Article pubs.acs.org/Langmuir
Myelin Structures Formed by Thermotropic Smectic Liquid Crystals Karthik Peddireddy, Pramoda Kumar,† Shashi Thutupalli,‡ Stephan Herminghaus, and Christian Bahr* Max Planck Institute for Dynamics and Self-Organization (MPIDS), 37077 Goettingen, Germany S Supporting Information *
ABSTRACT: We report on transient structures, formed by thermotropic smectic-A liquid crystals, resembling the myelin figures of lyotropic lamellar liquid crystals. The thermotropic myelin structures form during the solubilization of a smectic-A droplet in an aqueous phase containing a cationic surfactant at concentrations above the critical micelle concentration. Similar to the lyotropic myelin figures, the thermotropic myelins appear in an optical microscope as flexible tubelike structures growing at the smectic/aqueous interface. Polarizing microscopy and confocal fluorescence microscopy show that the smectic layers are parallel to the tube surface and form a cylindrically bent arrangement around a central line defect in the tube. We study the growth behavior of this new type of myelins and discuss similarities to and differences from the classical lyotropic myelin figures.
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INTRODUCTION Myelin figures are multilamellar tubular microstructures which can form spontaneously when a lamellar lyotropic liquid crystal, the Lα phase, which consists of a stack of alternating surfactant and water layers, is brought in contact with pure water. Since the first description of myelin figures by Virchow in 1854,1 numerous studies were concerned with their formation,2−7 structures,8−11 and properties.12−15 One condition for the formation of myelin figures is that the Lα phase should exhibit a low solubility in water; that is, in the binary phase diagram made up by water and the corresponding surfactant, there should be a large miscibility gap between the micellar L1 and the lamellar Lα phase. Although the detailed mechanism of their formation is still to be clarified, myelin figures can be seen as transient structures occurring during the swelling and dissolution process of the Lα phase, their structure corresponding to cylindrically bent water/surfactant bilayers with the maximum water content for which the Lα phase is still stable. The spontaneous formation of myelin figures has been observed for various experimental settings. For instance, instead of contacting the Lα phase with water, one can also bring the pure surfactant in contact with water. Myelins were also observed in mixtures containing cholesterol and aqueous ionic surfactant solutions,16 or oil, nonionic surfactants, and water.17,18 Also in these latter cases, the myelins are based on the lyotropic Lα phase. The spontaneous formation of tubelike structures has been observed also in thermotropic smectic liquid crystals. Smectic phases are liquids in which the rodlike molecules form a layered structure. The simplest phase, denoted as smectic-A, can be seen a stack of molecular layers, each layer corresponding to a two-dimensional liquid. Tubelike structures can be observed when the smectic-A phase coexists with its isotropic melt. In © 2013 American Chemical Society
most cases, the smectic-A phase nucleates in the form of socalled batonnets19 on cooling from the isotropic state. In some systems, however, such as mixtures of smectic compounds with nonmesogenic compounds like aliphatic alcohols20−22 or silicone oil,23,24 mixtures of nematic and smectic compounds with a chiral component,25 but also single smectic compounds,26 the smectic-A phase nucleates in the shape of cylindrical filaments (denoted by some authors as “nematoids” because of their threadlike appearance) with a diameter of a few micrometers. The length of the filaments grows considerably as the temperature is lowered through the coexistence region. The length growth at constant diameter has been attributed to the ambition of the system to avoid the generation of additional smectic layers (which are concentrically wrapped around the tube axis).20,27 The formation of elongated thermotropic liquid crystal structures has been observed also in nematic or cholesteric droplets: Cholesteric droplets were observed to develop cylindrically shaped protuberances in the pretransitional range near the phase change to the smectic-A state,28 and nematic droplets floating in aqueous phases containing a surfactant with a mesogenic moiety were found to exhibit temperature-induced shape transformations to long filaments.29 In the present study, we report on a new experimental configuration leading to the formation of tubelike structures of smectic-A liquid crystals. In our setting, the smectic-A liquid crystal is not coexisting with its isotropic melt, rather the conditions are similar to those for the formation of the classical lyotropic myelin figures; that is, we are contacting a lamellar phase with an aqueous phase, and the lamellar phase is, to a Received: October 7, 2013 Revised: November 22, 2013 Published: November 25, 2013 15682
