Investigation of the L3 Phase in Systems Containing Calcium Dodecyl

is located in that region of the phase diagram in which L3 phases are classically ... At 5% surfactant and octanol as cosurfactant, the L3 phase disap...
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Langmuir 2001, 17, 6113-6118

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Investigation of the L3 Phase in Systems Containing Calcium Dodecyl Sulfate, Alcohol, and Water A. Zapf, U. Hornfeck, G. Platz,* and H. Hoffmann Physikalische Chemie I, University of Bayreuth, 95440 Bayreuth, Germany Received April 12, 2001. In Final Form: June 18, 2001 An isotropic, clear phase at high alcohol content is found in a system composed of calcium dodecyl sulfate, water, and an alcohol of medium chain length, like pentanol, hexanol, heptanol, or octanol. This phase is located in that region of the phase diagram in which L3 phases are classically formed. Most ionic surfactants, however, only form sponge phases when the ionic charge of the surfactant molecules is sufficiently shielded by excess salt. At 5% surfactant and octanol as cosurfactant, the L3 phase disappears if more than 5% of the calcium ions are substituted by sodium. Calcium dodecyl sulfate, in some aspects, behaves like a nonionic or double-chain surfactant. The phase discussed in this article is completely transparent, has low viscosity, and exhibits neither birefringence nor yield stress. Viscosity, electrical conductivity, and freezefracture electron microscopy indicate the sponge structure of the phase. In contrast to typical L3 phases, small-angle neutron-scattering experiments show a peak the position of which is nearly unshifted compared with that of the neighboring lamellar phase.

Introduction Studies of the L3 phase have attracted considerable attention in the past years. The term “L3” has been used since 1984 for “anomalous isotropic phases” that are located above swollen lamellar phases at higher cosurfactant concentrations.1,21,2 The L3 phase typically has a low viscosity and exhibits strong light scattering and streaming birefringence.3-5 It can be described as a sponge phase consisting of a bicontinuous network of highly interconnected nonoriented surfactant bilayers.6-9 This structure has been predicted theoretically, the stabilizing factors being entropic effects.10,11 A typical L3 phase behavior was reported in the short-chain unshielded ionic system of sodium octanoate/octanoic acid/water as early as 1969.12 The L2 phase extends into the water corner in a narrow channel. Scattering data indicated that there are “aggregates probably of colloid dimensions”. More detailed measurements were performed in 1986.13 In 1988, it was proposed that the X-ray peak should be a hint that this anomalous isotropic phase has a structural relationship to the adjacent lamellar phase.14 In systems with * To whom correspondence should be addressed. (1) Nilsson, P. G.; Lindman, B. J. Phys. Chem. 1984, 88, 8(20), 47644769. (2) Persson, P. K. T.; Stenius, P. J. Colloid Interface Sci. 1984, 102(2), 527-532. (3) Miller, C. A.; Gradzielski, M.; Hoffmann, H.; Kraemer, U.; Thunig, C. Prog. Colloid Polym. Sci. 1991, 84, 243-249. (4) Lavrentovich, O. D.; Quilliet, C.; Kle´man, M. J. Phys. Chem. 1989, 93, 4243. (5) Snabre, P.; Porte, G. Europhys. Lett. 1990, 13(7), 641-645. (6) Benton, W. J.; Miller, C. A. J. Phys. Chem. 1983, 87, 7(24), 49814991. (7) Porte, G.; Appell, J.; Bassereau, P.; Marignan, J. J. Physiol. (Paris) 1989, 50(11), 1335-1347. (8) Gazeau, D.; Bellocq, A. M.; Roux, D.; Zemb, T. Prog. Colloid Polym. Sci. 1989, 79, 226-232. (9) Anderson, D.; Wennerstro¨m, H.; Olsson, U. J. Phys. Chem. 1989, 93(10), 4243-4253. (10) Cates, M. E.; Roux, D.; Andelman, D.; Milner, S. T.; Safran, S. A. Europhys. Lett. 1988, 5(8), 733-739. (11) Coulon, C.; Roux, D.; Bellocq, A. M. Phys. Rev. Lett. 1991, 66(13), 1709-1712. (12) Ekwall, P.; Mandell, L. Kolloid Z. Z. Polym. 1969, 233(1-2) 938-944. (13) Ekwall, P.; Mandell, L.; Fontell, K. Colloid Polym. Sci. 1986, 264, 542-551. (14) Ekwall, P.; Fontell, K. Colloid Polym. Sci. 1988, 266, 184-191.

