Article pubs.acs.org/Langmuir
Emulsion of Aqueous-Based Nonspherical Droplets in Aqueous Solutions by Single-Chain Surfactants: Templated Assembly by Nonamphiphilic Lyotropic Liquid Crystals in Water Nisha Varghese, Gauri S. Shetye, Debjyoti Bandyopadhyay, Nemal Gobalasingham,§ JinAm Seo,§ Jo-Han Wang,§ Barbara Theiler,‡ and Yan-Yeung Luk*,† †
Department of Chemistry, Department of Biomedical and Chemical Engineering, Syracuse University, Syracuse, New York 13244, United States S Supporting Information *
ABSTRACT: Single-chain surfactants usually emulsify and stabilize oily substances into droplets in an aqueous solution. Here, we report a coassembly system, in which single types of anionic or non-ionic surfactants emulsify a class of water-soluble nonamphiphilic organic salts with fused aromatic rings in aqueous solutions. The nonamphiphilic organic salts are in turn promoted to form droplets of water-based liquid crystals (chromonic liquid crystals) encapsulated by single-chain surfactants. The droplets, stabilized against coalescence by encapsulated in a layer (or layers) of single chain surfactants, are of both nonspherical tactoid (elongated ellipsoid with pointy ends) and spherical shapes. The tactoids have an average long axis of ∼9 μm and a short axis of ∼3.5 μm with the liquid crystal aligning parallel to the droplet surface. The spherical droplets are 5−10 μm in diameter and have the liquid crystal aligning perpendicular to the droplet surface and a point defect in the center. Cationic and zwitterionic surfactants studied in this work did not promote the organic salt to form droplets. These results illustrate the complex interplay of self-association and thermodynamic incompatibility of molecules in water, which can cause new assembly behavior, including potential formation of vesicles or other assemblies, from surfactants that usually form only micelles. These unprecedented tactoidal shaped droplets also provide potential for the fabrication of new soft organic microcapsules.
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different phases under slightly different aging conditions.8 The result demonstrates another example that polymorphism is not restricted to the solid phase, but can also manifest in different phases, including liquid crystals.8 Furthermore, a nonamphiphilic molecular structure is necessary for the liquid crystal formation from molecules similar to disodium cromoglycate (5′DSCG). The assembled noncovalent polymers are heavily hydrated, and form a liquid crystal phase when the amount of mesogens present in the sample reaches certain concentrations at ambient conditions. When two different polymers are mixed in a solvent, they often exhibit demixing and phase separation even when both polymers are completely soluble in the solvent.9−11 Interestingly, the noncovalent polymers formed by 5′DSCG also phaseseparate from covalent polymers that have different functional groups, such as polyacrylamide, polyvinyl alcohol,6 polyvinyl pyrrolidone,12 and poly(ethylene glycol).7 The phase separation leads to the formation of water-based liquid crystal droplets that are presumably coated and stabilized by the polymers.6 These results indicate that, if the polymer differs
INTRODUCTION Colloidal phenomena involving liquid crystals often reveal interesting fundamental phenomena. For example, alignment of inverse micelles in thermotropic liquid crystals1 and selective liquid crystallization of DNA duplex at high concentrations.2−5 Water-based droplets of nonamphiphilic lyotropic liquid crystals can be stabilized in water by non-ionic covalent polymers giving rise to unusual water-in-water emulsions.6,7 Here, we describe a coassembly system in which single types of anionic or non-ionic surfactants are templated to form a distinct class of assemblies at neutral pH by a class of nonamphiphilic, fused aromatic organic salts. The organic salts in turn are promoted by the surfactants to form hydrated noncovalent polymers that further condense into nonamphiphilic lyotropic liquid crystals, traditionally called chromonic liquid crystals. Due to the liquid crystal properties, both spherical and nonspherical assemblies were obtained in the coassembly system. The nonspherical assembly by the single-chain surfactants assumed a “tactoidal” shape (an elongated ellipsoid with pointy ends), which is unusual for amphiphilic molecules. By synthesizing and studying a series of structurally related dichromonyl molecules, we recently discovered that molecules that form nonamphiphilic lyotropic liquid crystals can exist in © 2012 American Chemical Society
