Self-assembly of surfactants and phospholipids at interfaces between aqueous phases and thermotropic liquid crystals

Current Opinion in Colloid & Interface Science 10 (2005) 111 – 120 www.elsevier.com/locate/cocis

Self-assembly of surfactants and phospholipids at interfaces between aqueous phases and thermotropic liquid crystals Nathan A. Lockwood, Nicholas L. Abbott * Department of Chemical and Biological Engineering, University of Wisconsin, Madison, WI 53706, USA Available online 14 July 2005

Abstract Recent studies have reported on the self-assembly of surfactants and phospholipids at interfaces between aqueous phases and thermotropic liquid crystals. These studies have been enabled by an experimental system that permits the preparation of stable and easily-imaged interfaces between liquid crystals and aqueous phases. The influence of the molecular structure of surfactants and their interfacial organization on the ordering of the liquid crystals has been revealed. Because the liquid crystals reorder on time-scales of seconds or less, the time-dependent behavior of the liquid crystals can be used to follow dynamic phenomena at these interfaces, including the adsorption and desorption of surfactants. At these same aqueous-liquid crystal interfaces, phospholipids have been shown to organize into assemblies that possess the mobility of biological membranes and are coupled to the order within the liquid crystal. Specific protein binding events and enzymecatalyzed reactions at phospholipid-decorated interfaces of liquid crystals lead to changes in the organization of the phospholipids that are ‘‘reported’’ as ordering transitions in the liquid crystal. D 2005 Elsevier Ltd. All rights reserved. Keywords: Surfactants; Phospholipids; Liquid crystals; Adsorption; Biomolecular interactions

1. Introduction Studies of the adsorption of surfactants at interfaces have progressed on various fronts over the past decade, enabled in part by development of new instrumentation that permits the characterization of surfactant-decorated interfaces. For example, atomic force microscopy has permitted the observation of a variety of different surfactant nanostructures at solid – liquid interfaces [1,2], and non-linear optical methods have yielded insights into the behavior of surfactants at solid – water [3] and oil – water [4 –7] interfaces, the latter of which are particularly hard to study by conventional methods because of the buried and deformable nature of the interface. While these experimental studies have provided important insights into surfactant adsorption at Ftraditional_ interfaces, this article

* Corresponding author. Tel.: +1 608 265 5278; fax: +1 608 262 5434. E-mail address: [email protected] (N.L. Abbott). 1359-0294/$ - see front matter D 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.cocis.2005.06.002

focuses on a series of experimental studies of surfactant adsorption at a type of interface that has not received the same level of past attention – an interface between an aqueous phase and a thermotropic liquid crystal. This interface represents a special class of Foil – water_ interfaces in which the oil possesses a long range order that couples to the presence and organization of surfactant at the interface. It is also reasonable to suspect that the organization of the liquid crystalline phase imposes itself on the surfactant, although this aspect of the system remains largely unstudied. Dispersions of thermotropic liquid crystals and immiscible isotropic phases have been studied for several decades due to their potential application in liquid crystal display technologies. In the past ten years, a series of papers by Poulin, Weitz and others have reported on emulsions of liquid crystals and water [8&&,9]. Addition of the surfactant sodium dodecyl sulfate (SDS) to the aqueous phase was observed to cause the liquid crystal to assume a perpendicular (homeotropic) orientation at the interface between the aqueous phase and liquid crystal. When using a water-in-

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liquid crystal emulsion, elastic distortions and topological defects formed in the liquid crystal surrounding the water droplets and mediated droplet – droplet interactions that had dipolar and quadrupolar symmetries. In this article, we do not attempt to describe liquid crystal emulsions in detail, as they have been reviewed previously [9]. Instead, we focus on recent reports of studies that seek to characterize the coupling that occurs between surfactants and phospholipids adsorbed at approximately planar interfaces between aqueous phases and liquid crystals and the resulting orientations of the liquid crystals.