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Figure 1. Sandwich experiment. Micrographs (crossed polarizers) of an 8CB droplet sandwiched between two glass plates separated by 100 μm spacers. The droplet is contacted by an aqueous CTAB solution (c = 20 mM), and the growth of the myelin-like tubular structures is observed. The left image shows the droplet immediately after the contact with the CTAB solution, and the middle and right image after 100 and 200 s, respectively. The blue circles in the right image indicate the circular approximations of the interfaces between the myelin-filled region and the aqueous phase and the smectic droplet. The difference between the radii of the two circles was used as a measure of the witdth L of the myelin-filled region. The horizontal width of each image corresponds 1.75 mm. experiment, a glass capillary with a square cross section (CM Scientific, length = 5 cm, side length of the square cross section between 35 and 100 μm) is filled first completely with the surfactant solution. Then, one end of the capillary is dipped into a 8CB droplet so that a few millimeters of the capillary are filled with 8CB. Under certain conditions (described below), a single myelin tube was then observed to grow along the capillary axis. The myelins were studied using polarizing optical microscopy (Nikon Eclipse VL100, equipped with a video recording facility). For a few samples, we did in addition fluorescence confocal polarizing microscopy measurements (Leica TCS-SP2). For this purpose, the 8CB sample and the CTAB solution were doped with the fluorescent dyes nile red (concentration 0.005 wt %) and fluorescein (concentration 0.01 wt %), respectively. The fluorescence of the two dyes can be separately excited to obtain information of the distribution of the 8CB phase and the aqueous phase in the sample. The fluorescence is excited with linearly polarized laser light, and the intensity distribution of the emitted fluorescence is imaged. Since the nile red molecules align on average parallel to the 8CB molecules, the intensity distribution of the nile red fluorescence gives additional information on the orientation of the 8CB molecules. Further details are given in the following section.
small extent, soluble in the aqueous phase but separated by a large miscibility gap. In our case, the lamellar phase is the thermotropic smectic-A liquid crystal and the aqueous phase contains a surfactant at concentrations above the critical micelle concentration, resulting in a possible micellar solubilization of the smectic-A material. Usually, thermotropic liquid crystals and aqueous surfactant solutions behave as immiscible liquids forming a stable interface, although the surfactant molecules adsorbing at the interface can have pronounced effects on the interfacial orientation of the liquid crystal.30,31 Inverse surfactant micelles can form a microemulsion in a thermotropic nematic matrix.32,33 We have recently reported34 on the micellar solubilization behavior of thermotropic liquid crystal droplets which were immersed into aqueous solutions containing micelles of a ionic surfactant. Whereas nematic droplets showed a similar solubilization behavior as isotropic organic liquids,35 the solubilization of smectic droplets was accompanied by filament-like transient structures which strongly resembled the myelin figures of the lyotropic systems. In the present study, we describe basic properties concerning the structure and growth behavior of these new “thermotropic myelins” and discuss similarities and differences to the classical lyotropic myelins.
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RESULTS AND DISCUSSION Figure 1 illustrates a typical sandwich experiment. The three micrographs show a sandwiched 8CB droplet at three different times: immediately after the contact with an aqueous CTAB solution and 100 and 200 s later. Myelin-like tubular structures develop at the 8CB/aqueous interface, forming a ring-shaped myelin-filled region around the 8CB droplet. It is clearly discernible that this ring-shaped region grows with time not only into the aqueous environment but also toward the center of the initial 8CB droplet. To quantify the growth of the myelin-like structures, both the droplet/myelin interface and the myelin/aqueous interface are approximated by circles and the difference between their radii, that is, the width L of the myelin-filled region, is determined as a function of the time t which has elapsed since the initial contact between the smectic liquid crystal and the surfactant solution. The result is shown in Figure 2 as a double-logarhitmic plot of L versus t. After an initial period of about 10−20 s, L grows for a longer time interval as t0.5 until after about 500 s, where the data suggest a slowing of the growth. A diffusional growth behavior for a longer period is a common observation when lyotropic myelins are studied in contact experiments using the sandwich configuration.2,4,7,11,13,15 It is attributed to the collective diffusion of surfactant molecules induced by the hydration gradient in the Lα/water contact zone.
