Formation of a Lamellar Phase - American Chemical Society

Formation of a Lamellar Phase: Rearrangement of. Amphiphiles from the Bulk Isotropic Phase into a. Lamellar Fashion. Arjen Sein† and Jan B. F. N. En...
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Formation of a Lamellar Phase: Rearrangement of Amphiphiles from the Bulk Isotropic Phase into a Lamellar Fashion Arjen Sein† and Jan B. F. N. Engberts* Department of Organic and Molecular Inorganic Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands Received November 27, 1995. In Final Form: March 22, 1996X The dynamics of the formation of a lyotropic lamellar arrangement of surfactant molecules has been studied by means of a contact experiment. Technical grade dodecylbenzenesulfonic acid (HDoBS) was brought into contact with water or an aqueous solution containing sodium hydroxide or sodium hydroxide plus sodium chloride. The phase penetration was followed by light microscopy and freeze-fracture electron microscopy (FFEM). By the FFEM technique more insight has been obtained into the ordering process on a molecular level. The use of FFEM for such an application has not been presented before. In combination with phase diagrams, these techniques offered a mechanism for the formation of the lamellar phase, starting from the bulk surfactant situation. As a first step, water penetrates into bulk liquid isotropic HDoBS, thereby in passing inducing the lamellar order of surfactant molecules. Further maturing of the lamellar phase strongly depends on the type of aqueous phase offered.

Introduction In many liquid laundry detergents and household cleaning products the surface active agents are dispersed in units where the surfactant molecules are arranged in a lamellar fashion, so-called lamellar droplets.1 The processing of such products involves the combination of a bulk (undiluted/undissolved) surfactant mixture and the aqueous phase containing amongst others previously dissolved buffer and builder salts. In many cases the majority of the surfactant mixture consists of dodecylbenzenesulfonate (DoBS), in its acidic and therefore liquid state (HDoBS). This material needs to be neutralized during processing by previously dissolved sodium hydroxide. We are interested in how the lamellar arrangement of surfactant molecules is obtained,2 starting from the bulk isotropic liquid state (HDoBS), with this commercial application in mind. In fact this means that the route the amphiphile will follow through the phase diagram has to be examined, as well as the way water imposes the lamellar order on the amphiphiles. The phase diagrams through which this route leads have been described in a previous paper.3 Now the individual DoBS molecule will be followed on its course from the isotropic liquid bulk state toward the lamellar phase, and this will be marked in the phase diagram as presented before.3 This phase diagram exists, however, for a large part of a lamellar phase. Therefore a location where the route terminates is taken as the point of 1 molal HDoBS, 1 molal NaOH, and 1 molal NaCl. This corresponds to about 23 wt % NaDoBS in 1 molal NaCl, which is a two-phase system of electrolyte solution with a lamellar phase, built up from strongly coalesced (fused) lamellar units. Such a system is not a good representative * Author to whom correspondence should be addressed. E-mail: [email protected]. † Present address: Unilever Research Laboratory Vlaardingen, Product Principles Section, Olivier van Noortlaan 120, 3133 AT Vlaardingen, The Netherlands. X Abstract published in Advance ACS Abstracts, May 15, 1996. (1) Pas, J. C. van de. Ph.D. Thesis, University of Groningen, 1993. (2) Sein, A. Ph.D. Thesis, University of Groningen, 1995. (3) Sein, A.; Engberts, J. B. F. N.; Linden, E. van der; Pas, J. C. van de. Langmuir 1996, 12, 2913 (previous paper in this issue).