2. Experimental geometry The experimental system that is the focus of this article comprises an interface between an aqueous phase and a water-immiscible, thermotropic liquid crystal. A liquid crystal that has been studied in this experimental system is 4V-pentyl-4-cyanobiphenyl (5CB). 5CB forms a nematic liquid crystal at ambient temperatures. The liquid crystal is hosted in the pores of an electron microscopy grid, which is supported on a glass microscope slide treated with octadecyltrichorlosilane (Fig. 1A). Immersion of this system under an aqueous phase (e.g., in a Petri dish) leads to the formation of stable interfaces between the aqueous phase and liquid crystal.

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Brake and Abbott reported on the use of this experimental geometry to study the adsorption of surfactants and other amphiphiles at interfaces of liquid crystals [10&&]. Initial attempts to study the adsorption of surfactants at aqueous-liquid crystal interfaces were based on micrometerthick films of liquid crystals spread on the surface of water. These attempts were unsuccessful largely because the liquid crystals dewet the surface of the water upon addition of surfactants to the aqueous sub-phase. Hosting of the liquid crystal in the TEM grid stabilized the interface, and also permitted the aqueous phase in contact with the interface of the liquid crystal to be exchanged for a second aqueous phase of a different composition. The change in composition of the aqueous phase makes possible studies of surfactant adsorption as well as desorption. By observing the optical appearance of the liquid crystal with a polarized light microscope, the authors demonstrated the versatility of this simple experimental system for studying and quantifying equilibrium and dynamic adsorption of surfactants at interfaces between liquid crystals and aqueous phases. The system is more broadly useful for investigations in which liquid crystals are used to amplify interfacial phenomena at fluid interfaces into optical images. The interface of the liquid crystal defined by the experimental system shown in Fig. 1 differs substantially from the well-studied interface between liquid crystals and solids onto which surfactants and other amphiphiles have

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292 µm (copper) 283 µm (gold) Fig. 1. A) Experimental geometry used to create stable interfaces between aqueous phases and thermtropic liquid crystals at which the self-assembly of amphiphiles can be studied. Top and side views are shown. B – C) Optical images (crossed polars) of 5CB confined to a copper grid with a spacing of 292 Am that was supported on an OTS-coated glass slide and immersed under pure water (B) or an aqueous solution of 2.2 mM SDS (C). D – E) Schematic illustrations of the director profiles of the liquid crystal corresponding to planar anchoring of the liquid crystal at the interface with the aqueous phase (D) and homeotropic anchoring (E). Note that the orientation of the 5CB at the grid surface is normal to the grid-5CB interface. (B – C adapted with permission from Langmuir 2002, 18, 6101 – 6109. Copyright 2002 Am. Chem. Soc.; D – E adapted with permission from Langmuir ASAP article. Copyright 2005 Am. Chem. Soc.).