MATERIALS AND METHODS
The liquid crystal compounds used for the experiments were 4-octyl4′-cyanobiphenyl (8CB) and 4-dodecyl-4′-cyanobiphenyl (12CB), obtained from Synthon Chemicals and used as received. The compound 8CB is at room temperature in the smectic-A state, and at 33 °C it transforms into the nematic phase; 12CB is in the solid state at room temperature, and it melts into the smectic-A state at 46 °C and transforms to the isotropic state at 59 °C. Both compounds showed essentially the same behavior in our experiments, in the following we present mainly the results obtained for 8CB. As surfactant, we used hexadecyltrimethylammonium bromide (CTAB, Sigma Aldrich, used as received). Aqueous surfactant solutions are prepared by dissolving certain amounts of CTAB in Millipore water. The measurements with 8CB were conducted at a slightly elevated room temperature (24−25 °C) to stay above the Krafft point of CTAB (≈20 °C for our sample). Studies with 12CB were conducted at 50 °C. We study the myelin formation using two experimental configurations which we designate in the following as sandwich or capillary experiment. For the sandwich experiment, a small amount of 8CB is sandwiched between two glass slides which are separated by 100 μm spacers. Then, the remaining empty space between the glass slides is filled via capillary action with the aqueous surfactant solution, thereby contacting the smectic liquid crystal with the surfactant solution. Similar experimental configurations have been widely used to study the growth of lyotropic myelins.2,4,7,11,13,15 For the capillary 15683
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Figure 2. Double logarhitmic plot of the average width L of the region occupied by the growing myelin-like tubes as a function of time t for the sample shown in Figure 1. The slope of the solid line amounts to 1/2.
Figure 4. Mean diameter D̅ of the myelin tubes observed in the sandwich experiment as a function of the concentration of CTAB. The sample with cCTAB = 5 mM is not included in this diagram because the data indicate that the tube diameter would be larger than the thickness of the sample cell (100 μm).
We have carried out the sandwich experiment with different concentrations cCTAB of CTAB in the aqueous phase. For all studied values 5 mM ≤ cCTAB ≤ 80 mM, a linear increase of L2 with t is observed for a time interval between 20 s and at least 200 s and the growth rate can be quantified by the linear slope L2/t. The dependence of L2/t on cCTAB is shown in Figure 3: apparently, the growth rate first increases with increasing CTAB concentration, is maximal for cCTAB = 10 mM, and then decreases with increasing values of cCTAB.
growth behavior of the total surface area instead of the length of the myelin tubes. Figure 5 shows the result when the slope L2/t is replaced by the slope (S/A)2/t, indicating now that the growth velocity of the myelin tubes (based on their total surface area) increases monotonously with increasing CTAB concentration.
Figure 5. Same data as shown in Figure 3 with the mean length L of the myelin tubes replaced by the ratio S/A (surface area S of a myelin tube divided by its cross-section A, see text) for various concentrations of CTAB.
Figure 3. Slope L2/t of the linear parts of L2 versus t plots for various concentrations of CTAB. The inset shows L2 versus t plots for cCTAB = 10 mM (●) and 20 mM (○).