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of the ideal, colloidally stable dispersion that is supposed to mimic a commercial liquid laundry detergent. For such a system other ingredients are needed next to NaDoBS, water, and salt: these include a nonionic surfactant (for instance C13-15E〈7〉) and/or a decoupling polymer (for colloidal stability).1,4 The first step in which the amphiphiles are transformed from an isotropic phase into an ordered lyotropic phase is always the same, irrespective of the other ingredients next to HDoBS. The elucidation of the consecutive steps to such an ideal dispersion of lamellar droplets would need many more experiments. Except for a short section on a quasi five-component system containing C13-15E〈7〉, this paper will deal only with the HDoBS/water/NaOH/NaCl system. The liquid bulk nature of HDoBS is quite unique. Usually amphiphiles are crystalline solids in the bulk. Penetration of water and the subsequent formation of a lyotropic liquid-crystalline phase induces a decreased order, in contrast to the present case, which leads to increased order. A convenient procedure to study the process of alignment is by means of a contact or phase-penetration experiment, where amphiphilic molecules are brought into contact with water or an aqueous phase. The interpenetration is usually followed by light microscopy. From the birefringence pattern it can be determined which lyotropic phases are formed consecutively. It is a simple test for the lyotropic phase behavior. This type of experiment has proven to be fruitful and has been known since 1954.5 Many other examples can be found in the literature, where surfactant is brought into contact with water,6 a neutralizing acidic aqueous phase,7 or an electrolyte solution (4) Pas, J. C. van de; Oltshoorn, Th. M.; Schepers, F. J.; Vries, C. H. J. de; Buytenhek, C. J. Colloids Surf., A 1994, 85, 221. (5) Hyde, A. J.; Langbridge, D. M.; Lawrence, A. S. C. Discuss. Faraday Soc. 1954, 18, 239. (6) (a) Blackmore, E. S.; Tiddy, G. J. T. J. Chem. Soc., Faraday Trans. 2 1988, 84, 1115. (b) Rendall, K.; Tiddy, G. J. T.; Trevethan, M. A. J. Chem. Soc., Faraday Trans. 1 1983, 79, 637. (c) Laughlin, R. G.; Munyon, R. L. J. Phys. Chem. 1987, 91, 3299. (d) Hakemi, H.; Varanasi, P. P.; Tcheurekdjian, N. J. Phys. Chem. 1987, 91, 120. (e) Doren, H. A. van; Wingert, L. M. Recl. Trav. Chim. Pays-Bas 1994, 113, 260. (f) Nusselder, J. J. H.; Engberts, J. B. F. N.; Doren, H. A. van. Liq. Cryst. 1993, 13, 213. (g) Fonteijn, T. A. A.; Hoekstra, D.; Engberts, J. B. F. N. Langmuir 1992, 8, 2437. (7) Miller, D. D.; Belare, J. R.; Evans, D. F.; Talmon, Y.; Ninham, B. W. J. Phys. Chem. 1987, 91, 674.

© 1996 American Chemical Society

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containing multivalent ions.8 Also the process of soil removal has been studied by a contact experiment involving a micellar solution with an oil/long chain alcohol phase.9 Here the phase-penetration experiment will be employed to locate and characterize the phase-transition phenomena. Freeze-fracture electron microscopy (FFEM) has been employed as a tool to elucidate the mechanism underlying the process of rearrangement of molecules during the penetration process. Since the dynamic penetration cannot be studied as such in an electron microscope, the process should be frozen in, capturing the transitions in action. To the authors’ knowledge this is the first time that such an experiment has been reported. The dynamics of the growth of myelins has been illustrated by (cryo-) scanning electron microscopy.10 Previously, we have also employed cryo-transmission electron microscopy to visualize the penetration process.11 Note that such a phase-penetration experiment, studied by light microscopy or electron microscopy, is a twodimensional reflection of the process in an unrestricted environment of a real bulk system, such as that which occurs in the manufacturing process. The penetration is confined to the thin layer between the microscopy slide and the cover slip of the light microscopy preparation or the sandwich of copper holder plates used for the FFEM technique that induces orientation parallel to the slides. Moreover, such an experiment does not allow for agitation, which is an important factor for the formation and homogeneity of the lamellar phase. Neither does it take turbulence into account. Experimental Section Materials. The origin of HDoBS has been described previously.3 The nonionic surfactant C13-15E〈7〉 was a poly(ethylene oxide) alkyl monoether, with an alkyl chain of 13-15 carbon atoms and on an average seven ethylene oxide units (varying from 0-20) (SYNPERONIC A7, ICI), which was used without further purification. Water was demineralized and double distilled in an all-quartz distillation setup. NaOH and NaCl (p.a., Merck) were used as received. Light Microscopy. For the phase penetration studied by light microscopy, a droplet of HDoBS or a mixture of HDoBS and C13-15E〈7〉 (7:3 by weight) was placed on a glass microscopy slide, covered by a glass slip, and allowed to spread out and settle for a few minutes. A droplet of aqueous phase (water, 1 molal NaOH solution, or 1 molal NaCl plus 1 molal NaOH solution) was brought into contact by capillary action. The process was followed using a Zeiss Axioplan or a Zeiss Axioskop light microscope, usually in the crossed polars mode. Electron Microscopy. Replicas of the phase penetration were prepared in the following manner. A tiny droplet of bulk surfactant (HDoBS or HDoBS/C13-15E〈7〉 in a 7:3 weight ratio) was placed on a thin copper holder plate, cleaned prior to use by concentrated nitric acid. The holder plate was moreover slightly remodeled to obtain unevenness that could act as a spacer in order to create a narrow film (less than 0.5 mm) and to prevent the material from being squeezed out by further manual handling. The droplet was covered by another copper holder plate. This sandwich was kept between a pair of tweezers. Aqueous phase (water or 1 molal NaOH solution) was added by a syringe from the head-on side and by capillary action brought into contact with the bulk surfactant phase. Within one minute the penetration process was vitrified by rapidly immersing the sandwich into a nitrogen slush (a mixture of solid and liquid nitrogen). The sandwich was put into the fracture unit (kept in liquid nitrogen) (8) Tezˇak, D.; Babacˇic´, O.; Derek, V.; Galesic´, M.; Heimer, S.; Hrust, V.; Ivezic´, Z.; Jurkovic´, D.; Rupcˇic´, S.; Zelovic´, V. Colloids Surf., A 1994, 90, 261. (9) Miller, C. A.; Raney, K. H. Colloids Surf., A 1993, 74, 169. (10) Sakurai, I.; Suzuki, T.; Sakurai, S. Biochim. Biophys. Acta 1989, 985, 101. (11) Sein, A.; Breemen, J. F. L. van; Engberts, J. B. F. N. Langmuir 1995, 11, 3565.

