Langmuir 2007, 23, 1073-1080
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Aggregation and Phase Behavior of a Double-Chain Surfactant, N-Dodecyl-N-octyl-N-methylamine Oxide, as a Function of the Protonation Degree Yuji Yamashita,† Heinz Hoffmann,*,† Hiroshi Maeda,‡ Li Li,§ and Matthias Ballauff§ BZKG, UniVersity of Bayreuth, Gottlieb-Keim-Strasse 60, 95448 Bayreuth, Germany, Department of Chemistry, Faculty of Science, Kyushu UniVersity, Fukuoka 812-8581, Japan, and PC I, UniVersity of Bayreuth, 95440 Bayreuth, Germany ReceiVed July 10, 2006. In Final Form: October 30, 2006 The phase sequence of the N-dodecyl-N-octyl-N-methylamine oxide (C12C8MAO)/HCl/water system with increasing apparent degree of protonation, X, defined as [HCl]/[C12C8MAO], has been studied. For a 100 mM concentration of C12C8MAO the following sequence of phases has been observed: L1/L2, L1/LR/L2, L1/LR, LR, LR/L2. The single-phase LR region begins at X ) 0.007 and ends at X ) 0.35. The upper phase boundary, X*, depends strongly on the acid that is used for the protonation of the surfactant. It is shifted for increasing hydrophilicity of the acid to higher X values. For formic acid X* ) 0.95, and for HBr X* ) 0.05. A weakly protonated 1% solution of the surfactant is an iridescent LR phase. Both unilamellar vesicles and multilamellar vesicles are observed in cryo transmission electron microscopy and freeze fracture transmission electron microscopy images in the LR phase. The phase sequence with protonation differs from that of single-chain amine oxide surfactants. The synergism between the protonated and the nonprotonated species is very weak in the range X < X*, while the transition from the LR phase to the LR/L2 two-phase region is considered to be due to synergism. Little or no synergism is observed regarding the surface tension, but synergism does appear in the interfacial tension between decane and the aqueous solution. The viscoelastic properties of the vesicle/LR phase resemble those of densely packed hard spheres. The effects of electric charge on the elastic property of the vesicles could be understood in terms of the osmotic pressure of the solutions. The interlamellar spacing evaluated by small-angle X-ray scattering showed a minimum around X ≈ 0.1, which is interpreted as a result of two opposing contributions. One contribution is the suppression of undulation of bilayer membranes by introduction of electric charges, and the other comes from the increasing total bilayer thickness due to the increasing hydrogen bond formation with increasing X.
Introduction It is well-known that double-chain surfactants form stable bilayer structures in aqueous media. The lamellar morphology can be divided into three types: classical stacked lamellae, vesicles, and multiconnected bilayers. To the last type belongs the “sponge phase” or L3 phase.1 Relative stabilities of these three types of bilayers depend on, among others, the bending moduli of the bilayer.2 Usually, the difference in the stability among these states is rather subtle, as shown by the finding that a stacked lamella phase is easily transformed into a “vesicle phase” by a quite gentle stirring.3,4 A shear-induced lamellato-vesicle transition has been receiving attention in recent years.5,6 The conversion of the classic LR phase to the vesicle phase is also induced by the introduction of charge.7,8 Amine oxide surfactants in aqueous media are generally a mixture of protonated and nonprotonated species, and their relative * To whom correspondence should be addressed. Phone: +49 921 50736135. Fax: +49 921 50736 139. E-mail: heinz.hoffmann@ nmbgmbh.de. † BZKG, University of Bayreuth. ‡ Kyushu University. § PC I, University of Bayreuth. (1) Hassan, S.; Rowe, W.; Tiddy, G. J. T. Surfactant Liquid Crystals. In Handbook of Applied Surface and Colloid Chemistry; Holmberg, K., Ed.; John Wiley & Sons Ltd.: New York, 2002; Vol. 1, Chapter 21. (2) Helfrich, W. Prog. Colloid Polym. Sci. 1994, 95, 7. (3) Bergmeier, M.; Hoffmann, H.; Thunig, C. J. Phys. Chem. B 1997, 101, 5767. (4) Hao, J.; Hoffmann, H.; Horbaschek, K. J. Phys. Chem. B 2000, 104, 10144. (5) Diat, O.; Roux, D.; Nallet, F. J. Phys II 1993, 3, 1427. (6) Panizza, P.; Roux, D.; Vuillaume, V.; Lu, C.-Y. D. M.; Cates, E. Langmuir 1996, 12, 248.