longer chains, the L3 phase forms only in nonionic surfactant systems with or without cosurfactant3 and also with ionic surfactants, given that the ionic charges are shielded by addition of salt.6,7 No such phase has been found with sodium dodecyl sulfate (SDS),15 with use of either hexanol, heptanol, or octanol as a cosurfactant. A sponge phase has only been obtained in those systems by addition of salt. Guerin and Bellocq, who studied the SDS/ pentanol/brine/water system intensively,16 found that, in the system without salt, there is an isotropic phase above the LR phase with a continuous transition between the L1 and L2 phases. This channel is broken by addition of salt, and the typical phase behavior of L3 occurs. This short-chain behavior is similar to that found by Ekwall et al.13 In this article, a clear isotropic phase is described that is located at the classical position of the L3 phase in the phase diagram of a system composed of calcium dodecyl sulfate (CDS)/medium-chain alcohol/water. Electron micrographs proved the structure of the phase as being L3. Experimental Section The phase diagram was obtained by preparing the samples in 10-mL test tubes. The sample mass was 5 g in all cases. The surfactant was dissolved by heating and shaking before the tubes were allowed to equilibrate in a 25 °C water bath for 2 weeks after centrifugating for 5 min at 5000g to eliminate gas bubbles and foam. The macroscopic properties of the samples were assayed by use of crossed polarizers and polarization microscopy. Phase volume intersections and phase diagrams were plotted as described previously.17-19 CDS was prepared by following a standard protocol20; SDS was obtained from BDH Laboratories, London. All alcohols were (15) Natoli, J.; Benton, W. J.; Miller, C. A.; Fort, T. J. Dispersion Sci. Technol. 1986, 7(2), 215-229. (16) Guerin, G.; Bellocq, A. M. J. Phys. Chem. 1988, 92, 2(9), 25502557. (17) Hornfeck, U.; Gradzielski, M.; Mortensen, K.; Thunig, C.; Platz, G. Langmuir 1998, 14, 4(11), 2958-2964. (18) Platz, G.; Thunig, C.; Hoffmann, H. Ber. Bunsen-Ges. Phys. Chem. 1992, 96, 667. (19) Hoffmann, H.; Munkert, U.; Thunig, C.; Valiente, M. J. Colloid Interface Sci. 1994, 163, 217-228. (20) Moroi, Y.; Motomura, K.; Matuura, R. J. Colloid Interface Sci. 1974, 46(1), 111-117.

10.1021/la010540p CCC: $20.00 © 2001 American Chemical Society Published on Web 09/08/2001

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Figure 2. Phase volume intersection of 5 wt % CDS (88 mM) at 25 °C with increasing amount of octanol. The measuring precision on the abscissa is 0.5%; in regions with rapid phase transitions it is 0.1%. The error on the ordinate is 0.03.

Figure 1. Phase volume intersections of 10 wt % CDS (176 mM) at 25 °C with increasing amount of alcohol, using, from top to bottom, pentanol, hexanol, heptanol, and octanol. The measuring precision on the abscissa is 0.5%; in regions with rapid phase transitions it is 0.1%. The error on the ordinate is 0.03. purchased from Merck, Darmstadt. Deionized water was used throughout all experiments. Conductivity measurements were performed with a WTW LF 196 Microprocessor Conductivity Meter. For the continuous measurements, a setup of our own design was used. While slowly adding alcohol (ca. 0.01 mL/min) with a computer-controlled dispenser (Schott T90-20) under magnetic stirring, the conductivity of the sample was continuously measured with a WTW LF 3000 conductivity meter.17 Polarization microscopy was performed with a Zeiss microscope, viscosity measurements with an OCRD system by Chempro Paar. Neutron-scattering experiments were conducted in Grenoble, France. Freeze-fracture electron microscopy was performed with a Carl Zeiss CEM 902 microscope.