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Figure 1. Oversimplified graphic representations of observations by light microscopy of the penetration of (A) water, (B) 1 molal NaOH solution, and (C) 1 molal NaCl plus 1 molal NaCl solution into bulk HDoBS, showing the typical features of the three contacting substances. The curved line on the righthand side represents the outermost penetration front, where isotropic HDoBS is aligned by the penetrating water molecules into a lamellar arrangement, the curly line represents the melting-off of the planar lamellar phase to form micelles, the circles with crosses represent lamellar units, the hatched collars indicate oily streaks, and the area with crossed lines indicates a domain with polygonal arrays. in such a position that the fracture would start in the bulk surfactant phase (the fracture would go from right to left in Figure 1). While the sandwich is broken, a platinum-carbon shadow layer of approximately 20 Å was deposited under a 45° angle, followed by a carbon support of approximately 200 Å at normal incidence, using a Balzers EVM 052A electron beam evaporation device with evaporator head EK 552. The film thickness was measured on a Balzers crystal quartz thin-film monitor QSG 201D. The replicas were washed with double-distilled water. The replicas were examined in a Philips EM 201 electron microscope, operating at 60 kV.

Results Observations by Light Microscopy. The route from the starting point (bulk HDoBS) to the end point (1 molal HDoBS, 1 molal NaOH, 1 molal NaCl) in the phase diagram involves three variables: (i) water to hydrate the headgroups and to induce order; (ii) NaOH (dissolved in water) to neutralize the dodecylbenzenesulfonic acid; and (iii) NaCl (dissolved in water) to insure that the amphiphiles remain aggregated in the lamellar fashion. The influence of these three substances was studied by phase-penetration experiments with only water, with a 1 molal NaOH solution, and with an aqueous solution containing NaOH and NaCl (both 1 molal). The main observations by light microscopy (between crossed polars) are schematically represented in Figure 1. Note that this is an oversimplified cartoon; the contact preparations as observed by light microscopy contain too many details to be completely described in this context. Most remarkable is that the outermost penetration front is the same for the three cases. After the first penetration, a planar lamellar phase appears as a uniform dark-gray area, which indicates that the lamellae are oriented in a parallel fashion to the glass support. This can also be inferred from the light microscopy images (Figure 2AC). In this area dislocations in the parallel packing of the lamellae manifest themselves as brightly birefringent structures. The number of dislocations increases as the penetration occurs more rapidly. The outermost penetration front looks like a straight line. On a closer look two types of tiny birefringent details are seen at some places on the front, focal-conic-like details (see Figure 2B) and

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Figure 2. Light microscopy micrographs of the penetration of aqueous phase (water or 1 molal NaOH solution) into bulk HDoBS (on the right-hand side), observed in the crossed polars mode, except (D) by phase contrast. (A) Overview of the outermost penetration front. (B) Detail of this front, showing a focal-conic-like defect. (C) Idem, showing a baˆtonnet-like defect. (D) The planar-lamellarto-micellar phase boundary in the case of contact with water. (E) Idem, in the case of contact with a NaOH solution, showing the growth of lamellar units in the micellar phase. Bars represent 25 µm.