Received: March 18, 2012 Published: June 22, 2012 10797
dx.doi.org/10.1021/la302396c | Langmuir 2012, 28, 10797−10807
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structurally from the 5′DSCG molecules and lacks strong interactions, such as salt exchange reactions, with the 5′DSCG molecules, then the polymer has a high propensity to demix from 5′DSCG molecules in water. Apart from structural differences, we believe that the high affinity for self-association of 5′DSCG molecules is also critical for the observed phase separation with the mixed covalent polymers. These findings suggest that surfactant molecules may also demix and phase separate from mesogens of nonamphiphilic lyotropic liquid crystals, because surfactants and 5′DSCG have grossly different functional groups and they both self-associate but with different sets of molecular interactions. Assembly of surfactants has had wide-ranging applications since their discovery in ancient times. However, they continue to be a source of significant scientific exploration, including the development of biosensors13,14 and novel mesoporous materials,15−18 as well as core shell structures.19,20 The fundamental study of exploring the coassembly formed by single-chain surfactants will enable the development of new soft materials and biological applications.
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RESULTS AND DISCUSSION This work studies the coassemblies formed between generic single-chain surfactants and a special class of nonamphiphilic molecules. The study included surfactants with all four kinds of head groups, anionic, non-ionic, cationic, and zwitterionic. Anionic surfactants included sodium dodecylbenzene sulfonate (SDBS), sodium dodecyl sulfate (SDS), sodium-N-lauroyl sarcosinate (SLS), and sodium decanoate (SD). Non-ionic surfactants included n-dodecyl-β-D-maltoside (DM) and tetra(ethylene glycol) monododecyl ether (TEGMDE). A zwitterionic surfactant, N-tetradecyl-N,N-dimethyl-3-amino-1-propane sulfate (TDAPS), and two cationic surfactants, hexadecyltrimethyl ammonium chloride (HTAC) and dodecyltrimethyl ammonium chloride (DTAC), were also included in the study of the coassembly system. We also used a fluorescently tagged anionic surfactant, 12-N-methyl-(7-nitrobenz-2-oxa-1,3-diazol) aminostearate (12-NBD stearate), to examine the location of the surfactants in the coassembly. The nonamphiphilic molecules studied, both of which can form an unusual class of lyotropic liquid crystals in water,21 included disodium cromoglycate (5′DSCG)8,22−24and Sunset Yellow dye (SSY dye)25−30 (Figure 1). Droplet Formation of Water-Based Nonamphiphilic Lyotropic Liquid Crystal in the Presence of Single-Chain Surfactants. We prepared samples containing a single type of surfactant and a nonamphiphilic molecule in water by two different methods. The dilution method involved mixing two solutions each containing the surfactant and 5′DSCG to achieve the targeted concentrations for each component. The direct dissolution method involved adding water directly to solid residues of a neat surfactant and a nonamphiphilic molecule (5′DSCG or SSY dye). The effects of aging, agitation by a vortex, and heating were also studied for samples prepared by both methods. The optical images under crossed polarizers and parallel polarizers for a sample containing 10.9 wt % of sodium Nlauroyl sarcosinate (SLS) and 5.5 wt % of 5′DSCG in deionized water (prepared by dilution method) were shown in (Figure 2). Between crossed polarizers, we observed birefringent droplets with both radial and bipolar configurations. For the radial droplets, a dark cross imagea Maltese cross31was observed in spheres of average diameter of 9 ± 0.5 μm; when the samples were rotated between the crossed polarizers (Figure 2A−C),
Figure 1. Structures of the nonamphiphilic mesogens disodium cromoglycate (5′DSCG), sunset yellow dye (SSY dye); anionic surfactants, sodium dodecylbenzene sulfonate (SDBS), sodium dodecyl sulfate (SDS), sodium decanoate (SD), sodium N-lauroyl sarcosinate (SLS), fluorescent tagged amphiphile sodium salt of 12-Nmethyl-(7-nitrobenz-2-oxa-1,3-diazol) aminostearate (12-NBD stearate); non-ionic surfactants, n-dodecyl-β-D-maltoside (DM), tetra(ethylene glycol) monododecyl ether (TEGMDE); zwitterionic surfactant, N-tetradecyl-N,N-dimethyl-3-amino-1-propane sulfate (TDAPS); cationic surfactants, dodecyl trimethyl ammonium chloride (DTAC), hexadecyl trimethyl ammonium chloride (HTAC).