Results and Discussion The phase volume intersection in Figure 1 shows the development of the different phases upon addition of increasing amounts of alcohol to a 176 mM (10%) solution of CDS in water. At 0% alcohol, a crystalline suspension of CDS is formed with a supernatant of almost pure water. Upon addition of pentanol, the crystals dissolve to form an L1 phase that gives way to a vesicular dispersion when more alcohol is added. By contrast, in the systems with alcohols of length C6-C8, addition of alcohol leads to the formation of vesicular droplets of a condensed lamellar phase. The subsequent transition to the LR phase is marked by a three-phase region of condensed lamellar, swollen lamellar, and L1 phases. Especially in the hexanol system, these phases are not readily separated, instead a white vesicular dispersion is formed. In the pentanol and hexanol systems, the vesicular dispersion gives way to a swollen lamellar phase when more alcohol is added. A further increase in alcohol content leads to the separation of a clear phase of low viscosity, the L3 phase. This process continues until the phase fills out the whole sample volume. The boundary between the LR and L3 phases is hard to distinguish without crossed polarizers because of the similarity of the refractive index in the two phases. On the other hand, no swollen lamellar phase is formed with heptanol; when the alcohol content is increased, the lamellar droplets dissolve directly into the L3 phase.21

Upon further addition of alcohol to the system, an L2 phase forms on top of the L3 phase, taking up the excess alcohol. In the octanol system, one observes that the CDS crystals dissolve into a birefringent dispersion of lamellar droplets that becomes thicker and more turbid upon alcohol addition. In a narrow region, a swollen lamellar phase separates at the bottom of the tubes that soon gives way to the L3 phase as more alcohol is added to the system. Just before the L3 one-phase region is reached, a lightly turbid L1/L3 dispersion forms and quickly dissolves again with further alcohol addition. In all systems, the L2/L3 two-phase region is reached when the alcohol concentration is increased to above L3 phase levels. We observe a different behavior at 5% of surfactant in the octanol system, as shown in Figure 2: At that concentration, a lamellar one-phase region could still be obtained at one point in the phase diagram that is reached from the L1 phase by way of a lamellar dispersion upon alcohol addition. However, this phase seems to be only kinetically stable. After a month, a tiny fraction of it transitions to an L3 phase, which is why the lamellar onephase region is not shown in the phase diagram in Figure 8. The transition to L3 occurs rapidly with a phase boundary that is hard to distinguish without crossed polarizers because of adaption of refractive indices.4 When the L2 phase begins to form on top of the L3 phase upon addition of alcohol, the L3 phase “withers away” in favor of an LR phase forming underneath. The newly formed L2 phase takes up so much of the alcohol that the alcohol content of the L3 phase underneath drops below the limit necessary for sponge phase formation, and an LR phase is formed once again. This effect is probably facilitated by the fact that the L3 phase is very narrow in the octanol system.21 At a surfactant concentration of 2% (35.2 mM) CDS, no L3 one-phase region could be obtained any more; the LR phase, however, is of considerable dimension at this surfactant concentration. The macroscopic structure of the lamellar phase itself was identical in all samples, no matter what alcohol they contained. It is a highly viscous phase, in most cases exhibiting yield stress and always showing strong scattering and birefringence of visible light. In test tubes, an optical axis becomes visible between crossed polarizers that has its origin in a partial orientation of the lamellae parallel to the walls of the tubes. On the other hand, the L3 phase is considerably less viscous and clearer than the LR phase. Neither streaming birefringence nor yield stress were observed. Particularly interesting information on the inner structure of the phases can be obtained from conductivity measurements with a constant amount of CDS and (21) Hornfeck, U.; Hammel, R.; Platz, G. Langmuir 1999, 15, 5(16), 5232-5236.

L3 Phase in Systems with CDS, Alcohol, and Water

Figure 3. Conductivity of 10 wt % CDS in water at 25 °C. Solid line, pentanol; dashed line, hexanol; dotted line, heptanol; dashed/dotted line, octanol. The indicators “L1” and “L3” refer to the approximate locations of the L1 and L3 phases, respectively. All concentrations are given in percent of mass. Note that the alcohol percentage refers to the amount of alcohol added.