baˆtonnet-like details (see Figure 2C). Both types of defects are also numerous in the planar lamellar part, as can be seen in Figure 2A. Next to the isotropic-to-lamellar transition, the penetration of water into HDoBS reveals only a lamellarto-micellar transition. The lamellar phase simply melts off into micelles. This is represented by the curly line shown in Figure 1A, and is illustrated by the light microscopy (phase contrast) image shown in Figure 2D. A similar transition occurs when the NaOH solution penetrates into HDoBS. However, further on in the micellar phase, lamellar units reappear, as more and more HDoBS is neutralized. This part resembles the micellar plus lamellar two-phase area in the NaDoBS/water phase diagram; the lamellar units are identical to those seen in a 40 wt % NaDoBS sample by light microscopy image, described in an earlier paper.3 Further into the NaOH solution, on the left-hand side of Figure 1B, the concentration of NaOH is high and, being an electrolyte, it induces the formation of flocculated lamellar droplets. If the electrolyte concentration in the aqueous phase is increased further, as in the case of the NaOH plus NaCl contact preparation, depicted in Figure 1C, the system cannot pass through a micellar phase. The penetration occurs rather violently, leaving many dislocations in the planar lamellar part and extended domains with an irregular mosaic structure or polygonal arrays.12 On the aqueous-phase side (the left-hand side in Figure 1C) oily streaks appear.13-15 Both oily streaks and polygonal arrays are typical manifestations of the lamellar phase. No aggregates drift off into the aqueous phase. (12) Candeau, F.; Ballet, F.; Debeauvais, F.; Wittmann, J.-C. J. Colloid Interface Sci. 1982, 87, 356. (13) Kle´man, M. Points, Lines and Walls; Wiley: New York, 1983; Chapter 5, p 108. (14) Benton, W. J.; Miller, C. A. Prog. Colloid Polym. Sci. 1983, 68, 71. (15) Miller, C. A.; Ghosh, O.; Benton, W. J. Colloids Surf. 1986, 19, 197.

Observations by Freeze-Fracture Electron Microscopy. In a similar way phase-penetration experiments were conducted, employing freeze-fracture electron microscopy (FFEM). In short, a tiny droplet of HDoBS was sandwiched between two small copper holder plates. A droplet of aqueous phase was added from the head-on side and by capillary action brought into contact with HDoBS. This sandwich was rapidly vitrified and broken open, and immediately a replica was made of the fracture face (for further details, see Experimental Section). Since the scale of observation is much smaller than that with light microscopy and because the replica usually breaks down into smaller pieces, it is not possible to follow on one replica the whole penetration process from bulk HDoBS to only aqueous phase, passing through all intermediary stages (from right to left in Figure 1). Still, particular details that were observed by FFEM could be matched with the observations performed by light microscopy. Figure 3 shows several FFEM images with these details. Figure 3A shows a rather fortunate image of the penetration front. On the right-hand side of this image the fracture surface of a vitrified liquid can be seen; this is the HDoBS part. On the left-hand side a phase with lamellar character is apparent. This is the only image with the penetration front that has been found. Many other images were made showing dislocations in the lamellar packing. These contain information on the penetration process, since the dislocations have arisen during the penetration process, at the time the penetration front passed by. Some of these are shown in Figure 3B-F. The images look the same for either water penetration or NaOH solution penetration. Many of these dislocations occur when the water penetration induces lamellar order and the lamellae buckle into an eddy (a spiral) and come to a dead end on themselves. Typical examples of spirals that peter out in themselves are shown in Figure 3B and C. Spirals of lamellar phases have been observed before by FFEM.16 When phosphatidylserine vesicles are treated by Ca2+ ions and incubated for 1 h, the Ca2+ ions induce the formation

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Figure 3. Freeze-fracture electron microscopy micrographs of HDoBS/aqueous phase contact experiments. (A) Outermost penetration front: On the left-hand side are stacks of lamellae, and on the right-hand side is vitrified HDoBS (contact with NaOH solution). (B) Typical spiral penetration pattern (eddy) running dead on itself (water). (C) Idem (NaOH solution). (D) Large screw dislocation (indicated by the arrow) next to a domain with many dislocations with several clear grain boundaries between focal conic domains (marked with the asterisks) (water). (E and F) A collection of different kinds of dislocations (water). Bars represent 500 nm.