the cross image did not change. The result indicates that the optical axis of 5′DSCG liquid crystal aligns perpendicular to the surface of the droplet with a single topological point defect at the center of the droplet. The bipolar droplets assumed an elongated shape with the polar axes longer than the diameter of the equatorial plane that bisected droplets. Interestingly, the polar ends of the droplets were sharp and pointy making these droplets tactoids7,32,33 rather than ellipsoids,6 which have a smooth curvature at the polar ends. The appearance (birefringence) of the tactoidal droplets changed from bright to dark as the samples were rotated between the crossed polars. This result suggests that the optical axes of the liquid crystal align parallel to the surface of the tactoidal droplets with two defects at each of the polar ends. The optical images of the sample between parallel polarizers (Figure 2D−F) also showed boundaries for both radial and tactoidal droplets. The formation of these liquid crystal droplets is intriguing because solutions of 5.5 wt % of 5′DSCG alone without surfactants exist as isotropic solutions, while concentration of 11−12 wt % is required for 5′DSCG alone to form liquid crystal phase. To understand the mechanism/assembly structure that contributes to the formation of droplets at such low concentrations of 5′DSCG, we hypothesize that the singlechain surfactants are coated on the droplets to encapsulate liquid crystal phase of 5′DSCG. We believe that the coating of the surfactants on the droplets may adopt one of the three possible assemblies. First, the hydrophilic head groups are in contact with the surface of the water-based liquid crystals, 10798
dx.doi.org/10.1021/la302396c | Langmuir 2012, 28, 10797−10807
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Figure 2. Liquid crystal droplets in radial and tactoid configurations formed by mixture of 10.9 wt % SLS and 5.5 wt % 5′DSCG. Images of radial and tactoidal droplets between crossed polarizers (A−C) and parallel polarizers (D−F). The different orientations of the sample relative to one polarizer are indicated by the angles above the images. Spacer is 3 pieces of (Saran wrap) thickness ∼39 μm.
Figure 3. Z-stacked images of confocal fluorescence of 10.1 wt % SDBS, 5.9 wt % 5′DSCG, and 0.002 wt % 12-NBD stearate. Z-thickness is represented by numerical values on the top left corner.
fluorescent microscope. The fluorescent images of the confocal layers showed that the fluorescent signals were localized on the surface of both the radial and tactoidal droplets. Figure 3 shows eight (out of fifteen) confocal layers (z-slices) at 1, 3, 5, 7, 9, 11, 13, and 15 μm of the depth of the sample. For the z-slice at 1μm deep, the defect lines were seen outlining the shape of the tactoidal and radial droplets. These features were not fluorescent, and were likely due to the different refractive index at the surface of the droplets. However, a few fluorescence signals were seen on top of the tactoidal droplets. Observing the confocal layer at 3-μm deep, strong fluorescence was seen on the edges of the tactoid along with a few scattered fluorescent signals in the center, whereas for the radial droplet, only a few fluorescent signals were seen in the center of the confocal layer. Examining the 5-μm-deep confocal layer, the fluorescence decreased on the edge of the tactoid, but emerged in the center of radial droplet. The fluorescent domains within the 5′DSCG droplets are likely due to the fluorescence emitted by the micelles of the surfactant, which are be trapped within the water-based droplets. Examining the 7-μm-deep confocal layer, the fluorescence ceased completely on the tactoidal droplet, but appeared to be strong on the edge of the radial droplet. Further examining deeper confocal layers up to 15 μm, the fluorescent signal on the radial droplet also decreased gradually from the edge toward the center of the confocal layer.