increasing alcohol concentrations (cf. Figure 3). The low conductivity of the L1 supernatant in binary mixtures of CDS and water is given by the low solubility product of the CDS/water system. On addition of pentanol, the conductivity rises rapidly in the instant when the formation of mixed CDS/pentanol micelles becomes possible because of a lowered Krafft point. The conductivity passes through a maximum when all the CDS is solubilized in the L1 phase. The condensed lamellar droplets formed at higher alcohol concentrations lead to a decrease in conductivity. In the L3 phase, formed at about 13% pentanol, the conductivity is just as high as in the L1 phase. In other experiments, we found the same behavior in the magnesium dodecyl sulfate/hexanol/water system. Because of sterical hindrance, it is usually expected that the conductivity of the L3 phase is at about two-thirds of that of the L1 phase at the same surfactant content. However, this assumption should be valid only for nonionic surfactants with added electrolyte. In this case, where an L3 phase forms from an L1 phase containing micelles with bivalent counterions without added salt, the conductivity behavior is not that simple that it could be explained by the sterical factor of 2/3 alone. Considering this, with a deviation of 30%, the measurements still fit the model predictions reasonably well. Electron microscopy proved that there is really a sponge structure present (cf. Figure 10). At high alcohol concentrations, the conductivity decreases again, as the entire CDS is bound in the L2 phase. In the hexanol, heptanol, and octanol systems, a significant increase in conductivity is only found as the L3 phase is formed. The conductivity of the L3 phases with the three alcohols is about equal. This is a clue that all the L3 phases have a comparable microstructure. For better understanding, both the conductivity and the phase transitions are plotted against the alcohol concentration in Figure 4 for the octanol system. To investigate the concentration dependence of the four L3 phases, linear paths were laid out. To this end, the lowest and highest CDS concentrations were determined for which an L3 phase could be obtained. For all alcohols investigated, an upper limit to the L3 phase could be found; if there was a continuation, the channel would be so narrow that it was not resolved. Figure 5 shows the paths

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Figure 4. Conductivity-versus-concentration plot of 10 wt % CDS in water with octanol. The curve is the same as in Figure 3, and the phase behavior is explicitly stated.

Figure 5. Points in the phase diagrams of the four alcohols at which conductivity and viscosity measurements were taken. They also represent the stretch in the phase diagrams where we could find L3 phases. Pentanol, open circles; hexanol, solid circles; heptanol, open squares; octanol, solid squares.

determined and the points used for measuring the conductivity. The curves have a very similar slope, the pentanol curve is shifted toward higher alcohol concentrations because of the comparatively high solubility of pentanol in water (2.20 g/100 g at 25 °C22). The conductivities of the L3 phases, the compositions of which are given in Figure 5, fall just about together on a single, almost linear, curve (cf. Figure 6). From this it can be concluded that all four L3 phases must have very similar microstructures. Figure 7 is a plot of the viscosities of the four L3 phases. Because the viscosity is a function of the volume fraction of solute, it is plotted against the sum of the concentrations of CDS and alcohol. This is legitimate, because the densities of the four alcohols are very similar. A linear increase of the viscosity can be observed for pentanol, hexanol, and octanol, which is easily understood as being caused by an increasing volume fraction of surfactant and cosurfactant. This fact has been theoretically predicted and experimentally proven previously.5 As readily evident, solutions containing longer-chain alcohols exhibit some(22) Barton, A. F. M., Ed. Solubility Data Series. Vol. 15: Alcohols with Water; Pergamon Press: Oxford, 1984; p. 163.

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Figure 6. Conductivity measurements on L3 phases obtained with pentanol (open circles), hexanol (solid circles), heptanol (open squares), and octanol (solid squares) at 25 °C with CDS. The abscissa represents the CDS concentration.

Figure 7. Viscosity measurements on the L3 phase with pentanol (open circles), hexanol (solid circles), heptanol (open squares), and octanol (solid squares) at 25 °C. The sum of alcohol and CDS concentrations is plotted along the x axis.

what higher viscosity than those containing shorter-chain alcohols. With octanol, however, we encounter an entirely different situation. Toward the lower end of the L3 concentration range, the viscosity rises again and passes through a maximum shortly before the L3 one-phase region ends. This can be explained by a gradual transition of L3 to the LR phase with parallel lamellae forming in small volume elements. No phase transitions, however, could be observed in any sample for any alcohol; the yield stress equaled zero in all cases. The fact that CDS forms a sponge phase can be explained by the calcium ions, because of their electric charge of 2, being condensed onto the membrane, thus neutralizing the charge of the dodecyl sulfate ions. Consequently, the properties of those solutions are similar to those of nonionic, double-chain surfactant solutions. To verify this hypothesis, we investigated the behavior of mixtures of calcium and SDS and octanol at a constant total surfactant concentration of 5 wt %. The CDS side of the resulting phase diagram is shown in Figure 8. This phase diagram uses strict application of the Gibbs phase rule to improve accuracy. The phase rule implies that at a phase boundary, only one phase can be added or subtracted. Only at the