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been deposited [11– 15]. First, the thermodynamic states of the amphiphiles at these interfaces differ from each other: at equilibrium, the chemical potential of a surfactant in a surfactant-laden aqueous-liquid crystal interface is determined by the composition of the surfactant in the bulk aqueous phase. In contrast, amphiphiles deposited at solid surfaces and subsequently covered by liquid crystal are not generally in equilibrium with a bulk surfactant phase. Second, the mobility of the surfactants at aqueous-liquid crystal interfaces is substantially greater than the mobility of surfactants at solid-liquid crystal interfaces. The higher mobility permits reorganization and formation of domains in adsorbed monolayers. Third, the liquid crystal-aqueous interface is Fsoft_ and can deform under the influence of stresses developed in the liquid crystals, whereas most solid surfaces studied in the past are rigid and do not deform when in contact with liquid crystal. Finally, interfaces between aqueous phases and liquid crystals enable studies of the dynamics of surfactant adsorption, desorption, and lateral organization. These qualities of aqueous-liquid crystal interfaces, when combined, provide the basis of new approaches to studies of surfactants at interfaces as well as technological opportunities related to chemical and biological sensing. Using the above experimental system, Brake and Abbott initially confirmed that it was possible to observe the adsorption and desorption of surfactant at the interface between the liquid crystal and aqueous phase [10&&]. In the absence of surfactant, the orientation of the liquid crystal at the aqueous-liquid crystal interface is parallel to the interface. This orientation appears bright with dark radial lines when viewed with crossed polars (Fig. 1B). This appearance was explained by the authors in terms of the competing influences of the interfaces that confine the liquid crystal: The anchoring of the liquid crystal on the surfaces of the copper grids (along the edge of each grid square) is perpendicular to the vertical surface of the grid, and thus a converging radial profile is imposed on the liquid crystal. The liquid crystal, however, can minimize the elastic energy stored in this distorted state by Fescaping_ into the third dimension, thus also meeting the requirement of perpendicular anchoring on the treated glass surface (shown schematically in Fig. 1D). This leads to an escaped radial defect, which is also seen in other geometries (e.g., a cylindrical capillary filled with liquid crystal). It is interesting to note that the uniformity of the escaped radial defect appears to depend on the type of grid used to host the liquid crystal. Copper grids lead to regular, centrally-located defects, whereas the positions of the defects within liquid crystal hosted in gold grids varies across the grids. In contrast to the case of a surfactant-free aqueous phase, Brake and Abbott reported the addition of the surfactant SDS to the aqueous phase to result in a spontaneous change in the optical appearance of the

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liquid crystal (Fig. 1C). The dark optical appearance of the liquid crystal (viewed between crossed polars) is consistent with perpendicular (homeotropic) anchoring of the liquid crystal at the aqueous-liquid crystal interface (shown schematically in Fig. 1D). Note that each square of the copper grid possesses a bright edge; this corresponds to the region of the liquid crystal in which interactions with the surface of the grid perturb the orientation of the liquid crystal. The width of this region is comparable to the thickness of the film of the liquid crystal (¨20 Am). Note that the lateral dimension of each grid square is 280 –300 Am (Fig. 1A). A transition from planar to homeotropic orientation of the liquid crystal was also reported with increasing concentration of SDS in the bulk solution. Below ¨0.6 mM SDS, the orientation of 5CB at the aqueous-5CB interface was parallel to the interface, whereas concentrations of SDS  1 mM yielded a homeotropic orientation. The change in orientation of the liquid crystal caused by the addition of SDS could be reversed by decreasing the bulk concentration of SDS. The authors also observed the appearance of the 5CB upon addition of NaCl to the aqueous solution to be consistent with increased adsorption of SDS caused by screening of the electrostatic interactions between the SDS molecules adsorbed at the interface. This result suggests that the orientation of the 5CB is influenced by the areal density of SDS molecules at the liquid crystalaqueous interface. In an effort to explain these results, Rey has reported a thermodynamic model that connects the orientations of liquid crystals and the areal density of surfactant adsorbed at this interface [16]. The model uses an orientation-dependent form for the surface free energy to derive the adsorption isotherm for ionic surfactants at the aqueous-liquid crystal interface. The model predicts a discontinuous transition from planar to homeotropic anchoring with increasing bulk concentration of surfactant. In contrast, recent experimental measurements have revealed a continuous transition between planar and homeotropic anchoring as a function of the increasing concentration of surfactant, with intermediate surfactant concentrations leading to stable tilted states of the liquid crystal (see, for example, Fig. 3F, J) [17&]. Here we note that Rey considers only interfacial energies of the system in his model. His analysis does not include the influence of the elastic energy stored in the film of liquid crystal confined to the TEM grids, as is present in the experiments. Finally, Brake and Abbott observed the orientations of the liquid crystal to evolve over time following the addition of SDS to an aqueous solution. Although it has not yet been exhaustively investigated (see below), initial observations indicate that the aqueous-liquid crystal interface can report dynamic phenomena that occur with characteristics times of seconds (the liquid crystals themselves reorient within tens of milliseconds). This

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characteristic of the system suggests that it may form the basis of a useful tool for measuring the dynamics of surfactant adsorption and desorption.