which form the first layer of coated single-chain surfactants. The hydrophobic chains of this layer are in contact with the hydrophobic chains of a second layer of single-chain surfactants. These two layers of single-chain surfactants form a unilamellar vesicle. Second, the liquid crystals droplets are coated by multiple layers of such bilayers, that is, a multilamellar vesicle. Third, the droplets are coated by individual micelles in contact with each other. Single-chain surfactants usually do not form vesicles. These novel assembly systems present a new class of water-in-oil-in-water emulsion system that is similar to living cells for which concentrated aqueous solutions are encapsulated by a lipid system surrounded by a carrier aqueous solution. Fluorescent Images Reveal Surfactant Coating on Droplets Encapsulating 5′DSCG. To test the hypothesis that surfactants are coated on the droplets, we added a fluorescently tagged anionic surfactant 12-NBD stearate into the emulsion system, and used confocal fluorescence to identify the location of the fluorescent surfactant. The fluorescent anionic surfactants will be sequestered by the assembly formed by other surfactants in solution.34 We prepared a sample by directly dissolving solid residues of SDBS and 5′DSCG in a solution of 0.002 wt % of 12-NBD stearate in deionized water to obtain 10.1 wt % SDBS and 5.9 wt % 5′DSCG. The sample was agitated by vortex before examination under a confocal 10799
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Figure 4. Schematic representation of the assembly structure of surfactant vesicles templated by 5′DSCG liquid crystal droplets. (A) Optical image (between cross polarizers) of the sample prepared from 10.9 wt % SLS and 5.5 wt % 5′DSCG by dilution method, scale bar = 9.5 μm. (B) Scheme of the radial droplet configuration consists of liquid crystal phases formed by noncovalent threads of 5′DSCG aligned perpendicular to the surface of the droplet. (C) Scheme of the tactoid configuration with the threads aligned parallel to the surface of the droplet. (D) Assembly structure of molecular threads of 5′DSCG in the liquid crystal droplet. (E) A noninterdigitated or interdigitated bilayer or flocculated layer of micelles formed by single-chain surfactant (SLS) in the coassembly system.
Because the fluorescent surfactant 12-NBD stearate will be sequestered by SDBS, these results suggest that there is a layer of assembled surfactants coating the liquid crystal droplets, which leads to concentric rings of fluorescence from the circular slice of the droplet. Furthermore, the appearance of the fluorescent signal spanned from 2 to 6 μm for the tactoid, and 3 to 14 μm for the radial droplet. These distances matched well with the dimensions of the birefringent droplets observed under the polarizing microscope: the minor axis of the tactoid was ∼3.5 μm, and the diameter of the radial droplet was ∼9.5 μm. As both the droplets and the bulk solution are water-based, the coating of surfactants can exist in one of the several possible assemblies mentioned earlier including bilayers (either a noninterdigitated double layer or an interdigitated mixed bilayer structure) or a layer of connected micelles coated on the surface. We note that these structures can also exist in unilamellar or multilamellar vesicles (Figure 4). As different phases are possible for the bilayers of lipids,35−37 we believe that different assembly structures are also possible for singlechain surfactants. The images in the confocal z-slices in (Figure 3) also showed a protrusion on the spherical droplet. This protrusion was visible both in the bright field at the 5-μm-deep z-slice and with the fluorescent signal at the 7-μm-deep z-slice. The fluorescent signal disappeared at deeper z-slices of the sample. This observation is similar to the budding or fusion process of vesicles,38,39 a topic of our ongoing research. Templated by the droplets of nonamphiphilic liquid crystals, these superstructures indicate a spontaneous coassembly process between 5′DSCG molecules and the micelles formed by single-chain surfactants in solution (Figure 4). The 5′DSCG molecules form highly hydrated threads,8,40 which existed in the
liquid crystal phase within the droplet. These droplets assumed both radial and tactoidal configurations and were coated with surfactant molecules. We note that the concentration of 5′DSCG used to prepare these droplets (