amount depends on the pH. A synergism between the two species has been observed on several amine oxides with a single alkyl tail.9 The synergism is due to the hydrogen bond between the protonated and nonprotonated headgroups.10 In the present study, the effects of protonation on a double-tail amine oxide, N-dodecyl-N-octyl-N-methylamine oxide (C12C8MAO), will be examined. One objective of the present study is to see whether or not the synergism observed for single-tail amine oxides is operating for a double-tail amine oxide. Another aim is to see the effects of the amine oxide headgroup on the structures and properties of the bilayer aggregates of double-tail amphiphiles. In the present study, hydrochloric acid (HCl) is mostly used to protonate the amine oxide. A few investigations were carried out with other acids to study the influence of the different counterions on the stability of the LR phase. Recently, a study was reported on the effects of protonation on the aggregate structures of another double-tail amine oxide, didecylmethylamine oxide (2C10MAO).11 Experimental Section Materials. C12C8MAO was a gift from Clariant GmbH, recrystallized twice with acetone. The purity was confirmed by the surface tension, cmc, and melting point. HCl (>99%) was purchased from Sigma Co. Ltd. Distilled water was used. (7) Rydhag, L.; Stenius, P.; O ¨ dberg, L. J. Colloid Interface Sci. 1982, 86, 274. (8) Hoffmann, H.; Thunig, C.; Schmiedel, P.; Munkert, U. Langmuir 1994, 10, 3972. (9) Maeda, H.; Kakehashi, R. AdV. Colloid Interface Sci. 2000, 88, 275. (10) Kawasaki, H.; Maeda, H. Langmuir 2001, 17, 2278. (11) Kawasaki, H.; Garamus, V. M.; Almgren, M.; Maeda, H. J. Phys. Chem. B 2006, 110, 10177.
10.1021/la061991i CCC: $37.00 © 2007 American Chemical Society Published on Web 12/16/2006
1074 Langmuir, Vol. 23, No. 3, 2007 Phase Study. Different amounts of HCl were added to the solutions of C12C8MAO (100 mM) and mixed with a stirrer (ca. 3000 rpm, 5 min). The mixture was left in a water bath at 25 °C for at least two weeks, and then the phase state was determined by visual inspection. Birefringence was detected between two crossed polarizers. Equilibrium phase properties are expected by this way of preparation, except for the polymorphism of the lamella phase, as will be discussed later. When some samples were incubated at 25 °C for about 1 year, trapped bubbles were found to remain, indicating extremely slow rates of stress relaxation caused by densely packed vesicles. Rheological Measurements. Rheological measurements were performed using a Haake RS600 with a cone-plate sensor. Temperature was controlled at 25 °C to an accuracy of (0.1 °C by a thermocontroller (Haake TC81). Measurements were operated by the software Haake RheoWin Job Manager, and the obtained data were analyzed by Haake RheoWin Data Manager. The measuring frequency ranged from 0.01 to 10 Hz. Sample solutions were prepared as described for the phase study. Bubbles, if present, were removed by centrifugation (ca. 1000 rpm, 3-5 min) prior to the measurements. Interfacial Tension Measurements. Interfacial tension measurements were carried out at 25 °C with a spinning drop interfacial tensiometer (KRUESS) equipped with a water circulator on decane/ aqueous solution interfaces. The rotating angular speed was from 1500 to 4000 rpm. The concentration of the surfactant in aqueous solutions was 1 mM. Small-Angle X-ray Scattering (SAXS). All SAXS measurements were conducted using the improved Kratky camera described in ref 26. The generated X-ray (Cu KR ray, λ ) 0.154 nm) was collimated and irradiated the sample in a capillary-type sample holder made from glass (diameter 1 mm). The scattered radiation was counted by the one-dimensional detector. The raw data were corrected for the scattering of the serum and the sample holder. Desmearing of the scattering curves was done as described in ref 26. In all cases to be discussed here absolute scattering intensities were obtained. The distance from the sample holder to the detector was 41.4 cm. The SAXS measurements were carried out for bubble-free surfactant solutions (100 mM) at room temperature (∼20 °C), and the scattering vector (q) ranged from 0.07 to 3 nm-1. Transmission Electron Microscopy (TEM). The cryogenic preparations were performed by applying a drop of the sample on a copper TEM grid (600 mesh, Science Services, Munich, Germany), blotting most of the liquid, leaving a thin film stretched over the grid holes, and then plunging the grid rapidly in liquid ethane cooled to ca. 90 K by liquid nitrogen in a temperature-controlled chamber (Zeiss Cryobox, Zeiss NTS GmbH, Oberkochen, Germany) to a Zeiss EM 922 EFTEM (Zeiss NTS