3. Influence of molecular structure of surfactants on orientation of liquid crystals The molecular-level mechanisms by which surfactants self-assemble at aqueous interfaces of thermotropic liquid crystals and couple to the order within liquid crystals remain to be fully elucidated. Several studies, however, do provide insights into these mechanisms by reporting the dependence of anchoring on the architecture of surfactants. One study has exploited the experimental system described above to develop criteria for surfactants that give rise to a particular orientation of 5CB.[18] Surfactants that have a bolaform structure [(11-hydroxyundecyl)trimethyl ammonium bromide (HTAB), dodecyl-1,12-bis(trimethylammonium bromide) (DBTAB), 11-(ferrocenylundecyl)trimethylammonium bromide (FTMA)] adopt looped configurations at air-water/oilwater interfaces. Bolaform surfactants were demonstrated to cause planar anchoring of 5CB (Fig. 2A –D). In contrast, classical linear surfactants [SDS, alkyltrimethylammonium halides (Cn TABs, n > 8), and N,N-dimethylferrocenylalkylammonium bromides (FCn ABs, n > 12)] that assume tilted orientations at air – water/oil– water interfaces caused homeotropic orientation of 5CB at concentrations greater than those associated with the onset of adsorption to air – water interfaces. By comparing SDS, dodecyl trimethylammonium halide (DTAB), and tetraethylene glycol monododecyl ether (C12E4), this study did not measure the nature of the surfactant headgroups to directly influence the orientation of 5CB (Fig. 2E – J). However, the orientation of the 5CB was found to depend on the length of the aliphatic chain and the areal density of the adsorbed surfactant. Surfactants with short alkyl chains (n = 8 for Cn TAB and n = 7 and 12 for FCn AB), caused the orientation of 5CB to remain parallel to the interface up to concentrations at which the 5CB began to be solubilized by the surfactant. These results support the proposal that interactions between the aliphatic chains of the surfactant and 5CB, which are influenced by the conformations of the tails of the surfactants, largely dictate the orientation of the 5CB (see below). Additional support for this proposition was obtained in a subsequent study in which the effects of branching of the aliphatic tails of surfactants on the anchoring of liquid crystals were investigated. Lockwood and coworkers compared aqueous-liquid crystal interfaces that were laden with linear surfactants [SDS, sodium dodecanesulfonate (LDS), and isomerically pure linear sodium dodecylbenzenesulfonate (L-DBS)] to interfaces laden with branched sodium dodecylbenzenesulfonate (BR-

DBS) [17&]. Linear surfactants caused homeotropic anchoring of the nematic liquid crystal TL205 (a mixture of cyclohexane-fluorinated biphenyls and fluorinated terphenyls) above a threshold concentration of surfactant and planar anchoring at lower concentrations (Fig. 3A, B). In contrast, BR-DBS caused planar anchoring of TL205 at all concentrations up to the critical micelle concentration of the surfactant (Fig. 3C, D). The effect of branching of the aliphatic tail was proposed to hinder packing of surfactant at the interface into an orderly assembly, analogous to the decreased melting temperatures in branched versus linear alkanes [19]. A second portion of the study further examined the effect of surfactant organization on the orientations of liquid crystals. The authors used LDS and a commercial L-DBS to probe the influence of surfactant tail organization on the anchoring of liquid crystals. LDS caused homeotropic anchoring of both TL205 and 5CB, whereas commercial L-DBS caused predominantly planar and tilted orientations of both TL205 and 5CB (Fig. 3E –J). The authors relate these results to work by Richmond and coworkers, who used vibrational sum-frequency spectroscopy to probe the order of LDS and commercial L-DBS at water-CCl4 interfaces as part of their broader effort to characterize the behavior of surfactants at water – oil interfaces. Commercial L-DBS, which comprises a mixture of ortho and para-isomers, was characterized by Richmond_s group to form less ordered monolayers than sodium dodecanesulfonate at oil – water interfaces at room temperature [20,21]. Lockwood and coworkers attributed the lack of homeotropic anchoring by commercial L-DBS to the disordered nature of the tails at water – oil interfaces. The proposition that interactions between surfactant tails and liquid crystals largely dictate the resulting orientations of the liquid crystals is supported by a number of past studies, including studies by Barmentlo and Vrehen using second-harmonic generation and surface pressure measurements of mixed Langmuir monolayers of 4V-octyl-4cyanobiphenyl (8CB) and pentadecanoic acid on water [22]. Their results revealed that compression of the mixed monolayers leads to a decrease of the tilt angle (measured from the interface normal) of 8CB, which the authors attributed to interactions between 8CB and the tails of the pentadecanoic acid. It is worth noting, however, that these measurements were made with 8CB monolayers, not bulk liquid crystal.