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Figure 8. Partial phase diagram of the CDS/SDS/octanol/water system. [CDS] + [SDS] ) 5 wt % ) constant, investigated region: 4.7% e [CDS] e 5%, 0 e [SDS] e 0.3%, 4% e [Octanol] e 10%. The measuring precision on the abscissa is 0.0625%;, it varies from 0.5% to 0.05% on the ordinate, according to the complexity of phase behavior. Dashed line, incompletely resolved phase transition.

corners of phase regions, a change in two phases is possible. This principle was adhered to when measurements delivered data that can be connected in such a fashion. Phase transitions that cannot be explained by the Gibbs phase rule are marked by dashed lines. As is evident from the diagram, the L3 one-phase region ceases to exist when 4% of the CDS is substituted for SDS. Even when only such a small fraction of the calcium ions is substituted for sodium, the increased thickness of the electric double layer induced by the monovalent sodium ions suffices to drastically change the physical behavior of the mixture, inducing the classical properties of an ionic surfactant solution. This happens as soon as the sodium diffuse double layer is thin enough to produce only a weak overlap if the mean distance between two surfactant bilayers is assumed to be about 15 nm. Another way of testing the hypothesis of CDS behaving like a two-chain nonionic surfactant would be to investigate the behavior of the CDS/alcohol/water system upon addition of salt. Experiments addressing this are currently being performed at our laboratory. The results of small-angle neutron scattering (SANS) measurements performed on the CDS/hexanol/water system are consistent with the results previously determined for typical sponge phases23 in terms of peak shape. In the L3 phase, the peak is not as pronounced as in the lamellar phase (Figure 9); however, the scattering maximum is located in the same position as the maximum of the lamellar phase. It is expected24,25 and often found26 that the peak is shifted to the right by a factor between 1.2 and 1.5. This anomalous behavior could be explained by the L3 phase being asymmetric,25 i.e., that the bilayer separates two inequal water volumina, a behavior that is topologically possible for an ideal one-bilayer system. However, from the behavior observed, no contradiction (23) Gradzielski, M.; Valiente, M.; Hoffmann, H.; Egelhaaf, S. J. Colloid Interface Sci. 1998, 205, 149-160. (24) Porte, G.; Marignan, J.; Bassereau, P.; May, R. J. Phys. (Les Ulis, Fr.) 1988, 49(3), 511-519. (25) Barnes, I. S.; Derian, P. J.; Hyde, S. T.; Ninham, B. W.; Zemb, T. N. J. Phys. (Paris) 1990, 51(22), 2605-2628. (26) Skouri, M.; Marignan, J.; May, R. Colloid Polym. Sci. 1991, 269, 9(9), 929-937.

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Figure 9. SANS measurements at 25 °C. Solid squares, LR phase (6.91% CDS, 8.24% hexanol); open squares, L3 phase (6.91% CDS, 8.47% hexanol); solid circles, L3 phase (9.67% CDS, 12.03% hexanol); open circles, L3 phase (16.15% CDS, 21.3% hexanol).

Figure 11. Freeze-fracture microscopy of the neighboring LR phase (4.938% CDS, 0.062% SDS, 6.400% octanol). (a) Planar lamellae; (b) disordered lamellae; (c) spongelike structures.

Figure 10. Freeze-fracture microscopy of an L3 phase composed of 4.875 wt % CDS, 0.125 wt % SDS, 7.000 wt % octanol. No glycerol was added.

could be found to the presence of a real sponge structure. Considering this, the phase can only be referred to as L3. However, it cannot be proven that the real microstructure is not partially broken or that no other anomalous structures are present. To prove the existence of the sponge structure of bicontinuous double layers characteristic of the L3 phase, freeze-fracture electron micrographs were prepared. A typical result, obtained from the CDS/SDS/octanol/water system without glycerol at 5% of surfactant, is shown in Figure 10. This texture clearly shows the characteristic bicontinuous sponge structure. The shape of the LR SANS peak suggests that a certain amount of disorder could already be present in the LR phase so that a closer-than-usual structural relationship exists between LR and L3. For further clarification of this question and the unusual fact that there is no peak shift in the SANS measurements of the LR and L3 phases, electron micrographs of the neighboring LR phase were prepared. The lamellar structure of this phase can clearly be proven (Figure 11a). Both regions of planar lamellae and of vesicle-like unordered bilayers can be identified