GmbH, Oberkochen, Germany). The TEM instrument was operated at a voltage of 200 kV. Zero-loss filtered images (DE ) 0 eV) were taken under reduced dose conditions (100-1000 e/nm2). All images were registered digitally by a bottommounted CCD camera system (Ultrascan 1000, Gatan, Munich, Germany) combined and processed with a digital imaging processing system (Digital Micrograph 3.9 for GMS 1.4, Gatan, Munich, Germany). Freeeze Fracture Transmission Electron Microscopy (FFTEM). The microstructures of the samples were also examined by FF-TEM. For FF-TEM, small amounts of the sample were placed on a 0.1 mm thick copper disk covered with a second copper disk. The sample was frozen by plunging this sandwich into liquid propane (cooled by liquid nitrogen). Fracturing and replication were carried out in a freeze fracture apparatus (Balzers BAF 400, Germany) at a temperature of -140 °C. Platinum/carbon was deposited at an angle of 45° followed by a second deposition of carbon at 90°. The collected replicas were examined in a CEM 902 electron microscope (Zeiss, Germany).
Results Phase Behavior of the C12C8MAO/HCl/Water System. Figure 1 shows a phase sequence in C12C8MAO/HCl aqueous mixtures at 25 °C. The total concentration of the surfactants C
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Figure 1. Phase sequence of the C12C8MAO/HCl/water system at 25 °C. The total concentration of the surfactants is fixed at 100 mM. The axis represents the mixing ratio X of C12C8MAO with HCl, namely, X ) [H+]/[C12C8MAO], which can be well approximated with the degree of protonation.
(C12C8MAO + C12C8MAOH+Cl-) is fixed at 100 mM. The horizontal axis represents the mixing ratio X defined as X ) CHCl/C. At a high concentration of 100 mM, the degree of protonation of C12C8MAO is well approximated with X. Selected examples of the appearance of the mixtures at various X values are shown in Figure 2. At X ) 0, nonionic C12C8MAO is not soluble in water at 25 °C, and turbid suspensions were obtained. Two clear solution phases were observed, however, at temperatures higher than about 30 °C. The two solutions are most likely L2 and L1 (or water). A preference to form aggregates with negative curvatures is thus suggested for C12C8MAO. A transparent iridescent phase was observed in a rather narrow concentration range around C ) 60 mM in the middle of the two-phase region (Figure 3). Usually, iridescence from surfactant solutions is considered to be due to the Bragg reflection of lamella layers when interlayer separations are in the range of the wavelength of visible light.12,13 A very small extent of selfprotonation is likely at this concentration. The iridescent phase disappeared at temperatures higher than about 30 °C. When a small amount of HCl is added to the nonprotonated C12C8MAO (X ) 0), two nonbirefringent solutions appear (X < 0.002). Introduction of a small amount of charges widens the interbilayer space to allow water to come in. Then, a three-phase region is observed around X ) 0.0032 where a birefringent phase is sandwiched between two nonbirefringent phases. In a narrow range of X between X ≈ 0.004 and X ≈ 0.006, two solutions, one nonbirefringent solution on top of the other birefringent one, are observed. Above X ≈ 0.006, a clear single phase of strong birefringence is obtained. Due to the increasing ionic repulsion, the LR phase becomes more stable than structures of negative curvatures. The single LR phase extends up to X ) 0.35, where the solution becomes slightly turbid. In the LR phase, air bubbles begin to be trapped as X increases, and in some instances bubbles did not rise up even after 1 year (Figure 2). A charge-induced lamella-to-vesicle conversion7,8 is suggested, and it is likely that solutions consist of closely packed vesicles for a range of X ) 0.25-0.35. The photos of the samples in Figure 2 show that the extent of protonation of the samples changes the birefringence intensity and the birefringence patterns of the samples. For a small X the samples have the typical appearance of classic LR phases with small domains pointing in different directions. These are typical equilibrium patterns. When samples with these birefringence patterns are shaken, the appearance of the birefringence pattern changes with shear; usually the domains become larger. However, when the shear is removed, the visual pattern returns within a few seconds to the same appearance as before the shaking process. For samples with a larger X, the situation is different. For these samples no domains are visible. Within (12) Satoh, N.; Tsujii, K. J. Phys. Chem. 1987, 91, 6629. (13) Platz, G.; Thunig, C.; Hoffmann, H. Colloid Polym. Sci. 1990, 83, 167.