of a continuous planar lamellar membrane sheet that subsequently buckles into a spiral cylinder. The spiral structures observed in the present case, however, must be a direct consequence of the penetration of water into an isotropic bulk surfactant phase. In Figure 3D also a large screw dislocation is seen that presumably gives rise to a baˆtonnet-like birefringent pattern in the light microscopy equivalent. In addition, several grain boundaries between focal conic domains are seen in this image,

where a mismatch occurs between two stacks of bilayers.17 Such a row of defects also appears in the light microscopy image (see Figure 2A) as a row of Maltese crosses. In addition many other dislocations are seen that have also been elegantly described by Kle´man and co-workers.17 These defects have also been comprehensively described in geometrical terms.13,18-20 The typical large birefringent lamellar units that appear in the NaOH solution-penetration experiment were clearly

(16) Papahadjopoulos, D.; Vail, W. J.; Jacobson, K.; Poste, G. Biochim. Biophys. Acta 1975, 394, 483.

(17) Kle´man, M.; Williams, C. E.; Costello, M. J.; Gulik-Krzywicki, T. Philos. Mag. 1977, 35, 33.

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Figure 4. Freeze-fracture electron microscopy micrographs of lamellar units that appear in the NaOH solution penetration experiment. (A) Detail of large units showing the multilamellar character and the thin water layers. (B) Smaller droplets in vitrified surroundings of the isotropic micellar phase. Bars represent 500 nm.

inferred from FFEM, as shown in Figure 4. Many units are partially broken open, revealing the multilamellar character and the thin intralamellar water layers. Discussion The images presented above will now be combined with other characteristics of the lyotropic phase behavior of HDoBS (or NaDoBS), mainly presented in a previous paper.3 It is clear that the first step in all penetration processes is the same. Water molecules are the first to penetrate; later the hydroxide ions, the sodium ions, and eventually the chloride ions follow suit. Macroscopically, the water penetration is a diffusion process. On a molecular scale, however, water aligns the HDoBS amphiphile in a lamellar fashion; other substances do not seem to affect this alignment process. Diffusion. The driving force for the penetration of the aqueous phase is determined by several factors. The primary diffusion process is the entropy-driven diffusion described by Fick’s first law: J ) D′w(dC/dr).21 In addition, an enthalpy-driven process will play a role, because water can hydrate the headgroups and the acidic proton. When also hydroxide is present in the aqueous phase, it will follow the water molecules at distance, since electroneutrality requires that Na+ (including its hydration shell) is also dragged into the bulk surfactant phase. Still, the large favorable enthalpic effect of the neutralization is important. At water concentrations of less than three water molecules per HDoBS molecule, the water molecules will be dispersed homogeneously throughout the HDoBS isotropic phase. The water molecules slowly and separately squeeze their way through the bulk HDoBS, obstructed by the HDoBS molecules for their urge to be hydrated. This was also apparent from the slow selfdiffusion of water in a 90 wt % HDoBS/10 wt % water system.2 Such a molecular penetration of water into bulk HDoBS will easily be overtaken by a process where water penetrates in portions of at least three molecules per (18) (a) Zasadzinski, J. A. N. Biophys. J. 1986, 49, 1119. (b) Zasadzinski, J. A. N.; Scriven, L. E.; Davis, H. T. Philos. Mag. A 1985, 51, 287. (19) (a) Allain, M. J. Phys. Fr. 1985, 46, 225. (b) Allain, M.; Kle´man, M. J. Phys. Fr. 1987, 48, 1799. (20) Demus, D.; Richter, C. Textures of Liquid Crystals; VEB Deutscher Verlag fu¨r Grundstoffindustrie: Leipzig, 1980. (21) Atkins, P. W. Physical Chemistry, 4th ed.; Oxford University Press: Oxford; 1990.