4. Redox-active surfactants and reversible anchoring transitions As mentioned above, an interesting attribute of the experimental system described in this article is that the surfactants adsorbed at the interfaces of the liquid crystals are in equilibrium with bulk surfactant solutions.

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Fig. 2. Influence of surfactant structure on the anchoring of 5CB at an interface between an aqueous solution of surfactant and the liquid crystal. Optical images (crossed polars) of 5CB confined to a copper grid that was supported on an OTS-coated glass slide and contacted with aqueous solutions of A) 0.01 mM FTMA, B) 0.01 mM oxidized FTMA, C) 10 mM DBTAB, D) 10 mM HTAB, E) 0.1 mM SDS, F) 10 mM SDS, G) 0.1 mM DTAB, H) 10 mM DTAB, I) 1 AM C12E4, J) 100 AM C12E4. A – D correspond to aqueous solutions of surfactant with 0.1 M Li2SO4 at pH 2; E – J were obtained with the surfactants dissolved in pure water. Scale bars are 300 Am. (Adapted with permission from Langmuir 2003 19, 6436 – 6442. Copyright 2003 Am. Chem. Soc.).

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Fig. 3. A – D) Optical images (crossed polars) of TL205 confined to a gold grid supported on an OTS-coated glass slide and in contact with aqueous solutions of A) 0.01 mM pure L-DBS, B) 1.2 mM pure L-DBS, C) 0.3 mM BR-DBS, D) 3.1 mM BR-DBS at 60 -C. E – J) Optical images (crossed polars) of 5CB confined to a gold grid supported on an OTS-coated glass slide and in contact with aqueous solutions of E) 0.12 mM LDS, F) 1.2 mM LDS, G) 12 mM LDS, H) 2 AM commercial L-DBS, I) 20 AM commercial L-DBS, J) 200 AM commercial L-DBS at room temperature. G – I were obtained with the surfactants dissolved in aqueous 100 mM NaCl, all other images were obtained by dissolving the surfactants in water. Scale bars are 300 Am. (Adapted with permission from Langmuir ASAP article. Copyright 2005 Am. Chem. Soc.).

This makes possible strategies based on reversible adsorption of surfactant at the aqueous-liquid crystal interface to control the orientation of the liquid crystal. One study that has exploited this characteristic used the reversible adsorption of the redox-active surfactant FTMA (see Fig. 2A and B) at the interface between an aqueous solution of surfactant and 5CB [23]. As discussed above, the anchoring of 5CB is insensitive to the oxidation state of FTMA in aqueous solution, owing to the bolaform nature of reduced and oxidized FTMA [18]. However, oxidation of FTMA in aqueous mixtures of cetyltrimethylammonium bromide (CTAB) and FTMA does lead to a transition from near-planar to homeotropic orientation of 5CB. The change in orientation of 5CB results from the competitive adsorption of CTAB (which causes homeotropic anchoring) and either FTMA or oxidized FTMA (both of which cause near-planar anchoring). Oxidation of FTMA in a mixture containing CTAB and FTMA leads to an increase in the surface concentration of CTAB because oxidized FTMA is less surface-active than FTMA. Reversible control of the anchoring of 5CB was achieved by sequential contact of the liquid crystal with aqueous solutions containing CTAB and FTMA and then CTAB and oxidized FTMA. The addition of chemical oxidizing or reducing agents