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Figure 2. Sample appearances of C12C8MAO/HCl aqueous solutions with different values of X at 25 °C. The photographs at the bottom were taken of the solutions between two crossed polarizers.
a given sample, there actually are areas showing no birefringence. The different birefringence behaviors of the two types of samples become evident and can be differentiated when the two samples are slowly rotated between two polarizers. In the first case, the appearance does not depend on the rotation angle, whereas in the second case it depends very much on the rotation angle. When the sample with a large X is shaken, the birefringence depends on the intensity of shear and how long the sample was sheared and does not return to the starting situation. The cryoTEM micrographs show that the samples contain unilamellar vesicles (ULVs) and multilamellar vesicles (MLVs). These structures are a result of the shear history and the different bending constants of the samples. In the state of small X values, vesicles are formed under shear from an LR phase. Vesicles of low charge densities are soft and can release the strain in the system and return to the classic LR state when the shear is removed. For high charge density the bilayers are stiff and the samples have a yield stress value. For this reason the stress in the system cannot relax and the vesicle state remains after the shear is taken away. The number of shells in the vesicles depends on the shear history. With increasing shear rate, the number of shells decreases.14,15 The samples can actually be prepared in the classic LR state when shear in the preparation is avoided as discussed later in Protonation by Chemical Reaction.
Different appearances of the solutions (X ) 0.17) due to different morphologies of the LR phase are also seen when the solutions are diluted from 80 to 10 mM (Figure 4). At high concentrations, C ) 60 and 80 mM, the solutions are birefringent without small domains but with stripes which are the trails of entrapped bubbles in a centrifugal field applied to purge them. This suggests a state of densely packed vesicles and hence a single LR phase if the shear could have been avoided during the preparation. On dilution to a concentration below 50 mM, solutions become colorless under crossed polarizers, indicating no macroscopic deformation of bilayers at these concentrations. It is to be noted that no macroscopic phase separation takes place on dilution. In the range of X below about 0.35, the observed effect of X is what is expected from ordinary electric repulsion and the synergism associated with protonation which has been observed on single-chain amine oxides9 is not observed on C12C8MAO. In the range of X larger than about 0.45, a liquid-liquid phase separation occurs where the upper layer (most likely an L2 phase) seems to contain more material than the lower clear solution and the phase volumes do not depend much on X up to X ) 1. The two phases are isotropic in a macroscopic sense. In the intermediate range of X ≈ 0.40, solutions are turbid but eventually separate into two phases. One of the factors leading to a greater stability of LR than L2 is the translational entropy of counterions.11 As counterions bind strongly to the headgroups, this contribution becomes insignificant and the L2 phase becomes preferred to LR due to the intrinsic propensity to take structures of negative curvatures. In the present study, the critical composition X* for the phase separation was found to shift toward smaller X values when the counterions became less hydrophilic: formate (X* ≈ 0.95) > Cl (X* ≈ 0.35) > Br (X* ≈ 0.05). A good correlation is found between log X* and the free energy of hydration of ions as shown in Figure 5. The importance of the hydrophilicity of ions in the Hofmeister series has been pointed out by Ruckenstein and Manciu.16-18 Interfacial Tension Measurements. In the case of doubletailed amine oxides, an unfavorable packing of alkyl chains in the bilayers will be induced as a result of the hydrogen bond formation between the protonated and the nonprotonated headgroups. The same mechanism is likely to hold for the monolayers at the air/aqueous solution interface. No synergism is seen for the surface tension in Figure 6, suggesting few hydrogen bonds between the headgroups in the monolayer. For the monolayers
(14) Hao, J.; Hoffmann, H.; Horbaschek, K. Langmuir 2001, 17, 4151. (15) Gradzielski, M.; Bergmeier, M.; Mu¨ller, M.; Hoffmann, H. J. Phys. Chem. B 1997, 101, 1719.