In the sandwich experiment, the individual myelin tubes grow in an arbitrary way and they are to certain degree movable in the surfactant solution. While looking for an experimental configuration that would offer a stronger confinement, we found that a single myelin tube can be grown in a glass capillary possessing a square cross section, provided, the surfactant concentration in the aqueous phase is tuned so that the diameter of the growing tube is approximately equal to or slightly larger than the side length of the capillary cross section. The confinement in a capillary enables structural studies of a single tube using polarizing microscopy and fluorescence confocal microscopy. In thermotropic smectic phases, the smectic layer thickness can be regarded as constant, thereby restricting the possible elastic deformations of the structure. When a smectic liquid crystal is confined to the shape of a cylindrical tube or a sphere with a perpendicular anchoring of the molecules on the surface, the natural way to arrange the smectic layers is an onionlike structure in which the smectic layer planes are parallel to the surface and wrapped around either a line defect running along the central axis of the tube or a point defect in the center of the sphere. Figure 6 shows micrographs of a growing tube in a capillary between crossed
Intuitively, since the myelin tubes do not form when the surfactant concentration in the aqueous phase is zero, one would expect that the growth becomes faster with increasing surfactant concentration. However, in that context, a second experimental observation becomes important: increasing the CTAB concentration results in a decrease of the average diameter of the tubes (see Figure 4) while the packing density does not show an obvious change. In other words, with increasing CTAB concentration, the diameter of the myelins decreases but their number density increases. These observations suggest to use, instead of the length of the tubes, the surface area of the tubes per unit area of the initial smectic LC/ aqueous interface as a measure of the myelin growth. If we assume that the type of packing of the tubes remains the same when tube diameter changes, we can simply divide the surface area S of a tube (calculated from L and D̅ assuming a cylindrical geometry) by the area A of the cross section of the tube to obtain an appropriate measure. A plot of (S/A)2 versus t has, of course, the same shape as L2 versus t, but it quantifies the 15684
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Figure 7 shows fluorescence confocal polarizing micrographs of a smectic tube in a capillary. The 8CB sample was doped with the dye nile red which aligns with the 8CB molecules.36 The study of the fluorescence signal as a function of the polarization direction of the exciting laser light enables the determination of the orientation of the liquid crystal molecules:37 The intensity of the nile red fluorescence is maximum in those regions of the sample in which the 8CB molecules, and thus, the nile red molecules are parallel to the polarization of the exciting laser light. In the configuration shown in Figure 7, this parallel arrangement exists only in the tip of the smectic tube, where we observe accordingly the largest intensity of the nile red fluorescence. Again, the results are in exact accordance with the expected arrangement of the smectic layers, shown in Figure 7e and f. Since confocal microscopy enables three-dimensional imaging, we can also directly prove the cylindrical shape of the smectic tube (Figure 7c). Another important result of the fluorescence confocal microscopy is that we do not observe an indication of the presence of the aqueous phase, which was doped with the dye fluorescein, in the interior of the smectic tube. This observation is in contrast to the growth process of the lyotropic myelins which takes place by incorporating water into the lamellae of the Lα phase. Volumetric measurements provide further confirmation that the growth of the smectic tubes cannot be characterized as a swelling process. Figure 8 shows volume data for a single tube growing in the capillary configuration: the magnitude of the volume change ΔVt, by which the tube has grown in a given time interval, equals exactly the amount of the volume change ΔVr, by which the volume of the reservoir of the disordered bulklike smectic material has decreased, and the sum of the tube volume and the reservoir volume stays constant, at least on the time scale of several minutes.38 We have also compared the
Figure 6. Capillary experiment. Micrographs with crossed polarizers (two orientations, differing by 45 degrees as indicated on the upper and lower image) of a square capillary with an inner width of 50 μm. The capillary was first filled with an aqueous CTAB solution (c = 10 mM) and then dipped into a 8CB droplet. A well ordered smectic tube (central part of the capillary) is growing from the bulklike disordered 8CB (left-hand part of the capillary).
polarizers for two different orientations of the polarizers. The change of the dark/bright pattern on rotation of the polarizers is exactly in accordance with the expected arrangement of the smectic layers.
