concentration. The excimer emission band is only observed at a certain concentration range between C1 and C3, while it is absent below C1 and above C3. IE/I1 gradually increases with the concentration of AEC4-Na until it reaches a maximum at C2 (= cmc). Above C2, IE/I1 decreases steeply and disappears at C3.

Figure 3. Solubilization ability of AEC4-Na aqueous solution for otoluene-azo-β-naphthol at 25 °C. ABS indicates the absorbance of the dye at 486 nm. C1 and C2 (Regions I−III) are indicated in Figure 2.

and increases more steeply above C2. These results could indicate the formation of some aggregates that have a hydrophobic core above C1 and then another aggregate begins to form at C2. The slope in Region II is smaller than that in Region III. The oil-soluble dye used here is a very hydrophobic material and is expected to be solubilized in the hydrophobic core of the aggregates. Therefore, the hydrophobic core volume of the aggregates formed in Region II is small, so that the degree of solubilization is still low.27 In Regions II and III, ABS increases linearly with the concentration. The linear slope suggests that the aggregates in these concentration regions have the same size and shape, and are not affected by the concentration. Steady-State Fluorescence. The characteristic emission spectrum of the pyrene monomer in the visible region shows five maxima in the range of 370−400 nm. The intensity ratio for the first and third peaks of the pyrene emission spectrum (I1/I3) is known to indicate the polarity of the probe environment, and a decrease of I1/I3 denotes a decrease in the polarity.28 When pyrene molecules are solubilized in the 5084

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hydrodynamic radius (Rh), assuming the Stokes−Einstein equation, D=

kBT 6πη0D

(4)

where kB is the Boltzmann constant and η0 is viscosity. The smaller Rh and larger Rh are listed for each solution in Table 1. Table 1. Hydrodynamic Radii (Rh) for Premicelles and Micelles of AEC4-Na in Water Evaluated using DLS at 25 °C R

Figure 5. IE/I1 versus log C for AEC4-Na in aqueous solution with pyrene (1.0 × 10−6 mol dm−3) at 25 °C. IE indicates the maximum intensity of the pyrene emission around 470 nm. C1, C2, and C3 (Regions I−IV) are indicated in Figure 2.

h

/ nm

concentration / mol dm−3

premicelles (vesicles)

5.0 × 10−4 9.0 × 10−4 4.0 × 10−3

118 128 a

volume fraction micelles a

3.47 2.89

premicelles (vesicles):micelles 1.00:0.00 0.09:0.91 0.00:1.00

A certain Rh was suggested by the double-exponential fitted line. The volume fraction was quite small (≪0.01); therefore, the result was considered to be due to dust/impurities in the sample solution. a

These results provide much information regarding the aggregation behavior of AEC4-Na in water: (i) The premicelles, which have smaller hydrophobic cores than the micelles, are present from C1 to C3 (Regions II−III). (ii) The premicelles begin to transform into micelles at C2. (iii) The premicelles and micelles are coexistent in Region III above the cmc. (4) The premicelles are vesicle aggregates. DLS. To determine the hydrodynamic sizes of the aggregates, DLS measurements were performed at 25 °C for AEC4-Na aqueous solutions with concentrations of 5.0 × 10−4 (Region II), 9.0 × 10−4 (Region III), and 4.0 × 10−3 (Region IV) mol dm−3. Figure 6A shows the time correlation functions of the scattered field g(1)(q, τ) for the aqueous solution in Region II. Curve-fitting was performed using a doubleexponential distribution: g(1)(q , τ ) = A1 exp( −G1τ ) + A 2 exp( −G2τ )

In Region II (5.0 × 10−4 mol dm−3), the smaller Rh is 118 nm, which suggests that of vesicles. The volume fraction for smaller aggregates is almost 1.000, as calculated from A1 and A2. In Region III (9.0 × 10−4 mol dm−3), where vesicles and micelles are expected to coexist, the smaller Rh is 3.5 nm and the volume fraction is 0.91. The smaller aggregates are micelles of AEC4Na. However, the larger Rh is 128.1 nm, which agrees well with the size of the aggregates formed in Region II. The volume fraction of larger aggregates is decreased from that in Region II (=1.00) to 0.09. In Region IV (4.0 × 10−3 mol dm−3), where the solution should be a micellar solution, the smaller Rh is 2.9 nm, which is almost equal to that of Region III, and the volume fraction reaches 0.997; therefore, this region of AEC4-Na indicates a micellar solution. According to Tanford et al.,33 the maximum possible extension length of saturated hydrocarbon chains is as follows:

(3)

where A1 and A2 are the amplitude fraction for smaller and larger aggregates, respectively, and A1 + A2= 1, G is the decay rate, and τ is the damping time.24 For each scattering angle at which DLS measurements were performed, the A and G values that gave the best fitting to each g(1)(q, τ) plot were taken. The G values evaluated by fitting are plotted against q2 in Figure 6B, where G1 and G2 indicate the fast and slow modes, respectively. The translational diffusion coefficients D, were calculated from the slope of the straight line in Figure 6B and converted into

lmax = 0.154 + 0.1265n (nm)

(5)

where n is the number of carbon atoms of the hydrophobic chain embedded in the micellar core. The general radii of micelles have been reported to be approximately 80% of lmax.4,5 Thus, the micellar radius of AEC4-Na can be calculated to be ca. 1.23 nm (lmax = 1.54 nm) when only the hydrocarbon chains form the micellar core, or ca. 2.63 nm (lmax = 3.29 nm) when all

Figure 6. DLS results for AEC4-Na aqueous solution (5.0 × 10−4 mol dm−3). (A) Time correlation functions for the scattered field at 25 °C and scattering angles of 40, 50, 60, and 70°. Colored solid lines are double-exponential fitted lines. (B) Decay rate G1 (fast mode) and G2 (slow mode) versus q2 plot. The diffusion coefficient D, of premicelles (vesicles) was evaluated from the slope of the straight line. 5085

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Thermodynamically Stable Vesicle Formation. The most interesting characteristic of the single-component AEC4Na aqueous solution is the formation of thermodynamically stable vesicles below the cmc. This was clarified according to the low Krafft point (