(16) Manciu, M.; Ruckenstein, E. AdV. Colloid Interface Sci. 2003, 105, 63. (17) Ruckenstein, E.; Manciu, M. AdV. Colloid Interface Sci. 2003, 105, 177. (18) Manciu, M.; Ruckenstein, E. Langmuir 2005, 21, 11312.
Figure 3. An iridescent phase of C12C8MAO at C ) 60 mM and X ) 0.
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Figure 4. Appearance of solutions of different concentrations at X ) 0.17 between two crossed polarizers at 25 °C.
Figure 5. Values of log X* as a function of the free energy of hydration ∆Ghyd of ions at 25 °C and C ) 60 mM.
Figure 6. Surface tension (γS) and interfacial tension (γI) as a function of X in the water/C12C8MAO/HCl system at 25 °C and C ) 1 mM. γI refers to the decane/aqueous solution interface.
at the oil (decane)/aqueous solution interface, on the contrary, a good packing of alkyl chains is facilitated by the penetration of decane molecules. Then, the hydrogen bond is expected to be formed with an increasing X until a value beyond which electric repulsion outweighs the stabilization by the hydrogen bond. The expected synergism in this case is indeed shown in Figure 6. It is to be noted that the interfacial tension is extremely low compared with that of the air/solution interface: the minimum value is about 20 µN/m at X ≈ 0.4. The low interfacial tension is a result of the stabilization of the monolayer by the decane penetration. A synergism between two surfactants is usually reflected in the cmc. Well-defined break points in the surface tension method were found: they are 1.78, 1.23, and 3.73 × 10-5 M for X ) 0, 0.17, and 1, respectively. If the break points are identified as
the cmc, the cmc of the mixtures is indeed somewhat lower than that of the single surfactants, indicating a weak synergism between the unprotonated and protonated species. The cmc of the cationic species is greater than that of the nonionic species only by a factor of about 2. This small difference is characteristic of the amine oxide headgroup.9 TEM Observations. Cryo-TEM images of the LR phase at X ) 0.1 and 0.05 are shown in parts a and b, respectively, of Figure 7. MLVs are present in the LR region, and the vesicle size and shape are not uniform: fluidic aggregates fill up the space at low X, and the polydispersity is greater with decreasing X. The vesicle shape changes from indefinite shapes to the spherical one with protonation. At X ) 0.3, where the LR phase exhibits a yield stress of about 3.5 Pa, many vesicles, both ULVs and MLVs, are packed in a rather uniform manner as shown in an FF-TEM image, Figure 7c, although the sizes are polydisperse. It should be realized that the different morphologies reflect metastable states that are caused by shear and are at least partially frozen in that state by the yield stress of the systems, as pointed out before. SAXS Measurements. Figure 8 depicts desmeared SAXS spectra of the LR solutions of different X values at C ) 100 mM. The ratio of the correlation peaks, 1:1/2:1/3:1/4, represents a bilayer structure, consistent with TEM measurements. These scattering curves are fairly superposed in the high scattering vector (q) range, which would indicate the identical microscopic morphology for a range of X between 0.025 and 0.35. The first peak, corresponding to the interlayer spacing (d), is however influenced by protonation as shown in the inset of Figure 8. The d value goes down to a minimum and again rises up to the original level (X ≈ 0.05) as shown in Figure 9. This peculiar dependence will be discussed later. Viscoelastic Property of the Lamellar (LR) Phase. Figure 10 shows the rheogram of the lamellar phase (X ) 0.007-0.35) at C ) 100 mM. At X ) 0.007, which is nearly the low X limit of the LR phase, the phase behaves fluid-like. A solid-like behavior manifests itself as X increases further: the storage modulus (G′) is about 10 times as large as the loss modulus (G′′) over the measuring frequency range, and the two moduli are both independent of the frequency. This typical feature can be found commonly in a number of densely packed vesicle dispersions.8,14,15 The viscoelastic behavior resembles that found on a ternary mixture of single-chain surfactants consisting of an amine oxide, a cosurfactant, and a cationic surfactant,8 and it is consistent with the predicted vesicle phase indicated by cryo-TEM and SAXS measurements. Stress sweep measurements were also done to obtain the yield stress (σ0) at X ) 0.3 (not shown). The shear rate remains to be almost zero at an applied stress equal to 2.3 Pa, beyond which the shear rate proportionally increases with the shear stress. The breaking point is defined as σ0. We plot the shear modulus G′ (at 1 Hz) and σ0 against X in Figure 11. The two parameters increase strongly with X and then
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Figure 8. SAXS spectra for the LR phase in the water/C12C8MAO/ HCl system at different X values at 25 °C and C ) 100 mM. Inset: an enlarged presentation around the first maximum.