Figure 7. Fluorescence confocal polarizing micrographs of a smectic 8CB (doped with nile red) tube growing in a square capillary (inner width 35 μm) filled with an aqueous CTAB solution (c = 20 mM, doped with fluorescein). Red/yellow color indicates the fluorescence signal stemming from nile red (smectic phase), and green color indicates the fluorescence signal of fluorescein (aqueous phase). The color codes on the left and on the right describe the relative intensities of the nile red and fluorescein signals, respectively. The capillary axis is along the x diretion and the capillary is observed through the microscope along the z direction. (a) Cross section through the central region of the capillary along the xy plane (top view). The white double arrow indicates the polarization of the exciting laser light. The yellow color in the tip of the tube corresponds to a high intensity of the nile red signal, indicating that in this region the dye molecules and the liquid crystal molecules are parallel to the polarization of the laser light. (b) Cross section through the central region of the capillary along the xz plane (side view). The decrease of the fluorescence intensity from top to bottom is a result of the relatively large thickness (35 μm) of the sample. (c) Cross section perpendicular to the capillary axis along the blue line drawn in (a), demonstrating the cylindrical shape of the smectic tube. (d) Cross section perpendicular to the capillary axis in a region that is filled with disordered bulk-like smectic 8CB (not shown in (a)). (e) Schematic cross section of the smectic tube along the xy or xz plane. The black lines indicate the orientation of the smectic layer planes, the smectic molecules (not shown) are perpendicular to the layer planes. The red line indicates the topological line defect located at the tube axis. (f) Schematic cross section of the smectic tube along the yz plane (perpendicular to the tube axis). The smectic layers are concentrically wrapped around the central line defect (red dot). 15685
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Figure 8. Volume data for a growing smectic tube in the capillary configuration (8CB in contact with aqueous CTAB solution, cCTAB = 40 mM)). (○) Volume of the growing tube, (∗) volume of the disordered bulk-like 8CB material, and (●) sum of both volumes.
Figure 9. Micrograph (polarizers not completely crossed) of a smectic tube ejecting a thin secondary tube (12CB in aqueous CTAB solution, cCTAB = 280 mM, T = 50 °C). Scale bar: 30 μm.
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smectic−nematic transition temperature in the tubes and the bulklike smectic material by observing both in the optical microscope while the temperature was slowly increased. Within an intervall of 0.1 K, both structures showed the same transition temperature. These measurements, as well as the fluorescence confocal microscopy results, suggest that the growth of the smectic tubes takes place essentially without the incorporation of material from the aqueous phase (with the exception of the surfactant molecules adsorbed at the newly created smectic/ aqueous interface), rather it appears basically as a rearrangement of the smectic layers. Another feature of the smectic myelins is a kind of hierarchical formation of the tubes: On the surface of the “primary” smectic tubes, which were growing from the interface between bulklike 8CB and the aqueous surfactant solution, thinner “secondary” tubes can start to grow. The probability to observe these secondary tubes increases with increasing CTAB concentration in the aqueous phase. The growth of these secondary tubes can start spontaneously, or it can be initiated by creating a defect in the surface of the parental tube, using, for example, a short heat pulse generated by optical tweezers. This defect-initiated growth of smectic tubes is similar to the observation that the creation of a defect in the surface of a wellordered Lα phase can lead to the growth of lyotropic myelin tubes,6 however, for the lyotropic myelins a spontaneous growth of secondary myelins from the surface of primary myelins has, to our knowledge, not been reported so far. Preliminary observations indicate that also lyotropic myelins may exhibit this behavior when they are brought in contact with an aqueous CTAB solution instead of pure water.39 The diameter of the secondary tubes can be quite small (of the order of 1 μm), and their growth appears as an ejection process (see movie in the Supporting Information and Figure 9) by which a random coil of the secondary tube is formed. Long secondary tubes eventually decompose into smaller fragments, which can adopt different shapes such as short tubes, small droplets, or vesicle-like structures. A systematic investigation of these smaller fragments is beyond the scope of the present study and will be reported in a later paper. However, if the reservoir of the surrounding aqueous surfactant solution is large enough (i.e., if the fraction of the smectic material in the system is not larger than a few wt %34), all smectic tubes and fragments become completely solubilized and the final state is a microemulsion in which all liquid crystal molecules have been transferred into the surfactant micelles in the aqueous phase.