was also observed to change the orientation of the liquid crystal. A particularly interesting aspect of the study reported with FTMA was that time-dependent changes in the optical texture of 5CB were quantitatively interpreted in terms of the tilt angle of the 5CB at the aqueous-LC interface (Fig. 4). The transition from homeotropic to near-planar anchoring of 5CB (and vice versa) was accompanied by a continuous change in the interference colors of 5CB when viewed using white light. The change in color is related to a change in the tilt angle of the 5CB at the water-5CB interface during the competitive adsorption of FTMA and CTAB. Colors observed in the liquid crystal were matched to a Michel – Levy chart to estimate the effective birefringence of the liquid crystal, and the tilt angle was subsequently calculated. The tilt angle of 5CB at the aqueous-5CB interface changed continuously from homeotropic (¨0-) to near-planar (¨74-) during the competitive adsorption of the mixture of CTAB and reduced FTMA (Fig. 4). Similar (but inverted) changes in the effective birefringence and tilt angle were observed when exchanging solutions containing mixtures of CTAB and FTMA for solutions containing CTAB and oxidized FTMA. These results demonstrate that it is possible to quantify the orientation of the liquid crystal during

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Time [s] Fig. 4. Quantitation of the tilt angles of 5CB at the aqueous-5CB interface estimated from interference colors observed in the liquid crystal during surfactant adsorption and desorption. The optical images (crossed polars) correspond to films of 5CB confined to copper grids (292 Am hole size) that were supported on an OTS-coated glass slide, (A) equilibrated for 90 min in a solution of 5 AM CTAB and 50 AM oxidized FTMA in aqueous 0.1 M Li2SO4 at pH 2 and exchanged into a solution of 5 AM CTAB and 50 AM FTMA in aqueous 0.1 M Li2SO4 at pH 2 for (B) 2 min and (C) 5 min. (D) The corresponding effective birefringence (squares) and tilt angles (circles) are plotted as a function of time following solution exchange from FTMA to oxidized FTMA. (E) The corresponding effective birefringence (squares) and tilt angles (circles) are plotted as a function of time for the solution exchange from oxidized FTMA to FTMA. The time axis is identical for both panels D and E. The images were obtained by orthoscopic illumination of the sample between crossed nicols at 4 magnification. All scale bars represent 300 Am. (Reproduced with permission from Langmuir 2003 19, 8629 – 8637. Copyright 2003 Am. Chem. Soc.).

dynamic processes and suggest the basis for a novel approach for measuring the time-dependent adsorption of amphiphiles at surfaces.

5. Self-assembly of phospholipids and interactions of proteins with phospholipid-decorated interfaces of thermotropic liquid crystals Whereas the studies described above address the selfassembly of soluble surfactants at aqueous interfaces of thermotropic liquid crystals, a second series of papers have demonstrated that the same experimental system can be used to characterize fluid interfacial assemblies formed spontaneously by the self-assembly of phospholipids at aqueous-liquid crystal interfaces [24&&,25]. The presence and

lateral organization of the phospholipids 1-palmitoyl-2oleoylphosphatidylcholine (POPC), dipalmitoylphosphatidylcholine (DPPC), and dilauroylphosphatidylcholine (DLPC) at the aqueous-liquid crystal interface lead to interface-driven orientational transitions within the liquid crystal (Fig. 5). The process by which phospholipids and soluble surfactants cause the transition to homeotropic anchoring are clearly different from one another: soluble surfactants adsorb to the aqueous-liquid crystal interface largely as monomeric species causing a spatially uniform appearance of the liquid crystal throughout the adsorption process. Phospholipids, in contrast, are effectively insoluble in an aqueous solution, and were prepared as small unilamellar vesicles (SUVs). The process of transfer is seen to proceed through the presence of spatial domains of phospholipid at the interface (manifested as patterned