Figure 9. Change in the interlayer spacing (d) of lamellae as a function of the apparent protonation degree X in the water/C12C8MAO/HCl system at 25 °C and C ) 100 mM.
Figure 7. Transmission electron microscopy images of the LR phase in the water/C12C8MAO/HCl system at 25 °C and C ) 100 mM: (a) cryo-TEM image at X ) 0.1, (b) cryo-TEM image at X ) 0.05, and (c) FF-TEM image at X ) 0.3.
level off at around X ) 0.2. G′ is about 10 times larger than σ0. This means that when the vesicles are deformed about 10%, they can then pass each other under shear.8 Protonation by Chemical Reaction. In the “normal” preparation procedure employed in the studies described in the preceding sections, relevant amounts of HCl were added followed by centrifugation (∼300 rpm, 5 min) to ensure uniform protonation throughout the solutions. It has been established,
however, that even a very weak shaking can provide kinetic energy enough for stacked lamellae to transform into vesicles.3,4 It is interesting to examine the consequences of the protonation without any extent of shaking, which is effected by the addition of methyl formate followed by shaking. Then uniform protonation of the amine oxide takes place without shaking as methyl formate gradually disintegrates into formic acid that can protonate the amine oxide immediately. Methyl formate was added to an emulsion (X ) 0) of the two-phase state (L1 + L2) at C ) 100 mM. The amount of methyl formate was adjusted to give the state X ) 0.17 where vesicles are formed if enough shear force is applied as shown in Figures 2 and 4. The milky emulsion at X ) 0 changed into a transparent solution 3 days after preparation (Figure 12). In the time course between 70 and 180 min, the birefringence is weak and heterogeneous due probably to creaming of the L2 phase. The clear solution 3 days after the addition of methyl formate is birefringent and is expected to consist of lamellae rather than vesicles, as confirmed by the rheological properties. The solution prepared by the chemical reaction exhibits weaker viscoelastic properties than the presheared solution of the same composition as shown in Figure 13. Although the frequency dependence of all three properties G′, G′′, and complex viscosity is similar for both solutions, their magnitudes are smaller by more than 1 order for the former than the latter. In the solution prepared by the chemical reaction, a typical shear-induced lamella-to-vesicle (MLV) transition was observed,
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Figure 10. Changes in the storage modulus (G′), the loss modulus (G′′), and the complex viscosity (|η*|) as a function of frequency for C12C8MAO/HCl aqueous solutions at 25 °C and C ) 100 mM.
Discussion
Figure 11. Storage modulus (G′) and the yield stress (σ0) against X for the LR phase in the water/C12C8MAO/HCl system at 25 °C and C ) 100 mM.
as shown in Figure 14. In the transition range, the viscosity remained nearly constant irrespective of the shear rate. With a further increase of the shear rate, another transition from MLVs to ULVs was also observed. We can discriminate three different LR phases in terms of the different shear-thinning slopes in the respective regions. Similar shear rate dependent morphology transitions have been found previously.19 (19) Escalante, J. I.; Gradzielski, M.; Hoffmann, H.; Mortensen, K. Langmuir 2000, 16, 8653.