CONCLUSION We have reported on the structure and growth behavior of myelin-like structures which form when a thermotropic smectic liquid crystal is brought in contact with an aqueous phase containing a ionic surfactant at concentrations above the critical micelle concentration. There are several similarities and differences between the thermotropic system and the classical lyotropic myelin figures. In both systems, myelins appear as transient structures during the dissolution process of a solute phase (the thermotropic smectic or the lyotropic Lα phase) possessing a large miscibility gap to the solvent phase (the aqueous surfactant solution or the pure water phase). In both systems, the transient structures appear in an optical microscope as cylindrical flexible tubes of similar dimensions. The similar appearance is probably a result of the fact that both systems are layered phases with a similar layer periodicity and similar elastic properties. The elastic properties of both systems are characterized by a small ratio of bending to compression modulus; that is, the energy related to a bending deformation of the layers is several orders of magnitude smaller than that related to a compression of the layers, which promotes the formation of flexible cylindrical tubes in which the layers are wrapped around the tube axis. The growth kinetics in the sandwich configuration also appears similar in both systems. This observation is somewhat surprising because, as is discussed below, the growth of the smectic tube is not accompanied by a material transport from the aqueous to the smectic phase. An obvious difference between both systems consists in their microscopic structures: The thermotropic smectic-A phase is an orientationally ordered liquid possessing a one-dimensional density wave. It can be formed by a one-component system, consisting just of one species of rodlike organic molecules. The lyotropic Lα phase consists of at least two components, water and surfactant, which segregate on the nanometer scale by forming alternating water and surfactant layers. The Lα phase is stable within certain range of water/surfactant composition. These microscopic differences show up in the growth process of the myelin tubes. The growth of the lyotropic myelins is accompanied by the transport of water into the myelin tubes leading to a larger water content in the myelin tube compared to the initial Lα phase; the formation of the lyotropic myelins is thus connected with a swelling process of the Lα phase. In contrast, the thermotropic smectic myelins form without the incorporation of water, as is demonstrated by the constant total 15686
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volume of the tube- and bulklike smectic material which is observed during the initial formation of the smectic tubes. Since the final state is a microemulsion, it is obvious that there must be a transfer of liquid crystal molecules into the CTAB micelles in the aqueous phase. At present, we do not know the mechanism of the formation of the smectic myelin tubes and how the transport of liquid crystal molecules to the aqueous phase is related to it. The presence of surfactant micelles which are able to take up smectic molecules is certainly a necessary condition for the formation of the tubes. Our observations suggest also that defects in the smectic layer structure near the surface promote the growth of the smectic tubes: In bulklike smectic-A samples, such as the droplet in the sandwich configuration, defects are certainly present. In the case of well ordered samples, such as the primary tubes, defects have to be generated, for example, by an external heat pulse, or they might develop spontaneously as a consequence of a surface instability which the smectic-A phase suffers in the presence of surfactant micelles. The structure of the smectic tubes as shown in Figure 7e and f is the same as that in the tubelike structures occurring during the nucleation of smectic domains in their isotropic melt.20−22,25−27 In these systems, the tubes grow in length at constant tube diameter when the temperature is lowered, that is, when the smectic fraction is increased at the expense of the isotropic fraction. The tube growth may be seen as a convenient possibility of the system to increase the smectic volume without the generation of new smectic layers (which would cost a layer nucleation energy).20 In a similar way, the tube growth in our experiment may be seen as a possibility of the system to increase the area of the smectic/aqueous interface without the generation of new smectic layers. However, it is obvious that this would be just one aspect of the tube formation and that further studies are necessary to clarify the mechanism of the formation of the smectic myelins. Finally, we would like to point out an important difference in the optical properties of lyotropic and thermotropic myelin tubes. The thermotropic smectic tubes are strongly birefringent and their refractive indices are larger than that of the aqueous CTAB solution, whereas the lyotropic myelin figures possess nearly the same refractive index as that of their aqueous environment. Thus, the smectic myelins may be promising systems for soft matter photonic applications, similar to nematic40 or cholesteric41 droplets. Indeed, first studies42 have demonstrated optical wave guiding by the smectic tubes as well as whispering gallery mode lasing in a plane perpendicular to the tube, rendering the smectic myelins a new class of soft matter optical fibers.
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S.T.: Complex Fluids Group, Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, USA. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge financial support from the European Union (EC Marie Curie ITN project Hierarchy − PITN-CA-2008215851). We wish to thank a referee for the suggestion to compare the smectic−nematic transition temperature in the tubes and the smectic bulk material.
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ASSOCIATED CONTENT
S Supporting Information *
Movie demonstrating the ejection of a thin secondary tube from a primary tube. The experimental system is the same as that shown in Figure 9. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
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P.K.: Harvard School of Engineering and Applied Science, Cambridge, MA 02138, USA. 15687
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dx.doi.org/10.1021/la4038588 | Langmuir 2013, 29, 15682−15688