(Figure 11b), but one also finds structures at other locations of the same sample that deliver exactly the picture of the L3 phase (Figure 11c). In contrast to this, the L3 phase shows the same structure all across the sample. In a test tube, the sharp boundary between the lower LR and upper L3 phases can be distinguished only by use of crossed polarizers. The near-perfect adaptation of refractive indices (nLR ) 1.3477; nL3 ) 1.3480) and densities (L2 ) 0.99185 gm/L; L3 ) 0.99059 gm/L) indicates that the coexistent phases should have very similar compositions and critical demixing phenomena should be taken into account. By shaking, the coexistent phases mix into a single birefringent phase which separates extremely slowly when at rest. From this, it becomes clear why the electron micrographs show fragments of the coexistent L3 phase dispersed in the lamellar phase. Assuming that the pure LR and L3 phases show peaks with the expected shift, for the sample observed one could expect either two peaks or a peak shift. The possible assumption that there is only a small fraction of LR dispersed in L3is contradicted by the considerable peak height of LR. Considering this, the assumption of the L3 phase being asymmetric25 is more likely. Sponge phases can only form if the forces acting between the bilayers are not predominantly electrostatic in origin. Because the interaction of two charged planar surfactant

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bilayers, along with their diffuse double layers of counterions, is always repulsive, the bilayers are kept in a parallel orientation, which is why L3 phases are not usually formed in an ionic surfactant system. This fact also accounts for the increased stiffness and lower swelling capability of ionic LR phases compared with their nonionic counterparts. In the swelling process, electrostatic repulsion as a stabilizing factor fades away, whereas undulation forces27 play an increasingly important role. LR phases that are stabilized by undulation show lower viscosities than electrostatically stabilized lamellar structures. Undulation effects are also found in the L3 phase. Along with topological effects,5 this also accounts for their low viscosity. Recent investigations at our laboratory have shown that with decanol, there is no L3 phase at 25 °C, but only at elevated temperatures. From this it can be concluded that the L3 phase is stabilized by decreasing chain length of the surfactant or cosurfactant, because the consequent increasing chain fluctuations lead to lower membrane stiffness. From this, one can understand why the unshielded sodium octanoate/octanoic acid system can form an L3 phase, whereas the SDS/alcohol system cannot. The same should apply for the SDS/pentanol system. The behavior observed is in full accordance to Ekwall’s conclusion that “the temperature at which [the L3 phase forms] decreases as the chain-length of the components”.12 Conclusions In the CDS/medium-chain alcohol/water system we observe exactly the qualities typically associated with a (27) Helfrich, W. Z. Naturforsch. 1978, 33a, 305-315.

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nonionic surfactant, i.e., formation of an L3 phase and a high swelling capacity of the LR phase. This is explained by the fact that the electric double layer is significantly thinner with calcium counterions, compared with the sodium system. Thus, the counterions hover at very short distances from the lamellar surface and neutralize the charge of the bilayer. Consequently, the lamellae take on the physical properties of a very weakly charged system. Also the low membrane stiffness of the calcium system is a result of the small dimension of the diffuse electric double layer, because the bending energies of the two counterion Stern layers on both sides of the membrane being bent do not cancel out. It is clear that to induce deformation, an extended double layer needs more energy than a thin one. Therefore, in ionic systems, undulations are less pronounced, which is why their ability to swell or form an L3 phase is lower than that of nonionic systems. The changes the CDS system undergoes upon addition of a small fraction of SDS are dramatic. One can assume that the diffuse electric double layers formed by the calcium and sodium system can be treated separately, with the calcium double layer neutralizing its fraction of the lamellar charge at very short distances from the lamellar surface, and the sodium double layer stretching far into the solution, thus building up electrostatic repulsion between the bilayers and, in that manner, inducing the typical physical properties of an ionic surfactant system. Acknowledgment. The authors would like to thank the Deutsche Forschungsgemeinschaft for financial support of this study. LA010540P