Change of the Interlayer Spacing with Protonation. The change of the interlayer spacing with protonation shown in Figure 9 takes place under constant volume fractions of the surfactant and water in a single-phase region. The result indicates that two opposing forces, attraction and repulsion, are operating. In dilute lamellar phases undulation forces and electrostatic forces have to be considered for the repulsive forces and the van der Waals forces as the attractive force. For uncharged bilayers the electrostatic repulsion can be neglected, and it can be assumed that the swelling of the bilayer is mainly given by the undulation force. The bending constant of surfactant bilayers is usually on the order of kT. Measurements on such bilayers have shown that, with small charge densities on the order of 1% or even smaller, the bilayers stiffen and the undulation forces disappear and are replaced by electrostatic forces. The consequence is a decrease of the interlamellar distance. This effect can simply be observed by the blue shift of the color of iridescent phases when the phases are charged by adding ionic surfactants.20 The initial reduction of the interlayer spacing with protonation in the present study would be a consequence of reduced undulation by stiffening of bilayers on charging. This effect has to saturate when the bilayer can be considered flat. With a further increase of the charge density, the repulsive interaction increases as long as the interlamellar distance is in the range of the electric double layer without the counterion (20) Schoma¨cker, R.; Strey, R. J. Phys. Chem. 1994, 98, 3908.
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Figure 12. Change of the appearance of a C12C8MAO/methyl formate mixture (X ) 0.17) with time at 25 °C and C ) 100 mM. The photographs were taken with (lower panel) and without (upper panel) two crossed polarizers.
Figure 13. Dynamic rheological measurements at 25 °C on two solutions of C12C8MAO (X ) 0.17 and C ) 100 mM) prepared differently. Open and filled symbols refer to the solutions prepared normally (with a preshear) and by a chemical reaction, respectively.
Figure 14. Shear rate dependent transformations of LR phases of C12C8MAO (X ) 0.17 and C ) 100 mM) at 25 °C. Regions I, II, III, and IV correspond, respectively, to LR (stacked lamella), a transition region, MLVs, and ULVs.
condensation. However, when the counterions begin to condense, the repulsive interaction should remain constant also. With the bilayers of alkylamine oxides, the situation is more
complicated. It is known that the added protons form hydrogen bonds between the headgroups and change the headgroup packing and hence also the packing of the whole bilayer. This effect is reflected in the interfacial tension of the surfactant against a hydrocarbon. This effect leads to an increase of the bilayer thickness and eventually of the interlayer spacing. It is believed that these two effects, the suppression of the undulation force and the increasing thickness of the bilayer with increasing charge density, lead to the minimum of the interlayer distance. This model can even be further extended to explain the collapse of the LR phase with further protonation and explain that the critical protonation degree X* depends on where the counterion stands in the Hofmeister series. When the counterions condense above a critical charge density of the bilayer, they no longer increase the electrostatic repulsion. They influence however the van der Waals interaction. The further the counterions are on the hydrophobic side in the series, the earlier they condense on the charged bilayer and the larger is their contribution to the attractive forces. This explains why the LR phase is observed over the almost complete neutralization range with formic acid but only for a very small range with bromic acid. In this context it is interesting to note that other double-chain surfactants with ionic headgroups form LR phases in the binary system. Typical examples are the dialkyldimethylammonium halides.21-23 The reason for this obviously different behavior lies probably in the structure of the headgroup. It is likely that double-chain cationic amine oxides bind the counterions more strongly than the double-chain alkyldimethylammonium ions. The same is true also for the single-chain analogues: alkyldimethylamine oxide and alkyltrimethylammonium ions. Solutions with 1% hexadecyldimethylamine oxide protonated with HBr are viscoelastic solutions, while solutions of the same concentration of hexadecyltrimethylammonium bromide (CTAB) are lowviscosity solutions.24 Comparison of the Viscoelastic Properties with Those of Tetradecyldimethylamine Oxide (C14DMAO)/Tetradecyltrimethylammonium Bromide (C14TMABr)/Hexanol (C6OH) Mixtures. It is interesting to compare the viscoelastic properties observed in the present study with those found for C14DMAO/ (21) Shinoda, K.; Kunieda, H. J. Phys. Chem. 1978, 82, 1711. (22) Dubois, M.; Zemb, T. Langmuir 1991, 7, 1352. (23) Marques, E. F.; Regev, Or.; Khan, A.; Miguel, M. D. G.; Lindman, B. J. Phys. Chem. B. 1999, 103, 8353. (24) Hartel, G.; Hoffmann, H. Liq. Cryst. 1989, 5, 1883.
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Table 1. Comparison of Vesicle Radii Calculated by Eq 1 and Estimated from TEM Observations X
G′ (Pa)
γeff (mN/m)
R (eq 1)
R (TEM obs)
0.05 0.1 0.3
1.31 4.184 46.01
0.08685 0.05556 0.01685
66 µm 13 µm 366 nm
∼1 µm