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Fig. 5. Optical images (crossed polars) of 5CB during the transfer of L-DLPC to the aqueous-5CB interface from aqueous dispersions of vesicles containing 0.1 mM L-DLPC and 0.001 mM TR-DPPE in TBS. The polarized light micrographs (A – D) and corresponding epif luorescence micrographs (E – H) of the phospholipids at the aqueous-5CB interfaces were recorded approximately (A and E) 0 min, (B and F) 15 min, (C and G) 45 min, and (D and H) 120 min after contact with the dispersion of phospholipid. The epifluorescence micrographs shown in parts F and G have been digitally enhanced to accentuate the contrast between domains. The actual intensities of the dark and bright regions were similar to those shown in parts E and H, respectively. Scale bar, 150 Am. (Reproduced with permission from Langmuir 2005 21, 2218 – 2228. Copyright 2005 Am. Chem. Soc.).

orientations in the liquid crystal – see, for example Fig. 5B – C) during the adsorption process rather than a uniform coverage of lipid. The adsorption of lipids to the aqueousliquid crystal interface proceeds most likely by fusion of SUVs to the interface, with little subsequent desorption of the lipid [26,27]. In an effort to characterize the formation of lipid layers at aqueous-liquid crystal interfaces, Brake and coworkers doped DPPC or DLPC with a fluorescently-labeled lipid (Texas Red-dipalmitoylphosphoethanolamine (TR-DPPE)) and used quantitative epifluorescence microscopy to measure the saturation coverage of lipid at the interface to be 49 T 8% relative to hydrated lipid bilayers, suggesting monolayer coverage at the interface [25&]. The areal density of the lipid could be controlled by adsorbing lipids to the aqueous-5CB interface from either lipid vesicles or mixed micelles of dodecyltrimethylammonium and lipid (from 42 T 10% to 102 T 18% of saturation coverage). Fluorescence recovery after photobleaching (FRAP) measurements revealed the lateral mobility of lipids assembled at the interface with the liquid crystal to be (6 T 1)  10 12 m2/s for densely packed monolayers, values that are comparable to the mobility of lipids in cell membranes [28]. The lateral diffusivity of the lipids varied from (0.2 T 0.1)  10 12 to (15 T 2)  10 12 m2/s as a function of the surface coverage and composition of lipid. The authors also demonstrated that the lipids were compartmentalized within individual grid squares and created patterned arrays of different lipids among adjacent grid squares. Given the evidence (described above) that the primary interactions dictating the orientations of liquid crystals at

surfactant-decorated interfaces are between the surfactant tails and the liquid crystal, it is likely that the tails of phospholipids adsorbed at aqueous-liquid crystal interfaces also play a dominant role. The results of recent simulations of 5CB and an analog of TL205 solubilized in hydrated phospholipid bilayers support this proposition [29]. The simulations show ordering of the lipid tails surrounding mesogens within the bilayer. The mesogens also demonstrate a preferred orientation that is parallel to the lipid tail. It is interesting to note that simulations of lipid bilayers containing cholesterol, also a molecule capable of forming a liquid crystal, have demonstrated a coupling between the order of aliphatic tails and the alignment of cholesteric mesogens [30]. The formation of phospholipid monolayers at aqueous-liquid crystal interfaces is of particular interest because proteins are known to bind and reorganize phospholipid-laden interfaces. To investigate the possible coupling of proteins and lipids at interfaces of liquid crystals, Brake and coworkers studied the interactions of proteins with monolayers of DPPC and DLPC spontaneously assembled at aqueous-5CB interfaces (Fig. 6) [24&&]. Specific binding of the protein phospholipase A2 to D-DPPC at the interface lead to observable orientational transitions in the liquid crystal. In contrast, nonspecific interactions between proteins such as albumin, lysozyme, and cytochrome-c and the lipid-laden interface did not produce observable changes in the ordering of the liquid crystal. Concurrent epifluorescence and polarized light imaging of labeled lipids and proteins at the aqueous-liquid crystal interface also demonstrated that spatially-patterned orientations of liquid crystals observed