HDoBS molecule, with which water in passing induces the lamellar order in the HDoBS. In this case the headgroups are largely satisfied in their need for water, but more importantly, the interlamellar water layer acts as a channel through which the water can be transported to the HDoBS parts deeper in the sample. This process will be more rapid than that when the molecules should wriggle their way individually through the bulk HDoBS, despite the dimensional restrictions in such a lamellar phase. This leads to the important conclusion that, irrespective of the type of aqueous phase that is offered, water always penetrates first and that, with such quantities of water molecules, the lamellar order of amphiphiles is induced simultaneously. Once the lamellae have been formed, more water will penetrate, the water layers will swell, and, if the aqueous phase consists of only water, the HDoBS lamellae will melt off into micelles. When the aqueous phase contains NaOH, HDoBS will soon be neutralized, and a normal NaDoBS/water lyotropic phase behavior is observed: a two-phase micellar plus lamellar phase system. When NaCl is present as well, the formation of micelles is circumvented. No micellar phase has been observed in this type of penetration experiment. Predominantly planar lamellar phases are seen, terraces oriented by the surfaces of the microscopy slides. Extra agitation is essential in order to obtain smaller patches of the lamellar phase. This agitation is necessary to speed up the penetration on a larger scale, as is required in the manufacturing process. When arrived at the final equilibrium situation, the electrolyte concentration in the interlamellar water layers will be lower than that in the continuous electrolyte solution.1 Experimentally it appears that the rate of penetration, as observed by the light microscope, is predominantly determined by the thickness of the preparation. The thinner the preparation, the faster the penetration, and moreover, more defects will occur in the lamellar phase, and thus more birefringent particles will appear. The rate of penetration might also depend on the type of aqueous phase that has been offered. However, no noteworthy differences are observed. If these rates should be determined, a standard penetration chamber with known dimensions is required.6c,d Route. Now a route can be marked down in the phase diagram presented before.3 This is shown in Figure 5. The route starts in the bulk HDoBS corner and terminates at the point when HDoBS, NaOH, and NaCl are present

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Figure 5. Proposed route from the isotropic bulk HDoBS toward a two-phase lamellar plus electrolyte solution at 23 wt % of NaDoBS in a 1 molal NaCl solution.

in a concentration of 1 molal, corresponding to 23 wt % of NaDoBS in 1 molal NaCl solution. Subsequently an HDoBS molecule experiences the following situations. In the bulk HDoBS phase, the amphiphiles are oriented randomly as in any liquid. When the water penetration front passes, the headgroup of the amphiphile receives about three water molecules and the acidic proton probably dissociates. Moreover, the water molecules orient in passing the amphiphile in a lamellar order, with its headgroup oriented toward the water channel. Below more light will be shed on the molecular aspects of this transition. As more water penetrates, the distance between the bilayers increases (swelling). Later hydroxide ions will arrive (and concomitantly sodium ions) and the sulfonic acid will be neutralized. The sodium ion can bind to the bilayer surface. If the majority of the headgroups have been neutralized, the bilayers experience a mutual attraction (or more properly, the electrical double-layer repulsion will be reduced), causing deswelling. The bilayers approach each other, and water molecules will be squeezed out or obstructed to penetrate further. As a consequence, the penetration process stagnates, illustrating the need for agitation and breaking up the bulk HDoBS into smaller units. Such a swelling/deswelling behavior might be measurable by a time-dependent X-ray diffraction phase-penetration experiment, similar to the one that has been described in the literature.22 In the absence of NaCl, the amphiphile can become so strongly hydrated that it can even form micelles. However, the presence of NaCl prevents micellization. The amphiphiles will see only a small number of Cl ions passing through the water layer. Toward the equilibrium situation, the distribution of water molecules in the electrolyte solution and those that are still tolerated in the thin intralamellar water layers will balance out. Molecular Aspects of a Lamellar Alignment. An intriguing question is how water is able to display its directional action on the amphiphiles. As it penetrates, it aligns the amphiphiles in a lamellar fashion. Light microscopy cannot visualize such a transition on a molecular scale. In general, the penetration front appears as a straight line, clearly restricted by the orientation imposed on the diffusion by the microscopy slides. The most plausible way of penetration is probably also the most obvious and straightforward one. It is schematically depicted in Figure 6; drawn as the side view, the direction of penetration is perpendicular to the penetration front and the direction of observation. In reality the water layer will be much thinner, because, to establish the lamellar order, it takes only three tiny water molecules per HDoBS (22) Chung, Y. J.; Jeffrey, G. A. Biochim. Biophys. Acta 1989, 985, 300.