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Fig. 6. Optical images of nematic 5CB caused by specific interactions of proteins with phospholipid-decorated interfaces of the liquid crystal. (A – C) Optical images (crossed polars) of 5CB following the introduction of 1 nM PLA2 into an aqueous solution of TBS containing 5 mM CaCl2 contacting the 5CB interface laden with L-DLPC and 1 mol% TR-DHPE. Images were obtained at (A) 0 min, (B) 45 min, and (C) 90 min. Scale bar, 150 Am. The black regions correspond to a homeotropic orientation of the LC at the interface between the LC and aqueous phase and the bright domains correspond to tilted or planar orientations. D) The epifluorescence micrograph of the TR-DHPE in the lipid-laden aqueous-5CB interface corresponding to the optical texture of 5CB shown in (B). E) Optical image of 5CB after 90 min of exposure of a L-DPPC-laden aqueous-5CB (TBS with 5 mM CaCl2) interface to 1 nM PLA2. F) Optical image of 5CB after 90 min of exposure of its L-DPPC-laden aqueous (TBS with 5 mM ethylenediaminetetraacetic acid) interface to 1 nM PLA2 in the absence of Ca2+. (Reproduced with permission from Science 2003 302, 2096 – 2097. Copyright 2003 Science).

during specific binding of phospholipase A2 to the interface, and during the subsequent hydrolysis of lipids by phospholipase A2, reflect the lateral organization (domains) of the lipids and proteins at the aqueousliquid crystal interface (Fig. 6B, D). The observation that the orientations of liquid crystals are coupled to the presence and organization of the lipid and protein leads to the possibility of creating a variety of biomimetic layers at aqueous-liquid crystal interfaces at which biomolecular phenomena are amplified into ordering transitions in the liquid crystals.

6. Conclusions and outlook The main conclusion of this article is that the simple experimental system shown in Fig. 1 provides the basis of a versatile set up for studying the self-assembly of soluble surfactants and phospholipids at interfaces of thermotropic liquid crystals. The geometry of this experimental system makes optical observation of the ordering of the liquid crystal straightforward. The studies summarized in this article demonstrate that the ordering of the liquid crystal is indeed strongly coupled to both the presence and organization of amphiphiles at the interface. To date, a primary factor influencing ordering of the liquid crystal in the presence of either surfactants or phospholipids appears to be the structure and organization of the tails of the amphiphiles. Interfaces that mimic some properties of biological membranes can be prepared by incorporating

phospholipids at the aqueous-liquid crystal interface (Fig. 6B, D). The biomimetic nature of this interface presents many opportunities to develop schemes for sensing biomolecular phenomena. The use of biologically-relevant lipids (either naturally-occurring functional lipids or those synthetically produced to incorporate a biological activity) will expand the types of biomolecular interactions touched on in the work described here.

Acknowledgements We thank Jugal Gupta for helpful discussions in preparing this manuscript. This research was partially supported by a grant from the National Science Foundation (CTS-0327489) and the donors of the Petroleum Research Fund (ACS-PRF 35409-AC7).

References and recommended readings [1] S. Manne, H.E. Gaub, Molecular organization of surfactants at solid – liquid interfaces, Science 270 (1995) 1480 – 1482. [2] M.M. Knock, G.R. Bell, E.K. Hill, H.J. Turner, C.D. Bain, Sumfrequency spectroscopy of surfactant monolayers at the oil – water interface, J. Phys. Chem. B 107 (2003) 10801 – 10814. [3] C.D. Bain, Sum-frequency vibrational spectroscopy of the solid/liquid interface, J. Chem. Soc. Faraday Trans. 91 (1995) 1281 – 1296. [4] J.C. Conboy, M.C. Messmer, G.L. Richmond, Dependence of alkyl chain conformation of simple ionic surfactants on head group functionality as studied by vibrational sum-frequency spectroscopy, J. Phys. Chem. B 34 (1997) 6724 – 6733.

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