Figure 6. (A) Side-view cartoon of the most common penetration site of water into bulk HDoBS; the penetration front runs perpendicular to the direction of penetration (into the paper). (B) Schematic representation of the penetration channels that cause a focal-conic-like penetration defect, as seen in Figure 2. This cartoon is inspired by the FFEM image presented in Figure 3A. Note that here the lines are water channels and not bilayers. HII* stands for an inverse-hexagonal-like structure in the center of such a defect (see text).

molecule. Three water molecules occupy only little space (the bilayer repeat distance for 86 wt % HDoBS3 is only 2.65 nm). Such a penetration front can also be inferred from the electron micrograph presented in Figure 3A. If one layer is formed, it most likely acts immediately as a template for the next one by facilitating the alignment of the tails of the amphiphiles in the other half of the bilayer. The first layer might be guided by the glass surface. Another obvious mechanism might involve a sort of inverse hexagonal penetration channel that diverges into a water layer as water moves further. Such a mechanism has been illustratively presented in the literature.23,24 It is not valid here, because such a transition site would clearly be borne out by the typical (inverse) hexagonal birefringence pattern when observed by light microscopy and a tubular structure in the FFEM experiments. Moreover, neither the inverse hexagonal phase nor a cubic phase has been detected in the DoBS/water lyotropic phase diagrams, which indicates the obstinacy of the amphiphile to assume such a packing. It might occur in an exceptional case, as a defect, as will be discussed below. Some birefringent defects appear at the penetration front that remain present in the planar lamellar part (see Figure 2A-C). Two types of defects occur, the focal-coniclike and the baˆtonnet-like defects. The latter one might be caused by a screw dislocation (see Figure 3D), resulting in a tilted orientation of the bilayer stack with respect to the other bilayers and the glass slides. Being not perpendicular to the optical axis, it appears as a brightly (23) Venetie¨, R. van; Verkleij, A. J. Biochim. Biophys. Acta 1981, 645, 262. (24) Fifield, R. New Sci. 1980, 88, 150.

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Figure 7. FFEM contact experiment of aqueous phase into a mixture of HDoBS/C13-15E〈7〉. (A) Longitudinal cut-through image of a myelin. (B and C) Penetration eddies, bound to run dead on themselves. Bar represents 500 nm.

birefringent patch by light microscopy.25 The focal-coniclike type is caused by a certain radial symmetry of the bilayers. Here the penetration front is not straight but is locally bent forward, induced by the pushing of a deformation that appears as the inverse of a myelin (water layers growing into the bulk surfactant). This can be seen in the electron micrograph presented in Figure 3A, on the left-hand side of the penetration front. The layer structure of this figure is schematically depicted in Figure 6B, with an inverse-hexagonal-like structure or a cylindrically shaped water channel in the center (marked in Figure 6B by HII*) that at a certain point must diverge into a water layer. Note that here the lines represent the water channels and not the bilayer. Such an inverse-myelin deformation involves a radial symmetry of the bilayers that also causes the focal-conic-like domain as seen by light microscopy (see Figure 2B). This mechanism will not survive for a longer time, since the inner parts of the inverse-myelin structure should receive the amphiphiles that are necessary for the growth of the structure, from the side where the aqueous phase comes from. Soon the supply of these amphiphiles will stagnate, the water in the penetration channels at the center will break through the curved parts of the outer layers of the structure, and the penetration process will continue as described above for the most frequently encountered case. A highly remarkable observation in the FFEM phasepenetration experiment is the occurrence of penetration eddies (see Figure 3B and C). The formation of such an eddy is a consequence of the penetration process, and they might level out upon aging. They have not been observed in the equilibrated lamellar phases described in the previous report.3 Effect of an Added Nonionic. In the light of the practical system, as a liquid laundry detergent, it would be quite informative to know how the penetration process occurs in a mixture of HDoBS and a nonionic surfactant such as C13-15E〈7〉. First a pseudo-four-component phase diagram should be worked out. The phase behavior of the [HDoBS/C13-15E〈7〉 (7:3)]/water and the [NaDoBS/ (25) Hartshorne, N. H. In Liquid Crystals and Plastic Crystals; Gray, G. W., Winsor, P. A., Eds.; Ellis Horwood: Chichester, 1974; Vol. 2, p 24.

C13-15E〈7〉 (7:3)]/water systems can be described rather briefly, since both systems have a swelling behavior comparable to that of the HDoBS/water system. The difference with the HDoBS/water system is, however, that in the presence of a nonionic surfactant the bilayers seem to be rather flexible; presumably they have a low bending rigidity. As a result, the bilayers are easily oriented parallel to the microscopy slides. The defects that usually cause the birefringent appearance by light microscopy can easily be smoothed out because of the flexibility of the bilayers. Defects of opposite sign can now come together easily and annihilate one another. Although macroscopically evidently a lamellar phase, it appears isotropic by light microscopy. Only in the wake of a moving air bubble or on the edges of a preparation can birefringence be seen.1,12 The most interesting observation, concerning the penetration process of the aqueous phase into a mixture of HDoBS and C13-15E〈7〉, is that the outermost penetration front looks exactly the same as in the case of solely HDoBS, again irrespective of the type of aqueous phase that has been offered. A noteworthy difference is that the penetration of both a 1 molal NaOH solution and the NaOH + NaCl solution induces extensive myelin formation in the planar lamellar phase; the myelins do not grow into the aqueous phase. These myelins are extremely flexible, but when they get trapped and age for some minutes, the myelins turn into oily streaks and focal conic domains. It seems as if myelins form a prestage of the oily streaks and focal conic domains. The myelins were also observed in an FFEM contact experiment. Shown in Figure 7A is a longitudinal cross-sectional view of a myelin. A transverse cross-sectional view has been presented in the literature.10 Also many spiral penetration dead ends are seen; some are shown in Figure 7B and C. These penetration eddies occur frequently and are probably a consequence of the low bending rigidity of the bilayer. Also the planar parts of stacks of bilayers show many corrugations by FFEM (not shown). After the penetration of the aqueous phase, the solution conditions for the nonionic surfactant will be good (even with the NaCl solution as the contacting substance). The PEO coils of the headgroups will occupy a rather large

Rearrangement of Amphiphiles into a Lamellar Fashion

volume. Hence the nonionic can exert a large steric repulsion between the bilayers. Probably more than three molecules of water per amphiphile are needed to induce the lamellar alignment of amphiphiles, because the PEO headgroup needs more water molecules for hydration.26 As a consequence, a fairly large interbilayer distance will be present from the beginning, that facilitates the transport of water molecules and hydroxide. Also after the neutralization of the HDoBS to NaDoBS, which caused the stagnation of the penetration of water in the HDoBS case, the nonionic can now keep the gap between the bilayers wide enough to allow for easy water penetration. Concluding Remarks From phase-penetration experiments the most likely route through the phase diagram has been obtained that starts at bulk HDoBS and terminates at the point of 23 wt % NaDoBS in a 1 molal NaCl solution, which is a lamellar plus electrolyte solution two-phase system. First water penetrates, irrespective of the offered aqueous phase. The penetration of at least three water molecules per HDoBS is necessary to induce the lamellar arrangement of HDoBS amphiphiles. This paves the way for more water for the bulk HDoBS parts further on. Such a penetration through an interbilayer water layer is faster than the individual penetration of water molecules through the bulk HDoBS. Later also neutralizing hydroxide ions, sodium ions, and eventually some chloride ions will penetrate. The contact with an electrolyte solution circumvents the formation of a micellar phase. When the majority of the HDoBS has been neutralized, a net attraction between the bilayers causes a deswelling (26) (a) Jonstro¨mer, M.; Jo¨nsson, B.; Lindman, B. J. Phys. Chem. 1991, 95, 3293. (b) Tanford, C.; Nozaki, Y.; Rohde, M. F. J. Phys. Chem. 1977, 81, 1555.

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that hampers further penetration. In order to get a more homogeneous mixture and to prevent stagnation of the penetration, the system needs to be agitated. The route is schematically marked down in the phase diagram shown in Figure 5. The mechanism by which water aligns the HDoBS amphiphiles most likely comprises a straight penetration front, immediately inducing the lamellar order (see Figure 6A). The first-formed flat water layer will act as a template for the next one. The orientation of the water layers is probably affected by the glass support slides. Dislocations in the lamellar stacks occur when the penetration outruns itself. Two types of dislocations are apparent from light microscopy, baˆtonnet-like and focal-conic-like defects. These dislocations have also been elucidated by freezefracture electron microscopy. Next to these, also many penetration eddies have been observed, where the penetration buckles and runs dead on itself. The mixture of HDoBS and a nonionic surfactant (C13-15E〈7〉) appears to follow the same mechanism in the first, lamellar arrangement-inducing water penetration step. Later in the penetration process, myelin formation occurs. In conclusion, the study of the penetration process by FFEM has proven to be fruitful, and the results exhibit a pleasing consistency. Acknowledgment. Dr. J. C. van de Pas and Dr. E. van der Linden of the Unilever Research Laboratory, Vlaardingen, The Netherlands, are gratefully acknowledged for stimulating discussions and valuable suggestions. We are indebted to Unilever Research Laboratory, Vlaardingen, The Netherlands, for financial support. LA951075X